Doc. no. | D???? |
Date: | 2025-05-24 |
Project: | Programming Language C++ |
Reply to: | Jonathan Wakely <lwgchair@gmail.com> |
Revised 2025-05-24 at 10:56:29 UTC
Reference ISO/IEC IS 14882:2020(E)
Also see:
This document contains only library issues which have been closed by the Library Working Group (LWG) after being found to be defects in the standard. That is, issues which have a status of DR, TC1, C++11, C++14, C++17, or Resolved. See the Library Closed Issues List for issues closed as non-defects. See the Library Active Issues List for active issues and more information. The introductory material in that document also applies to this document.
Section: 16.4.3.3 [using.linkage] Status: TC1 Submitter: Beman Dawes Opened: 1997-11-16 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
The change specified in the proposed resolution below did not make it into the Standard. This change was accepted in principle at the London meeting, and the exact wording below was accepted at the Morristown meeting.
Proposed resolution:
Change 16.4.3.3 [using.linkage] paragraph 2 from:
It is unspecified whether a name from the Standard C library declared with external linkage has either extern "C" or extern "C++" linkage.
to:
Whether a name from the Standard C library declared with external linkage has extern "C" or extern "C++" linkage is implementation defined. It is recommended that an implementation use extern "C++" linkage for this purpose.
Section: 17.5 [support.start.term] Status: TC1 Submitter: Steve Clamage Opened: 1997-12-12 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
We appear not to have covered all the possibilities of
exit processing with respect to
atexit registration.
Example 1: (C and C++)
#include <stdlib.h> void f1() { } void f2() { atexit(f1); } int main() { atexit(f2); // the only use of f2 return 0; // for C compatibility }
At program exit, f2 gets called due to its registration in main. Running f2 causes f1 to be newly registered during the exit processing. Is this a valid program? If so, what are its semantics?
Interestingly, neither the C standard, nor the C++ draft standard nor the forthcoming C9X Committee Draft says directly whether you can register a function with atexit during exit processing.
All 3 standards say that functions are run in reverse order of their registration. Since f1 is registered last, it ought to be run first, but by the time it is registered, it is too late to be first.
If the program is valid, the standards are self-contradictory about its semantics.
Example 2: (C++ only)
void F() { static T t; } // type T has a destructor int main() { atexit(F); // the only use of F }
Function F registered with atexit has a local static variable t, and F is called for the first time during exit processing. A local static object is initialized the first time control flow passes through its definition, and all static objects are destroyed during exit processing. Is the code valid? If so, what are its semantics?
Section 18.3 "Start and termination" says that if a function F is registered with atexit before a static object t is initialized, F will not be called until after t's destructor completes.
In example 2, function F is registered with atexit before its local static object O could possibly be initialized. On that basis, it must not be called by exit processing until after O's destructor completes. But the destructor cannot be run until after F is called, since otherwise the object could not be constructed in the first place.
If the program is valid, the standard is self-contradictory about its semantics.
I plan to submit Example 1 as a public comment on the C9X CD, with a recommendation that the results be undefined. (Alternative: make it unspecified. I don't think it is worthwhile to specify the case where f1 itself registers additional functions, each of which registers still more functions.)
I think we should resolve the situation in the whatever way the C committee decides.
For Example 2, I recommend we declare the results undefined.
[See reflector message lib-6500 for further discussion.]
Proposed resolution:
Change section 18.3/8 from:
First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit are called in the reverse order of their registration. A function registered with atexit before an object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after an object obj2 of static storage duration is initialized will be called before obj2's destruction starts.
to:
First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Non-local objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit are called in the reverse order of their registration, except that a function is called after any previously registered functions that had already been called at the time it was registered. A function registered with atexit before a non-local object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after a non-local object obj2 of static storage duration is initialized will be called before obj2's destruction starts. A local static object obj3 is destroyed at the same time it would be if a function calling the obj3 destructor were registered with atexit at the completion of the obj3 constructor.
Rationale:
See 99-0039/N1215, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.
Section: 27.4.3.7.8 [string.swap] Status: TC1 Submitter: Jack Reeves Opened: 1997-12-11 Last modified: 2016-11-12
Priority: Not Prioritized
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Duplicate of: 87
Discussion:
At the very end of the basic_string class definition is the signature: int compare(size_type pos1, size_type n1, const charT* s, size_type n2 = npos) const; In the following text this is defined as: returns basic_string<charT,traits,Allocator>(*this,pos1,n1).compare( basic_string<charT,traits,Allocator>(s,n2);
Since the constructor basic_string(const charT* s, size_type n, const Allocator& a = Allocator()) clearly requires that s != NULL and n < npos and further states that it throws length_error if n == npos, it appears the compare() signature above should always throw length error if invoked like so: str.compare(1, str.size()-1, s); where 's' is some null terminated character array.
This appears to be a typo since the obvious intent is to allow either the call above or something like: str.compare(1, str.size()-1, s, strlen(s)-1);
This would imply that what was really intended was two signatures int compare(size_type pos1, size_type n1, const charT* s) const int compare(size_type pos1, size_type n1, const charT* s, size_type n2) const; each defined in terms of the corresponding constructor.
Proposed resolution:
Replace the compare signature in 27.4.3 [basic.string] (at the very end of the basic_string synopsis) which reads:
int compare(size_type pos1, size_type n1,
const charT* s, size_type n2 = npos) const;
with:
int compare(size_type pos1, size_type n1,
const charT* s) const;
int compare(size_type pos1, size_type n1,
const charT* s, size_type n2) const;
Replace the portion of 27.4.3.7.8 [string.swap] paragraphs 5 and 6 which read:
int compare(size_type pos, size_type n1,
Returns:
charT * s, size_type n2 = npos) const;
basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
basic_string<charT,traits,Allocator>( s, n2))
with:
int compare(size_type pos, size_type n1,
Returns:
const charT * s) const;
Returns:
basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
basic_string<charT,traits,Allocator>( s ))
int compare(size_type pos, size_type n1,
const charT * s, size_type n2) const;
basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
basic_string<charT,traits,Allocator>( s, n2))
Editors please note that in addition to splitting the signature, the third argument becomes const, matching the existing synopsis.
Rationale:
While the LWG dislikes adding signatures, this is a clear defect in the Standard which must be fixed. The same problem was also identified in issues 7 (item 5) and 87.
Section: 27 [strings] Status: TC1 Submitter: Matt Austern Opened: 1997-12-15 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
(1) In 27.4.3.7.4 [string.insert], the description of template <class InputIterator> insert(iterator, InputIterator, InputIterator) makes no sense. It refers to a member function that doesn't exist. It also talks about the return value of a void function.
(2) Several versions of basic_string::replace don't appear in the class synopsis.
(3) basic_string::push_back appears in the synopsis, but is never described elsewhere. In the synopsis its argument is const charT, which doesn't makes much sense; it should probably be charT, or possible const charT&.
(4) basic_string::pop_back is missing.
(5) int compare(size_type pos, size_type n1, charT* s, size_type n2 = npos) make no sense. First, it's const charT* in the synopsis and charT* in the description. Second, given what it says in RETURNS, leaving out the final argument will always result in an exception getting thrown. This is paragraphs 5 and 6 of 27.4.3.7.8 [string.swap]
(6) In table 37, in section 27.2.2 [char.traits.require], there's a note for X::move(s, p, n). It says "Copies correctly even where p is in [s, s+n)". This is correct as far as it goes, but it doesn't go far enough; it should also guarantee that the copy is correct even where s in in [p, p+n). These are two orthogonal guarantees, and neither one follows from the other. Both guarantees are necessary if X::move is supposed to have the same sort of semantics as memmove (which was clearly the intent), and both guarantees are necessary if X::move is actually supposed to be useful.
Proposed resolution:
ITEM 1: In 21.3.5.4 [lib.string::insert], change paragraph 16 to
EFFECTS: Equivalent to insert(p - begin(), basic_string(first, last)).
ITEM 2: Not a defect; the Standard is clear.. There are ten versions of replace() in
the synopsis, and ten versions in 21.3.5.6 [lib.string::replace].
ITEM 3: Change the declaration of push_back in the string synopsis (21.3,
[lib.basic.string]) from:
void push_back(const charT)
to
void push_back(charT)
Add the following text immediately after 21.3.5.2 [lib.string::append], paragraph 10.
void basic_string::push_back(charT c);
EFFECTS: Equivalent to append(static_cast<size_type>(1), c);
ITEM 4: Not a defect. The omission appears to have been deliberate.
ITEM 5: Duplicate; see issue 5 (and 87).
ITEM 6: In table 37, Replace:
"Copies correctly even where p is in [s, s+n)."
with:
"Copies correctly even where the ranges [p, p+n) and [s,
s+n) overlap."
Section: 28.3.3.1.6 [locale.statics] Status: TC1 Submitter: Matt Austern Opened: 1997-12-24 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
It appears there's an important guarantee missing from clause 22. We're told that invoking locale::global(L) sets the C locale if L has a name. However, we're not told whether or not invoking setlocale(s) sets the global C++ locale.
The intent, I think, is that it should not, but I can't find any such words anywhere.
Proposed resolution:
Add a sentence at the end of 28.3.3.1.6 [locale.statics], paragraph 2:
No library function other than
locale::global()
shall affect the value returned bylocale()
.
Section: 17.6.3 [new.delete] Status: TC1 Submitter: Steve Clamage Opened: 1998-01-04 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
Scott Meyers, in a comp.std.c++ posting: I just noticed that section 3.7.3.1 of CD2 seems to allow for the possibility that all calls to operator new(0) yield the same pointer, an implementation technique specifically prohibited by ARM 5.3.3.Was this prohibition really lifted? Does the FDIS agree with CD2 in the regard? [Issues list maintainer's note: the IS is the same.]
Proposed resolution:
Change the last paragraph of 3.7.3 from:
Any allocation and/or deallocation functions defined in a C++ program shall conform to the semantics specified in 3.7.3.1 and 3.7.3.2.
to:
Any allocation and/or deallocation functions defined in a C++ program, including the default versions in the library, shall conform to the semantics specified in 3.7.3.1 and 3.7.3.2.
Change 3.7.3.1/2, next-to-last sentence, from :
If the size of the space requested is zero, the value returned shall not be a null pointer value (4.10).
to:
Even if the size of the space requested is zero, the request can fail. If the request succeeds, the value returned shall be a non-null pointer value (4.10) p0 different from any previously returned value p1, unless that value p1 was since passed to an operator delete.
5.3.4/7 currently reads:
When the value of the expression in a direct-new-declarator is zero, the allocation function is called to allocate an array with no elements. The pointer returned by the new-expression is non-null. [Note: If the library allocation function is called, the pointer returned is distinct from the pointer to any other object.]
Retain the first sentence, and delete the remainder.
18.5.1 currently has no text. Add the following:
Except where otherwise specified, the provisions of 3.7.3 apply to the library versions of operator new and operator delete.
To 18.5.1.3, add the following text:
The provisions of 3.7.3 do not apply to these reserved placement forms of operator new and operator delete.
Rationale:
See 99-0040/N1216, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.
Section: 22.9.2 [template.bitset] Status: TC1 Submitter: Matt Austern Opened: 1998-01-22 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
(1) bitset<>::operator[] is mentioned in the class synopsis (23.3.5), but it is not documented in 23.3.5.2.
(2) The class synopsis only gives a single signature for bitset<>::operator[], reference operator[](size_t pos). This doesn't make much sense. It ought to be overloaded on const. reference operator[](size_t pos); bool operator[](size_t pos) const.
(3) Bitset's stream input function (23.3.5.3) ought to skip all whitespace before trying to extract 0s and 1s. The standard doesn't explicitly say that, though. This should go in the Effects clause.
Proposed resolution:
ITEMS 1 AND 2:
In the bitset synopsis (22.9.2 [template.bitset]),
replace the member function
reference operator[](size_t pos);
with the two member functions
bool operator[](size_t pos) const;
reference operator[](size_t pos);
Add the following text at the end of 22.9.2.3 [bitset.members],
immediately after paragraph 45:
bool operator[](size_t pos) const;
Requires: pos is valid
Throws: nothing
Returns:test(pos)
bitset<N>::reference operator[](size_t pos);
Requires: pos is valid
Throws: nothing
Returns: An object of typebitset<N>::reference
such that(*this)[pos] == this->test(pos)
, and such that(*this)[pos] = val
is equivalent tothis->set(pos, val);
Rationale:
The LWG believes Item 3 is not a defect. "Formatted input" implies the desired semantics. See 31.7.5.3 [istream.formatted].
Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: William M. Miller Opened: 1998-03-03 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
In 27.6.1.2.3, there is a reference to "eos", which is the only one in the whole draft (at least using Acrobat search), so it's undefined.
Proposed resolution:
In [istream::extractors], replace "eos" with "charT()"
Section: 28.3.3.1.4 [locale.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
locale::combine is the only member function of locale (other than constructors and destructor) that is not const. There is no reason for it not to be const, and good reasons why it should have been const. Furthermore, leaving it non-const conflicts with 22.1.1 paragraph 6: "An instance of a locale is immutable."
History: this member function originally was a constructor. it happened that the interface it specified had no corresponding language syntax, so it was changed to a member function. As constructors are never const, there was no "const" in the interface which was transformed into member "combine". It should have been added at that time, but the omission was not noticed.
Proposed resolution:
In 28.3.3.1 [locale] and also in 28.3.3.1.4 [locale.members], add "const" to the declaration of member combine:
template <class Facet> locale combine(const locale& other) const;
Section: 28.3.3.1.4 [locale.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
locale::name() is described as returning a string that can be passed to a locale constructor, but there is no matching constructor.
Proposed resolution:
In 28.3.3.1.4 [locale.members], paragraph 5, replace
"locale(name())
" with
"locale(name().c_str())
".
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
The new virtual members ctype_byname<char>::do_widen and do_narrow did not get edited in properly. Instead, the member do_widen appears four times, with wrong argument lists.
Proposed resolution:
The correct declarations for the overloaded members
do_narrow
and do_widen
should be copied
from 28.3.4.2.4 [facet.ctype.special].
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
This section describes the process of parsing a text boolean value from the input
stream. It does not say it recognizes either of the sequences "true" or
"false" and returns the corresponding bool value; instead, it says it recognizes
only one of those sequences, and chooses which according to the received value of a
reference argument intended for returning the result, and reports an error if the other
sequence is found. (!) Furthermore, it claims to get the names from the ctype<>
facet rather than the numpunct<> facet, and it examines the "boolalpha"
flag wrongly; it doesn't define the value "loc"; and finally, it computes
wrongly whether to use numeric or "alpha" parsing.
I believe the correct algorithm is "as if":
// in, err, val, and str are arguments. err = 0; const numpunct<charT>& np = use_facet<numpunct<charT> >(str.getloc()); const string_type t = np.truename(), f = np.falsename(); bool tm = true, fm = true; size_t pos = 0; while (tm && pos < t.size() || fm && pos < f.size()) { if (in == end) { err = str.eofbit; } bool matched = false; if (tm && pos < t.size()) { if (!err && t[pos] == *in) matched = true; else tm = false; } if (fm && pos < f.size()) { if (!err && f[pos] == *in) matched = true; else fm = false; } if (matched) { ++in; ++pos; } if (pos > t.size()) tm = false; if (pos > f.size()) fm = false; } if (tm == fm || pos == 0) { err |= str.failbit; } else { val = tm; } return in;
Notice this works reasonably when the candidate strings are both empty, or equal, or when one is a substring of the other. The proposed text below captures the logic of the code above.
Proposed resolution:
In 28.3.4.3.2.3 [facet.num.get.virtuals], in the first line of paragraph 14, change "&&" to "&".
Then, replace paragraphs 15 and 16 as follows:
Otherwise target sequences are determined "as if" by calling the members
falsename()
andtruename()
of the facet obtained byuse_facet<numpunct<charT> >(str.getloc())
. Successive characters in the range[in,end)
(see [lib.sequence.reqmts]) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iteratorin
is compared toend
only when necessary to obtain a character. If and only if a target sequence is uniquely matched,val
is set to the corresponding value.
The
in
iterator is always left pointing one position beyond the last character successfully matched. Ifval
is set, then err is set tostr.goodbit
; or tostr.eofbit
if, when seeking another character to match, it is found that(in==end)
. Ifval
is not set, then err is set tostr.failbit
; or to(str.failbit|str.eofbit)
if the reason for the failure was that(in==end)
. [Example: for targetstrue
:"a" andfalse
:"abb", the input sequence "a" yieldsval==true
anderr==str.eofbit
; the input sequence "abc" yieldserr=str.failbit
, within
ending at the 'c' element. For targetstrue
:"1" andfalse
:"0", the input sequence "1" yieldsval==true
anderr=str.goodbit
. For empty targets (""), any input sequence yieldserr==str.failbit
. --end example]
Section: 28.3.4.3.2.2 [facet.num.get.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
In the list of num_get<> non-virtual members on page 22-23, the member that parses bool values was omitted from the list of definitions of non-virtual members, though it is listed in the class definition and the corresponding virtual is listed everywhere appropriate.
Proposed resolution:
Add at the beginning of 28.3.4.3.2.2 [facet.num.get.members] another get member for bool&, copied from the entry in 28.3.4.3.2 [locale.num.get].
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Duplicate of: 10
Discussion:
In the definitions of codecvt<>::do_out and do_in, they are specified to return noconv if "no conversion is needed". This definition is too vague, and does not say normatively what is done with the buffers.
Proposed resolution:
Change the entry for noconv in the table under paragraph 4 in section 28.3.4.2.5.3 [locale.codecvt.virtuals] to read:
noconv
:internT
andexternT
are the same type, and input sequence is identical to converted sequence.
Change the Note in paragraph 2 to normative text as follows:
If returns
noconv
,internT
andexternT
are the same type and the converted sequence is identical to the input sequence[from,from_next)
.to_next
is set equal toto
, the value ofstate
is unchanged, and there are no changes to the values in[to, to_limit)
.
Section: 28.3.4.4.1.3 [facet.numpunct.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
The synopsis for numpunct<>::do_thousands_sep, and the definition of numpunct<>::thousands_sep which calls it, specify that it returns a value of type char_type. Here it is erroneously described as returning a "string_type".
Proposed resolution:
In 28.3.4.4.1.3 [facet.numpunct.virtuals], above paragraph 2, change "string_type" to "char_type".
Section: 28.3.3.1.2.1 [locale.category] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the second table in the section, captioned "Required instantiations", the instantiations for codecvt_byname<> have been omitted. These are necessary to allow users to construct a locale by name from facets.
Proposed resolution:
Add in 28.3.3.1.2.1 [locale.category] to the table captioned "Required instantiations", in the category "ctype" the lines
codecvt_byname<char,char,mbstate_t>, codecvt_byname<wchar_t,char,mbstate_t>
Section: 31.10.4.4 [ifstream.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of basic_istream<>::open leaves unanswered questions about how it responds to or changes flags in the error status for the stream. A strict reading indicates that it ignores the bits and does not change them, which confuses users who do not expect eofbit and failbit to remain set after a successful open. There are three reasonable resolutions: 1) status quo 2) fail if fail(), ignore eofbit 3) clear failbit and eofbit on call to open().
Proposed resolution:
In 31.10.4.4 [ifstream.members] paragraph 3, and in 31.10.5.4 [ofstream.members] paragraph 3, under open() effects, add a footnote:
A successful open does not change the error state.
Rationale:
This may seem surprising to some users, but it's just an instance of a general rule: error flags are never cleared by the implementation. The only way error flags are are ever cleared is if the user explicitly clears them by hand.
The LWG believed that preserving this general rule was important enough so that an exception shouldn't be made just for this one case. The resolution of this issue clarifies what the LWG believes to have been the original intent.
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current description of numeric input does not account for the possibility of overflow. This is an implicit result of changing the description to rely on the definition of scanf() (which fails to report overflow), and conflicts with the documented behavior of traditional and current implementations.
Users expect, when reading a character sequence that results in a value unrepresentable in the specified type, to have an error reported. The standard as written does not permit this.
Further comments from Dietmar:
I don't feel comfortable with the proposed resolution to issue 23: It kind of simplifies the issue to much. Here is what is going on:
Currently, the behavior of numeric overflow is rather counter intuitive and hard to trace, so I will describe it briefly:
failbit
is set if scanf()
would
return an input error; otherwise a value is converted to the rules
of scanf
.
scanf()
is defined in terms of fscanf()
.
fscanf()
returns an input failure if during conversion no
character matching the conversion specification could be extracted
before reaching EOF. This is the only reason for fscanf()
to fail due to an input error and clearly does not apply to the case
of overflow.
fscanf()
which basically says that strtod
,
strtol()
, etc. are to be used for the conversion.
strtod()
, strtol()
, etc. functions consume as
many matching characters as there are and on overflow continue to
consume matching characters but also return a value identical to
the maximum (or minimum for signed types if there was a leading minus)
value of the corresponding type and set errno
to ERANGE
.
errno
after reading an element and, of course, clearing errno
before trying a conversion. With the current wording, it can be
detected whether the overflow was due to a positive or negative
number for signed types.
Further discussion from Redmond:
The basic problem is that we've defined our behavior,
including our error-reporting behavior, in terms of C90. However,
C90's method of reporting overflow in scanf is not technically an
"input error". The strto_*
functions are more precise.
There was general consensus that failbit
should be set
upon overflow. We considered three options based on this:
errno
to
indicated the precise nature of the error.Straw poll: (1) 5; (2) 0; (3) 8.
Discussed at Lillehammer. General outline of what we want the solution to look like: we want to say that overflow is an error, and provide a way to distinguish overflow from other kinds of errors. Choose candidate field the same way scanf does, but don't describe the rest of the process in terms of format. If a finite input field is too large (positive or negative) to be represented as a finite value, then set failbit and assign the nearest representable value. Bill will provide wording.
Discussed at Toronto: N2327 is in alignment with the direction we wanted to go with in Lillehammer. Bill to work on.
Proposed resolution:
Change 28.3.4.3.2.3 [facet.num.get.virtuals], end of p3:
Stage 3:
The result of stage 2 processing can be one ofThe sequence ofchar
s accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header<cstdlib>
:
A sequence ofFor a signed integer value, the functionchar
s has been accumulated in stage 2 that is converted (according to the rules ofscanf
) to a value of the type of val. This value is stored in val andios_base::goodbit
is stored in err.strtoll
.The sequence ofFor an unsigned integer value, the functionchar
s accumulated in stage 2 would have causedscanf
to report an input failure.ios_base::failbit
is assigned to err.strtoull
.- For a floating-point value, the function
strtold
.The numeric value to be stored can be one of:
- zero, if the conversion function fails to convert the entire field.
ios_base::failbit
is assigned to err.- the most positive representable value, if the field represents a value too large positive to be represented in val.
ios_base::failbit
is assigned to err.- the most negative representable value (zero for unsigned integer), if the field represents a value too large negative to be represented in val.
ios_base::failbit
is assigned to err.- the converted value, otherwise.
The resultant numeric value is stored in val.
Change 28.3.4.3.2.3 [facet.num.get.virtuals], p6-p7:
iter_type do_get(iter_type in, iter_type end, ios_base& str, ios_base::iostate& err, bool& val) const;-6- Effects: If
(str.flags()&ios_base::boolalpha)==0
then input proceeds as it would for along
except that if a value is being stored into val, the value is determined according to the following: If the value to be stored is 0 thenfalse
is stored. If the value is 1 thentrue
is stored. Otherwiseerr|=ios_base::failbit
is performed and no valuetrue
is stored.andios_base::failbit
is assigned to err.-7- Otherwise target sequences are determined "as if" by calling the members
falsename()
andtruename()
of the facet obtained byuse_facet<numpunct<charT> >(str.getloc())
. Successive characters in the range[in,end)
(see 23.1.1) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iterator in is compared to end only when necessary to obtain a character. Ifand only ifa target sequence is uniquely matched, val is set to the corresponding value. Otherwisefalse
is stored andios_base::failbit
is assigned to err.
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 72
Discussion:
The description of codecvt<>::do_out and do_in mentions a symbol "do_convert" which is not defined in the standard. This is a leftover from an edit, and should be "do_in and do_out".
Proposed resolution:
In 28.3.4.2.5 [locale.codecvt], paragraph 3, change "do_convert" to "do_in or do_out". Also, in 28.3.4.2.5.3 [locale.codecvt.virtuals], change "do_convert()" to "do_in or do_out".
Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 67
Discussion:
In the description of operator<< applied to strings, the standard says that uses the smaller of os.width() and str.size(), to pad "as described in stage 3" elsewhere; but this is inconsistent, as this allows no possibility of space for padding.
Proposed resolution:
Change 27.4.4.4 [string.io] paragraph 4 from:
"... where n
is the smaller of os.width()
and str.size()
;
..."
to:
"... where n
is the larger of os.width()
and str.size()
;
..."
Section: 31.7.5.2.4 [istream.sentry] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
In paragraph 6, the code in the example:
template <class charT, class traits = char_traits<charT> > basic_istream<charT,traits>::sentry( basic_istream<charT,traits>& is, bool noskipws = false) { ... int_type c; typedef ctype<charT> ctype_type; const ctype_type& ctype = use_facet<ctype_type>(is.getloc()); while ((c = is.rdbuf()->snextc()) != traits::eof()) { if (ctype.is(ctype.space,c)==0) { is.rdbuf()->sputbackc (c); break; } } ... }
fails to demonstrate correct use of the facilities described. In particular, it fails to use traits operators, and specifies incorrect semantics. (E.g. it specifies skipping over the first character in the sequence without examining it.)
Proposed resolution:
Remove the example above from [istream::sentry] paragraph 6.
Rationale:
The originally proposed replacement code for the example was not correct. The LWG tried in Kona and again in Tokyo to correct it without success. In Tokyo, an implementor reported that actual working code ran over one page in length and was quite complicated. The LWG decided that it would be counter-productive to include such a lengthy example, which might well still contain errors.
Section: 27.4.3.7.5 [string.erase] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
The string::erase(iterator first, iterator last) is specified to return an element one place beyond the next element after the last one erased. E.g. for the string "abcde", erasing the range ['b'..'d') would yield an iterator for element 'e', while 'd' has not been erased.
Proposed resolution:
In 27.4.3.7.5 [string.erase], paragraph 10, change:
Returns: an iterator which points to the element immediately following _last_ prior to the element being erased.
to read
Returns: an iterator which points to the element pointed to by _last_ prior to the other elements being erased.
Section: 28.3.4.2.4.3 [facet.ctype.char.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 236
Discussion:
The description of the vector form of ctype<char>::is can be interpreted to mean something very different from what was intended. Paragraph 4 says
Effects: The second form, for all *p in the range [low, high), assigns vec[p-low] to table()[(unsigned char)*p].
This is intended to copy the value indexed from table()[] into the place identified in vec[].
Proposed resolution:
Change 28.3.4.2.4.3 [facet.ctype.char.members], paragraph 4, to read
Effects: The second form, for all *p in the range [low, high), assigns into vec[p-low] the value table()[(unsigned char)*p].
Section: 31.4.3 [narrow.stream.objects] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Sections 31.4.3 [narrow.stream.objects] and 31.4.4 [wide.stream.objects] mention a function ios_base::init, which is not defined. Probably they mean basic_ios<>::init, defined in 31.5.4.2 [basic.ios.cons], paragraph 3.
Proposed resolution:
[R12: modified to include paragraph 5.]
In 31.4.3 [narrow.stream.objects] paragraph 2 and 5, change
ios_base::init
to
basic_ios<char>::init
Also, make a similar change in 31.4.4 [wide.stream.objects] except it should read
basic_ios<wchar_t>::init
Section: 28.3.3.1.2.1 [locale.category] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 2 implies that the C macros LC_CTYPE etc. are defined in <cctype>, where they are in fact defined elsewhere to appear in <clocale>.
Proposed resolution:
In 28.3.3.1.2.1 [locale.category], paragraph 2, change "<cctype>" to read "<clocale>".
Section: 28.3.3.1 [locale] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 378
Discussion:
Paragraph 6, says "An instance of locale
is
immutable; once a facet reference is obtained from it,
...". This has caused some confusion, because locale variables
are manifestly assignable.
Proposed resolution:
In 28.3.3.1 [locale] replace paragraph 6
An instance of
locale
is immutable; once a facet reference is obtained from it, that reference remains usable as long as the locale value itself exists.
with
Once a facet reference is obtained from a locale object by calling use_facet<>, that reference remains usable, and the results from member functions of it may be cached and re-used, as long as some locale object refers to that facet.
Section: 31.6.3.5.4 [streambuf.virt.pback] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of the required state before calling virtual member basic_streambuf<>::pbackfail requirements is inconsistent with the conditions described in 27.5.2.2.4 [lib.streambuf.pub.pback] where member sputbackc calls it. Specifically, the latter says it calls pbackfail if:
traits::eq(c,gptr()[-1]) is false
where pbackfail claims to require:
traits::eq(*gptr(),traits::to_char_type(c)) returns false
It appears that the pbackfail description is wrong.
Proposed resolution:
In 31.6.3.5.4 [streambuf.virt.pback], paragraph 1, change:
"
traits::eq(*gptr(),traits::to_char_type( c))
"
to
"
traits::eq(traits::to_char_type(c),gptr()[-1])
"
Rationale:
Note deliberate reordering of arguments for clarity in addition to the correction of the argument value.
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 43
Discussion:
In the table defining the results from do_out and do_in, the specification for the result error says
encountered a from_type character it could not convert
but from_type is not defined. This clearly is intended to be an externT for do_in, or an internT for do_out.
Proposed resolution:
In 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 4, replace the definition in the table for the case of _error_ with
encountered a character in
[from,from_end)
that it could not convert.
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In paragraph 19, Effects:, members truename() and falsename are used from facet ctype<charT>, but it has no such members. Note that this is also a problem in 22.2.2.1.2, addressed in (4).
Proposed resolution:
In 28.3.4.3.3.3 [facet.num.put.virtuals], paragraph 19, in the Effects: clause for member put(...., bool), replace the initialization of the string_type value s as follows:
const numpunct& np = use_facet<numpunct<charT> >(loc); string_type s = val ? np.truename() : np.falsename();
Section: 31.5 [iostreams.base] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.5.5.1 [fmtflags.manip], we have a definition for a manipulator named "unitbuf". Unlike other manipulators, it's not listed in synopsis. Similarly for "nounitbuf".
Proposed resolution:
Add to the synopsis for <ios> in 31.5 [iostreams.base], after the entry for "nouppercase", the prototypes:
ios_base& unitbuf(ios_base& str); ios_base& nounitbuf(ios_base& str);
Section: 31.5.2.6 [ios.base.storage] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the definitions for ios_base::iword and pword, the lifetime of the storage is specified badly, so that an implementation which only keeps the last value stored appears to conform. In particular, it says:
The reference returned may become invalid after another call to the object's iword member with a different index ...
This is not idle speculation; at least one implementation was done this way.
Proposed resolution:
Add in 31.5.2.6 [ios.base.storage], in both paragraph 2 and also in paragraph 4, replace the sentence:
The reference returned may become invalid after another call to the object's iword [pword] member with a different index, after a call to its copyfmt member, or when the object is destroyed.
with:
The reference returned is invalid after any other operations on the object. However, the value of the storage referred to is retained, so that until the next call to copyfmt, calling iword [pword] with the same index yields another reference to the same value.
substituting "iword" or "pword" as appropriate.
Section: 28.3.3.1 [locale] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the overview of locale semantics, paragraph 4, is the sentence
If Facet is not present in a locale (or, failing that, in the global locale), it throws the standard exception bad_cast.
This is not supported by the definition of use_facet<>, and represents semantics from an old draft.
Proposed resolution:
In 28.3.3.1 [locale], paragraph 4, delete the parenthesized expression
(or, failing that, in the global locale)
Section: 28.3.3.2 [locale.global.templates] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It has been noticed by Esa Pulkkinen that the definition of "facet" is incomplete. In particular, a class derived from another facet, but which does not define a member id, cannot safely serve as the argument F to use_facet<F>(loc), because there is no guarantee that a reference to the facet instance stored in loc is safely convertible to F.
Proposed resolution:
In the definition of std::use_facet<>(), replace the text in paragraph 1 which reads:
Get a reference to a facet of a locale.
with:
Requires:
Facet
is a facet class whose definition contains the public static memberid
as defined in 28.3.3.1.2.2 [locale.facet].
[
Kona: strike as overspecification the text "(not inherits)"
from the original resolution, which read "... whose definition
contains (not inherits) the public static member
id
..."
]
Section: 24.6.4.4 [istreambuf.iterator.ops] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2017-11-29
Priority: Not Prioritized
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Discussion:
Following the definition of istreambuf_iterator<>::operator++(int) in paragraph 3, the standard contains three lines of garbage text left over from a previous edit.
istreambuf_iterator<charT,traits> tmp = *this; sbuf_->sbumpc(); return(tmp);
Proposed resolution:
In [istreambuf.iterator::op++], delete the three lines of code at the end of paragraph 3.
Section: 99 [facets.examples] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 3 of the locale examples is a description of part of an implementation technique that has lost its referent, and doesn't mean anything.
Proposed resolution:
Delete 99 [facets.examples] paragraph 3 which begins "This initialization/identification system depends...", or (at the editor's option) replace it with a place-holder to keep the paragraph numbering the same.
Section: 31.5.2 [ios.base] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 157
Discussion:
The description of ios_base::iword() and pword() in 31.5.2.5 [ios.members.static], say that if they fail, they "set badbit, which may throw an exception". However, ios_base offers no interface to set or to test badbit; those interfaces are defined in basic_ios<>.
Proposed resolution:
Change the description in 31.5.2.6 [ios.base.storage] in paragraph 2, and also in paragraph 4, as follows. Replace
If the function fails it sets badbit, which may throw an exception.
with
If the function fails, and
*this
is a base sub-object of abasic_ios<>
object or sub-object, the effect is equivalent to callingbasic_ios<>::setstate(badbit)
on the derived object (which may throwfailure
).
[Kona: LWG reviewed wording; setstate(failbit) changed to setstate(badbit).]
Section: 27.4.3 [basic.string] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The basic_string<> copy constructor:
basic_string(const basic_string& str, size_type pos = 0, size_type n = npos, const Allocator& a = Allocator());
specifies an Allocator argument default value that is counter-intuitive. The natural choice for a the allocator to copy from is str.get_allocator(). Though this cannot be expressed in default-argument notation, overloading suffices.
Alternatively, the other containers in Clause 23 (deque, list, vector) do not have this form of constructor, so it is inconsistent, and an evident source of confusion, for basic_string<> to have it, so it might better be removed.
Proposed resolution:
In 27.4.3 [basic.string], replace the declaration of the copy constructor as follows:
basic_string(const basic_string& str); basic_string(const basic_string& str, size_type pos, size_type n = npos, const Allocator& a = Allocator());
In 27.4.3.2 [string.require], replace the copy constructor declaration as above. Add to paragraph 5, Effects:
In the first form, the Allocator value used is copied from
str.get_allocator()
.
Rationale:
The LWG believes the constructor is actually broken, rather than just an unfortunate design choice.
The LWG considered two other possible resolutions:
A. In 27.4.3 [basic.string], replace the declaration of the copy constructor as follows:
basic_string(const basic_string& str, size_type pos = 0, size_type n = npos); basic_string(const basic_string& str, size_type pos, size_type n, const Allocator& a);
In 27.4.3.2 [string.require], replace the copy constructor declaration as above. Add to paragraph 5, Effects:
When no
Allocator
argument is provided, the string is constructed using the valuestr.get_allocator()
.
B. In 27.4.3 [basic.string], and also in 27.4.3.2 [string.require], replace the declaration of the copy constructor as follows:
basic_string(const basic_string& str, size_type pos = 0, size_type n = npos);
The proposed resolution reflects the original intent of the LWG. It was also noted by Pete Becker that this fix "will cause a small amount of existing code to now work correctly."
[ Kona: issue editing snafu fixed - the proposed resolution now correctly reflects the LWG consensus. ]
Section: 31 [input.output] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Many of the specifications for iostreams specify that character values or their int_type equivalents are compared using operators == or !=, though in other places traits::eq() or traits::eq_int_type is specified to be used throughout. This is an inconsistency; we should change uses of == and != to use the traits members instead.
Proposed resolution:
[Pre-Kona: Dietmar supplied wording]
List of changes to clause 27:
tofillch == fill()
traits::eq(fillch, fill())
toc == delim for the next available input character c
traits::eq(c, delim) for the next available input character c
toc == delim for the next available input character c
traits::eq(c, delim) for the next available input character c
toc == delim for the next available input character c
traits::eq(c, delim) for the next available input character c
toc == delim for the next available input character c
traits::eq_int_type(c, delim) for the next available input character c
toThe last condition will never occur if delim == traits::eof()
The last condition will never occur if traits::eq_int_type(delim, traits::eof()).
towhile ((c = is.rdbuf()->snextc()) != traits::eof()) {
while (!traits::eq_int_type(c = is.rdbuf()->snextc(), traits::eof())) {
List of changes to Chapter 21:
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toc == delim for the next available input character c
traits::eq(c, delim) for the next available input character c
Notes:
Section: 99 [depr.str.strstreams] Status: TC1 Submitter: Brendan Kehoe Opened: 1998-06-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
See lib-6522 and edit-814.
Proposed resolution:
Change 99 [depr.strstreambuf] (since streambuf is a typedef of basic_streambuf<char>) from:
virtual streambuf<char>* setbuf(char* s, streamsize n);
to:
virtual streambuf* setbuf(char* s, streamsize n);
In [depr.strstream] insert the semicolon now missing after int_type:
namespace std { class strstream : public basic_iostream<char> { public: // Types typedef char char_type; typedef typename char_traits<char>::int_type int_type typedef typename char_traits<char>::pos_type pos_type;
Section: 31.5.2.4 [ios.base.locales] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 27.4.2.3 specifies how imbue() and getloc() work. That section has two RETURNS clauses, and they make no sense as stated. They make perfect sense, though, if you swap them. Am I correct in thinking that paragraphs 2 and 4 just got mixed up by accident?
Proposed resolution:
In 31.5.2.4 [ios.base.locales] swap paragraphs 2 and 4.
Section: 31.5.2.2.1 [ios.failure] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
27.4.2.1.1, paragraph 2, says that class failure initializes the base class, exception, with exception(msg). Class exception (see 18.6.1) has no such constructor.
Proposed resolution:
Replace [ios::failure], paragraph 2, with
EFFECTS: Constructs an object of class
failure
.
Section: 31.5.2.5 [ios.members.static] Status: CD1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Two problems
(1) 27.4.2.4 doesn't say what ios_base::sync_with_stdio(f) returns. Does it return f, or does it return the previous synchronization state? My guess is the latter, but the standard doesn't say so.
(2) 27.4.2.4 doesn't say what it means for streams to be synchronized with stdio. Again, of course, I can make some guesses. (And I'm unhappy about the performance implications of those guesses, but that's another matter.)
Proposed resolution:
Change the following sentence in 31.5.2.5 [ios.members.static] returns clause from:
true
if the standard iostream objects (27.3) are synchronized and otherwise returnsfalse
.
to:
true
if the previous state of the standard iostream objects (27.3) was synchronized and otherwise returnsfalse
.
Add the following immediately after 31.5.2.5 [ios.members.static], paragraph 2:
When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by
fputc(f, c);is the same as the effect of
str.rdbuf()->sputc(c);for any sequence of characters; the effect of extracting a character c by
c = fgetc(f);is the same as the effect of:
c = str.rdbuf()->sbumpc(c);for any sequences of characters; and the effect of pushing back a character c by
ungetc(c, f);is the same as the effect of
str.rdbuf()->sputbackc(c);for any sequence of characters. [Footnote: This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream. In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer. --End Footnote]
[pre-Copenhagen: PJP and Matt contributed the definition of "synchronization"]
[post-Copenhagen: proposed resolution was revised slightly: text was added in the non-normative footnote to say that operations on the two streams can be mixed arbitrarily.]
Section: 31.5.2 [ios.base] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
As written, ios_base has a copy constructor and an assignment operator. (Nothing in the standard says it doesn't have one, and all classes have copy constructors and assignment operators unless you take specific steps to avoid them.) However, nothing in 27.4.2 says what the copy constructor and assignment operator do.
My guess is that this was an oversight, that ios_base is, like basic_ios, not supposed to have a copy constructor or an assignment operator.
Jerry Schwarz comments: Yes, its an oversight, but in the opposite sense to what you're suggesting. At one point there was a definite intention that you could copy ios_base. It's an easy way to save the entire state of a stream for future use. As you note, to carry out that intention would have required a explicit description of the semantics (e.g. what happens to the iarray and parray stuff).
Proposed resolution:
In 31.5.2 [ios.base], class ios_base, specify the copy constructor and operator= members as being private.
Rationale:
The LWG believes the difficulty of specifying correct semantics outweighs any benefit of allowing ios_base objects to be copyable.
Section: 23.2 [container.requirements] Status: TC1 Submitter: David Vandevoorde Opened: 1998-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The std::sort algorithm can in general only sort a given sequence by moving around values. The list<>::sort() member on the other hand could move around values or just update internal pointers. Either method can leave iterators into the list<> dereferencable, but they would point to different things.
Does the FDIS mandate anywhere which method should be used for list<>::sort()?
Matt Austern comments:
I think you've found an omission in the standard.
The library working group discussed this point, and there was supposed to be a general requirement saying that list, set, map, multiset, and multimap may not invalidate iterators, or change the values that iterators point to, except when an operation does it explicitly. So, for example, insert() doesn't invalidate any iterators and erase() and remove() only invalidate iterators pointing to the elements that are being erased.
I looked for that general requirement in the FDIS, and, while I found a limited form of it for the sorted associative containers, I didn't find it for list. It looks like it just got omitted.
The intention, though, is that list<>::sort does not invalidate any iterators and does not change the values that any iterator points to. There would be no reason to have the member function otherwise.
Proposed resolution:
Add a new paragraph at the end of 23.1:
Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.
Rationale:
This was US issue CD2-23-011; it was accepted in London but the change was not made due to an editing oversight. The wording in the proposed resolution below is somewhat updated from CD2-23-011, particularly the addition of the phrase "or change the values of"
Section: 31.5.3.3 [fpos.operations] Status: TC1 Submitter: Matt Austern Opened: 1998-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
First, 31.5.4.2 [basic.ios.cons], table 89. This is pretty obvious: it should be titled "basic_ios<>() effects", not "ios_base() effects".
[The second item is a duplicate; see issue 6(i) for resolution.]
Second, 31.5.3.3 [fpos.operations] table 88 . There are a couple different things wrong with it, some of which I've already discussed with Jerry, but the most obvious mechanical sort of error is that it uses expressions like P(i) and p(i), without ever defining what sort of thing "i" is.
(The other problem is that it requires support for streampos arithmetic. This is impossible on some systems, i.e. ones where file position is a complicated structure rather than just a number. Jerry tells me that the intention was to require syntactic support for streampos arithmetic, but that it wasn't actually supposed to do anything meaningful except on platforms, like Unix, where genuine arithmetic is possible.)
Proposed resolution:
Change 31.5.4.2 [basic.ios.cons] table 89 title from "ios_base() effects" to "basic_ios<>() effects".
Section: 31.5.4.2 [basic.ios.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There's nothing in 27.4.4 saying what basic_ios's destructor does. The important question is whether basic_ios::~basic_ios() destroys rdbuf().
Proposed resolution:
Add after 31.5.4.2 [basic.ios.cons] paragraph 2:
virtual ~basic_ios();
Notes: The destructor does not destroy
rdbuf()
.
Rationale:
The LWG reviewed the additional question of whether or not
rdbuf(0)
may set badbit
. The answer is
clearly yes; it may be set via clear()
. See 31.5.4.3 [basic.ios.members], paragraph 6. This issue was reviewed at length
by the LWG, which removed from the original proposed resolution a
footnote which incorrectly said "rdbuf(0)
does not set
badbit
".
Section: 31.6.3.2 [streambuf.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-06-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class synopsis for basic_streambuf shows a (virtual) destructor, but the standard doesn't say what that destructor does. My assumption is that it does nothing, but the standard should say so explicitly.
Proposed resolution:
Add after 31.6.3.2 [streambuf.cons] paragraph 2:
virtual ~basic_streambuf();
Effects: None.
Section: 31 [input.output] Status: TC1 Submitter: Matt Austern Opened: 1998-06-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Several member functions in clause 27 are defined in certain circumstances to return an "invalid stream position", a term that is defined nowhere in the standard. Two places (27.5.2.4.2, paragraph 4, and 27.8.1.4, paragraph 15) contain a cross-reference to a definition in _lib.iostreams.definitions_, a nonexistent section.
I suspect that the invalid stream position is just supposed to be pos_type(-1). Probably best to say explicitly in (for example) 27.5.2.4.2 that the return value is pos_type(-1), rather than to use the term "invalid stream position", define that term somewhere, and then put in a cross-reference.
The phrase "invalid stream position" appears ten times in the C++ Standard. In seven places it refers to a return value, and it should be changed. In three places it refers to an argument, and it should not be changed. Here are the three places where "invalid stream position" should not be changed:
31.8.2.5 [stringbuf.virtuals], paragraph 14
31.10.3.5 [filebuf.virtuals], paragraph 14
99 [depr.strstreambuf.virtuals], paragraph 17
Proposed resolution:
In 31.6.3.5.2 [streambuf.virt.buffer], paragraph 4, change "Returns an
object of class pos_type that stores an invalid stream position
(_lib.iostreams.definitions_)" to "Returns
pos_type(off_type(-1))
".
In 31.6.3.5.2 [streambuf.virt.buffer], paragraph 6, change "Returns
an object of class pos_type that stores an invalid stream
position" to "Returns pos_type(off_type(-1))
".
In 31.8.2.5 [stringbuf.virtuals], paragraph 13, change "the object
stores an invalid stream position" to "the return value is
pos_type(off_type(-1))
".
In 31.10.3.5 [filebuf.virtuals], paragraph 13, change "returns an
invalid stream position (27.4.3)" to "returns
pos_type(off_type(-1))
"
In 31.10.3.5 [filebuf.virtuals], paragraph 15, change "Otherwise
returns an invalid stream position (_lib.iostreams.definitions_)"
to "Otherwise returns pos_type(off_type(-1))
"
In 99 [depr.strstreambuf.virtuals], paragraph 15, change "the object
stores an invalid stream position" to "the return value is
pos_type(off_type(-1))
"
In 99 [depr.strstreambuf.virtuals], paragraph 18, change "the object
stores an invalid stream position" to "the return value is
pos_type(off_type(-1))
"
Section: 31.6.3 [streambuf] Status: TC1 Submitter: Matt Austern Opened: 1998-06-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class summary for basic_streambuf<>, in 27.5.2, says that showmanyc has return type int. However, 27.5.2.4.3 says that its return type is streamsize.
Proposed resolution:
Change showmanyc
's return type in the
31.6.3 [streambuf] class summary to streamsize
.
Section: 27.2.4.6 [char.traits.specializations.wchar.t] Status: TC1 Submitter: Matt Austern Opened: 1998-07-01 Last modified: 2016-01-28
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Discussion:
21.1.3.2, paragraph 3, says "The types streampos and wstreampos may be different if the implementation supports no shift encoding in narrow-oriented iostreams but supports one or more shift encodings in wide-oriented streams".
That's wrong: the two are the same type. The <iosfwd> summary in 27.2 says that streampos and wstreampos are, respectively, synonyms for fpos<char_traits<char>::state_type> and fpos<char_traits<wchar_t>::state_type>, and, flipping back to clause 21, we see in 21.1.3.1 and 21.1.3.2 that char_traits<char>::state_type and char_traits<wchar_t>::state_type must both be mbstate_t.
Proposed resolution:
Remove the sentence in 27.2.4.6 [char.traits.specializations.wchar.t] paragraph 3 which begins "The types streampos and wstreampos may be different..." .
Section: 31.6.3.4.2 [streambuf.get.area] Status: TC1 Submitter: Matt Austern Opened: 1998-07-28 Last modified: 2016-01-28
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Discussion:
27.5.2.3.1 says that basic_streambuf::gbump() "Advances the next pointer for the input sequence by n."
The straightforward interpretation is that it is just gptr() += n. An alternative interpretation, though, is that it behaves as if it calls sbumpc n times. (The issue, of course, is whether it might ever call underflow.) There is a similar ambiguity in the case of pbump.
(The "classic" AT&T implementation used the former interpretation.)
Proposed resolution:
Change 31.6.3.4.2 [streambuf.get.area] paragraph 4 gbump effects from:
Effects: Advances the next pointer for the input sequence by n.
to:
Effects: Adds
n
to the next pointer for the input sequence.
Make the same change to 31.6.3.4.3 [streambuf.put.area] paragraph 4 pbump effects.
Section: 31.7.5.3.1 [istream.formatted.reqmts] Status: TC1 Submitter: Matt Austern Opened: 1998-08-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 1 of 27.6.1.2.1 contains general requirements for all formatted input functions. Some of the functions defined in section 27.6.1.2 explicitly say that those requirements apply ("Behaves like a formatted input member (as described in 27.6.1.2.1)"), but others don't. The question: is 27.6.1.2.1 supposed to apply to everything in 27.6.1.2, or only to those member functions that explicitly say "behaves like a formatted input member"? Or to put it differently: are we to assume that everything that appears in a section called "Formatted input functions" really is a formatted input function? I assume that 27.6.1.2.1 is intended to apply to the arithmetic extractors (27.6.1.2.2), but I assume that it is not intended to apply to extractors like
basic_istream& operator>>(basic_istream& (*pf)(basic_istream&));
and
basic_istream& operator>>(basic_streammbuf*);
There is a similar ambiguity for unformatted input, formatted output, and unformatted output.
Comments from Judy Ward: It seems like the problem is that the basic_istream and basic_ostream operator <<()'s that are used for the manipulators and streambuf* are in the wrong section and should have their own separate section or be modified to make it clear that the "Common requirements" listed in section 27.6.1.2.1 (for basic_istream) and section 27.6.2.5.1 (for basic_ostream) do not apply to them.
Additional comments from Dietmar Kühl: It appears to be somewhat
nonsensical to consider the functions defined in [istream::extractors] paragraphs 1 to 5 to be "Formatted input
function" but since these functions are defined in a section
labeled "Formatted input functions" it is unclear to me
whether these operators are considered formatted input functions which
have to conform to the "common requirements" from 31.7.5.3.1 [istream.formatted.reqmts]: If this is the case, all manipulators, not
just ws
, would skip whitespace unless noskipws
is
set (... but setting noskipws
using the manipulator syntax
would also skip whitespace :-)
It is not clear which functions
are to be considered unformatted input functions. As written, it seems
that all functions in 31.7.5.4 [istream.unformatted] are unformatted input
functions. However, it does not really make much sense to construct a
sentry object for gcount()
, sync()
, ... Also it is
unclear what happens to the gcount()
if
eg. gcount()
, putback()
, unget()
, or
sync()
is called: These functions don't extract characters,
some of them even "unextract" a character. Should this still
be reflected in gcount()
? Of course, it could be read as if
after a call to gcount()
gcount()
return 0
(the last unformatted input function, gcount()
, didn't
extract any character) and after a call to putback()
gcount()
returns -1
(the last unformatted input
function putback()
did "extract" back into the
stream). Correspondingly for unget()
. Is this what is
intended? If so, this should be clarified. Otherwise, a corresponding
clarification should be used.
Proposed resolution:
In 27.6.1.2.2 [lib.istream.formatted.arithmetic], paragraph 1. Change the beginning of the second sentence from "The conversion occurs" to "These extractors behave as formatted input functions (as described in 27.6.1.2.1). After a sentry object is constructed, the conversion occurs"
In 27.6.1.2.3, [lib.istream::extractors], before paragraph 1. Add an effects clause. "Effects: None. This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).
In 27.6.1.2.3, [lib.istream::extractors], paragraph 2. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).
In 27.6.1.2.3, [lib.istream::extractors], paragraph 4. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).
In 27.6.1.2.3, [lib.istream::extractors], paragraph 12. Change the first two sentences from "If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). Extracts characters from *this..." to "Behaves as a formatted input function (as described in 27.6.1.2.1). If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). After a sentry object is constructed, extracts characters from *this...".
In 27.6.1.3, [lib.istream.unformatted], before paragraph 2. Add an effects clause. "Effects: none. This member function does not behave as an unformatted input function (as described in 27.6.1.3, paragraph 1)."
In 27.6.1.3, [lib.istream.unformatted], paragraph 3. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"
In 27.6.1.3, [lib.istream.unformatted], paragraph 5. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"
In 27.6.1.3, [lib.istream.unformatted], paragraph 7. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"
[No change needed in paragraph 10, because it refers to paragraph 7.]
In 27.6.1.3, [lib.istream.unformatted], paragraph 12. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"
[No change needed in paragraph 15.]
In 27.6.1.3, [lib.istream.unformatted], paragraph 17. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"
[No change needed in paragraph 23.]
In 27.6.1.3, [lib.istream.unformatted], paragraph 24. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"
In 27.6.1.3, [lib.istream.unformatted], before paragraph 27. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, reads but does not extract the current input character."
In 27.6.1.3, [lib.istream.unformatted], paragraph 28. Change the first sentence of the Effects clause from "If !good() calls" to Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"
In 27.6.1.3, [lib.istream.unformatted], paragraph 30. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"
In 27.6.1.3, [lib.istream.unformatted], paragraph 32. Change the first sentence of the Effects clause from "If !good() calls..." to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls..." Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"
In 27.6.1.3, [lib.istream.unformatted], paragraph 34. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls". Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"
In 27.6.1.3, [lib.istream.unformatted], paragraph 36. Change the first sentence of the Effects clause from "If !rdbuf() is" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if rdbuf() is"
In 27.6.1.3, [lib.istream.unformatted], before paragraph 37. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount()." Change the first sentence of paragraph 37 from "if fail()" to "after constructing a sentry object, if fail()".
In 27.6.1.3, [lib.istream.unformatted], paragraph 38. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()
In 27.6.1.3, [lib.istream.unformatted], paragraph 40. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()
In 27.6.2.5.2 [lib.ostream.inserters.arithmetic], paragraph 1. Change the beginning of the third sentence from "The formatting conversion" to "These extractors behave as formatted output functions (as described in 27.6.2.5.1). After the sentry object is constructed, the conversion occurs".
In 27.6.2.5.3 [lib.ostream.inserters], before paragraph 1. Add an effects clause: "Effects: None. Does not behave as a formatted output function (as described in 27.6.2.5.1).".
In 27.6.2.5.3 [lib.ostream.inserters], paragraph 2. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".
In 27.6.2.5.3 [lib.ostream.inserters], paragraph 4. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".
In 27.6.2.5.3 [lib.ostream.inserters], paragraph 6. Change the first sentence from "If sb" to "Behaves as a formatted output function (as described in 27.6.2.5.1). After the sentry object is constructed, if sb".
In 27.6.2.6 [lib.ostream.unformatted], paragraph 2. Change the first sentence from "Inserts the character" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, inserts the character".
In 27.6.2.6 [lib.ostream.unformatted], paragraph 5. Change the first sentence from "Obtains characters" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, obtains characters".
In 27.6.2.6 [lib.ostream.unformatted], paragraph 7. Add a new sentence at the end of the paragraph: "Does not behave as an unformatted output function (as described in 27.6.2.6, paragraph 1)."
Rationale:
See J16/99-0043==WG21/N1219, Proposed Resolution to Library Issue 60, by Judy Ward and Matt Austern. This proposed resolution is section VI of that paper.
Section: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The introduction to the section on unformatted input (27.6.1.3) says that every unformatted input function catches all exceptions that were thrown during input, sets badbit, and then conditionally rethrows the exception. That seems clear enough. Several of the specific functions, however, such as get() and read(), are documented in some circumstances as setting eofbit and/or failbit. (The standard notes, correctly, that setting eofbit or failbit can sometimes result in an exception being thrown.) The question: if one of these functions throws an exception triggered by setting failbit, is this an exception "thrown during input" and hence covered by 27.6.1.3, or does 27.6.1.3 only refer to a limited class of exceptions? Just to make this concrete, suppose you have the following snippet.
char buffer[N]; istream is; ... is.exceptions(istream::failbit); // Throw on failbit but not on badbit. is.read(buffer, N);
Now suppose we reach EOF before we've read N characters. What iostate bits can we expect to be set, and what exception (if any) will be thrown?
Proposed resolution:
In 27.6.1.3, paragraph 1, after the sentence that begins
"If an exception is thrown...", add the following
parenthetical comment: "(Exceptions thrown from
basic_ios<>::clear()
are not caught or rethrown.)"
Rationale:
The LWG looked to two alternative wordings, and choose the proposed resolution as better standardese.
Sync
's return valueSection: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The Effects clause for sync() (27.6.1.3, paragraph 36) says that it "calls rdbuf()->pubsync() and, if that function returns -1 ... returns traits::eof()."
That looks suspicious, because traits::eof() is of type traits::int_type while the return type of sync() is int.
Proposed resolution:
In 31.7.5.4 [istream.unformatted], paragraph 36, change "returns
traits::eof()
" to "returns -1
".
Section: 31.7.6.4 [ostream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Clause 27 details an exception-handling policy for formatted input, unformatted input, and formatted output. It says nothing for unformatted output (27.6.2.6). 27.6.2.6 should either include the same kind of exception-handling policy as in the other three places, or else it should have a footnote saying that the omission is deliberate.
Proposed resolution:
In 27.6.2.6, paragraph 1, replace the last sentence ("In any case, the unformatted output function ends by destroying the sentry object, then returning the value specified for the formatted output function.") with the following text:
If an exception is thrown during output, then
ios::badbit
is turned on [Footnote: without causing anios::failure
to be thrown.] in*this
's error state. If(exceptions() & badbit) != 0
then the exception is rethrown. In any case, the unformatted output function ends by destroying the sentry object, then, if no exception was thrown, returning the value specified for the formatted output function.
Rationale:
This exception-handling policy is consistent with that of formatted input, unformatted input, and formatted output.
basic_istream::operator>>(basic_streambuf*)
Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: Matt Austern Opened: 1998-08-11 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
27.6.1.2.3, paragraph 13, is ambiguous. It can be interpreted two different ways, depending on whether the second sentence is read as an elaboration of the first.
Proposed resolution:
Replace [istream::extractors], paragraph 13, which begins "If the function inserts no characters ..." with:
If the function inserts no characters, it calls
setstate(failbit)
, which may throwios_base::failure
(27.4.4.3). If it inserted no characters because it caught an exception thrown while extracting characters fromsb
andfailbit
is on inexceptions()
(27.4.4.3), then the caught exception is rethrown.
Section: 99 [depr.strstreambuf.virtuals] Status: TC1 Submitter: Matt Austern Opened: 1998-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
D.7.1.3, paragraph 19, says that strstreambuf::setbuf "Performs an operation that is defined separately for each class derived from strstreambuf". This is obviously an incorrect cut-and-paste from basic_streambuf. There are no classes derived from strstreambuf.
Proposed resolution:
99 [depr.strstreambuf.virtuals], paragraph 19, replace the setbuf effects clause which currently says "Performs an operation that is defined separately for each class derived from strstreambuf" with:
Effects: implementation defined, except that
setbuf(0,0)
has no effect.
Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: Angelika Langer Opened: 1998-07-14 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
Extractors for char* (27.6.1.2.3) do not store a null character after the extracted character sequence whereas the unformatted functions like get() do. Why is this?
Comment from Jerry Schwarz: There is apparently an editing glitch. You'll notice that the last item of the list of what stops extraction doesn't make any sense. It was supposed to be the line that said a null is stored.
Proposed resolution:
[istream::extractors], paragraph 7, change the last list item from:
A null byte (
charT()
) in the next position, which may be the first position if no characters were extracted.
to become a new paragraph which reads:
Operator>> then stores a null byte (
charT()
) in the next position, which may be the first position if no characters were extracted.
Section: 23.3.13 [vector] Status: TC1 Submitter: Andrew Koenig Opened: 1998-07-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The issue is this: Must the elements of a vector be in contiguous memory?
(Please note that this is entirely separate from the question of whether a vector iterator is required to be a pointer; the answer to that question is clearly "no," as it would rule out debugging implementations)
Proposed resolution:
Add the following text to the end of 23.3.13 [vector], paragraph 1.
The elements of a vector are stored contiguously, meaning that if v is a
vector<T, Allocator>
where T is some type other thanbool
, then it obeys the identity&v[n] == &v[0] + n
for all0 <= n < v.size()
.
Rationale:
The LWG feels that as a practical matter the answer is clearly "yes". There was considerable discussion as to the best way to express the concept of "contiguous", which is not directly defined in the standard. Discussion included:
Section: 17.9 [support.exception], 99 [uncaught] Status: TC1 Submitter: Steve Clamage Opened: 1998-08-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In article 3E04@pratique.fr, Valentin Bonnard writes:
uncaught_exception() doesn't have a throw specification.
It is intentional ? Does it means that one should be prepared to handle exceptions thrown from uncaught_exception() ?
uncaught_exception() is called in exception handling contexts where exception safety is very important.
Proposed resolution:
In [except.uncaught], paragraph 1, 17.9 [support.exception], and 99 [uncaught], add "throw()" to uncaught_exception().
Section: 28.3.4.6.2 [locale.time.get] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The locale facet member time_get<>::do_get_monthname
is described in 28.3.4.6.2.3 [locale.time.get.virtuals] with five arguments,
consistent with do_get_weekday and with its specified use by member
get_monthname. However, in the synopsis, it is specified instead with
four arguments. The missing argument is the "end" iterator
value.
Proposed resolution:
In 28.3.4.6.2 [locale.time.get], add an "end" argument to the declaration of member do_monthname as follows:
virtual iter_type do_get_monthname(iter_type s, iter_type end, ios_base&, ios_base::iostate& err, tm* t) const;
codecvt::do_max_length
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Matt Austern Opened: 1998-09-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The text of codecvt::do_max_length
's "Returns"
clause (22.2.1.5.2, paragraph 11) is garbled. It has unbalanced
parentheses and a spurious n.
Proposed resolution:
Replace 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 11 with the following:
Returns: The maximum value that
do_length(state, from, from_end, 1)
can return for any valid range[from, from_end)
andstateT
valuestate
. The specializationcodecvt<char, char, mbstate_t>::do_max_length()
returns 1.
codecvt::length
's argument typesSection: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Matt Austern Opened: 1998-09-18 Last modified: 2023-03-29
Priority: Not Prioritized
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Discussion:
The class synopses for classes codecvt<>
(22.2.1.5)
and codecvt_byname<>
(22.2.1.6) say that the first
parameter of the member functions length
and
do_length
is of type const stateT&
. The member
function descriptions, however (22.2.1.5.1, paragraph 6; 22.2.1.5.2,
paragraph 9) say that the type is stateT&
. Either the
synopsis or the summary must be changed.
If (as I believe) the member function descriptions are correct,
then we must also add text saying how do_length
changes its
stateT
argument.
Proposed resolution:
In 28.3.4.2.5 [locale.codecvt], and also in 28.3.4.2.6 [locale.codecvt.byname],
change the stateT
argument type on both member
length()
and member do_length()
from
const stateT&
to
stateT&
In 28.3.4.2.5.3 [locale.codecvt.virtuals], add to the definition for member
do_length
a paragraph:
Effects: The effect on the
state
argument is ``as if'' it calleddo_in(state, from, from_end, from, to, to+max, to)
forto
pointing to a buffer of at leastmax
elements.
codecvt
facet always convert one internal character at a time?Section: 28.3.4.2.5 [locale.codecvt] Status: CD1 Submitter: Matt Austern Opened: 1998-09-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This issue concerns the requirements on classes derived from
codecvt
, including user-defined classes. What are the
restrictions on the conversion from external characters
(e.g. char
) to internal characters (e.g. wchar_t
)?
Or, alternatively, what assumptions about codecvt
facets can
the I/O library make?
The question is whether it's possible to convert from internal characters to external characters one internal character at a time, and whether, given a valid sequence of external characters, it's possible to pick off internal characters one at a time. Or, to put it differently: given a sequence of external characters and the corresponding sequence of internal characters, does a position in the internal sequence correspond to some position in the external sequence?
To make this concrete, suppose that [first, last)
is a
sequence of M external characters and that [ifirst,
ilast)
is the corresponding sequence of N internal
characters, where N > 1. That is, my_encoding.in()
,
applied to [first, last)
, yields [ifirst,
ilast)
. Now the question: does there necessarily exist a
subsequence of external characters, [first, last_1)
, such
that the corresponding sequence of internal characters is the single
character *ifirst
?
(What a "no" answer would mean is that
my_encoding
translates sequences only as blocks. There's a
sequence of M external characters that maps to a sequence of
N internal characters, but that external sequence has no
subsequence that maps to N-1 internal characters.)
Some of the wording in the standard, such as the description of
codecvt::do_max_length
(28.3.4.2.5.3 [locale.codecvt.virtuals],
paragraph 11) and basic_filebuf::underflow
(31.10.3.5 [filebuf.virtuals], paragraph 3) suggests that it must always be
possible to pick off internal characters one at a time from a sequence
of external characters. However, this is never explicitly stated one
way or the other.
This issue seems (and is) quite technical, but it is important if
we expect users to provide their own encoding facets. This is an area
where the standard library calls user-supplied code, so a well-defined
set of requirements for the user-supplied code is crucial. Users must
be aware of the assumptions that the library makes. This issue affects
positioning operations on basic_filebuf
, unbuffered input,
and several of codecvt
's member functions.
Proposed resolution:
Add the following text as a new paragraph, following 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 2:
A
codecvt
facet that is used bybasic_filebuf
(31.10 [file.streams]) must have the property that ifdo_out(state, from, from_end, from_next, to, to_lim, to_next)would return
ok
, wherefrom != from_end
, thendo_out(state, from, from + 1, from_next, to, to_end, to_next)must also return
ok
, and that ifdo_in(state, from, from_end, from_next, to, to_lim, to_next)would return
ok
, whereto != to_lim
, thendo_in(state, from, from_end, from_next, to, to + 1, to_next)must also return
ok
. [Footnote: Informally, this means thatbasic_filebuf
assumes that the mapping from internal to external characters is 1 to N: acodecvt
that is used bybasic_filebuf
must be able to translate characters one internal character at a time. --End Footnote]
[Redmond: Minor change in proposed resolution. Original
proposed resolution talked about "success", with a parenthetical
comment that success meant returning ok
. New wording
removes all talk about "success", and just talks about the
return value.]
Rationale:
The proposed resoluion says that conversions can be performed one internal character at a time. This rules out some encodings that would otherwise be legal. The alternative answer would mean there would be some internal positions that do not correspond to any external file position.
An example of an encoding that this rules out is one where the
internT
and externT
are of the same type, and
where the internal sequence c1 c2
corresponds to the
external sequence c2 c1
.
It was generally agreed that basic_filebuf
relies
on this property: it was designed under the assumption that
the external-to-internal mapping is N-to-1, and it is not clear
that basic_filebuf
is implementable without that
restriction.
The proposed resolution is expressed as a restriction on
codecvt
when used by basic_filebuf
, rather
than a blanket restriction on all codecvt
facets,
because basic_filebuf
is the only other part of the
library that uses codecvt
. If a user wants to define
a codecvt
facet that implements a more general N-to-M
mapping, there is no reason to prohibit it, so long as the user
does not expect basic_filebuf
to be able to use it.
Section: 31.5.2 [ios.base] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
typo: event_call_back should be event_callback
Proposed resolution:
In the 31.5.2 [ios.base] synopsis change "event_call_back" to "event_callback".
Section: 29.4.2 [complex.syn], 29.4.7 [complex.value.ops] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 29.4.2 [complex.syn] polar is declared as follows:
template<class T> complex<T> polar(const T&, const T&);
In 29.4.7 [complex.value.ops] it is declared as follows:
template<class T> complex<T> polar(const T& rho, const T& theta = 0);
Thus whether the second parameter is optional is not clear.
Proposed resolution:
In 29.4.2 [complex.syn] change:
template<class T> complex<T> polar(const T&, const T&);
to:
template<class T> complex<T> polar(const T& rho, const T& theta = 0);
Section: 29.4.2 [complex.syn], 29.4.3 [complex] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Both 26.2.1 and 26.2.2 contain declarations of global operators for class complex. This redundancy should be removed.
Proposed resolution:
Reduce redundancy according to the general style of the standard.
Section: 27.4.3 [basic.string] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 89
Discussion:
Many string member functions throw if size is getting or exceeding npos. However, I wonder why they don't throw if size is getting or exceeding max_size() instead of npos. May be npos is known at compile time, while max_size() is known at runtime. However, what happens if size exceeds max_size() but not npos, then? It seems the standard lacks some clarifications here.
Proposed resolution:
After 27.4.3 [basic.string] paragraph 4 ("The functions described in this clause...") add a new paragraph:
For any string operation, if as a result of the operation,
size()
would exceedmax_size()
then the operation throwslength_error
.
Rationale:
The LWG believes length_error is the correct exception to throw.
Section: 27.4.3.2 [string.require] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The constructor from a range:
template<class InputIterator> basic_string(InputIterator begin, InputIterator end, const Allocator& a = Allocator());
lacks a throws clause. However, I would expect that it throws according to the other constructors if the numbers of characters in the range equals npos (or exceeds max_size(), see above).
Proposed resolution:
In 27.4.3.2 [string.require], Strike throws paragraphs for constructors which say "Throws: length_error if n == npos."
Rationale:
Throws clauses for length_error if n == npos are no longer needed because they are subsumed by the general wording added by the resolution for issue 83(i).
Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The effect of operator >> for strings contain the following item:
isspace(c,getloc())
is true for the next available input
character c.
Here getloc()
has to be replaced by is.getloc()
.
Proposed resolution:
In 27.4.4.4 [string.io] paragraph 1 Effects clause replace:
isspace(c,getloc())
is true for the next available input character c.
with:
isspace(c,is.getloc())
is true for the next available input character c.
Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Operator >> and getline() for strings read until eof() in the input stream is true. However, this might never happen, if the stream can't read anymore without reaching EOF. So shouldn't it be changed into that it reads until !good() ?
Proposed resolution:
In 27.4.4.4 [string.io], paragraph 1, replace:
Effects: Begins by constructing a sentry object k as if k were constructed by typename basic_istream<charT,traits>::sentry k( is). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:
with:
Effects: Behaves as a formatted input function (31.7.5.3.1 [istream.formatted.reqmts]). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:
In 27.4.4.4 [string.io], paragraph 6, replace
Effects: Begins by constructing a sentry object k as if by typename basic_istream<charT,traits>::sentry k( is, true). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c) until any of the following occurs:
with:
Effects: Behaves as an unformatted input function (31.7.5.4 [istream.unformatted]), except that it does not affect the value returned by subsequent calls to basic_istream<>::gcount(). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c) until any of the following occurs:
[Redmond: Made changes in proposed resolution. operator>>
should be a formatted input function, not an unformatted input function.
getline
should not be required to set gcount
, since
there is no mechanism for gcount
to be set except by one of
basic_istream
's member functions.]
[Curaçao: Nico agrees with proposed resolution.]
Rationale:
The real issue here is whether or not these string input functions get their characters from a streambuf, rather than by calling an istream's member functions, a streambuf signals failure either by returning eof or by throwing an exception; there are no other possibilities. The proposed resolution makes it clear that these two functions do get characters from a streambuf.
Section: 26 [algorithms] Status: CD1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard does not state, how often a function object is copied, called, or the order of calls inside an algorithm. This may lead to surprising/buggy behavior. Consider the following example:
class Nth { // function object that returns true for the nth element private: int nth; // element to return true for int count; // element counter public: Nth (int n) : nth(n), count(0) { } bool operator() (int) { return ++count == nth; } }; .... // remove third element list<int>::iterator pos; pos = remove_if(coll.begin(),coll.end(), // range Nth(3)), // remove criterion coll.erase(pos,coll.end());
This call, in fact removes the 3rd AND the 6th element. This happens because the usual implementation of the algorithm copies the function object internally:
template <class ForwIter, class Predicate> ForwIter std::remove_if(ForwIter beg, ForwIter end, Predicate op) { beg = find_if(beg, end, op); if (beg == end) { return beg; } else { ForwIter next = beg; return remove_copy_if(++next, end, beg, op); } }
The algorithm uses find_if() to find the first element that should be removed. However, it then uses a copy of the passed function object to process the resulting elements (if any). Here, Nth is used again and removes also the sixth element. This behavior compromises the advantage of function objects being able to have a state. Without any cost it could be avoided (just implement it directly instead of calling find_if()).
Proposed resolution:
Add a new paragraph following 26 [algorithms] paragraph 8:
[Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely. Programmers for whom object identity is important should consider using a wrapper class that points to a noncopied implementation object, or some equivalent solution.]
[Dublin: Pete Becker felt that this may not be a defect, but rather something that programmers need to be educated about. There was discussion of adding wording to the effect that the number and order of calls to function objects, including predicates, not affect the behavior of the function object.]
[Pre-Kona: Nico comments: It seems the problem is that we don't have a clear statement of "predicate" in the standard. People including me seemed to think "a function returning a Boolean value and being able to be called by an STL algorithm or be used as sorting criterion or ... is a predicate". But a predicate has more requirements: It should never change its behavior due to a call or being copied. IMHO we have to state this in the standard. If you like, see section 8.1.4 of my library book for a detailed discussion.]
[Kona: Nico will provide wording to the effect that "unless otherwise specified, the number of copies of and calls to function objects by algorithms is unspecified". Consider placing in 26 [algorithms] after paragraph 9.]
[Santa Cruz: The standard doesn't currently guarantee that
functions object won't be copied, and what isn't forbidden is
allowed. It is believed (especially since implementations that were
written in concert with the standard do make copies of function
objects) that this was intentional. Thus, no normative change is
needed. What we should put in is a non-normative note suggesting to
programmers that if they want to guarantee the lack of copying they
should use something like the ref
wrapper.]
[Oxford: Matt provided wording.]
Section: 24.3.5.3 [input.iterators] Status: CD1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 72 in 24.3.5.3 [input.iterators] specifies semantics for
*r++
of:
{ T tmp = *r; ++r; return tmp; }
There are two problems with this. First, the return type is specified to be "T", as opposed to something like "convertible to T". This is too specific: we want to allow *r++ to return an lvalue.
Second, writing the semantics in terms of code misleadingly suggests that the effects *r++ should precisely replicate the behavior of this code, including side effects. (Does this mean that *r++ should invoke the copy constructor exactly as many times as the sample code above would?) See issue 334(i) for a similar problem.
Proposed resolution:
In Table 72 in 24.3.5.3 [input.iterators], change the return type
for *r++
from T
to "convertible to T".
Rationale:
This issue has two parts: the return type, and the number of times the copy constructor is invoked.
The LWG believes the the first part is a real issue. It's
inappropriate for the return type to be specified so much more
precisely for *r++ than it is for *r. In particular, if r is of
(say) type int*
, then *r++ isn't int
,
but int&
.
The LWG does not believe that the number of times the copy constructor is invoked is a real issue. This can vary in any case, because of language rules on copy constructor elision. That's too much to read into these semantics clauses.
Additionally, as Dave Abrahams pointed out (c++std-lib-13703): since we're told (24.1/3) that forward iterators satisfy all the requirements of input iterators, we can't impose any requirements in the Input Iterator requirements table that forward iterators don't satisfy.
Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Set::iterator is described as implementation-defined with a reference to the container requirement; the container requirement says that const_iterator is an iterator pointing to const T and iterator an iterator pointing to T.
23.1.2 paragraph 2 implies that the keys should not be modified to break the ordering of elements. But that is not clearly specified. Especially considering that the current standard requires that iterator for associative containers be different from const_iterator. Set, for example, has the following:
typedef implementation defined iterator;
// See _lib.container.requirements_
23.2 [container.requirements] actually requires that iterator type pointing to T (table 65). Disallowing user modification of keys by changing the standard to require an iterator for associative container to be the same as const_iterator would be overkill since that will unnecessarily significantly restrict the usage of associative container. A class to be used as elements of set, for example, can no longer be modified easily without either redesigning the class (using mutable on fields that have nothing to do with ordering), or using const_cast, which defeats requiring iterator to be const_iterator. The proposed solution goes in line with trusting user knows what he is doing.
Other Options Evaluated:
Option A. In 23.2.7 [associative.reqmts], paragraph 2, after first sentence, and before "In addition,...", add one line:
Modification of keys shall not change their strict weak ordering.
Option B. Add three new sentences to 23.2.7 [associative.reqmts]:
At the end of paragraph 5: "Keys in an associative container are immutable." At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both
iterator
andconst_iterator
are constant iterators. It is unspecified whether or notiterator
andconst_iterator
are the same type."
Option C. To 23.2.7 [associative.reqmts], paragraph 3, which currently reads:
The phrase ``equivalence of keys'' means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 in the same container are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false.
add the following:
For any two keys k1 and k2 in the same container, comp(k1, k2) shall return the same value whenever it is evaluated. [Note: If k2 is removed from the container and later reinserted, comp(k1, k2) must still return a consistent value but this value may be different than it was the first time k1 and k2 were in the same container. This is intended to allow usage like a string key that contains a filename, where comp compares file contents; if k2 is removed, the file is changed, and the same k2 (filename) is reinserted, comp(k1, k2) must again return a consistent value but this value may be different than it was the previous time k2 was in the container.]
Proposed resolution:
Add the following to 23.2.7 [associative.reqmts] at the indicated location:
At the end of paragraph 3: "For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value."
At the end of paragraph 5: "Keys in an associative container are immutable."
At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both
iterator
andconst_iterator
are constant iterators. It is unspecified whether or notiterator
andconst_iterator
are the same type."
Rationale:
Several arguments were advanced for and against allowing set elements to be mutable as long as the ordering was not effected. The argument which swayed the LWG was one of safety; if elements were mutable, there would be no compile-time way to detect of a simple user oversight which caused ordering to be modified. There was a report that this had actually happened in practice, and had been painful to diagnose. If users need to modify elements, it is possible to use mutable members or const_cast.
Simply requiring that keys be immutable is not sufficient, because the comparison object may indirectly (via pointers) operate on values outside of the keys.
The types iterator
and const_iterator
are permitted
to be different types to allow for potential future work in which some
member functions might be overloaded between the two types. No such
member functions exist now, and the LWG believes that user functionality
will not be impaired by permitting the two types to be the same. A
function that operates on both iterator types can be defined for
const_iterator
alone, and can rely on the automatic
conversion from iterator
to const_iterator
.
[Tokyo: The LWG crafted the proposed resolution and rationale.]
Section: 29.6.5 [template.slice.array] Status: TC1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This is the only place in the whole standard where the implementation has to document something private.
Proposed resolution:
Remove the comment which says "// remainder implementation defined" from:
Section: 17.7.3 [type.info] Status: TC1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 18.6.1, paragraphs 8-9, the lifetime of the return value of exception::what() is left unspecified. This issue has implications with exception safety of exception handling: some exceptions should not throw bad_alloc.
Proposed resolution:
Add to 17.7.3 [type.info] paragraph 9 (exception::what notes clause) the sentence:
The return value remains valid until the exception object from which it is obtained is destroyed or a non-const member function of the exception object is called.
Rationale:
If an exception object has non-const members, they may be used
to set internal state that should affect the contents of the string
returned by what()
.
Section: 99 [depr.lib.binders] Status: CD1 Submitter: Bjarne Stroustrup Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are no versions of binders that apply to non-const elements of a sequence. This makes examples like for_each() using bind2nd() on page 521 of "The C++ Programming Language (3rd)" non-conforming. Suitable versions of the binders need to be added.
Further discussion from Nico:
What is probably meant here is shown in the following example:
class Elem { public: void print (int i) const { } void modify (int i) { } };
int main() { vector<Elem> coll(2); for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::print),42)); // OK for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::modify),42)); // ERROR }
The error results from the fact that bind2nd() passes its first argument (the argument of the sequence) as constant reference. See the following typical implementation:
template <class Operation> class binder2nd : public unary_function<typename Operation::first_argument_type, typename Operation::result_type> { protected: Operation op; typename Operation::second_argument_type value; public: binder2nd(const Operation& o, const typename Operation::second_argument_type& v) : op(o), value(v) {}typename Operation::result_type operator()(const typename Operation::first_argument_type& x) const { return op(x, value); } };
The solution is to overload operator () of bind2nd for non-constant arguments:
template <class Operation> class binder2nd : public unary_function<typename Operation::first_argument_type, typename Operation::result_type> { protected: Operation op; typename Operation::second_argument_type value; public: binder2nd(const Operation& o, const typename Operation::second_argument_type& v) : op(o), value(v) {}typename Operation::result_type operator()(const typename Operation::first_argument_type& x) const { return op(x, value); } typename Operation::result_type operator()(typename Operation::first_argument_type& x) const { return op(x, value); } };
Proposed resolution:
Howard believes there is a flaw in this resolution. See c++std-lib-9127. We may need to reopen this issue.
In 99 [depr.lib.binders] in the declaration of binder1st after:
typename Operation::result_type
operator()(const typename Operation::second_argument_type& x) const;
insert:
typename Operation::result_type
operator()(typename Operation::second_argument_type& x) const;
In 99 [depr.lib.binders] in the declaration of binder2nd after:
typename Operation::result_type
operator()(const typename Operation::first_argument_type& x) const;
insert:
typename Operation::result_type
operator()(typename Operation::first_argument_type& x) const;
[Kona: The LWG discussed this at some length.It was agreed that this is a mistake in the design, but there was no consensus on whether it was a defect in the Standard. Straw vote: NAD - 5. Accept proposed resolution - 3. Leave open - 6.]
[Copenhagen: It was generally agreed that this was a defect. Strap poll: NAD - 0. Accept proposed resolution - 10. Leave open - 1.]
Section: 24.6.4 [istreambuf.iterator], 24.6.4.4 [istreambuf.iterator.ops] Status: TC1 Submitter: Nathan Myers Opened: 1998-10-15 Last modified: 2023-02-07
Priority: Not Prioritized
View other active issues in [istreambuf.iterator].
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Discussion:
Member istreambuf_iterator<>::equal is not declared "const", yet [istreambuf.iterator::op==] says that operator==, which is const, calls it. This is contradictory.
Proposed resolution:
In 24.6.4 [istreambuf.iterator] and also in [istreambuf.iterator::equal], replace:
bool equal(istreambuf_iterator& b);
with:
bool equal(const istreambuf_iterator& b) const;
ostreambuf_iterator
constructorSection: 24.6.5.2 [ostreambuf.iter.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-10-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The requires clause for ostreambuf_iterator
's
constructor from an ostream_type
(24.5.4.1, paragraph 1)
reads "s is not null". However, s is a
reference, and references can't be null.
Proposed resolution:
In 24.6.5.2 [ostreambuf.iter.cons]:
Move the current paragraph 1, which reads "Requires: s is not null.", from the first constructor to the second constructor.
Insert a new paragraph 1 Requires clause for the first constructor reading:
Requires:
s.rdbuf()
is not null.
Section: 17.6.3.4 [new.delete.placement] Status: TC1 Submitter: Steve Clamage Opened: 1998-10-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 196
Discussion:
Section 18.5.1.3 contains the following example:
[Example: This can be useful for constructing an object at a known address: char place[sizeof(Something)]; Something* p = new (place) Something(); -end example]
First code line: "place" need not have any special alignment, and the following constructor could fail due to misaligned data.
Second code line: Aren't the parens on Something() incorrect? [Dublin: the LWG believes the () are correct.]
Examples are not normative, but nevertheless should not show code that is invalid or likely to fail.
Proposed resolution:
Replace the first line of code in the example in 17.6.3.4 [new.delete.placement] with:
void* place = operator new(sizeof(Something));
Section: 99 [depr.strstream.cons] Status: TC1 Submitter: Steve Clamage Opened: 1998-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
D.7.4.1 strstream constructors paragraph 2 says:
Effects: Constructs an object of class strstream, initializing the base class with iostream(& sb) and initializing sb with one of the two constructors:
- If mode&app==0, then s shall designate the first element of an array of n elements. The constructor is strstreambuf(s, n, s).
- If mode&app==0, then s shall designate the first element of an array of n elements that contains an NTBS whose first element is designated by s. The constructor is strstreambuf(s, n, s+std::strlen(s)).
Notice the second condition is the same as the first. I think the second condition should be "If mode&app==app", or "mode&app!=0", meaning that the append bit is set.
Proposed resolution:
In [depr.ostrstream.cons] paragraph 2 and 99 [depr.strstream.cons]
paragraph 2, change the first condition to (mode&app)==0
and the second condition to (mode&app)!=0
.
basic_ostream
uses nonexistent num_put
member functionsSection: 31.7.6.3.2 [ostream.inserters.arithmetic] Status: CD1 Submitter: Matt Austern Opened: 1998-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The effects clause for numeric inserters says that
insertion of a value x
, whose type is either bool
,
short
, unsigned short
, int
, unsigned
int
, long
, unsigned long
, float
,
double
, long double
, or const void*
, is
delegated to num_put
, and that insertion is performed as if
through the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), val). failed();
This doesn't work, because num_put<>
::put is only
overloaded for the types bool
, long
, unsigned
long
, double
, long double
, and const
void*
. That is, the code fragment in the standard is incorrect
(it is diagnosed as ambiguous at compile time) for the types
short
, unsigned short
, int
, unsigned
int
, and float
.
We must either add new member functions to num_put
, or
else change the description in ostream
so that it only calls
functions that are actually there. I prefer the latter.
Proposed resolution:
Replace 27.6.2.5.2, paragraph 1 with the following:
The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting. When val is of type bool, long, unsigned long, double, long double, or const void*, the formatting conversion occurs as if it performed the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), val). failed();When val is of type short the formatting conversion occurs as if it performed the following code fragment:
ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned short>(val)) : static_cast<long>(val)). failed();When val is of type int the formatting conversion occurs as if it performed the following code fragment:
ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned int>(val)) : static_cast<long>(val)). failed();When val is of type unsigned short or unsigned int the formatting conversion occurs as if it performed the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), static_cast<unsigned long>(val)). failed();When val is of type float the formatting conversion occurs as if it performed the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), static_cast<double>(val)). failed();
[post-Toronto: This differs from the previous proposed resolution; PJP provided the new wording. The differences are in signed short and int output.]
Rationale:
The original proposed resolution was to cast int and short to long, unsigned int and unsigned short to unsigned long, and float to double, thus ensuring that we don't try to use nonexistent num_put<> member functions. The current proposed resolution is more complicated, but gives more expected results for hex and octal output of signed short and signed int. (On a system with 16-bit short, for example, printing short(-1) in hex format should yield 0xffff.)
basic_istream
uses nonexistent num_get
member functionsSection: 31.7.5.3.2 [istream.formatted.arithmetic] Status: CD1 Submitter: Matt Austern Opened: 1998-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.formatted.arithmetic].
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Discussion:
Formatted input is defined for the types short
, unsigned short
, int
,
unsigned int
, long
, unsigned long
, float
, double
,
long double
, bool
, and void*
. According to section 27.6.1.2.2,
formatted input of a value x
is done as if by the following code fragment:
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; use_facet< numget >(loc).get(*this, 0, *this, err, val); setstate(err);
According to section 28.3.4.3.2.2 [facet.num.get.members], however,
num_get<>::get()
is only overloaded for the types
bool
, long
, unsigned short
, unsigned
int
, unsigned long
, unsigned long
,
float
, double
, long double
, and
void*
. Comparing the lists from the two sections, we find
that 27.6.1.2.2 is using a nonexistent function for types
short
and int
.
Proposed resolution:
In 31.7.5.3.2 [istream.formatted.arithmetic] Arithmetic Extractors, remove the two lines (1st and 3rd) which read:
operator>>(short& val); ... operator>>(int& val);
And add the following at the end of that section (27.6.1.2.2) :
operator>>(short& val);The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet< numget >(loc).get(*this, 0, *this, err, lval); if (err == 0 && (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval)) err = ios_base::failbit; setstate(err);operator>>(int& val);The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet< numget >(loc).get(*this, 0, *this, err, lval); if (err == 0 && (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval)) err = ios_base::failbit; setstate(err);
[Post-Tokyo: PJP provided the above wording.]
Section: 16.4.6.14 [res.on.exception.handling] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 16.4.6.14 [res.on.exception.handling] states:
"An implementation may strengthen the exception-specification for a function by removing listed exceptions."
The problem is that if an implementation is allowed to do this for virtual functions, then a library user cannot write a class that portably derives from that class.
For example, this would not compile if ios_base::failure::~failure had an empty exception specification:
#include <ios> #include <string> class D : public std::ios_base::failure { public: D(const std::string&); ~D(); // error - exception specification must be compatible with // overridden virtual function ios_base::failure::~failure() };
Proposed resolution:
Change Section 16.4.6.14 [res.on.exception.handling] from:
"may strengthen the exception-specification for a function"
to:
"may strengthen the exception-specification for a non-virtual function".
Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The original issue asked whether a library implementor could specialize standard library templates for built-in types. (This was an issue because users are permitted to explicitly instantiate standard library templates.)
Specializations are no longer a problem, because of the resolution to core issue 259. Under the proposed resolution, it will be legal for a translation unit to contain both a specialization and an explicit instantiation of the same template, provided that the specialization comes first. In such a case, the explicit instantiation will be ignored. Further discussion of library issue 120 assumes that the core 259 resolution will be adopted.
However, as noted in lib-7047, one piece of this issue still remains: what happens if a standard library implementor explicitly instantiates a standard library templates? It's illegal for a program to contain two different explicit instantiations of the same template for the same type in two different translation units (ODR violation), and the core working group doesn't believe it is practical to relax that restriction.
The issue, then, is: are users allowed to explicitly instantiate standard library templates for non-user defined types? The status quo answer is 'yes'. Changing it to 'no' would give library implementors more freedom.
This is an issue because, for performance reasons, library implementors often need to explicitly instantiate standard library templates. (for example, std::basic_string<char>) Does giving users freedom to explicitly instantiate standard library templates for non-user defined types make it impossible or painfully difficult for library implementors to do this?
John Spicer suggests, in lib-8957, that library implementors have a mechanism they can use for explicit instantiations that doesn't prevent users from performing their own explicit instantiations: put each explicit instantiation in its own object file. (Different solutions might be necessary for Unix DSOs or MS-Windows DLLs.) On some platforms, library implementors might not need to do anything special: the "undefined behavior" that results from having two different explicit instantiations might be harmless.
Proposed resolution:
Append to 16.4.5.3 [reserved.names] paragraph 1:
A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.
[Kona: changed the wording from "a user-defined name" to "the name of a user-defined type"]
Rationale:
The LWG considered another possible resolution:
In light of the resolution to core issue 259, no normative changes in the library clauses are necessary. Add the following non-normative note to the end of 16.4.5.3 [reserved.names] paragraph 1:
[Note: A program may explicitly instantiate standard library templates, even when an explicit instantiation does not depend on a user-defined name. --end note]
The LWG rejected this because it was believed that it would make it unnecessarily difficult for library implementors to write high-quality implementations. A program may not include an explicit instantiation of the same template, for the same template arguments, in two different translation units. If users are allowed to provide explicit instantiations of Standard Library templates for built-in types, then library implementors aren't, at least not without nonportable tricks.
The most serious problem is a class template that has writeable
static member variables. Unfortunately, such class templates are
important and, in existing Standard Library implementations, are
often explicitly specialized by library implementors: locale facets,
which have a writeable static member variable id
. If a
user's explicit instantiation collided with the implementations
explicit instantiation, iostream initialization could cause locales
to be constructed in an inconsistent state.
One proposed implementation technique was for Standard Library implementors to provide explicit instantiations in separate object files, so that they would not be picked up by the linker when the user also provides an explicit instantiation. However, this technique only applies for Standard Library implementations that are packaged as static archives. Most Standard Library implementations nowadays are packaged as dynamic libraries, so this technique would not apply.
The Committee is now considering standardization of dynamic linking. If there are such changes in the future, it may be appropriate to revisit this issue later.
Section: 31.6.3 [streambuf] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 27.5.2 describes the streambuf classes this way:
The class streambuf is a specialization of the template class basic_streambuf specialized for the type char.
The class wstreambuf is a specialization of the template class basic_streambuf specialized for the type wchar_t.
This implies that these classes must be template specializations, not typedefs.
It doesn't seem this was intended, since Section 27.5 has them declared as typedefs.
Proposed resolution:
Remove 31.6.3 [streambuf] paragraphs 2 and 3 (the above two sentences).
Rationale:
The streambuf
synopsis already has a declaration for the
typedefs and that is sufficient.
Section: 29.6.5.4 [slice.arr.fill], 29.6.7.4 [gslice.array.fill], 29.6.8.4 [mask.array.fill], 29.6.9.4 [indirect.array.fill] Status: CD1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
One of the operator= in the valarray helper arrays is const and one is not. For example, look at slice_array. This operator= in Section 29.6.5.2 [slice.arr.assign] is const:
void operator=(const valarray<T>&) const;
but this one in Section 29.6.5.4 [slice.arr.fill] is not:
void operator=(const T&);
The description of the semantics for these two functions is similar.
Proposed resolution:
29.6.5 [template.slice.array] Template class slice_array
In the class template definition for slice_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
29.6.5.4 [slice.arr.fill] slice_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
29.6.7 [template.gslice.array] Template class gslice_array
In the class template definition for gslice_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
29.6.7.4 [gslice.array.fill] gslice_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
29.6.8 [template.mask.array] Template class mask_array
In the class template definition for mask_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
29.6.8.4 [mask.array.fill] mask_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
29.6.9 [template.indirect.array] Template class indirect_array
In the class template definition for indirect_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
29.6.9.4 [indirect.array.fill] indirect_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
[Redmond: Robert provided wording.]
Rationale:
There's no good reason for one version of operator= being const and another one not. Because of issue 253(i), this now matters: these functions are now callable in more circumstances. In many existing implementations, both versions are already const.
Section: 28.3.4.2.3 [locale.ctype.byname] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 28.3.4.2.3 [locale.ctype.byname] ctype_byname<charT>::do_scan_is() and do_scan_not() are declared to return a const char* not a const charT*.
Proposed resolution:
Change Section 28.3.4.2.3 [locale.ctype.byname] do_scan_is()
and
do_scan_not()
to return a const
charT*
.
Section: 29.6.2 [template.valarray] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 29.6.2 [template.valarray] valarray<T>::operator!() is declared to return a valarray<T>, but in Section 29.6.2.6 [valarray.unary] it is declared to return a valarray<bool>. The latter appears to be correct.
Proposed resolution:
Change in Section 29.6.2 [template.valarray] the declaration of
operator!()
so that the return type is
valarray<bool>
.
Section: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Typos in 22.2.1.1.2 need to be fixed.
Proposed resolution:
In Section 28.3.4.2.2.3 [locale.ctype.virtuals] change:
do_widen(do_narrow(c),0) == c
to:
do_widen(do_narrow(c,0)) == c
and change:
(is(M,c) || !ctc.is(M, do_narrow(c),dfault) )
to:
(is(M,c) || !ctc.is(M, do_narrow(c,dfault)) )
Section: 99 [auto.ptr] Status: TC1 Submitter: Greg Colvin Opened: 1999-02-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are two problems with the current auto_ptr
wording
in the standard:
First, the auto_ptr_ref
definition cannot be nested
because auto_ptr<Derived>::auto_ptr_ref
is unrelated to
auto_ptr<Base>::auto_ptr_ref
. Also submitted by
Nathan Myers, with the same proposed resolution.
Second, there is no auto_ptr
assignment operator taking an
auto_ptr_ref
argument.
I have discussed these problems with my proposal coauthor, Bill Gibbons, and with some compiler and library implementors, and we believe that these problems are not desired or desirable implications of the standard.
25 Aug 1999: The proposed resolution now reflects changes suggested by Dave Abrahams, with Greg Colvin's concurrence; 1) changed "assignment operator" to "public assignment operator", 2) changed effects to specify use of release(), 3) made the conversion to auto_ptr_ref const.
2 Feb 2000: Lisa Lippincott comments: [The resolution of] this issue states that the conversion from auto_ptr to auto_ptr_ref should be const. This is not acceptable, because it would allow initialization and assignment from _any_ const auto_ptr! It also introduces an implementation difficulty in writing this conversion function -- namely, somewhere along the line, a const_cast will be necessary to remove that const so that release() may be called. This may result in undefined behavior [7.1.5.1/4]. The conversion operator does not have to be const, because a non-const implicit object parameter may be bound to an rvalue [13.3.3.1.4/3] [13.3.1/5].
Tokyo: The LWG removed the following from the proposed resolution:
In 21.3.5 [meta.unary], paragraph 2, and 21.3.5.4 [meta.unary.prop], paragraph 2, make the conversion to auto_ptr_ref const:
template<class Y> operator auto_ptr_ref<Y>() const throw();
Proposed resolution:
In 21.3.5 [meta.unary], paragraph 2, move
the auto_ptr_ref
definition to namespace scope.
In 21.3.5 [meta.unary], paragraph 2, add
a public assignment operator to the auto_ptr
definition:
auto_ptr& operator=(auto_ptr_ref<X> r) throw();
Also add the assignment operator to 21.3.5.4 [meta.unary.prop]:
auto_ptr& operator=(auto_ptr_ref<X> r) throw()Effects: Calls
reset(p.release())
for theauto_ptr p
thatr
holds a reference to.
Returns:*this
.
Section: 31.7.5.4 [istream.unformatted], 31.7.6.2.5 [ostream.seeks] Status: TC1 Submitter: Angelika Langer Opened: 1999-02-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Currently, the standard does not specify how seekg() and seekp() indicate failure. They are not required to set failbit, and they can't return an error indication because they must return *this, i.e. the stream. Hence, it is undefined what happens if they fail. And they can fail, for instance, when a file stream is disconnected from the underlying file (is_open()==false) or when a wide character file stream must perform a state-dependent code conversion, etc.
The stream functions seekg() and seekp() should set failbit in the stream state in case of failure.
Proposed resolution:
Add to the Effects: clause of seekg() in 31.7.5.4 [istream.unformatted] and to the Effects: clause of seekp() in 31.7.6.2.5 [ostream.seeks]:
In case of failure, the function calls
setstate(failbit)
(which may throwios_base::failure
).
Rationale:
Setting failbit is the usual error reporting mechanism for streams
Section: 23.2.7 [associative.reqmts], 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 1999-03-02 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [associative.reqmts].
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Duplicate of: 451
Discussion:
Table 67 (23.1.1) says that container::erase(iterator) returns an iterator. Table 69 (23.1.2) says that in addition to this requirement, associative containers also say that container::erase(iterator) returns void. That's not an addition; it's a change to the requirements, which has the effect of making associative containers fail to meet the requirements for containers.
Proposed resolution:
In 23.2.7 [associative.reqmts], in Table 69 Associative container
requirements, change the return type of a.erase(q)
from
void
to iterator
. Change the
assertion/not/pre/post-condition from "erases the element pointed to
by q
" to "erases the element pointed to by q
.
Returns an iterator pointing to the element immediately following q
prior to the element being erased. If no such element exists, a.end()
is returned."
In 23.2.7 [associative.reqmts], in Table 69 Associative container
requirements, change the return type of a.erase(q1, q2)
from void
to iterator
. Change the
assertion/not/pre/post-condition from "erases the elements in the
range [q1, q2)
" to "erases the elements in the range [q1,
q2)
. Returns q2."
In 23.4.3 [map], in the map
class synopsis; and
in 23.4.4 [multimap], in the multimap
class synopsis; and
in 23.4.6 [set], in the set
class synopsis; and
in 23.4.7 [multiset], in the multiset
class synopsis:
change the signature of the first erase
overload to
iterator erase(iterator position);
and change the signature of the third erase
overload to
iterator erase(iterator first, iterator last);
[Pre-Kona: reopened at the request of Howard Hinnant]
[Post-Kona: the LWG agrees the return type should be
iterator
, not void
. (Alex Stepanov agrees too.)
Matt provided wording.]
[ Sydney: the proposed wording went in the right direction, but it wasn't good enough. We want to return an iterator from the range form of erase as well as the single-iterator form. Also, the wording is slightly different from the wording we have for sequences; there's no good reason for having a difference. Matt provided new wording, (reflected above) which we will review at the next meeting. ]
[ Redmond: formally voted into WP. ]
Section: 23.3.11.3 [list.capacity] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description reads:
-1- Effects:
if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) erase(begin()+sz, end()); else ; // do nothing
Obviously list::resize should not be specified in terms of random access iterators.
Proposed resolution:
Change 23.3.11.3 [list.capacity] paragraph 1 to:
Effects:
if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) { iterator i = begin(); advance(i, sz); erase(i, end()); }
[Dublin: The LWG asked Howard to discuss exception safety offline with David Abrahams. They had a discussion and believe there is no issue of exception safety with the proposed resolution.]
Section: 23.4.3 [map] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The title says it all.
Proposed resolution:
Insert in 23.4.3 [map], paragraph 2, after operator= in the map declaration:
allocator_type get_allocator() const;
Section: 23.3.13.2 [vector.cons] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity description says: "It does at most 2N calls to the copy constructor of T and logN reallocations if they are just input iterators ...".
This appears to be overly restrictive, dictating the precise memory/performance tradeoff for the implementor.
Proposed resolution:
Change 23.3.13.2 [vector.cons], paragraph 1 to:
-1- Complexity: The constructor template <class InputIterator> vector(InputIterator first, InputIterator last) makes only N calls to the copy constructor of T (where N is the distance between first and last) and no reallocations if iterators first and last are of forward, bidirectional, or random access categories. It makes order N calls to the copy constructor of T and order logN reallocations if they are just input iterators.
Rationale:
"at most 2N calls" is correct only if the growth factor is greater than or equal to 2.
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I may be misunderstanding the intent, but should not seekg set only the input stream and seekp set only the output stream? The description seems to say that each should set both input and output streams. If that's really the intent, I withdraw this proposal.
Proposed resolution:
In section 27.6.1.3 change:
basic_istream<charT,traits>& seekg(pos_type pos); Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).
To:
basic_istream<charT,traits>& seekg(pos_type pos); Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::in).
In section 27.6.1.3 change:
basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir); Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).
To:
basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir); Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::in).
In section 27.6.2.4, paragraph 2 change:
-2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).
To:
-2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::out).
In section 27.6.2.4, paragraph 4 change:
-4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).
To:
-4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::out).
[Dublin: Dietmar Kühl thinks this is probably correct, but would like the opinion of more iostream experts before taking action.]
[Tokyo: Reviewed by the LWG. PJP noted that although his docs are incorrect, his implementation already implements the Proposed Resolution.]
[Post-Tokyo: Matt Austern comments:
Is it a problem with basic_istream and basic_ostream, or is it a problem
with basic_stringbuf?
We could resolve the issue either by changing basic_istream and
basic_ostream, or by changing basic_stringbuf. I prefer the latter
change (or maybe both changes): I don't see any reason for the standard to
require that std::stringbuf s(std::string("foo"), std::ios_base::in);
s.pubseekoff(0, std::ios_base::beg); must fail.
This requirement is a bit weird. There's no similar requirement
for basic_streambuf<>::seekpos, or for basic_filebuf<>::seekoff or
basic_filebuf<>::seekpos.]
Section: 28.3.3.1 [locale] Status: TC1 Submitter: Angelika Langer Opened: 1999-03-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 28.3.3.1 [locale] says:
-4- In the call to use_facet<Facet>(loc), the type argument chooses a facet, making available all members of the named type. If Facet is not present in a locale (or, failing that, in the global locale), it throws the standard exception bad_cast. A C++ program can check if a locale implements a particular facet with the template function has_facet<Facet>().
This contradicts the specification given in section
28.3.3.2 [locale.global.templates]:
template <class Facet> const Facet& use_facet(const
locale& loc);
-1- Get a reference to a facet of a locale.
-2- Returns: a reference to the corresponding facet of loc, if present.
-3- Throws: bad_cast if has_facet<Facet>(loc) is false.
-4- Notes: The reference returned remains valid at least as long as any copy of loc exists
Proposed resolution:
Remove the phrase "(or, failing that, in the global locale)" from section 22.1.1.
Rationale:
Needed for consistency with the way locales are handled elsewhere in the standard.
Section: 23.2.4 [sequence.reqmts] Status: TC1 Submitter: Andrew Koenig Opened: 1999-03-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The sentence introducing the Optional sequence operation table (23.1.1 paragraph 12) has two problems:
A. It says ``The operations in table 68 are provided only for the containers for which
they take constant time.''
That could be interpreted in two ways, one of them being ``Even though table 68 shows
particular operations as being provided, implementations are free to omit them if they
cannot implement them in constant time.''
B. That paragraph says nothing about amortized constant time, and it should.
Proposed resolution:
Replace the wording in 23.1.1 paragraph 12 which begins ``The operations in table 68 are provided only..." with:
Table 68 lists sequence operations that are provided for some types of sequential containers but not others. An implementation shall provide these operations for all container types shown in the ``container'' column, and shall implement them so as to take amortized constant time.
Section: 27.4.3.7.4 [string.insert], 27.4.3.7.6 [string.replace] Status: TC1 Submitter: Arch Robison Opened: 1999-04-28 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
Sections 21.3.6.4 paragraph 1 and 21.3.6.6 paragraph 1 surely have misprints where they
say:
xpos <= pos
and pos < size();
Surely the document meant to say ``xpos < size()
'' in both places.
[Judy Ward also sent in this issue for 21.3.6.4 with the same proposed resolution.]
Proposed resolution:
Change Sections 21.3.6.4 paragraph 1 and 21.3.6.6 paragraph 1, the line which says:
xpos <= pos
and pos < size();
to:
xpos <= pos
and xpos < size();
Section: 26.8.11 [alg.lex.comparison] Status: TC1 Submitter: Howard Hinnant Opened: 1999-06-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The lexicographical_compare complexity is specified as:
"At most min((last1 - first1), (last2 - first2))
applications of the corresponding comparison."
The best I can do is twice that expensive.
Nicolai Josuttis comments in lib-6862: You mean, to check for equality you have to check both < and >? Yes, IMO you are right! (and Matt states this complexity in his book)
Proposed resolution:
Change 26.8.11 [alg.lex.comparison] complexity to:
At most
2*min((last1 - first1), (last2 - first2))
applications of the corresponding comparison.
Change the example at the end of paragraph 3 to read:
[Example:
for ( ; first1 != last1 && first2 != last2 ; ++first1, ++first2) {
if (*first1 < *first2) return true;
if (*first2 < *first1) return false;
}
return first1 == last1 && first2 != last2;
--end example]
Section: 23.3.5.2 [deque.cons] Status: TC1 Submitter: Herb Sutter Opened: 1999-05-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 23.3.5.2 [deque.cons] paragraph 6, the deque ctor that takes an iterator range appears to have complexity requirements which are incorrect, and which contradict the complexity requirements for insert(). I suspect that the text in question, below, was taken from vector:
Complexity: If the iterators first and last are forward iterators, bidirectional iterators, or random access iterators the constructor makes only N calls to the copy constructor, and performs no reallocations, where N is last - first.
The word "reallocations" does not really apply to deque. Further, all of the following appears to be spurious:
It makes at most 2N calls to the copy constructor of T and log N reallocations if they are input iterators.1)
1) The complexity is greater in the case of input iterators because each element must be added individually: it is impossible to determine the distance between first abd last before doing the copying.
This makes perfect sense for vector, but not for deque. Why should deque gain an efficiency advantage from knowing in advance the number of elements to insert?
Proposed resolution:
In 23.3.5.2 [deque.cons] paragraph 6, replace the Complexity description, including the footnote, with the following text (which also corrects the "abd" typo):
Complexity: Makes last - first calls to the copy constructor of T.
Section: 29.4.6 [complex.ops] Status: TC1 Submitter: Angelika Langer Opened: 1999-05-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The extractor for complex numbers is specified as:
template<class T, class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, complex<T>& x);
Effects: Extracts a complex number x of the form: u, (u), or (u,v), where u is the real part and v is the imaginary part (lib.istream.formatted).
Requires: The input values be convertible to T. If bad input is encountered, calls is.setstate(ios::failbit) (which may throw ios::failure (lib.iostate.flags).
Returns: is .
Is it intended that the extractor for complex numbers does not skip
whitespace, unlike all other extractors in the standard library do?
Shouldn't a sentry be used?
The inserter for complex numbers is specified as:
template<class T, class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& o, const complex<T>& x);
Effects: inserts the complex number x onto the stream o as if it were implemented as follows:
template<class T, class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& o, const complex<T>& x)
{
basic_ostringstream<charT, traits> s;
s.flags(o.flags());
s.imbue(o.getloc());
s.precision(o.precision());
s << '(' << x.real() << "," << x.imag() << ')';
return o << s.str();
}
Is it intended that the inserter for complex numbers ignores the field width and does not do any padding? If, with the suggested implementation above, the field width were set in the stream then the opening parentheses would be adjusted, but the rest not, because the field width is reset to zero after each insertion.
I think that both operations should use sentries, for sake of consistency with the other inserters and extractors in the library. Regarding the issue of padding in the inserter, I don't know what the intent was.
Proposed resolution:
After 29.4.6 [complex.ops] paragraph 14 (operator>>), add a Notes clause:
Notes: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions.
Rationale:
For extractors, the note is added to make it clear that skipping whitespace follows an "all-or-none" rule.
For inserters, the LWG believes there is no defect; the standard is correct as written.
Section: 16.4.6.4 [global.functions] Status: TC1 Submitter: Lois Goldthwaite Opened: 1999-06-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The library had many global functions until 17.4.1.1 [lib.contents] paragraph 2 was added:
All library entities except macros, operator new and operator delete are defined within the namespace std or namespaces nested within namespace std.
It appears "global function" was never updated in the following:
17.4.4.3 - Global functions [lib.global.functions]
-1- It is unspecified whether any global functions in the C++ Standard Library are defined as inline (dcl.fct.spec).
-2- A call to a global function signature described in Clauses lib.language.support through lib.input.output behaves the same as if the implementation declares no additional global function signatures.*
[Footnote: A valid C++ program always calls the expected library global function. An implementation may also define additional global functions that would otherwise not be called by a valid C++ program. --- end footnote]
-3- A global function cannot be declared by the implementation as taking additional default arguments.
17.4.4.4 - Member functions [lib.member.functions]
-2- An implementation can declare additional non-virtual member function signatures within a class:-- by adding arguments with default values to a member function signature; The same latitude does not extend to the implementation of virtual or global functions, however.
Proposed resolution:
Change "global" to "global or non-member" in:
17.4.4.3 [lib.global.functions] section title,
17.4.4.3 [lib.global.functions] para 1,
17.4.4.3 [lib.global.functions] para 2 in 2 places plus 2 places in the footnote,
17.4.4.3 [lib.global.functions] para 3,
17.4.4.4 [lib.member.functions] para 2
Rationale:
Because operator new and delete are global, the proposed resolution was changed from "non-member" to "global or non-member.
Section: 99 [facets.examples] Status: TC1 Submitter: Jeremy Siek Opened: 1999-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 99 [facets.examples] paragraph 13, the do_truename() and do_falsename() functions in the example facet BoolNames should be const. The functions they are overriding in numpunct_byname<char> are const.
Proposed resolution:
In 99 [facets.examples] paragraph 13, insert "const" in two places:
string do_truename() const { return "Oui Oui!"; }
string do_falsename() const { return "Mais Non!"; }
Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Andrew Koenig Opened: 1999-06-28 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
Suppose that c and c1 are sequential containers and i is an iterator that refers to an element of c. Then I can insert a copy of c1's elements into c ahead of element i by executing
c.insert(i, c1.begin(), c1.end());
If c is a vector, it is fairly easy for me to find out where the newly inserted elements are, even though i is now invalid:
size_t i_loc = i - c.begin(); c.insert(i, c1.begin(), c1.end());
and now the first inserted element is at c.begin()+i_loc and one
past the last is at c.begin()+i_loc+c1.size().
But what if c is a list? I can still find the location of one
past the last inserted element, because i is still valid.
To find the location of the first inserted element, though,
I must execute something like
for (size_t n = c1.size(); n; --n) --i;
because i is now no longer a random-access iterator.
Alternatively, I might write something like
bool first = i == c.begin(); list<T>::iterator j = i; if (!first) --j; c.insert(i, c1.begin(), c1.end()); if (first) j = c.begin(); else ++j;
which, although wretched, requires less overhead.
But I think the right solution is to change the definition of insert
so that instead of returning void, it returns an iterator that refers
to the first element inserted, if any, and otherwise is a copy of its
first argument.
[ Summit: ]
Reopened by Alisdair.
[ Post Summit Alisdair adds: ]
In addition to the original rationale for C++03, this change also gives a consistent interface for all container insert operations i.e. they all return an iterator to the (first) inserted item.
Proposed wording provided.
[ 2009-07 Frankfurt ]
Q: why isn't this change also proposed for associative containers?
A: The returned iterator wouldn't necessarily point to a contiguous range.
Moved to Ready.
Proposed resolution:
23.2.4 [sequence.reqmts] Table 83
change return type from void
to iterator
for the following rows:
Table 83 — Sequence container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition a.insert(p,n,t)
voiditeratorInserts n
copies oft
beforep
.a.insert(p,i,j)
voiditeratorEach iterator in the range [i,j)
shall be dereferenced exactly once. pre:i
andj
are not iterators intoa
. Inserts copies of elements in[i, j)
beforep
a.insert(p,il)
voiditeratora.insert(p, il.begin(), il.end())
.
Add after p6 23.2.4 [sequence.reqmts]:
-6- ...
The iterator returned from
a.insert(p,n,t)
points to the copy of the first element inserted intoa
, orp
ifn == 0
.The iterator returned from
a.insert(p,i,j)
points to the copy of the first element inserted intoa
, orp
ifi == j
.The iterator returned from
a.insert(p,il)
points to the copy of the first element inserted intoa
, orp
ifil
is empty.
p2 23.3.5 [deque] Update class definition, change return type
from void
to iterator
:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);voiditerator insert(const_iterator position, initializer_list<T>);
23.3.5.4 [deque.modifiers] change return type from void
to iterator
on following declarations:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);
Add the following (missing) declaration
iterator insert(const_iterator position, initializer_list<T>);
[forwardlist] Update class definition, change return type
from void
to iterator
:
voiditerator insert_after(const_iterator position, initializer_list<T> il);voiditerator insert_after(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert_after(const_iterator position, InputIterator first, InputIterator last);
p8 [forwardlist.modifiers] change return type from void
to iterator
:
voiditerator insert_after(const_iterator position, size_type n, const T& x);
Add paragraph:
Returns: position.
p10 [forwardlist.modifiers] change return type from void
to iterator
:
template <class InputIterator>voiditerator insert_after(const_iterator position, InputIterator first, InputIterator last);
Add paragraph:
Returns: position.
p12 [forwardlist.modifiers] change return type from void
to iterator
on following declarations:
voiditerator insert_after(const_iterator position, initializer_list<T> il);
change return type from void
to iterator
on following declarations:
p2 23.3.11 [list] Update class definition, change return type from void
to iterator
:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);voiditerator insert(const_iterator position, initializer_list<T>);
23.3.11.4 [list.modifiers] change return type from void
to iterator
on following declarations:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);
Add the following (missing) declaration
iterator insert(const_iterator position, initializer_list<T>);
p2 23.3.13 [vector]
Update class definition, change return type from void
to iterator
:
voiditerator insert(const_iterator position, T&& x);voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);voiditerator insert(const_iterator position, initializer_list<T>);
23.3.13.5 [vector.modifiers] change return type from void
to iterator
on following declarations:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);
Add the following (missing) declaration
iterator insert(const_iterator position, initializer_list<T>);
p1 23.3.14 [vector.bool] Update class definition, change return type from void
to iterator
:
voiditerator insert (const_iterator position, size_type n, const bool& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);voiditerator insert(const_iterator position, initializer_list<bool> il);
p5 27.4.3 [basic.string] Update class definition, change return type from void
to iterator
:
voiditerator insert(const_iterator p, size_type n, charT c); template<class InputIterator>voiditerator insert(const_iterator p, InputIterator first, InputIterator last);voiditerator insert(const_iterator p, initializer_list<charT>);
p13 27.4.3.7.4 [string.insert] change return type from void
to iterator
:
voiditerator insert(const_iterator p, size_type n, charT c);
Add paragraph:
Returns: an iterator which refers to the copy of the first inserted character, or
p
ifn == 0
.
p15 27.4.3.7.4 [string.insert] change return type from void
to iterator
:
template<class InputIterator>voiditerator insert(const_iterator p, InputIterator first, InputIterator last);
Add paragraph:
Returns: an iterator which refers to the copy of the first inserted character, or
p
iffirst == last
.
p17 27.4.3.7.4 [string.insert] change return type from void
to iterator
:
voiditerator insert(const_iterator p, initializer_list<charT> il);
Add paragraph:
Returns: an iterator which refers to the copy of the first inserted character, or
p
ifil
is empty.
Rationale:
[ The following was the C++98/03 rationale and does not necessarily apply to the proposed resolution in the C++0X time frame: ]
The LWG believes this was an intentional design decision and so is not a defect. It may be worth revisiting for the next standard.
Section: 26.6.9 [alg.find.first.of] Status: TC1 Submitter: Matt McClure Opened: 1999-06-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Proposed resolution:
Change 26.6.9 [alg.find.first.of] paragraph 2 from:
Returns: The first iterator i in the range [first1, last1) such that for some integer j in the range [first2, last2) ...
to:
Returns: The first iterator i in the range [first1, last1) such that for some iterator j in the range [first2, last2) ...
Section: 23.2.4 [sequence.reqmts] Status: TC1 Submitter: Ed Brey Opened: 1999-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
For both sequences and associative containers, a.clear() has the
semantics of erase(a.begin(),a.end()), which is undefined for an empty
container since erase(q1,q2) requires that q1 be dereferenceable
(23.1.1,3 and 23.1.2,7). When the container is empty, a.begin() is
not dereferenceable.
The requirement that q1 be unconditionally dereferenceable causes many
operations to be intuitively undefined, of which clearing an empty
container is probably the most dire.
Since q1 and q2 are only referenced in the range [q1, q2), and [q1,
q2) is required to be a valid range, stating that q1 and q2 must be
iterators or certain kinds of iterators is unnecessary.
Proposed resolution:
In 23.1.1, paragraph 3, change:
p and q2 denote valid iterators to a, q and q1 denote valid dereferenceable iterators to a, [q1, q2) denotes a valid range
to:
p denotes a valid iterator to a, q denotes a valid dereferenceable iterator to a, [q1, q2) denotes a valid range in a
In 23.1.2, paragraph 7, change:
p and q2 are valid iterators to a, q and q1 are valid dereferenceable iterators to a, [q1, q2) is a valid range
to
p is a valid iterator to a, q is a valid dereferenceable iterator to a, [q1, q2) is a valid range into a
scan_is()
semanticsSection: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The semantics of scan_is()
(paragraphs 4 and 6) is not exactly described
because there is no function is()
which only takes a character as
argument. Also, in the effects clause (paragraph 3), the semantic is also kept
vague.
Proposed resolution:
In 28.3.4.2.2.3 [locale.ctype.virtuals] paragraphs 4 and 6, change the returns clause from:
"... such that
is(*p)
would..."
to: "... such that is(m, *p)
would...."
narrow()
semanticsSection: 28.3.4.2.4.3 [facet.ctype.char.members] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 207
Discussion:
The description of the array version of narrow()
(in
paragraph 11) is flawed: There is no member do_narrow()
which
takes only three arguments because in addition to the range a default
character is needed.
Additionally, for both widen
and narrow
we have
two signatures followed by a Returns clause that only addresses
one of them.
Proposed resolution:
Change the returns clause in 28.3.4.2.4.3 [facet.ctype.char.members] paragraph 10 from:
Returns: do_widen(low, high, to).
to:
Returns: do_widen(c) or do_widen(low, high, to), respectively.
Change 28.3.4.2.4.3 [facet.ctype.char.members] paragraph 10 and 11 from:
char narrow(char c, char /*dfault*/) const; const char* narrow(const char* low, const char* high, char /*dfault*/, char* to) const;
Returns: do_narrow(low, high, to).
to:
char narrow(char c, char dfault) const; const char* narrow(const char* low, const char* high, char dfault, char* to) const;
Returns: do_narrow(c, dfault) or do_narrow(low, high, dfault, to), respectively.
[Kona: 1) the problem occurs in additional places, 2) a user defined version could be different.]
[Post-Tokyo: Dietmar provided the above wording at the request of the LWG. He could find no other places the problem occurred. He asks for clarification of the Kona "a user defined version..." comment above. Perhaps it was a circuitous way of saying "dfault" needed to be uncommented?]
[Post-Toronto: the issues list maintainer has merged in the proposed resolution from issue 207(i), which addresses the same paragraphs.]
double
specifier for do_get()
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The table in paragraph 7 for the length modifier does not list the length
modifier l
to be applied if the type is double
. Thus, the
standard asks the implementation to do undefined things when using scanf()
(the missing length modifier for scanf()
when scanning double
s
is actually a problem I found quite often in production code, too).
Proposed resolution:
In 28.3.4.3.2.3 [facet.num.get.virtuals], paragraph 7, add a row in the Length
Modifier table to say that for double
a length modifier
l
is to be used.
Rationale:
The standard makes an embarrassing beginner's mistake.
Init
Section: 31.4 [iostream.objects] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are conflicting statements about where the class
Init
is defined. According to 31.4 [iostream.objects] paragraph 2
it is defined as basic_ios::Init
, according to 31.5.2 [ios.base] it is defined as ios_base::Init
.
Proposed resolution:
Change 31.4 [iostream.objects] paragraph 2 from
"basic_ios::Init"
to
"ios_base::Init"
.
Rationale:
Although not strictly wrong, the standard was misleading enough to warrant the change.
imbue()
descriptionSection: 31.5.2.4 [ios.base.locales] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is a small discrepancy between the declarations of
imbue()
: in 31.5.2 [ios.base] the argument is passed as
locale const&
(correct), in 31.5.2.4 [ios.base.locales] it
is passed as locale const
(wrong).
Proposed resolution:
In 31.5.2.4 [ios.base.locales] change the imbue
argument
from "locale const" to "locale
const&".
setbuf()
Section: 31.6.3.5.2 [streambuf.virt.buffer] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The default behavior of setbuf()
is described only for the
situation that gptr() != 0 && gptr() != egptr()
:
namely to do nothing. What has to be done in other situations
is not described although there is actually only one reasonable
approach, namely to do nothing, too.
Since changing the buffer would almost certainly mess up most
buffer management of derived classes unless these classes do it
themselves, the default behavior of setbuf()
should always be
to do nothing.
Proposed resolution:
Change 31.6.3.5.2 [streambuf.virt.buffer], paragraph 3, Default behavior, to: "Default behavior: Does nothing. Returns this."
underflow()
Section: 31.6.3.5.3 [streambuf.virt.get] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of the meaning of the result of
showmanyc()
seems to be rather strange: It uses calls to
underflow()
. Using underflow()
is strange because
this function only reads the current character but does not extract
it, uflow()
would extract the current character. This should
be fixed to use sbumpc()
instead.
Proposed resolution:
Change 31.6.3.5.3 [streambuf.virt.get] paragraph 1,
showmanyc()
returns clause, by replacing the word
"supplied" with the words "extracted from the
stream".
exception()
Section: 31.7.5.2 [istream] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The paragraph 4 refers to the function exception()
which
is not defined. Probably, the referred function is
basic_ios<>::exceptions()
.
Proposed resolution:
In 31.7.5.2 [istream], 31.7.5.4 [istream.unformatted], paragraph 1,
31.7.6.2 [ostream], paragraph 3, and 31.7.6.3.1 [ostream.formatted.reqmts],
paragraph 1, change "exception()" to
"exceptions()"
.
[Note to Editor: "exceptions" with an "s" is the correct spelling.]
istream_iterator
vs. istreambuf_iterator
Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The note in the second paragraph pretends that the first argument
is an object of type istream_iterator
. This is wrong: It is
an object of type istreambuf_iterator
.
Proposed resolution:
Change 31.7.5.3.2 [istream.formatted.arithmetic] from:
The first argument provides an object of the istream_iterator class...
to
The first argument provides an object of the istreambuf_iterator class...
Section: 28.3.4.6.4.3 [locale.time.put.virtuals] Status: TC1 Submitter: Angelika Langer Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 28.3.4.6.4.3 [locale.time.put.virtuals] the do_put() function is specified
as taking a fill character as an argument, but the description of the
function does not say whether the character is used at all and, if so,
in which way. The same holds for any format control parameters that
are accessible through the ios_base& argument, such as the
adjustment or the field width. Is strftime() supposed to use the fill
character in any way? In any case, the specification of
time_put.do_put() looks inconsistent to me.
Is the
signature of do_put() wrong, or is the effects clause incomplete?
Proposed resolution:
Add the following note after 28.3.4.6.4.3 [locale.time.put.virtuals] paragraph 2:
[Note: the
fill
argument may be used in the implementation-defined formats, or by derivations. A space character is a reasonable default for this argument. --end Note]
Rationale:
The LWG felt that while the normative text was correct,
users need some guidance on what to pass for the fill
argument since the standard doesn't say how it's used.
xsputn()
, pubsync()
never called by basic_ostream
members?Section: 31.7.6.2 [ostream] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 2 explicitly states that none of the basic_ostream
functions falling into one of the groups "formatted output functions"
and "unformatted output functions" calls any stream buffer function
which might call a virtual function other than overflow()
. Basically
this is fine but this implies that sputn()
(this function would call
the virtual function xsputn()
) is never called by any of the standard
output functions. Is this really intended? At minimum it would be convenient to
call xsputn()
for strings... Also, the statement that overflow()
is the only virtual member of basic_streambuf
called is in conflict
with the definition of flush()
which calls rdbuf()->pubsync()
and thereby the virtual function sync()
(flush()
is listed
under "unformatted output functions").
In addition, I guess that the sentence starting with "They may use other
public members of basic_ostream
..." probably was intended to
start with "They may use other public members of basic_streamuf
..."
although the problem with the virtual members exists in both cases.
I see two obvious resolutions:
xsputn()
will never be
called by any ostream member and that this is intended.overflow()
and xsputn()
.
Of course, the problem with flush()
has to be resolved in some way.Proposed resolution:
Change the last sentence of 27.6.2.1 (lib.ostream) paragraph 2 from:
They may use other public members of basic_ostream except that they do not invoke any virtual members of rdbuf() except overflow().
to:
They may use other public members of basic_ostream except that they shall not invoke any virtual members of rdbuf() except overflow(), xsputn(), and sync().
[Kona: the LWG believes this is a problem. Wish to ask Jerry or PJP why the standard is written this way.]
[Post-Tokyo: Dietmar supplied wording at the request of the LWG. He comments: The rules can be made a little bit more specific if necessary be explicitly spelling out what virtuals are allowed to be called from what functions and eg to state specifically that flush() is allowed to call sync() while other functions are not.]
traits_type::length()
Section: 31.7.6.3.4 [ostream.inserters.character] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 4 states that the length is determined using
traits::length(s)
. Unfortunately, this function is not
defined for example if the character type is wchar_t
and the
type of s
is char const*
. Similar problems exist if
the character type is char
and the type of s
is
either signed char const*
or unsigned char
const*
.
Proposed resolution:
Change 31.7.6.3.4 [ostream.inserters.character] paragraph 4 from:
Effects: Behaves like an formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts characters. The number of characters starting at s to be inserted is traits::length(s). Padding is determined as described in lib.facet.num.put.virtuals. The traits::length(s) characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).
to:
Effects: Behaves like a formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts n characters starting at s, where n is the number that would be computed as if by:
- traits::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*, and also for the overload where the first argument is of type basic_ostream<char, traits>& and the second is of type const char*.
- std::char_traits<char>::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*.
- traits::length(reinterpret_cast<const char*>(s)) for the other two overloads.
Padding is determined as described in lib.facet.num.put.virtuals. The n characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).
[Santa Cruz: Matt supplied new wording]
[Kona: changed "where n is" to " where n is the number that would be computed as if by"]
Rationale:
We have five separate cases. In two of them we can use the user-supplied traits class without any fuss. In the other three we try to use something as close to that user-supplied class as possible. In two cases we've got a traits class that's appropriate for char and what we've got is a const signed char* or a const unsigned char*; that's close enough so we can just use a reinterpret cast, and continue to use the user-supplied traits class. Finally, there's one case where we just have to give up: where we've got a traits class for some arbitrary charT type, and we somehow have to deal with a const char*. There's nothing better to do but fall back to char_traits<char>
Section: 31.7.6.4 [ostream.unformatted] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The first paragraph begins with a descriptions what has to be done in formatted output functions. Probably this is a typo and the paragraph really want to describe unformatted output functions...
Proposed resolution:
In 31.7.6.4 [ostream.unformatted] paragraph 1, the first and last sentences, change the word "formatted" to "unformatted":
"Each unformatted output function begins ..."
"... value specified for the unformatted output function."
overflow()
mandatedSection: 31.8.2.5 [stringbuf.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 8, Notes, of this section seems to mandate an extremely
inefficient way of buffer handling for basic_stringbuf
,
especially in view of the restriction that basic_ostream
member functions are not allowed to use xsputn()
(see 31.7.6.2 [ostream]): For each character to be inserted, a new buffer
is to be created.
Of course, the resolution below requires some handling of
simultaneous input and output since it is no longer possible to update
egptr()
whenever epptr()
is changed. A possible
solution is to handle this in underflow()
.
Proposed resolution:
In 31.8.2.5 [stringbuf.virtuals] paragraph 8, Notes, insert the words "at least" as in the following:
To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus at least one additional write position.
traits_type
Section: 31.8.5 [stringstream] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The classes basic_stringstream
(31.8.5 [stringstream]),
basic_istringstream
(31.8.3 [istringstream]), and
basic_ostringstream
(31.8.4 [ostringstream]) are inconsistent
in their definition of the type traits_type
: For
istringstream
, this type is defined, for the other two it is
not. This should be consistent.
Proposed resolution:
Proposed resolution:
To the declarations of
basic_ostringstream
(31.8.4 [ostringstream]) and
basic_stringstream
(31.8.5 [stringstream]) add:
typedef traits traits_type;
seekpos()
semantics due to joint positionSection: 31.10.3.5 [filebuf.virtuals] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Overridden virtual functions, seekpos()
In 31.10.3 [filebuf] paragraph 3, it is stated that a joint input and
output position is maintained by basic_filebuf
. Still, the
description of seekpos()
seems to talk about different file
positions. In particular, it is unclear (at least to me) what is
supposed to happen to the output buffer (if there is one) if only the
input position is changed. The standard seems to mandate that the
output buffer is kept and processed as if there was no positioning of
the output position (by changing the input position). Of course, this
can be exactly what you want if the flag ios_base::ate
is
set. However, I think, the standard should say something like
this:
(which & mode) == 0
neither read nor write position is
changed and the call fails. Otherwise, the joint read and write position is
altered to correspond to sp
.Plus the appropriate error handling, that is...
Proposed resolution:
Change the unnumbered paragraph in 27.8.1.4 (lib.filebuf.virtuals) before paragraph 14 from:
pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);
Alters the file position, if possible, to correspond to the position stored in sp (as described below).
- if (which&ios_base::in)!=0, set the file position to sp, then update the input sequence
- if (which&ios_base::out)!=0, then update the output sequence, write any unshift sequence, and set the file position to sp.
to:
pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);
Alters the file position, if possible, to correspond to the position stored in sp (as described below). Altering the file position performs as follows:
1. if (om & ios_base::out)!=0, then update the output sequence and write any unshift sequence;
2. set the file position to sp;
3. if (om & ios_base::in)!=0, then update the input sequence;
where om is the open mode passed to the last call to open(). The operation fails if is_open() returns false.
[Kona: Dietmar is working on a proposed resolution.]
[Post-Tokyo: Dietmar supplied the above wording.]
basic_istream::ignore()
Section: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Greg Comeau, Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.7.5.2 [istream] the function
ignore()
gets an object of type streamsize
as first
argument. However, in 31.7.5.4 [istream.unformatted]
paragraph 23 the first argument is of type int.
As far as I can see this is not really a contradiction because
everything is consistent if streamsize
is typedef to be
int
. However, this is almost certainly not what was
intended. The same thing happened to basic_filebuf::setbuf()
,
as described in issue 173(i).
Darin Adler also submitted this issue, commenting: Either 27.6.1.1 should be modified to show a first parameter of type int, or 27.6.1.3 should be modified to show a first parameter of type streamsize and use numeric_limits<streamsize>::max.
Proposed resolution:
In 31.7.5.4 [istream.unformatted] paragraph 23 and 24, change both uses
of int
in the description of ignore()
to
streamsize
.
basic_filebuf::setbuf()
Section: 31.10.3.5 [filebuf.virtuals] Status: TC1 Submitter: Greg Comeau, Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.10.3 [filebuf] the function setbuf()
gets an
object of type streamsize
as second argument. However, in
31.10.3.5 [filebuf.virtuals] paragraph 9 the second argument is of type
int
.
As far as I can see this is not really a contradiction because
everything is consistent if streamsize
is typedef to be
int
. However, this is almost certainly not what was
intended. The same thing happened to basic_istream::ignore()
,
as described in issue 172(i).
Proposed resolution:
In 31.10.3.5 [filebuf.virtuals] paragraph 9, change all uses of
int
in the description of setbuf()
to
streamsize
.
OFF_T
vs. POS_T
Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to paragraph 1 of this section, streampos
is the
type OFF_T
, the same type as streamoff
. However, in
paragraph 6 the streampos
gets the type POS_T
Proposed resolution:
Change 99 [depr.ios.members] paragraph 1 from "typedef
OFF_T streampos;
" to "typedef POS_T
streampos;
"
basic_streambuf::pubseekpos()
and a few other functions.Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to paragraph 8 of this section, the methods
basic_streambuf::pubseekpos()
,
basic_ifstream::open()
, and basic_ofstream::open
"may" be overloaded by a version of this function taking the
type ios_base::open_mode
as last argument argument instead of
ios_base::openmode
(ios_base::open_mode
is defined
in this section to be an alias for one of the integral types). The
clause specifies, that the last argument has a default argument in
three cases. However, this generates an ambiguity with the overloaded
version because now the arguments are absolutely identical if the last
argument is not specified.
Proposed resolution:
In 99 [depr.ios.members] paragraph 8, remove the default arguments for
basic_streambuf::pubseekpos()
,
basic_ifstream::open()
, and
basic_ofstream::open().
exceptions()
in ios_base
...?Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The "overload" for the function exceptions()
in
paragraph 8 gives the impression that there is another function of
this function defined in class ios_base
. However, this is not
the case. Thus, it is hard to tell how the semantics (paragraph 9) can
be implemented: "Call the corresponding member function specified
in clause 31 [input.output]."
Proposed resolution:
In 99 [depr.ios.members] paragraph 8, move the declaration of the
function exceptions()
into class basic_ios
.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Judy Ward Opened: 1998-07-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Currently the following will not compile on two well-known standard library implementations:
#include <set> using namespace std; void f(const set<int> &s) { set<int>::iterator i; if (i==s.end()); // s.end() returns a const_iterator }
The reason this doesn't compile is because operator== was implemented as a member function of the nested classes set:iterator and set::const_iterator, and there is no conversion from const_iterator to iterator. Surprisingly, (s.end() == i) does work, though, because of the conversion from iterator to const_iterator.
I don't see a requirement anywhere in the standard that this must work. Should there be one? If so, I think the requirement would need to be added to the tables in section 24.1.1. I'm not sure about the wording. If this requirement existed in the standard, I would think that implementors would have to make the comparison operators non-member functions.
This issues was also raised on comp.std.c++ by Darin Adler. The example given was:
bool check_equal(std::deque<int>::iterator i, std::deque<int>::const_iterator ci) { return i == ci; }
Comment from John Potter:
In case nobody has noticed, accepting it will break reverse_iterator.
The fix is to make the comparison operators templated on two types.
template <class Iterator1, class Iterator2> bool operator== (reverse_iterator<Iterator1> const& x, reverse_iterator<Iterator2> const& y);Obviously: return x.base() == y.base();
Currently, no reverse_iterator to const_reverse_iterator compares are valid.
BTW, I think the issue is in support of bad code. Compares should be between two iterators of the same type. All std::algorithms require the begin and end iterators to be of the same type.
Proposed resolution:
Insert this paragraph after 23.2 [container.requirements] paragraph 7:
In the expressions
i == j i != j i < j i <= j i >= j i > j i - jWhere i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.
[post-Toronto: Judy supplied a proposed resolution saying that
iterator
and const_iterator
could be freely mixed in
iterator comparison and difference operations.]
[Redmond: Dave and Howard supplied a new proposed resolution which explicitly listed expressions; there was concern that the previous proposed resolution was too informal.]
Rationale:
The LWG believes it is clear that the above wording applies only to
the nested types X::iterator
and X::const_iterator
,
where X
is a container. There is no requirement that
X::reverse_iterator
and X::const_reverse_iterator
can be mixed. If mixing them is considered important, that's a
separate issue. (Issue 280(i).)
Section: 27.4.3 [basic.string] Status: CD1 Submitter: Dave Abrahams Opened: 1999-07-01 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
It is the constness of the container which should control whether it can be modified through a member function such as erase(), not the constness of the iterators. The iterators only serve to give positioning information.
Here's a simple and typical example problem which is currently very difficult or impossible to solve without the change proposed below.
Wrap a standard container C in a class W which allows clients to find and read (but not modify) a subrange of (C.begin(), C.end()]. The only modification clients are allowed to make to elements in this subrange is to erase them from C through the use of a member function of W.
[ post Bellevue, Alisdair adds: ]
This issue was implemented by N2350 for everything but
basic_string
.Note that the specific example in this issue (
basic_string
) is the one place we forgot to amend in N2350, so we might open this issue for that single container?
[ Sophia Antipolis: ]
This was a fix that was intended for all standard library containers, and has been done for other containers, but string was missed.
The wording updated.
We did not make the change in
replace
, because this change would affect the implementation because the string may be written into. This is an issue that should be taken up by concepts.We note that the supplied wording addresses the initializer list provided in N2679.
Proposed resolution:
Update the following signature in the basic_string
class template definition in
27.4.3 [basic.string], p5:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { ... iterator insert(const_iterator p, charT c); void insert(const_iterator p, size_type n, charT c); template<class InputIterator> void insert(const_iterator p, InputIterator first, InputIterator last); void insert(const_iterator p, initializer_list<charT>); ... iterator erase(const_iterator const_position); iterator erase(const_iterator first, const_iterator last); ... }; }
Update the following signatures in 27.4.3.7.4 [string.insert]:
iterator insert(const_iterator p, charT c); void insert(const_iterator p, size_type n, charT c); template<class InputIterator> void insert(const_iterator p, InputIterator first, InputIterator last); void insert(const_iterator p, initializer_list<charT>);
Update the following signatures in 27.4.3.7.5 [string.erase]:
iterator erase(const_iterator const_position); iterator erase(const_iterator first, const_iterator last);
Rationale:
The issue was discussed at length. It was generally agreed that 1) There is no major technical argument against the change (although there is a minor argument that some obscure programs may break), and 2) Such a change would not break const correctness. The concerns about making the change were 1) it is user detectable (although only in boundary cases), 2) it changes a large number of signatures, and 3) it seems more of a design issue that an out-and-out defect.
The LWG believes that this issue should be considered as part of a general review of const issues for the next revision of the standard. Also see issue 200(i).
Section: 22.3 [pairs] Status: TC1 Submitter: Andrew Koenig Opened: 1999-08-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The claim has surfaced in Usenet that expressions such as
make_pair("abc", 3)
are illegal, notwithstanding their use in examples, because template instantiation tries to bind the first template
parameter to const char (&)[4]
, which type is uncopyable.
I doubt anyone intended that behavior...
Proposed resolution:
In 22.2 [utility], paragraph 1 change the following declaration of make_pair():
template <class T1, class T2> pair<T1,T2> make_pair(const T1&, const T2&);
to:
template <class T1, class T2> pair<T1,T2> make_pair(T1, T2);
In 22.3 [pairs] paragraph 7 and the line before, change:
template <class T1, class T2> pair<T1, T2> make_pair(const T1& x, const T2& y);
to:
template <class T1, class T2> pair<T1, T2> make_pair(T1 x, T2 y);
and add the following footnote to the effects clause:
According to 12.8 [class.copy], an implementation is permitted to not perform a copy of an argument, thus avoiding unnecessary copies.
Rationale:
Two potential fixes were suggested by Matt Austern and Dietmar Kühl, respectively, 1) overloading with array arguments, and 2) use of a reference_traits class with a specialization for arrays. Andy Koenig suggested changing to pass by value. In discussion, it appeared that this was a much smaller change to the standard that the other two suggestions, and any efficiency concerns were more than offset by the advantages of the solution. Two implementors reported that the proposed resolution passed their test suites.
Section: 16 [library] Status: CD1 Submitter: Al Stevens Opened: 1999-08-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Many references to size_t
throughout the document
omit the std::
namespace qualification.
For example, 16.4.5.6 [replacement.functions] paragraph 2:
operator new(size_t) operator new(size_t, const std::nothrow_t&) operator new[](size_t) operator new[](size_t, const std::nothrow_t&)
Proposed resolution:
In 16.4.5.6 [replacement.functions] paragraph 2: replace:
- operator new(size_t)
- operator new(size_t, const std::nothrow_t&)
- operator new[](size_t)
- operator new[](size_t, const std::nothrow_t&)
by:
- operator new(std::size_t) - operator new(std::size_t, const std::nothrow_t&) - operator new[](std::size_t) - operator new[](std::size_t, const std::nothrow_t&)
In [lib.allocator.requirements] 20.1.5, paragraph 4: replace:
The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*, T const*, size_t, and ptrdiff_t, respectively.
by:
The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*, T const*, std::size_t, and std::ptrdiff_t, respectively.
In [lib.allocator.members] 20.4.1.1, paragraphs 3 and 6: replace:
3 Notes: Uses ::operator new(size_t) (18.4.1).
6 Note: the storage is obtained by calling ::operator new(size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires.
by:
3 Notes: Uses ::operator new(std::size_t) (18.4.1).
6 Note: the storage is obtained by calling ::operator new(std::size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires.
In [lib.char.traits.require] 21.1.1, paragraph 1: replace:
In Table 37, X denotes a Traits class defining types and functions for the character container type CharT; c and d denote values of type CharT; p and q denote values of type const CharT*; s denotes a value of type CharT*; n, i and j denote values of type size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and state denotes a value of type X::state_type.
by:
In Table 37, X denotes a Traits class defining types and functions for the character container type CharT; c and d denote values of type CharT; p and q denote values of type const CharT*; s denotes a value of type CharT*; n, i and j denote values of type std::size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and state denotes a value of type X::state_type.
In [lib.char.traits.require] 21.1.1, table 37: replace the return type of X::length(p): "size_t" by "std::size_t".
In [lib.std.iterator.tags] 24.3.3, paragraph 2: replace:
typedef ptrdiff_t difference_type;
by:
typedef std::ptrdiff_t difference_type;
In [lib.locale.ctype] 22.2.1.1 put namespace std { ...} around the declaration of template <class charT> class ctype.
In [lib.iterator.traits] 24.3.1, paragraph 2 put namespace std { ...} around the declaration of:
template<class Iterator> struct iterator_traits
template<class T> struct iterator_traits<T*>
template<class T> struct iterator_traits<const T*>
Rationale:
The LWG believes correcting names like size_t
and
ptrdiff_t
to std::size_t
and std::ptrdiff_t
to be essentially editorial. There there can't be another size_t or
ptrdiff_t meant anyway because, according to 16.4.5.3.5 [extern.types],
For each type T from the Standard C library, the types ::T and std::T are reserved to the implementation and, when defined, ::T shall be identical to std::T.
The issue is treated as a Defect Report to make explicit the Project Editor's authority to make this change.
[Post-Tokyo: Nico Josuttis provided the above wording at the request of the LWG.]
[Toronto: This is tangentially related to issue 229(i), but only tangentially: the intent of this issue is to
address use of the name size_t
in contexts outside of
namespace std, such as in the description of ::operator new
.
The proposed changes should be reviewed to make sure they are
correct.]
[pre-Copenhagen: Nico has reviewed the changes and believes them to be correct.]
Section: 31.7.7 [std.manip] Status: CD1 Submitter: Andy Sawyer Opened: 1999-07-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
31.7.7 [std.manip] paragraph 3 says (clause numbering added for exposition):
Returns: An object s of unspecified type such that if [1] out is an (instance of) basic_ostream then the expression out<<s behaves as if f(s) were called, and if [2] in is an (instance of) basic_istream then the expression in>>s behaves as if f(s) were called. Where f can be defined as: ios_base& f(ios_base& str, ios_base::fmtflags mask) { // reset specified flags str.setf(ios_base::fmtflags(0), mask); return str; } [3] The expression out<<s has type ostream& and value out. [4] The expression in>>s has type istream& and value in.
Given the definitions [1] and [2] for out and in, surely [3] should read: "The expression out << s has type basic_ostream& ..." and [4] should read: "The expression in >> s has type basic_istream& ..."
If the wording in the standard is correct, I can see no way of implementing any of the manipulators so that they will work with wide character streams.
e.g. wcout << setbase( 16 );
Must have value 'wcout' (which makes sense) and type 'ostream&' (which doesn't).
The same "cut'n'paste" type also seems to occur in Paras 4,5,7 and 8. In addition, Para 6 [setfill] has a similar error, but relates only to ostreams.
I'd be happier if there was a better way of saying this, to make it clear that the value of the expression is "the same specialization of basic_ostream as out"&
Proposed resolution:
Replace section 31.7.7 [std.manip] except paragraph 1 with the following:
2- The type designated smanip in each of the following function descriptions is implementation-specified and may be different for each function.
smanip resetiosflags(ios_base::fmtflags mask);
-3- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:*
[Footnote: The expression cin >> resetiosflags(ios_base::skipws) clears ios_base::skipws in the format flags stored in the basic_istream<charT,traits> object cin (the same as cin >> noskipws), and the expression cout << resetiosflags(ios_base::showbase) clears ios_base::showbase in the format flags stored in the basic_ostream<charT,traits> object cout (the same as cout << noshowbase). --- end footnote]
ios_base& f(ios_base& str, ios_base::fmtflags mask)
{
// reset specified flags
str.setf(ios_base::fmtflags(0), mask);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setiosflags(ios_base::fmtflags mask);
-4- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:
ios_base& f(ios_base& str, ios_base::fmtflags mask)
{
// set specified flags
str.setf(mask);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setbase(int base);
-5- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, base) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, base) were called. The function f can be defined as:
ios_base& f(ios_base& str, int base)
{
// set basefield
str.setf(base == 8 ? ios_base::oct :
base == 10 ? ios_base::dec :
base == 16 ? ios_base::hex :
ios_base::fmtflags(0), ios_base::basefield);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setfill(char_type c);
-6- Returns: An object s of unspecified type such that if out is (or is derived from) basic_ostream<charT,traits> and c has type charT then the expression out<<s behaves as if f(s, c) were called. The function f can be defined as:
template<class charT, class traits>
basic_ios<charT,traits>& f(basic_ios<charT,traits>& str, charT c)
{
// set fill character
str.fill(c);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out.
smanip setprecision(int n);
-7- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
ios_base& f(ios_base& str, int n)
{
// set precision
str.precision(n);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in
.
smanip setw(int n);
-8- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
ios_base& f(ios_base& str, int n)
{
// set width
str.width(n);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
[Kona: Andy Sawyer and Beman Dawes will work to improve the wording of the proposed resolution.]
[Tokyo - The LWG noted that issue 216(i) involves the same paragraphs.]
[Post-Tokyo: The issues list maintainer combined the proposed resolution of this issue with the proposed resolution for issue 216(i) as they both involved the same paragraphs, and were so intertwined that dealing with them separately appear fraught with error. The full text was supplied by Bill Plauger; it was cross checked against changes supplied by Andy Sawyer. It should be further checked by the LWG.]
Section: 17.3.5.3 [numeric.special] Status: CD1 Submitter: Gabriel Dos Reis Opened: 1999-07-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
bools are defined by the standard to be of integer types, as per 6.8.2 [basic.fundamental] paragraph 7. However "integer types" seems to have a special meaning for the author of 18.2. The net effect is an unclear and confusing specification for numeric_limits<bool> as evidenced below.
18.2.1.2/7 says numeric_limits<>::digits is, for built-in integer types, the number of non-sign bits in the representation.
4.5/4 states that a bool promotes to int ; whereas 4.12/1 says any non zero arithmetical value converts to true.
I don't think it makes sense at all to require numeric_limits<bool>::digits and numeric_limits<bool>::digits10 to be meaningful.
The standard defines what constitutes a signed (resp. unsigned) integer types. It doesn't categorize bool as being signed or unsigned. And the set of values of bool type has only two elements.
I don't think it makes sense to require numeric_limits<bool>::is_signed to be meaningful.
18.2.1.2/18 for numeric_limits<integer_type>::radix says:
For integer types, specifies the base of the representation.186)
This disposition is at best misleading and confusing for the standard requires a "pure binary numeration system" for integer types as per 3.9.1/7
The footnote 186) says: "Distinguishes types with base other than 2 (e.g BCD)." This also erroneous as the standard never defines any integer types with base representation other than 2.
Furthermore, numeric_limits<bool>::is_modulo and numeric_limits<bool>::is_signed have similar problems.
Proposed resolution:
Append to the end of 17.3.5.3 [numeric.special]:
The specialization for bool shall be provided as follows:
namespace std { template<> class numeric_limits<bool> { public: static const bool is_specialized = true; static bool min() throw() { return false; } static bool max() throw() { return true; } static const int digits = 1; static const int digits10 = 0; static const bool is_signed = false; static const bool is_integer = true; static const bool is_exact = true; static const int radix = 2; static bool epsilon() throw() { return 0; } static bool round_error() throw() { return 0; } static const int min_exponent = 0; static const int min_exponent10 = 0; static const int max_exponent = 0; static const int max_exponent10 = 0; static const bool has_infinity = false; static const bool has_quiet_NaN = false; static const bool has_signaling_NaN = false; static const float_denorm_style has_denorm = denorm_absent; static const bool has_denorm_loss = false; static bool infinity() throw() { return 0; } static bool quiet_NaN() throw() { return 0; } static bool signaling_NaN() throw() { return 0; } static bool denorm_min() throw() { return 0; } static const bool is_iec559 = false; static const bool is_bounded = true; static const bool is_modulo = false; static const bool traps = false; static const bool tinyness_before = false; static const float_round_style round_style = round_toward_zero; }; }
[Tokyo: The LWG desires wording that specifies exact values rather than more general wording in the original proposed resolution.]
[Post-Tokyo: At the request of the LWG in Tokyo, Nico Josuttis provided the above wording.]
Section: 22.10 [function.objects] Status: CD1 Submitter: UK Panel Opened: 1999-07-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 4 of 22.10 [function.objects] says:
[Example: To negate every element of a: transform(a.begin(), a.end(), a.begin(), negate<double>()); The corresponding functions will inline the addition and the negation. end example]
(Note: The "addition" referred to in the above is in para 3) we can find no other wording, except this (non-normative) example which suggests that any "inlining" will take place in this case.
Indeed both:
17.4.4.3 Global Functions [lib.global.functions] 1 It is unspecified whether any global functions in the C++ Standard Library are defined as inline (7.1.2).
and
17.4.4.4 Member Functions [lib.member.functions] 1 It is unspecified whether any member functions in the C++ Standard Library are defined as inline (7.1.2).
take care to state that this may indeed NOT be the case.
Thus the example "mandates" behavior that is explicitly not required elsewhere.
Proposed resolution:
In 22.10 [function.objects] paragraph 1, remove the sentence:
They are important for the effective use of the library.
Remove 22.10 [function.objects] paragraph 2, which reads:
Using function objects together with function templates increases the expressive power of the library as well as making the resulting code much more efficient.
In 22.10 [function.objects] paragraph 4, remove the sentence:
The corresponding functions will inline the addition and the negation.
[Kona: The LWG agreed there was a defect.]
[Tokyo: The LWG crafted the proposed resolution.]
Section: 22.9.2.3 [bitset.members] Status: CD1 Submitter: Darin Adler Opened: 1999-08-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In section 22.9.2.3 [bitset.members], paragraph 13 defines the bitset::set operation to take a second parameter of type int. The function tests whether this value is non-zero to determine whether to set the bit to true or false. The type of this second parameter should be bool. For one thing, the intent is to specify a Boolean value. For another, the result type from test() is bool. In addition, it's possible to slice an integer that's larger than an int. This can't happen with bool, since conversion to bool has the semantic of translating 0 to false and any non-zero value to true.
Proposed resolution:
In 22.9.2 [template.bitset] Para 1 Replace:
bitset<N>& set(size_t pos, int val = true );
With:
bitset<N>& set(size_t pos, bool val = true );
In 22.9.2.3 [bitset.members] Para 12(.5) Replace:
bitset<N>& set(size_t pos, int val = 1 );
With:
bitset<N>& set(size_t pos, bool val = true );
[Kona: The LWG agrees with the description. Andy Sawyer will work on better P/R wording.]
[Post-Tokyo: Andy provided the above wording.]
Rationale:
bool
is a better choice. It is believed that binary
compatibility is not an issue, because this member function is
usually implemented as inline
, and because it is already
the case that users cannot rely on the type of a pointer to a
nonvirtual member of a standard library class.
Section: 26.7.3 [alg.swap] Status: CD1 Submitter: Andrew Koenig Opened: 1999-08-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of iter_swap in 25.2.2 paragraph 7,says that it
``exchanges the values'' of the objects to which two iterators
refer.
What it doesn't say is whether it does so using swap
or using the assignment operator and copy constructor.
This
question is an important one to answer, because swap is specialized to
work efficiently for standard containers.
For example:
vector<int> v1, v2; iter_swap(&v1, &v2);
Is this call to iter_swap equivalent to calling swap(v1, v2)? Or is it equivalent to
{ vector<int> temp = v1; v1 = v2; v2 = temp; }
The first alternative is O(1); the second is O(n).
A LWG member, Dave Abrahams, comments:
Not an objection necessarily, but I want to point out the cost of that requirement:
iter_swap(list<T>::iterator, list<T>::iterator)
can currently be specialized to be more efficient than iter_swap(T*,T*) for many T (by using splicing). Your proposal would make that optimization illegal.
[Kona: The LWG notes the original need for iter_swap was proxy iterators which are no longer permitted.]
Proposed resolution:
Change the effect clause of iter_swap in 25.2.2 paragraph 7 from:
Exchanges the values pointed to by the two iterators a and b.
to
swap(*a, *b)
.
Rationale:
It's useful to say just what iter_swap
does. There may be
some iterators for which we want to specialize iter_swap
,
but the fully general version should have a general specification.
Note that in the specific case of list<T>::iterator
,
iter_swap should not be specialized as suggested above. That would do
much more than exchanging the two iterators' values: it would change
predecessor/successor relationships, possibly moving the iterator from
one list to another. That would surely be inappropriate.
Section: 31.5.2.3 [fmtflags.state] Status: TC1 Submitter: Andrew Koenig Opened: 1999-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
27.4.2.2 paragraph 9 claims that setprecision() sets the precision,
and includes a parenthetical note saying that it is the number of
digits after the decimal point.
This claim is not strictly correct. For example, in the default
floating-point output format, setprecision sets the number of
significant digits printed, not the number of digits after the decimal
point.
I would like the committee to look at the definition carefully and
correct the statement in 27.4.2.2
Proposed resolution:
Remove from 31.5.2.3 [fmtflags.state], paragraph 9, the text "(number of digits after the decimal point)".
Section: 26.8.8 [alg.heap.operations] Status: TC1 Submitter: Markus Mauhart Opened: 1999-09-24 Last modified: 2024-01-25
Priority: Not Prioritized
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Duplicate of: 216
Discussion:
25.3.6 [lib.alg.heap.operations] states two key properties of a heap [a,b), the first of them
is
"(1) *a is the largest element"
I think this is incorrect and should be changed to the wording in the proposed
resolution.
Actually there are two independent changes:
A-"part of largest equivalence class" instead of "largest", cause 25.3 [lib.alg.sorting] asserts "strict weak ordering" for all its sub clauses.
B-Take 'an oldest' from that equivalence class, otherwise the heap functions could not be used for a priority queue as explained in 23.2.3.2.2 [lib.priqueue.members] (where I assume that a "priority queue" respects priority AND time).
Proposed resolution:
Change 26.8.8 [alg.heap.operations] property (1) from:
(1) *a is the largest element
to:
(1) There is no element greater than
*a
basic_istream::sentry
's constructor ever set eofbit?Section: 31.7.5.2.4 [istream.sentry] Status: TC1 Submitter: Matt Austern Opened: 1999-10-13 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
Suppose that is.flags() & ios_base::skipws
is nonzero.
What should basic_istream<>::sentry
's constructor do if it
reaches eof while skipping whitespace? 27.6.1.1.2/5 suggests it
should set failbit. Should it set eofbit as well? The standard
doesn't seem to answer that question.
On the one hand, nothing in [istream::sentry] says that
basic_istream<>::sentry
should ever set eofbit. On the
other hand, 31.7.5.2 [istream] paragraph 4 says that if
extraction from a streambuf
"returns
traits::eof()
, then the input function, except as explicitly
noted otherwise, completes its actions and does
setstate(eofbit)"
. So the question comes down to
whether basic_istream<>::sentry
's constructor is an
input function.
Comments from Jerry Schwarz:
It was always my intention that eofbit should be set any time that a virtual returned something to indicate eof, no matter what reason iostream code had for calling the virtual.
The motivation for this is that I did not want to require streambufs to behave consistently if their virtuals are called after they have signaled eof.
The classic case is a streambuf reading from a UNIX file. EOF isn't really a state for UNIX file descriptors. The convention is that a read on UNIX returns 0 bytes to indicate "EOF", but the file descriptor isn't shut down in any way and future reads do not necessarily also return 0 bytes. In particular, you can read from tty's on UNIX even after they have signaled "EOF". (It isn't always understood that a ^D on UNIX is not an EOF indicator, but an EOL indicator. By typing a "line" consisting solely of ^D you cause a read to return 0 bytes, and by convention this is interpreted as end of file.)
Proposed resolution:
Add a sentence to the end of 27.6.1.1.2 paragraph 2:
If
is.rdbuf()->sbumpc()
oris.rdbuf()->sgetc()
returnstraits::eof()
, the function callssetstate(failbit | eofbit)
(which may throwios_base::failure
).
Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Beman Dawes Opened: 1999-11-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is a pointer or reference obtained from an iterator still valid after destruction of the iterator?
Is a pointer or reference obtained from an iterator still valid after the value of the iterator changes?
#include <iostream> #include <vector> #include <iterator> int main() { typedef std::vector<int> vec_t; vec_t v; v.push_back( 1 ); // Is a pointer or reference obtained from an iterator still // valid after destruction of the iterator? int * p = &*v.begin(); std::cout << *p << '\n'; // OK? // Is a pointer or reference obtained from an iterator still // valid after the value of the iterator changes? vec_t::iterator iter( v.begin() ); p = &*iter++; std::cout << *p << '\n'; // OK? return 0; }
The standard doesn't appear to directly address these questions. The standard needs to be clarified. At least two real-world cases have been reported where library implementors wasted considerable effort because of the lack of clarity in the standard. The question is important because requiring pointers and references to remain valid has the effect for practical purposes of prohibiting iterators from pointing to cached rather than actual elements of containers.
The standard itself assumes that pointers and references obtained from an iterator are still valid after iterator destruction or change. The definition of reverse_iterator::operator*(), 24.5.1.5 [reverse.iter.conv], which returns a reference, defines effects:
Iterator tmp = current; return *--tmp;
The definition of reverse_iterator::operator->(), [reverse.iter.op.star], which returns a pointer, defines effects:
return &(operator*());
Because the standard itself assumes pointers and references remain valid after iterator destruction or change, the standard should say so explicitly. This will also reduce the chance of user code breaking unexpectedly when porting to a different standard library implementation.
Proposed resolution:
Add a new paragraph to 24.3.4 [iterator.concepts]:
Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.
Replace paragraph 1 of 24.5.1.5 [reverse.iter.conv] with:
Effects:
this->tmp = current; --this->tmp; return *this->tmp;[Note: This operation must use an auxiliary member variable, rather than a temporary variable, to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.3.4 [iterator.concepts].) The name of this member variable is shown for exposition only. --end note]
[Post-Tokyo: The issue has been reformulated purely in terms of iterators.]
[Pre-Toronto: Steve Cleary pointed out the no-invalidation assumption by reverse_iterator. The issue and proposed resolution was reformulated yet again to reflect this reality.]
[Copenhagen: Steve Cleary pointed out that reverse_iterator
assumes its underlying iterator has persistent pointers and
references. Andy Koenig pointed out that it is possible to rewrite
reverse_iterator so that it no longer makes such an assupmption.
However, this issue is related to issue 299(i). If we
decide it is intentional that p[n]
may return by value
instead of reference when p
is a Random Access Iterator,
other changes in reverse_iterator will be necessary.]
Rationale:
This issue has been discussed extensively. Note that it is
not an issue about the behavior of predefined iterators. It is
asking whether or not user-defined iterators are permitted to have
transient pointers and references. Several people presented examples
of useful user-defined iterators that have such a property; examples
include a B-tree iterator, and an "iota iterator" that doesn't point
to memory. Library implementors already seem to be able to cope with
such iterators: they take pains to avoid forming references to memory
that gets iterated past. The only place where this is a problem is
reverse_iterator
, so this issue changes
reverse_iterator
to make it work.
This resolution does not weaken any guarantees provided by
predefined iterators like list<int>::iterator
.
Clause 23 should be reviewed to make sure that guarantees for
predefined iterators are as strong as users expect.
allocate(0)
return?Section: 16.4.4.6 [allocator.requirements] Status: TC1 Submitter: Matt Austern Opened: 1999-11-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Suppose that A
is a class that conforms to the
Allocator requirements of Table 32, and a
is an
object of class A
What should be the return
value of a.allocate(0)
? Three reasonable
possibilities: forbid the argument 0
, return
a null pointer, or require that the return value be a
unique non-null pointer.
Proposed resolution:
Add a note to the allocate
row of Table 32:
"[Note: If n == 0
, the return value is unspecified. --end note]"
Rationale:
A key to understanding this issue is that the ultimate use of allocate() is to construct an iterator, and that iterator for zero length sequences must be the container's past-the-end representation. Since this already implies special case code, it would be over-specification to mandate the return value.
Section: 24.3.5.5 [forward.iterators] Status: CD1 Submitter: Matt Austern Opened: 1999-11-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In table 74, the return type of the expression *a
is given
as T&
, where T
is the iterator's value type.
For constant iterators, however, this is wrong. ("Value type"
is never defined very precisely, but it is clear that the value type
of, say, std::list<int>::const_iterator
is supposed to be
int
, not const int
.)
Proposed resolution:
In table 74, in the *a
and *r++
rows, change the
return type from "T&
" to "T&
if X
is mutable, otherwise const T&
".
In the a->m
row, change the return type from
"U&
" to "U&
if X
is mutable,
otherwise const U&
".
[Tokyo: The LWG believes this is the tip of a larger iceberg; there are multiple const problems with the STL portion of the library and that these should be addressed as a single package. Note that issue 180(i) has already been declared NAD Future for that very reason.]
[Redmond: the LWG thinks this is separable from other constness issues. This issue is just cleanup; it clarifies language that was written before we had iterator_traits. Proposed resolution was modified: the original version only discussed *a. It was pointed out that we also need to worry about *r++ and a->m.]
Section: 17.3 [support.limits] Status: CD1 Submitter: Stephen Cleary Opened: 1999-12-21 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
In some places in this section, the terms "fundamental types" and "scalar types" are used when the term "arithmetic types" is intended. The current usage is incorrect because void is a fundamental type and pointers are scalar types, neither of which should have specializations of numeric_limits.
[Lillehammer: it remains true that numeric_limits is using imprecise language. However, none of the proposals for changed wording are clearer. A redesign of numeric_limits is needed, but this is more a task than an open issue.]
Proposed resolution:
Change 17.3 [support.limits] to:
-1- The headers
<limits>
,<climits>
,<cfloat>
, and<cinttypes>
supply characteristics of implementation-dependentfundamentalarithmetic types (3.9.1).
Change [limits] to:
-1- The
numeric_limits
component provides a C++ program with information about various properties of the implementation's representation of thefundamentalarithmetic types.-2- Specializations shall be provided for each
fundamentalarithmetic type, both floating point and integer, includingbool
. The memberis_specialized
shall betrue
for all such specializations ofnumeric_limits
.-4- Non-
fundamentalarithmetic standard types, such ascomplex<T>
(26.3.2), shall not have specializations.
Change 17.3.5 [numeric.limits] to:
-1- The memberis_specialized
makes it possible to distinguish between fundamental types, which have specializations, and non-scalar types, which do not.
Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Andrew Koenig Opened: 2000-01-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
What should unique() do if you give it a predicate that is not an equivalence relation? There are at least two plausible answers:
1. You can't, because 25.2.8 says that it it "eliminates all but the first element from every consecutive group of equal elements..." and it wouldn't make sense to interpret "equal" as meaning anything but an equivalence relation. [It also doesn't make sense to interpret "equal" as meaning ==, because then there would never be any sense in giving a predicate as an argument at all.]
2. The word "equal" should be interpreted to mean whatever the predicate says, even if it is not an equivalence relation (and in particular, even if it is not transitive).
The example that raised this question is from Usenet:
int f[] = { 1, 3, 7, 1, 2 }; int* z = unique(f, f+5, greater<int>());
If one blindly applies the definition using the predicate greater<int>, and ignore the word "equal", you get:
Eliminates all but the first element from every consecutive group of elements referred to by the iterator i in the range [first, last) for which *i > *(i - 1).
The first surprise is the order of the comparison. If we wanted to
allow for the predicate not being an equivalence relation, then we
should surely compare elements the other way: pred(*(i - 1), *i). If
we do that, then the description would seem to say: "Break the
sequence into subsequences whose elements are in strictly increasing
order, and keep only the first element of each subsequence". So the
result would be 1, 1, 2. If we take the description at its word, it
would seem to call for strictly DEcreasing order, in which case the
result should be 1, 3, 7, 2.
In fact, the SGI implementation of unique() does neither: It yields 1,
3, 7.
Proposed resolution:
Change 26.7.9 [alg.unique] paragraph 1 to:
For a nonempty range, eliminates all but the first element from every consecutive group of equivalent elements referred to by the iterator
i
in the range [first+1, last) for which the following conditions hold:*(i-1) == *i
orpred(*(i-1), *i) != false
.
Also insert a new paragraph, paragraph 2a, that reads: "Requires: The comparison function must be an equivalence relation."
[Redmond: discussed arguments for and against requiring the comparison function to be an equivalence relation. Straw poll: 14-2-5. First number is to require that it be an equivalence relation, second number is to explicitly not require that it be an equivalence relation, third number is people who believe they need more time to consider the issue. A separate issue: Andy Sawyer pointed out that "i-1" is incorrect, since "i" can refer to the first iterator in the range. Matt provided wording to address this problem.]
[Curaçao: The LWG changed "... the range (first, last)..." to "... the range [first+1, last)..." for clarity. They considered this change close enough to editorial to not require another round of review.]
Rationale:
The LWG also considered an alternative resolution: change 26.7.9 [alg.unique] paragraph 1 to:
For a nonempty range, eliminates all but the first element from every consecutive group of elements referred to by the iterator
i
in the range (first, last) for which the following conditions hold:*(i-1) == *i
orpred(*(i-1), *i) != false
.
Also insert a new paragraph, paragraph 1a, that reads: "Notes: The comparison function need not be an equivalence relation."
Informally: the proposed resolution imposes an explicit requirement that the comparison function be an equivalence relation. The alternative resolution does not, and it gives enough information so that the behavior of unique() for a non-equivalence relation is specified. Both resolutions are consistent with the behavior of existing implementations.
Section: 17.6.3.2 [new.delete.single] Status: CD1 Submitter: Howard Hinnant Opened: 1999-08-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
As specified, the implementation of the nothrow version of operator new does not necessarily call the ordinary operator new, but may instead simply call the same underlying allocator and return a null pointer instead of throwing an exception in case of failure.
Such an implementation breaks code that replaces the ordinary version of new, but not the nothrow version. If the ordinary version of new/delete is replaced, and if the replaced delete is not compatible with pointers returned from the library versions of new, then when the replaced delete receives a pointer allocated by the library new(nothrow), crash follows.
The fix appears to be that the lib version of new(nothrow) must call the ordinary new. Thus when the ordinary new gets replaced, the lib version will call the replaced ordinary new and things will continue to work.
An alternative would be to have the ordinary new call new(nothrow). This seems sub-optimal to me as the ordinary version of new is the version most commonly replaced in practice. So one would still need to replace both ordinary and nothrow versions if one wanted to replace the ordinary version.
Another alternative is to put in clear text that if one version is replaced, then the other must also be replaced to maintain compatibility. Then the proposed resolution below would just be a quality of implementation issue. There is already such text in paragraph 7 (under the new(nothrow) version). But this nuance is easily missed if one reads only the paragraphs relating to the ordinary new.
N2158 has been written explaining the rationale for the proposed resolution below.
Proposed resolution:
Change 18.5.1.1 [new.delete.single]:
void* operator new(std::size_t size, const std::nothrow_t&) throw();-5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a
bad_alloc
exception.-6- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.
-7- Required behavior: Return a non-null pointer to suitably aligned storage (3.7.4), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the (possibly replaced) ordinary version. This requirement is binding on a replacement version of this function.
-8- Default behavior:
- Calls
operator new(size)
.- If the call to
operator new(size)
returns normally, returns the result of that call, else- if the call to
operator new(size)
throws an exception, returns a null pointer.Executes a loop: Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to the Standard C library functionmalloc
is unspecified.Returns a pointer to the allocated storage if the attempt is successful. Otherwise, if the last argument toset_new_handler()
was a null pointer, return a null pointer.Otherwise, the function calls the current new_handler (18.5.2.2). If the called function returns, the loop repeats.The loop terminates when an attempt to allocate the requested storage is successful or when a called new_handler function does not return. If the called new_handler function terminates by throwing abad_alloc exception
, the function returns a null pointer.-9- [Example:
T* p1 = new T; // throws bad_alloc if it fails T* p2 = new(nothrow) T; // returns 0 if it fails--end example]
void operator delete(void* ptr) throw();void operator delete(void* ptr, const std::nothrow_t&) throw();-10- Effects: The deallocation function (3.7.4.2) called by a delete-expression to render the value of
ptr
invalid.-11- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.
-12- Requires: the value of
ptr
is null or the value returned by an earlier call to thedefault(possibly replaced)operator new(std::size_t)
oroperator new(std::size_t, const std::nothrow_t&)
.-13- Default behavior:
- For a null value of
ptr
, do nothing.- Any other value of
ptr
shall be a value returned earlier by a call to the defaultoperator new
, which was not invalidated by an intervening call tooperator delete(void*)
(17.4.3.7). For such a non-null value ofptr
, reclaims storage allocated by the earlier call to the defaultoperator new
.-14- Remarks: It is unspecified under what conditions part or all of such reclaimed storage is allocated by a subsequent call to
operator new
or any ofcalloc
,malloc
, orrealloc
, declared in<cstdlib>
.void operator delete(void* ptr, const std::nothrow_t&) throw();-15- Effects: Same as above, except that it is called by the implementation when an exception propagates from a nothrow placement version of the new-expression (i.e. when the constructor throws an exception).
-16- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.
-17- Requires: the value of
ptr
is null or the value returned by an earlier call to the (possibly replaced)operator new(std::size_t)
oroperator new(std::size_t, const std::nothrow_t&)
.-18- Default behavior: Calls
operator delete(ptr)
.
Change 18.5.1.2 [new.delete.array]
void* operator new[](std::size_t size, const std::nothrow_t&) throw();-5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a
bad_alloc
exception.-6- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.
-7- Required behavior:
Same as for operatorReturn a non-null pointer to suitably aligned storage (3.7.4), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the (possibly replaced)new(std::size_t, const std::nothrow_t&)
. This nothrow version of operatornew[]
returns a pointer obtained as if acquired from the ordinary version.operator new[](std::size_t size)
. This requirement is binding on a replacement version of this function.-8- Default behavior:
Returnsoperator new(size, nothrow)
.
- Calls
operator new[](size)
.- If the call to
operator new[](size)
returns normally, returns the result of that call, else- if the call to
operator new[](size)
throws an exception, returns a null pointer.void operator delete[](void* ptr) throw(); void operator delete[](void* ptr, const std::nothrow_t&) throw();-9- Effects: The deallocation function (3.7.4.2) called by the array form of a delete-expression to render the value of
ptr
invalid.-10- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.
-11- Requires: the value of
ptr
is null or the value returned by an earlier call tooperator new[](std::size_t)
oroperator new[](std::size_t, const std::nothrow_t&)
.-12- Default behavior: Calls
operator delete(ptr)
oroperator delete[](ptr
respectively., std::nothrow)
Rationale:
Yes, they may become unlinked, and that is by design. If a user replaces one, the user should also replace the other.
[ Reopened due to a gcc conversation between Howard, Martin and Gaby. Forwarding or not is visible behavior to the client and it would be useful for the client to know which behavior it could depend on. ]
[ Batavia: Robert voiced serious reservations about backwards compatibility for his customers. ]
Section: 24.3.4 [iterator.concepts] Status: TC1 Submitter: Stephen Cleary Opened: 2000-02-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 24.1 paragraph 5, it is stated ". . . Dereferenceable and past-the-end values are always non-singular."
This places an unnecessary restriction on past-the-end iterators for containers with forward iterators (for example, a singly-linked list). If the past-the-end value on such a container was a well-known singular value, it would still satisfy all forward iterator requirements.
Removing this restriction would allow, for example, a singly-linked list without a "footer" node.
This would have an impact on existing code that expects past-the-end iterators obtained from different (generic) containers being not equal.
Proposed resolution:
Change 24.3.4 [iterator.concepts] paragraph 5, the last sentence, from:
Dereferenceable and past-the-end values are always non-singular.
to:
Dereferenceable values are always non-singular.
Rationale:
For some kinds of containers, including singly linked lists and zero-length vectors, null pointers are perfectly reasonable past-the-end iterators. Null pointers are singular.
Section: 27.4.3 [basic.string] Status: TC1 Submitter: Igor Stauder Opened: 2000-02-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 27.4.3 [basic.string] the basic_string member function declarations use a consistent style except for the following functions:
void push_back(const charT); basic_string& assign(const basic_string&); void swap(basic_string<charT,traits,Allocator>&);
- push_back, assign, swap: missing argument name
- push_back: use of const with charT (i.e. POD type passed by value
not by reference - should be charT or const charT& )
- swap: redundant use of template parameters in argument
basic_string<charT,traits,Allocator>&
Proposed resolution:
In Section 27.4.3 [basic.string] change the basic_string member function declarations push_back, assign, and swap to:
void push_back(charT c); basic_string& assign(const basic_string& str); void swap(basic_string& str);
Rationale:
Although the standard is in general not consistent in declaration style, the basic_string declarations are consistent other than the above. The LWG felt that this was sufficient reason to merit the change.
Section: 26 [algorithms] Status: TC1 Submitter: Lisa Lippincott Opened: 2000-02-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In paragraph 9 of section 26 [algorithms], it is written:
In the description of the algorithms operators + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of [...] a-b is the same as of
return distance(a, b);
Proposed resolution:
On the last line of paragraph 9 of section 26 [algorithms] change
"a-b"
to "b-a".
Rationale:
There are two ways to fix the defect; change the description to b-a or change the return to distance(b,a). The LWG preferred the former for consistency.
Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Scott Snyder Opened: 2000-02-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of the stream extraction operator for std::string (section 21.3.7.9 [lib.string.io]) does not contain a requirement that failbit be set in the case that the operator fails to extract any characters from the input stream.
This implies that the typical construction
std::istream is; std::string str; ... while (is >> str) ... ;
(which tests failbit) is not required to terminate at EOF.
Furthermore, this is inconsistent with other extraction operators, which do include this requirement. (See sections 31.7.5.3 [istream.formatted] and 31.7.5.4 [istream.unformatted]), where this requirement is present, either explicitly or implicitly, for the extraction operators. It is also present explicitly in the description of getline (istream&, string&, charT) in section 27.4.4.4 [string.io] paragraph 8.)
Proposed resolution:
Insert new paragraph after paragraph 2 in section 27.4.4.4 [string.io]:
If the function extracts no characters, it calls is.setstate(ios::failbit) which may throw ios_base::failure (27.4.4.3).
Section: 26.8.9 [alg.min.max] Status: TC1 Submitter: Nico Josuttis Opened: 2000-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard doesn't specify what min_element() and max_element() shall return if the range is empty (first equals last). The usual implementations return last. This problem seems also apply to partition(), stable_partition(), next_permutation(), and prev_permutation().
Proposed resolution:
In 26.8.9 [alg.min.max] - Minimum and maximum, paragraphs 7 and 9, append: Returns last if first==last.
Rationale:
The LWG looked in some detail at all of the above mentioned algorithms, but believes that except for min_element() and max_element() it is already clear that last is returned if first == last.
Section: 23.4.6 [set], 23.4.7 [multiset] Status: CD1 Submitter: Judy Ward Opened: 2000-02-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 450
Discussion:
The specification for the associative container requirements in Table 69 state that the find member function should "return iterator; const_iterator for constant a". The map and multimap container descriptions have two overloaded versions of find, but set and multiset do not, all they have is:
iterator find(const key_type & x) const;
Proposed resolution:
Change the prototypes for find(), lower_bound(), upper_bound(), and equal_range() in section 23.4.6 [set] and section 23.4.7 [multiset] to each have two overloads:
iterator find(const key_type & x); const_iterator find(const key_type & x) const;iterator lower_bound(const key_type & x); const_iterator lower_bound(const key_type & x) const;iterator upper_bound(const key_type & x); const_iterator upper_bound(const key_type & x) const;pair<iterator, iterator> equal_range(const key_type & x); pair<const_iterator, const_iterator> equal_range(const key_type & x) const;
[Tokyo: At the request of the LWG, Judy Ward provided wording extending the proposed resolution to lower_bound, upper_bound, and equal_range.]
Section: 99 [facets.examples] Status: TC1 Submitter: Martin Sebor Opened: 2000-02-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The example in 22.2.8, paragraph 11 contains the following errors:
1) The member function `My::JCtype::is_kanji()' is non-const; the function must be const in order for it to be callable on a const object (a reference to which which is what std::use_facet<>() returns).
2) In file filt.C, the definition of `JCtype::id' must be qualified with the name of the namespace `My'.
3) In the definition of `loc' and subsequently in the call to use_facet<>() in main(), the name of the facet is misspelled: it should read `My::JCtype' rather than `My::JCType'.
Proposed resolution:
Replace the "Classifying Japanese characters" example in 22.2.8, paragraph 11 with the following:
#include <locale>
namespace My { using namespace std; class JCtype : public locale::facet { public: static locale::id id; // required for use as a new locale facet bool is_kanji (wchar_t c) const; JCtype() {} protected: ~JCtype() {} }; }
// file: filt.C #include <iostream> #include <locale> #include "jctype" // above std::locale::id My::JCtype::id; // the static JCtype member declared above.
int main() { using namespace std; typedef ctype<wchar_t> wctype; locale loc(locale(""), // the user's preferred locale... new My::JCtype); // and a new feature ... wchar_t c = use_facet<wctype>(loc).widen('!'); if (!use_facet<My::JCtype>(loc).is_kanji(c)) cout << "no it isn't!" << endl; return 0; }
Section: 31.5.2.8 [ios.base.cons] Status: TC1 Submitter: Jonathan Schilling, Howard Hinnant Opened: 2000-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The pre-conditions for the ios_base destructor are described in 27.4.2.7 paragraph 2:
Effects: Destroys an object of class ios_base. Calls each registered callback pair (fn,index) (27.4.2.6) as (*fn)(erase_event,*this,index) at such time that any ios_base member function called from within fn has well defined results.
But what is not clear is: If no callback functions were ever registered, does it matter whether the ios_base members were ever initialized?
For instance, does this program have defined behavior:
#include <ios>class D : public std::ios_base { };int main() { D d; }
It seems that registration of a callback function would surely affect the state of an ios_base. That is, when you register a callback function with an ios_base, the ios_base must record that fact somehow.
But if after construction the ios_base is in an indeterminate state, and that state is not made determinate before the destructor is called, then how would the destructor know if any callbacks had indeed been registered? And if the number of callbacks that had been registered is indeterminate, then is not the behavior of the destructor undefined?
By comparison, the basic_ios class description in 27.4.4.1 paragraph 2 makes it explicit that destruction before initialization results in undefined behavior.
Proposed resolution:
Modify 27.4.2.7 paragraph 1 from
Effects: Each ios_base member has an indeterminate value after construction.
to
Effects: Each ios_base member has an indeterminate value after construction. These members must be initialized by calling basic_ios::init. If an ios_base object is destroyed before these initializations have taken place, the behavior is undefined.
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Matt Austern Opened: 2000-03-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Stage 2 processing of numeric conversion is broken.
Table 55 in 22.2.2.1.2 says that when basefield is 0 the integral conversion specifier is %i. A %i specifier determines a number's base by its prefix (0 for octal, 0x for hex), so the intention is clearly that a 0x prefix is allowed. Paragraph 8 in the same section, however, describes very precisely how characters are processed. (It must be done "as if" by a specified code fragment.) That description does not allow a 0x prefix to be recognized.
Very roughly, stage 2 processing reads a char_type ct. It converts ct to a char, not by using narrow but by looking it up in a translation table that was created by widening the string literal "0123456789abcdefABCDEF+-". The character "x" is not found in that table, so it can't be recognized by stage 2 processing.
Proposed resolution:
In 22.2.2.1.2 paragraph 8, replace the line:
static const char src[] = "0123456789abcdefABCDEF+-";
with the line:
static const char src[] = "0123456789abcdefxABCDEFX+-";
Rationale:
If we're using the technique of widening a string literal, the string literal must contain every character we wish to recognize. This technique has the consequence that alternate representations of digits will not be recognized. This design decision was made deliberately, with full knowledge of that limitation.
Section: 16.3.2.4 [structure.specifications] Status: TC1 Submitter: Judy Ward Opened: 2000-03-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 21.3.6.8 describes the basic_string::compare function this way:
21.3.6.8 - basic_string::compare [lib.string::compare] int compare(size_type pos1, size_type n1, const basic_string<charT,traits,Allocator>& str , size_type pos2 , size_type n2 ) const; -4- Returns: basic_string<charT,traits,Allocator>(*this,pos1,n1).compare( basic_string<charT,traits,Allocator>(str,pos2,n2)) .
and the constructor that's implicitly called by the above is defined to throw an out-of-range exception if pos > str.size(). See section 27.4.3.2 [string.require] paragraph 4.
On the other hand, the compare function descriptions themselves don't have "Throws: " clauses and according to 17.3.1.3, paragraph 3, elements that do not apply to a function are omitted.
So it seems there is an inconsistency in the standard -- are the "Effects" clauses correct, or are the "Throws" clauses missing?
Proposed resolution:
In 16.3.2.4 [structure.specifications] paragraph 3, the footnote 148 attached to the sentence "Descriptions of function semantics contain the following elements (as appropriate):", insert the word "further" so that the foot note reads:
To save space, items that do not apply to a function are omitted. For example, if a function does not specify any further preconditions, there will be no "Requires" paragraph.
Rationale:
The standard is somewhat inconsistent, but a failure to note a throw condition in a throws clause does not grant permission not to throw. The inconsistent wording is in a footnote, and thus non-normative. The proposed resolution from the LWG clarifies the footnote.
Section: 26.7.10 [alg.reverse] Status: TC1 Submitter: Dave Abrahams Opened: 2000-03-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Shouldn't the effects say "applies iter_swap to all pairs..."?
Proposed resolution:
In 26.7.10 [alg.reverse], replace:
Effects: For each non-negative integer i <= (last - first)/2, applies swap to all pairs of iterators first + i, (last - i) - 1.
with:
Effects: For each non-negative integer i <= (last - first)/2, applies iter_swap to all pairs of iterators first + i, (last - i) - 1.
Section: 23.2.7 [associative.reqmts] Status: TC1 Submitter: Ed Brey Opened: 2000-03-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the associative container requirements table in 23.1.2 paragraph 7, a.clear() has complexity "log(size()) + N". However, the meaning of N is not defined.
Proposed resolution:
In the associative container requirements table in 23.1.2 paragraph
7, the complexity of a.clear(), change "log(size()) + N" to
"linear in size()
".
Rationale:
It's the "log(size())", not the "N", that is in
error: there's no difference between O(N) and O(N +
log(N)). The text in the standard is probably an incorrect
cut-and-paste from the range version of erase
.
Section: 16.4.6.4 [global.functions] Status: CD1 Submitter: Dave Abrahams Opened: 2000-04-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Are algorithms in std:: allowed to use other algorithms without qualification, so functions in user namespaces might be found through Koenig lookup?
For example, a popular standard library implementation includes this implementation of std::unique:
namespace std { template <class _ForwardIter> _ForwardIter unique(_ForwardIter __first, _ForwardIter __last) { __first = adjacent_find(__first, __last); return unique_copy(__first, __last, __first); } }
Imagine two users on opposite sides of town, each using unique on his own sequences bounded by my_iterators . User1 looks at his standard library implementation and says, "I know how to implement a more efficient unique_copy for my_iterators", and writes:
namespace user1 { class my_iterator; // faster version for my_iterator my_iterator unique_copy(my_iterator, my_iterator, my_iterator); }
user1::unique_copy() is selected by Koenig lookup, as he intended.
User2 has other needs, and writes:
namespace user2 { class my_iterator; // Returns true iff *c is a unique copy of *a and *b. bool unique_copy(my_iterator a, my_iterator b, my_iterator c); }
User2 is shocked to find later that his fully-qualified use of std::unique(user2::my_iterator, user2::my_iterator, user2::my_iterator) fails to compile (if he's lucky). Looking in the standard, he sees the following Effects clause for unique():
Effects: Eliminates all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first, last) for which the following corresponding conditions hold: *i == *(i - 1) or pred(*i, *(i - 1)) != false
The standard gives user2 absolutely no reason to think he can interfere with std::unique by defining names in namespace user2. His standard library has been built with the template export feature, so he is unable to inspect the implementation. User1 eventually compiles his code with another compiler, and his version of unique_copy silently stops being called. Eventually, he realizes that he was depending on an implementation detail of his library and had no right to expect his unique_copy() to be called portably.
On the face of it, and given above scenario, it may seem obvious that the implementation of unique() shown is non-conforming because it uses unique_copy() rather than ::std::unique_copy(). Most standard library implementations, however, seem to disagree with this notion.
[Tokyo: Steve Adamczyk from the core working group indicates that "std::" is sufficient; leading "::" qualification is not required because any namespace qualification is sufficient to suppress Koenig lookup.]
Proposed resolution:
Add a paragraph and a note at the end of 16.4.6.4 [global.functions]:
Unless otherwise specified, no global or non-member function in the standard library shall use a function from another namespace which is found through argument-dependent name lookup (6.5.4 [basic.lookup.argdep]).
[Note: the phrase "unless otherwise specified" is intended to allow Koenig lookup in cases like that of ostream_iterators:
Effects:*out_stream << value;
if(delim != 0) *out_stream << delim;
return (*this);--end note]
[Tokyo: The LWG agrees that this is a defect in the standard, but is as yet unsure if the proposed resolution is the best solution. Furthermore, the LWG believes that the same problem of unqualified library names applies to wording in the standard itself, and has opened issue 229(i) accordingly. Any resolution of issue 225(i) should be coordinated with the resolution of issue 229(i).]
[Toronto: The LWG is not sure if this is a defect in the
standard. Most LWG members believe that an implementation of
std::unique
like the one quoted in this issue is already
illegal, since, under certain circumstances, its semantics are not
those specified in the standard. The standard's description of
unique
does not say that overloading adjacent_find
should have any effect.]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution for this issue is in accordance with Howard's paper.]
Rationale:
It could be argued that this proposed isn't strictly necessary, that the Standard doesn't grant implementors license to write a standard function that behaves differently than specified in the Standard just because of an unrelated user-defined name in some other namespace. However, this is at worst a clarification. It is surely right that algorithsm shouldn't pick up random names, that user-defined names should have no effect unless otherwise specified. Issue 226(i) deals with the question of when it is appropriate for the standard to explicitly specify otherwise.
Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Dave Abrahams Opened: 2000-04-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The issues are:
1. How can a 3rd party library implementor (lib1) write a version of a standard algorithm which is specialized to work with his own class template?
2. How can another library implementor (lib2) write a generic algorithm which will take advantage of the specialized algorithm in lib1?
This appears to be the only viable answer under current language rules:
namespace lib1 { // arbitrary-precision numbers using T as a basic unit template <class T> class big_num { //... };// defining this in namespace std is illegal (it would be an // overload), so we hope users will rely on Koenig lookup template <class T> void swap(big_int<T>&, big_int<T>&); }#include <algorithm> namespace lib2 { template <class T> void generic_sort(T* start, T* end) { ... // using-declaration required so we can work on built-in types using std::swap; // use Koenig lookup to find specialized algorithm if available swap(*x, *y); } }
This answer has some drawbacks. First of all, it makes writing lib2 difficult and somewhat slippery. The implementor needs to remember to write the using-declaration, or generic_sort will fail to compile when T is a built-in type. The second drawback is that the use of this style in lib2 effectively "reserves" names in any namespace which defines types which may eventually be used with lib2. This may seem innocuous at first when applied to names like swap, but consider more ambiguous names like unique_copy() instead. It is easy to imagine the user wanting to define these names differently in his own namespace. A definition with semantics incompatible with the standard library could cause serious problems (see issue 225(i)).
Why, you may ask, can't we just partially specialize std::swap()? It's because the language doesn't allow for partial specialization of function templates. If you write:
namespace std { template <class T> void swap(lib1::big_int<T>&, lib1::big_int<T>&); }
You have just overloaded std::swap, which is illegal under the current language rules. On the other hand, the following full specialization is legal:
namespace std { template <> void swap(lib1::other_type&, lib1::other_type&); }
This issue reflects concerns raised by the "Namespace issue with specialized swap" thread on comp.lang.c++.moderated. A similar set of concerns was earlier raised on the boost.org mailing list and the ACCU-general mailing list. Also see library reflector message c++std-lib-7354.
J. C. van Winkel points out (in c++std-lib-9565) another unexpected fact: it's impossible to output a container of std::pair's using copy and an ostream_iterator, as long as both pair-members are built-in or std:: types. That's because a user-defined operator<< for (for example) std::pair<const std::string, int> will not be found: lookup for operator<< will be performed only in namespace std. Opinions differed on whether or not this was a defect, and, if so, whether the defect is that something is wrong with user-defined functionality and std, or whether it's that the standard library does not provide an operator<< for std::pair<>.
Proposed resolution:
Adopt the wording proposed in Howard Hinnant's paper N1523=03-0106, "Proposed Resolution To LWG issues 225, 226, 229".
[Tokyo: Summary, "There is no conforming way to extend std::swap for user defined templates." The LWG agrees that there is a problem. Would like more information before proceeding. This may be a core issue. Core issue 229 has been opened to discuss the core aspects of this problem. It was also noted that submissions regarding this issue have been received from several sources, but too late to be integrated into the issues list. ]
[Post-Tokyo: A paper with several proposed resolutions, J16/00-0029==WG21/N1252, "Shades of namespace std functions " by Alan Griffiths, is in the Post-Tokyo mailing. It should be considered a part of this issue.]
[Toronto: Dave Abrahams and Peter Dimov have proposed a
resolution that involves core changes: it would add partial
specialization of function template. The Core Working Group is
reluctant to add partial specialization of function templates. It is
viewed as a large change, CWG believes that proposal presented leaves
some syntactic issues unanswered; if the CWG does add partial
specialization of function templates, it wishes to develop its own
proposal. The LWG continues to believe that there is a serious
problem: there is no good way for users to force the library to use
user specializations of generic standard library functions, and in
certain cases (e.g. transcendental functions called by
valarray
and complex
) this is important. Koenig
lookup isn't adequate, since names within the library must be
qualified with std
(see issue 225), specialization doesn't
work (we don't have partial specialization of function templates), and
users aren't permitted to add overloads within namespace std.
]
[Copenhagen: Discussed at length, with no consensus. Relevant
papers in the pre-Copenhagen mailing: N1289, N1295, N1296. Discussion
focused on four options. (1) Relax restrictions on overloads within
namespace std. (2) Mandate that the standard library use unqualified
calls for swap
and possibly other functions. (3) Introduce
helper class templates for swap
and possibly other functions.
(4) Introduce partial specialization of function templates. Every
option had both support and opposition. Straw poll (first number is
support, second is strongly opposed): (1) 6, 4; (2) 6, 7; (3) 3, 8;
(4) 4, 4.]
[Redmond: Discussed, again no consensus. Herb presented an
argument that a user who is defining a type T
with an
associated swap
should not be expected to put that
swap
in namespace std, either by overloading or by partial
specialization. The argument is that swap
is part of
T
's interface, and thus should to in the same namespace as
T
and only in that namespace. If we accept this argument,
the consequence is that standard library functions should use
unqualified call of swap
. (And which other functions? Any?)
A small group (Nathan, Howard, Jeremy, Dave, Matt, Walter, Marc) will
try to put together a proposal before the next meeting.]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution is the one proposed by Howard.]
[Santa Cruz: the LWG agreed with the general direction of Howard's paper, N1387. (Roughly: Koenig lookup is disabled unless we say otherwise; this issue is about when we do say otherwise.) However, there were concerns about wording. Howard will provide new wording. Bill and Jeremy will review it.]
[Kona: Howard proposed the new wording. The LWG accepted his proposed resolution.]
Rationale:
Informally: introduce a Swappable concept, and specify that the
value types of the iterators passed to certain standard algorithms
(such as iter_swap, swap_ranges, reverse, rotate, and sort) conform
to that concept. The Swappable concept will make it clear that
these algorithms use unqualified lookup for the calls
to swap
. Also, in 29.6.3.3 [valarray.transcend] paragraph 1,
state that the valarray transcendentals use unqualified lookup.
Section: 26.7.3 [alg.swap] Status: TC1 Submitter: Dave Abrahams Opened: 2000-04-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
25.2.2 reads:
template<class T> void swap(T& a, T& b);
Requires: Type T is Assignable (_lib.container.requirements_).
Effects: Exchanges values stored in two locations.
The only reasonable** generic implementation of swap requires construction of a new temporary copy of one of its arguments:
template<class T> void swap(T& a, T& b); { T tmp(a); a = b; b = tmp; }
But a type which is only Assignable cannot be swapped by this implementation.
**Yes, there's also an unreasonable implementation which would require T to be DefaultConstructible instead of CopyConstructible. I don't think this is worthy of consideration:
template<class T> void swap(T& a, T& b); { T tmp; tmp = a; a = b; b = tmp; }
Proposed resolution:
Change 25.2.2 paragraph 1 from:
Requires: Type T is Assignable (23.1).
to:
Requires: Type T is CopyConstructible (20.1.3) and Assignable (23.1)
Section: 28.3.4 [locale.categories] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-20 Last modified: 2018-08-10
Priority: Not Prioritized
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Discussion:
The sections 28.3.4.2.3 [locale.ctype.byname], 28.3.4.2.6 [locale.codecvt.byname], 28.3.4.4.2 [locale.numpunct.byname], 28.3.4.5.2 [locale.collate.byname], 28.3.4.6.5 [locale.time.put.byname], 28.3.4.7.5 [locale.moneypunct.byname], and 28.3.4.8.3 [locale.messages.byname] overspecify the definitions of the "..._byname" classes by listing a bunch of virtual functions. At the same time, no semantics of these functions are defined. Real implementations do not define these functions because the functional part of the facets is actually implemented in the corresponding base classes and the constructor of the "..._byname" version just provides suitable date used by these implementations. For example, the 'numpunct' methods just return values from a struct. The base class uses a statically initialized struct while the derived version reads the contents of this struct from a table. However, no virtual function is defined in 'numpunct_byname'.
For most classes this does not impose a problem but specifically for 'ctype' it does: The specialization for 'ctype_byname<char>' is required because otherwise the semantics would change due to the virtual functions defined in the general version for 'ctype_byname': In 'ctype<char>' the method 'do_is()' is not virtual but it is made virtual in both 'ctype<cT>' and 'ctype_byname<cT>'. Thus, a class derived from 'ctype_byname<char>' can tell whether this class is specialized or not under the current specification: Without the specialization, 'do_is()' is virtual while with specialization it is not virtual.
Proposed resolution:
Change section 22.2.1.2 (lib.locale.ctype.byname) to become:
namespace std { template <class charT> class ctype_byname : public ctype<charT> { public: typedef ctype<charT>::mask mask; explicit ctype_byname(const char*, size_t refs = 0); protected: ~ctype_byname(); // virtual }; }
Change section 22.2.1.6 (lib.locale.codecvt.byname) to become:
namespace std { template <class internT, class externT, class stateT> class codecvt_byname : public codecvt<internT, externT, stateT> { public: explicit codecvt_byname(const char*, size_t refs = 0); protected: ~codecvt_byname(); // virtual }; }
Change section 22.2.3.2 (lib.locale.numpunct.byname) to become:
namespace std { template <class charT> class numpunct_byname : public numpunct<charT> { // this class is specialized for char and wchar_t. public: typedef charT char_type; typedef basic_string<charT> string_type; explicit numpunct_byname(const char*, size_t refs = 0); protected: ~numpunct_byname(); // virtual }; }
Change section 22.2.4.2 (lib.locale.collate.byname) to become:
namespace std { template <class charT> class collate_byname : public collate<charT> { public: typedef basic_string<charT> string_type; explicit collate_byname(const char*, size_t refs = 0); protected: ~collate_byname(); // virtual }; }
Change section 22.2.5.2 (lib.locale.time.get.byname) to become:
namespace std { template <class charT, class InputIterator = istreambuf_iterator<charT> > class time_get_byname : public time_get<charT, InputIterator> { public: typedef time_base::dateorder dateorder; typedef InputIterator iter_type
explicit time_get_byname(const char*, size_t refs = 0); protected: ~time_get_byname(); // virtual }; }
Change section 22.2.5.4 (lib.locale.time.put.byname) to become:
namespace std { template <class charT, class OutputIterator = ostreambuf_iterator<charT> > class time_put_byname : public time_put<charT, OutputIterator> { public: typedef charT char_type; typedef OutputIterator iter_type;
explicit time_put_byname(const char*, size_t refs = 0); protected: ~time_put_byname(); // virtual }; }"
Change section 22.2.6.4 (lib.locale.moneypunct.byname) to become:
namespace std { template <class charT, bool Intl = false> class moneypunct_byname : public moneypunct<charT, Intl> { public: typedef money_base::pattern pattern; typedef basic_string<charT> string_type;
explicit moneypunct_byname(const char*, size_t refs = 0); protected: ~moneypunct_byname(); // virtual }; }
Change section 22.2.7.2 (lib.locale.messages.byname) to become:
namespace std { template <class charT> class messages_byname : public messages<charT> { public: typedef messages_base::catalog catalog; typedef basic_string<charT> string_type;
explicit messages_byname(const char*, size_t refs = 0); protected: ~messages_byname(); // virtual }; }
Remove section 28.3.4.2.5 [locale.codecvt] completely (because in this case only those members are defined to be virtual which are defined to be virtual in 'ctype<cT>'.)
[Post-Tokyo: Dietmar Kühl submitted this issue at the request of the LWG to solve the underlying problems raised by issue 138(i).]
[Copenhagen: proposed resolution was revised slightly, to remove
three last virtual functions from messages_byname
.]
Section: 16.4.2.2 [contents] Status: CD1 Submitter: Steve Clamage Opened: 2000-04-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Throughout the library chapters, the descriptions of library entities refer to other library entities without necessarily qualifying the names.
For example, section 25.2.2 "Swap" describes the effect of swap_ranges in terms of the unqualified name "swap". This section could reasonably be interpreted to mean that the library must be implemented so as to do a lookup of the unqualified name "swap", allowing users to override any ::std::swap function when Koenig lookup applies.
Although it would have been best to use explicit qualification with "::std::" throughout, too many lines in the standard would have to be adjusted to make that change in a Technical Corrigendum.
Issue 182(i), which addresses qualification of
size_t
, is a special case of this.
Proposed resolution:
To section 17.4.1.1 "Library contents" Add the following paragraph:
Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects section for library function F is described as calling library function G, the function ::std::G is meant.
[Post-Tokyo: Steve Clamage submitted this issue at the request of the LWG to solve a problem in the standard itself similar to the problem within implementations of library identified by issue 225(i). Any resolution of issue 225(i) should be coordinated with the resolution of this issue.]
[post-Toronto: Howard is undecided about whether it is
appropriate for all standard library function names referred to in
other standard library functions to be explicitly qualified by
std
: it is common advice that users should define global
functions that operate on their class in the same namespace as the
class, and this requires argument-dependent lookup if those functions
are intended to be called by library code. Several LWG members are
concerned that valarray appears to require argument-dependent lookup,
but that the wording may not be clear enough to fall under
"unless explicitly described otherwise".]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. This paper resolves issues 225 and 226. In light of that resolution, the proposed resolution for the current issue makes sense.]
Section: 16 [library] Status: CD1 Submitter: Beman Dawes Opened: 2000-04-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 227(i) identified an instance (std::swap) where Assignable was specified without also specifying CopyConstructible. The LWG asked that the standard be searched to determine if the same defect existed elsewhere.
There are a number of places (see proposed resolution below) where Assignable is specified without also specifying CopyConstructible. There are also several cases where both are specified. For example, 29.5.3 [rand.req].
Proposed resolution:
In 23.2 [container.requirements] table 65 for value_type: change "T is Assignable" to "T is CopyConstructible and Assignable"
In 23.2.7 [associative.reqmts] table 69 X::key_type; change
"Key is Assignable" to "Key is
CopyConstructible and Assignable"
In 24.3.5.4 [output.iterators] paragraph 1, change:
A class or a built-in type X satisfies the requirements of an output iterator if X is an Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:
to:
A class or a built-in type X satisfies the requirements of an output iterator if X is a CopyConstructible (20.1.3) and Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:
[Post-Tokyo: Beman Dawes submitted this issue at the request of the LWG. He asks that the 26.7.5 [alg.replace] and 26.7.6 [alg.fill] changes be studied carefully, as it is not clear that CopyConstructible is really a requirement and may be overspecification.]
[Portions of the resolution for issue 230 have been superceded by the resolution of issue 276(i).]
Rationale:
The original proposed resolution also included changes to input iterator, fill, and replace. The LWG believes that those changes are not necessary. The LWG considered some blanket statement, where an Assignable type was also required to be Copy Constructible, but decided against this because fill and replace really don't require the Copy Constructible property.
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: James Kanze, Stephen Clamage Opened: 2000-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
What is the following program supposed to output?
#include <iostream> int main() { std::cout.setf( std::ios::scientific , std::ios::floatfield ) ; std::cout.precision( 0 ) ; std::cout << 1.00 << '\n' ; return 0 ; }
From my C experience, I would expect "1e+00"; this is what
printf("%.0e" , 1.00 );
does. G++ outputs
"1.000000e+00".
The only indication I can find in the standard is 22.2.2.2.2/11, where it says "For conversion from a floating-point type, if (flags & fixed) != 0 or if str.precision() > 0, then str.precision() is specified in the conversion specification." This is an obvious error, however, fixed is not a mask for a field, but a value that a multi-bit field may take -- the results of and'ing fmtflags with ios::fixed are not defined, at least not if ios::scientific has been set. G++'s behavior corresponds to what might happen if you do use (flags & fixed) != 0 with a typical implementation (floatfield == 3 << something, fixed == 1 << something, and scientific == 2 << something).
Presumably, the intent is either (flags & floatfield) != 0, or (flags & floatfield) == fixed; the first gives something more or less like the effect of precision in a printf floating point conversion. Only more or less, of course. In order to implement printf formatting correctly, you must know whether the precision was explicitly set or not. Say by initializing it to -1, instead of 6, and stating that for floating point conversions, if precision < -1, 6 will be used, for fixed point, if precision < -1, 1 will be used, etc. Plus, of course, if precision == 0 and flags & floatfield == 0, 1 should be = used. But it probably isn't necessary to emulate all of the anomalies of printf:-).
Proposed resolution:
Replace 28.3.4.3.3.3 [facet.num.put.virtuals], paragraph 11, with the following sentence:
For conversion from a floating-point type,
str.precision()
is specified in the conversion specification.
Rationale:
The floatfield determines whether numbers are formatted as if
with %f, %e, or %g. If the fixed
bit is set, it's %f,
if scientific
it's %e, and if both bits are set, or
neither, it's %g.
Turning to the C standard, a precision of 0 is meaningful for %f and %e. For %g, precision 0 is taken to be the same as precision 1.
The proposed resolution has the effect that if neither
fixed
nor scientific
is set we'll be
specifying a precision of 0, which will be internally
turned into 1. There's no need to call it out as a special
case.
The output of the above program will be "1e+00".
[Post-Curaçao: Howard provided improved wording covering the case where precision is 0 and mode is %g.]
Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Peter Dimov Opened: 2000-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
17.4.3.1/1 uses the term "depends" to limit the set of allowed specializations of standard templates to those that "depend on a user-defined name of external linkage."
This term, however, is not adequately defined, making it possible to construct a specialization that is, I believe, technically legal according to 17.4.3.1/1, but that specializes a standard template for a built-in type such as 'int'.
The following code demonstrates the problem:
#include <algorithm>template<class T> struct X { typedef T type; };namespace std { template<> void swap(::X<int>::type& i, ::X<int>::type& j); }
Proposed resolution:
Change "user-defined name" to "user-defined type".
Rationale:
This terminology is used in section 2.5.2 and 4.1.1 of The C++ Programming Language. It disallows the example in the issue, since the underlying type itself is not user-defined. The only possible problem I can see is for non-type templates, but there's no possible way for a user to come up with a specialization for bitset, for example, that might not have already been specialized by the implementor?
[Toronto: this may be related to issue 120(i).]
[post-Toronto: Judy provided the above proposed resolution and rationale.]
Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 2000-04-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
If mm
is a multimap and p
is an iterator
into the multimap, then mm.insert(p, x)
inserts
x
into mm
with p
as a hint as
to where it should go. Table 69 claims that the execution time is
amortized constant if the insert winds up taking place adjacent to
p
, but does not say when, if ever, this is guaranteed to
happen. All it says it that p
is a hint as to where to
insert.
The question is whether there is any guarantee about the relationship
between p
and the insertion point, and, if so, what it
is.
I believe the present state is that there is no guarantee: The user
can supply p
, and the implementation is allowed to
disregard it entirely.
Additional comments from Nathan:
The vote [in Redmond] was on whether to elaborately specify the use of
the hint, or to require behavior only if the value could be inserted
adjacent to the hint. I would like to ensure that we have a chance to
vote for a deterministic treatment: "before, if possible, otherwise
after, otherwise anywhere appropriate", as an alternative to the
proposed "before or after, if possible, otherwise [...]".
[Toronto: there was general agreement that this is a real defect: when inserting an element x into a multiset that already contains several copies of x, there is no way to know whether the hint will be used. The proposed resolution was that the new element should always be inserted as close to the hint as possible. So, for example, if there is a subsequence of equivalent values, then providing a.begin() as the hint means that the new element should be inserted before the subsequence even if a.begin() is far away. JC van Winkel supplied precise wording for this proposed resolution, and also for an alternative resolution in which hints are only used when they are adjacent to the insertion point.]
[Copenhagen: the LWG agreed to the original proposed resolution, in which an insertion hint would be used even when it is far from the insertion point. This was contingent on seeing a example implementation showing that it is possible to implement this requirement without loss of efficiency. John Potter provided such a example implementation.]
[Redmond: The LWG was reluctant to adopt the proposal that emerged from Copenhagen: it seemed excessively complicated, and went beyond fixing the defect that we identified in Toronto. PJP provided the new wording described in this issue. Nathan agrees that we shouldn't adopt the more detailed semantics, and notes: "we know that you can do it efficiently enough with a red-black tree, but there are other (perhaps better) balanced tree techniques that might differ enough to make the detailed semantics hard to satisfy."]
[Curaçao: Nathan should give us the alternative wording he suggests so the LWG can decide between the two options.]
[Lillehammer: The LWG previously rejected the more detailed semantics, because it seemed more loike a new feature than like defect fixing. We're now more sympathetic to it, but we (especially Bill) are still worried about performance. N1780 describes a naive algorithm, but it's not clear whether there is a non-naive implementation. Is it possible to implement this as efficently as the current version of insert?]
[Post Lillehammer: N1780 updated in post meeting mailing with feedback from Lillehammer with more information regarding performance. ]
[ Batavia: 1780 accepted with minor wording changes in the proposed wording (reflected in the proposed resolution below). Concerns about the performance of the algorithm were satisfactorily met by 1780. 371(i) already handles the stability of equal ranges and so that part of the resolution from 1780 is no longer needed (or reflected in the proposed wording below). ]
Proposed resolution:
Change the indicated rows of the "Associative container requirements" Table in 23.2.7 [associative.reqmts] to:
expression | return type | assertion/note pre/post-condition |
complexity |
---|---|---|---|
a_eq.insert(t) |
iterator |
inserts t and returns the iterator pointing to the newly inserted
element. If a range containing elements equivalent to t exists in
a_eq , t is inserted at the end of that range.
|
logarithmic |
a.insert(p,t) |
iterator |
inserts t if and only if there is no element with key equivalent to the
key of t in containers with unique keys; always inserts t in containers
with equivalent keys. always returns the iterator pointing to the element with key
equivalent to the key of t . p is a hint pointing to where
the insert should start to search.t is inserted as close as possible
to the position just prior to p .
|
logarithmic in general, but amortized constant if t is inserted right p .
|
Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In paragraphs 12 and 13 the effects of construct()
and
destruct()
are described as returns but the functions actually
return void
.
Proposed resolution:
Substitute "Returns" by "Effect".
Section: 24.5.1.2 [reverse.iterator] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The declaration of reverse_iterator
lists a default
constructor. However, no specification is given what this constructor
should do.
Proposed resolution:
In section 24.5.1.4 [reverse.iter.cons] add the following paragraph:
reverse_iterator()
Default initializes
current
. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a default constructed iterator of typeIterator
.
[pre-Copenhagen: Dietmar provide wording for proposed resolution.]
Section: 23.3.5.2 [deque.cons] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity specification in paragraph 6 says that the complexity
is linear in first - last
. Even if operator-()
is
defined on iterators this term is in general undefined because it
would have to be last - first
.
Proposed resolution:
Change paragraph 6 from
Linear in first - last.
to become
Linear in distance(first, last).
Section: 31.8.2.2 [stringbuf.cons] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-05-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 27.7.1.1 paragraph 4 the results of calling the constructor of
'basic_stringbuf' are said to be str() == str
. This is fine
that far but consider this code:
std::basic_stringbuf<char> sbuf("hello, world", std::ios_base::openmode(0)); std::cout << "'" << sbuf.str() << "'\n";
Paragraph 3 of 27.7.1.1 basically says that in this case neither
the output sequence nor the input sequence is initialized and
paragraph 2 of 27.7.1.2 basically says that str()
either
returns the input or the output sequence. None of them is initialized,
ie. both are empty, in which case the return from str()
is
defined to be basic_string<cT>()
.
However, probably only test cases in some testsuites will detect this "problem"...
Proposed resolution:
Remove 27.7.1.1 paragraph 4.
Rationale:
We could fix 27.7.1.1 paragraph 4, but there would be no point. If we fixed it, it would say just the same thing as text that's already in the standard.
Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity of unique and unique_copy are inconsistent with each other and inconsistent with the implementations. The standard specifies:
for unique():
-3- Complexity: If the range (last - first) is not empty, exactly (last - first) - 1 applications of the corresponding predicate, otherwise no applications of the predicate.
for unique_copy():
-7- Complexity: Exactly last - first applications of the corresponding predicate.
The implementations do it the other way round: unique() applies the predicate last-first times and unique_copy() applies it last-first-1 times.
As both algorithms use the predicate for pair-wise comparison of sequence elements I don't see a justification for unique_copy() applying the predicate last-first times, especially since it is not specified to which pair in the sequence the predicate is applied twice.
Proposed resolution:
Change both complexity sections in 26.7.9 [alg.unique] to:
Complexity: For nonempty ranges, exactly last - first - 1 applications of the corresponding predicate.
Section: 26.6.10 [alg.adjacent.find] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity section of adjacent_find is defective:
template <class ForwardIterator> ForwardIterator adjacent_find(ForwardIterator first, ForwardIterator last BinaryPredicate pred);-1- Returns: The first iterator i such that both i and i + 1 are in the range [first, last) for which the following corresponding conditions hold: *i == *(i + 1), pred(*i, *(i + 1)) != false. Returns last if no such iterator is found.
-2- Complexity: Exactly find(first, last, value) - first applications of the corresponding predicate.
In the Complexity section, it is not defined what "value" is supposed to mean. My best guess is that "value" means an object for which one of the conditions pred(*i,value) or pred(value,*i) is true, where i is the iterator defined in the Returns section. However, the value type of the input sequence need not be equality-comparable and for this reason the term find(first, last, value) - first is meaningless.
A term such as find_if(first, last, bind2nd(pred,*i)) - first or find_if(first, last, bind1st(pred,*i)) - first might come closer to the intended specification. Binders can only be applied to function objects that have the function call operator declared const, which is not required of predicates because they can have non-const data members. For this reason, a specification using a binder could only be an "as-if" specification.
Proposed resolution:
Change the complexity section in 26.6.10 [alg.adjacent.find] to:
For a nonempty range, exactly
min((i - first) + 1, (last - first) - 1)
applications of the corresponding predicate, where i isadjacent_find
's return value.
[Copenhagen: the original resolution specified an upper bound. The LWG preferred an exact count.]
Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Some popular implementations of unique_copy() create temporary copies of values in the input sequence, at least if the input iterator is a pointer. Such an implementation is built on the assumption that the value type is CopyConstructible and Assignable.
It is common practice in the standard that algorithms explicitly specify any additional requirements that they impose on any of the types used by the algorithm. An example of an algorithm that creates temporary copies and correctly specifies the additional requirements is accumulate(), 29.5.3 [rand.req].
Since the specifications of unique() and unique_copy() do not require CopyConstructible and Assignable of the InputIterator's value type the above mentioned implementations are not standard-compliant. I cannot judge whether this is a defect in the standard or a defect in the implementations.
Proposed resolution:
In 25.2.8 change:
-4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap.
to:
-4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap. The expression *result = *first must be valid. If neither InputIterator nor OutputIterator meets the requirements of forward iterator then the value type of InputIterator must be copy constructible. Otherwise copy constructible is not required.
[Redmond: the original proposed resolution didn't impose an
explicit requirement that the iterator's value type must be copy
constructible, on the grounds that an input iterator's value type must
always be copy constructible. Not everyone in the LWG thought that
this requirement was clear from table 72. It has been suggested that
it might be possible to implement unique_copy
without
requiring assignability, although current implementations do impose
that requirement. Howard provided new wording.]
[ Curaçao: The LWG changed the PR editorially to specify "neither...nor...meet..." as clearer than "both...and...do not meet...". Change believed to be so minor as not to require re-review. ]
Section: 26.7.4 [alg.transform], 29.5 [rand] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The algorithms transform(), accumulate(), inner_product(), partial_sum(), and adjacent_difference() require that the function object supplied to them shall not have any side effects.
The standard defines a side effect in 6.9.1 [intro.execution] as:
-7- Accessing an object designated by a volatile lvalue (basic.lval), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.
As a consequence, the function call operator of a function object supplied to any of the algorithms listed above cannot modify data members, cannot invoke any function that has a side effect, and cannot even create and modify temporary objects. It is difficult to imagine a function object that is still useful under these severe limitations. For instance, any non-trivial transformator supplied to transform() might involve creation and modification of temporaries, which is prohibited according to the current wording of the standard.
On the other hand, popular implementations of these algorithms exhibit uniform and predictable behavior when invoked with a side-effect-producing function objects. It looks like the strong requirement is not needed for efficient implementation of these algorithms.
The requirement of side-effect-free function objects could be replaced by a more relaxed basic requirement (which would hold for all function objects supplied to any algorithm in the standard library):
A function objects supplied to an algorithm shall not invalidate any iterator or sequence that is used by the algorithm. Invalidation of the sequence includes destruction of the sorting order if the algorithm relies on the sorting order (see section 25.3 - Sorting and related operations [lib.alg.sorting]).
I can't judge whether it is intended that the function objects supplied to transform(), accumulate(), inner_product(), partial_sum(), or adjacent_difference() shall not modify sequence elements through dereferenced iterators.
It is debatable whether this issue is a defect or a change request. Since the consequences for user-supplied function objects are drastic and limit the usefulness of the algorithms significantly I would consider it a defect.
Proposed resolution:
Things to notice about these changes:
Change 25.2.3/2 from:
-2- Requires: op and binary_op shall not have any side effects.
to:
-2- Requires: in the ranges [first1, last1], [first2, first2 + (last1 - first1)] and [result, result + (last1- first1)], op and binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 25.2.3/2 from:
-2- Requires: op and binary_op shall not have any side effects.
to:
-2- Requires: op and binary_op shall not invalidate iterators or subranges, or modify elements in the ranges [first1, last1], [first2, first2 + (last1 - first1)], and [result, result + (last1 - first1)]. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.1/2 from:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op shall not cause side effects.
to:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the range [first, last], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of a fully closed range is intentional --end footnote]
Change 26.4.2/2 from:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op1 and binary_op2 shall not cause side effects.
to:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the ranges [first, last] and [first2, first2 + (last - first)], binary_op1 and binary_op2 shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.3/4 from:
-4- Requires: binary_op is expected not to have any side effects.
to:
-4- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.4/2 from:
-2- Requires: binary_op shall not have any side effects.
to:
-2- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
[Toronto: Dave Abrahams supplied wording.]
[Copenhagen: Proposed resolution was modified slightly. Matt added footnotes pointing out that the use of closed ranges was intentional.]
get
and getline
when sentry reports failureSection: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
basic_istream<>::get(), and basic_istream<>::getline(), are unclear with respect to the behavior and side-effects of the named functions in case of an error.
27.6.1.3, p1 states that "... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input..." It is not clear from this (or the rest of the paragraph) what precisely the behavior should be when the sentry ctor exits by throwing an exception or when the sentry object returns false. In particular, what is the number of characters extracted that gcount() returns supposed to be?
27.6.1.3 p8 and p19 say about the effects of get() and getline(): "... In any case, it then stores a null character (using charT()) into the next successive location of the array." Is not clear whether this sentence applies if either of the conditions above holds (i.e., when sentry fails).
Proposed resolution:
Add to 27.6.1.3, p1 after the sentence
"... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input."
the following
"Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array."
Rationale:
Although the general philosophy of the input functions is that the
argument should not be modified upon failure, getline
historically added a terminating null unconditionally. Most
implementations still do that. Earlier versions of the draft standard
had language that made this an unambiguous requirement; those words
were moved to a place where their context made them less clear. See
Jerry Schwarz's message c++std-lib-7618.
vector
, deque::insert
complexitySection: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Lisa Lippincott Opened: 2000-06-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 2 of 23.3.13.5 [vector.modifiers] describes the complexity
of vector::insert
:
Complexity: If first and last are forward iterators, bidirectional iterators, or random access iterators, the complexity is linear in the number of elements in the range [first, last) plus the distance to the end of the vector. If they are input iterators, the complexity is proportional to the number of elements in the range [first, last) times the distance to the end of the vector.
First, this fails to address the non-iterator forms of
insert
.
Second, the complexity for input iterators misses an edge case --
it requires that an arbitrary number of elements can be added at
the end of a vector
in constant time.
I looked to see if deque
had a similar problem, and was
surprised to find that deque
places no requirement on the
complexity of inserting multiple elements (23.3.5.4 [deque.modifiers],
paragraph 3):
Complexity: In the worst case, inserting a single element into a deque takes time linear in the minimum of the distance from the insertion point to the beginning of the deque and the distance from the insertion point to the end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to the copy constructor of T.
Proposed resolution:
Change Paragraph 2 of 23.3.13.5 [vector.modifiers] to
Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.
[For input iterators, one may achieve this complexity by first
inserting at the end of the vector
, and then using
rotate
.]
Change 23.3.5.4 [deque.modifiers], paragraph 3, to:
Complexity: The complexity is linear in the number of elements inserted plus the shorter of the distances to the beginning and end of the deque. Inserting a single element at either the beginning or the end of a deque causes a single call to the copy constructor of T.
Rationale:
This is a real defect, and proposed resolution fixes it: some complexities aren't specified that should be. This proposed resolution does constrain deque implementations (it rules out the most naive possible implementations), but the LWG doesn't see a reason to permit that implementation.
Section: 28.3.4.6 [category.time] Status: CD1 Submitter: Martin Sebor Opened: 2000-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is no requirement that any of time_get member functions set ios::eofbit when they reach the end iterator while parsing their input. Since members of both the num_get and money_get facets are required to do so (22.2.2.1.2, and 22.2.6.1.2, respectively), time_get members should follow the same requirement for consistency.
Proposed resolution:
Add paragraph 2 to section 22.2.5.1 with the following text:
If the end iterator is reached during parsing by any of the get() member functions, the member sets ios_base::eofbit in err.
Rationale:
Two alternative resolutions were proposed. The LWG chose this one because it was more consistent with the way eof is described for other input facets.
Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: Brian Parker Opened: 2000-07-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 23.3.11.5 [list.ops] states that
void splice(iterator position, list<T, Allocator>& x);
invalidates all iterators and references to list x
.
This is unnecessary and defeats an important feature of splice. In
fact, the SGI STL guarantees that iterators to x
remain valid
after splice
.
Proposed resolution:
Add a footnote to 23.3.11.5 [list.ops], paragraph 1:
[Footnote: As specified in [default.con.req], paragraphs 4-5, the semantics described in this clause applies only to the case where allocators compare equal. --end footnote]
In 23.3.11.5 [list.ops], replace paragraph 4 with:
Effects: Inserts the contents of x before position and x becomes empty. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
In 23.3.11.5 [list.ops], replace paragraph 7 with:
Effects: Inserts an element pointed to by i from list x before position and removes the element from x. The result is unchanged if position == i or position == ++i. Pointers and references to *i continue to refer to this same element but as a member of *this. Iterators to *i (including i itself) continue to refer to the same element, but now behave as iterators into *this, not into x.
In 23.3.11.5 [list.ops], replace paragraph 12 with:
Requires: [first, last) is a valid range in x. The result is undefined if position is an iterator in the range [first, last). Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
[pre-Copenhagen: Howard provided wording.]
Rationale:
The original proposed resolution said that iterators and references would remain "valid". The new proposed resolution clarifies what that means. Note that this only applies to the case of equal allocators. From [default.con.req] paragraph 4, the behavior of list when allocators compare nonequal is outside the scope of the standard.
Section: 31.8.2 [stringbuf] Status: CD1 Submitter: Martin Sebor Opened: 2000-07-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis for the template class basic_stringbuf
doesn't list a typedef for the template parameter
Allocator
. This makes it impossible to determine the type of
the allocator at compile time. It's also inconsistent with all other
template classes in the library that do provide a typedef for the
Allocator
parameter.
Proposed resolution:
Add to the synopses of the class templates basic_stringbuf (27.7.1), basic_istringstream (27.7.2), basic_ostringstream (27.7.3), and basic_stringstream (27.7.4) the typedef:
typedef Allocator allocator_type;
Section: 31.8 [string.streams] Status: CD1 Submitter: Martin Sebor Opened: 2000-07-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
27.7.2.2, p1 uses a C-style cast rather than the more appropriate const_cast<> in the Returns clause for basic_istringstream<>::rdbuf(). The same C-style cast is being used in 27.7.3.2, p1, D.7.2.2, p1, and D.7.3.2, p1, and perhaps elsewhere. 27.7.6, p1 and D.7.2.2, p1 are missing the cast altogether.
C-style casts have not been deprecated, so the first part of this issue is stylistic rather than a matter of correctness.
Proposed resolution:
In 27.7.2.2, p1 replace
-1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In 27.7.3.2, p1 replace
-1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In 27.7.6, p1, replace
-1- Returns: &sb
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In D.7.2.2, p1 replace
-2- Returns: &sb.
with
-2- Returns: const_cast<strstreambuf*>(&sb).
Section: 29.6.2.2 [valarray.cons], 29.6.2.3 [valarray.assign] Status: CD1 Submitter: Robert Klarer Opened: 2000-07-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This discussion is adapted from message c++std-lib-7056 posted November 11, 1999. I don't think that anyone can reasonably claim that the problem described below is NAD.
These valarray constructors can never be called:
template <class T> valarray<T>::valarray(const slice_array<T> &); template <class T> valarray<T>::valarray(const gslice_array<T> &); template <class T> valarray<T>::valarray(const mask_array<T> &); template <class T> valarray<T>::valarray(const indirect_array<T> &);
Similarly, these valarray assignment operators cannot be called:
template <class T> valarray<T> valarray<T>::operator=(const slice_array<T> &); template <class T> valarray<T> valarray<T>::operator=(const gslice_array<T> &); template <class T> valarray<T> valarray<T>::operator=(const mask_array<T> &); template <class T> valarray<T> valarray<T>::operator=(const indirect_array<T> &);
Please consider the following example:
#include <valarray> using namespace std; int main() { valarray<double> va1(12); valarray<double> va2(va1[slice(1,4,3)]); // line 1 }
Since the valarray va1 is non-const, the result of the sub-expression va1[slice(1,4,3)] at line 1 is an rvalue of type const std::slice_array<double>. This slice_array rvalue is then used to construct va2. The constructor that is used to construct va2 is declared like this:
template <class T> valarray<T>::valarray(const slice_array<T> &);
Notice the constructor's const reference parameter. When the constructor is called, a slice_array must be bound to this reference. The rules for binding an rvalue to a const reference are in 8.5.3, paragraph 5 (see also 13.3.3.1.4). Specifically, paragraph 5 indicates that a second slice_array rvalue is constructed (in this case copy-constructed) from the first one; it is this second rvalue that is bound to the reference parameter. Paragraph 5 also requires that the constructor that is used for this purpose be callable, regardless of whether the second rvalue is elided. The copy-constructor in this case is not callable, however, because it is private. Therefore, the compiler should report an error.
Since slice_arrays are always rvalues, the valarray constructor that has a parameter of type const slice_array<T> & can never be called. The same reasoning applies to the three other constructors and the four assignment operators that are listed at the beginning of this post. Furthermore, since these functions cannot be called, the valarray helper classes are almost entirely useless.
Proposed resolution:
slice_array:
gslice_array:
mask_array:
indirect_array:
[Proposed resolution was modified in Santa Cruz: explicitly make copy constructor and copy assignment operators public, instead of removing them.]
Rationale:
Keeping the valarray constructors private is untenable. Merely making valarray a friend of the helper classes isn't good enough, because access to the copy constructor is checked in the user's environment.
Making the assignment operator public is not strictly necessary to solve this problem. A majority of the LWG (straw poll: 13-4) believed we should make the assignment operators public, in addition to the copy constructors, for reasons of symmetry and user expectation.
std::string
Section: 19.2 [std.exceptions], 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: Dave Abrahams Opened: 2000-08-01 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
Many of the standard exception types which implementations are required to throw are constructed with a const std::string& parameter. For example:
19.1.5 Class out_of_range [lib.out.of.range] namespace std { class out_of_range : public logic_error { public: explicit out_of_range(const string& what_arg); }; } 1 The class out_of_range defines the type of objects thrown as excep- tions to report an argument value not in its expected range. out_of_range(const string& what_arg); Effects: Constructs an object of class out_of_range. Postcondition: strcmp(what(), what_arg.c_str()) == 0.
There are at least two problems with this:
There may be no cure for (1) other than changing the interface to out_of_range, though one could reasonably argue that (1) is not a defect. Personally I don't care that much if out-of-memory is reported when I only have 20 bytes left, in the case when out_of_range would have been reported. People who use exception-specifications might care a lot, though.
There is a cure for (2), but it isn't completely obvious. I think a note for implementors should be made in the standard. Avoiding possible termination in this case shouldn't be left up to chance. The cure is to use a reference-counted "string" implementation in the exception object. I am not necessarily referring to a std::string here; any simple reference-counting scheme for a NTBS would do.
Further discussion, in email:
...I'm not so concerned about (1). After all, a library implementation can add const char* constructors as an extension, and users don't need to avail themselves of the standard exceptions, though this is a lame position to be forced into. FWIW, std::exception and std::bad_alloc don't require a temporary basic_string.
...I don't think the fixed-size buffer is a solution to the problem,
strictly speaking, because you can't satisfy the postcondition
strcmp(what(), what_arg.c_str()) == 0
For all values of what_arg (i.e. very long values). That means that
the only truly conforming solution requires a dynamic allocation.
Further discussion, from Redmond:
The most important progress we made at the Redmond meeting was
realizing that there are two separable issues here: the const
string& constructor, and the copy constructor. If a user writes
something like throw std::out_of_range("foo")
, the const
string& constructor is invoked before anything gets thrown. The
copy constructor is potentially invoked during stack unwinding.
The copy constructor is a more serious problem, becuase failure
during stack unwinding invokes terminate
. The copy
constructor must be nothrow. Curaçao: Howard thinks this
requirement may already be present.
The fundamental problem is that it's difficult to get the nothrow requirement to work well with the requirement that the exception objects store a string of unbounded size, particularly if you also try to make the const string& constructor nothrow. Options discussed include:
(Not all of these options are mutually exclusive.)
Proposed resolution:
Change 19.2.3 [logic.error]
namespace std { class logic_error : public exception { public: explicit logic_error(const string& what_arg); explicit logic_error(const char* what_arg); }; }...
logic_error(const char* what_arg);
-4- Effects: Constructs an object of class
logic_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.4 [domain.error]
namespace std { class domain_error : public logic_error { public: explicit domain_error(const string& what_arg); explicit domain_error(const char* what_arg); }; }...
domain_error(const char* what_arg);
-4- Effects: Constructs an object of class
domain_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.5 [invalid.argument]
namespace std { class invalid_argument : public logic_error { public: explicit invalid_argument(const string& what_arg); explicit invalid_argument(const char* what_arg); }; }...
invalid_argument(const char* what_arg);
-4- Effects: Constructs an object of class
invalid_argument
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.6 [length.error]
namespace std { class length_error : public logic_error { public: explicit length_error(const string& what_arg); explicit length_error(const char* what_arg); }; }...
length_error(const char* what_arg);
-4- Effects: Constructs an object of class
length_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.7 [out.of.range]
namespace std { class out_of_range : public logic_error { public: explicit out_of_range(const string& what_arg); explicit out_of_range(const char* what_arg); }; }...
out_of_range(const char* what_arg);
-4- Effects: Constructs an object of class
out_of_range
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.8 [runtime.error]
namespace std { class runtime_error : public exception { public: explicit runtime_error(const string& what_arg); explicit runtime_error(const char* what_arg); }; }...
runtime_error(const char* what_arg);
-4- Effects: Constructs an object of class
runtime_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.9 [range.error]
namespace std { class range_error : public runtime_error { public: explicit range_error(const string& what_arg); explicit range_error(const char* what_arg); }; }...
range_error(const char* what_arg);
-4- Effects: Constructs an object of class
range_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.10 [overflow.error]
namespace std { class overflow_error : public runtime_error { public: explicit overflow_error(const string& what_arg); explicit overflow_error(const char* what_arg); }; }...
overflow_error(const char* what_arg);
-4- Effects: Constructs an object of class
overflow_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.11 [underflow.error]
namespace std { class underflow_error : public runtime_error { public: explicit underflow_error(const string& what_arg); explicit underflow_error(const char* what_arg); }; }...
underflow_error(const char* what_arg);
-4- Effects: Constructs an object of class
underflow_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change [ios::failure]
namespace std { class ios_base::failure : public exception { public: explicit failure(const string& msg); explicit failure(const char* msg); virtual const char* what() const throw(); }; }...
failure(const char* msg);
-4- Effects: Constructs an object of class
failure
.-5- Postcondition:
strcmp(what(), msg) == 0
.
Rationale:
Throwing a bad_alloc while trying to construct a message for another exception-derived class is not necessarily a bad thing. And the bad_alloc constructor already has a no throw spec on it (18.4.2.1).
Future:
All involved would like to see const char* constructors added, but this should probably be done for C++0X as opposed to a DR.
I believe the no throw specs currently decorating these functions could be improved by some kind of static no throw spec checking mechanism (in a future C++ language). As they stand, the copy constructors might fail via a call to unexpected. I think what is intended here is that the copy constructors can't fail.
[Pre-Sydney: reopened at the request of Howard Hinnant. Post-Redmond: James Kanze noticed that the copy constructors of exception-derived classes do not have nothrow clauses. Those classes have no copy constructors declared, meaning the compiler-generated implicit copy constructors are used, and those compiler-generated constructors might in principle throw anything.]
[
Batavia: Merged copy constructor and assignment operator spec into exception
and added ios::failure
into the proposed resolution.
]
[
Oxford: The proposed resolution simply addresses the issue of constructing
the exception objects with const char*
and string literals without
the need to explicit include or construct a std::string
.
]
Section: 31.5.4.3 [basic.ios.members] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
27.4.4.2, p17 says
-17- Before copying any parts of rhs, calls each registered callback pair (fn,index) as (*fn)(erase_event,*this,index). After all parts but exceptions() have been replaced, calls each callback pair that was copied from rhs as (*fn)(copy_event,*this,index).
The name copy_event isn't defined anywhere. The intended name was copyfmt_event.
Proposed resolution:
Replace copy_event with copyfmt_event in the named paragraph.
Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Matt Austern Opened: 2000-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
From lib-7752:
I've been assuming (and probably everyone else has been assuming) that allocator instances have a particular property, and I don't think that property can be deduced from anything in Table 32.
I think we have to assume that allocator type conversion is a homomorphism. That is, if x1 and x2 are of type X, where X::value_type is T, and if type Y is X::template rebind<U>::other, then Y(x1) == Y(x2) if and only if x1 == x2.
Further discussion: Howard Hinnant writes, in lib-7757:
I think I can prove that this is not provable by Table 32. And I agree it needs to be true except for the "and only if". If x1 != x2, I see no reason why it can't be true that Y(x1) == Y(x2). Admittedly I can't think of a practical instance where this would happen, or be valuable. But I also don't see a need to add that extra restriction. I think we only need:
if (x1 == x2) then Y(x1) == Y(x2)
If we decide that == on allocators is transitive, then I think I can prove the above. But I don't think == is necessarily transitive on allocators. That is:
Given x1 == x2 and x2 == x3, this does not mean x1 == x3.
Example:
x1 can deallocate pointers from: x1, x2, x3
x2 can deallocate pointers from: x1, x2, x4
x3 can deallocate pointers from: x1, x3
x4 can deallocate pointers from: x2, x4x1 == x2, and x2 == x4, but x1 != x4
[Toronto: LWG members offered multiple opinions. One
opinion is that it should not be required that x1 == x2
implies Y(x1) == Y(x2)
, and that it should not even be
required that X(x1) == x1
. Another opinion is that
the second line from the bottom in table 32 already implies the
desired property. This issue should be considered in light of
other issues related to allocator instances.]
Proposed resolution:
Accept proposed wording from N2436 part 3.
[Lillehammer: Same conclusion as before: this should be considered as part of an allocator redesign, not solved on its own.]
[ Batavia: An allocator redesign is not forthcoming and thus we fixed this one issue. ]
[ Toronto: Reopened at the request of the project editor (Pete) because the proposed wording did not fit within the indicated table. The intent of the resolution remains unchanged. Pablo to work with Pete on improved wording. ]
[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
basic_string::operator[]
and const correctnessSection: 27.4.3.5 [string.capacity] Status: CD1 Submitter: Chris Newton Opened: 2000-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paraphrased from a message that Chris Newton posted to comp.std.c++:
The standard's description of basic_string<>::operator[]
seems to violate const correctness.
The standard (21.3.4/1) says that "If pos < size()
,
returns data()[pos]
." The types don't work. The
return value of data()
is const charT*
, but
operator[]
has a non-const version whose return type is reference
.
Proposed resolution:
In section 21.3.4, paragraph 1, change
"data()[pos]
" to "*(begin() +
pos)
".
istream_iterator::operator++(int)
Section: 24.6.2.3 [istream.iterator.ops] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis of istream_iterator::operator++(int) in 24.5.1 shows it as returning the iterator by value. 24.5.1.2, p5 shows the same operator as returning the iterator by reference. That's incorrect given the Effects clause below (since a temporary is returned). The `&' is probably just a typo.
Proposed resolution:
Change the declaration in 24.5.1.2, p5 from
istream_iterator<T,charT,traits,Distance>& operator++(int);
to
istream_iterator<T,charT,traits,Distance> operator++(int);
(that is, remove the `&').
istream_iterator::operator!=
Section: 24.6.2.3 [istream.iterator.ops] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
24.5.1, p3 lists the synopsis for
template <class T, class charT, class traits, class Distance> bool operator!=(const istream_iterator<T,charT,traits,Distance>& x, const istream_iterator<T,charT,traits,Distance>& y);
but there is no description of what the operator does (i.e., no Effects or Returns clause) in 24.5.1.2.
Proposed resolution:
Add paragraph 7 to the end of section 24.5.1.2 with the following text:
template <class T, class charT, class traits, class Distance> bool operator!=(const istream_iterator<T,charT,traits,Distance>& x, const istream_iterator<T,charT,traits,Distance>& y);
-7- Returns: !(x == y).
Section: 16.3.3.3.3 [bitmask.types] Status: CD1 Submitter: Beman Dawes Opened: 2000-09-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The ~ operation should be applied after the cast to int_type.
Proposed resolution:
Change 17.3.2.1.2 [lib.bitmask.types] operator~ from:
bitmask operator~ ( bitmask X ) { return static_cast< bitmask>(static_cast<int_type>(~ X)); }
to:
bitmask operator~ ( bitmask X ) { return static_cast< bitmask>(~static_cast<int_type>(X)); }
basic_string
reference countingSection: 27.4.3 [basic.string] Status: CD1 Submitter: Kevlin Henney Opened: 2000-09-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The note in paragraph 6 suggests that the invalidation rules for references, pointers, and iterators in paragraph 5 permit a reference- counted implementation (actually, according to paragraph 6, they permit a "reference counted implementation", but this is a minor editorial fix).
However, the last sub-bullet is so worded as to make a reference-counted implementation unviable. In the following example none of the conditions for iterator invalidation are satisfied:
// first example: "*******************" should be printed twice string original = "some arbitrary text", copy = original; const string & alias = original; string::const_iterator i = alias.begin(), e = alias.end(); for(string::iterator j = original.begin(); j != original.end(); ++j) *j = '*'; while(i != e) cout << *i++; cout << endl; cout << original << endl;
Similarly, in the following example:
// second example: "some arbitrary text" should be printed out string original = "some arbitrary text", copy = original; const string & alias = original; string::const_iterator i = alias.begin(); original.begin(); while(i != alias.end()) cout << *i++;
I have tested this on three string implementations, two of which were reference counted. The reference-counted implementations gave "surprising behavior" because they invalidated iterators on the first call to non-const begin since construction. The current wording does not permit such invalidation because it does not take into account the first call since construction, only the first call since various member and non-member function calls.
Proposed resolution:
Change the following sentence in 21.3 paragraph 5 from
Subsequent to any of the above uses except the forms of insert() and erase() which return iterators, the first call to non-const member functions operator[](), at(), begin(), rbegin(), end(), or rend().
to
Following construction or any of the above uses, except the forms of insert() and erase() that return iterators, the first call to non- const member functions operator[](), at(), begin(), rbegin(), end(), or rend().
insert(i, j)
complexity requirements are not feasible.Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: John Potter Opened: 2000-09-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 102
Discussion:
Table 69 requires linear time if [i, j) is sorted. Sorted is necessary but not sufficient. Consider inserting a sorted range of even integers into a set<int> containing the odd integers in the same range.
Proposed resolution:
In Table 69, in section 23.1.2, change the complexity clause for insertion of a range from "N log(size() + N) (N is the distance from i to j) in general; linear if [i, j) is sorted according to value_comp()" to "N log(size() + N), where N is the distance from i to j".
[Copenhagen: Minor fix in proposed resolution: fixed unbalanced parens in the revised wording.]
Rationale:
Testing for valid insertions could be less efficient than simply inserting the elements when the range is not both sorted and between two adjacent existing elements; this could be a QOI issue.
The LWG considered two other options: (a) specifying that the complexity was linear if [i, j) is sorted according to value_comp() and between two adjacent existing elements; or (b) changing to Klog(size() + N) + (N - K) (N is the distance from i to j and K is the number of elements which do not insert immediately after the previous element from [i, j) including the first). The LWG felt that, since we can't guarantee linear time complexity whenever the range to be inserted is sorted, it's more trouble than it's worth to say that it's linear in some special cases.
Section: 22.3 [pairs] Status: CD1 Submitter: Martin Sebor Opened: 2000-09-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I don't see any requirements on the types of the elements of the std::pair container in 20.2.2. From the descriptions of the member functions it appears that they must at least satisfy the requirements of 20.1.3 [lib.copyconstructible] and 20.1.4 [lib.default.con.req], and in the case of the [in]equality operators also the requirements of 20.1.1 [lib.equalitycomparable] and 20.1.2 [lib.lessthancomparable].
I believe that the the CopyConstructible requirement is unnecessary in the case of 20.2.2, p2.
Proposed resolution:
Change the Effects clause in 20.2.2, p2 from
-2- Effects: Initializes its members as if implemented:
pair() : first(T1()), second(T2()) {}
to
-2- Effects: Initializes its members as if implemented:
pair() : first(), second() {}
Rationale:
The existing specification of pair's constructor appears to be a historical artifact: there was concern that pair's members be properly zero-initialized when they are built-in types. At one time there was uncertainty about whether they would be zero-initialized if the default constructor was written the obvious way. This has been clarified by core issue 178, and there is no longer any doubt that the straightforward implementation is correct.
Section: 17.9.4 [bad.exception] Status: CD1 Submitter: Martin Sebor Opened: 2000-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis for std::bad_exception lists the function ~bad_exception() but there is no description of what the function does (the Effects clause is missing).
Proposed resolution:
Remove the destructor from the class synopses of
bad_alloc
(17.6.4.1 [bad.alloc]),
bad_cast
(17.7.4 [bad.cast]),
bad_typeid
(17.7.5 [bad.typeid]),
and bad_exception
(17.9.4 [bad.exception]).
Rationale:
This is a general problem with the exception classes in clause 18. The proposed resolution is to remove the destructors from the class synopses, rather than to document the destructors' behavior, because removing them is more consistent with how exception classes are described in clause 19.
Section: 28.3.3.1 [locale] Status: CD1 Submitter: Martin Sebor Opened: 2000-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis of the class std::locale in 22.1.1 contains two typos: the semicolons after the declarations of the default ctor locale::locale() and the copy ctor locale::locale(const locale&) are missing.
Proposed resolution:
Add the missing semicolons, i.e., change
// construct/copy/destroy: locale() throw() locale(const locale& other) throw()
in the synopsis in 22.1.1 to
// construct/copy/destroy: locale() throw(); locale(const locale& other) throw();
Section: 26.8.4 [alg.binary.search] Status: CD1 Submitter: Matt Austern Opened: 2000-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 472
Discussion:
Each of the four binary search algorithms (lower_bound, upper_bound, equal_range, binary_search) has a form that allows the user to pass a comparison function object. According to 25.3, paragraph 2, that comparison function object has to be a strict weak ordering.
This requirement is slightly too strict. Suppose we are searching through a sequence containing objects of type X, where X is some large record with an integer key. We might reasonably want to look up a record by key, in which case we would want to write something like this:
struct key_comp { bool operator()(const X& x, int n) const { return x.key() < n; } } std::lower_bound(first, last, 47, key_comp());
key_comp is not a strict weak ordering, but there is no reason to prohibit its use in lower_bound.
There's no difficulty in implementing lower_bound so that it allows the use of something like key_comp. (It will probably work unless an implementor takes special pains to forbid it.) What's difficult is formulating language in the standard to specify what kind of comparison function is acceptable. We need a notion that's slightly more general than that of a strict weak ordering, one that can encompass a comparison function that involves different types. Expressing that notion may be complicated.
Additional questions raised at the Toronto meeting:
equal_range
.operator()
?Additional discussion from Copenhagen:
Proposed resolution:
Change 25.3 [lib.alg.sorting] paragraph 3 from:
3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For the algorithms to work correctly, comp has to induce a strict weak ordering on the values.
to:
3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For algorithms other than those described in lib.alg.binary.search (25.3.3) to work correctly, comp has to induce a strict weak ordering on the values.
Add the following paragraph after 25.3 [lib.alg.sorting] paragraph 5:
-6- A sequence [start, finish) is partitioned with respect to an expression f(e) if there exists an integer n such that for all 0 <= i < distance(start, finish), f(*(begin+i)) is true if and only if i < n.
Change 25.3.3 [lib.alg.binary.search] paragraph 1 from:
-1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is in order according to the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.
to:
-1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is partitioned with respect to an expression formed by binding the search key to an argument of the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.
Change 25.3.3.1 [lib.lower.bound] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expression e < value or comp(e, value)
Remove 25.3.3.1 [lib.lower.bound] paragraph 2:
-2- Effects: Finds the first position into which value can be inserted without violating the ordering.
Change 25.3.3.2 [lib.upper.bound] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expression !(value < e) or !comp(value, e)
Remove 25.3.3.2 [lib.upper.bound] paragraph 2:
-2- Effects: Finds the furthermost position into which value can be inserted without violating the ordering.
Change 25.3.3.3 [lib.equal.range] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)
Change 25.3.3.3 [lib.equal.range] paragraph 2 from:
-2- Effects: Finds the largest subrange [i, j) such that the value can be inserted at any iterator k in it without violating the ordering. k satisfies the corresponding conditions: !(*k < value) && !(value < *k) or comp(*k, value) == false && comp(value, *k) == false.
to:
-2- Returns: make_pair(lower_bound(first, last, value), upper_bound(first, last, value)) or make_pair(lower_bound(first, last, value, comp), upper_bound(first, last, value, comp))
Change 25.3.3.3 [lib.binary.search] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)
[Copenhagen: Dave Abrahams provided this wording]
[Redmond: Minor changes in wording. (Removed "non-negative", and changed the "other than those described in" wording.) Also, the LWG decided to accept the "optional" part.]
Rationale:
The proposed resolution reinterprets binary search. Instead of thinking about searching for a value in a sorted range, we view that as an important special case of a more general algorithm: searching for the partition point in a partitioned range.
We also add a guarantee that the old wording did not: we ensure that the upper bound is no earlier than the lower bound, that the pair returned by equal_range is a valid range, and that the first part of that pair is the lower bound.
Section: 31.7.5.7 [iostreamclass] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Class template basic_iostream has no typedefs. The typedefs it inherits from its base classes can't be used, since (for example) basic_iostream<T>::traits_type is ambiguous.
Proposed resolution:
Add the following to basic_iostream's class synopsis in
31.7.5.7 [iostreamclass], immediately after public
:
// types: typedef charT char_type; typedef typename traits::int_type int_type; typedef typename traits::pos_type pos_type; typedef typename traits::off_type off_type; typedef traits traits_type;
Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 569
Discussion:
27.4.4.3, p4 says about the postcondition of the function: If rdbuf()!=0 then state == rdstate(); otherwise rdstate()==state|ios_base::badbit.
The expression on the right-hand-side of the operator==() needs to be parenthesized in order for the whole expression to ever evaluate to anything but non-zero.
Proposed resolution:
Add parentheses like so: rdstate()==(state|ios_base::badbit).
Section: 31 [input.output] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
27.5.2.4.2, p4, and 27.8.1.6, p2, 27.8.1.7, p3, 27.8.1.9, p2, 27.8.1.10, p3 refer to in and/or out w/o ios_base:: qualification. That's incorrect since the names are members of a dependent base class (14.6.2 [temp.dep]) and thus not visible.
Proposed resolution:
Qualify the names with the name of the class of which they are members, i.e., ios_base.
Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I see that table 31 in 20.1.5, p3 allows T in std::allocator<T> to be of any type. But the synopsis in 20.4.1 calls for allocator<>::address() to be overloaded on reference and const_reference, which is ill-formed for all T = const U. In other words, this won't work:
template class std::allocator<const int>;
The obvious solution is to disallow specializations of allocators on const types. However, while containers' elements are required to be assignable (which rules out specializations on const T's), I think that allocators might perhaps be potentially useful for const values in other contexts. So if allocators are to allow const types a partial specialization of std::allocator<const T> would probably have to be provided.
Proposed resolution:
Change the text in row 1, column 2 of table 32 in 20.1.5, p3 from
any type
to
any non-const, non-reference type
[Redmond: previous proposed resolution was "any non-const, non-volatile, non-reference type". Got rid of the "non-volatile".]
Rationale:
Two resolutions were originally proposed: one that partially specialized std::allocator for const types, and one that said an allocator's value type may not be const. The LWG chose the second. The first wouldn't be appropriate, because allocators are intended for use by containers, and const value types don't work in containers. Encouraging the use of allocators with const value types would only lead to unsafe code.
The original text for proposed resolution 2 was modified so that it also forbids volatile types and reference types.
[Curaçao: LWG double checked and believes volatile is correctly excluded from the PR.]
Section: 28.3.4.3.2.2 [facet.num.get.members] Status: CD1 Submitter: Matt Austern Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 22.2.2.1.1, we have a list of overloads for num_get<>::get(). There are eight overloads, all of which are identical except for the last parameter. The overloads are:
There is a similar list, in 22.2.2.1.2, of overloads for num_get<>::do_get(). In this list, the last parameter has the types:
These two lists are not identical. They should be, since
get
is supposed to call do_get
with exactly
the arguments it was given.
Proposed resolution:
In 28.3.4.3.2.2 [facet.num.get.members], change
iter_type get(iter_type in, iter_type end, ios_base& str, ios_base::iostate& err, short& val) const;
to
iter_type get(iter_type in, iter_type end, ios_base& str, ios_base::iostate& err, float& val) const;
Section: 23.2 [container.requirements] Status: CD1 Submitter: Peter Dimov Opened: 2000-11-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.1/3 states that the objects stored in a container must be Assignable. 23.4.3 [map], paragraph 2, states that map satisfies all requirements for a container, while in the same time defining value_type as pair<const Key, T> - a type that is not Assignable.
It should be noted that there exists a valid and non-contradictory interpretation of the current text. The wording in 23.1/3 avoids mentioning value_type, referring instead to "objects stored in a container." One might argue that map does not store objects of type map::value_type, but of map::mapped_type instead, and that the Assignable requirement applies to map::mapped_type, not map::value_type.
However, this makes map a special case (other containers store objects of type value_type) and the Assignable requirement is needlessly restrictive in general.
For example, the proposed resolution of active library issue 103(i) is to make set::iterator a constant iterator; this means that no set operations can exploit the fact that the stored objects are Assignable.
This is related to, but slightly broader than, closed issue 140(i).
Proposed resolution:
23.1/3: Strike the trailing part of the sentence:
, and the additional requirements of Assignable types from 23.1/3
so that it reads:
-3- The type of objects stored in these components must meet the requirements of CopyConstructible types (lib.copyconstructible).
23.1/4: Modify to make clear that this requirement is not for all containers. Change to:
-4- Table 64 defines the Assignable requirement. Some containers require this property of the types to be stored in the container. T is the type used to instantiate the container. t is a value of T, and u is a value of (possibly const) T.
23.1, Table 65: in the first row, change "T is Assignable" to "T is CopyConstructible".
23.2.1/2: Add sentence for Assignable requirement. Change to:
-2- A deque satisfies all of the requirements of a container and of a reversible container (given in tables in lib.container.requirements) and of a sequence, including the optional sequence requirements (lib.sequence.reqmts). In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on deque that are not described in one of these tables or for operations where there is additional semantic information.
23.2.2/2: Add Assignable requirement to specific methods of list. Change to:
-2- A list satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the the optional sequence requirements (lib.sequence.reqmts). The exceptions are the operator[] and at member functions, which are not provided. [Footnote: These member functions are only provided by containers whose iterators are random access iterators. --- end foonote]
list does not require the stored type T to be Assignable unless the following methods are instantiated: [Footnote: Implementors are permitted but not required to take advantage of T's Assignable properties for these methods. -- end foonote]
list<T,Allocator>& operator=(const list<T,Allocator>& x ); template <class InputIterator> void assign(InputIterator first, InputIterator last); void assign(size_type n, const T& t);Descriptions are provided here only for operations on list that are not described in one of these tables or for operations where there is additional semantic information.
23.2.4/2: Add sentence for Assignable requirement. Change to:
-2- A vector satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the optional sequence requirements (lib.sequence.reqmts). The exceptions are the push_front and pop_front member functions, which are not provided. In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on vector that are not described in one of these tables or for operations where there is additional semantic information.
Rationale:
list, set, multiset, map, multimap are able to store non-Assignables.
However, there is some concern about list<T>
:
although in general there's no reason for T to be Assignable, some
implementations of the member functions operator=
and
assign
do rely on that requirement. The LWG does not want
to forbid such implementations.
Note that the type stored in a standard container must still satisfy the requirements of the container's allocator; this rules out, for example, such types as "const int". See issue 274(i) for more details.
In principle we could also relax the "Assignable" requirement for
individual vector
member functions, such as
push_back
. However, the LWG did not see great value in such
selective relaxation. Doing so would remove implementors' freedom to
implement vector::push_back
in terms of
vector::insert
.
Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: P.J. Plauger Opened: 2000-11-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 23.3.11.5 [list.ops] states that
void splice(iterator position, list<T, Allocator>& x);
invalidates all iterators and references to list x
.
But what does the C++ Standard mean by "invalidate"? You can still dereference the iterator to a spliced list element, but you'd better not use it to delimit a range within the original list. For the latter operation, it has definitely lost some of its validity.
If we accept the proposed resolution to issue 250(i), then we'd better clarify that a "valid" iterator need no longer designate an element within the same container as it once did. We then have to clarify what we mean by invalidating a past-the-end iterator, as when a vector or string grows by reallocation. Clearly, such an iterator has a different kind of validity. Perhaps we should introduce separate terms for the two kinds of "validity."
Proposed resolution:
Add the following text to the end of section 24.3.4 [iterator.concepts], after paragraph 5:
An invalid iterator is an iterator that may be singular. [Footnote: This definition applies to pointers, since pointers are iterators. The effect of dereferencing an iterator that has been invalidated is undefined.]
[post-Copenhagen: Matt provided wording.]
[Redmond: General agreement with the intent, some objections to the wording. Dave provided new wording.]
Rationale:
This resolution simply defines a term that the Standard uses but never defines, "invalid", in terms of a term that is defined, "singular".
Why do we say "may be singular", instead of "is singular"? That's becuase a valid iterator is one that is known to be nonsingular. Invalidating an iterator means changing it in such a way that it's no longer known to be nonsingular. An example: inserting an element into the middle of a vector is correctly said to invalidate all iterators pointing into the vector. That doesn't necessarily mean they all become singular.
Section: 24.5.1 [reverse.iterators] Status: CD1 Submitter: Steve Cleary Opened: 2000-11-27 Last modified: 2020-03-29
Priority: Not Prioritized
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Discussion:
This came from an email from Steve Cleary to Fergus in reference to issue 179(i). The library working group briefly discussed this in Toronto and believed it should be a separate issue. There was also some reservations about whether this was a worthwhile problem to fix.
Steve said: "Fixing reverse_iterator
. std::reverse_iterator
can
(and should) be changed to preserve these additional
requirements." He also said in email that it can be done without
breaking user's code: "If you take a look at my suggested
solution, reverse_iterator doesn't have to take two parameters; there
is no danger of breaking existing code, except someone taking the
address of one of the reverse_iterator global operator functions, and
I have to doubt if anyone has ever done that. . . But, just in
case they have, you can leave the old global functions in as well --
they won't interfere with the two-template-argument functions. With
that, I don't see how any user code could break."
Proposed resolution:
Section: 24.5.1.2 [reverse.iterator] add/change the following declarations:
A) Add a templated assignment operator, after the same manner as the templated copy constructor, i.e.: template < class U > reverse_iterator < Iterator >& operator=(const reverse_iterator< U >& u); B) Make all global functions (except the operator+) have two template parameters instead of one, that is, for operator ==, !=, <, >, <=, >=, - replace: template < class Iterator > typename reverse_iterator< Iterator >::difference_type operator-( const reverse_iterator< Iterator >& x, const reverse_iterator< Iterator >& y); with: template < class Iterator1, class Iterator2 > typename reverse_iterator < Iterator1 >::difference_type operator-( const reverse_iterator < Iterator1 > & x, const reverse_iterator < Iterator2 > & y);
Also make the addition/changes for these signatures in [reverse.iter.ops].
[ Copenhagen: The LWG is concerned that the proposed resolution introduces new overloads. Experience shows that introducing overloads is always risky, and that it would be inappropriate to make this change without implementation experience. It may be desirable to provide this feature in a different way. ]
[ Lillehammer: We now have implementation experience, and agree that this solution is safe and correct. ]
[2020-03-29; Jonathan Wakely comments]
The issue title is misleading, it is not about comparing to const
-qualified
reverse_iterator
s, but comparing to reverse_iterator<const-iterator>
.
Section: 26.8.9 [alg.min.max] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 486
Discussion:
The requirements in 25.3.7, p1 and 4 call for T to satisfy the
requirements of LessThanComparable
( [lessthancomparable])
and CopyConstructible
(16.4.4.2 [utility.arg.requirements]).
Since the functions take and return their arguments and result by
const reference, I believe the CopyConstructible
requirement
is unnecessary.
Proposed resolution:
Remove the CopyConstructible
requirement. Specifically, replace
25.3.7, p1 with
-1- Requires: Type T is LessThanComparable
( [lessthancomparable]).
and replace 25.3.7, p4 with
-4- Requires: Type T is LessThanComparable
( [lessthancomparable]).
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: Howard Hinnant Opened: 2000-12-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 16 mistakenly singles out integral types for inserting thousands_sep() characters. This conflicts with the syntax for floating point numbers described under 22.2.3.1/2.
Proposed resolution:
Change paragraph 16 from:
For integral types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 28.3.4.4.1.3 [facet.numpunct.virtuals].
To:
For arithmetic types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 28.3.4.4.1.3 [facet.numpunct.virtuals].
[ Copenhagen: Opinions were divided about whether this is actually an inconsistency, but at best it seems to have been unintentional. This is only an issue for floating-point output: The standard is unambiguous that implementations must parse thousands_sep characters when performing floating-point. The standard is also unambiguous that this requirement does not apply to the "C" locale. ]
[ A survey of existing practice is needed; it is believed that some implementations do insert thousands_sep characters for floating-point output and others fail to insert thousands_sep characters for floating-point input even though this is unambiguously required by the standard. ]
[Post-Curaçao: the above proposed resolution is the consensus of Howard, Bill, Pete, Benjamin, Nathan, Dietmar, Boris, and Martin.]
Section: 26.7.5 [alg.replace] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 483
Discussion:
(revision of the further discussion) There are a number of problems with the requires clauses for the algorithms in 25.1 and 25.2. The requires clause of each algorithm should describe the necessary and sufficient requirements on the inputs to the algorithm such that the algorithm compiles and runs properly. Many of the requires clauses fail to do this. Here is a summary of the kinds of mistakes:
Here is the list of algorithms that contain mistakes:
Also, in the requirements for EqualityComparable, the requirement that the operator be defined for const objects is lacking.
Proposed resolution:
20.1.1 Change p1 from
In Table 28, T
is a type to be supplied by a C++ program
instantiating a template, a
, b
, and c
are
values of type T
.
to
In Table 28, T
is a type to be supplied by a C++ program
instantiating a template, a
, b
, and c
are
values of type const T
.
25 Between p8 and p9
Add the following sentence:
When the description of an algorithm gives an expression such as
*first == value
for a condition, it is required that the expression
evaluate to either true or false in boolean contexts.
25.1.2 Change p1 by deleting the requires clause.
25.1.6 Change p1 by deleting the requires clause.
25.1.9
Change p4 from
-4- Requires: Type T
is EqualityComparable
(20.1.1), type Size is convertible to integral type (4.7.12.3).
to
-4- Requires: The type Size
is convertible to integral
type (4.7.12.3).
25.2.4 Change p1 from
-1- Requires: Type T
is Assignable
(23.1 ) (and, for replace()
, EqualityComparable
(20.1.1 )).
to
-1- Requires: The expression *first = new_value
must be valid.
and change p4 from
-4- Requires: Type T
is Assignable
(23.1) (and,
for replace_copy()
, EqualityComparable
(20.1.1)). The ranges [first, last)
and [result, result +
(last - first))
shall not overlap.
to
-4- Requires: The results of the expressions *first
and
new_value
must be writable to the result output iterator. The
ranges [first, last)
and [result, result + (last -
first))
shall not overlap.
25.2.5 Change p1 from
-1- Requires: Type T
is Assignable
(23.1). The
type Size
is convertible to an integral type (4.7.12.3).
to
-1- Requires: The expression value
must be is writable to
the output iterator. The type Size
is convertible to an
integral type (4.7.12.3).
25.2.7 Change p1 from
-1- Requires: Type T
is EqualityComparable
(20.1.1).
to
-1- Requires: The value type of the iterator must be
Assignable
(23.1).
Rationale:
The general idea of the proposed solution is to remove the faulty requires clauses and let the returns and effects clauses speak for themselves. That is, the returns clauses contain expressions that must be valid, and therefore already imply the correct requirements. In addition, a sentence is added at the beginning of chapter 25 saying that expressions given as conditions must evaluate to true or false in a boolean context. An alternative would be to say that the type of these condition expressions must be literally bool, but that would be imposing a greater restriction that what the standard currently says (which is convertible to bool).
Section: 22.10.8 [comparisons] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The example in 22.10.8 [comparisons], p6 shows how to use the C
library function strcmp()
with the function pointer adapter
ptr_fun()
. But since it's unspecified whether the C library
functions have extern "C"
or extern
"C++"
linkage [16.4.3.3 [using.linkage]], and since
function pointers with different the language linkage specifications
(9.12 [dcl.link]) are incompatible, whether this example is
well-formed is unspecified.
Proposed resolution:
Change 22.10.8 [comparisons] paragraph 6 from:
[Example:
replace_if(v.begin(), v.end(), not1(bind2nd(ptr_fun(strcmp), "C")), "C++");replaces each
C
withC++
in sequencev
.
to:
[Example:
int compare(const char*, const char*); replace_if(v.begin(), v.end(), not1(bind2nd(ptr_fun(compare), "abc")), "def");replaces each
abc
withdef
in sequencev
.
Also, remove footnote 215 in that same paragraph.
[Copenhagen: Minor change in the proposed resolution. Since this issue deals in part with C and C++ linkage, it was believed to be too confusing for the strings in the example to be "C" and "C++". ]
[Redmond: More minor changes. Got rid of the footnote (which seems to make a sweeping normative requirement, even though footnotes aren't normative), and changed the sentence after the footnote so that it corresponds to the new code fragment.]
Section: 31.10.4.2 [ifstream.cons] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
31.10.4.2 [ifstream.cons], p2, 31.10.5.2 [ofstream.cons], p2, and 31.10.6.2 [fstream.cons], p2 say about the effects of each constructor:
... If that function returns a null pointer, calls
setstate(failbit)
(which may throw ios_base::failure
).
The parenthetical note doesn't apply since the ctors cannot throw an
exception due to the requirement in 31.5.4.2 [basic.ios.cons], p3
that exceptions()
be initialized to ios_base::goodbit
.
Proposed resolution:
Strike the parenthetical note from the Effects clause in each of the paragraphs mentioned above.
Section: 26.13 [alg.c.library] Status: CD1 Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The <cstdlib> header file contains prototypes for bsearch and qsort (C++ Standard section 25.4 paragraphs 3 and 4) and other prototypes (C++ Standard section 21.4 paragraph 1 table 49) that require the typedef size_t. Yet size_t is not listed in the <cstdlib> synopsis table 78 in section 25.4.
Proposed resolution:
Add the type size_t to Table 78 (section 25.4) and add the type size_t <cstdlib> to Table 97 (section C.2).
Rationale:
Since size_t is in <stdlib.h>, it must also be in <cstdlib>.
Section: 19.4 [errno] Status: CD1 Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
ISO/IEC 9899:1990/Amendment1:1994 Section 4.3 States: "The list of macros defined in <errno.h> is adjusted to include a new macro, EILSEQ"
ISO/IEC 14882:1998(E) section 19.3 does not refer to the above amendment.
Proposed resolution:
Update Table 26 (section 19.3) "Header <cerrno> synopsis" and Table 95 (section C.2) "Standard Macros" to include EILSEQ.
Section: 26.8.7 [alg.set.operations] Status: CD1 Submitter: Matt Austern Opened: 2001-01-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard library contains four algorithms that compute set
operations on sorted ranges: set_union
, set_intersection
,
set_difference
, and set_symmetric_difference
. Each
of these algorithms takes two sorted ranges as inputs, and writes the
output of the appropriate set operation to an output range. The elements
in the output range are sorted.
The ordinary mathematical definitions are generalized so that they
apply to ranges containing multiple copies of a given element. Two
elements are considered to be "the same" if, according to an
ordering relation provided by the user, neither one is less than the
other. So, for example, if one input range contains five copies of an
element and another contains three, the output range of set_union
will contain five copies, the output range of
set_intersection
will contain three, the output range of
set_difference
will contain two, and the output range of
set_symmetric_difference
will contain two.
Because two elements can be "the same" for the purposes of these set algorithms, without being identical in other respects (consider, for example, strings under case-insensitive comparison), this raises a number of unanswered questions:
The standard should either answer these questions, or explicitly say that the answers are unspecified. I prefer the former option, since, as far as I know, all existing implementations behave the same way.
Proposed resolution:
Add the following to the end of 26.8.7.3 [set.union] paragraph 5:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then max(m, n) of these elements will be copied to the output range: all m of these elements from [first1, last1), and the last max(n-m, 0) of them from [first2, last2), in that order.
Add the following to the end of 26.8.7.4 [set.intersection] paragraph 5:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the first min(m, n) of those elements from [first1, last1) are copied to the output range.
Add a new paragraph, Notes, after 26.8.7.5 [set.difference] paragraph 4:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the last max(m-n, 0) elements from [first1, last1) are copied to the output range.
Add a new paragraph, Notes, after 26.8.7.6 [set.symmetric.difference] paragraph 4:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then |m - n| of those elements will be copied to the output range: the last m - n of these elements from [first1, last1) if m > n, and the last n - m of these elements from [first2, last2) if m < n.
[Santa Cruz: it's believed that this language is clearer than what's in the Standard. However, it's also believed that the Standard may already make these guarantees (although not quite in these words). Bill and Howard will check and see whether they think that some or all of these changes may be redundant. If so, we may close this issue as NAD.]
Rationale:
For simple cases, these descriptions are equivalent to what's already in the Standard. For more complicated cases, they describe the behavior of existing implementations.
Section: 31.5.4.3 [basic.ios.members] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [basic.ios.members].
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Discussion:
The Effects clause of the member function copyfmt()
in
27.4.4.2, p15 doesn't consider the case where the left-hand side
argument is identical to the argument on the right-hand side, that is
(this == &rhs)
. If the two arguments are identical there
is no need to copy any of the data members or call any callbacks
registered with register_callback()
. Also, as Howard Hinnant
points out in message c++std-lib-8149 it appears to be incorrect to
allow the object to fire erase_event
followed by
copyfmt_event
since the callback handling the latter event
may inadvertently attempt to access memory freed by the former.
Proposed resolution:
Change the Effects clause in 27.4.4.2, p15 from
-15- Effects:Assigns to the member objects of
*this
the corresponding member objects ofrhs
, except that...
to
-15- Effects:If
(this == &rhs)
does nothing. Otherwise assigns to the member objects of*this
the corresponding member objects ofrhs
, except that...
Section: 16.4.5.3.3 [macro.names] Status: CD1 Submitter: James Kanze Opened: 2001-01-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 2 of 16.4.5.3.3 [macro.names] reads: "A translation unit that includes a header shall not contain any macros that define names declared in that header." As I read this, it would mean that the following program is legal:
#define npos 3.14 #include <sstream>
since npos is not defined in <sstream>. It is, however, defined in <string>, and it is hard to imagine an implementation in which <sstream> didn't include <string>.
I think that this phrase was probably formulated before it was decided that a standard header may freely include other standard headers. The phrase would be perfectly appropriate for C, for example. In light of 16.4.6.2 [res.on.headers] paragraph 1, however, it isn't stringent enough.
Proposed resolution:
For 16.4.5.3.3 [macro.names], replace the current wording, which reads:
Each name defined as a macro in a header is reserved to the implementation for any use if the translation unit includes the header.168)
A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.
168) It is not permissible to remove a library macro definition by using the #undef directive.
with the wording:
A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.
A translation unit shall not #define or #undef names lexically identical to keywords.
[Lillehammer: Beman provided new wording]
Section: 29.7 [c.math] Status: CD1 Submitter: Jens Maurer Opened: 2001-01-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 80 lists the contents of the <cmath> header. It does not
list abs()
. However, 26.5, paragraph 6, which lists added
signatures present in <cmath>, does say that several overloads
of abs()
should be defined in <cmath>.
Proposed resolution:
Add abs
to Table 80. Also, remove the parenthetical list
of functions "(abs(), div(), rand(), srand())" from 29.6 [numarray],
paragraph 1.
[Copenhagen: Modified proposed resolution so that it also gets rid of that vestigial list of functions in paragraph 1.]
Rationale:
All this DR does is fix a typo; it's uncontroversial. A separate question is whether we're doing the right thing in putting some overloads in <cmath> that we aren't also putting in <cstdlib>. That's issue 323(i).
Section: 22.3 [pairs] Status: C++11 Submitter: Martin Sebor Opened: 2001-01-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis of the header <utility>
in 22.2 [utility]
lists the complete set of equality and relational operators for pair
but the section describing the template and the operators only describes
operator==()
and operator<()
, and it fails to mention
any requirements on the template arguments. The remaining operators are
not mentioned at all.
[ 2009-09-27 Alisdair reopens. ]
The issue is a lack of wording specifying the semantics of
std::pair
relational operators. The rationale is that this is covered by catch-all wording in the relops component, and that as relops directly precedespair
in the document this is an easy connection to make.Reading the current working paper I make two observations:
- relops no longer immediately precedes
pair
in the order of specification. However, even if it did, there is a lot ofpair
specification itself between the (apparently) unrelated relops and the relational operators forpair
. (The catch-all still requiresoperator==
andoperator<
to be specified explicitly)- No other library component relies on the catch-all clause. The following all explicitly document all six relational operators, usually in a manner that could have deferred to the relops clause.
tuple unique_ptr duration time_point basic_string queue stack move_iterator reverse_iterator regex submatch thread::idThe container components provide their own (equivalent) definition in 23.2.2 [container.requirements.general] Table 90 -- Container requirements and do so do not defer to relops.
Shared_ptr
explicitly documentsoperator!=
and does not supply the other 3 missing operators (>
,>=
,<=
) so does not meet the reqirements of the relops clause.
Weak_ptr
only supportsoperator<
so would not be covered by relops.At the very least I would request a note pointing to the relops clause we rely on to provide this definition. If this route is taken, I would recommend reducing many of the above listed clauses to a similar note rather than providing redundant specification.
My preference would be to supply the 4 missing specifications consistent with the rest of the library.
[
2009-10-11 Daniel opens 1233(i) which deals with the same issue as
it pertains to unique_ptr
.
]
[ 2009-10 Santa Cruz: ]
Move to Ready
Proposed resolution:
After p20 22.3 [pairs] add:
template <class T1, class T2> bool operator!=(const pair<T1,T2>& x, const pair<T1,T2>& y);Returns:
!(x==y)
template <class T1, class T2> bool operator> (const pair<T1,T2>& x, const pair<T1,T2>& y);Returns:
y < x
template <class T1, class T2> bool operator>=(const pair<T1,T2>& x, const pair<T1,T2>& y);Returns:
!(x < y)
template <class T1, class T2> bool operator<=(const pair<T1,T2>& x, const pair<T1,T2>& y);Returns:
!(y < x)
Rationale:
[operators] paragraph 10 already specifies the semantics.
That paragraph says that, if declarations of operator!=, operator>,
operator<=, and operator>= appear without definitions, they are
defined as specified in [operators]. There should be no user
confusion, since that paragraph happens to immediately precede the
specification of pair
.
Section: 22.10.10 [logical.operations] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The class templates const_mem_fun_t
in 20.5.8, p8 and
const_mem_fun1_t
in 20.5.8, p9 derive from unary_function<T*, S>
, and
binary_function<T*,
A, S>
, respectively. Consequently, their argument_type
, and
first_argument_type
members, respectively, are both defined to be T*
(non-const).
However, their function call member operator takes a const T*
argument. It is my opinion that argument_type
should be const
T*
instead, so that one can easily refer to it in generic code. The
example below derived from existing code fails to compile due to the
discrepancy:
template <class T>
void foo (typename T::argument_type arg) // #1
{
typename T::result_type (T::*pf) (typename
T::argument_type)
const = // #2
&T::operator();
}
struct X { /* ... */ };
int main ()
{
const X x;
foo<std::const_mem_fun_t<void, X>
>(&x);
// #3
}
#1 foo()
takes a plain unqualified X*
as an argument
#2 the type of the pointer is incompatible with the type of the member
function
#3 the address of a constant being passed to a function taking a non-const
X*
Proposed resolution:
Replace the top portion of the definition of the class template const_mem_fun_t in 20.5.8, p8
template <class S, class T> class const_mem_fun_t
: public
unary_function<T*, S> {
with
template <class S, class T> class const_mem_fun_t
: public
unary_function<const T*, S> {
Also replace the top portion of the definition of the class template const_mem_fun1_t in 20.5.8, p9
template <class S, class T, class A> class const_mem_fun1_t
: public
binary_function<T*, A, S> {
with
template <class S, class T, class A> class const_mem_fun1_t
: public
binary_function<const T*, A, S> {
Rationale:
This is simply a contradiction: the argument_type
typedef,
and the argument type itself, are not the same.
Section: 17.6.3.3 [new.delete.array] Status: CD1 Submitter: John A. Pedretti Opened: 2001-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The default behavior of operator delete[]
described in 18.5.1.2, p12 -
namely that for non-null value of ptr, the operator reclaims storage
allocated by the earlier call to the default operator new[]
- is not
correct in all cases. Since the specified operator new[]
default
behavior is to call operator new
(18.5.1.2, p4, p8), which can be
replaced, along with operator delete
, by the user, to implement their
own memory management, the specified default behavior of operator
delete[]
must be to call operator delete
.
Proposed resolution:
Change 18.5.1.2, p12 from
-12- Default behavior:
- For a null value of
ptr
, does nothing.- Any other value of
ptr
shall be a value returned earlier by a call to the defaultoperator new[](std::size_t)
. [Footnote: The value must not have been invalidated by an intervening call tooperator delete[](void*)
(16.4.5.9 [res.on.arguments]). --- end footnote] For such a non-null value ofptr
, reclaims storage allocated by the earlier call to the defaultoperator new[]
.
to
-12- Default behavior: Calls
operator delete(
ptr) oroperator delete(ptr, std::nothrow)
respectively.
and expunge paragraph 13.
Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: John Pedretti Opened: 2001-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [list.ops].
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Discussion:
The "Effects" clause for list::merge() (23.3.11.5 [list.ops], p23) appears to be incomplete: it doesn't cover the case where the argument list is identical to *this (i.e., this == &x). The requirement in the note in p24 (below) is that x be empty after the merge which is surely unintended in this case.
Proposed resolution:
In 23.3.11.5 [list.ops], replace paragraps 23-25 with:
23 Effects: if (&x == this) does nothing; otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()). The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i in the range other than the first, the condition comp(*i, *(i - 1)) will be false.
24 Notes: Stable: if (&x != this), then for equivalent elements in the two original ranges, the elements from the original range [begin(), end()) always precede the elements from the original range [x.begin(), x.end()). If (&x != this) the range [x.begin(), x.end()) is empty after the merge.
25 Complexity: At most size() + x.size() - 1 applications of comp if (&x ! = this); otherwise, no applications of comp are performed. If an exception is thrown other than by a comparison there are no effects.
[Copenhagen: The original proposed resolution did not fix all of
the problems in 23.3.11.5 [list.ops], p22-25. Three different
paragraphs (23, 24, 25) describe the effects of merge
.
Changing p23, without changing the other two, appears to introduce
contradictions. Additionally, "merges the argument list into the
list" is excessively vague.]
[Post-Curaçao: Robert Klarer provided new wording.]
Section: 27.4.3.2 [string.require] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The effects clause for the basic_string template ctor in 21.3.1, p15 leaves out the third argument of type Allocator. I believe this to be a mistake.
Proposed resolution:
Replace
-15- Effects: If
InputIterator
is an integral type, equivalent to
basic_string(static_cast<size_type>(begin), static_cast<value_type>(end))
with
-15- Effects: If
InputIterator
is an integral type, equivalent to
basic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)
Section: 22.9.4 [bitset.operators] Status: CD1 Submitter: Matt Austern Opened: 2001-02-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.operators].
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Discussion:
In 23.3.5.3, we are told that bitset
's input operator
"Extracts up to N (single-byte) characters from
is.", where is is a stream of type
basic_istream<charT, traits>
.
The standard does not say what it means to extract single byte
characters from a stream whose character type, charT
, is in
general not a single-byte character type. Existing implementations
differ.
A reasonable solution will probably involve widen()
and/or
narrow()
, since they are the supplied mechanism for
converting a single character between char
and
arbitrary charT
.
Narrowing the input characters is not the same as widening the
literals '0'
and '1'
, because there may be some
locales in which more than one wide character maps to the narrow
character '0'
. Narrowing means that alternate
representations may be used for bitset input, widening means that
they may not be.
Note that for numeric input, num_get<>
(22.2.2.1.2/8) compares input characters to widened version of narrow
character literals.
From Pete Becker, in c++std-lib-8224:
Different writing systems can have different representations for the digits that represent 0 and 1. For example, in the Unicode representation of the Devanagari script (used in many of the Indic languages) the digit 0 is 0x0966, and the digit 1 is 0x0967. Calling narrow would translate those into '0' and '1'. But Unicode also provides the ASCII values 0x0030 and 0x0031 for for the Latin representations of '0' and '1', as well as code points for the same numeric values in several other scripts (Tamil has no character for 0, but does have the digits 1-9), and any of these values would also be narrowed to '0' and '1'.
...
It's fairly common to intermix both native and Latin representations of numbers in a document. So I think the rule has to be that if a wide character represents a digit whose value is 0 then the bit should be cleared; if it represents a digit whose value is 1 then the bit should be set; otherwise throw an exception. So in a Devanagari locale, both 0x0966 and 0x0030 would clear the bit, and both 0x0967 and 0x0031 would set it. Widen can't do that. It would pick one of those two values, and exclude the other one.
From Jens Maurer, in c++std-lib-8233:
Whatever we decide, I would find it most surprising if bitset conversion worked differently from int conversion with regard to alternate local representations of numbers.
Thus, I think the options are:
- Have a new defect issue for 22.2.2.1.2/8 so that it will require the use of narrow().
- Have a defect issue for bitset() which describes clearly that widen() is to be used.
Proposed resolution:
Replace the first two sentences of paragraph 5 with:
Extracts up to N characters from is. Stores these characters in a temporary object str of type
basic_string<charT, traits>
, then evaluates the expressionx = bitset<N>(str)
.
Replace the third bullet item in paragraph 5 with:
is.widen(0)
nor is.widen(1)
(in which case the input character
is not extracted).
Rationale:
Input for bitset
should work the same way as numeric
input. Using widen
does mean that alternative digit
representations will not be recognized, but this was a known
consequence of the design choice.
Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Howard Hinnant Opened: 2001-01-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.codecvt.byname].
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Discussion:
22.2.1.5/3 introduces codecvt in part with:
codecvt<wchar_t,char,mbstate_t> converts between the native character sets for tiny and wide characters. Instantiations on mbstate_t perform conversion between encodings known to the library implementor.
But 22.2.1.5.2/10 describes do_length in part with:
... codecvt<wchar_t, char, mbstate_t> ... return(s) the lesser of max and (from_end-from).
The semantics of do_in and do_length are linked. What one does must be consistent with what the other does. 22.2.1.5/3 leads me to believe that the vendor is allowed to choose the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in performs so that it makes his customers happy on a given platform. But 22.2.1.5.2/10 explicitly says what codecvt<wchar_t,char,mbstate_t>::do_length must return. And thus indirectly specifies the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in must perform. I believe that this is not what was intended and is a defect.
Discussion from the -lib reflector:
This proposal would have the effect of making the semantics of
all of the virtual functions in codecvt<wchar_t, char,
mbstate_t>
implementation specified. Is that what we want, or
do we want to mandate specific behavior for the base class virtuals
and leave the implementation specified behavior for the codecvt_byname
derived class? The tradeoff is that former allows implementors to
write a base class that actually does something useful, while the
latter gives users a way to get known and specified---albeit
useless---behavior, and is consistent with the way the standard
handles other facets. It is not clear what the original intention
was.
Nathan has suggest a compromise: a character that is a widened version of the characters in the basic execution character set must be converted to a one-byte sequence, but there is no such requirement for characters that are not part of the basic execution character set.
Proposed resolution:
Change 22.2.1.5.2/5 from:
The instantiations required in Table 51 (lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and codecvt<char,char,mbstate_t>, store no characters. Stores no more than (to_limit-to) destination elements. It always leaves the to_next pointer pointing one beyond the last element successfully stored.
to:
Stores no more than (to_limit-to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. codecvt<char,char,mbstate_t> stores no characters.
Change 22.2.1.5.2/10 from:
-10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiations required in Table 51 (21.1.1.1.1), namely codecvt<wchar_t, char, mbstate_t> and codecvt<char, char, mbstate_t>, return the lesser of max and (from_end-from).
to:
-10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiation codecvt<char, char, mbstate_t> returns the lesser of max and (from_end-from).
[Redmond: Nathan suggested an alternative resolution: same as above, but require that, in the default encoding, a character from the basic execution character set would map to a single external character. The straw poll was 8-1 in favor of the proposed resolution.]
Rationale:
The default encoding should be whatever users of a given platform would expect to be the most natural. This varies from platform to platform. In many cases there is a preexisting C library, and users would expect the default encoding to be whatever C uses in the default "C" locale. We could impose a guarantee like the one Nathan suggested (a character from the basic execution character set must map to a single external character), but this would rule out important encodings that are in common use: it would rule out JIS, for example, and it would rule out a fixed-width encoding of UCS-4.
[Curaçao: fixed rationale typo at the request of Ichiro Koshida; "shift-JIS" changed to "JIS".]
Section: 17.2 [support.types] Status: CD1 Submitter: Steve Clamage Opened: 2001-02-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Spliced together from reflector messages c++std-lib-8294 and -8295:
18.1, paragraph 5, reads: "The macro offsetof
accepts a restricted set of type arguments in this
International Standard. type shall be a POD structure or a POD
union (clause 9). The result of applying the offsetof macro to a field
that is a static data member or a function member is
undefined."
For the POD requirement, it doesn't say "no diagnostic required" or "undefined behavior". I read 4.1 [intro.compliance], paragraph 1, to mean that a diagnostic is required. It's not clear whether this requirement was intended. While it's possible to provide such a diagnostic, the extra complication doesn't seem to add any value.
Proposed resolution:
Change 18.1, paragraph 5, to "If type is not a POD structure or a POD union the results are undefined."
[Copenhagen: straw poll was 7-4 in favor. It was generally agreed that requiring a diagnostic was inadvertent, but some LWG members thought that diagnostics should be required whenever possible.]
Section: 23.3.11 [list] Status: CD1 Submitter: Howard Hinnant Opened: 2001-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
From reflector message c++std-lib-8330. See also lib-8317.
The standard is currently inconsistent in 23.3.11.3 [list.capacity] paragraph 1 and 23.3.11.4 [list.modifiers] paragraph 1. 23.2.3.3/1, for example, says:
-1- Any sequence supporting operations back(), push_back() and pop_back() can be used to instantiate stack. In particular, vector (lib.vector), list (lib.list) and deque (lib.deque) can be used.
But this is false: vector<bool> can not be used, because the container adaptors return a T& rather than using the underlying container's reference type.
This is a contradiction that can be fixed by:
I propose 3. This does not preclude option 2 if we choose to do it later (see issue 96(i)); the issues are independent. Option 3 offers a small step towards support for proxied containers. This small step fixes a current contradiction, is easy for vendors to implement, is already implemented in at least one popular lib, and does not break any code.
Proposed resolution:
Summary: Add reference and const_reference typedefs to queue, priority_queue and stack. Change return types of "value_type&" to "reference". Change return types of "const value_type&" to "const_reference". Details:
Change 23.2.3.1/1 from:
namespace std { template <class T, class Container = deque<T> > class queue { public: typedef typename Container::value_type value_type; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit queue(const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } value_type& front() { return c.front(); } const value_type& front() const { return c.front(); } value_type& back() { return c.back(); } const value_type& back() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void pop() { c.pop_front(); } };
to:
namespace std { template <class T, class Container = deque<T> > class queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::value_type value_type; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit queue(const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } reference front() { return c.front(); } const_reference front() const { return c.front(); } reference back() { return c.back(); } const_reference back() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void pop() { c.pop_front(); } };
Change 23.2.3.2/1 from:
namespace std { template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue { public: typedef typename Container::value_type value_type; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; Compare comp; public: explicit priority_queue(const Compare& x = Compare(), const Container& = Container()); template <class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } const value_type& top() const { return c.front(); } void push(const value_type& x); void pop(); }; // no equality is provided }
to:
namespace std { template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; Compare comp; public: explicit priority_queue(const Compare& x = Compare(), const Container& = Container()); template <class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } const_reference top() const { return c.front(); } void push(const value_type& x); void pop(); }; // no equality is provided }
And change 23.2.3.3/1 from:
namespace std { template <class T, class Container = deque<T> > class stack { public: typedef typename Container::value_type value_type; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit stack(const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } value_type& top() { return c.back(); } const value_type& top() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void pop() { c.pop_back(); } }; template <class T, class Container> bool operator==(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator< (const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator> (const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y); }
to:
namespace std { template <class T, class Container = deque<T> > class stack { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit stack(const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } reference top() { return c.back(); } const_reference top() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void pop() { c.pop_back(); } }; template <class T, class Container> bool operator==(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator< (const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator> (const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y); }
[Copenhagen: This change was discussed before the IS was released and it was deliberately not adopted. Nevertheless, the LWG believes (straw poll: 10-2) that it is a genuine defect.]
Section: 31 [input.output] Status: CD1 Submitter: Martin Sebor Opened: 2001-03-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 82 in section 27 mentions the header <cstdlib> for String streams (31.8 [string.streams]) and the headers <cstdio> and <cwchar> for File streams (31.10 [file.streams]). It's not clear why these headers are mentioned in this context since they do not define any of the library entities described by the subclauses. According to 16.4.2.2 [contents], only such headers are to be listed in the summary.
Proposed resolution:
Remove <cstdlib> and <cwchar> from Table 82.
[Copenhagen: changed the proposed resolution slightly. The original proposed resolution also said to remove <cstdio> from Table 82. However, <cstdio> is mentioned several times within section 31.10 [file.streams], including 31.13 [c.files].]
Section: 16.4.2.3 [headers], 19.4 [errno] Status: CD1 Submitter: Steve Clamage Opened: 2001-03-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Exactly how should errno be declared in a conforming C++ header?
The C standard says in 7.1.4 that it is unspecified whether errno is a macro or an identifier with external linkage. In some implementations it can be either, depending on compile-time options. (E.g., on Solaris in multi-threading mode, errno is a macro that expands to a function call, but is an extern int otherwise. "Unspecified" allows such variability.)
The C++ standard:
I find no other references to errno.
We should either explicitly say that errno must be a macro, even
though it need not be a macro in C, or else explicitly leave it
unspecified. We also need to say something about namespace std.
A user who includes <cerrno> needs to know whether to write
errno
, or ::errno
, or std::errno
, or
else <cerrno> is useless.
Two acceptable fixes:
errno must be a macro. This is trivially satisfied by adding
#define errno (::std::errno)
to the headers if errno is not already a macro. You then always
write errno without any scope qualification, and it always expands
to a correct reference. Since it is always a macro, you know to
avoid using errno as a local identifer.
errno is in the global namespace. This fix is inferior, because ::errno is not guaranteed to be well-formed.
[ This issue was first raised in 1999, but it slipped through the cracks. ]
Proposed resolution:
Change the Note in section 17.4.1.2p5 from
Note: the names defined as macros in C include the following: assert, errno, offsetof, setjmp, va_arg, va_end, and va_start.
to
Note: the names defined as macros in C include the following: assert, offsetof, setjmp, va_arg, va_end, and va_start.
In section 19.3, change paragraph 2 from
The contents are the same as the Standard C library header <errno.h>.
to
The contents are the same as the Standard C library header <errno.h>, except that errno shall be defined as a macro.
Rationale:
C++ must not leave it up to the implementation to decide whether or not a name is a macro; it must explicitly specify exactly which names are required to be macros. The only one that really works is for it to be a macro.
[Curaçao: additional rationale added.]
Section: 31.7.6.2 [ostream] Status: CD1 Submitter: Andy Sawyer Opened: 2001-03-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.7.6.2 [ostream], the synopsis of class basic_ostream says:
// partial specializationss template<class traits> basic_ostream<char,traits>& operator<<( basic_ostream<char,traits>&, const char * );
Problems:
Proposed resolution:
In the synopsis in 31.7.6.2 [ostream], remove the // partial specializationss comment. Also remove the same comment (correctly spelled, but still incorrect) from the synopsis in 31.7.6.3.4 [ostream.inserters.character].
[ Pre-Redmond: added 31.7.6.3.4 [ostream.inserters.character] because of Martin's comment in c++std-lib-8939. ]
Section: 22 [utilities] Status: CD1 Submitter: Martin Sebor Opened: 2001-03-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 27 in section 20 lists the header <memory> (only) for Memory (lib.memory) but neglects to mention the headers <cstdlib> and <cstring> that are discussed in 21.3.7 [meta.rel].
Proposed resolution:
Add <cstdlib> and <cstring> to Table 27, in the same row as <memory>.
Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: Andy Sawyer Opened: 2001-05-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.3.11.5 [list.ops], Para 21 describes the complexity of list::unique as: "If the range (last - first) is not empty, exactly (last - first) -1 applications of the corresponding predicate, otherwise no applications of the predicate)".
"(last - first)" is not a range.
Proposed resolution:
Change the "range" from (last - first) to [first, last).
Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 69 says this about a_uniq.insert(t):
inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair indicates whether the insertion takes place and the iterator component of the pair points to the element with key equivalent to the key of t.
The description should be more specific about exactly how the bool component indicates whether the insertion takes place.
Proposed resolution:
Change the text in question to
...The bool component of the returned pair is true if and only if the insertion takes place...
Section: 28.3 [localization] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The localization section of the standard refers to specializations of the facet templates as instantiations even though the required facets are typically specialized rather than explicitly (or implicitly) instantiated. In the case of ctype<char> and ctype_byname<char> (and the wchar_t versions), these facets are actually required to be specialized. The terminology should be corrected to make it clear that the standard doesn't mandate explicit instantiation (the term specialization encompasses both explicit instantiations and specializations).
Proposed resolution:
In the following paragraphs, replace all occurrences of the word instantiation or instantiations with specialization or specializations, respectively:
22.1.1.1.1, p4, Table 52, 22.2.1.1, p2, 22.2.1.5, p3, 22.2.1.5.1, p5, 22.2.1.5.2, p10, 22.2.2, p2, 22.2.3.1, p1, 22.2.3.1.2, p1, p2 and p3, 22.2.4.1, p1, 22.2.4.1.2, p1, 22,2,5, p1, 22,2,6, p2, 22.2.6.3.2, p7, and Footnote 242.
And change the text in 22.1.1.1.1, p4 from
An implementation is required to provide those instantiations for facet templates identified as members of a category, and for those shown in Table 52:
to
An implementation is required to provide those specializations...
[Nathan will review these changes, and will look for places where explicit specialization is necessary.]
Rationale:
This is a simple matter of outdated language. The language to describe templates was clarified during the standardization process, but the wording in clause 22 was never updated to reflect that change.
Section: 28.3.4.4.2 [locale.numpunct.byname] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The definition of the numpunct_byname template contains the following comment:
namespace std { template <class charT> class numpunct_byname : public numpunct<charT> { // this class is specialized for char and wchar_t. ...
There is no documentation of the specializations and it seems conceivable that an implementation will not explicitly specialize the template at all, but simply provide the primary template.
Proposed resolution:
Remove the comment from the text in 22.2.3.2 and from the proposed resolution of library issue 228(i).
Section: 17.6.3.2 [new.delete.single], 17.6.3.3 [new.delete.array] Status: CD1 Submitter: Beman Dawes Opened: 2001-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard specifies 16.3.2.4 [structure.specifications] that "Required behavior" elements describe "the semantics of a function definition provided by either the implementation or a C++ program."
The standard specifies 16.3.2.4 [structure.specifications] that "Requires" elements describe "the preconditions for calling the function."
In the sections noted below, the current wording specifies "Required Behavior" for what are actually preconditions, and thus should be specified as "Requires".
Proposed resolution:
In 17.6.3.2 [new.delete.single] Para 12 Change:
Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...
to:
Requires: the value of ptr is null or the value returned by an earlier call ...
In 17.6.3.3 [new.delete.array] Para 11 Change:
Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...
to:
Requires: the value of ptr is null or the value returned by an earlier call ...
Section: 23.3.11.2 [list.cons] Status: CD1 Submitter: Howard Hinnant Opened: 2001-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 23.3.11.2 [list.cons], paragraphs 6-8 specify that list assign (both forms) have the "effects" of a call to erase followed by a call to insert.
I would like to document that implementers have the freedom to implement assign by other methods, as long as the end result is the same and the exception guarantee is as good or better than the basic guarantee.
The motivation for this is to use T's assignment operator to recycle existing nodes in the list instead of erasing them and reallocating them with new values. It is also worth noting that, with careful coding, most common cases of assign (everything but assignment with true input iterators) can elevate the exception safety to strong if T's assignment has a nothrow guarantee (with no extra memory cost). Metrowerks does this. However I do not propose that this subtlety be standardized. It is a QoI issue.
Existing practise: Metrowerks and SGI recycle nodes, Dinkumware and Rogue Wave don't.
Proposed resolution:
Change 23.3.11.2 [list.cons]/7 from:
Effects:
erase(begin(), end()); insert(begin(), first, last);
to:
Effects: Replaces the contents of the list with the range [first, last).
In 23.2.4 [sequence.reqmts], in Table 67 (sequence requirements), add two new rows:
a.assign(i,j) void pre: i,j are not iterators into a. Replaces elements in a with a copy of [i, j). a.assign(n,t) void pre: t is not a reference into a. Replaces elements in a with n copies of t.
Change 23.3.11.2 [list.cons]/8 from:
Effects:
erase(begin(), end()); insert(begin(), n, t);
to:
Effects: Replaces the contents of the list with n copies of t.
[Redmond: Proposed resolution was changed slightly. Previous version made explicit statement about exception safety, which wasn't consistent with the way exception safety is expressed elsewhere. Also, the change in the sequence requirements is new. Without that change, the proposed resolution would have required that assignment of a subrange would have to work. That too would have been overspecification; it would effectively mandate that assignment use a temporary. Howard provided wording. ]
[Curaçao: Made editorial improvement in wording; changed "Replaces elements in a with copies of elements in [i, j)." with "Replaces the elements of a with a copy of [i, j)." Changes not deemed serious enough to requre rereview.]
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Kevin Djang Opened: 2001-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 22.2.2.1.2 at p7 states that "A length specifier is added to the conversion function, if needed, as indicated in Table 56." However, Table 56 uses the term "length modifier", not "length specifier".
Proposed resolution:
In 22.2.2.1.2 at p7, change the text "A length specifier is added ..." to be "A length modifier is added ..."
Rationale:
C uses the term "length modifier". We should be consistent.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Matt Austern Opened: 2001-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It's widely assumed that, if X is a container, iterator_traits<X::iterator>::value_type and iterator_traits<X::const_iterator>::value_type should both be X::value_type. However, this is nowhere stated. The language in Table 65 is not precise about the iterators' value types (it predates iterator_traits), and could even be interpreted as saying that iterator_traits<X::const_iterator>::value_type should be "const X::value_type".
Proposed resolution:
In Table 65 ("Container Requirements"), change the return type for X::iterator to "iterator type whose value type is T". Change the return type for X::const_iterator to "constant iterator type whose value type is T".
Rationale:
This belongs as a container requirement, rather than an iterator requirement, because the whole notion of iterator/const_iterator pairs is specific to containers' iterator.
It is existing practice that (for example) iterator_traits<list<int>::const_iterator>::value_type is "int", rather than "const int". This is consistent with the way that const pointers are handled: the standard already requires that iterator_traits<const int*>::value_type is int.
Section: 24.3.5.4 [output.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2001-06-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 73 suggests that output iterators have value types. It requires the expression "*a = t". Additionally, although Table 73 never lists "a = t" or "X(a) = t" in the "expressions" column, it contains a note saying that "a = t" and "X(a) = t" have equivalent (but nowhere specified!) semantics.
According to 24.1/9, t is supposed to be "a value of value type T":
In the following sections, a and b denote values of X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T.
Two other parts of the standard that are relevant to whether output iterators have value types:
The first of these passages suggests that "*i" is supposed to return a useful value, which contradicts the note in 24.1.2/2 saying that the only valid use of "*i" for output iterators is in an expression of the form "*i = t". The second of these passages appears to contradict Table 73, because it suggests that "*i"'s return value should be void. The second passage is also broken in the case of a an iterator type, like non-const pointers, that satisfies both the output iterator requirements and the forward iterator requirements.
What should the standard say about *i
's return value when
i is an output iterator, and what should it say about that t is in the
expression "*i = t"? Finally, should the standard say anything about
output iterators' pointer and reference types?
Proposed resolution:
24.1 p1, change
All iterators
i
support the expression*i
, resulting in a value of some class, enumeration, or built-in typeT
, called the value type of the iterator.
to
All input iterators
i
support the expression*i
, resulting in a value of some class, enumeration, or built-in typeT
, called the value type of the iterator. All output iterators support the expression*i = o
whereo
is a value of some type that is in the set of types that are writable to the particular iterator type ofi
.
24.1 p9, add
o
denotes a value of some type that is writable to the output iterator.
Table 73, change
*a = t
to
*r = o
and change
*r++ = t
to
*r++ = o
[post-Redmond: Jeremy provided wording]
Rationale:
The LWG considered two options: change all of the language that seems to imply that output iterators have value types, thus making it clear that output iterators have no value types, or else define value types for output iterator consistently. The LWG chose the former option, because it seems clear that output iterators were never intended to have value types. This was a deliberate design decision, and any language suggesting otherwise is simply a mistake.
A future revision of the standard may wish to revisit this design decision.
Section: 28.3.4.7.4.3 [locale.moneypunct.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2001-07-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The Returns clause in 22.2.6.3.2, p3 says about moneypunct<charT>::do_grouping()
Returns: A pattern defined identically as the result of numpunct<charT>::do_grouping().241)
Footnote 241 then reads
This is most commonly the value "\003" (not "3").
The returns clause seems to imply that the two member functions must return an identical value which in reality may or may not be true, since the facets are usually implemented in terms of struct std::lconv and return the value of the grouping and mon_grouping, respectively. The footnote also implies that the member function of the moneypunct facet (rather than the overridden virtual functions in moneypunct_byname) most commonly return "\003", which contradicts the C standard which specifies the value of "" for the (most common) C locale.
Proposed resolution:
Replace the text in Returns clause in 22.2.6.3.2, p3 with the following:
Returns: A pattern defined identically as, but not necessarily equal to, the result of numpunct<charT>::do_grouping().241)
and replace the text in Footnote 241 with the following:
To specify grouping by 3s the value is "\003", not "3".
Rationale:
The fundamental problem is that the description of the locale facet virtuals serves two purposes: describing the behavior of the base class, and describing the meaning of and constraints on the behavior in arbitrary derived classes. The new wording makes that separation a little bit clearer. The footnote (which is nonnormative) is not supposed to say what the grouping is in the "C" locale or in any other locale. It is just a reminder that the values are interpreted as small integers, not ASCII characters.
Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: Tiki Wan Opened: 2001-07-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 447
Discussion:
The wchar_t
versions of time_get
and
time_get_byname
are listed incorrectly in table 52,
required instantiations. In both cases the second template
parameter is given as OutputIterator. It should instead be
InputIterator, since these are input facets.
Proposed resolution:
In table 52, required instantiations, in 28.3.3.1.2.1 [locale.category], change
time_get<wchar_t, OutputIterator> time_get_byname<wchar_t, OutputIterator>
to
time_get<wchar_t, InputIterator> time_get_byname<wchar_t, InputIterator>
[Redmond: Very minor change in proposed resolution. Original had a typo, wchart instead of wchar_t.]
Section: 28.3.4.7.3.3 [locale.money.put.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2001-07-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The sprintf format string , "%.01f" (that's the digit one), in the description of the do_put() member functions of the money_put facet in 22.2.6.2.2, p1 is incorrect. First, the f format specifier is wrong for values of type long double, and second, the precision of 01 doesn't seem to make sense. What was most likely intended was "%.0Lf"., that is a precision of zero followed by the L length modifier.
Proposed resolution:
Change the format string to "%.0Lf".
Rationale:
Fixes an obvious typo
Section: 23.3.13.3 [vector.capacity], 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Anthony Williams Opened: 2001-07-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is an apparent contradiction about which circumstances can cause a reallocation of a vector in Section 23.3.13.3 [vector.capacity] and section 23.3.13.5 [vector.modifiers].
23.3.13.3 [vector.capacity],p5 says:
Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the size specified in the most recent call to reserve().
Which implies if I do
std::vector<int> vec; vec.reserve(23); vec.reserve(0); vec.insert(vec.end(),1);
then the implementation may reallocate the vector for the insert, as the size specified in the previous call to reserve was zero.
However, the previous paragraphs (23.3.13.3 [vector.capacity], p1-2) state:
(capacity) Returns: The total number of elements the vector can hold without requiring reallocation
...After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise...
This implies that vec.capacity() is still 23, and so the insert() should not require a reallocation, as vec.size() is 0. This is backed up by 23.3.13.5 [vector.modifiers], p1:
(insert) Notes: Causes reallocation if the new size is greater than the old capacity.
Though this doesn't rule out reallocation if the new size is less than the old capacity, I think the intent is clear.
Proposed resolution:
Change the wording of 23.3.13.3 [vector.capacity] paragraph 5 to:
Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity().
[Redmond: original proposed resolution was modified slightly. In the original, the guarantee was that there would be no reallocation until the size would be greater than the value of capacity() after the most recent call to reserve(). The LWG did not believe that the "after the most recent call to reserve()" added any useful information.]
Rationale:
There was general agreement that, when reserve() is called twice in succession and the argument to the second invocation is smaller than the argument to the first, the intent was for the second invocation to have no effect. Wording implying that such cases have an effect on reallocation guarantees was inadvertant.
Section: 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: PremAnand M. Rao Opened: 2001-08-23 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
With the change in 16.4.6.14 [res.on.exception.handling] to state "An implementation may strengthen the exception-specification for a non-virtual function by removing listed exceptions." (issue 119(i)) and the following declaration of ~failure() in ios_base::failure
namespace std { class ios_base::failure : public exception { public: ... virtual ~failure(); ... }; }
the class failure cannot be implemented since in 17.7.3 [type.info] the destructor of class exception has an empty exception specification:
namespace std { class exception { public: ... virtual ~exception() throw(); ... }; }
Proposed resolution:
Remove the declaration of ~failure().
Rationale:
The proposed resolution is consistent with the way that destructors
of other classes derived from exception
are handled.
Section: 31.7.6.5 [ostream.manip] Status: CD1 Submitter: PremAnand M. Rao Opened: 2001-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A footnote in 31.7.6.5 [ostream.manip] states:
[Footnote: The effect of executing cout << endl is to insert a newline character in the output sequence controlled by cout, then synchronize it with any external file with which it might be associated. --- end foonote]
Does the term "file" here refer to the external device? This leads to some implementation ambiguity on systems with fully buffered files where a newline does not cause a flush to the device.
Choosing to sync with the device leads to significant performance penalties for each call to endl, while not sync-ing leads to errors under special circumstances.
I could not find any other statement that explicitly defined the behavior one way or the other.
Proposed resolution:
Remove footnote 300 from section 31.7.6.5 [ostream.manip].
Rationale:
We already have normative text saying what endl
does: it
inserts a newline character and calls flush
. This footnote
is at best redundant, at worst (as this issue says) misleading,
because it appears to make promises about what flush
does.
Section: 23.4.3.3 [map.access] Status: CD1 Submitter: Andrea Griffini Opened: 2001-09-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current standard describes map::operator[] using a code example. That code example is however quite inefficient because it requires several useless copies of both the passed key_type value and of default constructed mapped_type instances. My opinion is that was not meant by the comitee to require all those temporary copies.
Currently map::operator[] behaviour is specified as:
Returns: (*((insert(make_pair(x, T()))).first)).second.
This specification however uses make_pair that is a template function of which parameters in this case will be deduced being of type const key_type& and const T&. This will create a pair<key_type,T> that isn't the correct type expected by map::insert so another copy will be required using the template conversion constructor available in pair to build the required pair<const key_type,T> instance.
If we consider calling of key_type copy constructor and mapped_type default constructor and copy constructor as observable behaviour (as I think we should) then the standard is in this place requiring two copies of a key_type element plus a default construction and two copy construction of a mapped_type (supposing the addressed element is already present in the map; otherwise at least another copy construction for each type).
A simple (half) solution would be replacing the description with:
Returns: (*((insert(value_type(x, T()))).first)).second.
This will remove the wrong typed pair construction that requires one extra copy of both key and value.
However still the using of map::insert requires temporary objects while the operation, from a logical point of view, doesn't require any.
I think that a better solution would be leaving free an implementer to use a different approach than map::insert that, because of its interface, forces default constructed temporaries and copies in this case. The best solution in my opinion would be just requiring map::operator[] to return a reference to the mapped_type part of the contained element creating a default element with the specified key if no such an element is already present in the container. Also a logarithmic complexity requirement should be specified for the operation.
This would allow library implementers to write alternative implementations not using map::insert and reaching optimal performance in both cases of the addressed element being present or absent from the map (no temporaries at all and just the creation of a new pair inside the container if the element isn't present). Some implementer has already taken this option but I think that the current wording of the standard rules that as non-conforming.
Proposed resolution:
Replace 23.4.3.3 [map.access] paragraph 1 with
-1- Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.
-2- Returns: A reference to the mapped_type corresponding to x in *this.
-3- Complexity: logarithmic.
[This is the second option mentioned above. Howard provided wording. We may also wish to have a blanket statement somewhere in clause 17 saying that we do not intend the semantics of sample code fragments to be interpreted as specifing exactly how many copies are made. See issue 98(i) for a similar problem.]
Rationale:
This is the second solution described above; as noted, it is consistent with existing practice.
Note that we now need to specify the complexity explicitly, because
we are no longer defining operator[]
in terms of
insert
.
Section: 27.2.2 [char.traits.require] Status: CD1 Submitter: Andy Sawyer Opened: 2001-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 37, in 27.2.2 [char.traits.require], descibes char_traits::assign as:
X::assign(c,d) assigns c = d.
And para 1 says:
[...] c and d denote values of type CharT [...]
Naturally, if c and d are values, then the assignment is (effectively) meaningless. It's clearly intended that (in the case of assign, at least), 'c' is intended to be a reference type.
I did a quick survey of the four implementations I happened to have lying around, and sure enough they all have signatures:
assign( charT&, const charT& );
(or the equivalent). It's also described this way in Nico's book. (Not to mention the synopses of char_traits<char> in 21.1.3.1 and char_traits<wchar_t> in 21.1.3.2...)
Proposed resolution:
Add the following to 21.1.1 para 1:
r denotes an lvalue of CharT
and change the description of assign in the table to:
X::assign(r,d) assigns r = d
Section: 16 [library] Status: CD1 Submitter: Detlef Vollmann Opened: 2001-09-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
From c++std-edit-873:
16.4.2.3 [headers], Table 11. In this table, the header <strstream> is missing.
This shows a general problem: The whole clause 17 refers quite often to clauses 18 through 27, but D.7 is also a part of the standard library (though a deprecated one).
Proposed resolution:
To 16.4.2.3 [headers] Table 11, C++ Library Headers, add "<strstream>".
In the following places, change "clauses 17 through 27" to "clauses 17 through 27 and Annex D":
Section: 26.7.5 [alg.replace] Status: CD1 Submitter: Detlef Vollmann Opened: 2001-09-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
From c++std-edit-876:
In section 26.7.5 [alg.replace] before p4: The name of the first parameter of template replace_copy_if should be "InputIterator" instead of "Iterator". According to 16.3.3.3 [type.descriptions] p1 the parameter name conveys real normative meaning.
Proposed resolution:
Change Iterator
to InputIterator
.
Section: 28.3.4 [locale.categories] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
From Stage 2 processing in 28.3.4.3.2.3 [facet.num.get.virtuals], p8 and 9 (the original text or the text corrected by the proposed resolution of issue 221(i)) it seems clear that no whitespace is allowed within a number, but 28.3.4.4.1 [locale.numpunct], p2, which gives the format for integer and floating point values, says that whitespace is optional between a plusminus and a sign.
The text needs to be clarified to either consistently allow or disallow whitespace between a plusminus and a sign. It might be worthwhile to consider the fact that the C library stdio facility does not permit whitespace embedded in numbers and neither does the C or C++ core language (the syntax of integer-literals is given in 5.13.2 [lex.icon], that of floating-point-literals in 5.13.4 [lex.fcon] of the C++ standard).
Proposed resolution:
Change the first part of 28.3.4.4.1 [locale.numpunct] paragraph 2 from:
The syntax for number formats is as follows, where
digit
represents the radix set specified by thefmtflags
argument value,whitespace
is as determined by the facetctype<charT>
(22.2.1.1), andthousands-sep
anddecimal-point
are the results of correspondingnumpunct<charT>
members. Integer values have the format:integer ::= [sign] units sign ::= plusminus [whitespace] plusminus ::= '+' | '-' units ::= digits [thousands-sep units] digits ::= digit [digits]
to:
The syntax for number formats is as follows, where
digit
represents the radix set specified by thefmtflags
argument value, andthousands-sep
anddecimal-point
are the results of correspondingnumpunct<charT>
members. Integer values have the format:integer ::= [sign] units sign ::= plusminus plusminus ::= '+' | '-' units ::= digits [thousands-sep units] digits ::= digit [digits]
Rationale:
It's not clear whether the format described in 28.3.4.4.1 [locale.numpunct] paragraph 2 has any normative weight: nothing in the standard says how, or whether, it's used. However, there's no reason for it to differ gratuitously from the very specific description of numeric processing in 28.3.4.3.2.3 [facet.num.get.virtuals]. The proposed resolution removes all mention of "whitespace" from that format.
Section: 28.3.4.2 [category.ctype], 16.3.3.3.3 [bitmask.types] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The ctype_category::mask type is declared to be an enum in 28.3.4.2 [category.ctype] with p1 then stating that it is a bitmask type, most likely referring to the definition of bitmask type in 16.3.3.3.3 [bitmask.types], p1. However, the said definition only applies to clause 27, making the reference in 22.2.1 somewhat dubious.
Proposed resolution:
Clarify 17.3.2.1.2, p1 by changing the current text from
Several types defined in clause 27 are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (22.9.2 [template.bitset]).
to read
Several types defined in clauses lib.language.support through lib.input.output and Annex D are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (lib.template.bitset).
Additionally, change the definition in 22.2.1 to adopt the same convention as in clause 27 by replacing the existing text with the following (note, in particluar, the cross-reference to 17.3.2.1.2 in 22.2.1, p1):
22.2.1 The ctype category [lib.category.ctype]
namespace std { class ctype_base { public: typedef T mask; // numeric values are for exposition only. static const mask space = 1 << 0; static const mask print = 1 << 1; static const mask cntrl = 1 << 2; static const mask upper = 1 << 3; static const mask lower = 1 << 4; static const mask alpha = 1 << 5; static const mask digit = 1 << 6; static const mask punct = 1 << 7; static const mask xdigit = 1 << 8; static const mask alnum = alpha | digit; static const mask graph = alnum | punct; }; }The type
mask
is a bitmask type (16.3.3.3.3 [bitmask.types]).
[Curaçao: The LWG notes that T above should be bold-italics to be consistent with the rest of the standard.]
has_facet<Facet>(loc)
Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It's unclear whether 22.1.1.1.1, p3 says that
has_facet<Facet>(loc)
returns true for any Facet
from Table 51 or whether it includes Table 52 as well:
For any locale
loc
either constructed, or returned by locale::classic(), and any facetFacet
that is a member of a standard category,has_facet<Facet>(loc)
is true. Each locale member function which takes alocale::category
argument operates on the corresponding set of facets.
It seems that it comes down to which facets are considered to be members of a standard category. Intuitively, I would classify all the facets in Table 52 as members of their respective standard categories, but there are an unbounded set of them...
The paragraph implies that, for instance, has_facet<num_put<C,
OutputIterator> >(loc)
must always return true. I don't think that's
possible. If it were, then use_facet<num_put<C, OutputIterator>
>(loc)
would have to return a reference to a distinct object for each
valid specialization of num_put<C, OutputIteratory>
, which is
clearly impossible.
On the other hand, if none of the facets in Table 52 is a member of a standard category then none of the locale member functions that operate on entire categories of facets will work properly.
It seems that what p3 should mention that it's required (permitted?)
to hold only for specializations of Facet
from Table 52 on
C
from the set { char
, wchar_t
}, and
InputIterator
and OutputIterator
from the set of
{
{i,o}streambuf_iterator
<{char
,wchar_t
}>
}.
Proposed resolution:
In 28.3.3.1.2.1 [locale.category], paragraph 3, change "that is a member of a standard category" to "shown in Table 51".
Rationale:
The facets in Table 52 are an unbounded set. Locales should not be required to contain an infinite number of facets.
It's not necessary to talk about which values of InputIterator and OutputIterator must be supported. Table 51 already contains a complete list of the ones we need.
Section: 23.3.13.3 [vector.capacity] Status: CD1 Submitter: Anthony Williams Opened: 2001-09-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It is a common idiom to reduce the capacity of a vector by swapping it with an empty one:
std::vector<SomeType> vec; // fill vec with data std::vector<SomeType>().swap(vec); // vec is now empty, with minimal capacity
However, the wording of 23.3.13.3 [vector.capacity]paragraph 5 prevents the capacity of a vector being reduced, following a call to reserve(). This invalidates the idiom, as swap() is thus prevented from reducing the capacity. The proposed wording for issue 329(i) does not affect this. Consequently, the example above requires the temporary to be expanded to cater for the contents of vec, and the contents be copied across. This is a linear-time operation.
However, the container requirements state that swap must have constant complexity (23.2 [container.requirements] note to table 65).
This is an important issue, as reallocation affects the validity of references and iterators.
If the wording of 23.2.4.2p5 is taken to be the desired intent, then references and iterators remain valid after a call to swap, if they refer to an element before the new end() of the vector into which they originally pointed, in which case they refer to the element at the same index position. Iterators and references that referred to an element whose index position was beyond the new end of the vector are invalidated.
If the note to table 65 is taken as the desired intent, then there are two possibilities with regard to iterators and references:
Proposed resolution:
Add a new paragraph after 23.3.13.3 [vector.capacity] paragraph 5:
void swap(vector<T,Allocator>& x);Effects: Exchanges the contents and capacity() of
*this
with that ofx
.Complexity: Constant time.
[This solves the problem reported for this issue. We may also have a problem with a circular definition of swap() for other containers.]
Rationale:
swap should be constant time. The clear intent is that it should just do pointer twiddling, and that it should exchange all properties of the two vectors, including their reallocation guarantees.
Section: 16 [library] Status: Resolved Submitter: Martin Sebor Opened: 2001-10-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopses of the C++ library headers clearly show which names are required to be defined in each header. Since in order to implement the classes and templates defined in these headers declarations of other templates (but not necessarily their definitions) are typically necessary the standard in 17.4.4, p1 permits library implementers to include any headers needed to implement the definitions in each header.
For instance, although it is not explicitly specified in the synopsis of
<string>
, at the point of definition of the std::basic_string
template
the declaration of the std::allocator
template must be in scope. All
current implementations simply include <memory>
from within <string>
,
either directly or indirectly, to bring the declaration of
std::allocator
into scope.
Additionally, however, some implementation also include <istream>
and
<ostream>
at the top of <string>
to bring the declarations of
std::basic_istream
and std::basic_ostream
into scope (which are needed
in order to implement the string inserter and extractor operators
(21.3.7.9 [lib.string.io])). Other implementations only include
<iosfwd>
, since strictly speaking, only the declarations and not the
full definitions are necessary.
Obviously, it is possible to implement <string>
without actually
providing the full definitions of all the templates std::basic_string
uses (std::allocator
, std::basic_istream
, and std::basic_ostream
).
Furthermore, not only is it possible, doing so is likely to have a
positive effect on compile-time efficiency.
But while it may seem perfectly reasonable to expect a program that uses
the std::basic_string
insertion and extraction operators to also
explicitly include <istream>
or <ostream>
, respectively, it doesn't seem
reasonable to also expect it to explicitly include <memory>
. Since
what's reasonable and what isn't is highly subjective one would expect
the standard to specify what can and what cannot be assumed.
Unfortunately, that isn't the case.
The examples below demonstrate the issue.
Example 1:
It is not clear whether the following program is complete:
#include <string> extern std::basic_ostream<char> &strm; int main () { strm << std::string ("Hello, World!\n"); }
or whether one must explicitly include <memory>
or
<ostream>
(or both) in addition to <string>
in order for
the program to compile.
Example 2:
Similarly, it is unclear whether the following program is complete:
#include <istream> extern std::basic_iostream<char> &strm; int main () { strm << "Hello, World!\n"; }
or whether one needs to explicitly include <ostream>
, and
perhaps even other headers containing the definitions of other
required templates:
#include <ios> #include <istream> #include <ostream> #include <streambuf> extern std::basic_iostream<char> &strm; int main () { strm << "Hello, World!\n"; }
Example 3:
Likewise, it seems unclear whether the program below is complete:
#include <iterator> bool foo (std::istream_iterator<int> a, std::istream_iterator<int> b) { return a == b; } int main () { }
or whether one should be required to include <istream>
.
There are many more examples that demonstrate this lack of a requirement. I believe that in a good number of cases it would be unreasonable to require that a program explicitly include all the headers necessary for a particular template to be specialized, but I think that there are cases such as some of those above where it would be desirable to allow implementations to include only as much as necessary and not more.
[ post Bellevue: ]
Position taken in prior reviews is that the idea of a table of header dependencies is a good one. Our view is that a full paper is needed to do justice to this, and we've made that recommendation to the issue author.
[ 2009-07 Frankfurt ]
Proposed resolution:
For every C++ library header, supply a minimum set of other C++ library headers that are required to be included by that header. The proposed list is below (C++ headers for C Library Facilities, table 12 in 17.4.1.2, p3, are omitted):
+------------+--------------------+ | C++ header |required to include | +============+====================+ |<algorithm> | | +------------+--------------------+ |<bitset> | | +------------+--------------------+ |<complex> | | +------------+--------------------+ |<deque> |<memory> | +------------+--------------------+ |<exception> | | +------------+--------------------+ |<fstream> |<ios> | +------------+--------------------+ |<functional>| | +------------+--------------------+ |<iomanip> |<ios> | +------------+--------------------+ |<ios> |<streambuf> | +------------+--------------------+ |<iosfwd> | | +------------+--------------------+ |<iostream> |<istream>, <ostream>| +------------+--------------------+ |<istream> |<ios> | +------------+--------------------+ |<iterator> | | +------------+--------------------+ |<limits> | | +------------+--------------------+ |<list> |<memory> | +------------+--------------------+ |<locale> | | +------------+--------------------+ |<map> |<memory> | +------------+--------------------+ |<memory> | | +------------+--------------------+ |<new> |<exception> | +------------+--------------------+ |<numeric> | | +------------+--------------------+ |<ostream> |<ios> | +------------+--------------------+ |<queue> |<deque> | +------------+--------------------+ |<set> |<memory> | +------------+--------------------+ |<sstream> |<ios>, <string> | +------------+--------------------+ |<stack> |<deque> | +------------+--------------------+ |<stdexcept> | | +------------+--------------------+ |<streambuf> |<ios> | +------------+--------------------+ |<string> |<memory> | +------------+--------------------+ |<strstream> | | +------------+--------------------+ |<typeinfo> |<exception> | +------------+--------------------+ |<utility> | | +------------+--------------------+ |<valarray> | | +------------+--------------------+ |<vector> |<memory> | +------------+--------------------+
Rationale:
The portability problem is real. A program that works correctly on one implementation might fail on another, because of different header dependencies. This problem was understood before the standard was completed, and it was a conscious design choice.
One possible way to deal with this, as a library extension, would
be an <all>
header.
Hinnant: It's time we dealt with this issue for C++0X. Reopened.
Section: 27.5 [c.strings] Status: CD1 Submitter: Clark Nelson Opened: 2001-10-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
C99, and presumably amendment 1 to C90, specify that <wchar.h> declares struct tm as an incomplete type. However, table 48 in 27.5 [c.strings] does not mention the type tm as being declared in <cwchar>. Is this omission intentional or accidental?
Proposed resolution:
In section 27.5 [c.strings], add "tm" to table 48.
Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Jeremy Siek Opened: 2001-10-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Iterator member functions and operators that do not change the state of the iterator should be defined as const member functions or as functions that take iterators either by const reference or by value. The standard does not explicitly state which functions should be const. Since this a fairly common mistake, the following changes are suggested to make this explicit.
The tables almost indicate constness properly through naming: r for non-const and a,b for const iterators. The following changes make this more explicit and also fix a couple problems.
Proposed resolution:
In 24.3.4 [iterator.concepts] Change the first section of p9 from "In the following sections, a and b denote values of X..." to "In the following sections, a and b denote values of type const X...".
In Table 73, change
a->m U& ...
to
a->m const U& ... r->m U& ...
In Table 73 expression column, change
*a = t
to
*r = t
[Redmond: The container requirements should be reviewed to see if the same problem appears there.]
Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: P.J. Plauger, Nathan Myers Opened: 2001-10-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 28.3.3.1.2.1 [locale.category] paragraph 1, the category members
are described as bitmask elements. In fact, the bitmask requirements
in 16.3.3.3.3 [bitmask.types] don't seem quite right: none
and all
are bitmask constants, not bitmask elements.
In particular, the requirements for none
interact poorly
with the requirement that the LC_* constants from the C library must
be recognizable as C++ locale category constants. LC_* values should
not be mixed with these values to make category values.
We have two options for the proposed resolution. Informally:
option 1 removes the requirement that LC_* values be recognized as
category arguments. Option 2 changes the category type so that this
requirement is implementable, by allowing none
to be some
value such as 0x1000 instead of 0.
Nathan writes: "I believe my proposed resolution [Option 2] merely re-expresses the status quo more clearly, without introducing any changes beyond resolving the DR.
Proposed resolution:
Replace the first two paragraphs of 28.3.3.1.2 [locale.types] with:
typedef int category;Valid category values include the
locale
member bitmask elementscollate
,ctype
,monetary
,numeric
,time
, andmessages
, each of which represents a single locale category. In addition,locale
member bitmask constantnone
is defined as zero and represents no category. And locale member bitmask constantall
is defined such that the expression(collate | ctype | monetary | numeric | time | messages | all) == allis
true
, and represents the union of all categories. Further the expression(X | Y)
, whereX
andY
each represent a single category, represents the union of the two categories.
locale
member functions expecting acategory
argument require one of thecategory
values defined above, or the union of two or more such values. Such acategory
argument identifies a set of locale categories. Each locale category, in turn, identifies a set of locale facets, including at least those shown in Table 51:
[Curaçao: need input from locale experts.]
Rationale:
The LWG considered, and rejected, an alternate proposal (described as "Option 2" in the discussion). The main reason for rejecting it was that library implementors were concerened about implementation difficult, given that getting a C++ library to work smoothly with a separately written C library is already a delicate business. Some library implementers were also concerned about the issue of adding extra locale categories.
Option 2:
Replace the first paragraph of 28.3.3.1.2 [locale.types] with:Valid category values include the enumerated values. In addition, the result of applying commutative operators | and & to any two valid values is valid, and results in the setwise union and intersection, respectively, of the argument categories. The values
all
andnone
are defined such that for any valid valuecat
, the expressions(cat | all == all)
,(cat & all == cat)
,(cat | none == cat)
and(cat & none == none)
are true. For non-equal valuescat1
andcat2
of the remaining enumerated values,(cat1 & cat2 == none)
is true. For any valid categoriescat1
andcat2
, the result of(cat1 & ~cat2)
is valid, and equals the setwise union of those categories found incat1
but not found incat2
. [Footnote: it is not required thatall
equal the setwise union of the other enumerated values; implementations may add extra categories.]
Section: 24.6.3 [ostream.iterator] Status: CD1 Submitter: Andy Sawyer Opened: 2001-10-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
24.5.2 [lib.ostream.iterator] states:
[...] private: // basic_ostream<charT,traits>* out_stream; exposition only // const char* delim; exposition only
Whilst it's clearly marked "exposition only", I suspect 'delim' should be of type 'const charT*'.
Proposed resolution:
In 24.6.3 [ostream.iterator], replace const char* delim
with
const charT* delim
.
Section: 27.2.3 [char.traits.typedefs] Status: CD1 Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
(1)
There are no requirements on the stateT
template parameter of
fpos
listed in 27.4.3. The interface appears to require that
the type be at least Assignable and CopyConstructible (27.4.3.1, p1),
and I think also DefaultConstructible (to implement the operations in
Table 88).
21.1.2, p3, however, only requires that
char_traits<charT>::state_type
meet the requirements of
CopyConstructible types.
(2)
Additionally, the stateT
template argument has no
corresponding typedef in fpos which might make it difficult to use in
generic code.
Proposed resolution:
Modify 21.1.2, p4 from
Requires: state_type
shall meet the requirements of
CopyConstructible types (20.1.3).
Requires: state_type shall meet the requirements of Assignable (23.1, p4), CopyConstructible (20.1.3), and DefaultConstructible (20.1.4) types.
Rationale:
The LWG feels this is two issues, as indicated above. The first is a defect---std::basic_fstream is unimplementable without these additional requirements---and the proposed resolution fixes it. The second is questionable; who would use that typedef? The class template fpos is used only in a very few places, all of which know the state type already. Unless motivation is provided, the second should be considered NAD.
std::pair
missing template assignmentSection: 22.3 [pairs] Status: Resolved Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class template std::pair
defines a template ctor (20.2.2, p4) but
no template assignment operator. This may lead to inefficient code since
assigning an object of pair<C, D>
to pair<A, B>
where the types C
and D
are distinct from but convertible to
A
and B
, respectively, results in a call to the template copy
ctor to construct an unnamed temporary of type pair<A, B>
followed by an ordinary (perhaps implicitly defined) assignment operator,
instead of just a straight assignment.
Proposed resolution:
Add the following declaration to the definition of std::pair
:
template<class U, class V> pair& operator=(const pair<U, V> &p);
And also add a paragraph describing the effects of the function template to the end of 20.2.2:
template<class U, class V> pair& operator=(const pair<U, V> &p);
Effects: first = p.first;
second = p.second;
Returns: *this
[Curaçao: There is no indication this is was anything other than a design decision, and thus NAD. May be appropriate for a future standard.]
[
Pre Bellevue: It was recognized that this was taken care of by
N1856,
and thus moved from NAD Future to NAD EditorialResolved.
]
Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Hans Aberg Opened: 2001-12-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Discussions in the thread "Associative container lower/upper bound requirements" on comp.std.c++ suggests that there is a defect in the C++ standard, Table 69 of section 23.1.2, "Associative containers", [lib.associative.reqmts]. It currently says:
a.find(k): returns an iterator pointing to an element with the key equivalent to k, or a.end() if such an element is not found.
a.lower_bound(k): returns an iterator pointing to the first element with key not less than k.
a.upper_bound(k): returns an iterator pointing to the first element with key greater than k.
We have "or a.end() if such an element is not found" for
find
, but not for upper_bound
or
lower_bound
. As the text stands, one would be forced to
insert a new element into the container and return an iterator to that
in case the sought iterator does not exist, which does not seem to be
the intention (and not possible with the "const" versions).
Proposed resolution:
Change Table 69 of section 23.2.7 [associative.reqmts] indicated entries to:
a.lower_bound(k): returns an iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.
a.upper_bound(k): returns an iterator pointing to the first element with key greater than k, or a.end() if such an element is not found.
[Curaçao: LWG reviewed PR.]
Section: 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Yaroslav Mironov Opened: 2002-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 68 "Optional Sequence Operations" in 23.1.1/12 specifies operational semantics for "a.back()" as "*--a.end()", which may be ill-formed [because calling operator-- on a temporary (the return) of a built-in type is ill-formed], provided a.end() returns a simple pointer rvalue (this is almost always the case for std::vector::end(), for example). Thus, the specification is not only incorrect, it demonstrates a dangerous construct: "--a.end()" may successfully compile and run as intended, but after changing the type of the container or the mode of compilation it may produce compile-time error.
Proposed resolution:
Change the specification in table 68 "Optional Sequence Operations" in 23.1.1/12 for "a.back()" from
*--a.end()
to
{ iterator tmp = a.end(); --tmp; return *tmp; }
and the specification for "a.pop_back()" from
a.erase(--a.end())
to
{ iterator tmp = a.end(); --tmp; a.erase(tmp); }
[Curaçao: LWG changed PR from "{ X::iterator tmp = a.end(); return *--tmp; }" to "*a.rbegin()", and from "{ X::iterator tmp = a.end(); a.erase(--tmp); }" to "a.erase(rbegin())".]
[There is a second possible defect; table 68 "Optional Sequence Operations" in the "Operational Semantics" column uses operations present only in the "Reversible Container" requirements, yet there is no stated dependency between these separate requirements tables. Ask in Santa Cruz if the LWG would like a new issue opened.]
[Santa Cruz: the proposed resolution is even worse than what's in the current standard: erase is undefined for reverse iterator. If we're going to make the change, we need to define a temporary and use operator--. Additionally, we don't know how prevalent this is: do we need to make this change in more than one place? Martin has volunteered to review the standard and see if this problem occurs elsewhere.]
[Oxford: Matt provided new wording to address the concerns raised in Santa Cruz. It does not appear that this problem appears anywhere else in clauses 23 or 24.]
[Kona: In definition of operational semantics of back(), change "*tmp" to "return *tmp;"]
thousands_sep
after a decimal_point
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I don't think thousands_sep
is being treated correctly after
decimal_point has been seen. Since grouping applies only to the
integral part of the number, the first such occurrence should, IMO,
terminate Stage 2. (If it does not terminate it, then 22.2.2.1.2, p12
and 22.2.3.1.2, p3 need to explain how thousands_sep
is to be
interpreted in the fractional part of a number.)
The easiest change I can think of that resolves this issue would be something like below.
Proposed resolution:
Change the first sentence of 22.2.2.1.2, p9 from
If discard is true then the position of the character is remembered, but the character is otherwise ignored. If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.
to
If
discard
is true, then if'.'
has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if'.'
has already been accumulated, the character is discarded and Stage 2 terminates. ...
Rationale:
We believe this reflects the intent of the Standard. Thousands sep characters after the decimal point are not useful in any locale. Some formatting conventions do group digits that follow the decimal point, but they usually introduce a different grouping character instead of reusing the thousand sep character. If we want to add support for such conventions, we need to do so explicitly.
Section: 28.3.4.3.3.2 [facet.num.put.members] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
22.2.2.2.1, p1:
iter_type put (iter_type out, ios_base& str, char_type fill, bool val) const; ... 1 Returns: do_put (out, str, fill, val).
AFAICS, the behavior of do_put (..., bool) is not documented anywhere, however, 22.2.2.2.2, p23:
iter_type put (iter_type out, ios_base& str, char_type fill, bool val) const;Effects: If (str.flags() & ios_base::boolalpha) == 0 then do out = do_put(out, str, fill, (int)val) Otherwise do
string_type s = val ? use_facet<ctype<charT> >(loc).truename() : use_facet<ctype<charT> >(loc).falsename();and then insert the characters of s into out. out.
This means that the bool overload of do_put()
will never be called,
which contradicts the first paragraph. Perhaps the declaration
should read do_put()
, and not put()
?
Note also that there is no Returns clause for this function, which should probably be corrected, just as should the second occurrence of "out." in the text.
I think the least invasive change to fix it would be something like the following:
Proposed resolution:
In 28.3.4.3.3.3 [facet.num.put.virtuals], just above paragraph 1, remove
the bool
overload.
In 28.3.4.3.3.3 [facet.num.put.virtuals], p23, make the following changes
Replace
put()
withdo_put()
in the declaration of the member function.
Change the Effects clause to a Returns clause (to avoid the requirement to call
do_put(..., int)
fromdo_put (..., bool))
like so:
23 Returns: If
(str.flags() & ios_base::boolalpha) == 0
thendo_put (out, str, fill, (long)val)
Otherwise the function obtains a strings
as if bystring_type s = val ? use_facet<ctype<charT> >(loc).truename() : use_facet<ctype<charT> >(loc).falsename();and then inserts each character
c
of s into out via*out++ = c
and returnsout
.
Rationale:
This fixes a couple of obvious typos, and also fixes what appears to be a requirement of gratuitous inefficiency.
Section: 28.3.3.1 [locale] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
22.1.1, p7 (copied below) allows iostream formatters and extractors
to make assumptions about the values returned from facet members.
However, such assumptions are apparently not guaranteed to hold
in other cases (e.g., when the facet members are being called directly
rather than as a result of iostream calls, or between successive
calls to the same iostream functions with no interevening calls to
imbue()
, or even when the facet member functions are called
from other member functions of other facets). This restriction
prevents locale from being implemented efficiently.
Proposed resolution:
Change the first sentence in 22.1.1, p7 from
In successive calls to a locale facet member function during a call to an iostream inserter or extractor or a streambuf member function, the returned result shall be identical. [Note: This implies that such results may safely be reused without calling the locale facet member function again, and that member functions of iostream classes cannot safely call
imbue()
themselves, except as specified elsewhere. --end note]
to
In successive calls to a locale facet member function on a facet object installed in the same locale, the returned result shall be identical. ...
Rationale:
This change is reasonable becuase it clarifies the intent of this part of the standard.
Section: 99 [depr.lib.binders] Status: CD1 Submitter: Andrew Demkin Opened: 2002-04-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The definition of bind1st() (99 [depr.lib.binders]) can result in the construction of an unsafe binding between incompatible pointer types. For example, given a function whose first parameter type is 'pointer to T', it's possible without error to bind an argument of type 'pointer to U' when U does not derive from T:
foo(T*, int); struct T {}; struct U {}; U u; int* p; int* q; for_each(p, q, bind1st(ptr_fun(foo), &u)); // unsafe binding
The definition of bind1st() includes a functional-style conversion to map its argument to the expected argument type of the bound function (see below):
typename Operation::first_argument_type(x)
A functional-style conversion (99 [depr.lib.binders]) is defined to be semantically equivalent to an explicit cast expression (99 [depr.lib.binders]), which may (according to 5.4, paragraph 5) be interpreted as a reinterpret_cast, thus masking the error.
The problem and proposed change also apply to 99 [depr.lib.binders].
Proposed resolution:
Add this sentence to the end of 99 [depr.lib.binders]/1:
"Binders bind1st
and bind2nd
are deprecated in
favor of std::tr1::bind
."
(Notes to editor: (1) when and if tr1::bind is incorporated into the standard, "std::tr1::bind" should be changed to "std::bind". (2) 20.5.6 should probably be moved to Annex D.
Rationale:
There is no point in fixing bind1st and bind2nd. tr1::bind is a superior solution. It solves this problem and others.
Section: 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: Walter Brown and Marc Paterno Opened: 2002-05-20 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
The destructor of ios_base::failure should have an empty throw specification, because the destructor of its base class, exception, is declared in this way.
Proposed resolution:
Change the destructor to
virtual ~failure() throw();
Rationale:
Fixes an obvious glitch. This is almost editorial.
Section: 31.6.3.5.2 [streambuf.virt.buffer] Status: CD1 Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
31.6.3.5.2 [streambuf.virt.buffer] paragraph 1 is inconsistent with the Effects clause for seekoff.
Proposed resolution:
Make this paragraph, the Effects clause for setbuf, consistent in wording with the Effects clause for seekoff in paragraph 3 by amending paragraph 1 to indicate the purpose of setbuf:
Original text:
1 Effects: Performs an operation that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).
Proposed text:
1 Effects: Influences stream buffering in a way that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).
Rationale:
The LWG doesn't believe there is any normative difference between the existing wording and what's in the proposed resolution, but the change may make the intent clearer.
Section: 31 [input.output] Status: CD1 Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Some stream and streambuf member functions are declared non-const, even thought they appear only to report information rather than to change an object's logical state. They should be declared const. See document N1360 for details and rationale.
The list of member functions under discussion: in_avail
,
showmanyc
, tellg
, tellp
, is_open
.
Proposed resolution:
In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13
Replace
bool is_open();
with
bool is_open() const;
Rationale:
Of the changes proposed in N1360, the only one that is safe is changing the filestreams' is_open to const. The LWG believed that this was NAD the first time it considered this issue (issue 73(i)), but now thinks otherwise. The corresponding streambuf member function, after all,is already const.
The other proposed changes are less safe, because some streambuf functions that appear merely to report a value do actually perform mutating operations. It's not even clear that they should be considered "logically const", because streambuf has two interfaces, a public one and a protected one. These functions may, and often do, change the state as exposed by the protected interface, even if the state exposed by the public interface is unchanged.
Note that implementers can make this change in a binary compatible way by providing both overloads; this would be a conforming extension.
Section: 31.4 [iostream.objects] Status: CD1 Submitter: Ruslan Abdikeev Opened: 2002-07-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is it safe to use standard iostream objects from constructors of static objects? Are standard iostream objects constructed and are their associations established at that time?
Surpisingly enough, Standard does NOT require that.
27.3/2 [lib.iostream.objects] guarantees that standard iostream objects are constructed and their associations are established before the body of main() begins execution. It also refers to ios_base::Init class as the panacea for constructors of static objects.
However, there's nothing in 27.3 [lib.iostream.objects], in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init], that would require implementations to allow access to standard iostream objects from constructors of static objects.
Details:
Core text refers to some magic object ios_base::Init, which will be discussed below:
"The [standard iostream] objects are constructed, and their associations are established at some time prior to or during first time an object of class basic_ios<charT,traits>::Init is constructed, and in any case before the body of main begins execution." (27.3/2 [lib.iostream.objects])
The first non-normative footnote encourages implementations to initialize standard iostream objects earlier than required.
However, the second non-normative footnote makes an explicit and unsupported claim:
"Constructors and destructors for static objects can access these [standard iostream] objects to read input from stdin or write output to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])
The only bit of magic is related to that ios_base::Init class. AFAIK, the rationale behind ios_base::Init was to bring an instance of this class to each translation unit which #included <iostream> or related header. Such an inclusion would support the claim of footnote quoted above, because in order to use some standard iostream object it is necessary to #include <iostream>.
However, while Standard explicitly describes ios_base::Init as an appropriate class for doing the trick, I failed to found a mention of an _instance_ of ios_base::Init in Standard.
Proposed resolution:
Add to 31.4 [iostream.objects], p2, immediately before the last sentence of the paragraph, the following two sentences:
If a translation unit includes <iostream>, or explicitly constructs an ios_base::Init object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.
[Lillehammer: Matt provided wording.]
[Mont Tremblant: Matt provided revised wording.]
Rationale:
The original proposed resolution unconditionally required implementations to define an ios_base::Init object of some implementation-defined name in the header <iostream>. That's an overspecification. First, defining the object may be unnecessary and even detrimental to performance if an implementation can guarantee that the 8 standard iostream objects will be initialized before any other user-defined object in a program. Second, there is no need to require implementations to document the name of the object.
The new proposed resolution gives users guidance on what they need to do to ensure that stream objects are constructed during startup.
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Ray Lischner Opened: 2002-07-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Defect report for description of basic_istream::get (section 31.7.5.4 [istream.unformatted]), paragraph 15. The description for the get function with the following signature:
basic_istream<charT,traits>& get(basic_streambuf<char_type,traits>& sb);
is incorrect. It reads
Effects: Calls get(s,n,widen('\n'))
which I believe should be:
Effects: Calls get(sb,widen('\n'))
Proposed resolution:
Change the Effects paragraph to:
Effects: Calls get(sb,this->widen('\n'))
[Pre-Oxford: Minor correction from Howard: replaced 'widen' with 'this->widen'.]
Rationale:
Fixes an obvious typo.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Frank Compagner Opened: 2002-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The requirements for multiset and multimap containers (23.1 [lib.containers.requirements], 23.1.2 [lib.associative.reqmnts], 23.3.2 [lib.multimap] and 23.3.4 [lib.multiset]) make no mention of the stability of the required (mutating) member functions. It appears the standard allows these functions to reorder equivalent elements of the container at will, yet the pervasive red-black tree implementation appears to provide stable behaviour.
This is of most concern when considering the behaviour of erase(). A stability requirement would guarantee the correct working of the following 'idiom' that removes elements based on a certain predicate function.
multimap<int, int> m; multimap<int, int>::iterator i = m.begin(); while (i != m.end()) { if (pred(i)) m.erase (i++); else ++i; }
Although clause 23.1.2/8 guarantees that i remains a valid iterator througout this loop, absence of the stability requirement could potentially result in elements being skipped. This would make this code incorrect, and, furthermore, means that there is no way of erasing these elements without iterating first over the entire container, and second over the elements to be erased. This would be unfortunate, and have a negative impact on both performance and code simplicity.
If the stability requirement is intended, it should be made explicit (probably through an extra paragraph in clause 23.1.2).
If it turns out stability cannot be guaranteed, i'd argue that a remark or footnote is called for (also somewhere in clause 23.1.2) to warn against relying on stable behaviour (as demonstrated by the code above). If most implementations will display stable behaviour, any problems emerging on an implementation without stable behaviour will be hard to track down by users. This would also make the need for an erase_if() member function that much greater.
This issue is somewhat related to LWG issue 130(i).
Proposed resolution:
Add the following to the end of 23.2.7 [associative.reqmts] paragraph 4:
"For multiset
and multimap
, insert
and erase
are stable: they preserve the relative ordering of equivalent
elements.
[Lillehammer: Matt provided wording]
[Joe Gottman points out that the provided wording does not address multimap and multiset. N1780 also addresses this issue and suggests wording.]
[Mont Tremblant: Changed set and map to multiset and multimap.]
Rationale:
The LWG agrees that this guarantee is necessary for common user idioms to work, and that all existing implementations provide this property. Note that this resolution guarantees stability for multimap and multiset, not for all associative containers in general.
Section: 31.7.5.3.1 [istream.formatted.reqmts], 31.7.6.3.1 [ostream.formatted.reqmts] Status: CD1 Submitter: Keith Baker Opened: 2002-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.7.5.3.1 [istream.formatted.reqmts] and 31.7.6.3.1 [ostream.formatted.reqmts] (exception()&badbit) != 0 is used in testing for rethrow, yet exception() is the constructor to class std::exception in 17.7.3 [type.info] that has no return type. Should member function exceptions() found in 31.5.4 [ios] be used instead?
Proposed resolution:
In 31.7.5.3.1 [istream.formatted.reqmts] and 31.7.6.3.1 [ostream.formatted.reqmts], change "(exception()&badbit) != 0" to "(exceptions()&badbit) != 0".
Rationale:
Fixes an obvious typo.
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Ray Lischner Opened: 2002-08-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 31.8.2.5 [stringbuf.virtuals]: Table 90, Table 91, and paragraph 14 all contain references to "basic_ios::" which should be "ios_base::".
Proposed resolution:
Change all references to "basic_ios" in Table 90, Table 91, and paragraph 14 to "ios_base".
Rationale:
Fixes an obvious typo.
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Ray Lischner Opened: 2002-08-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 31.8.2.5 [stringbuf.virtuals], Table 90, the implication is that the four conditions should be mutually exclusive, but they are not. The first two cases, as written, are subcases of the third.
As written, it is unclear what should be the result if cases 1 and 2 are both true, but case 3 is false.
Proposed resolution:
Rewrite these conditions as:
(which & (ios_base::in|ios_base::out)) == ios_base::in
(which & (ios_base::in|ios_base::out)) == ios_base::out
(which & (ios_base::in|ios_base::out)) == (ios_base::in|ios_base::out) and way == either ios_base::beg or ios_base::end
Otherwise
Rationale:
It's clear what we wanted to say, we just failed to say it. This fixes it.
Section: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The last sentence in 22.2.1.1.2, p11 below doesn't seem to make sense.
charT do_widen (char c) const; -11- Effects: Applies the simplest reasonable transformation from a char value or sequence of char values to the corresponding charT value or values. The only characters for which unique transformations are required are those in the basic source character set (2.2). For any named ctype category with a ctype<charT> facet ctw and valid ctype_base::mask value M (is(M, c) || !ctw.is(M, do_widen(c))) is true.
Shouldn't the last sentence instead read
For any named ctype category with a ctype<char> facet ctc and valid ctype_base::mask value M (ctc.is(M, c) || !is(M, do_widen(c))) is true.
I.e., if the narrow character c is not a member of a class of characters then neither is the widened form of c. (To paraphrase footnote 224.)
Proposed resolution:
Replace the last sentence of 28.3.4.2.2.3 [locale.ctype.virtuals], p11 with the following text:
For any named ctype category with a ctype<char> facet ctc and valid ctype_base::mask value M, (ctc.is(M, c) || !is(M, do_widen(c))) is true.
[Kona: Minor edit. Added a comma after the M for clarity.]
Rationale:
The LWG believes this is just a typo, and that this is the correct fix.
Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Tables 53 and 54 in 28.3.4.2.6 [locale.codecvt.byname] are both titled "convert result values," when surely "do_in/do_out result values" must have been intended for Table 53 and "do_unshift result values" for Table 54.
Table 54, row 3 says that the meaning of partial is "more characters needed to be supplied to complete termination." The function is not supplied any characters, it is given a buffer which it fills with characters or, more precisely, destination elements (i.e., an escape sequence). So partial means that space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state.
Proposed resolution:
Change the title of Table 53 to "do_in/do_out result values" and the title of Table 54 to "do_unshift result values."
Change the text in Table 54, row 3 (the partial row), under the heading Meaning, to "space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state."
Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
All but one codecvt member functions that take a state_type argument list as one of their preconditions that the state_type argument have a valid value. However, according to 22.2.1.5.2, p6, codecvt::do_unshift() is the only codecvt member that is supposed to return error if the state_type object is invalid.
It seems to me that the treatment of state_type by all codecvt member functions should be the same and the current requirements should be changed. Since the detection of invalid state_type values may be difficult in general or computationally expensive in some specific cases, I propose the following:
Proposed resolution:
Add a new paragraph before 22.2.1.5.2, p5, and after the function declaration below
result do_unshift(stateT& state, externT* to, externT* to_limit, externT*& to_next) const;
as follows:
Requires: (to <= to_end) well defined and true; state initialized, if at the beginning of a sequence, or else equal to the result of converting the preceding characters in the sequence.
and change the text in Table 54, row 4, the error row, under the heading Meaning, from
state has invalid value
to
an unspecified error has occurred
Rationale:
The intent is that implementations should not be required to detect invalid state values; such a requirement appears nowhere else. An invalid state value is a precondition violation, i.e. undefined behavior. Implementations that do choose to detect invalid state values, or that choose to detect any other kind of error, may return error as an indication.
Section: 24.3.5.6 [bidirectional.iterators] Status: CD1 Submitter: ysapir (submitted via comp.std.c++) Opened: 2002-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Following a discussion on the boost list regarding end iterators and the possibility of performing operator--() on them, it seems to me that there is a typo in the standard. This typo has nothing to do with that discussion.
I have checked this newsgroup, as well as attempted a search of the Active/Defect/Closed Issues List on the site for the words "s is derefer" so I believe this has not been proposed before. Furthermore, the "Lists by Index" mentions only DR 299(i) on section 24.1.4, and DR 299(i) is not related to this issue.
The standard makes the following assertion on bidirectional iterators, in section 24.1.4 [lib.bidirectional.iterators], Table 75:
operational assertion/note expression return type semantics pre/post-condition --r X& pre: there exists s such that r == ++s. post: s is dereferenceable. --(++r) == r. --r == --s implies r == s. &r == &--r.
(See http://lists.boost.org/Archives/boost/2002/10/37636.php.)
In particular, "s is dereferenceable" seems to be in error. It seems that the intention was to say "r is dereferenceable".
If it were to say "r is dereferenceable" it would make perfect sense. Since s must be dereferenceable prior to operator++, then the natural result of operator-- (to undo operator++) would be to make r dereferenceable. Furthermore, without other assertions, and basing only on precondition and postconditions, we could not otherwise know this. So it is also interesting information.
Proposed resolution:
Change the guarantee to "postcondition: r is dereferenceable."
Rationale:
Fixes an obvious typo
Section: 26.8.4.4 [equal.range] Status: CD1 Submitter: Hans Bos Opened: 2002-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 26.8.4.4 [equal.range] states that at most 2 * log(last - first) + 1 comparisons are allowed for equal_range.
It is not possible to implement equal_range with these constraints.
In a range of one element as in:
int x = 1; equal_range(&x, &x + 1, 1)
it is easy to see that at least 2 comparison operations are needed.
For this case at most 2 * log(1) + 1 = 1 comparison is allowed.
I have checked a few libraries and they all use the same (nonconforming) algorithm for equal_range that has a complexity of
2* log(distance(first, last)) + 2.
I guess this is the algorithm that the standard assumes for equal_range.
It is easy to see that 2 * log(distance) + 2 comparisons are enough since equal range can be implemented with lower_bound and upper_bound (both log(distance) + 1).
I think it is better to require something like 2log(distance) + O(1) (or even logarithmic as multiset::equal_range). Then an implementation has more room to optimize for certain cases (e.g. have log(distance) characteristics when at most match is found in the range but 2log(distance) + 4 for the worst case).
Proposed resolution:
In 26.8.4.2 [lower.bound]/4, change log(last - first) + 1
to log2(last - first) + O(1)
.
In 26.8.4.3 [upper.bound]/4, change log(last - first) + 1
to log2(last - first) + O(1)
.
In 26.8.4.4 [equal.range]/4, change 2*log(last - first) + 1
to 2*log2(last - first) + O(1)
.
[Matt provided wording]
Rationale:
The LWG considered just saying O(log n) for all three, but
decided that threw away too much valuable information. The fact
that lower_bound is twice as fast as equal_range is important.
However, it's better to allow an arbitrary additive constant than to
specify an exact count. An exact count would have to
involve floor
or ceil
. It would be too easy to
get this wrong, and don't provide any substantial value for users.
Section: 24.5.1.6 [reverse.iter.elem] Status: CD1 Submitter: Matt Austern Opened: 2002-10-23 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
In [reverse.iter.op-=], reverse_iterator<>::operator[]
is specified as having a return type of reverse_iterator::reference
,
which is the same as iterator_traits<Iterator>::reference
.
(Where Iterator
is the underlying iterator type.)
The trouble is that Iterator
's own operator[] doesn't
necessarily have a return type
of iterator_traits<Iterator>::reference
. Its
return type is merely required to be convertible
to Iterator
's value type. The return type specified for
reverse_iterator's operator[] would thus appear to be impossible.
With the resolution of issue 299(i), the type of
a[n]
will continue to be required (for random access
iterators) to be convertible to the value type, and also a[n] =
t
will be a valid expression. Implementations of
reverse_iterator
will likely need to return a proxy from
operator[]
to meet these requirements. As mentioned in the
comment from Dave Abrahams, the simplest way to specify that
reverse_iterator
meet this requirement to just mandate
it and leave the return type of operator[]
unspecified.
Proposed resolution:
In 24.5.1.3 [reverse.iter.requirements] change:
reference operator[](difference_type n) const;
to:
unspecified operator[](difference_type n) const; // see 24.3.5.7 [random.access.iterators]
[ Comments from Dave Abrahams: IMO we should resolve 386 by just saying that the return type of reverse_iterator's operator[] is unspecified, allowing the random access iterator requirements to impose an appropriate return type. If we accept 299's proposed resolution (and I think we should), the return type will be readable and writable, which is about as good as we can do. ]
std::complex
over-encapsulatedSection: 29.4 [complex.numbers] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The absence of explicit description of std::complex<T>
layout
makes it imposible to reuse existing software developed in traditional
languages like Fortran or C with unambigous and commonly accepted
layout assumptions. There ought to be a way for practitioners to
predict with confidence the layout of std::complex<T>
whenever T
is a numerical datatype. The absence of ways to access individual
parts of a std::complex<T>
object as lvalues unduly promotes
severe pessimizations. For example, the only way to change,
independently, the real and imaginary parts is to write something like
complex<T> z; // ... // set the real part to r z = complex<T>(r, z.imag()); // ... // set the imaginary part to i z = complex<T>(z.real(), i);
At this point, it seems appropriate to recall that a complex number
is, in effect, just a pair of numbers with no particular invariant to
maintain. Existing practice in numerical computations has it that a
complex number datatype is usually represented by Cartesian
coordinates. Therefore the over-encapsulation put in the specification
of std::complex<>
is not justified.
Proposed resolution:
Add the following requirements to 29.4 [complex.numbers] as 26.3/4:
If
z
is an lvalue expression of type cvstd::complex<T>
then
- the expression
reinterpret_cast<cv T(&)[2]>(z)
is well-formed; andreinterpret_cast<cv T(&)[2]>(z)[0]
designates the real part ofz
; andreinterpret_cast<cv T(&)[2]>(z)[1]
designates the imaginary part ofz
.Moreover, if
a
is an expression of pointer type cvcomplex<T>*
and the expressiona[i]
is well-defined for an integer expressioni
then:
reinterpret_cast<cv T*>(a)[2*i]
designates the real part ofa[i]
; andreinterpret_cast<cv T*>(a)[2*i+1]
designates the imaginary part ofa[i]
.
In 29.4.3 [complex] and [complex.special] add the following member functions
(changing T
to concrete types as appropriate for the specializations).
void real(T); void imag(T);
Add to 29.4.4 [complex.members]
T real() const;Returns: the value of the real component
void real(T val);Assigns
val
to the real component.T imag() const;Returns: the value of the imaginary component
void imag(T val);Assigns
val
to the imaginary component.
[Kona: The layout guarantee is absolutely necessary for C
compatibility. However, there was disagreement about the other part
of this proposal: retrieving elements of the complex number as
lvalues. An alternative: continue to have real()
and imag()
return
rvalues, but add set_real()
and set_imag()
. Straw poll: return
lvalues - 2, add setter functions - 5. Related issue: do we want
reinterpret_cast as the interface for converting a complex to an
array of two reals, or do we want to provide a more explicit way of
doing it? Howard will try to resolve this issue for the next
meeting.]
[pre-Sydney: Howard summarized the options in n1589.]
[ Bellevue: ]
Second half of proposed wording replaced and moved to Ready.
[ Pre-Sophia Antipolis, Howard adds: ]
Added the members to [complex.special] and changed from Ready to Review.
[ Post-Sophia Antipolis: ]
Moved from WP back to Ready so that the "and [complex.special]" in the proposed resolution can be officially applied.
Rationale:
The LWG believes that C99 compatibility would be enough justification for this change even without other considerations. All existing implementations already have the layout proposed here.
Section: 29.6.2.4 [valarray.access] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 77
Discussion:
Consider the following program:
#include <iostream> #include <ostream> #include <vector> #include <valarray> #include <algorithm> #include <iterator> template<typename Array> void print(const Array& a) { using namespace std; typedef typename Array::value_type T; copy(&a[0], &a[0] + a.size(), ostream_iterator<T>(std::cout, " ")); } template<typename T, unsigned N> unsigned size(T(&)[N]) { return N; } int main() { double array[] = { 0.89, 9.3, 7, 6.23 }; std::vector<double> v(array, array + size(array)); std::valarray<double> w(array, size(array)); print(v); // #1 std::cout << std::endl; print(w); // #2 std::cout << std::endl; }
While the call numbered #1 succeeds, the call numbered #2 fails because the const version of the member function valarray<T>::operator[](size_t) returns a value instead of a const-reference. That seems to be so for no apparent reason, no benefit. Not only does that defeats users' expectation but it also does hinder existing software (written either in C or Fortran) integration within programs written in C++. There is no reason why subscripting an expression of type valarray<T> that is const-qualified should not return a const T&.
Proposed resolution:
In the class synopsis in 29.6.2 [template.valarray], and in 29.6.2.4 [valarray.access] just above paragraph 1, change
T operator[](size_t const);
to
const T& operator[](size_t const);
[Kona: fixed a minor typo: put semicolon at the end of the line wehre it belongs.]
Rationale:
Return by value seems to serve no purpose. Valaray was explicitly designed to have a specified layout so that it could easily be integrated with libraries in other languages, and return by value defeats that purpose. It is believed that this change will have no impact on allowable optimizations.
Section: 28.3.3.3.2 [conversions.character] Status: CD1 Submitter: James Kanze Opened: 2002-12-10 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
The specifications of toupper and tolower both specify the functions as const, althought they are not member functions, and are not specified as const in the header file synopsis in section 28.3.3 [locales].
Proposed resolution:
In [conversions], remove const
from the function
declarations of std::toupper and std::tolower
Rationale:
Fixes an obvious typo
Section: 29.7 [c.math] Status: CD1 Submitter: James Kanze Opened: 2003-01-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 29.7 [c.math], the C++ standard refers to the C standard for the definition of rand(); in the C standard, it is written that "The implementation shall behave as if no library function calls the rand function."
In 26.7.13 [alg.random.shuffle], there is no specification as to how the two parameter version of the function generates its random value. I believe that all current implementations in fact call rand() (in contradiction with the requirement avove); if an implementation does not call rand(), there is the question of how whatever random generator it does use is seeded. Something is missing.
Proposed resolution:
In [lib.c.math], add a paragraph specifying that the C definition of rand shal be modified to say that "Unless otherwise specified, the implementation shall behave as if no library function calls the rand function."
In [lib.alg.random.shuffle], add a sentence to the effect that "In
the two argument form of the function, the underlying source of
random numbers is implementation defined. [Note: in particular, an
implementation is permitted to use rand
.]
Rationale:
The original proposed resolution proposed requiring the
two-argument from of random_shuffle
to
use rand
. We don't want to do that, because some existing
implementations already use something else: gcc
uses lrand48
, for example. Using rand
presents a
problem if the number of elements in the sequence is greater than
RAND_MAX.
Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.3.5.1, p6 [lib.bitset.cons] talks about a generic character having the value of 0 or 1 but there is no definition of what that means for charT other than char and wchar_t. And even for those two types, the values 0 and 1 are not actually what is intended -- the values '0' and '1' are. This, along with the converse problem in the description of to_string() in 23.3.5.2, p33, looks like a defect remotely related to DR 303(i).
23.3.5.1: -6- An element of the constructed string has value zero if the corresponding character in str, beginning at position pos, is 0. Otherwise, the element has the value one.
23.3.5.2: -33- Effects: Constructs a string object of the appropriate type and initializes it to a string of length N characters. Each character is determined by the value of its corresponding bit position in *this. Character position N ?- 1 corresponds to bit position zero. Subsequent decreasing character positions correspond to increasing bit positions. Bit value zero becomes the character 0, bit value one becomes the character 1.
Also note the typo in 23.3.5.1, p6: the object under construction is a bitset, not a string.
[ Sophia Antipolis: ]
We note that
bitset
has been moved from section 23 to section 20, by another issue (842(i)) previously resolved at this meeting.Disposition: move to ready.
We request that Howard submit a separate issue regarding the three
to_string
overloads.
[ post Bellevue: ]
We are happy with the resolution as proposed, and we move this to Ready.
[ Howard adds: ]
The proposed wording neglects the 3 newer
to_string
overloads.
Proposed resolution:
Change the constructor's function declaration immediately before 22.9.2.2 [bitset.cons] p3 to:
template <class charT, class traits, class Allocator> explicit bitset(const basic_string<charT, traits, Allocator>& str, typename basic_string<charT, traits, Allocator>::size_type pos = 0, typename basic_string<charT, traits, Allocator>::size_type n = basic_string<charT, traits, Allocator>::npos, charT zero = charT('0'), charT one = charT('1'))
Change the first two sentences of 22.9.2.2 [bitset.cons] p6 to: "An element of the constructed string has value 0 if the corresponding character in str, beginning at position pos, is zero. Otherwise, the element has the value 1.
Change the text of the second sentence in 23.3.5.1, p5 to read: "The function then throws invalid_argument if any of the rlen characters in str beginning at position pos is other than zero or one. The function uses traits::eq() to compare the character values."
Change the declaration of the to_string
member function
immediately before 22.9.2.3 [bitset.members] p33 to:
template <class charT, class traits, class Allocator> basic_string<charT, traits, Allocator> to_string(charT zero = charT('0'), charT one = charT('1')) const;
Change the last sentence of 22.9.2.3 [bitset.members] p33 to: "Bit
value 0 becomes the character zero
, bit value 1 becomes the
character one
.
Change 22.9.4 [bitset.operators] p8 to:
Returns:
os << x.template to_string<charT,traits,allocator<charT> >( use_facet<ctype<charT> >(os.getloc()).widen('0'), use_facet<ctype<charT> >(os.getloc()).widen('1'));
Rationale:
There is a real problem here: we need the character values of '0' and '1', and we have no way to get them since strings don't have imbued locales. In principle the "right" solution would be to provide an extra object, either a ctype facet or a full locale, which would be used to widen '0' and '1'. However, there was some discomfort about using such a heavyweight mechanism. The proposed resolution allows those users who care about this issue to get it right.
We fix the inserter to use the new arguments. Note that we already fixed the analogous problem with the extractor in issue 303(i).
Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.2.10.2 [allocator.members] allocator members, contains the following 3 lines:
12 Returns: new((void *) p) T( val) void destroy(pointer p); 13 Returns: ((T*) p)->~T()
The type cast "(T*) p" in the last line is redundant cause we know that std::allocator<T>::pointer is a typedef for T*.
Proposed resolution:
Replace "((T*) p)" with "p".
Rationale:
Just a typo, this is really editorial.
Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I think that in par2 of [default.con.req] the last two lines of table 32 contain two incorrect type casts. The lines are ...
a.construct(p,t) Effect: new((void*)p) T(t) a.destroy(p) Effect: ((T*)p)?->~T()
.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:
alloc<T> a ;// an allocator for T alloc<T>::pointer p ;// random access iterator // (may be different from T*) alloc<T>::reference r = *p;// T& T const& t ;
For that two type casts ("(void*)p" and "(T*)p") to be well-formed this would require then conversions to T* and void* for all alloc<T>::pointer, so it would implicitely introduce extra requirements for alloc<T>::pointer, additionally to the only current requirement (being a random access iterator).
Proposed resolution:
Accept proposed wording from N2436 part 1.
Note: Actually I would prefer to replace "((T*)p)?->dtor_name" with "p?->dtor_name", but AFAICS this is not possible cause of an omission in 12.4.6 [over.ref] (for which I have filed another DR on 29.11.2002).
[Kona: The LWG thinks this is somewhere on the border between
Open and NAD. The intend is clear: construct
constructs an
object at the location p. It's reading too much into the
description to think that literally calling new
is
required. Tweaking this description is low priority until we can do
a thorough review of allocators, and, in particular, allocators with
non-default pointer types.]
[ Batavia: Proposed resolution changed to less code and more description. ]
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 16.4.4.6 [allocator.requirements], 20.2.10.2 [allocator.members] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This applies to the new expression that is contained in both par12 of 20.2.10.2 [allocator.members] and in par2 (table 32) of [default.con.req]. I think this new expression is wrong, involving unintended side effects.
20.2.10.2 [allocator.members] contains the following 3 lines:
11 Returns: the largest value N for which the call allocate(N,0) might succeed. void construct(pointer p, const_reference val); 12 Returns: new((void *) p) T( val)
[default.con.req] in table 32 has the following line:
a.construct(p,t) Effect: new((void*)p) T(t)
.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:
alloc<T> a ;// an allocator for T alloc<T>::pointer p ;// random access iterator // (may be different from T*) alloc<T>::reference r = *p;// T& T const& t ;
Cause of using "new" but not "::new", any existing "T::operator new" function will hide the global placement new function. When there is no "T::operator new" with adequate signature, every_alloc<T>::construct(..) is ill-formed, and most std::container<T,every_alloc<T>> use it; a workaround would be adding placement new and delete functions with adequate signature and semantic to class T, but class T might come from another party. Maybe even worse is the case when T has placement new and delete functions with adequate signature but with "unknown" semantic: I dont like to speculate about it, but whoever implements any_container<T,any_alloc> and wants to use construct(..) probably must think about it.
Proposed resolution:
Replace "new" with "::new" in both cases.
Section: 27.4.3.7.8 [string.swap] Status: CD1 Submitter: Beman Dawes Opened: 2003-03-25 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
std::basic_string, 27.4.3 [basic.string] paragraph 2 says that basic_string "conforms to the requirements of a Sequence, as specified in (23.1.1)." The sequence requirements specified in (23.1.1) to not include any prohibition on swap members throwing exceptions.
Section 23.2 [container.requirements] paragraph 10 does limit conditions under which exceptions may be thrown, but applies only to "all container types defined in this clause" and so excludes basic_string::swap because it is defined elsewhere.
Eric Niebler points out that 27.4.3 [basic.string] paragraph 5 explicitly permits basic_string::swap to invalidates iterators, which is disallowed by 23.2 [container.requirements] paragraph 10. Thus the standard would be contradictory if it were read or extended to read as having basic_string meet 23.2 [container.requirements] paragraph 10 requirements.
Yet several LWG members have expressed the belief that the original intent was that basic_string::swap should not throw exceptions as specified by 23.2 [container.requirements] paragraph 10, and that the standard is unclear on this issue. The complexity of basic_string::swap is specified as "constant time", indicating the intent was to avoid copying (which could cause a bad_alloc or other exception). An important use of swap is to ensure that exceptions are not thrown in exception-safe code.
Note: There remains long standing concern over whether or not it is possible to reasonably meet the 23.2 [container.requirements] paragraph 10 swap requirements when allocators are unequal. The specification of basic_string::swap exception requirements is in no way intended to address, prejudice, or otherwise impact that concern.
Proposed resolution:
In 27.4.3.7.8 [string.swap], add a throws clause:
Throws: Shall not throw exceptions.
Section: 16.4.5.6 [replacement.functions], 17.6.3 [new.delete] Status: CD1 Submitter: Matt Austern Opened: 2003-04-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The eight basic dynamic memory allocation functions (single-object and array versions of ::operator new and ::operator delete, in the ordinary and nothrow forms) are replaceable. A C++ program may provide an alternative definition for any of them, which will be used in preference to the implementation's definition.
Three different parts of the standard mention requirements on replacement functions: 16.4.5.6 [replacement.functions], 17.6.3.2 [new.delete.single] and 17.6.3.3 [new.delete.array], and 6.7.6.4 [basic.stc.auto].
None of these three places say whether a replacement function may
be declared inline. 17.6.3.2 [new.delete.single] paragraph 2 specifies a
signature for the replacement function, but that's not enough:
the inline
specifier is not part of a function's signature.
One might also reason from 9.2.3 [dcl.fct.spec] paragraph 2, which
requires that "an inline function shall be defined in every
translation unit in which it is used," but this may not be quite
specific enough either. We should either explicitly allow or
explicitly forbid inline replacement memory allocation
functions.
Proposed resolution:
Add a new sentence to the end of 16.4.5.6 [replacement.functions] paragraph 3:
"The program's definitions shall not be specified as inline
.
No diagnostic is required."
[Kona: added "no diagnostic is required"]
Rationale:
The fact that inline
isn't mentioned appears to have been
nothing more than an oversight. Existing implementations do not
permit inline functions as replacement memory allocation functions.
Providing this functionality would be difficult in some cases, and is
believed to be of limited value.
Section: 26.13 [alg.c.library] Status: CD1 Submitter: Ray Lischner Opened: 2003-04-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 26.13 [alg.c.library] describes bsearch and qsort, from the C standard library. Paragraph 4 does not list any restrictions on qsort, but it should limit the base parameter to point to POD. Presumably, qsort sorts the array by copying bytes, which requires POD.
Proposed resolution:
In 26.13 [alg.c.library] paragraph 4, just after the declarations and before the nonnormative note, add these words: "both of which have the same behavior as the original declaration. The behavior is undefined unless the objects in the array pointed to by base are of POD type."
[Something along these lines is clearly necessary. Matt provided wording.]
Section: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Dave Abrahams Opened: 2003-04-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is a possible defect in the standard: the standard text was never intended to prevent arbitrary ForwardIterators, whose operations may throw exceptions, from being passed, and it also wasn't intended to require a temporary buffer in the case where ForwardIterators were passed (and I think most implementations don't use one). As is, the standard appears to impose requirements that aren't met by any existing implementation.
Proposed resolution:
Replace 23.3.13.5 [vector.modifiers] paragraph 1 with:
1- Notes: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor or assignment operator of T or by any InputIterator operation there are no effects.
[We probably need to say something similar for deque.]
Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Clause 24.3.4 [iterator.concepts], paragraph 5, says that the only expression that is defined for a singular iterator is "an assignment of a non-singular value to an iterator that holds a singular value". This means that destroying a singular iterator (e.g. letting an automatic variable go out of scope) is technically undefined behavior. This seems overly strict, and probably unintentional.
Proposed resolution:
Change the sentence in question to "... the only exceptions are destroying an iterator that holds a singular value, or the assignment of a non-singular value to an iterator that holds a singular value."
Section: 31.10.4.4 [ifstream.members], 31.10.5.4 [ofstream.members] Status: CD1 Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A strict reading of [fstreams] shows that opening or closing a basic_[io]fstream does not affect the error bits. This means, for example, that if you read through a file up to EOF, and then close the stream and reopen it at the beginning of the file, the EOF bit in the stream's error state is still set. This is counterintuitive.
The LWG considered this issue once before, as issue 22(i), and put in a footnote to clarify that the strict reading was indeed correct. We did that because we believed the standard was unambiguous and consistent, and that we should not make architectural changes in a TC. Now that we're working on a new revision of the language, those considerations no longer apply.
Proposed resolution:
Change 31.10.4.4 [ifstream.members], para. 3 from:
Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)].
to:
Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().
Change 31.10.5.4 [ofstream.members], para. 3 from:
Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)).
to:
Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().
Change 31.10.6.4 [fstream.members], para. 3 from:
Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) )
to:
Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) ), else calls clear().
[Kona: the LWG agrees this is a good idea. Post-Kona: Bill provided wording. He suggests having open, not close, clear the error flags.]
[Post-Sydney: Howard provided a new proposed resolution. The old one didn't make sense because it proposed to fix this at the level of basic_filebuf, which doesn't have access to the stream's error state. Howard's proposed resolution fixes this at the level of the three fstream class template instead.]
Section: 23.3.11.2 [list.cons], 23.3.11.4 [list.modifiers] Status: CD1 Submitter: Hans Bos Opened: 2003-06-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Sections 23.3.11.2 [list.cons] and 23.3.11.4 [list.modifiers] list comparison operators (==, !=, <, <=, >, =>) for queue and stack. Only the semantics for queue::operator== (23.3.11.2 [list.cons] par2) and queue::operator< (23.3.11.2 [list.cons] par3) are defined.
Proposed resolution:
Add the following new paragraphs after 23.3.11.2 [list.cons] paragraph 3:
operator!=Returns:
x.c != y.c
operator>Returns:
x.c > y.c
operator<=Returns:
x.c <= y.c
operator>=Returns:
x.c >= y.c
Add the following paragraphs at the end of 23.3.11.4 [list.modifiers]:
operator==Returns:
x.c == y.c
operator<Returns:
x.c < y.c
operator!=Returns:
x.c != y.c
operator>Returns:
x.c > y.c
operator<=Returns:
x.c <= y.c
operator>=Returns:
x.c >= y.c
[Kona: Matt provided wording.]
Rationale:
There isn't any real doubt about what these operators are supposed to do, but we ought to spell it out.
Section: 26.8.7 [alg.set.operations] Status: CD1 Submitter: Daniel Frey Opened: 2003-07-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
26.8.7 [alg.set.operations] paragraph 1 reads: "The semantics of the set operations are generalized to multisets in a standard way by defining union() to contain the maximum number of occurrences of every element, intersection() to contain the minimum, and so on."
This is wrong. The name of the functions are set_union() and set_intersection(), not union() and intersection().
Proposed resolution:
Change that sentence to use the correct names.
Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2003-07-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 429
Discussion:
The Effects clause in 31.5.4.4 [iostate.flags] paragraph 5 says that the function only throws if the respective bits are already set prior to the function call. That's obviously not the intent. The typo ought to be corrected and the text reworded as: "If (state & exceptions()) == 0, returns. ..."
Proposed resolution:
In 31.5.4.4 [iostate.flags] paragraph 5, replace "If (rdstate() & exceptions()) == 0" with "If ((state | (rdbuf() ? goodbit : badbit)) & exceptions()) == 0".
[Kona: the original proposed resolution wasn't quite right. We really do mean rdstate(); the ambiguity is that the wording in the standard doesn't make it clear whether we mean rdstate() before setting the new state, or rdsate() after setting it. We intend the latter, of course. Post-Kona: Martin provided wording.]
Section: 31.7.5.3.3 [istream.extractors] Status: CD1 Submitter: Bo Persson Opened: 2003-07-13 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
The second sentence of the proposed resolution says:
"If it inserted no characters because it caught an exception thrown while extracting characters from sb and ..."
However, we are not extracting from sb, but extracting from the basic_istream (*this) and inserting into sb. I can't really tell if "extracting" or "sb" is a typo.
[ Sydney: Definitely a real issue. We are, indeed, extracting characters from an istream and not from sb. The problem was there in the FDIS and wasn't fixed by issue 64(i). Probably what was intended was to have *this instead of sb. We're talking about the exception flag state of a basic_istream object, and there's only one basic_istream object in this discussion, so that would be a consistent interpretation. (But we need to be careful: the exception policy of this member function must be consistent with that of other extractors.) PJP will provide wording. ]
Proposed resolution:
Change the sentence from:
If it inserted no characters because it caught an exception thrown while extracting characters from sb and failbit is on in exceptions(), then the caught exception is rethrown.
to:
If it inserted no characters because it caught an exception thrown while extracting characters from *this and failbit is on in exceptions(), then the caught exception is rethrown.
Section: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Matt Austern Opened: 2003-08-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Consider the following code fragment:
int A[8] = { 1,3,5,7,9,8,4,2 }; std::vector<int> v(A, A+8); std::vector<int>::iterator i1 = v.begin() + 3; std::vector<int>::iterator i2 = v.begin() + 4; v.erase(i1);
Which iterators are invalidated by v.erase(i1)
: i1, i2,
both, or neither?
On all existing implementations that I know of, the status of i1 and i2 is the same: both of them will be iterators that point to some elements of the vector (albeit not the same elements they did before). You won't get a crash if you use them. Depending on exactly what you mean by "invalidate", you might say that neither one has been invalidated because they still point to something, or you might say that both have been invalidated because in both cases the elements they point to have been changed out from under the iterator.
The standard doesn't say either of those things. It says that erase invalidates all iterators and references "after the point of the erase". This doesn't include i1, since it's at the point of the erase instead of after it. I can't think of any sensible definition of invalidation by which one can say that i2 is invalidated but i1 isn't.
(This issue is important if you try to reason about iterator validity based only on the guarantees in the standard, rather than reasoning from typical implementation techniques. Strict debugging modes, which some programmers find useful, do not use typical implementation techniques.)
Proposed resolution:
In 23.3.13.5 [vector.modifiers] paragraph 3, change "Invalidates all the iterators and references after the point of the erase" to "Invalidates iterators and references at or after the point of the erase".
Rationale:
I believe this was essentially a typographical error, and that it was taken for granted that erasing an element invalidates iterators that point to it. The effects clause in question treats iterators and references in parallel, and it would seem counterintuitive to say that a reference to an erased value remains valid.
Section: 31.7.5.5 [istream.manip] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to 27.6.1.4, the ws() manipulator is not required to construct the sentry object. The manipulator is also not a member function so the text in 27.6.1, p1 through 4 that describes the exception policy for istream member functions does not apply. That seems inconsistent with the rest of extractors and all the other input functions (i.e., ws will not cause a tied stream to be flushed before extraction, it doesn't check the stream's exceptions or catch exceptions thrown during input, and it doesn't affect the stream's gcount).
Proposed resolution:
Add to 31.7.5.5 [istream.manip], immediately before the first sentence of paragraph 1, the following text:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to is.gcount(). After constructing a sentry object...
[Post-Kona: Martin provided wording]
Section: 17.3.6 [climits.syn] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
Given two overloads of the function foo(), one taking an argument of type
int and the other taking a long, which one will the call foo(LONG_MAX)
resolve to? The expected answer should be foo(long), but whether that
is true depends on the #defintion of the LONG_MAX macro, specifically
its type. This issue is about the fact that the type of these macros
is not actually required to be the same as the the type each respective
limit.
Section 18.2.2 of the C++ Standard does not specify the exact types of
the XXX_MIN and XXX_MAX macros #defined in the <climits> and <limits.h>
headers such as INT_MAX and LONG_MAX and instead defers to the C standard.
Section 5.2.4.2.1, p1 of the C standard specifies that "The values [of
these constants] shall be replaced by constant expressions suitable for use
in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX,
the following shall be replaced by expressions that have the same type as
would an expression that is an object of the corresponding type converted
according to the integer promotions."
The "corresponding type converted according to the integer promotions" for
LONG_MAX is, according to 6.4.4.1, p5 of the C standard, the type of long
converted to the first of the following set of types that can represent it:
int, long int, long long int. So on an implementation where (sizeof(long)
== sizeof(int)) this type is actually int, while on an implementation where
(sizeof(long) > sizeof(int)) holds this type will be long.
This is not an issue in C since the type of the macro cannot be detected
by any conforming C program, but it presents a portability problem in C++
where the actual type is easily detectable by overload resolution.
[Kona: the LWG does not believe this is a defect. The C macro
definitions are what they are; we've got a better
mechanism, std::numeric_limits
, that is specified more
precisely than the C limit macros. At most we should add a
nonnormative note recommending that users who care about the exact
types of limit quantities should use <limits> instead of
<climits>.]
Proposed resolution:
Change [c.limits], paragraph 2:
-2- The contents are the same as the Standard C library header
<limits.h>
. [Note: The types of the macros in<climits>
are not guaranteed to match the type to which they refer.--end note]
failbit
if eofbit
is already setSection: 31.7.5.2.4 [istream.sentry] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
[istream::sentry], p2 says that istream::sentry
ctor prepares for input if is.good()
is true. p4 then goes on to say that the ctor sets the sentry::ok_
member to
true if the stream state is good after any preparation. 31.7.5.3.1 [istream.formatted.reqmts], p1 then
says that a formatted input function endeavors to obtain the requested input
if the sentry's operator bool()
returns true.
Given these requirements, no formatted extractor should ever set failbit
if
the initial stream rdstate() == eofbit
. That is contrary to the behavior of
all implementations I tested. The program below prints out
eof = 1, fail = 0 eof = 1, fail = 1
on all of them.
#include <sstream> #include <cstdio> int main() { std::istringstream strm ("1"); int i = 0; strm >> i; std::printf ("eof = %d, fail = %d\n", !!strm.eof (), !!strm.fail ()); strm >> i; std::printf ("eof = %d, fail = %d\n", !!strm.eof (), !!strm.fail ()); }
Comments from Jerry Schwarz (c++std-lib-11373):
Jerry Schwarz wrote:
I don't know where (if anywhere) it says it in the standard, but the
formatted extractors are supposed to set failbit
if they don't extract
any characters. If they didn't then simple loops like
while (cin >> x);
would loop forever.
Further comments from Martin Sebor:
The question is which part of the extraction should prevent this from happening
by setting failbit
when eofbit
is already set. It could either be the sentry
object or the extractor. It seems that most implementations have chosen to
set failbit
in the sentry [...] so that's the text that will need to be
corrected.
Pre Berlin: This issue is related to 342(i). If the sentry
sets failbit
when it finds eofbit
already set, then
you can never seek away from the end of stream.
Kona: Possibly NAD. If eofbit
is set then good()
will return false. We
then set ok to false. We believe that the sentry's
constructor should always set failbit
when ok is false, and
we also think the standard already says that. Possibly it could be
clearer.
[ 2009-07 Frankfurt ]
Moved to Ready.
Proposed resolution:
Change [istream::sentry], p2 to:
explicit sentry(basic_istream<charT,traits>& is , bool noskipws = false);-2- Effects: If
is.good()
istrue
false
, callsis.setstate(failbit)
. Otherwise prepares for formatted or unformatted input. ...
Section: 31.10 [file.streams] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
7.19.1, p2, of C99 requires that the FILE type only be declared in <stdio.h>. None of the (implementation-defined) members of the struct is mentioned anywhere for obvious reasons.
C++ says in 27.8.1, p2 that FILE is a type that's defined in <cstdio>. Is it really the intent that FILE be a complete type or is an implementation allowed to just declare it without providing a full definition?
Proposed resolution:
In the first sentence of [fstreams] paragraph 2, change "defined" to "declared".
Rationale:
We don't want to impose any restrictions beyond what the C standard already says. We don't want to make anything implementation defined, because that imposes new requirements in implementations.
Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It has been suggested that 17.4.3.1, p1 may or may not allow programs to explicitly specialize members of standard templates on user-defined types. The answer to the question might have an impact where library requirements are given using the "as if" rule. I.e., if programs are allowed to specialize member functions they will be able to detect an implementation's strict conformance to Effects clauses that describe the behavior of the function in terms of the other member function (the one explicitly specialized by the program) by relying on the "as if" rule.
Proposed resolution:
Add the following sentence to 16.4.5.3 [reserved.names], p1:
It is undefined for a C++ program to add declarations or definitions to namespace std or namespaces within namespace
std
unless otherwise specified. A program may add template specializations for any standard library template to namespacestd
. Such a specialization (complete or partial) of a standard library template results in undefined behavior unless the declaration depends on a user-defined type of external linkage and unless the specialization meets the standard library requirements for the original template.168) A program has undefined behavior if it declares
- an explicit specialization of any member function of a standard library class template, or
- an explicit specialization of any member function template of a standard library class or class template, or
- an explicit or partial specialization of any member class template of a standard library class or class template.
A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.
[Kona: straw poll was 6-1 that user programs should not be allowed to specialize individual member functions of standard library class templates, and that doing so invokes undefined behavior. Post-Kona: Martin provided wording.]
[Sydney: The LWG agrees that the standard shouldn't permit users to specialize individual member functions unless they specialize the whole class, but we're not sure these words say what we want them to; they could be read as prohibiting the specialization of any standard library class templates. We need to consult with CWG to make sure we use the right wording.]
Section: 99 [depr.temporary.buffer] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
The standard is not clear about the requirements on the value returned from a call to get_temporary_buffer(0). In particular, it fails to specify whether the call should return a distinct pointer each time it is called (like operator new), or whether the value is unspecified (as if returned by malloc). The standard also fails to mention what the required behavior is when the argument is less than 0.
Proposed resolution:
Change 21.3.4 [meta.help] paragraph 2 from "...or a pair of 0 values if no storage can be obtained" to "...or a pair of 0 values if no storage can be obtained or if n <= 0."
[Kona: Matt provided wording]
Section: 26.6.15 [alg.search], 26.7.6 [alg.fill], 26.7.7 [alg.generate] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity requirements for these function templates are incorrect (or don't even make sense) for negative n:
25.1.9, p7 (search_n):
Complexity: At most (last1 - first1) * count applications
of the corresponding predicate.
25.2.5, p3 (fill_n):
Complexity: Exactly last - first (or n) assignments.
25.2.6, p3 (generate_n):
Complexity: Exactly last - first (or n) assignments.
In addition, the Requirements or the Effects clauses for the latter two templates don't say anything about the behavior when n is negative.
Proposed resolution:
Change 25.1.9, p7 to
Complexity: At most (last1 - first1) * count applications of the corresponding predicate if count is positive, or 0 otherwise.
Change 25.2.5, p2 to
Effects: Assigns value through all the iterators in the range [first, last), or [first, first + n) if n is positive, none otherwise.
Change 25.2.5, p3 to:
Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.
Change 25.2.6, p1 to (notice the correction for the misspelled "through"):
Effects: Invokes the function object genand assigns the return value of gen through all the iterators in the range [first, last), or [first, first + n) if n is positive, or [first, first) otherwise.
Change 25.2.6, p3 to:
Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.
Rationale:
Informally, we want to say that whenever we see a negative number we treat it the same as if it were zero. We believe the above changes do that (although they may not be the minimal way of saying so). The LWG considered and rejected the alternative of saying that negative numbers are undefined behavior.
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The requirements specified in Stage 2 and reiterated in the rationale of DR 221 (and echoed again in DR 303) specify that num_get<charT>:: do_get() compares characters on the stream against the widened elements of "012...abc...ABCX+-"
An implementation is required to allow programs to instantiate the num_get template on any charT that satisfies the requirements on a user-defined character type. These requirements do not include the ability of the character type to be equality comparable (the char_traits template must be used to perform tests for equality). Hence, the num_get template cannot be implemented to support any arbitrary character type. The num_get template must either make the assumption that the character type is equality-comparable (as some popular implementations do), or it may use char_traits<charT> to do the comparisons (some other popular implementations do that). This diversity of approaches makes it difficult to write portable programs that attempt to instantiate the num_get template on user-defined types.
[Kona: the heart of the problem is that we're theoretically
supposed to use traits classes for all fundamental character
operations like assignment and comparison, but facets don't have
traits parameters. This is a fundamental design flaw and it
appears all over the place, not just in this one place. It's not
clear what the correct solution is, but a thorough review of facets
and traits is in order. The LWG considered and rejected the
possibility of changing numeric facets to use narrowing instead of
widening. This may be a good idea for other reasons (see issue
459(i)), but it doesn't solve the problem raised by this
issue. Whether we use widen or narrow the num_get
facet
still has no idea which traits class the user wants to use for
the comparison, because only streams, not facets, are passed traits
classes. The standard does not require that two different
traits classes with the same char_type
must necessarily
have the same behavior.]
Informally, one possibility: require that some of the basic
character operations, such as eq
, lt
,
and assign
, must behave the same way for all traits classes
with the same char_type
. If we accept that limitation on
traits classes, then the facet could reasonably be required to
use char_traits<charT>
.
[ 2009-07 Frankfurt ]
There was general agreement that the standard only needs to specify the behavior when the character type is char or wchar_t.
Beman: we don't need to worry about C++1x because there is a non-zero possibility that we would have a replacement facility for iostreams that would solve these problems.
We need to change the following sentence in [locale.category], paragraph 6 to specify that C is char and wchar_t:
"A template formal parameter with name C represents the set of all possible specializations on a parameter that satisfies the requirements for a character on which any member of the iostream components can be instantiated."
We also need to specify in 27 that the basic character operations, such as eq, lt, and assign use std::char_traits.
Daniel volunteered to provide wording.
[ 2009-09-19 Daniel provided wording. ]
[ 2009-10 Santa Cruz: ]
Leave as Open. Alisdair and/or Tom will provide wording based on discussions. We want to clearly state that streams and locales work just on
char
andwchar_t
(except where otherwise specified).
[ 2010-02-06 Tom updated the proposed wording. ]
[ The original proposed wording is preserved here: ]
Change 28.3.3.1.2.1 [locale.category]/6:
[..] A template formal parameter with name
C
represents the set of all possible specializations on achar
orwchar_t
parameterthat satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]Add the following sentence to the end of 28.3.4.3 [category.numeric]/2:
[..] These specializations refer to [..], and also for the
ctype<>
facet to perform character classification. Implementations are encouraged but not required to use thechar_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations.Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3:
Stage 2: If
in==end
then stage 2 terminates. Otherwise acharT
is taken fromin
and local variables are initialized as if bychar_type ct = *in; using tr = char_traits<char_type>; const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct); char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atomspos ? pos - atoms : sizeof(src) - 1]; if (tr::eq(ct,ct ==use_facet<numpunct<charT>(loc).decimal_point())) c = '.'; bool discard = tr::eq(ct,ct ==use_facet<numpunct<charT>(loc).thousands_sep()) && use_facet<numpunct<charT> >(loc).grouping().length() != 0;where the values
src
andatoms
are defined as if by: [..][Remark of the author: I considered to replace the initialization "
char_type ct = *in;
" by the sequence "char_type ct; tr::assign(ct, *in);
", but decided against it, because it is a copy-initialization context, not an assignment]Add the following sentence to the end of 28.3.4.6 [category.time]/1:
[..] Their members use [..] , to determine formatting details. Implementations are encouraged but not required to use the
char_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations.Change 28.3.4.6.2.2 [locale.time.get.members]/8 bullet 4:
The next element ofFor the next elementfmt
is equal to'%'
c
offmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true
, [..]Add the following sentence to the end of 28.3.4.7 [category.monetary]/2:
Their members use [..] to determine formatting details. Implementations are encouraged but not required to use the
char_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations.Change 28.3.4.7.2.3 [locale.money.get.virtuals]/4:
[..] The value
units
is produced as if by:for (int i = 0; i < n; ++i) buf2[i] = src[char_traits<charT>::find(atoms,atoms+sizeof(src), buf1[i]) - atoms]; buf2[n] = 0; sscanf(buf2, "%Lf", &units);Change 28.3.4.7.3.3 [locale.money.put.virtuals]/1:
[..] for character buffers
buf1
andbuf2
. If for the first characterc
indigits
orbuf2
is equal toct.widen('-')
char_traits<charT>::eq(c, ct.widen('-')) == true
, [..]Add a footnote to the first sentence of 31.7.5.3.2 [istream.formatted.arithmetic]/1:
As in the case of the inserters, these extractors depend on the locale's
num_get<>
(22.4.2.1) object to perform parsing the input stream data.(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote to the second sentence of 31.7.6.3.2 [ostream.inserters.arithmetic]/1:
Effects: The classes
num_get<>
andnum_put<>
handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/4:
Returns: An object of unspecified type such that if in is an object of type
basic_istream<charT, traits>
then the expressionin >> get_money(mon, intl)
behaves as if it calledf(in, mon, intl)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/5:
Returns: An object of unspecified type such that if
out
is an object of typebasic_ostream<charT, traits>
then the expressionout << put_money(mon, intl)
behaves as a formatted input function that callsf(out, mon, intl)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.13) Add a footnote after the first sentence of 31.7.8 [ext.manip]/8:
Returns: An object of unspecified type such that if
in
is an object of type basic_istream<charT, traits>
then the expressionin >>get_time(tmb, fmt)
behaves as if it calledf(in, tmb, fmt)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/10:
Returns: An object of unspecified type such that if
out
is an object of typebasic_ostream<charT, traits>
then the expressionout <<put_time(tmb, fmt)
behaves as if it calledf(out, tmb, fmt)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.
[ 2010 Pittsburgh: ]
Moved to Ready with only two of the bullets. The original wording is preserved here:
Change 28.3.3.1.2.1 [locale.category]/6:
[..] A template formal parameter with name
C
represents the setof all possible specializations on aof types containingchar
,wchar_t
, and any other implementation-defined character typeparameterthat satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]Add the following sentence to the end of 28.3.4.3 [category.numeric]/2:
[..] These specializations refer to [..], and also for the
ctype<>
facet to perform character classification. [Note: Implementations are encouraged but not required to use thechar_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations - end note].Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3:
Stage 2: If
in==end
then stage 2 terminates. Otherwise acharT
is taken fromin
and local variables are initialized as if bychar_type ct = *in; using tr = char_traits<char_type>; const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct); char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atomspos ? pos - atoms : sizeof(src) - 1]; if (tr::eq(ct,ct ==use_facet<numpunct<charT>(loc).decimal_point())) c = '.'; bool discard = tr::eq(ct,ct ==use_facet<numpunct<charT>(loc).thousands_sep()) && use_facet<numpunct<charT> >(loc).grouping().length() != 0;where the values
src
andatoms
are defined as if by: [..][Remark of the author: I considered to replace the initialization "
char_type ct = *in;
" by the sequence "char_type ct; tr::assign(ct, *in);
", but decided against it, because it is a copy-initialization context, not an assignment]Add the following sentence to the end of 28.3.4.6 [category.time]/1:
[..] Their members use [..] , to determine formatting details. [Note: Implementations are encouraged but not required to use the
char_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations - end note].Change 28.3.4.6.2.2 [locale.time.get.members]/8 bullet 4:
The next element ofFor the next elementfmt
is equal to'%'
c
offmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true
, [..]Add the following sentence to the end of 28.3.4.7 [category.monetary]/2:
Their members use [..] to determine formatting details. [Note: Implementations are encouraged but not required to use the
char_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations - end note].Change 28.3.4.7.2.3 [locale.money.get.virtuals]/4:
[..] The value
units
is produced as if by:for (int i = 0; i < n; ++i) buf2[i] = src[char_traits<charT>::find(atoms,atoms+sizeof(src), buf1[i]) - atoms]; buf2[n] = 0; sscanf(buf2, "%Lf", &units);Change 28.3.4.7.3.3 [locale.money.put.virtuals]/1:
[..] for character buffers
buf1
andbuf2
. If for the first characterc
indigits
orbuf2
is equal toct.widen('-')
char_traits<charT>::eq(c, ct.widen('-')) == true
, [..]Add a new paragraph after the first paragraph of 31.2.3 [iostreams.limits.pos]/1:
In the classes of clause 27, a template formal parameter with name
charT
represents one of the set of types containingchar
,wchar_t
, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.Add a footnote to the first sentence of 31.7.5.3.2 [istream.formatted.arithmetic]/1:
As in the case of the inserters, these extractors depend on the locale's
num_get<>
(22.4.2.1) object to perform parsing the input stream data.(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote to the second sentence of 31.7.6.3.2 [ostream.inserters.arithmetic]/1:
Effects: The classes
num_get<>
andnum_put<>
handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/4:
Returns: An object of unspecified type such that if in is an object of type
basic_istream<charT, traits>
then the expressionin >> get_money(mon, intl)
behaves as if it calledf(in, mon, intl)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/5:
Returns: An object of unspecified type such that if
out
is an object of typebasic_ostream<charT, traits>
then the expressionout << put_money(mon, intl)
behaves as a formatted input function that callsf(out, mon, intl)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/8:
Returns: An object of unspecified type such that if
in
is an object of type basic_istream<charT, traits>
then the expressionin >>get_time(tmb, fmt)
behaves as if it calledf(in, tmb, fmt)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/10:
Returns: An object of unspecified type such that if
out
is an object of typebasic_ostream<charT, traits>
then the expressionout <<put_time(tmb, fmt)
behaves as if it calledf(out, tmb, fmt)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.
Proposed resolution:
Change 28.3.3.1.2.1 [locale.category]/6:
[..] A template formal parameter with name
C
represents the setof all possible specializations on aof types containingchar
,wchar_t
, and any other implementation-defined character typeparameterthat satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]
Add a new paragraph after the first paragraph of 31.2.3 [iostreams.limits.pos]/1:
In the classes of clause 27, a template formal parameter with name
charT
represents one of the set of types containingchar
,wchar_t
, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.
Section: 27.4.3.7.5 [string.erase] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-11-12
Priority: Not Prioritized
View all other issues in [string.erase].
View all issues with CD1 status.
Discussion:
23.1.1, p3 along with Table 67 specify as a prerequisite for a.erase(q) that q must be a valid dereferenceable iterator into the sequence a.
However, 21.3.5.5, p5 describing string::erase(p) only requires that p be a valid iterator.
This may be interepreted as a relaxation of the general requirement, which is most likely not the intent.
Proposed resolution:
Remove 27.4.3.7.5 [string.erase] paragraph 5.
Rationale:
The LWG considered two options: changing the string requirements to match the general container requirements, or just removing the erroneous string requirements altogether. The LWG chose the latter option, on the grounds that duplicating text always risks the possibility that it might be duplicated incorrectly.
valarray
subset operationsSection: 29.6.2.5 [valarray.sub] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
The standard fails to specify the behavior of valarray::operator[](slice) and other valarray subset operations when they are passed an "invalid" slice object, i.e., either a slice that doesn't make sense at all (e.g., slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray object (e.g., slice (2, 1, 1) for a valarray of size 1).
[Kona: the LWG believes that invalid slices should invoke undefined behavior. Valarrays are supposed to be designed for high performance, so we don't want to require specific checking. We need wording to express this decision.]
[ Bellevue: ]
Please note that the standard also fails to specify the behavior of slice_array and gslice_array in the valid case. Bill Plauger will endeavor to provide revised wording for
slice_array
andgslice_array
.
[ post-Bellevue: Bill provided wording. ]
[ 2009-07 Frankfurt ]
Move to Ready.
[ 2009-11-04 Pete opens: ]
The resolution to LWG issue 430(i) has not been applied — there have been changes to the underlying text, and the resolution needs to be reworked.
[ 2010-03-09 Matt updated wording. ]
[ 2010 Pittsburgh: Moved to Ready for Pittsburgh. ]
Proposed resolution:
Replace 29.6.2.5 [valarray.sub], with the following:
The member operator is overloaded to provide several ways to select sequences of elements from among those controlled by
*this
. Each of these operations returns a subset of the array. The const-qualified versions return this subset as a newvalarray
. The non-const versions return a class template object which has reference semantics to the original array, working in conjunction with various overloads ofoperator=
(and other assigning operators) to allow selective replacement (slicing) of the controlled sequence. In each case the selected element(s) must exist.valarray<T> operator[](slice slicearr) const;This function returns an object of class
valarray<T>
containing those elements of the controlled sequence designated byslicearr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDE", 5); v0[slice(2, 5, 3)] = v1; // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)end example]
valarray<T> operator[](slice slicearr);This function selects those elements of the controlled sequence designated by
slicearr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDE", 5); v0[slice(2, 5, 3)] = v1; // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)end example]
valarray<T> operator[](const gslice& gslicearr) const;This function returns an object of class
valarray<T>
containing those elements of the controlled sequence designated bygslicearr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); const size_t lv[] = {2, 3}; const size_t dv[] = {7, 2}; const valarray<size_t> len(lv, 2), str(dv, 2); // v0[gslice(3, len, str)] returns // valarray<char>("dfhkmo", 6)end example]
gslice_array<T> operator[](const gslice& gslicearr);This function selects those elements of the controlled sequence designated by
gslicearr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDEF", 6); const size_t lv[] = {2, 3}; const size_t dv[] = {7, 2}; const valarray<size_t> len(lv, 2), str(dv, 2); v0[gslice(3, len, str)] = v1; // v0 == valarray<char>("abcAeBgCijDlEnFp", 16)end example]
valarray<T> operator[](const valarray<bool>& boolarr) const;This function returns an object of class
valarray<T>
containing those elements of the controlled sequence designated byboolarr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); const bool vb[] = {false, false, true, true, false, true}; // v0[valarray<bool>(vb, 6)] returns // valarray<char>("cdf", 3)end example]
mask_array<T> operator[](const valarray<bool>& boolarr);This function selects those elements of the controlled sequence designated by
boolarr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABC", 3); const bool vb[] = {false, false, true, true, false, true}; v0[valarray<bool>(vb, 6)] = v1; // v0 == valarray<char>("abABeCghijklmnop", 16)end example]
valarray<T> operator[](const valarray<size_t>& indarr) const;This function returns an object of class
valarray<T>
containing those elements of the controlled sequence designated byindarr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); const size_t vi[] = {7, 5, 2, 3, 8}; // v0[valarray<size_t>(vi, 5)] returns // valarray<char>("hfcdi", 5)end example]
indirect_array<T> operator[](const valarray<size_t>& indarr);This function selects those elements of the controlled sequence designated by
indarr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDE", 5); const size_t vi[] = {7, 5, 2, 3, 8}; v0[valarray<size_t>(vi, 5)] = v1; // v0 == valarray<char>("abCDeBgAEjklmnop", 16)end example]
Section: 16.4.4.6 [allocator.requirements], 26 [algorithms] Status: Resolved Submitter: Matt Austern Opened: 2003-09-20 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with Resolved status.
Discussion:
Clause 16.4.4.6 [allocator.requirements] paragraph 4 says that implementations are permitted to supply containers that are unable to cope with allocator instances and that container implementations may assume that all instances of an allocator type compare equal. We gave implementers this latitude as a temporary hack, and eventually we want to get rid of it. What happens when we're dealing with allocators that don't compare equal?
In particular: suppose that v1
and v2
are both
objects of type vector<int, my_alloc>
and that
v1.get_allocator() != v2.get_allocator()
. What happens if
we write v1.swap(v2)
? Informally, three possibilities:
1. This operation is illegal. Perhaps we could say that an implementation is required to check and to throw an exception, or perhaps we could say it's undefined behavior.
2. The operation performs a slow swap (i.e. using three
invocations of operator=
, leaving each allocator with its
original container. This would be an O(N) operation.
3. The operation swaps both the vectors' contents and their allocators. This would be an O(1) operation. That is:
my_alloc a1(...); my_alloc a2(...); assert(a1 != a2); vector<int, my_alloc> v1(a1); vector<int, my_alloc> v2(a2); assert(a1 == v1.get_allocator()); assert(a2 == v2.get_allocator()); v1.swap(v2); assert(a1 == v2.get_allocator()); assert(a2 == v1.get_allocator());
[Kona: This is part of a general problem. We need a paper saying how to deal with unequal allocators in general.]
[pre-Sydney: Howard argues for option 3 in N1599. ]
[ 2007-01-12, Howard: This issue will now tend to come up more often with move constructors and move assignment operators. For containers, these members transfer resources (i.e. the allocated memory) just like swap. ]
[
Batavia: There is agreement to overload the container swap
on the allocator's Swappable
requirement using concepts. If the allocator supports Swappable, then container's swap will
swap allocators, else it will perform a "slow swap" using copy construction and copy assignment.
]
[ 2009-04-28 Pablo adds: ]
Fixed in N2525. I argued for marking this Tentatively-Ready right after Bellevue, but there was a concern that N2525 would break in the presence of the RVO. (That breakage had nothing to do with swap, but never-the-less). I addressed that breakage in in N2840 (Summit) by means of a non-normative reference:
[Note: in situations where the copy constructor for a container is elided, this function is not called. The behavior in these cases is as if
select_on_container_copy_construction
returnedx
— end note]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Christian W Brock Opened: 2003-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [stringbuf.virtuals].
View all other issues in [stringbuf.virtuals].
View all issues with CD1 status.
Discussion:
27.7.1.3 par 8 says:
Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).
The sentences "plus one additional write position." and especially "(as does the write end pointer epptr())" COULD by interpreted (and is interpreted by at least my library vendor) as:
post-condition: epptr() == pptr()+1
This WOULD force sputc() to call the virtual overflow() each time.
The proposed change also affects Defect Report 169.
Proposed resolution:
27.7.1.1/2 Change:
2- Notes: The function allocates no array object.
to:
2- Postcondition: str() == "".
27.7.1.1/3 Change:
-3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which . Then copies the content of str into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to which. If which & ios_base::out is true, initializes the output sequence with the underlying sequence. If which & ios_base::in is true, initializes the input sequence with the underlying sequence.
to:
-3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which. Then copies the content of str into the basic_stringbuf underlying character sequence. If which & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (which & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If which & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.
27.7.1.2/1 Change:
-1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the buffer is only created in input mode, the underlying character sequence is equal to the input sequence; otherwise, it is equal to the output sequence. In case of an empty underlying character sequence, the function returns basic_string<charT,traits,Allocator>().
to:
-1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the basic_stringbuf was created only in input mode, the resultant basic_string contains the character sequence in the range [eback(), egptr()). If the basic_stringbuf was created with (which & ios_base::out) being true then the resultant basic_string contains the character sequence in the range [pbase(), high_mark) where high_mark represents the position one past the highest initialized character in the buffer. Characters can be initialized either through writing to the stream, or by constructing the basic_stringbuf with a basic_string, or by calling the str(basic_string) member function. In the case of calling the str(basic_string) member function, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_string). Otherwise the basic_stringbuf has been created in neither input nor output mode and a zero length basic_string is returned.
27.7.1.2/2 Change:
-2- Effects: If the basic_stringbuf's underlying character sequence is not empty, deallocates it. Then copies the content of s into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to the mode stored when creating the basic_stringbuf object. If (mode&ios_base::out) is true, then initializes the output sequence with the underlying sequence. If (mode&ios_base::in) is true, then initializes the input sequence with the underlying sequence.
to:
-2- Effects: Copies the content of s into the basic_stringbuf underlying character sequence. If mode & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (mode & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If mode & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.
Remove 27.2.1.2/3. (Same rationale as issue 238: incorrect and unnecessary.)
27.7.1.3/1 Change:
1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof().
to:
1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof(). Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.
27.7.1.3/9 Change:
-9- Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).
to:
-9- The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position.
27.7.1.3/12 Change:
-12- _ If (newoff + off) < 0, or (xend - xbeg) < (newoff + off), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .
to:
-12- _ If (newoff + off) < 0, or if (newoff + off) refers to an uninitialized character (as defined in 31.8.2.4 [stringbuf.members] paragraph 1), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .
[post-Kona: Howard provided wording. At Kona the LWG agreed that something along these lines was a good idea, but the original proposed resolution didn't say enough about the effect of various member functions on the underlying character sequences.]
Rationale:
The current basic_stringbuf description is over-constrained in such a way as to prohibit vendors from making this the high-performance in-memory stream it was meant to be. The fundamental problem is that the pointers: eback(), gptr(), egptr(), pbase(), pptr(), epptr() are observable from a derived client, and the current description restricts the range [pbase(), epptr()) from being grown geometrically. This change allows, but does not require, geometric growth of this range.
Backwards compatibility issues: These changes will break code that derives from basic_stringbuf, observes epptr(), and depends upon [pbase(), epptr()) growing by one character on each call to overflow() (i.e. test suites). Otherwise there are no backwards compatibility issues.
27.7.1.1/2: The non-normative note is non-binding, and if it were binding, would be over specification. The recommended change focuses on the important observable fact.
27.7.1.1/3: This change does two things: 1. It describes exactly what must happen in terms of the sequences. The terms "input sequence" and "output sequence" are not well defined. 2. It introduces a common extension: open with app or ate mode. I concur with issue 238 that paragraph 4 is both wrong and unnecessary.
27.7.1.2/1: This change is the crux of the efficiency issue. The resultant basic_string is not dependent upon epptr(), and thus implementors are free to grow the underlying buffer geometrically during overflow() *and* place epptr() at the end of that buffer.
27.7.1.2/2: Made consistent with the proposed 27.7.1.1/3.
27.7.1.3/1: Clarifies that characters written to the stream beyond the initially specified string are available for reading in an i/o basic_streambuf.
27.7.1.3/9: Made normative by removing "Notes:", and removed the trailing parenthetical comment concerning epptr().
27.7.1.3/12: Restricting the positioning to [xbeg, xend) is no longer allowable since [pbase(), epptr()) may now contain uninitialized characters. Positioning is only allowable over the initialized range.
Section: 22.9.2.3 [bitset.members] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.members].
View all issues with CD1 status.
Discussion:
It has been pointed out a number of times that the bitset to_string() member function template is tedious to use since callers must explicitly specify the entire template argument list (3 arguments). At least two implementations provide a number of overloads of this template to make it easier to use.
Proposed resolution:
In order to allow callers to specify no template arguments at all, just the first one (charT), or the first 2 (charT and traits), in addition to all three template arguments, add the following three overloads to both the interface (declarations only) of the class template bitset as well as to section 23.3.5.2, immediately after p34, the Returns clause of the existing to_string() member function template:
template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string () const; -34.1- Returns: to_string<charT, traits, allocator<charT> >(). template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string () const; -34.2- Returns: to_string<charT, char_traits<charT>, allocator<charT> >(). basic_string<char, char_traits<char>, allocator<char> > to_string () const; -34.3- Returns: to_string<char, char_traits<char>, allocator<char> >().
[Kona: the LWG agrees that this is an improvement over the status quo. Dietmar thought about an alternative using a proxy object but now believes that the proposed resolution above is the right choice. ]
Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.io].
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Discussion:
It has been pointed out that the proposed resolution in DR 25(i) may not be
quite up to snuff:
http://gcc.gnu.org/ml/libstdc++/2003-09/msg00147.html
It looks like Petur is right. The complete corrected text is copied below. I think we may have have been confused by the reference to 22.2.2.2.2 and the subsequent description of `n' which actually talks about the second argument to sputn(), not about the number of fill characters to pad with.
So the question is: was the original text correct? If the intent was to follow classic iostreams then it most likely wasn't, since setting width() to less than the length of the string doesn't truncate it on output. This is also the behavior of most implementations (except for SGI's standard iostreams where the operator does truncate).
Proposed resolution:
Change the text in 21.3.7.9, p4 from
If bool(k) is true, inserts characters as if by calling os.rdbuf()->sputn(str.data(), n), padding as described in stage 3 of lib.facet.num.put.virtuals, where n is the larger of os.width() and str.size();
to
If bool(k) is true, determines padding as described in lib.facet.num.put.virtuals, and then inserts the resulting sequence of characters
seq
as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size()
;
[Kona: it appears that neither the original wording, DR25, nor the proposed resolution, is quite what we want. We want to say that the string will be output, padded to os.width() if necessary. We don't want to duplicate the padding rules in clause 22, because they're complicated, but we need to be careful because they weren't quite written with quite this case in mind. We need to say what the character sequence is, and then defer to clause 22. Post-Kona: Benjamin provided wording.]
Section: 28.3.3.1.2.2 [locale.facet] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is "const std::ctype<char>" a valid template argument to has_facet, use_facet, and the locale template ctor? And if so, does it designate the same Facet as the non-const "std::ctype<char>?" What about "volatile std::ctype<char>?" Different implementations behave differently: some fail to compile, others accept such types but behave inconsistently.
Proposed resolution:
Change 22.1.1.1.2, p1 to read:
Template parameters in this clause which are required to be facets are those named Facet in declarations. A program that passes a type that is not a facet, or a type that refers to volatile-qualified facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed. A const-qualified facet is a valid template argument to any locale function that expects a Facet template parameter.
[Kona: changed the last sentence from a footnote to normative text.]
Section: 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Howard Hinnant Opened: 2003-10-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 23.2.4 [sequence.reqmts], paragraphs 9-11, fixed up the problem noticed with statements like:
vector<int> v(10, 1);
The intent of the above statement was to construct with:
vector(size_type, const value_type&);
but early implementations failed to compile as they bound to:
template <class InputIterator> vector(InputIterator f, InputIterator l);
instead.
Paragraphs 9-11 say that if InputIterator is an integral type, then the member template constructor will have the same effect as:
vector<static_cast<size_type>(f), static_cast<value_type>(l));
(and similarly for the other member template functions of sequences).
There is also a note that describes one implementation technique:
One way that sequence implementors can satisfy this requirement is to specialize the member template for every integral type.
This might look something like:
template <class T> struct vector { typedef unsigned size_type; explicit vector(size_type) {} vector(size_type, const T&) {} template <class I> vector(I, I); // ... }; template <class T> template <class I> vector<T>::vector(I, I) { ... } template <> template <> vector<int>::vector(int, int) { ... } template <> template <> vector<int>::vector(unsigned, unsigned) { ... } // ...
Label this solution 'A'.
The standard also says:
Less cumbersome implementation techniques also exist.
A popular technique is to not specialize as above, but instead catch every call with the member template, detect the type of InputIterator, and then redirect to the correct logic. Something like:
template <class T> template <class I> vector<T>::vector(I f, I l) { choose_init(f, l, int2type<is_integral<I>::value>()); } template <class T> template <class I> vector<T>::choose_init(I f, I l, int2type<false>) { // construct with iterators } template <class T> template <class I> vector<T>::choose_init(I f, I l, int2type<true>) { size_type sz = static_cast<size_type>(f); value_type v = static_cast<value_type>(l); // construct with sz,v }
Label this solution 'B'.
Both of these solutions solve the case the standard specifically mentions:
vector<int> v(10, 1); // ok, vector size 10, initialized to 1
However, (and here is the problem), the two solutions have different behavior in some cases where the value_type of the sequence is not an integral type. For example consider:
pair<char, char> p('a', 'b'); vector<vector<pair<char, char> > > d('a', 'b');
The second line of this snippet is likely an error. Solution A catches the error and refuses to compile. The reason is that there is no specialization of the member template constructor that looks like:
template <> template <> vector<vector<pair<char, char> > >::vector(char, char) { ... }
So the expression binds to the unspecialized member template constructor, and then fails (compile time) because char is not an InputIterator.
Solution B compiles the above example though. 'a' is casted to an unsigned integral type and used to size the outer vector. 'b' is static casted to the inner vector using it's explicit constructor:
explicit vector(size_type n);
and so you end up with a static_cast<size_type>('a') by static_cast<size_type>('b') matrix.
It is certainly possible that this is what the coder intended. But the explicit qualifier on the inner vector has been thwarted at any rate.
The standard is not clear whether the expression:
vector<vector<pair<char, char> > > d('a', 'b');
(and similar expressions) are:
My preference is listed in the order presented.
There are still other techniques for implementing the requirements of paragraphs 9-11, namely the "restricted template technique" (e.g. enable_if). This technique is the most compact and easy way of coding the requirements, and has the behavior of #2 (rejects the above expression).
Choosing 1 would allow all implementation techniques I'm aware of. Choosing 2 would allow only solution 'A' and the enable_if technique. Choosing 3 would allow only solution 'B'.
Possible wording for a future standard if we wanted to actively reject the expression above would be to change "static_cast" in paragraphs 9-11 to "implicit_cast" where that is defined by:
template <class T, class U> inline T implicit_cast(const U& u) { return u; }
Proposed resolution:
Replace 23.2.4 [sequence.reqmts] paragraphs 9 - 11 with:
For every sequence defined in this clause and in clause lib.strings:
If the constructor
template <class InputIterator> X(InputIterator f, InputIterator l, const allocator_type& a = allocator_type())
is called with a type InputIterator that does not qualify as an input iterator, then the constructor will behave as if the overloaded constructor:
X(size_type, const value_type& = value_type(), const allocator_type& = allocator_type())
were called instead, with the arguments static_cast<size_type>(f), l and a, respectively.
If the member functions of the forms:
template <class InputIterator> // such as insert() rt fx1(iterator p, InputIterator f, InputIterator l); template <class InputIterator> // such as append(), assign() rt fx2(InputIterator f, InputIterator l); template <class InputIterator> // such as replace() rt fx3(iterator i1, iterator i2, InputIterator f, InputIterator l);
are called with a type InputIterator that does not qualify as an input iterator, then these functions will behave as if the overloaded member functions:
rt fx1(iterator, size_type, const value_type&); rt fx2(size_type, const value_type&); rt fx3(iterator, iterator, size_type, const value_type&);
were called instead, with the same arguments.
In the previous paragraph the alternative binding will fail if f is not implicitly convertible to X::size_type or if l is not implicitly convertible to X::value_type.
The extent to which an implementation determines that a type cannot be an input iterator is unspecified, except that as a minimum integral types shall not qualify as input iterators.
[
Kona: agreed that the current standard requires v('a', 'b')
to be accepted, and also agreed that this is surprising behavior. The
LWG considered several options, including something like
implicit_cast, which doesn't appear to be quite what we want. We
considered Howards three options: allow acceptance or rejection,
require rejection as a compile time error, and require acceptance. By
straw poll (1-6-1), we chose to require a compile time error.
Post-Kona: Howard provided wording.
]
[ Sydney: The LWG agreed with this general direction, but there was some discomfort with the wording in the original proposed resolution. Howard submitted new wording, and we will review this again in Redmond. ]
[Redmond: one very small change in wording: the first argument
is cast to size_t. This fixes the problem of something like
vector<vector<int> >(5, 5)
, where int is not
implicitly convertible to the value type.]
Rationale:
The proposed resolution fixes:
vector<int> v(10, 1);
since as integral types 10 and 1 must be disqualified as input iterators and therefore the (size,value) constructor is called (as if).
The proposed resolution breaks:
vector<vector<T> > v(10, 1);
because the integral type 1 is not *implicitly* convertible to vector<T>. The wording above requires a diagnostic.
The proposed resolution leaves the behavior of the following code unspecified.
struct A { operator int () const {return 10;} }; struct B { B(A) {} }; vector<B> v(A(), A());
The implementation may or may not detect that A is not an input iterator and employee the (size,value) constructor. Note though that in the above example if the B(A) constructor is qualified explicit, then the implementation must reject the constructor as A is no longer implicitly convertible to B.
Section: 31.5.3 [fpos] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In section 31.5.3.2 [fpos.members] fpos<stateT>::state() is declared non const, but in section 31.5.3 [fpos] it is declared const.
Proposed resolution:
In section 31.5.3.2 [fpos.members], change the declaration of
fpos<stateT>::state()
to const.
Section: 31.7.6.2.4 [ostream.sentry] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-18 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
In section [ostream::sentry] paragraph 4, in description part basic_ostream<charT, traits>::sentry::operator bool() is declared as non const, but in section 27.6.2.3, in synopsis it is declared const.
Proposed resolution:
In section [ostream::sentry] paragraph 4, change the declaration
of sentry::operator bool()
to const.
Section: 31.10.3.4 [filebuf.members] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In section 31.10.3.4 [filebuf.members] par6, in effects description of basic_filebuf<charT, traits>::close(), overflow(EOF) is used twice; should be overflow(traits::eof()).
Proposed resolution:
Change overflow(EOF) to overflow(traits::eof()).
Section: 31.10 [file.streams] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-20 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
31.10.4.4 [ifstream.members] p1, 31.10.5.4 [ofstream.members] p1, 31.10.6.4 [fstream.members] p1 seems have same problem as exposed in LWG issue 252(i).
Proposed resolution:
[Sydney: Genuine defect. 27.8.1.13 needs a cast to cast away constness. The other two places are stylistic: we could change the C-style casts to const_cast. Post-Sydney: Howard provided wording. ]
Change 27.8.1.7/1 from:
Returns: (basic_filebuf<charT,traits>*)&sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Change 27.8.1.10/1 from:
Returns: (basic_filebuf<charT,traits>*)&sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Change 27.8.1.13/1 from:
Returns: &sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Section: 24.3.2.3 [iterator.traits] Status: CD1 Submitter: Dave Abrahams Opened: 2003-12-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard places no restrictions at all on the reference type of input, output, or forward iterators (for forward iterators it only specifies that *x must be value_type& and doesn't mention the reference type). Bidirectional iterators' reference type is restricted only by implication, since the base iterator's reference type is used as the return type of reverse_iterator's operator*, which must be T& in order to be a conforming forward iterator.
Here's what I think we ought to be able to expect from an input or forward iterator's reference type R, where a is an iterator and V is its value_type
A mutable forward iterator ought to satisfy, for x of type V:
{ R r = *a; r = x; } is equivalent to *a = x;
I think these requirements capture existing container iterators (including vector<bool>'s), but render istream_iterator invalid; its reference type would have to be changed to a constant reference.
(Jeremy Siek) During the discussion in Sydney, it was felt that a
simpler long term solution for this was needed. The solution proposed
was to require reference
to be the same type as *a
and pointer
to be the same type as a->
. Most
iterators in the Standard Library already meet this requirement. Some
iterators are output iterators, and do not need to meet the
requirement, and others are only specified through the general
iterator requirements (which will change with this resolution). The
sole case where there is an explicit definition of the reference type
that will need to change is istreambuf_iterator
which returns
charT
from operator*
but has a reference type of
charT&
. We propose changing the reference type of
istreambuf_iterator
to charT
.
The other option for resolving the issue with pointer
,
mentioned in the note below, is to remove pointer
altogether. I prefer placing requirements on pointer
to
removing it for two reasons. First, pointer
will become
useful for implementing iterator adaptors and in particular,
reverse_iterator
will become more well defined. Second,
removing pointer
is a rather drastic and publicly-visible
action to take.
The proposed resolution technically enlarges the requirements for
iterators, which means there are existing iterators (such as
istreambuf_iterator
, and potentially some programmer-defined
iterators) that will no longer meet the requirements. Will this break
existing code? The scenario in which it would is if an algorithm
implementation (say in the Standard Library) is changed to rely on
iterator_traits::reference
, and then is used with one of the
iterators that do not have an appropriately defined
iterator_traits::reference
.
The proposed resolution makes one other subtle change. Previously,
it was required that output iterators have a difference_type
and value_type
of void
, which means that a forward
iterator could not be an output iterator. This is clearly a mistake,
so I've changed the wording to say that those types may be
void
.
Proposed resolution:
In 24.3.2.3 [iterator.traits], after:
be defined as the iterator's difference type, value type and iterator category, respectively.
add
In addition, the types
iterator_traits<Iterator>::reference iterator_traits<Iterator>::pointermust be defined as the iterator's reference and pointer types, that is, the same type as the type of
*a
anda->
, respectively.
In 24.3.2.3 [iterator.traits], change:
In the case of an output iterator, the types
iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_typeare both defined as
void
.
to:
In the case of an output iterator, the types
iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_type iterator_traits<Iterator>::reference iterator_traits<Iterator>::pointermay be defined as
void
.
In 24.6.4 [istreambuf.iterator], change:
typename traits::off_type, charT*, charT&>
to:
typename traits::off_type, charT*, charT>
[ Redmond: there was concern in Sydney that this might not be the only place where things were underspecified and needed to be changed. Jeremy reviewed iterators in the standard and confirmed that nothing else needed to be changed. ]
Section: 24.3.5.7 [random.access.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2004-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 76, the random access iterator requirement table, says that the return type of a[n] must be "convertible to T". When an iterator's value_type T is an abstract class, nothing is convertible to T. Surely this isn't an intended restriction?
Proposed resolution:
Change the return type to "convertible to T const&".
Section: 17.2 [support.types] Status: CD1 Submitter: Pete Becker Opened: 2004-01-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Original text:
The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9). The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."
Revised text:
"If type is not a POD structure or a POD union the results are undefined."
Looks to me like the revised text should have replaced only the second sentence. It doesn't make sense standing alone.
Proposed resolution:
Change 18.1, paragraph 5, to:
The macro offsetof accepts a restricted set of type arguments in this International Standard. If type is not a POD structure or a POD union the results are undefined. The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
pos_type basic_stringbuf::seekoff(off_type, ios_base::seekdir, ios_base::openmode);
is obliged to fail if nothing has been inserted into the stream. This is unnecessary and undesirable. It should be permissible to seek to an effective offset of zero.
[ Sydney: Agreed that this is an annoying problem: seeking to zero should be legal. Bill will provide wording. ]
Proposed resolution:
Change the sentence from:
For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer, the positioning operation fails.
to:
For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails.
Section: 31.4 [iostream.objects] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Both cerr::tie() and wcerr::tie() are obliged to be null at program startup. This is overspecification and overkill. It is both traditional and useful to tie cerr to cout, to ensure that standard output is drained whenever an error message is written. This behavior should at least be permitted if not required. Same for wcerr::tie().
Proposed resolution:
Add to the description of cerr:
After the object cerr is initialized, cerr.tie() returns &cout. Its state is otherwise the same as required for basic_ios<char>::init (lib.basic.ios.cons).
Add to the description of wcerr:
After the object wcerr is initialized, wcerr.tie() returns &wcout. Its state is otherwise the same as required for basic_ios<wchar_t>::init (lib.basic.ios.cons).
[Sydney: straw poll (3-1): we should require, not just permit, cout and cerr to be tied on startup. Pre-Redmond: Bill will provide wording.]
Section: 16.4.2.3 [headers] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The C++ Standard effectively requires that the traditional C headers (of the form <xxx.h>) be defined in terms of the newer C++ headers (of the form <cxxx>). Clauses 17.4.1.2/4 and D.5 combine to require that:
The rules were left in this form despited repeated and heated objections from several compiler vendors. The C headers are often beyond the direct control of C++ implementors. In some organizations, it's all they can do to get a few #ifdef __cplusplus tests added. Third-party library vendors can perhaps wrap the C headers. But neither of these approaches supports the drastic restructuring required by the C++ Standard. As a result, it is still widespread practice to ignore this conformance requirement, nearly seven years after the committee last debated this topic. Instead, what is often implemented is:
The practical benefit for implementors with the second approach is that they can use existing C library headers, as they are pretty much obliged to do. The practical cost for programmers facing a mix of implementations is that they have to assume weaker rules:
There also exists the possibility of subtle differences due to Koenig lookup, but there are so few non-builtin types defined in the C headers that I've yet to see an example of any real problems in this area.
It is worth observing that the rate at which programmers fall afoul of these differences has remained small, at least as measured by newsgroup postings and our own bug reports. (By an overwhelming margin, the commonest problem is still that programmers include <string> and can't understand why the typename string isn't defined -- this a decade after the committee invented namespace std, nominally for the benefit of all programmers.)
We should accept the fact that we made a serious mistake and rectify it, however belatedly, by explicitly allowing either of the two schemes for declaring C names in headers.
[Sydney: This issue has been debated many times, and will
certainly have to be discussed in full committee before any action
can be taken. However, the preliminary sentiment of the LWG was in
favor of the change. (6 yes, 0 no, 2 abstain) Robert Klarer
suggests that we might also want to undeprecate the
C-style .h
headers.]
Proposed resolution:
Add to 16.4.2.3 [headers], para. 4:
Except as noted in clauses 18 through 27 and Annex D, the contents of each header cname shall be the same as that of the corresponding header name.h, as specified in ISO/IEC 9899:1990 Programming Languages C (Clause 7), or ISO/IEC:1990 Programming Languages-C AMENDMENT 1: C Integrity, (Clause 7), as appropriate, as if by inclusion. In the C++ Standard Library, however, the declarations
and definitions(except for names which are defined as macros in C) are within namespace scope (3.3.5) of the namespace std. It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations (9.10 [namespace.udecl]).
Change [depr.c.headers], para. 2-3:
-2- Every C header, each of which has a name of the form name.h, behaves as if each name placed in the Standard library namespace by the corresponding cname header is
alsoplaced within the global namespace scope.of the namespaceIt is unspecified whether these names are first declared or defined within namespace scope (6.4.6 [basic.scope.namespace]) of the namespacestd
and is followed by an explicit using-declaration (9.10 [namespace.udecl]).std
and are then injected into the global namespace scope by explicit using-declarations (9.10 [namespace.udecl]).-3- [Example: The header
<cstdlib>
assuredly provides its declarations and definitions within the namespacestd
. It may also provide these names within the global namespace. The header<stdlib.h>
makes these available also inassuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespacestd
. -- end example]
Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Dag Henriksson Opened: 2004-01-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The constructor from unsigned long says it initializes "the first M bit positions to the corresponding bit values in val. M is the smaller of N and the value CHAR_BIT * sizeof(unsigned long)."
Object-representation vs. value-representation strikes again. CHAR_BIT * sizeof (unsigned long) does not give us the number of bits an unsigned long uses to hold the value. Thus, the first M bit position above is not guaranteed to have any corresponding bit values in val.
Proposed resolution:
In 22.9.2.2 [bitset.cons] paragraph 2, change "M is the smaller of
N and the value CHAR_BIT * sizeof (unsigned long). (249)" to
"M
is the smaller of N
and the number of bits in
the value representation (section 6.8 [basic.types]) of unsigned
long
."
Section: 31.10 [file.streams] Status: CD1 Submitter: Ben Hutchings Opened: 2004-04-01 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
The second parameters of the non-default constructor and of the open member function for basic_fstream, named "mode", are optional according to the class declaration in 27.8.1.11 [lib.fstream]. The specifications of these members in 27.8.1.12 [lib.fstream.cons] and 27.8.1.13 lib.fstream.members] disagree with this, though the constructor declaration has the "explicit" function-specifier implying that it is intended to be callable with one argument.
Proposed resolution:
In 31.10.6.2 [fstream.cons], change
explicit basic_fstream(const char* s, ios_base::openmode mode);
to
explicit basic_fstream(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out);
In 31.10.6.4 [fstream.members], change
void open(const char*s, ios_base::openmode mode);
to
void open(const char*s, ios_base::openmode mode = ios_base::in|ios_base::out);
Section: 28.3.4.6.2.3 [locale.time.get.virtuals] Status: CD1 Submitter: Bill Plauger Opened: 2004-03-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Template time_get currently contains difficult, if not impossible, requirements for do_date_order, do_get_time, and do_get_date. All require the implementation to scan a field generated by the %x or %X conversion specifier in strftime. Yes, do_date_order can always return no_order, but that doesn't help the other functions. The problem is that %x can be nearly anything, and it can vary widely with locales. It's horribly onerous to have to parse "third sunday after Michaelmas in the year of our Lord two thousand and three," but that's what we currently ask of do_get_date. More practically, it leads some people to think that if %x produces 10.2.04, we should know to look for dots as separators. Still not easy.
Note that this is the opposite effect from the intent stated in the footnote earlier in this subclause:
"In other words, user confirmation is required for reliable parsing of user-entered dates and times, but machine-generated formats can be parsed reliably. This allows parsers to be aggressive about interpreting user variations on standard formats."
We should give both implementers and users an easier and more reliable alternative: provide a (short) list of alternative delimiters and say what the default date order is for no_order. For backward compatibility, and maximum latitude, we can permit an implementation to parse whatever %x or %X generates, but we shouldn't require it.
Proposed resolution:
In the description:
iter_type do_get_time(iter_type s, iter_type end, ios_base& str, ios_base::iostate& err, tm* t) const;
2 Effects: Reads characters starting at suntil it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce the format specified by 'X', or until it encounters an error or end of sequence.
change: 'X'
to: "%H:%M:%S"
Change
iter_type do_get_date(iter_type s, iter_type end, ios_base& str, ios_base::iostate& err, tm* t) const; 4 Effects: Reads characters starting at s until it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce the format specified by 'x', or until it encounters an error.
to
iter_type do_get_date(iter_type s, iter_type end, ios_base& str, ios_base::iostate& err, tm* t) const;
4 Effects: Reads characters starting at s until it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce one of the following formats, or until it encounters an error. The format depends on the value returned by date_order() as follows:
date_order() format no_order "%m/%d/%y" dmy "%d/%m/%y" mdy "%m/%d/%y" ymd "%y/%m/%d" ydm "%y/%d/%m"
An implementation may also accept additional implementation-defined formats.
[Redmond: agreed that this is a real problem. The solution is probably to match C99's parsing rules. Bill provided wording. ]
Section: 23.3.13 [vector], 23.4.3 [map] Status: CD1 Submitter: Thorsten Ottosen Opened: 2004-05-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [vector].
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Discussion:
To add slightly more convenience to vector<T> and map<Key,T> we should consider to add
Rationale:
Proposed resolution:
In 23.3.13 [vector], add the following to the vector
synopsis after "element access" and before "modifiers":
// [lib.vector.data] data access pointer data(); const_pointer data() const;
Add a new subsection of 23.3.13 [vector]:
23.2.4.x
vector
data accesspointer data(); const_pointer data() const;Returns: A pointer such that [data(), data() + size()) is a valid range. For a non-empty vector, data() == &front().
Complexity: Constant time.
Throws: Nothing.
In 23.4.3 [map], add the following to the map
synopsis immediately after the line for operator[]:
T& at(const key_type& x); const T& at(const key_type& x) const;
Add the following to 23.4.3.3 [map.access]:
T& at(const key_type& x); const T& at(const key_type& x) const;Returns: A reference to the element whose key is equivalent to x, if such an element is present in the map.
Throws:
out_of_range
if no such element is present.
Rationale:
Neither of these additions provides any new functionality but the
LWG agreed that they are convenient, especially for novices. The
exception type chosen for at
, std::out_of_range
,
was chosen to match vector::at
.
Section: 16.4.2.3 [headers] Status: CD1 Submitter: Steve Clamage Opened: 2004-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
C header <iso646.h> defines macros for some operators, such as not_eq for !=.
Section 16.4.2.3 [headers] "Headers" says that except as noted in clauses 18 through 27, the <cname> C++ header contents are the same as the C header <name.h>. In particular, table 12 lists <ciso646> as a C++ header.
I don't find any other mention of <ciso646>, or any mention of <iso646.h>, in clauses 17 thorough 27. That implies that the contents of <ciso646> are the same as C header <iso646.h>.
Annex C (informative, not normative) in [diff.header.iso646.h] C.2.2.2 "Header <iso646.h>" says that the alternative tokens are not defined as macros in <ciso646>, but does not mention the contents of <iso646.h>.
I don't find any normative text to support C.2.2.2.
Proposed resolution:
Add to section 17.4.1.2 Headers [lib.headers] a new paragraph after paragraph 6 (the one about functions must be functions):
Identifiers that are keywords or operators in C++ shall not be defined as macros in C++ standard library headers. [Footnote:In particular, including the standard header <iso646.h> or <ciso646> has no effect.
[post-Redmond: Steve provided wording.]
Section: 27.2.4.2 [char.traits.specializations.char] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [char.traits.specializations.char].
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Discussion:
Table 37 describes the requirements on Traits::compare() in terms of those on Traits::lt(). 21.1.3.1, p6 requires char_traits<char>::lt() to yield the same result as operator<(char, char).
Most, if not all, implementations of char_traits<char>::compare() call memcmp() for efficiency. However, the C standard requires both memcmp() and strcmp() to interpret characters under comparison as unsigned, regardless of the signedness of char. As a result, all these char_traits implementations fail to meet the requirement imposed by Table 37 on compare() when char is signed.
Read email thread starting with c++std-lib-13499 for more.
Proposed resolution:
Change 21.1.3.1, p6 from
The two-argument members assign, eq, and lt are defined identically to the built-in operators =, ==, and < respectively.
to
The two-argument member assign is defined identically to the built-in operator =. The two argument members eq and lt are defined identically to the built-in operators == and < for type unsigned char.
[Redmond: The LWG agreed with this general direction, but we
also need to change eq
to be consistent with this change.
Post-Redmond: Martin provided wording.]
Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The program below is required to compile but when run it typically produces unexpected results due to the user-defined conversion from std::cout or any object derived from basic_ios to void*.
#include <cassert> #include <iostream> int main () { assert (std::cin.tie () == std::cout); // calls std::cout.ios::operator void*() }
Proposed resolution:
Replace std::basic_ios<charT, traits>::operator void*() with another conversion operator to some unspecified type that is guaranteed not to be convertible to any other type except for bool (a pointer-to-member might be one such suitable type). In addition, make it clear that the pointer type need not be a pointer to a complete type and when non-null, the value need not be valid.
Specifically, change in [lib.ios] the signature of
operator void*() const;
to
operator unspecified-bool-type() const;
and change [lib.iostate.flags], p1 from
operator void*() const;
to
operator unspecified-bool-type() const; -1- Returns: if fail() then a value that will evaluate false in a boolean context; otherwise a value that will evaluate true in a boolean context. The value type returned shall not be convertible to int. -2- [Note: This conversion can be used in contexts where a bool is expected (e.g., an if condition); however, implicit conversions (e.g., to int) that can occur with bool are not allowed, eliminating some sources of user error. One possible implementation choice for this type is pointer-to-member. - end note]
[Redmond: 5-4 straw poll in favor of doing this.]
[Lillehammer: Doug provided revised wording for "unspecified-bool-type".]
Section: 23.3.13 [vector] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The overloads of relational operators for vector<bool> specified in [lib.vector.bool] are redundant (they are semantically identical to those provided for the vector primary template) and may even be diagnosed as ill-formed (refer to Daveed Vandevoorde's explanation in c++std-lib-13647).
Proposed resolution:
Remove all overloads of overloads of relational operators for vector<bool> from [lib.vector.bool].
what()
implementation-definedSection: 17.9.3 [exception] Status: C++11 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
[lib.exception] specifies the following:
exception (const exception&) throw(); exception& operator= (const exception&) throw(); -4- Effects: Copies an exception object. -5- Notes: The effects of calling what() after assignment are implementation-defined.
First, does the Note only apply to the assignment operator? If so, what are the effects of calling what() on a copy of an object? Is the returned pointer supposed to point to an identical copy of the NTBS returned by what() called on the original object or not?
Second, is this Note intended to extend to all the derived classes in section 19? I.e., does the standard provide any guarantee for the effects of what() called on a copy of any of the derived class described in section 19?
Finally, if the answer to the first question is no, I believe it constitutes a defect since throwing an exception object typically implies invoking the copy ctor on the object. If the answer is yes, then I believe the standard ought to be clarified to spell out exactly what the effects are on the copy (i.e., after the copy ctor was called).
[Redmond: Yes, this is fuzzy. The issue of derived classes is fuzzy too.]
[ Batavia: Howard provided wording. ]
[ Bellevue: ]
Eric concerned this is unimplementable, due to nothrow guarantees. Suggested implementation would involve reference counting.
Is the implied reference counting subtle enough to call out a note on implementation? Probably not.
If reference counting required, could we tighten specification further to require same pointer value? Probably an overspecification, especially if exception classes defer evalutation of final string to calls to what().
Remember issue moved open and not resolved at Batavia, but cannot remember who objected to canvas a disenting opinion - please speak up if you disagree while reading these minutes!
Move to Ready as we are accepting words unmodified.
[ Sophia Antipolis: ]
The issue was pulled from Ready. It needs to make clear that only homogenous copying is intended to be supported, not coping from a derived to a base.
[ Batavia (2009-05): ]
Howard supplied the following replacement wording for paragraph 7 of the proposed resolution:
-7- Postcondition:
what()
shall return the same NTBS as would be obtained by usingstatic_cast
to cast the rhs to the same types as the lhs and then callingwhat()
on that possibly sliced object.Pete asks what "the same NTBS" means.
[ 2009-07-30 Niels adds: ]
Further discussion in the thread starting with c++std-lib-24512.
[ 2009-09-24 Niels provided updated wording: ]
I think the resolution should at least guarantee that the result of
what()
is independent of whether the compiler does copy-elision. And for any class derived fromstd::excepion
that has a constructor that allows specifying awhat_arg
, it should make sure that the text of a user-providedwhat_arg
is preserved, when the object is copied. Note that all the implementations I've tested already appear to satisfy the proposed resolution, including MSVC 2008 SP1, Apache stdcxx-4.2.1, GCC 4.1.2, GCC 4.3.2, and CodeGear C++ 6.13.The proposed resolution was updated with help from Daniel Krügler; the update aims to clarify that the proposed postcondition only applies to homogeneous copying.
[ 2009-10 Santa Cruz: ]
Moved to Ready after inserting "publicly accessible" in two places.
Proposed resolution:
Change 17.9.3 [exception] to:
-1- The class
exception
defines the base class for the types of objects thrown as exceptions by C++ standard library components, and certain expressions, to report errors detected during program execution.Each standard library class
T
that derives from classexception
shall have a publicly accessible copy constructor and a publicly accessible copy assignment operator that do not exit with an exception. These member functions shall preserve the following postcondition: If two objects lhs and rhs both have dynamic typeT
, and lhs is a copy of rhs, thenstrcmp(lhs.what(), rhs.what()) == 0
....
exception(const exception& rhs) throw(); exception& operator=(const exception& rhs) throw();-4- Effects: Copies an exception object.
-5- Remarks: The effects of callingwhat()
after assignment are implementation-defined.-5- Postcondition: If
*this
and rhs both have dynamic typeexception
thenstrcmp(what(), rhs.what()) == 0
.
ctype
callsSection: 28.3.4.2.2 [locale.ctype] Status: C++11 Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Most ctype
member functions come in two forms: one that operates
on a single character at a time and another form that operates
on a range of characters. Both forms are typically described by
a single Effects and/or Returns clause.
The Returns clause of each of the single-character non-virtual forms suggests that the function calls the corresponding single character virtual function, and that the array form calls the corresponding virtual array form. Neither of the two forms of each virtual member function is required to be implemented in terms of the other.
There are three problems:
1. One is that while the standard does suggest that each non-virtual member function calls the corresponding form of the virtual function, it doesn't actually explicitly require it.
Implementations that cache results from some of the virtual member functions for some or all values of their arguments might want to call the array form from the non-array form the first time to fill the cache and avoid any or most subsequent virtual calls. Programs that rely on each form of the virtual function being called from the corresponding non-virtual function will see unexpected behavior when using such implementations.
2. The second problem is that either form of each of the virtual functions can be overridden by a user-defined function in a derived class to return a value that is different from the one produced by the virtual function of the alternate form that has not been overriden.
Thus, it might be possible for, say, ctype::widen(c)
to return one
value, while for ctype::widen(&c, &c + 1, &wc)
to set
wc
to another value. This is almost certainly not intended. Both
forms of every function should be required to return the same result
for the same character, otherwise the same program using an
implementation that calls one form of the functions will behave
differently than when using another implementation that calls the
other form of the function "under the hood."
3. The last problem is that the standard text fails to specify whether one form of any of the virtual functions is permitted to be implemented in terms of the other form or not, and if so, whether it is required or permitted to call the overridden virtual function or not.
Thus, a program that overrides one of the virtual functions so that it calls the other form which then calls the base member might end up in an infinite loop if the called form of the base implementation of the function in turn calls the other form.
Lillehammer: Part of this isn't a real problem. We already talk about
caching. 22.1.1/6 But part is a real problem. ctype
virtuals may call
each other, so users don't know which ones to override to avoid avoid
infinite loops.
This is a problem for all facet virtuals, not just ctype
virtuals,
so we probably want a blanket statement in clause 22 for all
facets. The LWG is leaning toward a blanket prohibition, that a
facet's virtuals may never call each other. We might want to do that
in clause 27 too, for that matter. A review is necessary. Bill will
provide wording.
[ 2009-07 Frankfurt, Howard provided wording directed by consensus. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add paragraph 3 to 28.3.4 [locale.categories]:
-3- Within this clause it is unspecified if one virtual function calls another virtual function.
Rationale:
We are explicitly not addressing bullet item #2, thus giving implementors more latitude. Users will have to override both virtual functions, not just one.
Section: 31.7.6.3.4 [ostream.inserters.character] Status: CD1 Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I think Footnote 297 is confused. The paragraph it applies to seems quite clear in that widen() is only called if the object is not a char stream (i.e., not basic_ostream<char>), so it's irrelevant what the value of widen(c) is otherwise.
Proposed resolution:
I propose to strike the Footnote.
Section: 26.6.5 [alg.foreach] Status: CD1 Submitter: Stephan T. Lavavej, Jaakko Jarvi Opened: 2004-07-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It is not clear whether the function object passed to for_each is allowed to modify the elements of the sequence being iterated over.
for_each is classified without explanation in [lib.alg.nonmodifying], "25.1 Non-modifying sequence operations". 'Non-modifying sequence operation' is never defined.
25(5) says: "If an algorithm's Effects section says that a value pointed to by any iterator passed as an argument is modified, then that algorithm has an additional type requirement: The type of that argument shall satisfy the requirements of a mutable iterator (24.1)."
for_each's Effects section does not mention whether arguments can be modified:
"Effects: Applies f to the result of dereferencing every iterator in the range [first, last), starting from first and proceeding to last - 1."
Every other algorithm in [lib.alg.nonmodifying] is "really" non-modifying in the sense that neither the algorithms themselves nor the function objects passed to the algorithms may modify the sequences or elements in any way. This DR affects only for_each.
We suspect that for_each's classification in "non-modifying sequence operations" means that the algorithm itself does not inherently modify the sequence or the elements in the sequence, but that the function object passed to it may modify the elements it operates on.
The original STL document by Stepanov and Lee explicitly prohibited the function object from modifying its argument. The "obvious" implementation of for_each found in several standard library implementations, however, does not impose this restriction. As a result, we suspect that the use of for_each with function objects that modify their arguments is wide-spread. If the restriction was reinstated, all such code would become non-conforming. Further, none of the other algorithms in the Standard could serve the purpose of for_each (transform does not guarantee the order in which its function object is called).
We suggest that the standard be clarified to explicitly allow the function object passed to for_each modify its argument.
Proposed resolution:
Add a nonnormative note to the Effects in 26.6.5 [alg.foreach]: If the type of 'first' satisfies the requirements of a mutable iterator, 'f' may apply nonconstant functions through the dereferenced iterators passed to it.
Rationale:
The LWG believes that nothing in the standard prohibits function objects that modify the sequence elements. The problem is that for_each is in a secion entitled "nonmutating algorithms", and the title may be confusing. A nonnormative note should clarify that.
Section: 24.3.5.5 [forward.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2004-07-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 477
Discussion:
The Forward Iterator requirements table contains the following:
expression return type operational precondition semantics ========== ================== =========== ========================== a->m U& if X is mutable, (*a).m pre: (*a).m is well-defined. otherwise const U& r->m U& (*r).m pre: (*r).m is well-defined.
The second line may be unnecessary. Paragraph 11 of [lib.iterator.requirements] says:
In the following sections, a and b denote values of type const X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T, o denotes a value of some type that is writable to the output iterator.
Because operators can be overloaded on an iterator's const-ness, the current requirements allow iterators to make many of the operations specified using the identifiers a and b invalid for non-const iterators.
Proposed resolution:
Remove the "r->m" line from the Forward Iterator requirements table. Change
"const X"
to
"X or const X"
in paragraph 11 of [lib.iterator.requirements].
Rationale:
This is a defect because it constrains an lvalue to returning a modifiable lvalue.
Section: 22.3 [pairs], 22.4 [tuple] Status: Resolved Submitter: Andrew Koenig Opened: 2004-09-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
(Based on recent comp.std.c++ discussion)
Pair (and tuple) should specialize std::swap to work in terms of std::swap on their components. For example, there's no obvious reason why swapping two objects of type pair<vector<int>, list<double> > should not take O(1).
[Lillehammer: We agree it should be swappable. Howard will provide wording.]
[
Post Oxford: We got swap
for pair
but accidently
missed tuple
. tuple::swap
is being tracked by 522(i).
]
Proposed resolution:
Wording provided in N1856.
Rationale:
Recommend NADResolved, fixed by
N1856.
Section: 24.3.5.4 [output.iterators] Status: Resolved Submitter: Chris Jefferson Opened: 2004-10-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The note on 24.1.2 Output iterators insufficiently limits what can be performed on output iterators. While it requires that each iterator is progressed through only once and that each iterator is written to only once, it does not require the following things:
Note: Here it is assumed that x
is an output iterator of type X
which
has not yet been assigned to.
a) That each value of the output iterator is written to:
The standard allows:
++x; ++x; ++x;
b) That assignments to the output iterator are made in order
X a(x); ++a; *a=1; *x=2;
is allowed
c) Chains of output iterators cannot be constructed:
X a(x); ++a; X b(a); ++b; X c(b); ++c;
is allowed, and under the current
wording (I believe) x,a,b,c
could be written to in any order.
I do not believe this was the intension of the standard?
[Lillehammer: Real issue. There are lots of constraints we intended but didn't specify. Should be solved as part of iterator redesign.]
[ 2009-07 Frankfurt ]
Bill provided wording according to consensus.
[ 2009-07-21 Alisdair requests change from Review to Open. See thread starting with c++std-lib-24459 for discussion. ]
[ 2009-10 Santa Cruz: ]
Modified wording. Set to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready after looking at again in a larger group in Santa Cruz.
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3066.
Proposed resolution:
Change Table 101 — Output iterator requirements in 24.3.5.4 [output.iterators]:
Table 101 — Output iterator requirements Expression Return type Operational semantics Assertion/note pre-/post-condition X(a)
a = t
is equivalent toX(a) = t
. note: a destructor is assumed.X u(a);
X u = a;
*r = o
result is not used Post: r
is not required to be dereferenceable.r
is incrementable.++r
X&
&r == &++r
Post:r
is dereferenceable, unless otherwise specified.r
is not required to be incrementable.r++
convertible to const X&
{X tmp = r;
++r;
return tmp;}Post: r
is dereferenceable, unless otherwise specified.r
is not required to be incrementable.*r++ = o;
result is not used
Section: 26.7.11 [alg.rotate] Status: CD1 Submitter: Howard Hinnant Opened: 2004-11-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
rotate takes 3 iterators: first, middle and last which point into a sequence, and rearranges the sequence such that the subrange [middle, last) is now at the beginning of the sequence and the subrange [first, middle) follows. The return type is void.
In many use cases of rotate, the client needs to know where the subrange [first, middle) starts after the rotate is performed. This might look like:
rotate(first, middle, last); Iterator i = advance(first, distance(middle, last));
Unless the iterators are random access, the computation to find the start of the subrange [first, middle) has linear complexity. However, it is not difficult for rotate to return this information with negligible additional computation expense. So the client could code:
Iterator i = rotate(first, middle, last);
and the resulting program becomes significantly more efficient.
While the backwards compatibility hit with this change is not zero, it is very small (similar to that of lwg 130(i)), and there is a significant benefit to the change.
Proposed resolution:
In 26 [algorithms] p2, change:
template<class ForwardIterator>voidForwardIterator rotate(ForwardIterator first, ForwardIterator middle, ForwardIterator last);
In 26.7.11 [alg.rotate], change:
template<class ForwardIterator>voidForwardIterator rotate(ForwardIterator first, ForwardIterator middle, ForwardIterator last);
In 26.7.11 [alg.rotate] insert a new paragraph after p1:
Returns:
first + (last - middle)
.
[ The LWG agrees with this idea, but has one quibble: we want to make sure not to give the impression that the function "advance" is actually called, just that the nth iterator is returned. (Calling advance is observable behavior, since users can specialize it for their own iterators.) Howard will provide wording. ]
[Howard provided wording for mid-meeting-mailing Jun. 2005.]
[ Toronto: moved to Ready. ]
Section: 28.3 [localization] Status: CD1 Submitter: Beman Dawes Opened: 2005-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It appears that there are no requirements specified for many of the template parameters in clause 22. It looks like this issue has never come up, except perhaps for Facet.
Clause 22 isn't even listed in 17.3.2.1 [lib.type.descriptions], either, which is the wording that allows requirements on template parameters to be identified by name.
So one issue is that 17.3.2.1 [lib.type.descriptions] Should be changed to cover clause 22. A better change, which will cover us in the future, would be to say that it applies to all the library clauses. Then if a template gets added to any library clause we are covered.
charT, InputIterator, and other names with requirements defined elsewhere are fine, assuming the 17.3.2.1 [lib.type.descriptions] fix. But there are a few template arguments names which I don't think have requirements given elsewhere:
Proposed resolution:
Change 16.3.3.3 [type.descriptions], paragraph 1, from:
The Requirements subclauses may describe names that are used to specify constraints on template arguments.153) These names are used in clauses 20, 23, 25, and 26 to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.
to:
The Requirements subclauses may describe names that are used to specify constraints on template arguments.153) These names are used in library clauses to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.
In the front matter of class 22, locales, add:
Template parameter types internT and externT shall meet the requirements of charT (described in 27 [strings]).
Rationale:
Again, a blanket clause isn't blanket enough. Also, we've got a couple of names that we don't have blanket requirement statements for. The only issue is what to do about stateT. This wording is thin, but probably adequate.
Section: 23.3.13 [vector] Status: CD1 Submitter: richard@ex-parrot.com Opened: 2005-02-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the synopsis of the std::vector<bool> specialisation in 23.3.13 [vector], the non-template assign() function has the signature
void assign( size_type n, const T& t );
The type, T, is not defined in this context.
Proposed resolution:
Replace "T" with "value_type".
Section: 17.3.5.2 [numeric.limits.members] Status: CD1 Submitter: Martin Sebor Opened: 2005-03-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
18.2.1.2, p59 says this much about the traps member of numeric_limits:
static const bool traps;
-59- true if trapping is implemented for the type.204)
Footnote 204: Required by LIA-1.
It's not clear what is meant by "is implemented" here.
In the context of floating point numbers it seems reasonable to expect to be able to use traps to determine whether a program can "safely" use infinity(), quiet_NaN(), etc., in arithmetic expressions, that is without causing a trap (i.e., on UNIX without having to worry about getting a signal). When traps is true, I would expect any of the operations in section 7 of IEEE 754 to cause a trap (and my program to get a SIGFPE). So, for example, on Alpha, I would expect traps to be true by default (unless I compiled my program with the -ieee option), false by default on most other popular architectures, including IA64, MIPS, PA-RISC, PPC, SPARC, and x86 which require traps to be explicitly enabled by the program.
Another possible interpretation of p59 is that traps should be true on any implementation that supports traps regardless of whether they are enabled by default or not. I don't think such an interpretation makes the traps member very useful, even though that is how traps is implemented on several platforms. It is also the only way to implement traps on platforms that allow programs to enable and disable trapping at runtime.
Proposed resolution:
Change p59 to read:
True if, at program startup, there exists a value of the type that would cause an arithmetic operation using that value to trap.
Rationale:
Real issue, since trapping can be turned on and off. Unclear what a static query can say about a dynamic issue. The real advice we should give users is to use cfenv for these sorts of queries. But this new proposed resolution is at least consistent and slightly better than nothing.
Section: 26.8.5 [alg.partitions] Status: C++11 Submitter: Sean Parent, Joe Gottman Opened: 2005-05-04 Last modified: 2025-03-13
Priority: Not Prioritized
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Discussion:
Problem: The iterator requirements for partition() and stable_partition() [25.2.12] are listed as BidirectionalIterator, however, there are efficient algorithms for these functions that only require ForwardIterator that have been known since before the standard existed. The SGI implementation includes these (see https://www.boost.org/sgi/stl/partition.html and https://www.boost.org/sgi/stl/stable_partition.html).
[ 2009-04-30 Alisdair adds: ]
Now we have concepts this is easier to express!
Proposed resolution:
Add the following signature to:
Header
<algorithm>
synopsis [algorithms.syn]
p3 Partitions 26.8.5 [alg.partitions]template<ForwardIterator Iter, Predicate<auto, Iter::value_type> Pred> requires ShuffleIterator<Iter> && CopyConstructible<Pred> Iter partition(Iter first, Iter last, Pred pred);Update p3 Partitions 26.8.5 [alg.partitions]:
Complexity:
At mostIf(last - first)/2
swaps. Exactlylast - first
applications of the predicate are done.Iter
satisfiesBidirectionalIterator
, at most(last - first)/2
swaps. Exactlylast - first
applications of the predicate are done.If
Iter
merely satisfiedForwardIterator
at most(last - first)
swaps are done. Exactly(last - first)
applications of the predicate are done.[Editorial note: I looked for existing precedent in how we might call out distinct overloads overloads from a set of constrained templates, but there is not much existing practice to lean on. advance/distance were the only algorithms I could find, and that wording is no clearer.]
[ 2009-07 Frankfurt ]
Hinnant: if you want to partition your std::forward_list, you'll need partition() to accept ForwardIterators.
No objection to Ready.
Move to Ready.
Proposed resolution:
Change 25.2.12 from
template<class BidirectionalIterator, class Predicate> BidirectionalIterator partition(BidirectionalIterato r first, BidirectionalIterator last, Predicate pred);
to
template<class ForwardIterator, class Predicate> ForwardIterator partition(ForwardIterator first, ForwardIterator last, Predicate pred);
Change the complexity from
At most (last - first)/2 swaps are done. Exactly (last - first) applications of the predicate are done.
to
If ForwardIterator is a bidirectional_iterator, at most (last - first)/2 swaps are done; otherwise at most (last - first) swaps are done. Exactly (last - first) applications of the predicate are done.
Rationale:
Partition is a "foundation" algorithm useful in many contexts (like sorting as just one example) - my motivation for extending it to include forward iterators is foward_list - without this extension you can't partition an foward_list (without writing your own partition). Holes like this in the standard library weaken the argument for generic programming (ideally I'd be able to provide a library that would refine std::partition() to other concepts without fear of conflicting with other libraries doing the same - but that is a digression). I consider the fact that partition isn't defined to work for ForwardIterator a minor embarrassment.
[Mont Tremblant: Moved to Open, request motivation and use cases by next meeting. Sean provided further rationale by post-meeting mailing.]
Section: 29.5.3 [rand.req], 99 [tr.rand.req] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 17: Random distribution requirements
Row 1 requires that each random distribution provide a nested type "input_type"; this type denotes the type of the values that the distribution consumes.
Inspection of all distributions in [tr.rand.dist] reveals that each distribution provides a second typedef ("result_type") that denotes the type of the values the distribution produces when called.
Proposed resolution:
It seems to me that this is also a requirement for all distributions and should therefore be indicated as such via a new second row to this table 17:
X::result_type | T | --- | compile-time |
[ Berlin: Voted to WP. N1932 adopts the proposed resolution: see Table 5 row 1. ]
Section: 29.5 [rand], 99 [tr.rand.var] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 11 of [tr.rand.var] equires that the member template
template<class T> result_type operator() (T value);
return
distribution()(e, value)
However, not all distributions have an operator() with a corresponding signature.
[ Berlin: As a working group we voted in favor of N1932 which makes this moot: variate_generator has been eliminated. Then in full committee we voted to give this issue WP status (mistakenly). ]
Proposed resolution:
We therefore recommend that we insert the following precondition before paragraph 11:
Precondition:
distribution().operator()(e,value)
is well-formed.
Section: 29.5.6 [rand.predef], 99 [tr.rand.predef] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The fifth of these engines with predefined parameters, ranlux64_base_01, appears to have an unintentional error for which there is a simple correction. The two pre-defined subtract_with_carry_01 engines are given as:
typedef subtract_with_carry_01<float, 24, 10, 24> ranlux_base_01; typedef subtract_with_carry_01<double, 48, 10, 24> ranlux64_base_01;
We demonstrate below that ranlux64_base_01 fails to meet the intent of the random number generation proposal, but that the simple correction to
typedef subtract_with_carry_01<double, 48, 5, 12> ranlux64_base_01;
does meet the intent of defining well-known good parameterizations.
The ranlux64_base_01 engine as presented fails to meet the intent for predefined engines, stated in proposal N1398 (section E):
In order to make good random numbers available to a large number of library users, this proposal not only defines generic random-number engines, but also provides a number of predefined well-known good parameterizations for those.
The predefined ranlux_base_01 engine has been proven [1,2,3] to have a very long period and so meets this criterion. This property makes it suitable for use in the excellent discard_block engines defined subsequently. The proof of long period relies on the fact (proven in [1]) that 2**(w*r) - 2**(w*s) + 1 is prime (w, r, and s are template parameters to subtract_with_carry_01, as defined in [tr.rand.eng.sub1]).
The ranlux64_base_01 engine as presented in [tr.rand.predef] uses w=48, r=24, s=10. For these numbers, the combination 2**(w*r)-2**(w*s)+1 is non-prime (though explicit factorization would be a challenge). In consequence, while it is certainly possible for some seeding states that this engine would have a very long period, it is not at all "well-known" that this is the case. The intent in the N1398 proposal involved the base of the ranlux64 engine, which finds heavy use in the physics community. This is isomorphic to the predefined ranlux_base_01, but exploits the ability of double variables to hold (at least) 48 bits of mantissa, to deliver 48 random bits at a time rather than 24.
Proposed resolution:
To achieve this intended behavior, the correct template parameteriztion would be:
typedef subtract_with_carry_01<double, 48, 5, 12> ranlux64_base_01;
The sequence of mantissa bits delivered by this is isomorphic (treating each double as having the bits of two floats) to that delivered by ranlux_base_01.
References:
[ Berlin: Voted to WP. N1932 adopts the proposed resolution in 26.3.5, just above paragraph 5. ]
Section: 23.2.8 [unord.req], 99 [tr.unord.req] Status: CD1 Submitter: Matt Austern Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 371 deals with stability of multiset/multimap under insert and erase (i.e. do they preserve the relative order in ranges of equal elements). The same issue applies to unordered_multiset and unordered_multimap.
[ Moved to open (from review): There is no resolution. ]
[ Toronto: We have a resolution now. Moved to Review. Some concern was noted as to whether this conflicted with existing practice or not. An additional concern was in specifying (partial) ordering for an unordered container. ]
Proposed resolution:
Wording for the proposed resolution is taken from the equivalent text for associative containers.
Change 23.2.8 [unord.req], Unordered associative containers, paragraph 6 to:
An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys.
unordered_set
andunordered_map
support unique keys.unordered_multiset
andunordered_multimap
support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other. Forunordered_multiset
andunordered_multimap
,insert
anderase
preserve the relative ordering of equivalent elements.
Change 23.2.8 [unord.req], Unordered associative containers, paragraph 8 to:
The elements of an unordered associative container are organized into buckets. Keys with the same hash code appear in the same bucket. The number of buckets is automatically increased as elements are added to an unordered associative container, so that the average number of elements per bucket is kept below a bound. Rehashing invalidates iterators, changes ordering between elements, and changes which buckets elements appear in, but does not invalidate pointers or references to elements. For
unordered_multiset
andunordered_multimap
, rehashing preserves the relative ordering of equivalent elements.
Section: 23.3.3 [array], 99 [tr.array.array] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
array<>::data()
is present in the class synopsis, but not documented.
Proposed resolution:
Add a new section, after 6.2.2.3:
T* data() const T* data() const;
Returns: elems
.
Change 6.2.2.4/2 to:
In the case where
N == 0
,begin() == end()
. The return value ofdata()
is unspecified.
Section: 22.10.15 [func.bind], 99 [tr.func.bind] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the original proposal for binders, the return type of bind() when called with a pointer to member data as it's callable object was defined to be mem_fn(ptr); when Peter Dimov and I unified the descriptions of the TR1 function objects we hoisted the descriptions of return types into the INVOKE pseudo-function and into result_of. Unfortunately, we left pointer to member data out of result_of, so bind doesn't have any specified behavior when called with a pointer to member data.
Proposed resolution:
[ Pete and Peter will provide wording. ]
In 20.5.4 [lib.func.ret] ([tr.func.ret]) p3 add the following bullet after bullet 2:
F
is a member data pointer type R T::*
, type
shall be cv R&
when T1
is cv U1&
,
R
otherwise.[ Peter provided wording. ]
Section: 22.10.6 [refwrap], 99 [tr.util.refwrp.refwrp] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
2.1.2/3, second bullet item currently says that reference_wrapper<T> is derived from unary_function<T, R> if T is:
a pointer to member function type with cv-qualifier cv and no arguments; the type T1 is cv T* and R is the return type of the pointer to member function;
The type of T1 can't be cv T*, 'cause that's a pointer to a pointer to member function. It should be a pointer to the class that T is a pointer to member of. Like this:
a pointer to a member function R T0::f() cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*
Similarly, bullet item 2 in 2.1.2/4 should be:
a pointer to a member function R T0::f(T2) cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*
Proposed resolution:
Change bullet item 2 in 2.1.2/3:
- a pointer to member function
type with cv-qualifiercv
and no arguments; the typeT1
iscv T*
andR
is the return type of the pointer to member functionR T0::f() cv
(wherecv
represents the member function's cv-qualifiers); the typeT1
iscv T0*
Change bullet item 2 in 2.1.2/4:
- a pointer to member function
with cv-qualifiercv
and taking one argument of typeT2
; the typeT1
iscv T*
andR
is the return type of the pointer to member functionR T0::f(T2) cv
(wherecv
represents the member function's cv-qualifiers); the typeT1
iscv T0*
Section: 22.4 [tuple], 99 [tr.tuple] Status: CD1 Submitter: Andy Koenig Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Tuple doesn't define swap(). It should.
[ Berlin: Doug to provide wording. ]
[ Batavia: Howard to provide wording. ]
[ Toronto: Howard to provide wording (really this time). ]
[ Bellevue: Alisdair provided wording. ]
Proposed resolution:
Add these signatures to 22.4 [tuple]
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>&& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>&& y);
Add this signature to 22.4.4 [tuple.tuple]
void swap(tuple&&);
Add the following two sections to the end of the tuple clauses
20.3.1.7 tuple swap [tuple.swap]
void swap(tuple&& rhs);Requires: Each type in
Types
shall beSwappable
.Effects: Calls
swap
for each element in*this
and its corresponding element inrhs
.Throws: Nothing, unless one of the element-wise
swap
calls throw an exception.20.3.1.8 tuple specialized algorithms [tuple.special]
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>&& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>&& y);Effects: x.swap(y)
Section: 28.6 [re] Status: CD1 Submitter: Eric Niebler Opened: 2005-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This defect is also being discussed on the Boost developers list. The full discussion can be found here: http://lists.boost.org/boost/2005/07/29546.php
-- Begin original message --
Also, I may have found another issue, closely related to the one under discussion. It regards case-insensitive matching of named character classes. The regex_traits<> provides two functions for working with named char classes: lookup_classname and isctype. To match a char class such as [[:alpha:]], you pass "alpha" to lookup_classname and get a bitmask. Later, you pass a char and the bitmask to isctype and get a bool yes/no answer.
But how does case-insensitivity work in this scenario? Suppose we're doing a case-insensitive match on [[:lower:]]. It should behave as if it were [[:lower:][:upper:]], right? But there doesn't seem to be enough smarts in the regex_traits interface to do this.
Imagine I write a traits class which recognizes [[:fubar:]], and the "fubar" char class happens to be case-sensitive. How is the regex engine to know that? And how should it do a case-insensitive match of a character against the [[:fubar:]] char class? John, can you confirm this is a legitimate problem?
I see two options:
1) Add a bool icase parameter to lookup_classname. Then, lookup_classname( "upper", true ) will know to return lower|upper instead of just upper.
2) Add a isctype_nocase function
I prefer (1) because the extra computation happens at the time the pattern is compiled rather than when it is executed.
-- End original message --
For what it's worth, John has also expressed his preference for option (1) above.
Proposed resolution:
Adopt the proposed resolution in N2409.
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 21.3.5 [meta.unary], 99 [tr.meta.unary] Status: Resolved Submitter: Robert Klarer Opened: 2005-07-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It is not completely clear how the primary type traits deal with cv-qualified types. And several of the secondary type traits seem to be lacking a definition.
[ Berlin: Howard to provide wording. ]
Proposed resolution:
Wording provided in N2028. A revision (N2157) provides more detail for motivation.
Rationale:
Solved by revision (N2157) in the WP.
tr1::bind
has lost its Throws clauseSection: 22.10.15.4 [func.bind.bind], 99 [tr.func.bind.bind] Status: CD1 Submitter: Peter Dimov Opened: 2005-10-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The original bind proposal gives the guarantee that tr1::bind(f, t1, ..., tN)
does not throw when the copy constructors of f, t1, ..., tN
don't.
This guarantee is not present in the final version of TR1.
I'm pretty certain that we never removed it on purpose. Editorial omission? :-)
[ Berlin: not quite editorial, needs proposed wording. ]
[ Batavia: Doug to translate wording to variadic templates. ]
[ Toronto: We agree but aren't quite happy with the wording. The "t"'s no longer refer to anything. Alan to provide improved wording. ]
[ Pre-Bellevue: Alisdair provided wording. ]
TR1 proposed resolution:
In 99 [tr.func.bind.bind], add a new paragraph after p2:
Throws: Nothing unless one of the copy constructors of
f, t1, t2, ..., tN
throws an exception.Add a new paragraph after p4:
Throws: nothing unless one of the copy constructors of
f, t1, t2, ..., tN
throws an exception.
Proposed resolution:
In 22.10.15.4 [func.bind.bind], add a new paragraph after p2:
Throws: Nothing unless the copy constructor of
F
or of one of the types in theBoundArgs...
pack expansion throws an exception.
In 22.10.15.4 [func.bind.bind], add a new paragraph after p4:
Throws: Nothing unless the copy constructor of
F
or of one of the types in theBoundArgs...
pack expansion throws an exception.
Section: 27.4.3 [basic.string] Status: CD1 Submitter: Matt Austern Opened: 2005-11-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 69(i), which was incorporated into C++03, mandated
that the elements of a vector must be stored in contiguous memory.
Should the same also apply to basic_string
?
We almost require contiguity already. Clause 23.4.7 [multiset]
defines operator[]
as data()[pos]
. What's missing
is a similar guarantee if we access the string's elements via the
iterator interface.
Given the existence of data()
, and the definition of
operator[]
and at
in terms of data
,
I don't believe it's possible to write a useful and standard-
conforming basic_string
that isn't contiguous. I'm not
aware of any non-contiguous implementation. We should just require
it.
Proposed resolution:
Add the following text to the end of 27.4.3 [basic.string], paragraph 2.
The characters in a string are stored contiguously, meaning that if
s
is abasic_string<charT, Allocator>
, then it obeys the identity&*(s.begin() + n) == &*s.begin() + n
for all0 <= n < s.size()
.
Rationale:
Not standardizing this existing practice does not give implementors more freedom. We thought it might a decade ago. But the vendors have spoken both with their implementations, and with their voice at the LWG meetings. The implementations are going to be contiguous no matter what the standard says. So the standard might as well give string clients more design choices.
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2005-11-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The array forms of unformatted input functions don't seem to have well-defined
semantics for zero-element arrays in a couple of cases. The affected ones
(istream::get()
and istream::getline()
) are supposed to
terminate when (n - 1)
characters are stored, which obviously can
never be true when (n == 0)
holds to start with. See
c++std-lib-16071.
Proposed resolution:
I suggest changing 27.6.1.3, p7 (istream::get()
), bullet 1 to read:
(n < 1)
is true or (n - 1)
characters
are stored;
Change 27.6.1.3, p9:
If the function stores no characters, it calls
setstate(failbit)
(which may throwios_base::failure
(27.4.4.3)). In any case, if(n > 0)
is true it then stores a null character into the next successive location of the array.
and similarly p17 (istream::getline()
), bullet 3 to:
(n < 1)
is true or (n - 1)
characters
are stored (in which case the function calls
setstate(failbit)
).
In addition, to clarify that istream::getline()
must not store the
terminating NUL character unless the the array has non-zero size, Robert
Klarer suggests in c++std-lib-16082 to change 27.6.1.3, p20 to read:
In any case, provided
(n > 0)
is true, it then stores a null character (using charT()) into the next successive location of the array.
[
post-Redmond: Pete noticed that the current resolution for get
requires
writing to out of bounds memory when n == 0
. Martin provided fix.
]
Section: 20.3.2.2.11 [util.smartptr.getdeleter], 99 [tr.util.smartptr.getdeleter] Status: CD1 Submitter: Paolo Carlini Opened: 2005-11-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I'm seeing something that looks like a typo. The Return of get_deleter
says:
If
*this
owns a deleterd
...
but get_deleter
is a free function!
Proposed resolution:
Therefore, I think should be:
If
owns a deleter
*thispd
...
Section: 27.4.3 [basic.string] Status: CD1 Submitter: Alisdair Meredith Opened: 2005-11-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
OK, we all know std::basic_string is bloated and already has way too many members. However, I propose it is missing 3 useful members that are often expected by users believing it is a close approximation of the container concept. All 3 are listed in table 71 as 'optional'
i/ pop_back.
This is the one I feel most strongly about, as I only just discovered it was missing as we are switching to a more conforming standard library <g>
I find it particularly inconsistent to support push_back, but not pop_back.
ii/ back.
There are certainly cases where I want to examine the last character of a string before deciding to append, or to trim trailing path separators from directory names etc. *rbegin() somehow feels inelegant.
iii/ front
This one I don't feel strongly about, but if I can get the first two, this one feels that it should be added as a 'me too' for consistency.
I believe this would be similarly useful to the data() member recently added to vector, or at() member added to the maps.
Proposed resolution:
Add the following members to definition of class template basic_string, 21.3p7
void pop_back () const charT & front() const charT & front() const charT & back() const charT & back()
Add the following paragraphs to basic_string description
21.3.4p5
const charT & front() const charT & front()Precondition:
!empty()
Effects: Equivalent to
operator[](0)
.
21.3.4p6
const charT & back() const charT & back()Precondition:
!empty()
Effects: Equivalent to
operator[]( size() - 1)
.
21.3.5.5p10
void pop_back ()Precondition:
!empty()
Effects: Equivalent to
erase( size() - 1, 1 )
.
Update Table 71: (optional sequence operations) Add basic_string to the list of containers for the following operations.
a.front() a.back() a.push_back() a.pop_back() a[n]
[ Berlin: Has support. Alisdair provided wording. ]
Section: 27.4.3.7.8 [string.swap] Status: CD1 Submitter: Beman Dawes Opened: 2005-12-14 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
std::string::swap currently says for effects and postcondition:
Effects: Swaps the contents of the two strings.
Postcondition:
*this
contains the characters that were ins
,s
contains the characters that were in*this
.
Specifying both Effects and Postcondition seems redundant, and the postcondition needs to be made stronger. Users would be unhappy if the characters were not in the same order after the swap.
Proposed resolution:
Effects: Swaps the contents of the two strings.Postcondition:
*this
contains the same sequence of characters thatwerewas ins
,s
contains the same sequence of characters thatwerewas in*this
.
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Paolo Carlini Opened: 2006-02-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the most recent working draft, I'm still seeing:
seekg(off_type& off, ios_base::seekdir dir)
and
seekp(pos_type& pos) seekp(off_type& off, ios_base::seekdir dir)
that is, by reference off and pos arguments.
Proposed resolution:
After 27.6.1.3p42 change:
basic_istream<charT,traits>& seekg(off_type&off, ios_base::seekdir dir);
After 27.6.2.4p1 change:
basic_ostream<charT,traits>& seekp(pos_type&pos);
After 27.6.2.4p3 change:
basic_ostream<charT,traits>& seekp(off_type&off, ios_base::seekdir dir);
Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I believe I botched the resolution of 241(i) "Does unique_copy() require CopyConstructible and Assignable?" which now has WP status.
This talks about unique_copy
requirements and currently reads:
-5- Requires: The ranges
[first, last)
and[result, result+(last-first))
shall not overlap. The expression*result = *first
shall be valid. If neitherInputIterator
norOutputIterator
meets the requirements of forward iterator then the value type ofInputIterator
must be CopyConstructible (20.1.3). Otherwise CopyConstructible is not required.
The problem (which Paolo discovered) is that when the iterators are at their
most restrictive (InputIterator
, OutputIterator
), then we want
InputIterator::value_type
to be both CopyConstructible
and
CopyAssignable
(for the most efficient implementation). However this
proposed resolution only makes it clear that it is CopyConstructible
,
and that one can assign from *first
to *result
.
This latter requirement does not necessarily imply that you can:
*first = *first;
Proposed resolution:
-5- Requires: The ranges
[first, last)
and[result, result+(last-first))
shall not overlap. The expression*result = *first
shall be valid. If neitherInputIterator
norOutputIterator
meets the requirements of forward iterator then thevalue typevalue_type
ofInputIterator
must be CopyConstructible (20.1.3) and Assignable. Otherwise CopyConstructible is not required.
partial_sum
and adjacent_difference
should mention requirementsSection: 26.10.7 [partial.sum] Status: C++11 Submitter: Marc Schoolderman Opened: 2006-02-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
There are some problems in the definition of partial_sum
and
adjacent_difference
in 26.4 [lib.numeric.ops]
Unlike accumulate
and inner_product
, these functions are not
parametrized on a "type T", instead, 26.4.3 [lib.partial.sum] simply
specifies the effects clause as;
Assigns to every element referred to by iterator
i
in the range[result,result + (last - first))
a value correspondingly equal to((...(* first + *( first + 1)) + ...) + *( first + ( i - result )))
And similarly for BinaryOperation. Using just this definition, it seems logical to expect that:
char i_array[4] = { 100, 100, 100, 100 }; int o_array[4]; std::partial_sum(i_array, i_array+4, o_array);
Is equivalent to
int o_array[4] = { 100, 100+100, 100+100+100, 100+100+100+100 };
i.e. 100, 200, 300, 400, with addition happening in the result type
,
int
.
Yet all implementations I have tested produce 100, -56, 44, -112,
because they are using an accumulator of the InputIterator
's
value_type
, which in this case is char
, not int
.
The issue becomes more noticeable when the result of the expression *i +
*(i+1)
or binary_op(*i, *i-1)
can't be converted to the
value_type
. In a contrived example:
enum not_int { x = 1, y = 2 }; ... not_int e_array[4] = { x, x, y, y }; std::partial_sum(e_array, e_array+4, o_array);
Is it the intent that the operations happen in the input type
, or in
the result type
?
If the intent is that operations happen in the result type
, something
like this should be added to the "Requires" clause of 26.4.3/4
[lib.partial.sum]:
The type of
*i + *(i+1)
orbinary_op(*i, *(i+1))
shall meet the requirements ofCopyConstructible
(20.1.3) andAssignable
(23.1) types.
(As also required for T
in 26.4.1 [lib.accumulate] and 26.4.2
[lib.inner.product].)
The "auto initializer" feature proposed in
N1894
is not required to
implement partial_sum
this way. The 'narrowing' behaviour can still be
obtained by using the std::plus<>
function object.
If the intent is that operations happen in the input type
, then
something like this should be added instead;
The type of *first shall meet the requirements of
CopyConstructible
(20.1.3) andAssignable
(23.1) types. The result of*i + *(i+1)
orbinary_op(*i, *(i+1))
shall be convertible to this type.
The 'widening' behaviour can then be obtained by writing a custom proxy iterator, which is somewhat involved.
In both cases, the semantics should probably be clarified.
26.4.4 [lib.adjacent.difference] is similarly underspecified, although all implementations seem to perform operations in the 'result' type:
unsigned char i_array[4] = { 4, 3, 2, 1 }; int o_array[4]; std::adjacent_difference(i_array, i_array+4, o_array);
o_array is 4, -1, -1, -1 as expected, not 4, 255, 255, 255.
In any case, adjacent_difference
doesn't mention the requirements on the
value_type
; it can be brought in line with the rest of 26.4
[lib.numeric.ops] by adding the following to 26.4.4/2
[lib.adjacent.difference]:
The type of
*first
shall meet the requirements ofCopyConstructible
(20.1.3) andAssignable
(23.1) types."
[ Berlin: Giving output iterator's value_types very controversial. Suggestion of adding signatures to allow user to specify "accumulator". ]
[ Bellevue: ]
The intent of the algorithms is to perform their calculations using the type of the input iterator. Proposed wording provided.
[ Sophia Antipolis: ]
We did not agree that the proposed resolution was correct. For example, when the arguments are types
(float*, float*, double*)
, the highest-quality solution would use double as the type of the accumulator. If the intent of the wording is to require that the type of the accumulator must be theinput_iterator
'svalue_type
, the wording should specify it.
[ 2009-05-09 Alisdair adds: ]
Now that we have the facility, the 'best' accumulator type could probably be deduced as:
std::common_type<InIter::value_type, OutIter::reference>::typeThis type would then have additional requirements of constructability and incrementability/assignability.
If this extracting an accumulator type from a pair/set of iterators (with additional requirements on that type) is a problem for multiple functions, it might be worth extracting into a SharedAccumulator concept or similar.
I'll go no further in writing up wording now, until the group gives a clearer indication of preferred direction.
[ 2009-07 Frankfurt ]
The proposed resolution isn't quite right. For example, "the type of
*first
" should be changed to "iterator::value_type
" or similar. Daniel volunteered to correct the wording.
[ 2009-07-29 Daniel corrected wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change 26.10.7 [partial.sum]/1 as indicated:
Effects: Let
VT
beInputIterator
's value type. For a nonempty range, initializes an accumulatoracc
of typeVT
with*first
and performs*result = acc
. For every iteratori
in[first + 1, last)
in order,acc
is then modified byacc = acc + *i
oracc = binary_op(acc, *i)
and is assigned to*(result + (i - first))
.Assigns to every element referred to by iteratori
in the range[result,result + (last - first))
a value correspondingly equal to((...(*first + *(first + 1)) + ...) + *(first + (i - result)))
orbinary_op(binary_op(..., binary_op(*first, *(first + 1)),...), *(first + (i - result)))
Change 26.10.7 [partial.sum]/3 as indicated:
Complexity: Exactly
max((last - first) - 1, 0)
applications ofthe binary operation.
binary_op
Change 26.10.7 [partial.sum]/4 as indicated:
Requires:
VT
shall be constructible from the type of*first
, the result ofacc + *i
orbinary_op(acc, *i)
shall be implicitly convertible toVT
, and the result of the expressionacc
shall be writable to theresult
output iterator. In the ranges[first,last]
and[result,result + (last - first)]
[..]
Change 26.10.12 [adjacent.difference]/1 as indicated:
Effects: Let
VT
beInputIterator
's value type. For a nonempty range, initializes an accumulatoracc
of typeVT
with*first
and performs*result = acc
. For every iteratori
in[first + 1, last)
in order, initializes a valueval
of typeVT
with*i
, assigns the result ofval - acc
orbinary_op(val, acc)
to*(result + (i - first))
and modifiesacc = std::move(val)
.Assigns to every element referred to by iteratori
in the range[result + 1, result + (last - first))
a value correspondingly equal to*(first + (i - result)) - *(first + (i - result) - 1)
orbinary_op(*(first + (i - result)), *(first + (i - result) - 1)).
result gets the value of *first.
Change 26.10.12 [adjacent.difference]/2 as indicated:
Requires:
VT
shall beMoveAssignable
([moveassignable]) and shall be constructible from the type of*first
. The result of the expressionacc
and the result of the expressionval - acc
orbinary_op(val, acc)
shall be writable to theresult
output iterator. In the ranges[first,last]
[..]
Change 26.10.12 [adjacent.difference]/5 as indicated:
Complexity: Exactly
max((last - first) - 1, 0)
applications ofthe binary operation.binary_op
Section: 20.3.2.2.6 [util.smartptr.shared.obs], 99 [tr.util.smartptr.shared.obs] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.obs].
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Discussion:
I'm trying to reconcile the note in tr.util.smartptr.shared.obs, p6 that talks about the operator*() member function of shared_ptr:
Notes: When T is void, attempting to instantiate this member function renders the program ill-formed. [Note: Instantiating shared_ptr<void> does not necessarily result in instantiating this member function. --end note]
with the requirement in temp.inst, p1:
The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions...
I assume that what the note is really trying to say is that "instantiating shared_ptr<void> *must not* result in instantiating this member function." That is, that this function must not be declared a member of shared_ptr<void>. Is my interpretation correct?
Proposed resolution:
Change 2.2.3.5p6
-6-
Notes:WhenT
isvoid
,attempting to instantiate this member function renders the program ill-formed. [Note: Instantiatingit is unspecified whether this member function is declared or not, and if so, what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.shared_ptr<void>
does not necessarily result in instantiating this member function. --end note]
Section: 20.3.2.2 [util.smartptr.shared], 99 [tr.util.smartptr.shared] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is the void specialization of the template assignment operator taking a shared_ptr<void> as an argument supposed be well-formed?
I.e., is this snippet well-formed:
shared_ptr<void> p; p.operator=<void>(p);
Gcc complains about auto_ptr<void>::operator*() returning a reference to void. I suspect it's because shared_ptr has two template assignment operators, one of which takes auto_ptr, and the auto_ptr template gets implicitly instantiated in the process of overload resolution.
The only way I see around it is to do the same trick with auto_ptr<void> operator*() as with the same operator in shared_ptr<void>.
PS Strangely enough, the EDG front end doesn't mind the code, even though in a small test case (below) I can reproduce the error with it as well.
template <class T> struct A { T& operator*() { return *(T*)0; } }; template <class T> struct B { void operator= (const B&) { } template <class U> void operator= (const B<U>&) { } template <class U> void operator= (const A<U>&) { } }; int main () { B<void> b; b.operator=<void>(b); }
Proposed resolution:
In [lib.memory] change:
template<class X> class auto_ptr; template<> class auto_ptr<void>;
In [lib.auto.ptr]/2 add the following before the last closing brace:
template<> class auto_ptr<void> { public: typedef void element_type; };
Section: 20.3.2.2.6 [util.smartptr.shared.obs], 99 [tr.util.smartptr.shared.obs] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.obs].
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Discussion:
Peter Dimov wrote: To: C++ libraries mailing list Message c++std-lib-15614 [...] The intent is for both use_count() and unique() to work in a threaded environment. They are intrinsically prone to race conditions, but they never return garbage.
This is a crucial piece of information that I really wish were captured in the text. Having this in a non-normative note would have made everything crystal clear to me and probably stopped me from ever starting this discussion :) Instead, the sentence in p12 "use only for debugging and testing purposes, not for production code" very strongly suggests that implementations can and even are encouraged to return garbage (when threads are involved) for performance reasons.
How about adding an informative note along these lines:
Note: Implementations are encouraged to provide well-defined behavior for use_count() and unique() even in the presence of multiple threads.
I don't necessarily insist on the exact wording, just that we capture the intent.
Proposed resolution:
Change 20.3.2.2.6 [util.smartptr.shared.obs] p12:
[Note:
use_count()
is not necessarily efficient.Use only for debugging and testing purposes, not for production code.--end note]
Change 20.3.2.3.6 [util.smartptr.weak.obs] p3:
[Note:
use_count()
is not necessarily efficient.Use only for debugging and testing purposes, not for production code.--end note]
Section: 29.6.4 [class.slice] Status: CD1 Submitter: Howard Hinnant Opened: 2005-11-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
If one explicitly constructs a slice or glice with the default constructor, does the standard require this slice to have any usable state? It says "creates a slice which specifies no elements", which could be interpreted two ways:
Here is a bit of code to illustrate:
#include <iostream> #include <valarray> int main() { std::valarray<int> v(10); std::valarray<int> v2 = v[std::slice()]; std::cout << "v[slice()].size() = " << v2.size() << '\n'; }
Is the behavior undefined? Or should the output be:
v[slice()].size() = 0
There is a similar question and wording for gslice at 26.3.6.1p1.
Proposed resolution:
[Martin suggests removing the second sentence in 29.6.4.2 [cons.slice] as well.]
Change 29.6.4.2 [cons.slice]:
1 -
The default constructor forThe default constructor is equivalent toslice
creates aslice
which specifies no elements.slice(0, 0, 0)
. A default constructor is provided only to permit the declaration of arrays of slices. The constructor with arguments for a slice takes a start, length, and stride parameter.
Change 29.6.6.2 [gslice.cons]:
1 -
The default constructor creates aThe default constructor is equivalent togslice
which specifies no elements.gslice(0, valarray<size_t>(), valarray<size_t>())
. The constructor with arguments builds agslice
based on a specification of start, lengths, and strides, as explained in the previous section.
Section: 20.3.2.2.11 [util.smartptr.getdeleter], 99 [tr.util.smartptr.shared.dest] Status: CD1 Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of ~shared_ptr doesn't say when the shared_ptr's deleter, if any, is destroyed. In principle there are two possibilities: it is destroyed unconditionally whenever ~shared_ptr is executed (which, from an implementation standpoint, means that the deleter is copied whenever the shared_ptr is copied), or it is destroyed immediately after the owned pointer is destroyed (which, from an implementation standpoint, means that the deleter object is shared between instances). We should say which it is.
Proposed resolution:
Add after the first sentence of 20.3.2.2.11 [util.smartptr.getdeleter]/1:
The returned pointer remains valid as long as there exists a
shared_ptr
instance that ownsd
.[Note: it is unspecified whether the pointer remains valid longer than that. This can happen if the implementation doesn't destroy the deleter until all
weak_ptr
instances in the ownership group are destroyed. -- end note]
pow(float,int)
be?Section: 29.7 [c.math] Status: CD1 Submitter: Howard Hinnant Opened: 2006-01-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Assuming we adopt the C compatibility package from C99 what should be the return type of the following signature be:
? pow(float, int);
C++03 says that the return type should be float
.
TR1 and C90/99 say the return type should be double
. This can put
clients into a situation where C++03 provides answers that are not as high
quality as C90/C99/TR1. For example:
#include <math.h> int main() { float x = 2080703.375F; double y = pow(x, 2); }
Assuming an IEEE 32 bit float and IEEE 64 bit double, C90/C99/TR1 all suggest:
y = 4329326534736.390625
which is exactly right. While C++98/C++03 demands:
y = 4329326510080.
which is only approximately right.
I recommend that C++0X adopt the mixed mode arithmetic already adopted by
Fortran, C and TR1 and make the return type of pow(float,int)
be
double
.
[
Kona (2007): Other functions that are affected by this issue include
ldexp
, scalbln
, and scalbn
. We also believe that there is a typo in
26.7/10: float nexttoward(float, long double);
[sic] should be float
nexttoward(float, float);
Proposed Disposition: Review (the proposed
resolution appears above, rather than below, the heading "Proposed
resolution")
]
[Howard, post Kona:]
Unfortunately I strongly disagree with a part of the resolution from Kona. I am moving from New to Open instead of to Review because I do not believe we have consensus on the intent of the resolution.
This issue does not include
ldexp
,scalbln
, andscalbn
because the second integral parameter in each of these signatures (from C99) is not a generic parameter according to C99 7.22p2. The corresponding C++ overloads are intended (as far as I know) to correspond directly to C99's definition of generic parameter.For similar reasons, I do not believe that the second
long double
parameter ofnexttoward
, nor the return type of this function, is in error. I believe the correct signature is:float nexttoward(float, long double);which is what both the C++0X working paper and C99 state (as far as I currently understand).
This is really only about
pow(float, int)
. And this is because C++98 took one route (withpow
only) and C99 took another (with many math functions in<tgmath.h>
. The proposed resolution basically says: C++98 got it wrong and C99 got it right; let's go with C99.
[ Bellevue: ]
This signature was not picked up from C99. Instead, if one types
pow(2.0f,2)
, the promotion rules will invoke "double pow(double, double)", which generally gives special treatment for integral exponents, preserving full accuracy of the result. New proposed wording provided.
Proposed resolution:
Change 29.7 [c.math] p10:
The added signatures are:
...float pow(float, int);...double pow(double, int);...long double pow(long double, int);
Section: 17.15 [support.c.headers] Status: CD1 Submitter: Howard Hinnant Opened: 2006-01-23 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
Previously xxx.h was parsable by C++. But in the case of C99's <complex.h> it isn't. Otherwise we could model it just like <string.h>, <cstring>, <string>:
In the case of C's complex, the C API won't compile in C++. So we have:
The ? can't refer to the C API. TR1 currently says:
Proposed resolution:
Change 26.3.11 [cmplxh]:
The header behaves as if it includes the header
<ccomplex>
., and provides sufficient using declarations to declare in the global namespace all function and type names declared or defined in the neader[Note:<complex>
.<complex.h>
does not promote any interface into the global namespace as there is no C interface to promote. --end note]
Section: 26.7.13 [alg.random.shuffle] Status: CD1 Submitter: Martin Sebor Opened: 2006-01-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
...is specified to shuffle its range by calling swap but not how (or even that) it's supposed to use the RandomNumberGenerator argument passed to it.
Shouldn't we require that the generator object actually be used by the algorithm to obtain a series of random numbers and specify how many times its operator() should be invoked by the algorithm?
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Compare
a BinaryPredicate
?Section: 26.8 [alg.sorting] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 25, p8 we allow BinaryPredicates to return a type that's convertible to bool but need not actually be bool. That allows predicates to return things like proxies and requires that implementations be careful about what kinds of expressions they use the result of the predicate in (e.g., the expression in if (!pred(a, b)) need not be well-formed since the negation operator may be inaccessible or return a type that's not convertible to bool).
Here's the text for reference:
...if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct if (binary_pred(*first1, first2)){...}.
In 25.3, p2 we require that the Compare function object return true of false, which would seem to preclude such proxies. The relevant text is here:
Compare is used as a function object which returns true if the first argument is less than the second, and false otherwise...
[ Portland: Jack to define "convertible to bool" such that short circuiting isn't destroyed. ]
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Move to Review once wording received. Stefanus to send proposed wording.
[ 2009-10 Santa Cruz: ]
Move to Review once wording received. Stefanus to send proposed wording.
[ 2009-10-24 Stefanus supplied wording. ]
Move to Review once wording received. Stefanus to send proposed wording. Old proposed wording here:
I think we could fix this by rewording 25.3, p2 to read somthing like:
-2-
Compare
isused as a function object which returnsatrue
if the first argumentBinaryPredicate
. The return value of the function call operator applied to an object of typeCompare
, when converted to typebool
, yieldstrue
if the first argument of the call is less than the second, andfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.
[ 2010-01-17: ]
Howard expresses concern that the current direction of the proposed wording outlaws expressions such as:
if (!comp(x, y))Daniel provides wording which addresses that concern.
The previous wording is saved here:
Change 26.8 [alg.sorting] p2:
Compare
is used as a function object. The return value of the function call operator applied to an object of type Compare, when converted to type bool, yields true if the first argument of the callwhich returnsis less than the second, andtrue
if the first argumentfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.
[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 26.1 [algorithms.general]/7+8 as indicated. [This change is
recommended to bring the return value requirements of BinaryPredicate
and Compare
in sync.]
7 The
Predicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing the corresponding iterator returns a value testable astrue
. In other words, if an algorithm takesPredicate pred
as its argument andfirst
as its iterator argument, it should work correctly in the constructif(pred(*first)){...}
pred(*first)
contextually converted tobool
(7.3 [conv]). The function objectpred
shall not apply any nonconstant function through the dereferenced iterator. This function object may be a pointer to function, or an object of a type with an appropriate function call operator.8 The
BinaryPredicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and typeT
whenT
is part of the signature returns a value testable astrue
. In other words, if an algorithm takesBinaryPredicate
binary_pred
as its argument andfirst1
andfirst2
as its iterator arguments, it should work correctly in the constructif (binary_pred(*first1, *first2)){...}
binary_pred(*first1, *first2)
contextually converted tobool
(7.3 [conv]).BinaryPredicate
always takes the first iterator type as its first argument, that is, in those cases whenT value
is part of the signature, it should work correctly in thecontext ofconstructif (binary_pred(*first1, value)){...}
binary_pred(*first1, value)
contextually converted tobool
(7.3 [conv]).binary_pred
shall not apply any non-constant function through the dereferenced iterators.
Change 26.8 [alg.sorting]/2 as indicated:
2
Compare
isused asa function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of typeCompare
, when contextually converted to typebool
(7.3 [conv]), yieldstrue
if the first argument of the callwhich returnsis less than the second, andtrue
if the first argumentfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.
Section: 17.3.5 [numeric.limits] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-19 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
[limits], p2 requires implementations to provide specializations of the
numeric_limits
template for each scalar type. While this
could be interepreted to include cv-qualified forms of such types such
an interepretation is not reflected in the synopsis of the
<limits>
header.
The absence of specializations of the template on cv-qualified forms
of fundamental types makes numeric_limits
difficult to
use in generic code where the constness (or volatility) of a type is
not always immediately apparent. In such contexts, the primary
template ends up being instantiated instead of the provided
specialization, typically yielding unexpected behavior.
Require that specializations of numeric_limits
on
cv-qualified fundamental types have the same semantics as those on the
unqualifed forms of the same types.
Proposed resolution:
Add to the synopsis of the <limits>
header,
immediately below the declaration of the primary template, the
following:
template <class T> class numeric_limits<const T>; template <class T> class numeric_limits<volatile T>; template <class T> class numeric_limits<const volatile T>;
Add a new paragraph to the end of 17.3.5 [numeric.limits], with the following text:
-new-para- The value of each member of a numeric_limits
specialization on a cv-qualified T is equal to the value of the same
member of numeric_limits<T>
.
[ Portland: Martin will clarify that user-defined types get cv-specializations automatically. ]
Section: 24.5.2.4.3 [inserter] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The declaration of std::inserter
is:
template <class Container, class Iterator> insert_iterator<Container> inserter(Container& x, Iterator i);
The template parameter Iterator
in this function is completely unrelated
to the template parameter Container
when it doesn't need to be. This
causes the code to be overly generic. That is, any type at all can be deduced
as Iterator
, whether or not it makes sense. Now the same is true of
Container
. However, for every free (unconstrained) template parameter
one has in a signature, the opportunity for a mistaken binding grows geometrically.
It would be much better if inserter
had the following signature instead:
template <class Container> insert_iterator<Container> inserter(Container& x, typename Container::iterator i);
Now there is only one free template parameter. And the second argument to
inserter
must be implicitly convertible to the container's iterator,
else the call will not be a viable overload (allowing other functions in the
overload set to take precedence). Furthermore, the first parameter must have a
nested type named iterator
, or again the binding to std::inserter
is not viable. Contrast this with the current situation
where any type can bind to Container
or Iterator
and those
types need not be anything closely related to containers or iterators.
This can adversely impact well written code. Consider:
#include <iterator> #include <string> namespace my { template <class String> struct my_type {}; struct my_container { template <class String> void push_back(const my_type<String>&); }; template <class String> void inserter(const my_type<String>& m, my_container& c) {c.push_back(m);} } // my int main() { my::my_container c; my::my_type<std::string> m; inserter(m, c); }
Today this code fails because the call to inserter
binds to
std::inserter
instead of to my::inserter
. However with the
proposed change std::inserter
will no longer be a viable function which
leaves only my::inserter
in the overload resolution set. Everything
works as the client intends.
To make matters a little more insidious, the above example works today if you simply change the first argument to an rvalue:
inserter(my::my_type(), c);
It will also work if instantiated with some string type other than
std::string
(or any other std
type). It will also work if
<iterator>
happens to not get included.
And it will fail again for such inocuous reaons as my_type
or
my_container
privately deriving from any std
type.
It seems unfortunate that such simple changes in the client's code can result in such radically differing behavior.
Proposed resolution:
Change 24.2:
24.2 Header
<iterator>
synopsis... template <class Container, class Iterator> insert_iterator<Container> inserter(Container& x,Iteratortypename Container::iterator i); ...
Change 24.4.2.5:
24.4.2.5 Class template
insert_iterator
... template <class Container, class Iterator> insert_iterator<Container> inserter(Container& x,Iteratortypename Container::iterator i); ...
Change 24.4.2.6.5:
24.4.2.6.5
inserter
template <class Container, class Inserter> insert_iterator<Container> inserter(Container& x,Insertertypename Container::iterator i);-1- Returns:
insert_iterator<Container>(x,
.typename Container::iterator(i))
[ Kona (2007): This issue will probably be addressed as a part of the concepts overhaul of the library anyway, but the proposed resolution is correct in the absence of concepts. Proposed Disposition: Ready ]
Section: 31.8 [string.streams] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.streams].
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Discussion:
For better efficiency, the requirement on the stringbuf ctor that
takes a string argument should be loosened up to let it set
epptr()
beyond just one past the last initialized
character just like overflow()
has been changed to be
allowed to do (see issue 432). That way the first call to
sputc()
on an object won't necessarily cause a call to
overflow
. The corresponding change should be made to the
string overload of the str()
member function.
Proposed resolution:
Change 27.7.1.1, p3 of the Working Draft, N1804, as follows:
explicit basic_stringbuf(const basic_string<charT,traits,Allocator>& str, ios_base::openmode which = ios_base::in | ios_base::out);-3- Effects: Constructs an object of class
basic_stringbuf
, initializing the base class withbasic_streambuf()
(27.5.2.1), and initializingmode
withwhich
. Then callsstr(s)
.copies the content of str into thebasic_stringbuf
underlying character sequence. Ifwhich & ios_base::out
is true, initializes the output sequence such thatpbase()
points to the first underlying character,epptr()
points one past the last underlying character, andpptr()
is equal toepptr()
ifwhich & ios_base::ate
is true, otherwisepptr()
is equal topbase()
. Ifwhich & ios_base::in
is true, initializes the input sequence such thateback()
andgptr()
point to the first underlying character andegptr()
points one past the last underlying character.
Change the Effects clause of the str()
in 27.7.1.2, p2 to
read:
-2- Effects: Copies the contents of
s
into thebasic_stringbuf
underlying character sequence and initializes the input and output sequences according tomode
.Ifmode & ios_base::out
is true, initializes the output sequence such thatpbase()
points to the first underlying character,epptr()
points one past the last underlying character, andpptr()
is equal toepptr()
ifmode & ios_base::in
is true, otherwisepptr()
is equal topbase()
. Ifmode & ios_base::in
is true, initializes the input sequence such thateback()
andgptr()
point to the first underlying character andegptr()
points one past the last underlying character.-3- Postconditions: If
mode & ios_base::out
is true,pbase()
points to the first underlying character and(epptr() >= pbase() + s.size())
holds; in addition, ifmode & ios_base::in
is true,(pptr() == pbase() + s.data())
holds, otherwise(pptr() == pbase())
is true. Ifmode & ios_base::in
is true,eback()
points to the first underlying character, and(gptr() == eback())
and(egptr() == eback() + s.size())
hold.
[ Kona (2007) Moved to Ready. ]
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [stringbuf.virtuals].
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Discussion:
According to Table 92 (unchanged by issue 432), when (way ==
end)
the newoff
value in out mode is computed as
the difference between epptr()
and pbase()
.
This value isn't meaningful unless the value of epptr()
can be precisely controlled by a program. That used to be possible
until we accepted the resolution of issue 432, but since then the
requirements on overflow()
have been relaxed to allow it
to make more than 1 write position available (i.e., by setting
epptr()
to some unspecified value past
pptr()
). So after the first call to
overflow()
positioning the output sequence relative to
end will have unspecified results.
In addition, in in|out
mode, since (egptr() ==
epptr())
need not hold, there are two different possible values
for newoff
: epptr() - pbase()
and
egptr() - eback()
.
Proposed resolution:
Change the newoff
column in the last row of Table 94 to
read:
the
endhigh mark pointer minus the beginning pointer ().
xendhigh_mark - xbeg
[ Kona (2007) Moved to Ready. ]
stringbuf
seekpos underspecifiedSection: 31.8.2.5 [stringbuf.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [stringbuf.virtuals].
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Discussion:
The effects of the seekpos()
member function of
basic_stringbuf
simply say that the function positions
the input and/or output sequences but fail to spell out exactly
how. This is in contrast to the detail in which seekoff()
is described.
[ 2009-07 Frankfurt ]
Move to Ready.
Proposed resolution:
Change 27.7.1.3, p13 to read:
-13- Effects: Equivalent to
seekoff(off_type(sp), ios_base::beg, which)
.Alters the stream position within the controlled sequences, if possible, to correspond to the stream position stored insp
(as described below).
If(which & ios_base::in) != 0
, positions the input sequence.If(which & ios_base::out) != 0
, positions the output sequence.Ifsp
is an invalid stream position, or if the function positions neither sequence, the positioning operation fails. Ifsp
has not been obtained by a previous successful call to one of the positioning functions (seekoff
,seekpos
,tellg
,tellp
) the effect is undefined.
[
Kona (2007): A pos_type
is a position in a stream by
definition, so there is no ambiguity as to what it means. Proposed
Disposition: NAD
]
[
Post-Kona Martin adds:
I'm afraid I disagree
with the Kona '07 rationale for marking it NAD. The only text
that describes precisely what it means to position the input
or output sequence is in seekoff()
. The seekpos()
Effects
clause is inadequate in comparison and the proposed resolution
plugs the hole by specifying seekpos()
in terms of seekoff()
.
]
xsputn
inefficientSection: 31.6.3.5.5 [streambuf.virt.put] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
streambuf::xsputn()
is specified to have the effect of
"writing up to n
characters to the output sequence as if by
repeated calls to sputc(c)
."
Since sputc()
is required to call overflow()
when
(pptr() == epptr())
is true, strictly speaking
xsputn()
should do the same. However, doing so would be
suboptimal in some interesting cases, such as in unbuffered mode or
when the buffer is basic_stringbuf
.
Assuming calling overflow()
is not really intended to be
required and the wording is simply meant to describe the general
effect of appending to the end of the sequence it would be worthwhile
to mention in xsputn()
that the function is not actually
required to cause a call to overflow()
.
[ 2009-07 Frankfurt ]
Move to Ready.
Proposed resolution:
Add the following sentence to the xsputn()
Effects clause in
27.5.2.4.5, p1 (N1804):
-1- Effects: Writes up to
n
characters to the output sequence as if by repeated calls tosputc(c)
. The characters written are obtained from successive elements of the array whose first element is designated bys
. Writing stops when eithern
characters have been written or a call tosputc(c)
would returntraits::eof()
. It is uspecified whether the function callsoverflow()
when(pptr() == epptr())
becomes true or whether it achieves the same effects by other means.
In addition, I suggest to add a footnote to this function with the
same text as Footnote 292 to make it extra clear that derived classes
are permitted to override xsputn()
for efficiency.
[
Kona (2007): We want to permit a streambuf
that streams output directly
to a device without making calls to sputc
or overflow
. We believe that
has always been the intention of the committee. We believe that the
proposed wording doesn't accomplish that. Proposed Disposition: Open
]
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.unformatted].
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Discussion:
The array forms of unformatted input functions don't have well-defined
semantics for zero-element arrays in a couple of cases. The affected
ones (istream::get()
and getline()
) are supposed to
terminate when (n - 1)
characters are stored, which obviously
can never be true when (n == 0)
to start with.
Proposed resolution:
I propose the following changes (references are relative to the Working Draft (document N1804).
Change 27.6.1.3, p8 (istream::get()
), bullet 1 as follows:
if
(n < 1)
is true or(n - 1)
characters are stored;
Similarly, change 27.6.1.3, p18 (istream::getline()
), bullet
3 as follows:
(n < 1)
is true or(n - 1)
characters are stored (in which case the function callssetstate(failbit)
).
Finally, change p21 as follows:
In any case, provided
(n > 0)
is true, it then stores a null character (using charT()) into the next successive location of the array.
Section: 31.7 [iostream.format] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 60 explicitly made the extractor and inserter operators that
take a basic_streambuf*
argument formatted input and output
functions, respectively. I believe that's wrong, certainly in the
case of the extractor, since formatted functions begin by extracting
and discarding whitespace. The extractor should not discard any
characters.
Proposed resolution:
I propose to change each operator to behave as unformatted input and output function, respectively. The changes below are relative to the working draft document number N1804.
Specifically, change 27.6.1.2.3, p14 as follows:
Effects: Behaves as an unformatted input function (as described in
27.6.1.2.127.6.1.3, paragraph 1).
And change 27.6.2.5.3, p7 as follows:
Effects: Behaves as an unformatted output function (as described in
27.6.2.5.127.6.2.6, paragraph 1).
[ Kona (2007): Proposed Disposition: Ready ]
Section: 31.4 [iostream.objects] Status: CD1 Submitter: Pete Becker Opened: 2006-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
lib.iostream.objects requires that the standard stream objects are never destroyed, and it requires that they be destroyed.
DR 369 adds words to say that we really mean for ios_base::Init
objects to force
construction of standard stream objects. It ends, though, with the phrase "these
stream objects shall be destroyed after the destruction of dynamically ...".
However, the rule for destruction is stated in the standard: "The objects are
not destroyed during program execution."
Proposed resolution:
Change 31.4 [iostream.objects]/1:
-2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class
ios_base::Init
is constructed, and in any case before the body of main begins execution.290) The objects are not destroyed during program execution.291) If a translation unit includes<iostream>
or explicitly constructs anios_base::Init
object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.
[ Kona (2007): From 31.4 [iostream.objects]/2, strike the words "...and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit." Proposed Disposition: Review ]
Section: 20.3.2.2.3 [util.smartptr.shared.dest], 99 [tr.util.smartptr.shared.dest] Status: CD1 Submitter: Peter Dimov Opened: 2006-04-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.dest].
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Discussion:
[tr.util.smartptr.shared.dest] says in its second bullet:
"If *this shares ownership with another shared_ptr instance (use_count() > 1), decrements that instance's use count."
The problem with this formulation is that it presupposes the existence of an "use count" variable that can be decremented and that is part of the state of a shared_ptr instance (because of the "that instance's use count".)
This is contrary to the spirit of the rest of the specification that carefully avoids to require an use count variable. Instead, use_count() is specified to return a value, a number of instances.
In multithreaded code, the usual implicit assumption is that a shared variable should not be accessed by more than one thread without explicit synchronization, and by introducing the concept of an "use count" variable, the current wording implies that two shared_ptr instances that share ownership cannot be destroyed simultaneously.
In addition, if we allow the interpretation that an use count variable is part of shared_ptr's state, this would lead to other undesirable consequences WRT multiple threads. For example,
p1 = p2;
would now visibly modify the state of p2, a "write" operation, requiring a lock.
Proposed resolution:
Change the first two bullets of [lib.util.smartptr.shared.dest]/1 to:
- If
*this
is empty or shares ownership with anothershared_ptr
instance (use_count() > 1
), there are no side effects.If*this
shares ownership with anothershared_ptr
instance (use_count() > 1
), decrements that instance's use count.
Add the following paragraph after [lib.util.smartptr.shared.dest]/1:
[Note: since the destruction of
*this
decreases the number of instances in*this
's ownership group by one, allshared_ptr
instances that share ownership with*this
will report anuse_count()
that is one lower than its previous value after*this
is destroyed. --end note]
Section: 26.6.9 [alg.find.first.of] Status: CD1 Submitter: Doug Gregor Opened: 2006-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.find.first.of].
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Discussion:
In 25.1.4 Find First [lib.alg.find.first], the two iterator type parameters to find_first_of are specified to require Forward Iterators, as follows:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 find_first_of(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 find_first_of(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);
However, ForwardIterator1 need not actually be a Forward Iterator; an Input Iterator suffices, because we do not need the multi-pass property of the Forward Iterator or a true reference.
Proposed resolution:
Change the declarations of find_first_of
to:
template<classForwardIterator1InputIterator1, class ForwardIterator2>ForwardIterator1InputIterator1 find_first_of(ForwardIterator1InputIterator1 first1,ForwardIterator1InputIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<classForwardIterator1InputIterator1, class ForwardIterator2, class BinaryPredicate>ForwardIterator1InputIterator1 find_first_of(ForwardIterator1InputIterator1 first1,ForwardIterator1InputIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);
Section: 26.8.4.3 [upper.bound] Status: CD1 Submitter: Seungbeom Kim Opened: 2006-05-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
ISO/IEC 14882:2003 says:
25.3.3.2 upper_bound
Returns: The furthermost iterator
i
in the range[first, last)
such that for any iteratorj
in the range[first, i)
the following corresponding conditions hold:!(value < *j)
orcomp(value, *j) == false
.
From the description above, upper_bound cannot return last, since it's not in the interval [first, last). This seems to be a typo, because if value is greater than or equal to any other values in the range, or if the range is empty, returning last seems to be the intended behaviour. The corresponding interval for lower_bound is also [first, last].
Proposed resolution:
Change [lib.upper.bound]:
Returns: The furthermost iterator
i
in the range[first, last
such that for any iterator)]j
in the range[first, i)
the following corresponding conditions hold:!(value < *j)
orcomp(value, *j) == false
.
Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Martin Sebor Opened: 2006-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of the allocator member function
allocate()
requires that the hint argument be
either 0 or a value previously returned from allocate()
.
Footnote 227 further suggests that containers may pass the address of
an adjacent element as this argument.
I believe that either the footnote is wrong or the normative
requirement that the argument be a value previously returned from a
call to allocate()
is wrong. The latter is supported by
the resolution to issue 20-004 proposed in c++std-lib-3736 by Nathan
Myers. In addition, the hint is an ordinary void* and not the
pointer
type returned by allocate()
, with
the two types potentially being incompatible and the requirement
impossible to satisfy.
See also c++std-lib-14323 for some more context on where this came up (again).
Proposed resolution:
Remove the requirement in 20.6.1.1, p4 that the hint be a value
previously returned from allocate()
. Specifically, change
the paragraph as follows:
Requires: hint either 0 or previously obtained from member
[Note: The value hint may be used by an
implementation to help improve performance. -- end note]
allocate
and not yet passed to member deallocate
.
The value hint may be used by an implementation to help improve performance
223).
[Footnote: 223)In a container member function, the address of an adjacent element is often a good choice to pass for this argument.
flush()
not unformatted functionSection: 31.7.6.4 [ostream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2006-06-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The resolution of issue 60 changed basic_ostream::flush()
so as not to require it to behave as an unformatted output function.
That has at least two in my opinion problematic consequences:
First, flush()
now calls rdbuf()->pubsync()
unconditionally, without regard to the state of the stream. I can't
think of any reason why flush()
should behave differently
from the vast majority of stream functions in this respect.
Second, flush()
is not required to catch exceptions from
pubsync()
or set badbit
in response to such
events. That doesn't seem right either, as most other stream functions
do so.
Proposed resolution:
I propose to revert the resolution of issue 60 with respect to
flush()
. Specifically, I propose to change 27.6.2.6, p7
as follows:
Effects: Behaves as an unformatted output function (as described
in 27.6.2.6, paragraph 1). If rdbuf()
is not a null
pointer, constructs a sentry object. If this object returns
true
when converted to a value of type bool the function
calls rdbuf()->pubsync()
. If that function returns
-1 calls setstate(badbit)
(which may throw
ios_base::failure
(27.4.4.3)). Otherwise, if the
sentry object returns false
, does nothing.Does
not behave as an unformatted output function (as described in
27.6.2.6, paragraph 1).
[ Kona (2007): Proposed Disposition: Ready ]
Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Martin Sebor Opened: 2006-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section and paragraph numbers in this paper are relative to the working draft document number N2009 from 4/21/2006.
The basic_string
extractor in 21.3.7.9, p1 is clearly
required to behave as a formatted input function, as is the
std::getline()
overload for string described in p7.
However, the basic_string
inserter described in p5 of the
same section has no such requirement. This has implications on how the
operator responds to exceptions thrown from xsputn()
(formatted output functions are required to set badbit
and swallow the exception unless badbit
is also set in
exceptions()
; the string inserter doesn't have any such
requirement).
I don't see anything in the spec for the string inserter that would justify requiring it to treat exceptions differently from all other similar operators. (If it did, I think it should be made this explicit by saying that the operator "does not behave as a formatted output function" as has been made customary by the adoption of the resolution of issue 60).
Proposed resolution:
I propose to change the Effects clause in 21.3.7.9, p5, as follows:
Effects:
Begins by constructing a sentry object k as if k were constructed by typenameBehaves as a formatted output function (27.6.2.5.1). After constructing abasic_ostream<charT, traits>::sentry k (os)
. Ifbool(k)
istrue
,sentry
object, if this object returnstrue
when converted to a value of typebool
, determines padding as described in 22.2.2.2.2, then inserts the resulting sequence of charactersseq
as if by callingos.rdbuf()->sputn(seq , n)
, wheren
is the larger ofos.width()
andstr.size()
; then callsos.width(0)
.If the call to sputn fails, callsos.setstate(ios_base::failbit)
.
This proposed resilution assumes the resolution of issue 394 (i.e.,
that all formatted output functions are required to set
ios_base::badbit
in response to any kind of streambuf
failure), and implicitly assumes that a return value of
sputn(seq, n)
other than n
indicates a failure.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Peter Dimov Opened: 2006-08-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 536
Discussion:
There appears to be no requirements on the InputIterators used in sequences in 23.1.1 in terms of their value_type, and the requirements in 23.1.2 appear to be overly strict (requires InputIterator::value_type be the same type as the container's value_type).
Proposed resolution:
Change 23.1.1 p3:
In Tables 82 and 83,
X
denotes a sequence class,a
denotes a value ofX
,i
andj
denote iterators satisfying input iterator requirements and refer to elements implicitly convertible tovalue_type
,[i, j)
denotes a valid range,n
denotes a value ofX::size_type
,p
denotes a valid iterator toa
,q
denotes a valid dereferenceable iterator toa
,[q1, q2)
denotes a valid range ina
, andt
denotes a value ofX::value_type
.
Change 23.1.2 p7:
In Table 84,
X
is an associative container class,a
is a value ofX
,a_uniq
is a value ofX
whenX
supports unique keys, anda_eq
is a value ofX
whenX
supports multiple keys,i
andj
satisfy input iterator requirements and refer to elementsofimplicitly convertible tovalue_type
,[i, j)
is a valid range,p
is a valid iterator toa
,q
is a valid dereferenceable iterator toa
,[q1, q2)
is a valid range ina
,t
is a value ofX::value_type
,k
is a value ofX::key_type
andc
is a value of typeX::key_compare
.
Rationale:
Concepts will probably come in and rewrite this section anyway. But just in case it is easy to fix this up as a safety net and as a clear statement of intent.
Section: 17.4.1 [cstdint.syn], 99 [tr.c99.cstdint] Status: CD1 Submitter: Walter Brown Opened: 2006-08-28 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
Clause 18.3 of the current Working Paper (N2009) deals with the new C++ headers <cstdint> and <stdint.h>. These are of course based on the C99 header <stdint.h>, and were part of TR1.
Per 18.3.1/1, these headers define a number of macros and function macros. While the WP does not mention __STDC_CONSTANT_MACROS in this context, C99 footnotes do mention __STDC_CONSTANT_MACROS. Further, 18.3.1/2 states that "The header defines all ... macros the same as C99 subclause 7.18."
Therefore, if I wish to have the above-referenced macros and function macros defined, must I #define __STDC_CONSTANT_MACROS before I #include <cstdint>, or does the C++ header define these macros/function macros unconditionally?
Proposed resolution:
To put this issue to rest for C++0X, I propose the following addition to 18.3.1/2 of the Working Paper N2009:
[Note: The macros defined by <cstdint> are provided unconditionally: in particular, the symbols __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS (mentioned in C99 footnotes 219, 220, and 222) play no role in C++. --end note]
Swappable
in terms of CopyConstructible
and Assignable
Section: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Niels Dekker Opened: 2006-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It seems undesirable to define the Swappable
requirement in terms of
CopyConstructible
and Assignable
requirements. And likewise, once the
MoveConstructible
and MoveAssignable
requirements (N1860) have made it
into the Working Draft, it seems undesirable to define the Swappable
requirement in terms of those requirements. Instead, it appears
preferable to have the Swappable
requirement defined exclusively in
terms of the existence of an appropriate swap function.
Section 20.1.4 [lib.swappable] of the current Working Draft (N2009) says:
The Swappable requirement is met by satisfying one or more of the following conditions:
- T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);
- T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33.
I can think of three disadvantages of this definition:
If a client's type T satisfies the first condition (T is both CopyConstructible and Assignable), the client cannot stop T from satisfying the Swappable requirement without stopping T from satisfying the first condition.
A client might want to stop T from satisfying the Swappable requirement, because swapping by means of copy construction and assignment might throw an exception, and she might find a throwing swap unacceptable for her type. On the other hand, she might not feel the need to fully implement her own swap function for this type. In this case she would want to be able to simply prevent algorithms that would swap objects of type T from being used, e.g., by declaring a swap function for T, and leaving this function purposely undefined. This would trigger a link error, if an attempt would be made to use such an algorithm for this type. For most standard library implementations, this practice would indeed have the effect of stopping T from satisfying the Swappable requirement.
A client's type T that does not satisfy the first condition can not be made Swappable by providing a specialization of std::swap for T.
While I'm aware about the fact that people have mixed feelings about providing a specialization of std::swap, it is well-defined to do so. It sounds rather counter-intuitive to say that T is not Swappable, if it has a valid and semantically correct specialization of std::swap. Also in practice, providing such a specialization will have the same effect as satisfying the Swappable requirement.
For a client's type T that satisfies both conditions of the Swappable requirement, it is not specified which of the two conditions prevails. After reading section 20.1.4 [lib.swappable], one might wonder whether objects of T will be swapped by doing copy construction and assignments, or by calling the swap function of T.
I'm aware that the intention of the Draft is to prefer calling the swap function of T over doing copy construction and assignments. Still in my opinion, it would be better to make this clear in the wording of the definition of Swappable.
I would like to have the Swappable requirement defined in such a way that the following code fragment will correctly swap two objects of a type T, if and only if T is Swappable:
using std::swap; swap(t, u); // t and u are of type T.
This is also the way Scott Meyers recommends calling a swap function, in Effective C++, Third Edition, item 25.
Most aspects of this issue have been dealt with in a discussion on comp.std.c++ about the Swappable requirement, from 13 September to 4 October 2006, including valuable input by David Abrahams, Pete Becker, Greg Herlihy, Howard Hinnant and others.
[ San Francisco: ]
Recommend
NADResolved. Solved by N2774.
[ 2009-07 Frankfurt ]
Moved to Open. Waiting for non-concepts draft.
[ 2009-11-08 Howard adds: ]
[ 2010-02-03 Sean Hunt adds: ]
While reading N3000, I independently came across Issue 594. Having seen that it's an issue under discussion, I think the proposed wording needs fixing to something more like "...function call swap(t,u) that includes std::swap in its overload set is valid...", because "...is valid within the namespace std..." does not allow other libraries to simply use the Swappable requirement by referring to the standard's definition, since they cannot actually perform any calls within std.
This wording I suggested would also make overloads visible in the same scope as the
using std::swap
valid for Swappable requirements; a more complex wording limiting the non-ADL overload set to std::swap might be required.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added.
Rationale:
Solved by N3048.
Proposed resolution:
Change section 20.1.4 [lib.swappable] as follows:
The Swappable requirement is met by satisfying
one or more of the following conditions:the following condition:
T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33.T is Swappable if an unqualified function call swap(t,u) is valid within the namespace std, and has the semantics described in Table 33.
fabs(complex<T>)
redundant / wrongly specifiedSection: 29.4.7 [complex.value.ops] Status: CD1 Submitter: Stefan Große Pawig Opened: 2006-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [complex.value.ops].
View all issues with CD1 status.
Discussion:
TR1 introduced, in the C compatibility chapter, the function
fabs(complex<T>)
:
----- SNIP ----- 8.1.1 Synopsis [tr.c99.cmplx.syn] namespace std { namespace tr1 { [...] template<class T> complex<T> fabs(const complex<T>& x); } // namespace tr1 } // namespace std [...] 8.1.8 Function fabs [tr.c99.cmplx.fabs] 1 Effects: Behaves the same as C99 function cabs, defined in subclause 7.3.8.1. ----- SNIP -----
The current C++0X draft document (n2009.pdf) adopted this definition in chapter 26.3.1 (under the comment // 26.3.7 values) and 26.3.7/7.
But in C99 (ISO/IEC 9899:1999 as well as the 9899:TC2 draft document n1124), the referenced subclause reads
----- SNIP ----- 7.3.8.1 The cabs functions Synopsis 1 #include <complex.h> double cabs(double complex z); float cabsf(float complex z); long double cabsl(long double z); Description 2 The cabs functions compute the complex absolute value (also called norm, modulus, or magnitude) of z. Returns 3 The cabs functions return the complex absolute value. ----- SNIP -----
Note that the return type of the cabs*() functions is not a complex type. Thus, they are equivalent to the already well established template<class T> T abs(const complex<T>& x); (26.2.7/2 in ISO/IEC 14882:1998, 26.3.7/2 in the current draft document n2009.pdf).
So either the return value of fabs() is specified wrongly, or fabs() does not behave the same as C99's cabs*().
Possible Resolutions
This depends on the intention behind the introduction of fabs().
If the intention was to provide a /complex/ valued function that calculates the magnitude of its argument, this should be explicitly specified. In TR1, the categorization under "C compatibility" is definitely wrong, since C99 does not provide such a complex valued function.
Also, it remains questionable if such a complex valued function is really needed, since complex<T> supports construction and assignment from real valued arguments. There is no difference in observable behaviour between
complex<double> x, y; y = fabs(x); complex<double> z(fabs(x));
and
complex<double> x, y; y = abs(x); complex<double> z(abs(x));
If on the other hand the intention was to provide the intended functionality of C99, fabs() should be either declared deprecated or (for C++0X) removed from the standard, since the functionality is already provided by the corresponding overloads of abs().
[ Bellevue: ]
Bill believes that
abs()
is a suitable overload. We should removefabs()
.
Proposed resolution:
Change the synopsis in 29.4.2 [complex.syn]:
template<class T> complex<T> fabs(const complex<T>&);
Remove 29.4.7 [complex.value.ops], p7:
template<class T> complex<T> fabs(const complex<T>& x);
-7- Effects: Behaves the same as C99 functioncabs
, defined in subclause 7.3.8.1.
[
Kona (2007): Change the return type of fabs(complex)
to T
.
Proposed Disposition: Ready
]
Section: 31.10.3.4 [filebuf.members] Status: CD1 Submitter: Thomas Plum Opened: 2006-09-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [filebuf.members].
View all issues with CD1 status.
Discussion:
In testing 31.10.3.4 [filebuf.members], Table 112 (in the latest N2009 draft), we invoke
ostr.open("somename", ios_base::out | ios_base::in | ios_base::app)
and we expect the open to fail, because out|in|app is not listed in Table 92, and just before the table we see very specific words:
If mode is not some combination of flags shown in the table then the open fails.
But the corresponding table in the C standard, 7.19.5.3, provides two modes "a+" and "a+b", to which the C++ modes out|in|app and out|in|app|binary would presumably apply.
We would like to argue that the intent of Table 112 was to match the semantics of 7.19.5.3 and that the omission of "a+" and "a+b" was unintentional. (Otherwise there would be valid and useful behaviors available in C file I/O which are unavailable using C++, for no valid functional reason.)
We further request that the missing modes be explicitly restored to the WP, for inclusion in C++0x.
[ Martin adds: ]
...besides "a+" and "a+b" the C++ table is also missing a row for a lone app bit which in at least two current implementation as well as in Classic Iostreams corresponds to the C stdio "a" mode and has been traditionally documented as implying ios::out. Which means the table should also have a row for in|app meaning the same thing as "a+" already proposed in the issue.
Proposed resolution:
Add to the table "File open modes" in 31.10.3.4 [filebuf.members]:
File open modes ios_base
Flag combinationstdio
equivalentbinary
in
out
trunc
app
+
"w"
+
+
"a"
+
"a"
+
+
"w"
+
"r"
+
+
"r+"
+
+
+
"w+"
+
+
+
"a+"
+
+
"a+"
+
+
"wb"
+
+
+
"ab"
+
+
"ab"
+
+
+
"wb"
+
+
"rb"
+
+
+
"r+b"
+
+
+
+
"w+b"
+
+
+
+
"a+b"
+
+
+
"a+b"
[ Kona (2007) Added proposed wording and moved to Review. ]
Section: 3.2 [dec.tr::trdec.types.types] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all other issues in [dec.tr::trdec.types.types].
View all issues with TRDec status.
Discussion:
In a private email, Daniel writes:
I would like to ask, what where the reason for the decision to define the semantics of the integral conversion of the decimal types, namely
"operator long long() const; Returns: Returns the result of the conversion of *this to the type long long, as if performed by the expression llrounddXX(*this)."where XX stands for either 32, 64, or 128, corresponding to the proper decimal type. The exact meaning of llrounddXX is not given in that paper, so I compared it to the corresponding definition given in C99, 2nd edition (ISO 9899), which says in 7.12.9.7 p. 2:
"The lround and llround functions round their argument to the nearest integer value, rounding halfway cases away from zero, regardless of the current rounding direction. [..]"
Now considering the fact that integral conversion of the usual floating-point types ("4.9 Floating-integral conversions") has truncation semantic I wonder why this conversion behaviour has not been transferred for the decimal types.
Robert comments:
Also, there is a further error in the Returns: clause for converting decimal::decimal128
to long long
. It currently calls llroundd64
, not llroundd128
.
Proposed resolution:
Change the Returns: clause in 3.2.2.4 to:
Returns: Returns the result of the conversion of
*this
to the typelong long
, as if performed by the expressionllroundd32(*this)
while the decimal rounding direction mode [3.5.2]FE_DEC_TOWARD_ZERO
is in effect.
Change the Returns: clause in 3.2.3.4 to:
Returns: Returns the result of the conversion of
*this
to the typelong long
, as if performed by the expressionllroundd64(*this)
while the decimal rounding direction mode [3.5.2]FE_DEC_TOWARD_ZERO
is in effect.
Change the Returns: clause in 3.2.4.4 to:
Returns: Returns the result of the conversion of
*this
to the typelong long
, as if performed by the expressionllroundd64(*this)
llroundd128(*this)
while the decimal rounding direction mode [3.5.2]FE_DEC_TOWARD_ZERO
is in effect.
Section: 3.1 [dec.tr::trdec.types.encodings] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all issues with TRDec status.
Discussion:
Daniel writes in a private email:
- 3.1 'Decimal type encodings' says in its note:
"this implies that sizeof(std::decimal::decimal32) == 4, sizeof(std::decimal::decimal64) == 8, and sizeof(std::decimal::decimal128) == 16."This is a wrong assertion, because the definition of 'byte' in 1.7 'The C+ + memory model' of ISO 14882 (2nd edition) does not specify that a byte must be necessarily 8 bits large, which would be necessary to compare with the specified bit sizes of the types decimal32, decimal64, and decimal128.
Proposed resolution:
Change 3.1 as follows:
The three decimal encoding formats defined in IEEE-754R correspond to the three decimal floating types as follows:
- decimal32 is a decimal32 number, which is encoded in four consecutive
bytesoctets (32 bits)- decimal64 is a decimal64 number, which is encoded in eight consecutive
bytesoctets (64 bits)- decimal128 is a decimal128 number, which is encoded in 16 consecutive
bytesoctets (128 bits)
[Note: this implies thatsizeof(std::decimal::decimal32) == 4
,sizeof(std::decimal::decimal64) == 8
, andsizeof(std::decimal::decimal128) == 16
. --end note]
Section: 3.9 [dec.tr::trdec.types.cwchar] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all issues with TRDec status.
Discussion:
Daniel writes:
- 3.9.1 'Additions to <cwchar>' provides wrong signatures to the wcstod32, wcstod64, and wcstod128 functions ([the parameters have type pointer-to-] char instead of wchar_t).
Proposed resolution:
Change "3.9.1 Additions to <cwchar>
synopsis" to:
namespace std { namespace decimal { // 3.9.2 wcstod functions: decimal32 wcstod32 (constcharwchar_t * nptr,charwchar_t ** endptr); decimal64 wcstod64 (constcharwchar_t * nptr,charwchar_t ** endptr); decimal128 wcstod128 (constcharwchar_t * nptr,charwchar_t ** endptr); } }
Section: 3.3 [dec.tr::trdec.types.limits] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all issues with TRDec status.
Discussion:
Daniel writes in a private email:
- 3.3 'Additions to header <limits>' contains two errors in the specialisation of numeric_limits<decimal::decimal128>:
- The static member max() returns DEC128_MIN, this should be DEC128_MAX.
- The static member digits is assigned to 384, this should be 34 (Probably mixed up with the max. exponent for decimal::decimal64).
Proposed resolution:
In "3.3 Additions to header <limits>
" change numeric_limits<decimal::decimal128> as follows:
template<> class numeric_limits<decimal::decimal128> { public: static const bool is_specialized = true; static decimal::decimal128 min() throw() { return DEC128_MIN; } static decimal::decimal128 max() throw() { returnDEC128_MIN;DEC128_MAX; } static const int digits =38434; /* ... */
Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Daniel Krügler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all other issues in [dec.tr::trdec.types].
View all issues with TRDec status.
Discussion:
The document uses the term "generic floating types," but defines it nowhere.
Proposed resolution:
Change the first paragraph of "3 Decimal floating-point types" as follows:
This Technical Report introduces three decimal floating-point types, named decimal32, decimal64, and decimal128. The set of values of type decimal32 is a subset of the set of values of type decimal64; the set of values of the type decimal64 is a subset of the set of values of the type decimal128. Support for decimal128 is optional. These types supplement the Standard C++ types
float
,double
, andlong double
, which are collectively described as the basic floating types.
Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Martin Sebor Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all other issues in [dec.tr::trdec.types].
View all issues with TRDec status.
Discussion:
In c++std-lib-17198, Martin writes:
Each of the three classes proposed in the paper (decimal32, decimal64, and decimal128) explicitly declares and specifies the semantics of its copy constructor, copy assignment operator, and destructor. Since the semantics of all three functions are identical to the trivial versions implicitly generated by the compiler in the absence of any declarations it is safe to drop them from the spec. This change would make the proposed classes consistent with other similar classes already in the standard (e.g., std::complex).
Proposed resolution:
Change "3.2.2 Class decimal32
" as follows:
namespace std { namespace decimal { class decimal32 { public: // 3.2.2.1 construct/copy/destroy: decimal32();decimal32(const decimal32 & d32);decimal32 & operator=(const decimal32 & d32);~decimal32();/* ... */
Change "3.2.2.1 construct/copy/destroy" as follows:
decimal32(); Effects: Constructs an object of type decimal32 with the value 0;decimal32(const decimal32 & d32);decimal32 & operator=(const decimal32 & d32);Effects: Copies an object of type decimal32.~decimal32();Effects: Destroys an object of type decimal32.
Change "3.2.3 Class decimal64
" as follows:
namespace std { namespace decimal { class decimal64 { public: // 3.2.3.1 construct/copy/destroy: decimal64();decimal64(const decimal64 & d64);decimal64 & operator=(const decimal64 & d64);~decimal64();/* ... */
Change "3.2.3.1 construct/copy/destroy" as follows:
decimal64(); Effects: Constructs an object of type decimal64 with the value 0;decimal64(const decimal64 & d64);decimal64 & operator=(const decimal64 & d64);Effects: Copies an object of type decimal64.~decimal64();Effects: Destroys an object of type decimal64.
Change "3.2.4 Class decimal128
" as follows:
namespace std { namespace decimal { class decimal128 { public: // 3.2.4.1 construct/copy/destroy: decimal128();decimal128(const decimal128 & d128);decimal128 & operator=(const decimal128 & d128);~decimal128();/* ... */
Change "3.2.4.1 construct/copy/destroy" as follows:
decimal128(); Effects: Constructs an object of type decimal128 with the value 0;decimal128(const decimal128 & d128);decimal128 & operator=(const decimal128 & d128);Effects: Copies an object of type decimal128.~decimal128();Effects: Destroys an object of type decimal128.
Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Martin Sebor Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all other issues in [dec.tr::trdec.types].
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Discussion:
In c++std-lib-17197, Martin writes:
The extended_num_get and extended_num_put facets are designed to store a reference to a num_get or num_put facet which the extended facets delegate the parsing and formatting of types other than decimal. One form of the extended facet's ctor (the default ctor and the size_t overload) obtains the reference from the global C++ locale while the other form takes this reference as an argument.
The problem with storing a reference to a facet in another object (as opposed to storing the locale object in which the facet is installed) is that doing so bypasses the reference counting mechanism designed to prevent a facet that is still being referenced (i.e., one that is still installed in some locale) from being destroyed when another locale that contains it is destroyed. Separating a facet reference from the locale it comes from van make it cumbersome (and in some cases might even make it impossible) for programs to prevent invalidating the reference. (The danger of this design is highlighted in the paper.)
This problem could be easily avoided by having the extended facets store a copy of the locale from which they would extract the base facet either at construction time or when needed. To make it possible, the forms of ctors of the extended facets that take a reference to the base facet would need to be changed to take a locale argument instead.
Proposed resolution:
1. Change the extended_num_get
synopsis in 3.10.2 as follows:
extended_num_get(conststd::num_get<charT, InputIterator>std::locale & b, size_t refs = 0); /* ... */// const std::num_get<charT, InputIterator> & base; exposition only// std::locale baseloc; exposition only
2. Change the description of the above constructor in 3.10.2.1:
extended_num_get(conststd::num_get<charT, InputIterator>std::locale & b, size_t refs = 0);
Effects: Constructs an
extended_num_get
facet as if by:extended_num_get(conststd::num_get<charT, InputIterator>std::locale & b, size_t refs = 0) : facet(refs), baseloc(b) { /* ... */ }
Notes: Care must be taken by the implementation to ensure that the lifetime of the facet referenced by base exceeds that of the resultingextended_num_get
facet.
3. Change the Returns: clause for do_get(iter_type, iter_type, ios_base &, ios_base::iostate &, bool &) const
, et al to
Returns:
.
basestd::use_facet<std::num_get<charT, InputIterator> >(baseloc).get(in, end, str, err, val)
4. Change the extended_num_put
synopsis in 3.10.3 as follows:
extended_num_put(conststd::num_put<charT, OutputIterator>std::locale & b, size_t refs = 0); /* ... */// const std::num_put<charT, OutputIterator> & base; exposition only// std::locale baseloc; exposition only
5. Change the description of the above constructor in 3.10.3.1:
extended_num_put(conststd::num_put<charT, OutputIterator>std::locale & b, size_t refs = 0);
Effects: Constructs an
extended_num_put
facet as if by:extended_num_put(conststd::num_put<charT, OutputIterator>std::locale & b, size_t refs = 0) : facet(refs), baseloc(b) { /* ... */ }
Notes: Care must be taken by the implementation to ensure that the lifetime of the facet referenced by base exceeds that of the resultingextended_num_put
facet.
6. Change the Returns: clause for do_put(iter_type, ios_base &, char_type, bool &) const
, et al to
Returns:
.
basestd::use_facet<std::num_put<charT, OutputIterator> >(baseloc).put(s, f, fill, val)
[ Redmond: We would prefer to rename "extended" to "decimal". ]
Section: 3.4 [dec.tr::trdec.types.cdecfloat] Status: TRDec Submitter: Robert Klarer Opened: 2006-10-17 Last modified: 2016-01-31
Priority: Not Prioritized
View all issues with TRDec status.
Discussion:
In Berlin, WG14 decided to drop the <decfloat.h> header. The contents of that header have been moved into <float.h>. For the sake of C compatibility, we should make corresponding changes.
Proposed resolution:
1. Change the heading of subclause 3.4, "Headers <cdecfloat>
and <decfloat.h>
" to "Additions to headers <cfloat>
and <float.h>
."
2. Change the text of subclause 3.4 as follows:
The standard C++ headers<cfloat>
and<float.h>
define characteristics of the floating-point typesfloat
,double
, andlong double
. Their contents remain unchanged by this Technical Report.
Headers<cdecfloat>
and<decfloat.h>
define characteristics of the decimal floating-point typesdecimal32
,decimal64
, anddecimal128
. As well,<decfloat.h>
defines the convenience typedefs_Decimal32
,_Decimal64
, and_Decimal128
, for compatibilty with the C programming language.The header
<cfloat>
is described in [tr.c99.cfloat]. The header<float.h>
is described in [tr.c99.floath]. These headers are extended by this Technical Report to define characteristics of the decimal floating-point typesdecimal32
,decimal64
, anddecimal128
. As well,<float.h>
is extended to define the convenience typedefs_Decimal32
,_Decimal64
, and_Decimal128
for compatibility with the C programming language.
3. Change the heading of subclause 3.4.1, "Header <cdecfloat>
synopsis" to "Additions to header <cfloat>
synopsis."
4. Change the heading of subclause 3.4.2, "Header <decfloat.h>
synopsis" to "Additions to header <float.h>
synopsis."
5. Change the contents of 3.4.2 as follows:
#include <cdecfloat>// C-compatibility convenience typedefs: typedef std::decimal::decimal32 _Decimal32; typedef std::decimal::decimal64 _Decimal64; typedef std::decimal::decimal128 _Decimal128;
Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2006-10-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
View all issues with CD1 status.
Discussion:
Short seed vectors of 32-bit quantities all result in different states. However this is not true of seed vectors of 16-bit (or smaller) quantities. For example these two seeds
unsigned short seed = {1, 2, 3}; unsigned short seed = {1, 2, 3, 0};
both pack to
unsigned seed = {0x20001, 0x3};
yielding the same state.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2006-10-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
View all issues with CD1 status.
Discussion:
In 26.4.7.1 [rand.util.seedseq] /6, the order of packing the inputs into b and the treatment of signed quantities is unclear. Better to spell it out.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.5.3 [rand.adapt.ibits], 99 [tr.rand] Status: CD1 Submitter: Walter E. Brown Opened: 2006-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
In preparing N2111, an error on my part resulted in the omission of the following line from the template synopsis in the cited section:
static const size_t word_size = w;
(This same constant is found, for example, in 26.4.3.3 [rand.eng.sub].)
Proposed resolution:
Add the above declaration as the first line after the comment in [rand.adapt.ibits] p4:
// engine characteristics static const size_t word_size = w;
and accept my apologies for the oversight.
Section: 22.10.17.3.2 [func.wrap.func.con], 99 [tr.func.wrap.func.con] Status: CD1 Submitter: Scott Meyers Opened: 2006-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.con].
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Discussion:
My suggestion is that implementers of both tr1::function and its official C++0x successor be explicitly encouraged (but not required) to optimize for the cases mentioned above, i.e., function pointers and small function objects. They could do this by using a small internal buffer akin to the buffer used by implementations of the small string optimization. (That would make this the small functor optimization -- SFO :-}) The form of this encouragement could be a note in the standard akin to footnote 214 of the current standard.
Dave Abrahams notes:
"shall not throw exceptions" should really be "nothing," both to be more grammatical and to be consistent with existing wording in the standard.
Doug Gregor comments: I think this is a good idea. Currently, implementations of tr1::function are required to have non-throwing constructors and assignment operators when the target function object is a function pointer or a reference_wrapper. The common case, however, is for a tr1::function to store either an empty function object or a member pointer + an object pointer.
The function implementation in the upcoming Boost 1.34.0 uses the "SFO", so that the function objects for typical bind expressions like
bind(&X::f, this, _1, _2, _3)
do not require heap allocation when stored in a boost::function. I believe Dinkumware's implementation also performs this optimization.
Proposed resolution:
Revise 20.5.14.2.1 p6 [func.wrap.func.con] to add a note as follows:
Throws: shall not throw exceptions if
f
's target is a function pointer or a function object passed viareference_wrapper
. Otherwise, may throwbad_alloc
or any exception thrown by the copy constructor of the stored function object.Note: Implementations are encouraged to avoid the use of dynamically allocated memory for "small" function objects, e.g., where
f
's target is an object holding only a pointer or reference to an object and a member function pointer (a "bound member function").
Section: 16.4.5.8 [res.on.functions] Status: CD1 Submitter: Nicola Musatti Opened: 2006-11-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the latest available draft standard (N2134) § 17.4.3.6 [res.on.functions] states:
-1- In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ Standard Library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.
-2- In particular, the effects are undefined in the following cases:
[...]
- if an incomplete type (3.9) is used as a template argument when instantiating a template component.
This is contradicted by § 20.6.6.2/2 [util.smartptr.shared] which states:
[...]
The template parameter
T
ofshared_ptr
may be an incomplete type.
Proposed resolution:
Modify the last bullet of § 17.4.3.6/2 [res.on.functions] to allow for exceptions:
- if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for the component.
Section: 17.3.5.2 [numeric.limits.members] Status: CD1 Submitter: Chris Jefferson Opened: 2006-11-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
18.2.1.2 55 states that "A type is modulo if it is possible to add two positive numbers together and have a result that wraps around to a third number that is less". This seems insufficient for the following reasons:
[ Batavia: Related to N2144. Pete: is there an ISO definition of modulo? Underflow on signed behavior is undefined. ]
[ Bellevue: accept resolution, move to ready status. Does this mandate that is_modulo be true on platforms for which int happens to b modulo? A: the standard already seems to require that. ]
Proposed resolution:
Suggest 17.3.5.2 [numeric.limits.members], paragraph 57 is amended to:
A type is modulo if,
it is possible to add two positive numbers and have a result that wraps around to a third number that is less.given any operation involving +,- or * on values of that type whose value would fall outside the range[min(), max()]
, then the value returned differs from the true value by an integer multiple of(max() - min() + 1)
.
Section: 17.3.5.3 [numeric.special] Status: CD1 Submitter: Bo Persson Opened: 2006-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 17.3.5.3 [numeric.special] starts out by saying that "All members shall be provided for all specializations."
Then it goes on to show specializations for float and bool, where one member is missing (max_digits10).
Maarten Kronenburg adds:
I agree, just adding the comment that the exact number of decimal digits is digits * ln(radix) / ln(10), where probably this real number is rounded downward for digits10, and rounded upward for max_digits10 (when radix=10, then digits10=max_digits10). Why not add this exact definition also to the standard, so the user knows what these numbers exactly mean.
Howard adds:
For reference, here are the correct formulas from N1822:
digits10 = floor((digits-1) * log10(2)) max_digits10 = ceil((1 + digits) * log10(2))
We are also missing a statement regarding for what specializations this member has meaning.
Proposed resolution:
Change and add after 17.3.5.2 [numeric.limits.members], p11:
static const int max_digits10;-11- Number of base 10 digits required to ensure that values which differ
by only one epsilonare always differentiated.-12- Meaningful for all floating point types.
Change 17.3.5.3 [numeric.special], p2:
template<> class numeric_limits<float> { public: static const bool is_specialized = true; ... static const int digits10 = 6; static const int max_digits10 = 9; ...
Change 17.3.5.3 [numeric.special], p3:
template<> class numeric_limits<bool> { public: static const bool is_specialized = true; ... static const int digits10 = 0; static const int max_digits10 = 0; ...
Section: 28.3.4.2.3 [locale.ctype.byname] Status: CD1 Submitter: Bo Persson Opened: 2006-12-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 22.2.1.2 defines the ctype_byname class template. It contains the line
typedef ctype<charT>::mask mask;
Proposed resolution:
as this is a dependent type, it should obviously be
typedef typename ctype<charT>::mask mask;
Section: 29.6.2.8 [valarray.members] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2007-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
I would respectfully request an issue be opened with the intention to
clarify the wording for size() == 0
for cshift
.
Proposed resolution:
Change 29.6.2.8 [valarray.members], paragraph 10:
valarray<T> cshift(int n) const;This function returns an object of class
valarray<T>
, of lengthsize()
,each of whose elementsthat is a circular shift ofI
is(*this)[(I + n ) % size()]
. Thus, if element zero is taken as the leftmost element, a positive value of n shifts the elements circularly left n places.*this
. If element zero is taken as the leftmost element, a non-negative value of n shifts the elements circularly left n places and a negative value of n shifts the elements circularly right -n places.
Rationale:
We do not believe that there is any real ambiguity about what happens
when size() == 0
, but we do believe that spelling this out as a C++
expression causes more trouble that it solves. The expression is
certainly wrong when n < 0
, since the sign of % with negative arguments
is implementation defined.
[ Kona (2007) Changed proposed wording, added rationale and set to Review. ]
Section: 17.14 [support.runtime] Status: CD1 Submitter: Lawrence Crowl Opened: 2007-01-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The wording for longjmp
is confusing.
17.14 [support.runtime] -4- Other runtime support
The function signature
longjmp(jmp_buf jbuf, int val)
has more restricted behavior in this International Standard. If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call tolongjmp(jbuf, val)
that the throw point that transfers control to the same (destination) point has undefined behavior.
Someone at Google thinks that should say "then a call to longjmp(jbuf, val)
*at* the throw point that transfers control".
Bill Gibbons thinks it should say something like "If any automatic objects would be destroyed by an exception thrown at the point of the longjmp and caught only at the point of the setjmp, the behavior is undefined."
Proposed resolution:
In general, accept Bill Gibbons' recommendation, but add "call" to indicate that the undefined behavior comes from the dynamic call, not from its presence in the code. In 17.14 [support.runtime] paragraph 4, change
The function signature
longjmp(jmp_buf jbuf, int val)
has more restricted behavior in this International Standard.If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call toAlongjmp(jbuf, val)
that the throw point that transfers control to the same (destination) point has undefined behavior.setjmp
/longjmp
call pair has undefined behavior if replacing thesetjmp
andlongjmp
bycatch
andthrow
would destroy any automatic objects.
Section: 29.6.2.2 [valarray.cons] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The Effects clause for the default valarray
ctor
suggests that it is possible to increase the size of an empty
valarray
object by calling other non-const member
functions of the class besides resize()
. However, such an
interpretation would be contradicted by the requirement on the copy
assignment operator (and apparently also that on the computed
assignments) that the assigned arrays be the same size. See the
reflector discussion starting with c++std-lib-17871.
In addition, Footnote 280 uses some questionable normative language.
Proposed resolution:
Reword the Effects clause and Footnote 280 as follows (29.6.2.2 [valarray.cons]):
valarray();
Effects: Constructs an object of class
valarray<T>
,279) which has zero lengthuntil it is passed into a library function as a modifiable lvalue or through a non-constant this pointer.280)Postcondition:
size() == 0
.Footnote 280: This default constructor is essential, since arrays of
valarray
are likely to prove useful. There shall also be a way to change the size of an array after initialization; this is supplied by the semanticsmay be useful. The length of an empty array can be increased after initialization by means of theresize()
member function.
Section: 29.6 [numarray] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The computed and "fill" assignment operators of valarray
helper array class templates (slice_array
,
gslice_array
, mask_array
, and
indirect_array
) are const member functions of each class
template (the latter by the resolution of 123(i)
since they have reference semantics and thus do not affect
the state of the object on which they are called. However, the copy
assignment operators of these class templates, which also have
reference semantics, are non-const. The absence of constness opens
the door to speculation about whether they really are intended to have
reference semantics (existing implementations vary widely).
Pre-Kona, Martin adds:
I realized that adding the const qualifier to the functions as I suggested would break the const correctness of the classes. A few possible solutions come to mind:
Proposed resolution:
Declare the copy assignment operators of all four helper array class templates const.
Specifically, make the following edits:
Change the signature in 29.6.5 [template.slice.array] and 29.6.5.2 [slice.arr.assign] as follows:
const slice_array& operator= (const slice_array&) const;
Change the signature in 29.6.7 [template.gslice.array] and 29.6.7.2 [gslice.array.assign] as follows:
const gslice_array& operator= (const gslice_array&) const;
Change the signature in 29.6.8 [template.mask.array] and 29.6.8.2 [mask.array.assign] as follows:
const mask_array& operator= (const mask_array&) const;
Change the signature in 29.6.9 [template.indirect.array] and 29.6.9.2 [indirect.array.assign] as follows:
const indirect_array& operator= (const indirect_array&) const;
[ Kona (2007) Added const qualification to the return types and set to Ready. ]
filebuf
dtor and close
on errorSection: 31.10.6.4 [fstream.members] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
basic_filebuf
dtor is specified to have the following
straightforward effects:
Effects: Destroys an object of class
basic_filebuf
. Callsclose()
.
close()
does a lot of potentially complicated processing,
including calling overflow()
to write out the termination
sequence (to bring the output sequence to its initial shift
state). Since any of the functions called during the processing can
throw an exception, what should the effects of an exception be on the
dtor? Should the dtor catch and swallow it or should it propagate it
to the caller? The text doesn't seem to provide any guidance in this
regard other than the general restriction on throwing (but not
propagating) exceptions from destructors of library classes in
16.4.6.14 [res.on.exception.handling].
Further, the last thing close()
is specified to do is
call fclose()
to close the FILE
pointer. The
last sentence of the Effects clause reads:
... If any of the calls to
overflow
orstd::fclose
fails thenclose
fails.
This suggests that close()
might be required to call
fclose()
if and only if none of the calls to
overflow()
fails, and avoid closing the FILE
otherwise. This way, if overflow()
failed to flush out
the data, the caller would have the opportunity to try to flush it
again (perhaps after trying to deal with whatever problem may have
caused the failure), rather than losing it outright.
On the other hand, the function's Postcondition specifies that
is_open() == false
, which suggests that it should call
fclose()
unconditionally. However, since
Postcondition clauses are specified for many functions in the
standard, including constructors where they obviously cannot apply
after an exception, it's not clear whether this Postcondition
clause is intended to apply even after an exception.
It might be worth noting that the traditional behavior (Classic
Iostreams fstream::close()
and C fclose()
)
is to close the FILE
unconditionally, regardless of
errors.
[ See 397(i) and 418(i) for related issues. ]
Proposed resolution:
After discussing this on the reflector (see the thread starting with
c++std-lib-17650) we propose that close()
be clarified to
match the traditional behavior, that is to close the FILE
unconditionally, even after errors or exceptions. In addition, we
propose the dtor description be amended so as to explicitly require it
to catch and swallow any exceptions thrown by close()
.
Specifically, we propose to make the following edits in 31.10.3.4 [filebuf.members]:
basic_filebuf<charT,traits>* close();
Effects: If
is_open() == false
, returns a null pointer. If a put area exists, callsoverflow(traits::eof())
to flush characters. If the last virtual member function called on*this
(betweenunderflow
,overflow
,seekoff
, andseekpos
) wasoverflow
then callsa_codecvt.unshift
(possibly several times) to determine a termination sequence, inserts those characters and callsoverflow(traits::eof())
again. Finally, regardless of whether any of the preceding calls fails or throws an exception, the functionitcloses the file ("as if" by callingstd::fclose(file)
).334) If any of the calls made by the functionto, includingoverflow
orstd::fclose
, fails thenclose
fails by returning a null pointer. If one of these calls throws an exception, the exception is caught and rethrown after closing the file.
And to make the following edits in 31.10.3.2 [filebuf.cons].
virtual ~basic_filebuf();
Effects: Destroys an object of class
basic_filebuf<charT,traits>
. Callsclose()
. If an exception occurs during the destruction of the object, including the call toclose()
, the exception is caught but not rethrown (see 16.4.6.14 [res.on.exception.handling]).
pubimbue
forbidden to call imbue
Section: 31.2.1 [iostream.limits.imbue] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
31.2.1 [iostream.limits.imbue] specifies that "no function described in
clause 27 except for ios_base::imbue
causes any instance
of basic_ios::imbue
or
basic_streambuf::imbue
to be called."
That contradicts the Effects clause for
basic_streambuf::pubimbue()
which requires the function
to do just that: call basic_streambuf::imbue()
.
Proposed resolution:
To fix this, rephrase the sentence above to allow
pubimbue
to do what it was designed to do. Specifically.
change 31.2.1 [iostream.limits.imbue], p1 to read:
No function described in clause 27 except for
ios_base::imbue
andbasic_filebuf::pubimbue
causes any instance ofbasic_ios::imbue
orbasic_streambuf::imbue
to be called. ...
valarray
assignment and arrays of unequal lengthSection: 29.6.2.3 [valarray.assign] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The behavior of the valarray
copy assignment operator is
defined only when both sides have the same number of elements and the
spec is explicit about assignments of arrays of unequal lengths having
undefined behavior.
However, the generalized subscripting assignment operators overloaded
on slice_array
et al (29.6.2.3 [valarray.assign]) don't have any
such restriction, leading the reader to believe that the behavior of
these overloads is well defined regardless of the lengths of the
arguments.
For example, based on the reading of the spec the behavior of the snippet below can be expected to be well-defined:
const std::slice from_0_to_3 (0, 3, 1); // refers to elements 0, 1, 2 const std::valarray<int> a (1, 3); // a = { 1, 1, 1 } std::valarray<int> b (2, 4); // b = { 2, 2, 2, 2 } b = a [from_0_to_3];
In practice, b
may end up being { 1, 1, 1 }
,
{ 1, 1, 1, 2 }
, or anything else, indicating that
existing implementations vary.
Quoting from Section 3.4, Assignment operators, of Al Vermeulen's Proposal for Standard C++ Array Classes (see c++std-lib-704; N0308):
...if the size of the array on the right hand side of the equal sign differs from the size of the array on the left, a run time error occurs. How this error is handled is implementation dependent; for compilers which support it, throwing an exception would be reasonable.
And see more history in N0280.
It has been argued in discussions on the committee's reflector that
the semantics of all valarray
assignment operators should
be permitted to be undefined unless the length of the arrays being
assigned is the same as the length of the one being assigned from. See
the thread starting at c++std-lib-17786.
In order to reflect such views, the standard must specify that the size of the array referred to by the argument of the assignment must match the size of the array under assignment, for example by adding a Requires clause to 29.6.2.3 [valarray.assign] as follows:
Requires: The length of the array to which the argument refers equals
size()
.
Note that it's far from clear that such leeway is necessary in order
to implement valarray
efficiently.
Proposed resolution:
Insert new paragraph into 29.6.2.3 [valarray.assign]:
valarray<T>& operator=(const slice_array<T>&); valarray<T>& operator=(const gslice_array<T>&); valarray<T>& operator=(const mask_array<T>&); valarray<T>& operator=(const indirect_array<T>&);Requires: The length of the array to which the argument refers equals
size()
.These operators allow the results of a generalized subscripting operation to be assigned directly to a
valarray
.
Section: 16 [library] Status: Resolved Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 895
Discussion:
Many member functions of basic_string
are overloaded,
with some of the overloads taking a string
argument,
others value_type*
, others size_type
, and
others still iterators
. Often, the requirements on one of
the overloads are expressed in the form of Effects,
Throws, and in the Working Paper
(N2134)
also Remark clauses, while those on the rest of the overloads
via a reference to this overload and using a Returns clause.
The difference between the two forms of specification is that per 16.3.2.4 [structure.specifications], p3, an Effects clause specifies "actions performed by the functions," i.e., its observable effects, while a Returns clause is "a description of the return value(s) of a function" that does not impose any requirements on the function's observable effects.
Since only Notes are explicitly defined to be informative and all other paragraphs are explicitly defined to be normative, like Effects and Returns, the new Remark clauses also impose normative requirements.
So by this strict reading of the standard there are some member
functions of basic_string
that are required to throw an
exception under some conditions or use specific traits members while
many other otherwise equivalent overloads, while obliged to return the
same values, aren't required to follow the exact same requirements
with regards to the observable effects.
Here's an example of this problem that was precipitated by the change from informative Notes to normative Remarks (presumably made to address 424(i)):
In the Working Paper, find(string, size_type)
contains a
Remark clause (which is just a Note in the current
standard) requiring it to use traits::eq()
.
find(const charT *s, size_type pos)
is specified to
return find(string(s), pos)
by a Returns clause
and so it is not required to use traits::eq()
. However,
the Working Paper has replaced the original informative Note
about the function using traits::length()
with a
normative requirement in the form of a Remark. Calling
traits::length()
may be suboptimal, for example when the
argument is a very long array whose initial substring doesn't appear
anywhere in *this
.
Here's another similar example, one that existed even prior to the introduction of Remarks:
insert(size_type pos, string, size_type, size_type)
is
required to throw out_of_range
if pos >
size()
.
insert(size_type pos, string str)
is specified to return
insert(pos, str, 0, npos)
by a Returns clause and
so its effects when pos > size()
are strictly speaking
unspecified.
I believe a careful review of the current Effects and Returns clauses is needed in order to identify all such problematic cases. In addition, a review of the Working Paper should be done to make sure that the newly introduced normative Remark clauses do not impose any undesirable normative requirements in place of the original informative Notes.
[ Batavia: Alan and Pete to work. ]
[ Bellevue: Marked as NAD Editorial. ]
[ Post-Sophia Antipolis: Martin indicates there is still work to be done on this issue. Reopened. ]
[ Batavia (2009-05): ]
Tom proposes we say that, unless specified otherwise, it is always the caller's responsibility to verify that supplied arguments meet the called function's requirements. If further semantics are specified (e.g., that the function throws under certain conditions), then it is up to the implementer to check those conditions. Alan feels strongly that our current use of Requires in this context is confusing, especially now that
requires
is a new keyword.
[ 2009-07 Frankfurt ]
Move to Tentatively NAD.
[ 2009 Santa Cruz: ]
Move to Open. Martin will work on proposed wording.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial, solved by revision to N3021.
Rationale:
Solved by revision to N3021.
Proposed resolution:
Section: 28.6.7 [re.regex] Status: CD1 Submitter: Bo Persson Opened: 2007-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 28.6.7 [re.regex] lists a constructor
template<class InputIterator> basic_regex(InputIterator first, InputIterator last, flag_type f = regex_constants::ECMAScript);
However, in section 28.6.7.2 [re.regex.construct], this constructor takes a
pair of ForwardIterator
.
Proposed resolution:
Change 28.6.7.2 [re.regex.construct]:
template <classForwardIteratorInputIterator> basic_regex(ForwardIteratorInputIterator first,ForwardIteratorInputIterator last, flag_type f = regex_constants::ECMAScript);
complex<T>
insertion and locale dependenceSection: 29.4.6 [complex.ops] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2007-01-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
is there an issue opened for (0,3) as complex number with the French local? With the English local, the above parses as an imaginery complex number. With the French locale it parses as a real complex number.
Further notes/ideas from the lib-reflector, messages 17982-17984:
Add additional entries in
num_punct
to cover the complex separator (French would be ';').Insert a space before the comma, which should eliminate the ambiguity.
Solve the problem for ordered sequences in general, perhaps with a dedicated facet. Then complex should use that solution.
[ Bellevue: ]
After much discussion, we agreed on the following: Add a footnote:
[In a locale in which comma is being used as a decimal point character, inserting "showbase" into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]
And move this to READY status.
[ Pre-Sophia Antipolis, Howard adds: ]
Changed "showbase" to "showpoint" and changed from Ready to Review.
[ Post-Sophia Antipolis: ]
I neglected to pull this issue from the formal motions page after the "showbase" to "showpoint" change. In Sophia Antipolis this change was reviewed by the LWG and the issue was set to Ready. We subsequently voted the footnote into the WP with "showbase".
I'm changing from WP back to Ready to pick up the "showbase" to "showpoint" change.
Proposed resolution:
Add a footnote to 29.4.6 [complex.ops] p16:
[In a locale in which comma is being used as a decimal point character, inserting
showpoint
into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]
valarray
Section: 29.6.2.2 [valarray.cons] Status: C++11 Submitter: Martin Sebor Opened: 2007-01-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 29.2 [numeric.requirements], p1 suggests that a
valarray
specialization on a type T
that
satisfies the requirements enumerated in the paragraph is itself a
valid type on which valarray
may be instantiated
(Footnote 269 makes this clear). I.e.,
valarray<valarray<T> >
is valid as long as
T
is valid. However, since implementations of
valarray
are permitted to initialize storage allocated by
the class by invoking the default ctor of T
followed by
the copy assignment operator, such implementations of
valarray
wouldn't work with (perhaps user-defined)
specializations of valarray
whose assignment operator had
undefined behavior when the size of its argument didn't match the size
of *this
. By "wouldn't work" I mean that it would
be impossible to resize such an array of arrays by calling the
resize()
member function on it if the function used the
copy assignment operator after constructing all elements using the
default ctor (e.g., by invoking new value_type[N]
) to
obtain default-initialized storage) as it's permitted to do.
Stated more generally, the problem is that
valarray<valarray<T> >::resize(size_t)
isn't
required or guaranteed to have well-defined semantics for every type
T
that satisfies all requirements in
29.2 [numeric.requirements].
I believe this problem was introduced by the adoption of the
resolution outlined in N0857,
Assignment of valarrays, from 1996. The copy assignment
operator of the original numerical array classes proposed in N0280,
as well as the one proposed in N0308
(both from 1993), had well-defined semantics for arrays of unequal
size (the latter explicitly only when *this
was empty;
assignment of non empty arrays of unequal size was a runtime error).
The justification for the change given in N0857 was the "loss of performance [deemed] only significant for very simple operations on small arrays or for architectures with very few registers."
Since tiny arrays on a limited subset of hardware architectures are
likely to be an exceedingly rare case (despite the continued
popularity of x86) I propose to revert the resolution and make the
behavior of all valarray
assignment operators
well-defined even for non-conformal arrays (i.e., arrays of unequal
size). I have implemented this change and measured no significant
degradation in performance in the common case (non-empty arrays of
equal size). I have measured a 50% (and in some cases even greater)
speedup in the case of assignments to empty arrays versus calling
resize()
first followed by an invocation of the copy
assignment operator.
[ Bellevue: ]
If no proposed wording by June meeting, this issue should be closed NAD.
[ 2009-07 Frankfurt ]
Move resolution 1 to Ready.
Howard: second resolution has been commented out (made invisible). Can be brought back on demand.
Proposed resolution:
Change 29.6.2.3 [valarray.assign], p1 as follows:
valarray<T>& operator=(const valarray<T>& x);
-1- Each element of the
*this
array is assigned the value of the corresponding element of the argument array.The resulting behavior is undefined ifWhen the length of the argument array is not equal to the length of the *this array.resizes*this
to make the two arrays the same length, as if by callingresize(x.size())
, before performing the assignment.
And add a new paragraph just below paragraph 1 with the following text:
-2- Postcondition:
size() == x.size()
.
Also add the following paragraph to 29.6.2.3 [valarray.assign], immediately after p4:
-?- When the length,
N
of the array referred to by the argument is not equal to the length of*this
, the operator resizes*this
to make the two arrays the same length, as if by callingresize(N)
, before performing the assignment.
[ pre-Sophia Antipolis, Martin adds the following compromise wording, but prefers the original proposed resolution: ]
[
Kona (2007): Gaby to propose wording for an alternative resolution in
which you can assign to a valarray
of size 0, but not to any other
valarray
whose size is unequal to the right hand side of the assignment.
]
allocator.address()
doesn't work for types overloading operator&
Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Howard Hinnant Opened: 2007-02-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.members].
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Duplicate of: 350
Discussion:
20.2.10.2 [allocator.members] says:
pointer address(reference x) const;-1- Returns:
&x
.
20.2.10.2 [allocator.members] defines CopyConstructible
which currently not
only defines the semantics of copy construction, but also restricts what an overloaded
operator&
may do. I believe proposals are in the works (such as concepts
and rvalue reference) to decouple these two requirements. Indeed it is not evident
that we should disallow overloading operator&
to return something other
than the address of *this
.
An example of when you want to overload operator&
to return something
other than the object's address is proxy references such as vector<bool>
(or its replacement, currently code-named bit_vector
). Taking the address of
such a proxy reference should logically yield a proxy pointer, which when dereferenced,
yields a copy of the original proxy reference again.
On the other hand, some code truly needs the address of an object, and not a proxy
(typically for determining the identity of an object compared to a reference object).
boost has long recognized this dilemma and solved it with
boost::addressof
.
It appears to me that this would be useful functionality for the default allocator. Adopting
this definition for allocator::address
would free the standard of requiring
anything special from types which overload operator&
. Issue 580(i)
is expected to make use of allocator::address
mandatory for containers.
Proposed resolution:
Change 20.2.10.2 [allocator.members]:
pointer address(reference x) const;-1- Returns:
The actual address of object referenced by x, even in the presence of an overloaded&x
.operator&
.const_pointer address(address(const_reference x) const;-2- Returns:
The actual address of object referenced by x, even in the presence of an overloaded&x
.operator&
.
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
allocator::address
Section: 16.4.4.6 [allocator.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-02-08 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
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Discussion:
The table of allocator requirements in 16.4.4.6 [allocator.requirements] describes
allocator::address
as:
a.address(r) a.address(s)
where r
and s
are described as:
a value of type
X::reference
obtained by the expression*p
.
and p
is
a value of type
X::pointer
, obtained by callinga1.allocate
, wherea1 == a
This all implies that to get the address of some value of type T
that
value must have been allocated by this allocator or a copy of it.
However sometimes container code needs to compare the address of an external value of
type T
with an internal value. For example list::remove(const T& t)
may want to compare the address of the external value t
with that of a value
stored within the list. Similarly vector
or deque insert
may
want to make similar comparisons (to check for self-referencing calls).
Mandating that allocator::address
can only be called for values which the
allocator allocated seems overly restrictive.
[ post San Francisco: ]
Pablo recommends NAD Editorial, solved by N2768.
[ 2009-04-28 Pablo adds: ]
Tentatively-ready NAD Editorial as fixed by N2768.
[ 2009-07 Frankfurt ]
Fixed by N2768.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
Change 16.4.4.6 [allocator.requirements]:
r
: a value of typeX::reference
obtained by the expression *p.
s
: a value of typeX::const_reference
obtained by the expression.*q
or by conversion from a valuer
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): This issue is section 8 of N2387. There was some discussion of it but no resolution to this issue was recorded. Moved to Open. ]
deque
end invalidation during eraseSection: 23.3.5.4 [deque.modifiers] Status: CD1 Submitter: Steve LoBasso Opened: 2007-02-17 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [deque.modifiers].
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Discussion:
The standard states at 23.3.5.4 [deque.modifiers]/4:
deque erase(...)Effects: ... An erase at either end of the deque invalidates only the iterators and the references to the erased elements.
This does not state that iterators to end will be invalidated. It needs to be amended in such a way as to account for end invalidation.
Something like:
Any time the last element is erased, iterators to end are invalidated.
This would handle situations like:
erase(begin(), end()) erase(end() - 1) pop_back() resize(n, ...) where n < size() pop_front() with size() == 1
[ Post Kona, Steve LoBasso notes: ]
My only issue with the proposed resolution is that it might not be clear that
pop_front()
[wheresize() == 1
] can invalidate past-the-end iterators.
[ Kona (2007): Proposed wording added and moved to Review. ]
[ Bellevue: ]
Note that there is existing code that relies on iterators not being invalidated, but there are also existing implementations that do invalidate iterators. Thus, such code is not portable in any case. There is a
pop_front()
note, which should possibly be a separate issue. Mike Spertus to evaluate and, if need be, file an issue.
Proposed resolution:
Change 23.3.5.4 [deque.modifiers], p4:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last);-4- Effects: An erase in the middle of the
deque
invalidates all the iterators and references to elements of thedeque
and the past-the-end iterator. An erase at either end of thedeque
invalidates only the iterators and the references to the erased elements, except that erasing at the end also invalidates the past-the-end iterator.
(unsigned) long long
Section: 31.7.6.3.2 [ostream.inserters.arithmetic] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ostream.inserters.arithmetic].
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Discussion:
The arithmetic inserters are described in 31.7.6.3.2 [ostream.inserters.arithmetic]. Although the section starts with a listing of the inserters including the new ones:
operator<<(long long val ); operator<<(unsigned long long val );
the text in paragraph 1, which describes the corresponding effects
of the inserters, depending on the actual type of val, does not
handle the types long long
and unsigned long long
.
[ Alisdair: In addition to the (unsigned) long long problem, that whole paragraph misses any reference to extended integral types supplied by the implementation - one of the additions by core a couple of working papers back. ]
Proposed resolution:
In 31.7.6.3.2 [ostream.inserters.arithmetic]/1 change the third sentence
When val is of type
bool
,long
,unsigned long
, long long, unsigned long long,double
,long double
, orconst void*
, the formatting conversion occurs as if it performed the following code fragment:
Section: 31.10.3 [filebuf], 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The current standard 14882:2003(E) as well as N2134 have the following defects:
31.10.3 [filebuf]/5 says:
In order to support file I/O and multibyte/wide character conversion, conversions are performed using members of a facet, referred to as
a_codecvt
in following sections, obtained "as if" bycodecvt<charT,char,typename traits::state_type> a_codecvt = use_facet<codecvt<charT,char,typename traits::state_type> >(getloc());
use_facet
returns a const facet
reference and no facet is
copyconstructible, so the codecvt construction should fail to compile.
A similar issue arises in 28.3.4.3.3.3 [facet.num.put.virtuals]/15 for num_punct
.
Proposed resolution:
In 31.10.3 [filebuf]/5 change the "as if" code
const codecvt<charT,char,typename traits::state_type>& a_codecvt = use_facet<codecvt<charT,char,typename traits::state_type> >(getloc());
In 28.3.4.3.3.3 [facet.num.put.virtuals]/15 (This is para 5 in N2134) change
A local variable
punct
is initialized viaconst numpunct<charT>& punct = use_facet< numpunct<charT> >(str.getloc() );
(Please note also the additional provided trailing semicolon)
Section: 28.6.9.6 [re.results.form] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
28.6.9.6 [re.results.form] (root and para 3) in N2134 defines the two function template members format as non-const functions, although they are declared as const in 28.6.9 [re.results]/3.
Proposed resolution:
Add the missing const
specifier to both format
overloads described
in section 28.6.9.6 [re.results.form].
Section: 28.6.11.2 [re.tokiter] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.tokiter].
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Discussion:
Both the class definition of regex_token_iterator (28.6.11.2 [re.tokiter]/6) and the latter member specifications (28.6.11.2.3 [re.tokiter.comp]/1+2) declare both comparison operators as non-const functions. Furtheron, both dereference operators are unexpectedly also declared as non-const in 28.6.11.2 [re.tokiter]/6 as well as in (28.6.11.2.4 [re.tokiter.deref]/1+2).
Proposed resolution:
1) In (28.6.11.2 [re.tokiter]/6) change the current declarations
bool operator==(const regex_token_iterator&) const; bool operator!=(const regex_token_iterator&) const; const value_type& operator*() const; const value_type* operator->() const;
2) In 28.6.11.2.3 [re.tokiter.comp] change the following declarations
bool operator==(const regex_token_iterator& right) const; bool operator!=(const regex_token_iterator& right) const;
3) In 28.6.11.2.4 [re.tokiter.deref] change the following declarations
const value_type& operator*() const; const value_type* operator->() const;
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 28.6.11.2.2 [re.tokiter.cnstr] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.tokiter.cnstr].
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Discussion:
The text provided in 28.6.11.2.2 [re.tokiter.cnstr]/2+3 describes the effects of the three non-default constructors of class template regex_token_iterator but is does not clarify which values are legal values for submatch/submatches. This becomes an issue, if one takes 28.6.11.2 [re.tokiter]/9 into account, which explains the notion of a "current match" by saying:
The current match is
(*position).prefix()
ifsubs[N] == -1
, or(*position)[subs[N]]
for any other value ofsubs[N]
.
It's not clear to me, whether other negative values except -1 are legal arguments or not - it seems they are not.
Proposed resolution:
Add the following precondition paragraph just before the current 28.6.11.2.2 [re.tokiter.cnstr]/2:
Requires: Each of the initialization values of
subs
must be>= -1
.
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 28.6.11.1 [re.regiter] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
Both the class definition of regex_iterator (28.6.11.1 [re.regiter]/1) and the latter member specification (28.6.11.1.3 [re.regiter.comp]/1+2) declare both comparison operators as non-const functions. Furtheron, both dereference operators are unexpectedly also declared as non-const in 28.6.11.1 [re.regiter]/1 as well as in (28.6.11.1.4 [re.regiter.deref]/1+2).
Proposed resolution:
1) In (28.6.11.1 [re.regiter]/1) change the current declarations
bool operator==(const regex_iterator&) const; bool operator!=(const regex_iterator&) const; const value_type& operator*() const; const value_type* operator->() const;
2) In 28.6.11.1.4 [re.regiter.deref] change the following declarations
const value_type& operator*() const; const value_type* operator->() const;
3) In 28.6.11.1.3 [re.regiter.comp] change the following declarations
bool operator==(const regex_iterator& right) const; bool operator!=(const regex_iterator& right) const;
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.3.4 [rand.req.eng] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.req.eng].
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Discussion:
Table 98 and para 5 in 29.5.3.4 [rand.req.eng] specify the IO insertion and extraction semantic of random number engines. It can be shown, v.i., that the specification of the extractor cannot guarantee to fulfill the requirement from para 5:
If a textual representation written via os << x was subsequently read via is >> v, then x == v provided that there have been no intervening invocations of x or of v.
The problem is, that the extraction process described in table 98 misses to specify that it will initially set the if.fmtflags to ios_base::dec, see table 104:
dec: converts integer input or generates integer output in decimal base
Proof: The following small program demonstrates the violation of requirements (exception safety not fulfilled):
#include <cassert> #include <ostream> #include <iostream> #include <iomanip> #include <sstream> class RanNumEngine { int state; public: RanNumEngine() : state(42) {} bool operator==(RanNumEngine other) const { return state == other.state; } template <typename Ch, typename Tr> friend std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& os, RanNumEngine engine) { Ch old = os.fill(os.widen(' ')); // Sets space character std::ios_base::fmtflags f = os.flags(); os << std::dec << std::left << engine.state; // Adds ios_base::dec|ios_base::left os.fill(old); // Undo os.flags(f); return os; } template <typename Ch, typename Tr> friend std::basic_istream<Ch, Tr>& operator>>(std::basic_istream<Ch, Tr>& is, RanNumEngine& engine) { // Uncomment only for the fix. //std::ios_base::fmtflags f = is.flags(); //is >> std::dec; is >> engine.state; //is.flags(f); return is; } }; int main() { std::stringstream s; s << std::setfill('#'); // No problem s << std::oct; // Yikes! // Here starts para 5 requirements: RanNumEngine x; s << x; RanNumEngine v; s >> v; assert(x == v); // Fails: 42 == 34 }
A second, minor issue seems to be, that the insertion description from table 98 unnecessarily requires the addition of ios_base::fixed (which only influences floating-point numbers). Its not entirely clear to me whether the proposed standard does require that the state of random number engines is stored in integral types or not, but I have the impression that this is the indent, see e.g. p. 3
The specification of each random number engine defines the size of its state in multiples of the size of its result_type.
If other types than integrals are supported, then I wonder why no requirements are specified for the precision of the stream.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.8.2 [rand.util.canonical] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.canonical].
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Discussion:
In 29.5.2 [rand.synopsis] we have the declaration
template<class RealType, class UniformRandomNumberGenerator, size_t bits> result_type generate_canonical(UniformRandomNumberGenerator& g);
Besides the "result_type" issue (already recognized by Bo Persson at Sun, 11 Feb 2007 05:26:47 GMT in this group) it's clear, that the template parameter order is not reasonably choosen: Obviously one always needs to specify all three parameters, although usually only two are required, namely the result type RealType and the wanted bits, because UniformRandomNumberGenerator can usually be deduced.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 22.10 [function.objects] Status: Resolved Submitter: Daniel Krügler Opened: 2007-03-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [function.objects].
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Discussion:
The header <functional>
synopsis in 22.10 [function.objects]
contains the following two free comparison operator templates
for the function
class template
template<class Function1, class Function2> void operator==(const function<Function1>&, const function<Function2>&); template<class Function1, class Function2> void operator!=(const function<Function1>&, const function<Function2>&);
which are nowhere described. I assume that they are relicts before the corresponding two private and undefined member templates in the function template (see 22.10.17.3 [func.wrap.func] and [func.wrap.func.undef]) have been introduced. The original free function templates should be removed, because using an undefined entity would lead to an ODR violation of the user.
Proposed resolution:
Remove the above mentioned two function templates from
the header <functional>
synopsis (22.10 [function.objects])
template<class Function1, class Function2> void operator==(const function<Function1>&, const function<Function2>&); template<class Function1, class Function2> void operator!=(const function<Function1>&, const function<Function2>&);
Rationale:
Fixed by N2292 Standard Library Applications for Deleted Functions.
istreambuf_iterator
should have an operator->()
Section: 24.6.4 [istreambuf.iterator] Status: C++11 Submitter: Niels Dekker Opened: 2007-03-25 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [istreambuf.iterator].
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Discussion:
Greg Herlihy has clearly demonstrated that a user defined input
iterator should have an operator->()
, even if its
value type is a built-in type (comp.std.c++, "Re: Should any iterator
have an operator->()
in C++0x?", March 2007). And as Howard
Hinnant remarked in the same thread that the input iterator
istreambuf_iterator
doesn't have one, this must be a
defect!
Based on Greg's example, the following code demonstrates the issue:
#include <iostream> #include <fstream> #include <streambuf> typedef char C; int main () { std::ifstream s("filename", std::ios::in); std::istreambuf_iterator<char> i(s); (*i).~C(); // This is well-formed... i->~C(); // ... so this should be supported! }
Of course, operator->
is also needed when the value_type
of
istreambuf_iterator
is a class.
The operator->
could be implemented in various ways. For instance,
by storing the current value inside the iterator, and returning its
address. Or by returning a proxy, like operator_arrow_proxy
, from
http://www.boost.org/boost/iterator/iterator_facade.hpp
I hope that the resolution of this issue will contribute to getting a clear and consistent definition of iterator concepts.
[
Kona (2007): The proposed resolution is inconsistent because the return
type of istreambuf_iterator::operator->()
is specified to be pointer
,
but the proposed text also states that "operator->
may return a proxy."
]
[ Niels Dekker (mailed to Howard Hinnant): ]
The proposed resolution does not seem inconsistent to me.
istreambuf_iterator::operator->()
should haveistreambuf_iterator::pointer
as return type, and this return type may in fact be a proxy.AFAIK, the resolution of 445(i) ("
iterator_traits::reference
unspecified for some iterator categories") implies that for any iterator classIter
, the return type ofoperator->()
isIter::pointer
, by definition. I don't thinkIter::pointer
needs to be a raw pointer.Still I wouldn't mind if the text "
operator->
may return a proxy" would be removed from the resolution. I think it's up to the library implementation, how to implementistreambuf_iterator::operator->()
. As longs as it behaves as expected:i->m
should have the same effect as(*i).m
. Even for an explicit destructor call,i->~C()
. The main issue is just:istreambuf_iterator
should have anoperator->()
!
[ 2009-04-30 Alisdair adds: ]
Note that
operator->
is now a requirement in theInputIterator
concept, so this issue cannot be ignored or existing valid programs will break when compiled with an 0x library.
[ 2009-05-29 Alisdair adds: ]
I agree with the observation that in principle the type 'pointer' may be a proxy, and the words highlighting this are redundant.
However, in the current draught
pointer
is required to be exactly 'charT *
' by the derivation fromstd::iterator
. At a minimum, the 4th parameter of this base class template should become unspecified. That permits the introduction of a proxy as a nested class in some further undocumented (not even exposition-only) base.It also permits the
istream_iterator
approach where the cached value is stored in the iterator itself, and the iterator serves as its own proxy for post-incrementoperator++
- removing the need for the existing exposition-only nested classproxy
.Note that the current
proxy
class also has exactly the right properties to serve as the pointerproxy
too. This is likely to be a common case where anInputIterator
does not hold internal state but delegates to another class.Proposed Resolution:
In addition to the current proposal:
24.6.4 [istreambuf.iterator]
template<class charT, class traits = char_traits<charT> > class istreambuf_iterator : public iterator<input_iterator_tag, charT, typename traits::off_type,charT*unspecified, charT> {
[ 2009-07 Frankfurt ]
Move the additional part into the proposed resolution, and wrap the descriptive text in a Note.
[Howard: done.]
Move to Ready.
Proposed resolution:
Add to the synopsis in 24.6.4 [istreambuf.iterator]:
charT operator*() const; pointer operator->() const; istreambuf_iterator<charT,traits>& operator++();
24.6.4 [istreambuf.iterator]
template<class charT, class traits = char_traits<charT> > class istreambuf_iterator : public iterator<input_iterator_tag, charT, typename traits::off_type,charT*unspecified, charT> {
Change 24.6.4 [istreambuf.iterator], p1:
The class template
istreambuf_iterator
reads successive characters from thestreambuf
for which it was constructed.operator*
provides access to the current input character, if any. [Note:operator->
may return a proxy. — end note] Each timeoperator++
is evaluated, the iterator advances to the next input character. If the end of stream is reached (streambuf_type::sgetc()
returnstraits::eof()
), the iterator becomes equal to the end of stream iterator value. The default constructoristreambuf_iterator()
and the constructoristreambuf_iterator(0)
both construct an end of stream iterator object suitable for use as an end-of-range.
Section: 22.10 [function.objects] Status: CD1 Submitter: Beman Dawes Opened: 2007-04-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 22.10 [function.objects] provides function objects for some unary and binary operations, but others are missing. In a LWG reflector discussion, beginning with c++std-lib-18078, pros and cons of adding some of the missing operations were discussed. Bjarne Stroustrup commented "Why standardize what isn't used? Yes, I see the chicken and egg problems here, but it would be nice to see a couple of genuine uses before making additions."
A number of libraries, including Rogue Wave, GNU, Adobe ASL, and Boost, have already added these functions, either publicly or for internal use. For example, Doug Gregor commented: "Boost will also add ... (|, &, ^) in 1.35.0, because we need those function objects to represent various parallel collective operations (reductions, prefix reductions, etc.) in the new Message Passing Interface (MPI) library."
Because the bitwise operators have the strongest use cases, the proposed resolution is limited to them.
Proposed resolution:
To 22.10 [function.objects], Function objects, paragraph 2, add to the header <functional> synopsis:
template <class T> struct bit_and; template <class T> struct bit_or; template <class T> struct bit_xor;
At a location in clause 20 to be determined by the Project Editor, add:
The library provides basic function object classes for all of the bitwise operators in the language ([expr.bit.and], [expr.or], [exp.xor]).
template <class T> struct bit_and : binary_function<T,T,T> { T operator()(const T& x , const T& y ) const; };
operator()
returnsx & y
.template <class T> struct bit_or : binary_function<T,T,T> { T operator()(const T& x , const T& y ) const; };
operator()
returnsx | y
.template <class T> struct bit_xor : binary_function<T,T,T> { T operator()(const T& x , const T& y ) const; };
operator()
returnsx ^ y
.
Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: CD1 Submitter: Daniel Krügler Opened: 2007-04-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.formatted.arithmetic].
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Discussion:
To the more drastic changes of 31.7.5.3.2 [istream.formatted.arithmetic] in the current draft N2134 belong the explicit description of the extraction of the types short and int in terms of as-if code fragments.
Proposed resolution:
In 31.7.5.3.2 [istream.formatted.arithmetic]/2 change the current as-if code fragment
typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval ); if (err == 0) {&&if (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval))err = ios_base::failbit; else val = static_cast<short>(lval); } setstate(err);
Similarily in 31.7.5.3.2 [istream.formatted.arithmetic]/3 change the current as-if fragment
typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval ); if (err == 0) {&&if (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval))err = ios_base::failbit; else val = static_cast<int>(lval); } setstate(err);
[
Kona (2007): Note to the editor: the name lval in the call to use_facet
is incorrectly italicized in the code fragments corresponding to
operator>>(short &)
and operator >>(int &)
. Also, val -- which appears
twice on the line with the static_cast
in the proposed resolution --
should be italicized. Also, in response to part two of the issue: this
is deliberate.
]
do_unshift
for codecvt<char, char, mbstate_t>
Section: 28.3.4.2.5.3 [locale.codecvt.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.codecvt.virtuals].
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Discussion:
28.3.4.2.5.3 [locale.codecvt.virtuals], para 7 says (regarding do_unshift
):
Effects: Places characters starting at to that should be appended to terminate a sequence when the current
stateT
is given bystate
.237) Stores no more than(to_limit - to)
destination elements, and leaves theto_next
pointer pointing one beyond the last element successfully stored.codecvt<char, char, mbstate_t>
stores no characters.
The following objection has been raised:
Since the C++ Standard permits a nontrivial conversion for the required instantiations of
codecvt
, it is overly restrictive to say thatdo_unshift
must store no characters and returnnoconv
.
[Plum ref _222152Y50]
Proposed resolution:
Change 28.3.4.2.5.3 [locale.codecvt.virtuals], p7:
Effects: Places characters starting at to that should be appended to terminate a sequence when the current
stateT
is given by state.237) Stores no more than (to_limit -to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored.codecvt<char, char, mbstate_t>
stores no characters.
do_unshift
return valueSection: 28.3.4.2.5.3 [locale.codecvt.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.codecvt.virtuals].
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Discussion:
28.3.4.2.5.3 [locale.codecvt.virtuals], para 8 says:
codecvt<char,char,mbstate_t>
, returnsnoconv
.
The following objection has been raised:
Despite what the C++ Standard says,
unshift
can't always returnnoconv
for the default facets, since they can be nontrivial. At least one implementation does whatever the C functions do.
[Plum ref _222152Y62]
Proposed resolution:
Change 28.3.4.2.5.3 [locale.codecvt.virtuals], p8:
Returns: An enumeration value, as summarized in Table 76:
...
codecvt<char,char,mbstate_t>
, returnsnoconv
.
moneypunct::do_curr_symbol()
Section: 28.3.4.7.4.3 [locale.moneypunct.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.moneypunct.virtuals].
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Discussion:
28.3.4.7.4.3 [locale.moneypunct.virtuals], para 4 footnote 257 says
257) For international specializations (second template parameter
true
) this is always four characters long, usually three letters and a space.
The following objection has been raised:
The international currency symbol is whatever the underlying locale says it is, not necessarily four characters long.
[Plum ref _222632Y41]
Proposed resolution:
Change footnote 253 in 28.3.4.7.4.3 [locale.moneypunct.virtuals]:
253) For international specializations (second template parameter
true
) this isalwaystypically four characters long, usually three letters and a space.
hexfloat
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++11 Submitter: John Salmon Opened: 2007-04-20 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [facet.num.put.virtuals].
View all other issues in [facet.num.put.virtuals].
View all issues with C++11 status.
Discussion:
I am trying to understand how TR1 supports hex float (%a) output.
As far as I can tell, it does so via the following:
8.15 Additions to header <locale>
[tr.c99.locale]
In subclause 28.3.4.3.3.3 [facet.num.put.virtuals], Table 58 Floating-point conversions, after
the line:
floatfield == ios_base::scientific %E
add the two lines:
floatfield == ios_base::fixed | ios_base::scientific && !uppercase %a floatfield == ios_base::fixed | ios_base::scientific %A 2
[Note: The additional requirements on print and scan functions, later in this clause, ensure that the print functions generate hexadecimal floating-point fields with a %a or %A conversion specifier, and that the scan functions match hexadecimal floating-point fields with a %g conversion specifier. end note]
Following the thread, in 28.3.4.3.3.3 [facet.num.put.virtuals], we find:
For conversion from a floating-point type, if (flags & fixed) != 0
or
if str.precision() > 0
, then str.precision()
is specified in the
conversion specification.
This would seem to imply that when floatfield == fixed|scientific
, the
precision of the conversion specifier is to be taken from
str.precision()
. Is this really what's intended? I sincerely hope
that I'm either missing something or this is an oversight. Please
tell me that the committee did not intend to mandate that hex floats
(and doubles) should by default be printed as if by %.6a.
[ Howard: I think the fundamental issue we overlooked was that with %f, %e, %g, the default precision was always 6. With %a the default precision is not 6, it is infinity. So for the first time, we need to distinguish between the default value of precision, and the precision value 6. ]
[ 2009-07 Frankfurt ]
Leave this open for Robert and Daniel to work on.
Straw poll: Disposition?
- Default is %.6a (i.e. NAD): 2
- Always %a (no precision): 6
- precision(-1) == %a: 3
Daniel and Robert have direction to write up wording for the "always %a" solution.
[ 2009-07-15 Robert provided wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change 28.3.4.3.3.3 [facet.num.put.virtuals], Stage 1, under p5 (near the end of Stage 1):
For conversion from a floating-point type,
str.precision()
is specified as precision in the conversion specification iffloatfield != (ios_base::fixed | ios_base::scientific)
, else no precision is specified.
[ Kona (2007): Robert volunteers to propose wording. ]
Section: 16.4.4.2 [utility.arg.requirements] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
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Discussion:
The current Swappable
is:
Table 37: Swappable
requirements [swappable]expression return type post-condition swap(s,t)
void
t
has the value originally held byu
, andu
has the value originally held byt
The Swappable requirement is met by satisfying one or more of the following conditions:
T
is Swappable ifT
satisfies theCopyConstructible
requirements (Table 34) and theCopyAssignable
requirements (Table 36);T
is Swappable if a namespace scope function namedswap
exists in the same namespace as the definition ofT
, such that the expressionswap(t,u)
is valid and has the semantics described in this table.
With the passage of rvalue reference into the language, Swappable
needs to be updated to
require only MoveConstructible
and MoveAssignable
. This is a minimum.
Additionally we may want to support proxy references such that the following code is acceptable:
namespace Mine { template <class T> struct proxy {...}; template <class T> struct proxied_iterator { typedef T value_type; typedef proxy<T> reference; reference operator*() const; ... }; struct A { // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable void swap(A&); ... }; void swap(A&, A&); void swap(proxy<A>, A&); void swap(A&, proxy<A>); void swap(proxy<A>, proxy<A>); } // Mine ... Mine::proxied_iterator<Mine::A> i(...) Mine::A a; swap(*i1, a);
I.e. here is a call to swap
which the user enables swapping between a proxy to a class and the class
itself. We do not need to anything in terms of implementation except not block their way with overly
constrained concepts. That is, the Swappable
concept should be expanded to allow swapping
between two different types for the case that one is binding to a user-defined swap
.
Proposed resolution:
Change 16.4.4.2 [utility.arg.requirements]:
-1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables,
T
is a type to be supplied by a C++ program instantiating a template;a
,b
, andc
are values of typeconst T
;s
andt
are modifiable lvalues of typeT
;u
is a value of type (possiblyconst
)T
; andrv
is a non-const
rvalue of typeT
.
Table 37: Swappable
requirements [swappable]expression return type post-condition swap(s,t)
void
t
has the value originally held byu
, andu
has the value originally held byt
The
Swappable
requirement is met by satisfying one or more of the following conditions:
T
isSwappable
ifT
satisfies theMoveConstructible requirements (TableCopyConstructible
3433) and theMoveAssignable requirements (TableCopyAssignable
3635);T
isSwappable
if a namespace scope function namedswap
exists in the same namespace as the definition ofT
, such that the expressionswap(t,u)
is valid and has the semantics described in this table.
[
Kona (2007): We like the change to the Swappable
requirements to use
move semantics. The issue relating to the support of proxies is
separable from the one relating to move semantics, and it's bigger than
just swap. We'd like to address only the move semantics changes under
this issue, and open a separated issue (742(i)) to handle proxies. Also, there
may be a third issue, in that the current definition of Swappable
does
not permit rvalues to be operands to a swap operation, and Howard's
proposed resolution would allow the right-most operand to be an rvalue,
but it would not allow the left-most operand to be an rvalue (some swap
functions in the library have been overloaded to permit left operands to
swap to be rvalues).
]
unique_ptr
updateSection: 20.3.1 [unique.ptr] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr].
View all issues with CD1 status.
Discussion:
Since the publication of
N1856
there have been a few small but significant advances which should be included into
unique_ptr
. There exists a
example implementation
for all of these changes.
Even though unique_ptr<void>
is not a valid use case (unlike for shared_ptr<void>
),
unexpected cases to crop up which require the instantiation of the interface of unique_ptr<void>
even if it is never used. For example see 541(i) for how this accidently happened to auto_ptr
.
I believe the most robust way to protect unique_ptr
against this
type of failure is to augment the return type of unique_ptr<T>:operator*()
with
add_lvalue_reference<T>::type
. This means that given an instantiated unique_ptr<void>
the act of dereferencing it will simply return void
instead of causing a compile time failure.
This is simpler than creating a unique_ptr<void>
specialization which isn't robust in the
face of cv-
qualified void
types.
This resolution also supports instantiations such as unique_ptr<void, free_deleter>
which could be very useful to the client.
Efforts have been made to better support containers and smart pointers in shared
memory contexts. One of the key hurdles in such support is not assuming that a
pointer type is actually a T*
. This can easily be accomplished
for unique_ptr
by having the deleter define the pointer type:
D::pointer
. Furthermore this type can easily be defaulted to
T*
should the deleter D
choose not to define a pointer
type (example implementation
here).
This change has no run time overhead. It has no interface overhead on
authors of custom delter types. It simply allows (but not requires)
authors of custom deleter types to define a smart pointer for the
storage type of unique_ptr
if they find such functionality
useful. std::default_delete
is an example of a deleter which
defaults pointer
to T*
by simply ignoring this issue
and not including a pointer typedef
.
When the deleter type is a function pointer then it is unsafe to construct
a unique_ptr
without specifying the function pointer in the constructor.
This case is easy to check for with a static_assert
assuring that the
deleter is not a pointer type in those constructors which do not accept deleters.
unique_ptr<A, void(*)(void*)> p(new A); // error, no function given to delete the pointer!
[ Kona (2007): We don't like the solution given to the first bullet in light of concepts. The second bullet solves the problem of supporting fancy pointers for one library component only. The full LWG needs to decide whether to solve the problem of supporting fancy pointers piecemeal, or whether a paper addressing the whole library is needed. We think that the third bullet is correct. ]
[ Post Kona: Howard adds example user code related to the first bullet: ]
void legacy_code(void*, std::size_t); void foo(std::size_t N) { std::unique_ptr<void, void(*)(void*)> ptr(std::malloc(N), std::free); legacy_code(ptr.get(), N); } // unique_ptr used for exception safety purposes
I.e. unique_ptr<void>
is a useful tool that we don't want
to disable with concepts. The only part of unique_ptr<void>
we
want to disable (with concepts or by other means) are the two member functions:
T& operator*() const; T* operator->() const;
Proposed resolution:
[ I am grateful for the generous aid of Peter Dimov and Ion Gaztañaga in helping formulate and review the proposed resolutions below. ]
Change 20.3.1.3 [unique.ptr.single]:
template <class T, class D = default_delete<T>> class unique_ptr { ...T&typename add_lvalue_reference<T>::type operator*() const; ... };
Change 20.3.1.3.5 [unique.ptr.single.observers]:
T&typename add_lvalue_reference<T>::type operator*() const;
Change 20.3.1.3 [unique.ptr.single]:
template <class T, class D = default_delete<T>> class unique_ptr { public: typedef implementation (see description below) pointer; ... explicit unique_ptr(T*pointer p); ... unique_ptr(T*pointer p, implementation defined (see description below) d); unique_ptr(T*pointer p, implementation defined (see description below) d); ...T*pointer operator->() const;T*pointer get() const; ...T*pointer release(); void reset(T*pointer p =0pointer()); };
-3- If the type remove_reference<D>::type::pointer
exists, then unique_ptr<T, D>::pointer
is a typedef to
remove_reference<D>::type::pointer
. Otherwise
unique_ptr<T, D>::pointer
is a typedef to T*
.
The type unique_ptr<T, D>::pointer
shall be CopyConstructible
and CopyAssignable
.
Change 20.3.1.3.2 [unique.ptr.single.ctor]:
unique_ptr(T*pointer p); ... unique_ptr(T*pointer p, implementation defined d); unique_ptr(T*pointer p, implementation defined d); ... unique_ptr(T*pointer p, const A& d); unique_ptr(T*pointer p, A&& d); ... unique_ptr(T*pointer p, A& d); unique_ptr(T*pointer p, A&& d); ... unique_ptr(T*pointer p, const A& d); unique_ptr(T*pointer p, const A&& d); ...
-23- Requires: If D
is not a reference type,
construction of the deleter D
from an rvalue of type E
must shall be well formed and not throw an exception. If D
is a
reference type, then E
must shall be the same type as D
(diagnostic required). U*
unique_ptr<U,E>::pointer
must shall be implicitly convertible to
pointer.
T*
-25- Postconditions: get() == value u.get()
had before
the construction, modulo any required offset adjustments resulting from
the cast from U*
unique_ptr<U,E>::pointer
to
pointer. T*
get_deleter()
returns a reference to the
internally stored deleter which was constructed from
u.get_deleter()
.
Change 20.3.1.3.4 [unique.ptr.single.asgn]:
-8- Requires: Assignment of the deleter
D
from an rvalueD
mustshall not throw an exception.U*
unique_ptr<U,E>::pointer
mustshall be implicitly convertible topointer.T*
Change 20.3.1.3.5 [unique.ptr.single.observers]:
T*pointer operator->() const;...
T*pointer get() const;
Change 20.3.1.3.6 [unique.ptr.single.modifiers]:
T*pointer release();...
void reset(T*pointer p =0pointer());
Change 20.3.1.4 [unique.ptr.runtime]:
template <class T, class D> class unique_ptr<T[], D> { public: typedef implementation pointer; ... explicit unique_ptr(T*pointer p); ... unique_ptr(T*pointer p, implementation defined d); unique_ptr(T*pointer p, implementation defined d); ...T*pointer get() const; ...T*pointer release(); void reset(T*pointer p =0pointer()); };
Change 20.3.1.4.2 [unique.ptr.runtime.ctor]:
unique_ptr(T*pointer p); unique_ptr(T*pointer p, implementation defined d); unique_ptr(T*pointer p, implementation defined d);These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to
T*
pointer
. [Note: One implementation technique is to create private templated overloads of these members. -- end note]
Change 20.3.1.4.5 [unique.ptr.runtime.modifiers]:
void reset(T*pointer p =0pointer());-1- Requires: Does not accept pointer types which are convertible to
T*
pointer
(diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]
Change 20.3.1.3.2 [unique.ptr.single.ctor]:
unique_ptr();Requires:
D
mustshall be default constructible, and that constructionmustshall not throw an exception.D
mustshall not be a reference type or pointer type (diagnostic required).unique_ptr(T*pointer p);Requires: The expression
D()(p)
mustshall be well formed. The default constructor ofD
mustshall not throw an exception.D
mustshall not be a reference type or pointer type (diagnostic required).
shared_ptr
interface changes for consistency with N1856Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
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Discussion:
N1856 does not propose
any changes to shared_ptr
. It needs to be updated to use a rvalue reference where appropriate
and to interoperate with unique_ptr
as it does with auto_ptr
.
Proposed resolution:
Change 20.3.2.2 [util.smartptr.shared] as follows:
template<class Y> explicit shared_ptr(auto_ptr<Y>&&& r); template<class Y, class D> explicit shared_ptr(const unique_ptr<Y,D>& r) = delete; template<class Y, class D> explicit shared_ptr(unique_ptr<Y,D>&& r); ... template<class Y> shared_ptr& operator=(auto_ptr<Y>&&& r); template<class Y, class D> shared_ptr& operator=(const unique_ptr<Y,D>& r) = delete; template<class Y, class D> shared_ptr& operator=(unique_ptr<Y,D>&& r);
Change 20.3.2.2.2 [util.smartptr.shared.const] as follows:
template<class Y> shared_ptr(auto_ptr<Y>&&& r);
Add to 20.3.2.2.2 [util.smartptr.shared.const]:
template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);Effects: Equivalent to
shared_ptr( r.release(), r.get_deleter() )
whenD
is not a reference type,shared_ptr( r.release(), ref( r.get_deleter() ) )
otherwise.Exception safety: If an exception is thrown, the constructor has no effect.
Change 20.3.2.2.4 [util.smartptr.shared.assign] as follows:
template<class Y> shared_ptr& operator=(auto_ptr<Y>&&& r);
Add to 20.3.2.2.4 [util.smartptr.shared.assign]:
template<class Y, class D> shared_ptr& operator=(unique_ptr<Y,D>&& r);-4- Effects: Equivalent to
shared_ptr(std::move(r)).swap(*this)
.-5- Returns:
*this
.
[
Kona (2007): We may need to open an issue (743(i)) to deal with the question of
whether shared_ptr
needs an rvalue swap
.
]
Section: 23.2 [container.requirements] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
View all issues with CD1 status.
Discussion:
James Hopkin pointed out to me that if vector<T>
move assignment is O(1)
(just a swap
) then containers such as vector<shared_ptr<ostream>>
might have
the wrong semantics under move assignment when the source is not truly an rvalue, but a
moved-from lvalue (destructors could run late).
vector<shared_ptr<ostream>>
v1;vector<shared_ptr<ostream>>
v2; ... v1 = v2; // #1 v1 = std::move(v2); // #2
Move semantics means not caring what happens to the source (v2
in this example).
It doesn't mean not caring what happens to the target (v1
). In the above example
both assignments should have the same effect on v1
. Any non-shared ostream
's
v1
owns before the assignment should be closed, whether v1
is undergoing
copy assignment or move assignment.
This implies that the semantics of move assignment of a generic container should be
clear, swap
instead of just swap. An alternative which could achieve the same
effect would be to move assign each element. In either case, the complexity of move
assignment needs to be relaxed to O(v1.size())
.
The performance hit of this change is not nearly as drastic as it sounds.
In practice, the target of a move assignment has always just been move constructed
or move assigned from. Therefore under clear, swap
semantics (in
this common use case) we are still achieving O(1) complexity.
Proposed resolution:
Change 23.2 [container.requirements]:
Table 89: Container requirements expression return type operational semantics assertion/note pre/post-condition complexity a = rv;
X&
All existing elements of a
are either move assigned or destructeda
shall be equal to the value thatrv
had before this construction(Note C)linearNotes: the algorithms
swap()
,equal()
andlexicographical_compare()
are defined in clause 25. Those entries marked "(Note A)" should have constant complexity. Those entries marked "(Note B)" have constant complexity unlessallocator_propagate_never<X::allocator_type>::value
istrue
, in which case they have linear complexity.Those entries marked "(Note C)" have constant complexity ifa.get_allocator() == rv.get_allocator()
or if eitherallocator_propagate_on_move_assignment<X::allocator_type>::value
istrue
orallocator_propagate_on_copy_assignment<X::allocator_type>::value
istrue
and linear complexity otherwise.
[ post Bellevue Howard adds: ]
This issue was voted to WP in Bellevue, but accidently got stepped on by N2525 which was voted to WP simulataneously. Moving back to Open for the purpose of getting the wording right. The intent of this issue and N2525 are not in conflict.
[ post Sophia Antipolis Howard updated proposed wording: ]
Section: 23.5 [unord] Status: C++11 Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord].
View all issues with C++11 status.
Discussion:
Move semantics are missing from the unordered
containers. The proposed
resolution below adds move-support consistent with
N1858
and the current working draft.
The current proposed resolution simply lists the requirements for each function. These might better be hoisted into the requirements table for unordered associative containers. Futhermore a mild reorganization of the container requirements could well be in order. This defect report is purposefully ignoring these larger issues and just focusing on getting the unordered containers "moved".
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10-17 Removed rvalue-swaps from wording. ]
[ 2009-10 Santa Cruz: ]
Move to Review. Alisdair will review proposed wording.
[ 2009-10-29 Daniel updates wording. ]
[ 2010-01-26 Alisdair updates wording. ]
[ 2010-02-10 Howard updates wording to reference the unordered container requirements table (modified by 704(i)) as much as possible. ]
[ Voted to WP in Bellevue. ]
[ post Bellevue, Pete notes: ]
Please remind people who are reviewing issues to check that the text modifications match the current draft. Issue 676, for example, adds two overloads for unordered_map::insert taking a hint. One takes a const_iterator and returns a const_iterator, and the other takes an iterator and returns an iterator. This was correct at the time the issue was written, but was changed in Toronto so there is only one hint overload, taking a const_iterator and returning an iterator.
This issue is not ready. In addition to the relatively minor signature problem I mentioned earlier, it puts requirements in the wrong places. Instead of duplicating requirements throughout the template specifications, it should put them in the front matter that talks about requirements for unordered containers in general. This presentation problem is editorial, but I'm not willing to do the extensive rewrite that it requires. Please put it back into Open status.
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-24 Pete moved to Open: ]
The descriptions of the semantics of the added
insert
functions belong in the requirements table. That's where the rest of theinsert
functions are.
[ 2010 Pittsburgh: ]
Move issue 676 to Ready for Pittsburgh. Nico to send Howard an issue for the broader problem.
Rationale:
[ San Francisco: ]
Solved by N2776.
[ Rationale is obsolete. ]
Proposed resolution:
unordered_map
Change 23.5.3 [unord.map]:
class unordered_map { ... unordered_map(const unordered_map&); unordered_map(unordered_map&&); unordered_map(const Allocator&); unordered_map(const unordered_map&, const Allocator&); unordered_map(unordered_map&&, const Allocator&); ... unordered_map& operator=(const unordered_map&); unordered_map& operator=(unordered_map&&); ... // modifiers ...std::pair<iterator, bool> insert(const value_type& obj); template <class P> pair<iterator, bool> insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); template <class P> iterator insert(const_iterator hint, P&& obj); ... mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k); ... };
Add to 23.5.3.3 [unord.map.elem]:
mapped_type& operator[](const key_type& k);...
Requires:
key_type
shall beCopyConstructible
andmapped_type
shall beDefaultConstructible
.Complexity: Average case
O(1)
, worst caseO(size())
.mapped_type& operator[](key_type&& k);Requires:
key_type
shall beMoveConstructible
andmapped_type
shall beDefaultConstructible
.Effects: If the
unordered_map
does not already contain an element whose key is equivalent tok
, inserts the valuevalue_type(std::move(k), mapped_type())
.Returns: A reference to
x.second
, wherex
is the (unique) element whose key is equivalent tok
.Complexity: Average case
O(1)
, worst caseO(size())
.
Add new section [unord.map.modifiers]:
template <class P> pair<iterator, bool> insert(P&& x);Requires:
value_type
is constructible fromstd::forward<P>(x)
.Effects: Inserts
x
converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(x)
.Returns: The
bool
component of the returnedpair
indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key ofvalue_type(x)
.Complexity: Average case
O(1)
, worst caseO(size())
.Remarks:
P
shall be implicitly convertible tovalue_type
, else this signature shall not participate in overload resolution.template <class P> iterator insert(const_iterator hint, P&& x);Requires:
value_type
is constructible fromstd::forward<P>(x)
.Effects: Inserts
x
converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(x)
. The iteratorhint
is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.Returns: An iterator pointing to the element with key equivalent to the key of
value_type(x)
.Complexity: Average case
O(1)
, worst caseO(size())
.Remarks:
P
shall be implicitly convertible tovalue_type
, else this signature shall not participate in overload resolution.
unordered_multimap
Change 23.5.4 [unord.multimap]:
class unordered_multimap { ... unordered_multimap(const unordered_multimap&); unordered_multimap(unordered_multimap&&); unordered_multimap(const Allocator&); unordered_multimap(const unordered_multimap&, const Allocator&); unordered_multimap(unordered_multimap&&, const Allocator&); ... unordered_multimap& operator=(const unordered_multimap&); unordered_multimap& operator=(unordered_multimap&&); ... // modifiers ... iterator insert(const value_type& obj); template <class P> iterator insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); template <class P> iterator insert(const_iterator hint, P&& obj); ... };
Add new section [unord.multimap.modifiers]:
template <class P> iterator insert(P&& x);Requires:
value_type
is constructible fromstd::forward<P>(x)
.Effects: Inserts
x
converted tovalue_type
.Returns: An iterator pointing to the element with key equivalent to the key of
value_type(x)
.Complexity: Average case
O(1)
, worst caseO(size())
.Remarks:
P
shall be implicitly convertible tovalue_type
, else this signature shall not participate in overload resolution.template <class P> iterator insert(const_iterator hint, P&& x);Requires:
value_type
is constructible fromstd::forward<P>(x)
.Effects: Inserts
x
converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(x)
. The iteratorhint
is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.Returns: An iterator pointing to the element with key equivalent to the key of
value_type(x)
.Complexity: Average case
O(1)
, worst caseO(size())
.Remarks:
P
shall be implicitly convertible tovalue_type
, else this signature shall not participate in overload resolution.
unordered_set
Change 23.5.6 [unord.set]:
class unordered_set { ... unordered_set(const unordered_set&); unordered_set(unordered_set&&); unordered_set(const Allocator&); unordered_set(const unordered_set&, const Allocator&); unordered_set(unordered_set&&, const Allocator&); ... unordered_set& operator=(const unordered_set&); unordered_set& operator=(unordered_set&&); ... // modifiers ...std::pair<iterator, bool> insert(const value_type& obj); pair<iterator, bool> insert(value_type&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); ... };
unordered_multiset
Change 23.5.7 [unord.multiset]:
class unordered_multiset { ... unordered_multiset(const unordered_multiset&); unordered_multiset(unordered_multiset&&); unordered_multiset(const Allocator&); unordered_multiset(const unordered_multiset&, const Allocator&); unordered_multiset(unordered_multiset&&, const Allocator&); ... unordered_multiset& operator=(const unordered_multiset&); unordered_multiset& operator=(unordered_multiset&&); ... // modifiers ... iterator insert(const value_type& obj); iterator insert(value_type&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); ... };
Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2007-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
seed_seq::randomize
provides a mechanism for initializing random number
engines which ideally would yield "distant" states when given "close"
seeds. The algorithm for seed_seq::randomize
given in the current
Working Draft for C++,
N2284
(2007-05-08), has 3 weaknesses
Collisions in state. Because of the way the state is initialized, seeds of different lengths may result in the same state. The current version of seed_seq has the following properties:
s <= n
, each of the 2^(32s) seed vectors results in a
distinct state.The proposed algorithm (below) has the considerably stronger properties:
(2^(32n)-1)/(2^32-1)
seed vectors of lengths s < n
result in distinct states.
2^(32n)
seed vectors of length s == n
result in
distinct states.
Poor mixing of v'
s entropy into the state. Consider v.size() == n
and hold v[n/2]
thru v[n-1]
fixed while varying v[0]
thru v[n/2-1]
,
a total of 2^(16n)
possibilities. Because of the simple recursion
used in seed_seq
, begin[n/2]
thru begin[n-1]
can take on only 2^64
possible states.
The proposed algorithm uses a more complex recursion which results in much better mixing.
seed_seq::randomize
is undefined for v.size() == 0
. The proposed
algorithm remedies this.
The current algorithm for seed_seq::randomize
is adapted by me from the
initialization procedure for the Mersenne Twister by Makoto Matsumoto
and Takuji Nishimura. The weakness (2) given above was communicated to
me by Matsumoto last year.
The proposed replacement for seed_seq::randomize
is due to Mutsuo Saito,
a student of Matsumoto, and is given in the implementation of the
SIMD-oriented Fast Mersenne Twister random number generator SFMT.
http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/index.html
http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/SFMT-src-1.2.tar.gz
See Mutsuo Saito, An Application of Finite Field: Design and Implementation of 128-bit Instruction-Based Fast Pseudorandom Number Generator, Master's Thesis, Dept. of Math., Hiroshima University (Feb. 2007) http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/M062821.pdf
One change has been made here, namely to treat the case of small n
(setting t = (n-1)/2
for n < 7
).
Since seed_seq
was introduced relatively recently there is little cost
in making this incompatible improvement to it.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.3.4 [rand.req.eng] Status: CD1 Submitter: Charles Karney Opened: 2007-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.req.eng].
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Discussion:
Section 29.5.3.4 [rand.req.eng] Random number engine requirements:
This change follows naturally from the proposed change to
seed_seq::randomize
in 677(i).
In table 104 the description of X(q)
contains a special treatment of
the case q.size() == 0
. This is undesirable for 4 reasons:
X()
.X(q)
with q.size() > 0
.X(q)
in
paragraphs 29.5.4.2 [rand.eng.lcong] p5, 29.5.4.3 [rand.eng.mers] p8, and 29.5.4.4 [rand.eng.sub] p10 where
there is no special treatment of q.size() == 0
.seed_seq::randomize
given above
allows for the case q.size() == 0
.See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 23.3 [sequences] Status: CD1 Submitter: Howard Hinnant Opened: 2007-06-11 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [sequences].
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Discussion:
The C++98 standard specifies that one member function alone of the containers
passes its parameter (T
) by value instead of by const reference:
void resize(size_type sz, T c = T());
This fact has been discussed / debated repeatedly over the years, the first time being even before C++98 was ratified. The rationale for passing this parameter by value has been:
So that self referencing statements are guaranteed to work, for example:
v.resize(v.size() + 1, v[0]);
However this rationale is not convincing as the signature for push_back
is:
void push_back(const T& x);
And push_back
has similar semantics to resize
(append).
And push_back
must also work in the self referencing case:
v.push_back(v[0]); // must work
The problem with passing T
by value is that it can be significantly more
expensive than passing by reference. The converse is also true, however when it is
true it is usually far less dramatic (e.g. for scalar types).
Even with move semantics available, passing this parameter by value can be expensive.
Consider for example vector<vector<int>>
:
std::vector<int> x(1000); std::vector<std::vector<int>> v; ... v.resize(v.size()+1, x);
In the pass-by-value case, x
is copied once to the parameter of
resize
. And then internally, since the code can not know at compile
time by how much resize
is growing the vector
, x
is
usually copied (not moved) a second time from resize
's parameter into its proper place
within the vector
.
With pass-by-const-reference, the x
in the above example need be copied
only once. In this case, x
has an expensive copy constructor and so any
copies that can be saved represents a significant savings.
If we can be efficient for push_back
, we should be efficient for resize
as well. The resize taking a reference parameter has been coded and shipped in the
CodeWarrior library with no reports of problems which I am aware of.
Proposed resolution:
Change 23.3.5 [deque], p2:
class deque { ... void resize(size_type sz, const T& c);
Change 23.3.5.3 [deque.capacity], p3:
void resize(size_type sz, const T& c);
Change 23.3.11 [list], p2:
class list { ... void resize(size_type sz, const T& c);
Change 23.3.11.3 [list.capacity], p3:
void resize(size_type sz, const T& c);
Change 23.3.13 [vector], p2:
class vector { ... void resize(size_type sz, const T& c);
Change 23.3.13.3 [vector.capacity], p11:
void resize(size_type sz, const T& c);
operator->
returnSection: 24.5.4.2 [move.iterator] Status: CD1 Submitter: Howard Hinnant Opened: 2007-06-11 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [move.iterator].
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Discussion:
move_iterator
's operator->
return type pointer
does not consistently match the type which is returned in the description
in [move.iter.op.ref].
template <class Iterator> class move_iterator { public: ... typedef typename iterator_traits<Iterator>::pointer pointer; ... pointer operator->() const {return current;} ... private: Iterator current; // exposition only };
There are two possible fixes.
pointer operator->() const {return &*current;}
typedef Iterator pointer;
The first solution is the one chosen by reverse_iterator
. A potential
disadvantage of this is it may not work well with iterators which return a
proxy on dereference and that proxy has overloaded operator&()
. Proxy
references often need to overloaad operator&()
to return a proxy
pointer. That proxy pointer may or may not be the same type as the iterator's
pointer
type.
By simply returning the Iterator
and taking advantage of the fact that
the language forwards calls to operator->
automatically until it
finds a non-class type, the second solution avoids the issue of an overloaded
operator&()
entirely.
Proposed resolution:
Change the synopsis in 24.5.4.2 [move.iterator]:
typedeftypename iterator_traits<Iterator>::pointerpointer;
Section: 28.6.8.3 [re.submatch.op] Status: CD1 Submitter: Nozomu Katoo Opened: 2007-05-27 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.submatch.op].
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Discussion:
In 28.6.8.3 [re.submatch.op] of N2284, operator functions numbered 31-42 seem impossible to compare. E.g.:
template <class BiIter> bool operator==(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);-31- Returns:
lhs == rhs.str()
.
When char*
is used as BiIter
, iterator_traits<BiIter>::value_type
would be
char
, so that lhs == rhs.str()
ends up comparing a char
value and an object
of std::basic_string<char>
. However, the behaviour of comparison between
these two types is not defined in 27.4.4 [string.nonmembers] of N2284.
This applies when wchar_t*
is used as BiIter
.
Proposed resolution:
Adopt the proposed resolution in N2409.
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 28.6.7.2 [re.regex.construct] Status: CD1 Submitter: Eric Niebler Opened: 2007-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [re.regex.construct].
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Discussion:
Looking at N2284, 28.6.7 [re.regex], p3 basic_regex
class template synopsis shows this
constructor:
template <class InputIterator> basic_regex(InputIterator first, InputIterator last, flag_type f = regex_constants::ECMAScript);
In 28.6.7.2 [re.regex.construct], p15, the constructor appears with this signature:
template <class ForwardIterator> basic_regex(ForwardIterator first, ForwardIterator last, flag_type f = regex_constants::ECMAScript);
ForwardIterator
is probably correct, so the synopsis is wrong.
[ John adds: ]
I think either could be implemented? Although an input iterator would probably require an internal copy of the string being made.
I have no strong feelings either way, although I think my original intent was
InputIterator
.
Proposed resolution:
Adopt the proposed resolution in N2409.
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 24.5.1.9 [reverse.iter.nonmember], 24.5.4.9 [move.iter.nonmember] Status: CD1 Submitter: Bo Persson Opened: 2007-06-10 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
In C++03 the difference between two reverse_iterators
ri1 - ri2
is possible to compute only if both iterators have the same base
iterator. The result type is the difference_type
of the base iterator.
In the current draft, the operator is defined as [reverse.iter.opdiff]
template<class Iterator1, class Iterator2> typename reverse_iterator<Iterator>::difference_type operator-(const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);
The return type is the same as the C++03 one, based on the no longer
present Iterator
template parameter.
Besides being slightly invalid, should this operator work only when
Iterator1
and Iterator2
has the same difference_type
? Or should the
implementation choose one of them? Which one?
The same problem now also appears in operator-()
for move_iterator
24.5.4.9 [move.iter.nonmember].
Proposed resolution:
Change the synopsis in 24.5.1.2 [reverse.iterator]:
template <class Iterator1, class Iterator2>typename reverse_iterator<Iterator>::difference_typeauto operator-( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);
Change [reverse.iter.opdiff]:
template <class Iterator1, class Iterator2>typename reverse_iterator<Iterator>::difference_typeauto operator-( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);Returns:
y.current - x.current
.
Change the synopsis in 24.5.4.2 [move.iterator]:
template <class Iterator1, class Iterator2>typename move_iterator<Iterator>::difference_typeauto operator-( const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());
Change 24.5.4.9 [move.iter.nonmember]:
template <class Iterator1, class Iterator2>typename move_iterator<Iterator>::difference_typeauto operator-( const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());Returns:
x.base() - y.base()
.
[
Pre Bellevue: This issue needs to wait until the auto -> return
language feature
goes in.
]
Section: 20.3.2.2.2 [util.smartptr.shared.const], 20.3.2.3.2 [util.smartptr.weak.const] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
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Discussion:
Since all conversions from shared_ptr<T>
to shared_ptr<U>
have the same
rank regardless of the relationship between T
and U
, reasonable user
code that works with raw pointers fails with shared_ptr
:
void f( shared_ptr<void> ); void f( shared_ptr<int> ); int main() { f( shared_ptr<double>() ); // ambiguous }
Now that we officially have enable_if
, we can constrain the constructor
and the corresponding assignment operator to only participate in the
overload resolution when the pointer types are compatible.
Proposed resolution:
In 20.3.2.2.2 [util.smartptr.shared.const], change:
-14- Requires:
For the second constructorThe second constructor shall not participate in the overload resolution unlessY*
shall beis implicitly convertible toT*
.
In 20.3.2.3.2 [util.smartptr.weak.const], change:
template<class Y> weak_ptr(shared_ptr<Y> const& r);weak_ptr(weak_ptr const& r);template<class Y> weak_ptr(weak_ptr<Y> const& r);weak_ptr(weak_ptr const& r); template<class Y> weak_ptr(weak_ptr<Y> const& r); template<class Y> weak_ptr(shared_ptr<Y> const& r);-4- Requires:
FortThe second and third constructors,shall not participate in the overload resolution unlessY*
shall beis implicitly convertible toT*
.
Section: 22.10.6.2 [refwrap.const] Status: C++11 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [refwrap.const].
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Discussion:
A reference_wrapper
can be constructed from an rvalue, either by using
the constructor, or via cref
(and ref
in some corner cases). This leads
to a dangling reference being stored into the reference_wrapper
object.
Now that we have a mechanism to detect an rvalue, we can fix them to
disallow this source of undefined behavior.
Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.
[ 2009-05-09 Alisdair adds: ]
Now that
ref/cref
are constained thatT
must be anObjectType
, I do not believe there is any risk of bindingref
to a temporary (which would rely on deducingT
to be an rvalue reference type)However, the problem for
cref
remains, so I recommend retaining that deleted overload.
[ 2009-05-10 Howard adds: ]
Without:
template <class T> void ref(const T&& t) = delete;I believe this program will compile:
#include <functional> struct A {}; const A source() {return A();} int main() { std::reference_wrapper<const A> r = std::ref(source()); }I.e. in:
template <ObjectType T> reference_wrapper<T> ref(T& t);this:
ref(source())deduces
T
asconst A
, and so:ref(const A& t)will bind to a temporary (tested with a pre-concepts rvalue-ref enabled compiler).
Therefore I think we still need the ref-protection. I respectfully disagree with Alisdair's comment and am in favor of the proposed wording as it stands. Also, CWG 606 (noted below) has now been "favorably" resolved.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 22.10 [function.objects], add the following two signatures to the synopsis:
template <class T> void ref(const T&& t) = delete; template <class T> void cref(const T&& t) = delete;
[ N2292 addresses the first part of the resolution but not the second. ]
[ Bellevue: Doug noticed problems with the current wording. ]
[ post Bellevue: Howard and Peter provided revised wording. ]
[ This resolution depends on a "favorable" resolution of CWG 606: that is, the "special deduction rule" is disabled with the const T&& pattern. ]
Section: 22.10.6.2 [refwrap.const] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [refwrap.const].
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Discussion:
The constructor of reference_wrapper
is currently explicit
. The primary
motivation behind this is the safety problem with respect to rvalues,
which is addressed by the proposed resolution of the previous issue.
Therefore we should consider relaxing the requirements on the
constructor since requests for the implicit conversion keep resurfacing.
Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.
Proposed resolution:
Remove the explicit
from the constructor of reference_wrapper
. If the
proposed resolution of the previous issue is accepted, remove the
explicit
from the T&&
constructor as well to keep them in sync.
Section: 23.5 [unord], 99 [tr.hash] Status: CD1 Submitter: Joaquín M López Muñoz Opened: 2007-06-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord].
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Discussion:
The last version of TR1 does not include the following member functions for unordered containers:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
which looks like an oversight to me. I've checked th TR1 issues lists and the latest working draft of the C++0x std (N2284) and haven't found any mention to these menfuns or to their absence.
Is this really an oversight, or am I missing something?
Proposed resolution:
Add the following two rows to table 93 (unordered associative container requirements) in section 23.2.8 [unord.req]:
Unordered associative container requirements (in addition to container) expression return type assertion/note pre/post-condition complexity b.cbegin(n)
const_local_iterator
n
shall be in the range[0, bucket_count())
. Note:[b.cbegin(n), b.cend(n))
is a valid range containing all of the elements in then
th bucket.Constant b.cend(n)
const_local_iterator
n
shall be in the range[0, bucket_count())
.Constant
Add to the synopsis in 23.5.3 [unord.map]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.5.4 [unord.multimap]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.5.6 [unord.set]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.5.7 [unord.multiset]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
get_money
and put_money
should be formatted I/O functionsSection: 31.7.8 [ext.manip] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ext.manip].
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Discussion:
In a private email Bill Plauger notes:
I believe that the function that implements
get_money
[from N2072] should behave as a formatted input function, and the function that implementsput_money
should behave as a formatted output function. This has implications regarding the skipping of whitespace and the handling of errors, among other things.The words don't say that right now and I'm far from convinced that such a change is editorial.
Martin's response:
I agree that the manipulators should handle exceptions the same way as formatted I/O functions do. The text in N2072 assumes so but the Returns clause explicitly omits exception handling for the sake of brevity. The spec should be clarified to that effect.
As for dealing with whitespace, I also agree it would make sense for the extractors and inserters involving the new manipulators to treat it the same way as formatted I/O.
Proposed resolution:
Add a new paragraph immediately above p4 of 31.7.8 [ext.manip] with the following text:
Effects: The expression
in >> get_money(mon, intl)
described below behaves as a formatted input function (as described in 31.7.5.3.1 [istream.formatted.reqmts]).
Also change p4 of 31.7.8 [ext.manip] as follows:
Returns: An object
s
of unspecified type such that ifin
is an object of typebasic_istream<charT, traits>
then the expressionin >> get_money(mon, intl)
behaves as a formatted input function that callsf(in, mon, intl)
were called. The functionf
can be defined as...
[ post Bellevue: ]
We recommend moving immediately to Review. We've looked at the issue and have a consensus that the proposed resolution is correct, but want an iostream expert to sign off. Alisdair has taken the action item to putt this up on the reflector for possible movement by Howard to Tenatively Ready.
std::bitset::all()
missingSection: 22.9.2 [template.bitset] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The bitset
class template provides the member function
any()
to determine whether an object of the type has any
bits set, and the member function none()
to determine
whether all of an object's bits are clear. However, the template does
not provide a corresponding function to discover whether a
bitset
object has all its bits set. While it is
possible, even easy, to obtain this information by comparing the
result of count()
with the result of size()
for equality (i.e., via b.count() == b.size()
) the
operation is less efficient than a member function designed
specifically for that purpose could be. (count()
must
count all non-zero bits in a bitset
a word at a time
while all()
could stop counting as soon as it encountered
the first word with a zero bit).
Proposed resolution:
Add a declaration of the new member function all()
to the
defintion of the bitset
template in 22.9.2 [template.bitset], p1,
right above the declaration of any()
as shown below:
bool operator!=(const bitset<N>& rhs) const; bool test(size_t pos) const; bool all() const; bool any() const; bool none() const;
Add a description of the new member function to the end of 22.9.2.3 [bitset.members] with the following text:
bool all() const;
Returns:
count() == size()
.
In addition, change the description of any()
and
none()
for consistency with all()
as
follows:
bool any() const;
Returns:
true
if any bit in*this
is onecount() != 0
.
bool none() const;
Returns:
true
if no bit in*this
is onecount() == 0
.
std::bitset
and long long
Section: 22.9.2 [template.bitset] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [template.bitset].
View all other issues in [template.bitset].
View all issues with CD1 status.
Discussion:
Objects of the bitset
class template specializations can
be constructed from and explicitly converted to values of the widest
C++ integer type, unsigned long
. With the introduction
of long long
into the language the template should be
enhanced to make it possible to interoperate with values of this type
as well, or perhaps uintmax_t
. See c++std-lib-18274 for
a brief discussion in support of this change.
Proposed resolution:
For simplicity, instead of adding overloads for unsigned long
long
and dealing with possible ambiguities in the spec, replace
the bitset
ctor that takes an unsigned long
argument with one taking unsigned long long
in the
definition of the template as shown below. (The standard permits
implementations to add overloads on other integer types or employ
template tricks to achieve the same effect provided they don't cause
ambiguities or changes in behavior.)
// [bitset.cons] constructors: bitset(); bitset(unsigned long long val); template<class charT, class traits, class Allocator> explicit bitset( const basic_string<charT,traits,Allocator>& str, typename basic_string<charT,traits,Allocator>::size_type pos = 0, typename basic_string<charT,traits,Allocator>::size_type n = basic_string<charT,traits,Allocator>::npos);
Make a corresponding change in 22.9.2.2 [bitset.cons], p2:
bitset(unsigned long long val);
Effects: Constructs an object of class bitset<N>, initializing the first
M
bit positions to the corresponding bit values inval
.M
is the smaller ofN
and the number of bits in the value representation (section [basic.types]) ofunsigned long long
. IfM < N
istrue
, the remaining bit positions are initialized to zero.
Additionally, introduce a new member function to_ullong()
to make it possible to convert bitset
to values of the
new type. Add the following declaration to the definition of the
template, immediate after the declaration of to_ulong()
in 22.9.2 [template.bitset], p1, as shown below:
// element access: bool operator[](size_t pos) const; // for b[i]; reference operator[](size_t pos); // for b[i]; unsigned long to_ulong() const; unsigned long long to_ullong() const; template <class charT, class traits, class Allocator> basic_string<charT, traits, Allocator> to_string() const;
And add a description of the new member function to 22.9.2.3 [bitset.members],
below the description of the existing to_ulong()
(if
possible), with the following text:
unsigned long long to_ullong() const;
Throws:
overflow_error
if the integral valuex
corresponding to the bits in*this
cannot be represented as typeunsigned long long
.Returns:
x
.
Section: 28.3.4.2.4 [facet.ctype.special] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The ctype<char>::classic_table()
static member
function returns a pointer to an array of const
ctype_base::mask
objects (enums) that contains
ctype<char>::table_size
elements. The table
describes the properties of the character set in the "C" locale (i.e.,
whether a character at an index given by its value is alpha, digit,
punct, etc.), and is typically used to initialize the
ctype<char>
facet in the classic "C" locale (the
protected ctype<char>
member function
table()
then returns the same value as
classic_table()
).
However, while ctype<char>::table_size
(the size of
the table) is a public static const member of the
ctype<char>
specialization, the
classic_table()
static member function is protected. That
makes getting at the classic data less than convenient (i.e., one has
to create a whole derived class just to get at the masks array). It
makes little sense to expose the size of the table in the public
interface while making the table itself protected, especially when the
table is a constant object.
The same argument can be made for the non-static protected member
function table()
.
Proposed resolution:
Make the ctype<char>::classic_table()
and
ctype<char>::table()
member functions public by
moving their declarations into the public section of the definition of
specialization in 28.3.4.2.4 [facet.ctype.special] as shown below:
static locale::id id; static const size_t table_size = IMPLEMENTATION_DEFINED;protected:const mask* table() const throw(); static const mask* classic_table() throw(); protected: ~ctype(); // virtual virtual char do_toupper(char c) const;
istream::operator>>(int&)
brokenSection: 31.7.5.3.2 [istream.formatted.arithmetic] Status: C++11 Submitter: Martin Sebor Opened: 2007-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.formatted.arithmetic].
View all issues with C++11 status.
Discussion:
From message c++std-lib-17897:
The code shown in 31.7.5.3.2 [istream.formatted.arithmetic] as the "as if"
implementation of the two arithmetic extractors that don't have a
corresponding num_get
interface (i.e., the
short
and int
overloads) is subtly buggy in
how it deals with EOF
, overflow, and other similar
conditions (in addition to containing a few typos).
One problem is that if num_get::get()
reaches the EOF
after reading in an otherwise valid value that exceeds the limits of
the narrower type (but not LONG_MIN
or
LONG_MAX
), it will set err
to
eofbit
. Because of the if condition testing for
(err == 0)
, the extractor won't set
failbit
(and presumably, return a bogus value to the
caller).
Another problem with the code is that it never actually sets the
argument to the extracted value. It can't happen after the call to
setstate()
since the function may throw, so we need to
show when and how it's done (we can't just punt as say: "it happens
afterwards"). However, it turns out that showing how it's done isn't
quite so easy since the argument is normally left unchanged by the
facet on error except when the error is due to a misplaced thousands
separator, which causes failbit
to be set but doesn't
prevent the facet from storing the value.
[ Batavia (2009-05): ]
We believe this part of the Standard has been recently adjusted and that this issue was addressed during that rewrite.
Move to NAD.
[ 2009-05-28 Howard adds: ]
I've moved this issue from Tentatively NAD to Open.
The current wording of N2857 in 28.3.4.3.2.3 [facet.num.get.virtuals] p3, stage 3 appears to indicate that in parsing arithmetic types, the value is always set, but sometimes in addition to setting
failbit
.
- If there is a range error, the value is set to min or max, else
- if there is a conversion error, the value is set to 0, else
- if there is a grouping error, the value is set to whatever it would be if grouping were ignored, else
- the value is set to its error-free result.
However there is a contradictory sentence in 28.3.4.3.2.3 [facet.num.get.virtuals] p1.
31.7.5.3.2 [istream.formatted.arithmetic] should mimic the behavior of 28.3.4.3.2.3 [facet.num.get.virtuals] (whatever we decide that behavior is) for
int
andshort
, and currently does not. I believe that the correct code fragment should look like:typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = ios_base::goodbit; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval); if (lval < numeric_limits<int>::min()) { err |= ios_base::failbit; val = numeric_limits<int>::min(); } else if (lval > numeric_limits<int>::max()) { err |= ios_base::failbit; val = numeric_limits<int>::max(); } else val = static_cast<int>(lval); setstate(err);
[ 2009-07 Frankfurt ]
Move to Ready.
Proposed resolution:
Change 28.3.4.3.2.3 [facet.num.get.virtuals], p1:
-1- Effects: Reads characters from
in
, interpreting them according tostr.flags()
,use_facet<ctype<charT> >(loc)
, anduse_facet< numpunct<charT> >(loc)
, whereloc
isstr.getloc()
.If an error occurs,val
is unchanged; otherwise it is set to the resulting value.
Change 31.7.5.3.2 [istream.formatted.arithmetic], p2 and p3:
operator>>(short& val);-2- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = iostate_base::goodbit; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval);if (err != 0) ; else if (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval) err = ios_base::failbit;if (lval < numeric_limits<short>::min()) { err |= ios_base::failbit; val = numeric_limits<short>::min(); } else if (lval > numeric_limits<short>::max()) { err |= ios_base::failbit; val = numeric_limits<short>::max(); } else val = static_cast<short>(lval); setstate(err);operator>>(int& val);-3- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = iostate_base::goodbit; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval);if (err != 0) ; else if (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval) err = ios_base::failbit;if (lval < numeric_limits<int>::min()) { err |= ios_base::failbit; val = numeric_limits<int>::min(); } else if (lval > numeric_limits<int>::max()) { err |= ios_base::failbit; val = numeric_limits<int>::max(); } else val = static_cast<int>(lval); setstate(err);
<system_error>
header leads to name clashesSection: 19.5 [syserr] Status: Resolved Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [syserr].
View all issues with Resolved status.
Discussion:
The most recent state of
N2241
as well as the current draft
N2284
(section 19.5 [syserr], p.2) proposes a
new
enumeration type posix_errno
immediatly in the namespace std
. One of
the enumerators has the name invalid_argument
, or fully qualified:
std::invalid_argument
. This name clashes with the exception type
std::invalid_argument
, see 19.2 [std.exceptions]/p.3. This clash makes
e.g. the following snippet invalid:
#include <system_error> #include <stdexcept> void foo() { throw std::invalid_argument("Don't call us - we call you!"); }
I propose that this enumeration type (and probably the remaining parts
of
<system_error>
as well) should be moved into one additional inner
namespace, e.g. sys
or system
to reduce foreseeable future clashes
due
to the great number of members that std::posix_errno
already contains
(Btw.: Why has the already proposed std::sys
sub-namespace from
N2066
been rejected?). A further clash candidate seems to be
std::protocol_error
(a reasonable name for an exception related to a std network library,
I guess).
Another possible resolution would rely on the proposed strongly typed enums, as described in N2213. But maybe the forbidden implicit conversion to integral types would make these enumerators less attractive in this special case?
Proposed resolution:
Fixed by issue 7 of N2422.
system_error
needs const char*
constructorsSection: 19.5.8.1 [syserr.syserr.overview] Status: CD1 Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
In 19.5.8.1 [syserr.syserr.overview] we have the class definition of
std::system_error
. In contrast to all exception classes, which
are constructible with a what_arg string
(see 19.2 [std.exceptions],
or ios_base::failure
in [ios::failure]), only overloads with with
const string&
are possible. For consistency with the re-designed
remaining exception classes this class should also provide
c'tors which accept a const char* what_arg
string.
Please note that this proposed addition makes sense even
considering the given implementation hint for what()
, because
what_arg
is required to be set as what_arg
of the base class
runtime_error
, which now has the additional c'tor overload
accepting a const char*
.
Proposed resolution:
This proposed wording assumes issue 832(i) has been accepted and applied to the working paper.
Change 19.5.8.1 [syserr.syserr.overview] Class system_error overview, as indicated:
public: system_error(error_code ec, const string& what_arg); system_error(error_code ec, const char* what_arg); system_error(error_code ec); system_error(int ev, const error_category* ecat, const string& what_arg); system_error(int ev, const error_category* ecat, const char* what_arg); system_error(int ev, const error_category* ecat);
To 19.5.8.2 [syserr.syserr.members] Class system_error members add:
system_error(error_code ec, const char* what_arg);Effects: Constructs an object of class
system_error
.Postconditions:
code() == ec
andstrcmp(runtime_error::what(), what_arg) == 0
.system_error(int ev, const error_category* ecat, const char* what_arg);Effects: Constructs an object of class
system_error
.Postconditions:
code() == error_code(ev, ecat)
andstrcmp(runtime_error::what(), what_arg) == 0
.
Section: 29.5 [rand] Status: CD1 Submitter: P.J. Plauger Opened: 2007-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand].
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Discussion:
N2111
changes min/max
in several places in random from member
functions to static data members. I believe this introduces
a needless backward compatibility problem between C++0X and
TR1. I'd like us to find new names for the static data members,
or perhaps change min/max
to constexpr
s in C++0X.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
identity
Section: 22.2.4 [forward] Status: CD1 Submitter: P.J. Plauger Opened: 2007-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [forward].
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Discussion:
N1856
defines struct identity
in <utility>
which clashes with
the traditional definition of struct identity
in <functional>
(not standard, but a common extension from old STL). Be nice
if we could avoid this name clash for backward compatibility.
Proposed resolution:
Change 22.2.4 [forward]:
template <class T> struct identity { typedef T type; const T& operator()(const T& x) const; };const T& operator()(const T& x) const;Returns:
x
.
map::at()
need a complexity specificationSection: 23.4.3.3 [map.access] Status: CD1 Submitter: Joe Gottman Opened: 2007-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [map.access].
View all issues with CD1 status.
Discussion:
map::at()
need a complexity specification.
Proposed resolution:
Add the following to the specification of map::at()
, 23.4.3.3 [map.access]:
Complexity: logarithmic.
MoveAssignable
requirement for container value type overly strictSection: 23.2 [container.requirements] Status: C++11 Submitter: Howard Hinnant Opened: 2007-05-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
View all issues with C++11 status.
Discussion:
The move-related changes inadvertently overwrote the intent of 276(i).
Issue 276(i) removed the requirement of CopyAssignable
from
most of the member functions of node-based containers. But the move-related changes
unnecessarily introduced the MoveAssignable
requirement for those members which used to
require CopyAssignable
.
We also discussed (c++std-lib-18722) the possibility of dropping MoveAssignable
from some of the sequence requirements. Additionally the in-place construction
work may further reduce requirements. For purposes of an easy reference, here are the
minimum sequence requirements as I currently understand them. Those items in requirements
table in the working draft which do not appear below have been purposefully omitted for
brevity as they do not have any requirements of this nature. Some items which do not
have any requirements of this nature are included below just to confirm that they were
not omitted by mistake.
X u(a) | value_type must be CopyConstructible |
X u(rv) | array requires value_type to be CopyConstructible |
a = u | Sequences require value_type to be CopyConstructible and CopyAssignable .
Associative containers require value_type to be CopyConstructible . |
a = rv | array requires value_type to be CopyAssignable .
Sequences containers with propagate_on_container_move_assignment == false allocators require value_type to be MoveConstructible and MoveAssignable .
Associative containers with propagate_on_container_move_assignment == false allocators require value_type to be MoveConstructible . |
swap(a,u) | array requires value_type to be Swappable . |
X(n) | value_type must be DefaultConstructible |
X(n, t) | value_type must be CopyConstructible |
X(i, j) | Sequences require value_type to be constructible from *i . Additionally if input_iterators
are used, vector and deque require MoveContructible and MoveAssignable . |
a.insert(p, t) | The value_type must be CopyConstructible .
The sequences vector and deque also require the value_type to be CopyAssignable . |
a.insert(p, rv) | The value_type must be MoveConstructible .
The sequences vector and deque also require the value_type to be MoveAssignable . |
a.insert(p, n, t) | The value_type must be CopyConstructible .
The sequences vector and deque also require the value_type to be CopyAssignable . |
a.insert(p, i, j) | If the iterators return an lvalue the value_type must be CopyConstructible .
The sequences vector and deque also require the value_type to be CopyAssignable when the iterators return an lvalue.
If the iterators return an rvalue the value_type must be MoveConstructible .
The sequences vector and deque also require the value_type to be MoveAssignable when the iterators return an rvalue. |
a.erase(p) | The sequences vector and deque require the value_type to be MoveAssignable . |
a.erase(q1, q2) | The sequences vector and deque require the value_type to be MoveAssignable . |
a.clear() | |
a.assign(i, j) | If the iterators return an lvalue the value_type must be CopyConstructible and CopyAssignable .
If the iterators return an rvalue the value_type must be MoveConstructible and MoveAssignable . |
a.assign(n, t) | The value_type must be CopyConstructible and CopyAssignable . |
a.resize(n) | The value_type must be DefaultConstructible .
The sequence vector also requires the value_type to be MoveConstructible . |
a.resize(n, t) | The value_type must be CopyConstructible . |
a.front() | |
a.back() | |
a.push_front(t) | The value_type must be CopyConstructible . |
a.push_front(rv) | The value_type must be MoveConstructible . |
a.push_back(t) | The value_type must be CopyConstructible . |
a.push_back(rv) | The value_type must be MoveConstructible . |
a.pop_front() | |
a.pop_back() | |
a[n] | |
a.at[n] |
X(i, j) | If the iterators return an lvalue the value_type must be CopyConstructible .
If the iterators return an rvalue the value_type must be MoveConstructible . |
a_uniq.insert(t) | The value_type must be CopyConstructible . |
a_uniq.insert(rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a_eq.insert(t) | The value_type must be CopyConstructible . |
a_eq.insert(rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a.insert(p, t) | The value_type must be CopyConstructible . |
a.insert(p, rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a.insert(i, j) | If the iterators return an lvalue the value_type must be CopyConstructible .
If the iterators return an rvalue the key_type and the mapped_type (if it exists) must be MoveConstructible .. |
X(i, j, n, hf, eq) | If the iterators return an lvalue the value_type must be CopyConstructible .
If the iterators return an rvalue the value_type must be MoveConstructible . |
a_uniq.insert(t) | The value_type must be CopyConstructible . |
a_uniq.insert(rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a_eq.insert(t) | The value_type must be CopyConstructible . |
a_eq.insert(rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a.insert(p, t) | The value_type must be CopyConstructible . |
a.insert(p, rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a.insert(i, j) | If the iterators return an lvalue the value_type must be CopyConstructible .
If the iterators return an rvalue the key_type and the mapped_type (if it exists) must be MoveConstructible .. |
map[lvalue-key] | The key_type must be CopyConstructible .
The mapped_type must be DefaultConstructible and MoveConstructible . |
map[rvalue-key] | The key_type must be MoveConstructible .
The mapped_type must be DefaultConstructible and MoveConstructible . |
[ Kona (2007): Howard and Alan to update requirements table in issue with emplace signatures. ]
[ Bellevue: This should be handled as part of the concepts work. ]
[ 2009-07-20 Reopened by Howard: ]
This is one of the issues that was "solved by concepts" and is now no longer solved.
In a nutshell, concepts adopted the "minimum requirements" philosophy outlined in the discussion of this issue, and enforced it. My strong suggestion is that we translate the concepts specification into documentation for the containers.
What this means for vendors is that they will have to implement container members being careful to only use those characteristics of a type that the concepts specification formally allowed. Note that I am not talking about
enable_if
'ing everything. I am simply suggesting that (for example) we tell the vendor he can't callT's
copy constructor or move constructor within theemplace
member function, etc.What this means for customers is that they will be able to use types within C++03 containers which are sometimes not CopyConstructible, and sometimes not even MoveConstructible, etc.
[ 2009-10 Santa Cruz: ]
Leave open. Howard to provide wording.
[ 2010-02-06 Howard provides wording. ]
[ 2010-02-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Howard opened. I neglected to reduce the requirements on value_type for the insert function of the ordered and unordered associative containers when the argument is an rvalue. Fixed it. ]
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-03-08 Nico opens: ]
I took the task to see whether 868(i) is covered by 704 already. However, by doing that I have the impression that 704 is a big mistake.
Take e.g. the second change of 868(i):
Change 23.3.5.2 [deque.cons] para 5:
Effects: Constructs a
deque
withn
default constructed elements.where "default constructed" should be replaced by "value-initialized". This is the constructor out of a number of elements:
ContType c(num)704 says:
Remove the entire section 23.3.5.2 [deque.cons].
[ This section is already specified by the requirements tables. ]
BUT, there is no requirement table that lists this constructor at all, which means that we would lose the entire specification of this function !!!
In fact, I found with further investigation, if we follow 704 to remove 23.3.2.1 we
- have no semantics for
ContType c(num)
- have no complexity and no allocator specification for
ContType c(num,val)
- have no semantics for
ContType c(num,val,alloc)
- - have no complexity and no allocator specification for
ContType c(beg,end)
- - have no semantics for
ContType c(beg,end,alloc)
- - have different wording (which might or might not give the same guarantees) for the
assign
functionsbecause all these guarantees are given in the removed section but nowhere else (as far as I saw).
Looks to me that 704 need a significant review before we take that change, because chances are high that there are similar flaws in other proposed changes there (provided I am not missing anything).
[ 2010 Pittsburgh: ]
Removed the parts from the proposed wording that removed existing sections, and set to Ready for Pittsburgh.
Rationale:
[ post San Francisco: ]
Solved by N2776.
This rationale is obsolete.
Proposed resolution:
Change 23.2.2 [container.requirements.general]/4:
4 In Tables 91 and 92,
X
denotes a container class containing objects of typeT
,a
andb
denote values of typeX
,u
denotes an identifier,r
denotesan lvalue or a const rvaluea non-const value of typeX
, andrv
denotes a non-const rvalue of typeX
.
Change the following rows in Table 91 — Container requirements 23.2.2 [container.requirements.general]:
Table 91 — Container requirements Expression Return type Assertion/note
pre-/post-conditionComplexity X::value_type
T
Requires: T
isDestructible
.compile time
Change 23.2.2 [container.requirements.general]/10:
Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.2.3, and 23.3.6.4) all container types defined in this Clause meet the following additional requirements:
…
no
erase()
,clear()
,pop_back()
orpop_front()
function throws an exception.…
Insert a new paragraph prior to 23.2.2 [container.requirements.general]/14:
The descriptions of the requirements of the type
T
in this section use the termsCopyConstructible
,MoveConstructible
, constructible from*i
, and constructible fromargs
. These terms are equivalent to the following expression using the appropriate arguments:allocator_traits<allocator_type>::construct(x.get_allocator(), q, args...);where
x
is a non-const lvalue of some container typeX
andq
has typeX::value_type*
.[Example: The container is going to move construct a
T
, so will call:allocator_traits<allocator_type>::construct(get_allocator(), q, std::move(t));The default implementation of construct will call:
::new (q) T(std::forward<T>(t)); // where forward is the same as move here, cast to rvalueBut the allocator author may override the above definition of
construct
and do the construction ofT
by some other means. — end example]14 ...
Add to 23.2.2 [container.requirements.general]/14:
14 In Table 93,
X
denotes an allocator-aware container class with avalue_type
ofT
using allocator of typeA
,u
denotes a variable,a
andb
denote non-const lvalues of typeX
,t
denotes an lvalue or a const rvalue of typeX
,rv
denotes a non-const rvalue of typeX
,m
is a value of typeA
, andQ
is an allocator type.
Change or add the following rows in Table 93 — Allocator-aware container requirements in 23.2.2 [container.requirements.general]:
Table 93 — Allocator-aware container requirements Expression Return type Assertion/note
pre-/post-conditionComplexity X(t, m)
X u(t, m);Requires: T
isCopyConstructible
.
post:u == t
,
get_allocator() == m
linear X(rv, m)
X u(rv, m);Requires: T
isMoveConstructible
.
post:u
shall have the same elements, or copies of the elements, thatrv
had before this construction,
get_allocator() == m
constant if m == rv.get_allocator()
, otherwise lineara = t
X&
Requires: T
isCopyConstructible
andCopyAssignable
post:a == t
.linear a = rv
X&
Requires: If allocator_traits< allocator_type > ::propagate_on_container_move_assignment ::value
isfalse
,T
isMoveConstructible
andMoveAssignable
.
All existing elements ofa
are either move assigned to or destroyed.
a
shall be equal to the value thatrv
had before this assignmentlinear a.swap(b);
void
exchanges the contents of a
andb
constant
Change the following rows in Table 94 — Sequence container requirements (in addition to container) in 23.2.4 [sequence.reqmts]:
Table 94 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionX(i, j)
X a(i, j)Requires: If the iterator's dereference operation returns an lvalue or a const rvalue,T
shall beCopyConstructible
.T
shall be constructible from*i
.
If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), thenvector
also requiresT
to beMoveConstructible
.
Each iterator in the range[i,j)
shall be dereferenced exactly once.
post:size() ==
distance betweeni
andj
Constructs a sequence container equal to the range[i, j)
a = il;
X&
Requires: T
isCopyConstructible
andCopyAssignable
.
a = X(il);
Assigns the range[il.begin(), il.end())
intoa
. All existing elements ofa
are either assigned or destroyed.
rReturns*this;
a.emplace(p, args);
iterator
Requires: ConstructibleAsElement<A, T, Args>
.T
is constructible fromargs
.vector
anddeque
also requireT
to beMoveConstructible
andMoveAssignable
. Inserts an object of typeT
constructed withstd::forward<Args>(args)...
beforep
.a.insert(p, t);
iterator
Requires: ConstructibleAsElement<A, T, Args>
andT
shall beCopyAssignable
.T
shall beCopyConstructible
.vector
anddeque
also requireT
to beCopyAssignable
. Inserts a copyt
beforep
.a.insert(p, rv);
iterator
Requires: ConstructibleAsElement<A, T, T&&>
andT
shall beMoveAssignable
.T
shall beMoveConstructible
.vector
anddeque
also requireT
to beMoveAssignable
. Inserts a copyrv
beforep
.a.insert(p, i, j)
iterator
Requires: If the iterator's dereference operation returns an lvalue or a const rvalue,T
shall beCopyConstructible
.T
shall be constructible from*i
.
If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), thenvector
also requiresT
to beMoveConstructible
andMoveAssignable
.
Each iterator in the range[i,j)
shall be dereferenced exactly once.
pre:i
andj
are not iterators intoa
.
Inserts copies of elements in[i, j)
beforep
a.erase(q);
iterator
Requires: T
andT
shall beMoveAssignable
.vector
anddeque
requireT
to beMoveAssignable
. Erases the element pointed to byq
.a.erase(q1, q2);
iterator
Requires: T
andT
shall beMoveAssignable
.vector
anddeque
requireT
to beMoveAssignable
. Erases the elements in the range[q1, q2)
.a.clear();
void
erase(begin(), end())
Destroys all elements ina
. Invalidates all references, pointers, and iterators referring to the elements ofa
and may invalidate the past-the-end iterator.
post:
size() == 0a.empty() == truea.assign(i, j)
void
Requires: If the iterator's dereference operation returns an lvalue or a const rvalue,T
shall beCopyConstructible
andCopyAssignable
.T
shall be constructible and assignable from*i
. If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), thenvector
also requiresT
to beMoveConstructible
.
Each iterator in the range[i,j)
shall be dereferenced exactly once.
pre:i
,j
are not iterators intoa
.
Replaces elements ina
with a copy of[i, j)
.
Change the following rows in Table 95 — Optional sequence container operations in 23.2.4 [sequence.reqmts]:
Table 95 — Optional sequence container operations Expression Return type Operational semantics Container a.emplace_front(args)
void
a.emplace(a.begin(), std::forward<Args>(args)...)
Prepends an object of typeT
constructed withstd::forward<Args>(args)...
.
Requires:ConstructibleAsElement<A, T, Args>
T
shall be constructible fromargs
.list
,deque
,forward_list
a.emplace_back(args)
void
a.emplace(a.end(), std::forward<Args>(args)...)
Appends an object of typeT
constructed withstd::forward<Args>(args)...
.
Requires:ConstructibleAsElement<A, T, Args>
T
shall be constructible fromargs
.vector
also requiresT
to beMoveConstructible
.list
,deque
,vector
a.push_front(t)
void
a.insert(a.begin(), t)
Prepends a copy oft
.
Requires:ConstructibleAsElement<A, T, T>
andT
shall beCopyAssignable
.T
shall beCopyConstructible
.list
,deque
,forward_list
a.push_front(rv)
void
a.insert(a.begin(), t)
Prepends a copy ofrv
.
Requires:ConstructibleAsElement<A, T, T&&>
andT
shall beMoveAssignable
.T
shall beMoveConstructible
.list
,deque
,forward_list
a.push_back(t)
void
a.insert(a.end(), t)
Appends a copy oft
.
Requires:ConstructibleAsElement<A, T, T>
andT
shall beCopyAssignable
.T
shall beCopyConstructible
.vector
,list
,deque
,basic_string
a.push_back(rv)
void
a.insert(a.end(), t)
Appends a copy ofrv
.
Requires:ConstructibleAsElement<A, T, T&&>
andT
shall beMoveAssignable
.T
shall beMoveConstructible
.vector
,list
,deque
,basic_string
a.pop_front()
void
a.erase(a.begin())
Destroys the first element.
Requires:a.empty()
shall befalse
.list
,deque
,forward_list
a.pop_back()
void
{ iterator tmp = a.end();
--tmp;
a.erase(tmp); }
Destroys the last element.
Requires:a.empty()
shall befalse
.vector
,list
,deque
,basic_string
Insert a new paragraph prior to 23.2.7 [associative.reqmts]/7, and edit paragraph 7:
The associative containers meet all of the requirements of Allocator-aware containers (23.2.2 [container.requirements.general]), except for the containers
map
andmultimap
, the requirements placed onvalue_type
in Table 93 apply instead directly tokey_type
andmapped_type
. [Note: For examplekey_type
andmapped_type
are sometimes required to beCopyAssignable
even though thevalue_type
(pair<const key_type, mapped_type>
) is notCopyAssignable
. — end note]7 In Table 96,
X
denotes an associative container class, a denotes a value ofX
,a_uniq
denotes a value ofX
whenX
supports unique keys,a_eq
denotes a value ofX
whenX
supports multiple keys,u
denotes an identifier,r
denotes an lvalue or a const rvalue of typeX
,rv
denotes a non-const rvalue of typeX
,i
andj
satisfy input iterator requirements and refer to elements implicitly convertible tovalue_type
,[i,j)
denotes a valid range,p
denotes a valid const iterator toa
,q
denotes a valid dereferenceable const iterator toa
,[q1, q2)
denotes a valid range of const iterators ina
,il
designates an object of typeinitializer_list<value_type>
,t
denotes a value ofX::value_type
,k
denotes a value ofX::key_type
andc
denotes a value of typeX::key_compare
.A
denotes the storage allocator used byX
, if any, orstd::allocator<X::value_type>
otherwise, andm
denotes an allocator of a type convertible toA
.
Change or add the following rows in Table 96 — Associative container requirements (in addition to container) in 23.2.7 [associative.reqmts]:
Table 96 — Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity X::key_type
Key
Requires: Key
isCopyConstructible
andCopyAssignable
Destructible
compile time X::mapped_type
(map
andmultimap
only)T
Requires: T
isDestructible
compile time X(c)
X a(c);Requires: .ConstructibleAsElement<A, key_compare, key_compare>
key_compare
isCopyConstructible
.
Constructs an empty container.
Uses a copy ofc
as a comparison object.constant X()
X a;Requires: .ConstructibleAsElement<A, key_compare, key_compare>
key_compare
isDefaultConstructible
.
Constructs an empty container.
UsesCompare()
as a comparison object.constant X(i, j, c)
X a(i, j, c);Requires: .ConstructibleAsElement<A, key_compare, key_compare>
key_compare
isCopyConstructible
.value_type
shall be constructible from*i
.
Constructs an empty container ans inserts elements from the range[i, j)
into it; usesc
as a comparison object.N
logN
in general (N
is the distance fromi
toj
); linear if[i, j)
is sorted withvalue_comp()
X(i, j)
X a(i, j);Requires: .ConstructibleAsElement<A, key_compare, key_compare>
value_type
shall be constructible from*i
.key_compare
isDefaultConstructible
.
Same as above, but usesCompare()
as a comparison object.same as above a = il
X&
a = X(il);
return *this;
Requires:T
isCopyConstructible
andCopyAssignable
.
Assigns the range[il.begin(), il.end())
intoa
. All existing elements ofa
are either assigned or destroyed.Same as.
a = X(il)
N
logN
in general (N
isil.size()
added to the existing size ofa
); linear if[il.begin(), il.end())
is sorted withvalue_comp()
a_uniq.emplace(args)
pair<iterator, bool>
Requires: T
shall be constructible fromargs
inserts aT
objectt
constructed withstd::forward<Args>(args)...
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key oft
.logarithmic a_eq.emplace(args)
iterator
Requires: T
shall be constructible fromargs
inserts aT
objectt
constructed withstd::forward<Args>(args)...
and returns the iterator pointing to the newly inserted element.logarithmic a_uniq.insert(t)
pair<iterator, bool>
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
insertst
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key oft
.logarithmic a_eq.insert(t)
iterator
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
insertst
and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent tot
exists ina_eq
,t
is inserted at the end of that range.logarithmic a.insert(p, t)
iterator
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
insertst
if and only if there is no element with key equivalent to the key oft
in containers with unique keys; always insertst
in containers with equivalent keys; always returns the iterator pointing to the element with key equivalent to the key oft
.t
is inserted as close as possible to the position just prior top
.logarithmic in general, but amortized constant if t
is inserted right beforep
.a.insert(i, j)
void
Requires: T
shall be constructible from*i
.
pre:i
,j
are not iterators intoa
. inserts each element from the range[i,j)
if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys.N log(size() + N ) (N is the distance from i to j)
Insert a new paragraph prior to 23.2.8 [unord.req]/9:
The unordered associative containers meet all of the requirements of Allocator-aware containers (23.2.2 [container.requirements.general]), except for the containers
unordered_map
andunordered_multimap
, the requirements placed onvalue_type
in Table 93 apply instead directly tokey_type
andmapped_type
. [Note: For examplekey_type
andmapped_type
are sometimes required to beCopyAssignable
even though thevalue_type
(pair<const key_type, mapped_type>
) is notCopyAssignable
. — end note]9 ...
Change or add the following rows in Table 98 — Unordered associative container requirements (in addition to container) in 23.2.8 [unord.req]:
Table 98 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity X::key_type
Key
Requires: Key
shall beCopyAssignable
andCopyConstructible
Destructible
compile time X::mapped_type
(unordered_map
andunordered_multimap
only)T
Requires: T
isDestructible
compile time X(n, hf, eq)
X a(n, hf, eq)X
Requires: hasher
andkey_equal
areCopyConstructible
. Constructs an empty container with at leastn
buckets, usinghf
as the hash function andeq
as the key equality predicate.O(N)
X(n, hf)
X a(n, hf)X
Requires: hasher
isCopyConstructible
andkey_equal
isDefaultConstructible
. Constructs an empty container with at leastn
buckets, usinghf
as the hash function andkey_equal()
as the key equality predicate.O(N)
X(n)
X a(n)X
Requires: hasher
andkey_equal
areDefaultConstructible
. Constructs an empty container with at leastn
buckets, usinghasher()
as the hash function andkey_equal()
as the key equality predicate.O(N)
X()
X aX
Requires: hasher
andkey_equal
areDefaultConstructible
. Constructs an empty container an unspecified number of buckets, usinghasher()
as the hash function andkey_equal()
as the key equality predicate.constant X(i, j, n, hf, eq)
X a(i, j, n, hf, eq)X
Requires: value_type
is constructible from*i
.hasher
andkey_equal
areCopyConstructible
.
Constructs an empty container with at leastn
buckets, usinghf
as the hash function andeq
as the key equality predicate, and inserts elements from[i, j)
into it.Average case O(N)
(N
isdistance(i, j)
), worst caseO(N2)
X(i, j, n, hf)
X a(i, j, n, hf)X
Requires: value_type
is constructible from*i
.hasher
isCopyConstructible
andkey_equal
isDefaultConstructible
.
Constructs an empty container with at leastn
buckets, usinghf
as the hash function andkey_equal()
as the key equality predicate, and inserts elements from[i, j)
into it.Average case O(N)
(N
isdistance(i, j)
), worst caseO(N2)
X(i, j, n)
X a(i, j, n)X
Requires: value_type
is constructible from*i
.hasher
andkey_equal
areDefaultConstructible
.
Constructs an empty container with at leastn
buckets, usinghasher()
as the hash function andkey_equal()
as the key equality predicate, and inserts elements from[i, j)
into it.Average case O(N)
(N
isdistance(i, j)
), worst caseO(N2)
X(i, j)
X a(i, j)X
Requires: value_type
is constructible from*i
.hasher
andkey_equal
areDefaultConstructible
.
Constructs an empty container with an unspecified number of buckets, usinghasher()
as the hash function andkey_equal()
as the key equality predicate, and inserts elements from[i, j)
into it.Average case O(N)
(N
isdistance(i, j)
), worst caseO(N2)
X(b)
X a(b)X
Copy constructor. In addition to the contained elementsrequirements of Table 93 (23.2.2 [container.requirements.general]), copies the hash function, predicate, and maximum load factor.Average case linear in b.size()
, worst case quadratic.a = b
X&
Copy assignment operator. In addition to the contained elementsrequirements of Table 93 (23.2.2 [container.requirements.general]), copies the hash function, predicate, and maximum load factor.Average case linear in b.size()
, worst case quadratic.a = il
X&
a = X(il); return *this;
Requires:T
isCopyConstructible
andCopyAssignable
.
Assigns the range[il.begin(), il.end())
intoa
. All existing elements ofa
are either assigned or destroyed.Average case linear in il.size()
, worst case quadratic.a_uniq.emplace(args)
pair<iterator, bool>
Requires: T
shall be constructible fromargs
inserts aT
objectt
constructed withstd::forward<Args>(args)...
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key oft
.Average case O(1), worst case O( a_uniq.size()
).a_eq.emplace(args)
iterator
Requires: T
shall be constructible fromargs
inserts aT
objectt
constructed withstd::forward<Args>(args)...
and returns the iterator pointing to the newly inserted element.Average case O(1), worst case O( a_eq.size()
).a.emplace_hint(p, args)
iterator
Requires: T
shall be constructible fromargs
equivalent toa.emplace( std::forward<Args>(args)...)
. Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. Theconst_iterator p
is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.Average case O(1), worst case O( a.size()
).a_uniq.insert(t)
pair<iterator, bool>
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
Insertst
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key oft
.Average case O(1), worst case O( a_uniq.size()
).a_eq.insert(t)
iterator
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
Insertst
, and returns an iterator pointing to the newly inserted element.Average case O(1), worst case O( a_uniq.size()
).a.insert(q, t)
iterator
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
Equivalent toa.insert(t)
. Return value is an iterator pointing to the element with the key equivalent to that oft
. The iteratorq
is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.Average case O(1), worst case O( a_uniq.size()
).a.insert(i, j)
void
Requires: T
shall be constructible from*i
.
Pre:i
andj
are not iterators ina
. Equivalent toa.insert(t)
for each element in[i,j)
.Average case O( N
), whereN
isdistance(i, j)
. Worst case O(N * a.size()
).
Change [forwardlist]/2:
2 A
forward_list
satisfies all of the requirements of a container (table 91), except that thesize()
member function is not provided. Aforward_list
also satisfies all of the requirements of an allocator-aware container (table 93). Andforward_list
provides theassign
member functions as specified in Table 94, Sequence container requirements, and several of the optional sequence container requirements (Table 95). Descriptions are provided here only for operations onforward_list
that are not described in that table or for operations where there is additional semantic information.
Add a new paragraph after [forwardlist.modifiers]/23:
void clear();23 Effects: Erases all elements in the range
[begin(),end())
.Remarks: Does not invalidate past-the-end iterators.
Change 23.3.13.3 [vector.capacity]/13:
void resize(size_type sz, const T& c);13 Requires:
T
shall beCopyConstructible
. Ifvalue_type
has a move constructor, that constructor shall not throw any exceptions.
In 23.5.6 [unord.set] and 23.5.7 [unord.multiset] substitute
"Key
" for "Value
".
[ The above substitution is normative as it ties into the requirements table. ]
decay
incompletely specifiedSection: 21.3.8.7 [meta.trans.other] Status: CD1 Submitter: Thorsten Ottosen Opened: 2007-07-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
View all issues with CD1 status.
Discussion:
The current working draft has a type-trait decay
in 21.3.8.7 [meta.trans.other].
Its use is to turn C++03 pass-by-value parameters into efficient C++0x pass-by-rvalue-reference parameters. However, the current definition introduces an incompatible change where the cv-qualification of the parameter type is retained. The deduced type should loose such cv-qualification, as pass-by-value does.
Proposed resolution:
In 21.3.8.7 [meta.trans.other] change the last sentence:
Otherwise the member typedef
type
equalsremove_cv<U>::type
.
In 22.4.5 [tuple.creation]/1 change:
where eachLetVi
inVTypes
isX&
if, for the corresponding typeTi
inTypes
,remove_cv<remove_reference<Ti>::type>::type
equalsreference_wrapper<X>
, otherwiseVi
isdecay<Ti>::type
.Ui
bedecay<Ti>::type
for eachTi
inTypes
. Then eachVi
inVTypes
isX&
ifUi
equalsreference_wrapper<X>
, otherwiseVi
isUi
.
make_pair()
should behave as make_tuple()
wrt. reference_wrapper()
Section: 22.3 [pairs] Status: CD1 Submitter: Thorsten Ottosen Opened: 2007-07-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [pairs].
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Discussion:
The current draft has make_pair()
in 22.3 [pairs]/16
and make_tuple()
in 22.4.5 [tuple.creation].
make_tuple()
detects the presence of
reference_wrapper<X>
arguments and "unwraps" the reference in
such cases. make_pair()
would OTOH create a
reference_wrapper<X>
member. I suggest that the two
functions are made to behave similar in this respect to minimize
confusion.
Proposed resolution:
In 22.2 [utility] change the synopsis for make_pair() to read
template <class T1, class T2> pair<typename decay<T1>::typeV1,typename decay<T2>::typeV2> make_pair(T1&&, T2&&);
In 22.3 [pairs]/16 change the declaration to match the above synopsis. Then change the 22.3 [pairs]/17 to:
Returns:
pair<
wheretypename decay<T1>::typeV1,typename decay<T2>::typeV2>(forward<T1>(x),forward<T2>(y))V1
andV2
are determined as follows: LetUi
bedecay<Ti>::type
for eachTi
. Then eachVi
isX&
ifUi
equalsreference_wrapper<X>
, otherwiseVi
isUi
.
char_traits::not_eof
has wrong signatureSection: 27.2.4 [char.traits.specializations] Status: CD1 Submitter: Bo Persson Opened: 2007-08-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [char.traits.specializations].
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Discussion:
The changes made for constexpr
in 27.2.4 [char.traits.specializations] have
not only changed the not_eof
function from pass by const reference to
pass by value, it has also changed the parameter type from int_type
to
char_type
.
This doesn't work for type char
, and is inconsistent with the
requirements in Table 56, Traits requirements, 27.2.2 [char.traits.require].
Pete adds:
For what it's worth, that may not have been an intentional change. N2349, which detailed the changes for adding constant expressions to the library, has strikeout bars through the
const
and the&
that surround thechar_type
argument, but none throughchar_type
itself. So the intention may have been just to change to pass by value, with text incorrectly copied from the standard.
Proposed resolution:
Change the signature in 27.2.4.2 [char.traits.specializations.char], [char.traits.specializations.char16_t], [char.traits.specializations.char32_t], and 27.2.4.6 [char.traits.specializations.wchar.t] to
static constexpr int_type not_eof(char_typeint_type c);
[ Bellevue: ]
Resolution: NAD editorial - up to Pete's judgment
[ Post Sophia Antipolis ]
Moved from Pending NAD Editorial to Review. The proposed wording appears to be correct but non-editorial.
Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
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Discussion:
A discussion on
comp.std.c++
has identified a contradiction in the shared_ptr
specification.
The shared_ptr
move constructor and the cast functions are
missing postconditions for the get()
accessor.
[ Bellevue: ]
Move to "ready", adopting the first (Peter's) proposed resolution.
Note to the project editor: there is an editorial issue here. The wording for the postconditions of the casts is slightly awkward, and the editor should consider rewording "If w is the return value...", e. g. as "For a return value w...".
Proposed resolution:
Add to 20.3.2.2.2 [util.smartptr.shared.const]:
shared_ptr(shared_ptr&& r); template<class Y> shared_ptr(shared_ptr<Y>&& r);Postconditions:
*this
shall contain the old value ofr
.r
shall be empty.r.get() == 0
.
Add to 20.3.2.2.10 [util.smartptr.shared.cast]:
template<class T, class U> shared_ptr<T> static_pointer_cast(shared_ptr<U> const& r);Postconditions: If
w
is the return value,w.get() == static_cast<T*>(r.get()) && w.use_count() == r.use_count()
.
template<class T, class U> shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const& r);Postconditions: If
w
is the return value,w.get() == dynamic_cast<T*>(r.get())
.
template<class T, class U> shared_ptr<T> const_pointer_cast(shared_ptr<U> const& r);Postconditions: If
w
is the return value,w.get() == const_cast<T*>(r.get()) && w.use_count() == r.use_count()
.
Alberto Ganesh Barbati has written an alternative proposal where he suggests (among other things) that the casts be respecified in terms of the aliasing constructor as follows:
Change 20.3.2.2.10 [util.smartptr.shared.cast]:
-2- Returns:
Ifr
is empty, anempty shared_ptr<T>;
otherwise, ashared_ptr<T>
object that storesstatic_cast<T*>(r.get())
and shares ownership withr
.shared_ptr<T>(r, static_cast<T*>(r.get())
.
-6- Returns:
Whendynamic_cast<T*>(r.get())
returns a nonzero value, ashared_ptr<T>
object that stores a copy of it and shares ownership withr
;Otherwise, an emptyshared_ptr<T>
object.- If
p = dynamic_cast<T*>(r.get())
is a non-null pointer,shared_ptr<T>(r, p);
- Otherwise,
shared_ptr<T>()
.
-10- Returns:
Ifr
is empty, anempty shared_ptr<T>;
otherwise, ashared_ptr<T>
object that storesconst_cast<T*>(r.get())
and shares ownership withr
.shared_ptr<T>(r, const_cast<T*>(r.get())
.
This takes care of the missing postconditions for the casts by bringing in the aliasing constructor postcondition "by reference".
shared_ptr
Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: C++11 Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.obs].
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Discussion:
A discussion on
comp.std.c++
has identified a contradiction in the shared_ptr
specification.
The note:
[ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -end note ]
after the aliasing constructor
template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);
reflects the intent of
N2351
to, well, allow the creation of an empty shared_ptr
with a non-NULL stored pointer.
This is contradicted by the second sentence in the Returns clause of 20.3.2.2.6 [util.smartptr.shared.obs]:
T* get() const;Returns: the stored pointer. Returns a null pointer if
*this
is empty.
[ Bellevue: ]
Adopt option 1 and move to review, not ready.
There was a lot of confusion about what an empty
shared_ptr
is (the term isn't defined anywhere), and whether we have a good mental model for how one behaves. We think it might be possible to deduce what the definition should be, but the words just aren't there. We need to open an issue on the use of this undefined term. (The resolution of that issue might affect the resolution of issue 711(i).)The LWG is getting more uncomfortable with the aliasing proposal (N2351) now that we realize some of its implications, and we need to keep an eye on it, but there isn't support for removing this feature at this time.
[ Sophia Antipolis: ]
We heard from Peter Dimov, who explained his reason for preferring solution 1.
Because it doesn't seem to add anything. It simply makes the behavior for p = 0 undefined. For programmers who don't create empty pointers with p = 0, there is no difference. Those who do insist on creating them presumably have a good reason, and it costs nothing for us to define the behavior in this case.
The aliasing constructor is sharp enough as it is, so "protecting" users doesn't make much sense in this particular case.
> Do you have a use case for r being empty and r being non-null?
I have received a few requests for it from "performance-conscious" people (you should be familiar with this mindset) who don't like the overhead of allocating and maintaining a control block when a null deleter is used to approximate a raw pointer. It is obviously an "at your own risk", low-level feature; essentially a raw pointer behind a shared_ptr facade.
We could not agree upon a resolution to the issue; some of us thought that Peter's description above is supporting an undesirable behavior.
[ 2009-07 Frankfurt: ]
We favor option 1, move to Ready.
[ Howard: Option 2 commented out for clarity, and can be brought back. ]
Proposed resolution:
In keeping the N2351 spirit and obviously my preference, change 20.3.2.2.6 [util.smartptr.shared.obs]:
T* get() const;Returns: the stored pointer.
Returns a null pointer if*this
is empty.
seed_seq::size
no longer usefulSection: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Marc Paterno Opened: 2007-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
One of the motivations for incorporating seed_seq::size()
was to simplify the wording
in other parts of 29.5 [rand].
As a side effect of resolving related issues,
all such references
to seed_seq::size()
will have been excised.
More importantly,
the present specification is contradictory,
as "The number of 32-bit units the object can deliver"
is not the same as "the result of v.size()
."
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
sort()
complexity is too laxSection: 26.8.2.1 [sort] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity of sort()
is specified as "Approximately N
log(N)
(where N == last - first
) comparisons on the
average", with no worst case complicity specified. The intention was to
allow a median-of-three quicksort implementation, which is usually O(N
log N)
but can be quadratic for pathological inputs. However, there is
no longer any reason to allow implementers the freedom to have a
worst-cast-quadratic sort algorithm. Implementers who want to use
quicksort can use a variant like David Musser's "Introsort" (Software
Practice and Experience 27:983-993, 1997), which is guaranteed to be O(N
log N)
in the worst case without incurring additional overhead in the
average case. Most C++ library implementers already do this, and there
is no reason not to guarantee it in the standard.
Proposed resolution:
In 26.8.2.1 [sort], change the complexity to "O(N log N)", and remove footnote 266:
Complexity:
ApproximatelyO(N log(N)) (where N == last - first ) comparisonson the average.266)
266) If the worst case behavior is importantstable_sort()
(25.3.1.2) orpartial_sort()
(25.3.1.3) should be used.
search_n
complexity is too laxSection: 26.6.15 [alg.search] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity for search_n
(26.6.15 [alg.search] par 7) is specified as "At most
(last - first ) * count applications of the corresponding predicate if
count is positive, or 0 otherwise." This is unnecessarily pessimistic.
Regardless of the value of count, there is no reason to examine any
element in the range more than once.
Proposed resolution:
Change the complexity to "At most (last - first) applications of the corresponding predicate".
template<class ForwardIterator, class Size, class T> ForwardIterator search_n(ForwardIterator first , ForwardIterator last , Size count , const T& value ); template<class ForwardIterator, class Size, class T, class BinaryPredicate> ForwardIterator search_n(ForwardIterator first , ForwardIterator last , Size count , const T& value , BinaryPredicate pred );Complexity: At most
(last - first )
applications of the corresponding predicate* countif.count
is positive, or 0 otherwise
minmax_element
complexity is too laxSection: 26.8.9 [alg.min.max] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity for minmax_element
(26.8.9 [alg.min.max] par 16) says "At most max(2 *
(last - first ) - 2, 0)
applications of the corresponding comparisons",
i.e. the worst case complexity is no better than calling min_element
and
max_element
separately. This is gratuitously inefficient. There is a
well known technique that does better: see section 9.1 of CLRS
(Introduction to Algorithms, by Cormen, Leiserson, Rivest, and Stein).
Proposed resolution:
Change 26.8.9 [alg.min.max] to:
template<class ForwardIterator> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first , ForwardIterator last); template<class ForwardIterator, class Compare> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first , ForwardIterator last , Compare comp);Returns:
make_pair(m, M)
, wherem
isthe first iterator inmin_element(first, last)
ormin_element(first, last, comp)
[first, last)
such that no iterator in the range refers to a smaller element, and whereM
isthe last iterator inmax_element(first, last)
ormax_element(first, last, comp)
[first, last)
such that no iterator in the range refers to a larger element.Complexity: At most
max(2 * (last - first ) - 2, 0)
max(⌊(3/2) (N-1)⌋, 0)
applications of the correspondingcomparisonspredicate, whereN
isdistance(first, last)
.
Section: 28.6.12 [re.grammar] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2007-08-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
[tr.re.grammar]/3 and C++0x WP 28.6.12 [re.grammar]/3 say:
The following productions within the ECMAScript grammar are modified as follows:
CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] [ ^ ClassRanges ]
This definition for CharacterClass
appears to be exactly identical to that in ECMA-262.
Was an actual modification intended here and accidentally omitted, or was this production accidentally included?
[ Batavia (2009-05): ]
We agree that what is specified is identical to what ECMA-262 specifies. Pete would like to take a bit of time to assess whether we had intended, but failed, to make a change. It would also be useful to hear from John Maddock on the issue.
Move to Open.
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
Remove this mention of the CharacterClass production.
CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] [ ^ ClassRanges ]
std::is_literal
type traits should be providedSection: 21 [meta] Status: Resolved Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 750
Discussion:
Since the inclusion of constexpr
in the standard draft N2369 we have
a new type category "literal", which is defined in 6.8 [basic.types]/p.11:
-11- A type is a literal type if it is:
- a scalar type; or
a class type (clause 9) with
- a trivial copy constructor,
- a trivial destructor,
- at least one constexpr constructor other than the copy constructor,
- no virtual base classes, and
- all non-static data members and base classes of literal types; or
- an array of literal type.
I strongly suggest that the standard provides a type traits for literal types in 21.3.5.4 [meta.unary.prop] for several reasons:
The special problem of reason (c) is that I don't see currently a way to portably test the condition for literal class types:
- at least one constexpr constructor other than the copy constructor,
[ Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits. ]
[ 2009-07 Frankfurt: ]
Beman, Daniel, and Alisdair will work on a paper proposing new type traits.
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2984.
Proposed resolution:
In 21.3.3 [meta.type.synop] in the group "type properties", just below the line
template <class T> struct is_pod;
add a new one:
template <class T> struct is_literal;
In 21.3.5.4 [meta.unary.prop], table Type Property Predicates, just
below the line for the is_pod
property add a new line:
Template | Condition | Preconditions |
---|---|---|
template <class T> struct is_literal; |
T is a literal type (3.9) |
T shall be a complete type, an
array of unknown bound, or
(possibly cv-qualified) void . |
Section: 23.3.3 [array], 22.9.2 [template.bitset] Status: CD1 Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
bool array<T,N>::empty() const
should be a
constexpr
because this is easily to proof and to implement following it's operational
semantics defined by Table 87 (Container requirements) which says: a.size() == 0
.
bool bitset<N>::test() const
must be a
constexpr
(otherwise it would violate the specification of constexpr
bitset<N>::operator[](size_t) const
, because it's return clause delegates to test()
).
bitset<N>::bitset(unsigned long)
can
be declared as a constexpr
. Current implementations usually have no such bitset
c'tor which would fulfill the requirements of a constexpr
c'tor because they have a
non-empty c'tor body that typically contains for-loops or memcpy
to compute the
initialisation. What have I overlooked here?
[ Sophia Antipolis: ]
We handle this as two parts
- The proposed resolution is correct; move to ready.
- The issue points out a real problem, but the issue is larger than just this solution. We believe a paper is needed, applying the full new features of C++ (including extensible literals) to update
std::bitset
. We note that we do not consider this new work, and that is should be handled by the Library Working Group.In order to have a consistent working paper, Alisdair and Daniel produced a new wording for the resolution.
Proposed resolution:
In the class template definition of 23.3.3 [array]/p. 3 change
constexpr bool empty() const;
In the class template definition of 22.9.2 [template.bitset]/p. 1 change
constexpr bool test(size_t pos ) const;
and in 22.9.2.3 [bitset.members] change
constexpr bool test(size_t pos ) const;
nanf
and nanl
Section: 29.7 [c.math] Status: CD1 Submitter: Daniel Krügler Opened: 2007-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the listing of 29.7 [c.math], table 108: Header <cmath>
synopsis I miss
the following C99 functions (from 7.12.11.2):
float nanf(const char *tagp); long double nanl(const char *tagp);
(Note: These functions cannot be overloaded and they are also not listed anywhere else)
Proposed resolution:
In 29.7 [c.math], table 108, section "Functions", add nanf
and nanl
just after the existing entry nan
.
basic_regex
should be moveableSection: 28.6.7 [re.regex] Status: C++11 Submitter: Daniel Krügler Opened: 2007-08-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 316
According to the current state of the standard draft, the class
template basic_regex
, as described in 28.6.7 [re.regex]/3, is
neither MoveConstructible
nor MoveAssignable
.
IMO it should be, because typical regex state machines tend
to have a rather large data quantum and I have seen several
use cases, where a factory function returns regex values,
which would take advantage of moveabilities.
[ Sophia Antipolis: ]
Needs wording for the semantics, the idea is agreed upon.
[ Post Summit Daniel updated wording to reflect new "swap rules". ]
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
In the class definition of basic_regex
, just below 28.6.7 [re.regex]/3,
perform the following changes:
Just after basic_regex(const basic_regex&);
insert:
basic_regex(basic_regex&&);
Just after basic_regex& operator=(const basic_regex&);
insert:
basic_regex& operator=(basic_regex&&);
Just after basic_regex& assign(const basic_regex& that);
insert:
basic_regex& assign(basic_regex&& that);
In 28.6.7.2 [re.regex.construct], just after p.11 add the following new member definition:
basic_regex(basic_regex&& e);Effects: Move-constructs a
basic_regex
instance frome
.Postconditions:
flags()
andmark_count()
returne.flags()
ande.mark_count()
, respectively, thate
had before construction, leavinge
in a valid state with an unspecified value.Throws: nothing.
Also in 28.6.7.2 [re.regex.construct], just after p.18 add the following new member definition:
basic_regex& operator=(basic_regex&& e);Effects: Returns the result of
assign(std::move(e))
.
In 28.6.7.3 [re.regex.assign], just after p. 2 add the following new member definition:
basic_regex& assign(basic_regex&& rhs);Effects: Move-assigns a
basic_regex
instance fromrhs
and returns*this
.Postconditions:
flags()
andmark_count()
returnrhs.flags()
andrhs.mark_count()
, respectively, thatrhs
had before assignment, leavingrhs
in a valid state with an unspecified value.Throws: nothing.
DefaultConstructible
is not definedSection: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Pablo Halpern Opened: 2007-09-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
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Discussion:
The DefaultConstructible
requirement is referenced in
several places in the August 2007 working draft
N2369,
but is not defined anywhere.
[ Bellevue: ]
Walking into the default/value-initialization mess...
Why two lines? Because we need both expressions to be valid.
AJM not sure what the phrase "default constructed" means. This is unfortunate, as the phrase is already used 24 times in the library!
Example:
const int
would not accept first line, but will accept the second.This is an issue that must be solved by concepts, but we might need to solve it independantly first.
It seems that the requirements are the syntax in the proposed first column is valid, but not clear what semantics we need.
A table where there is no post-condition seems odd, but appears to sum up our position best.
At a minimum an object is declared and is destructible.
Move to open, as no-one happy to produce wording on the fly.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-08-17 Daniel adds "[defaultconstructible]" to table title. 408(i) depends upon this issue. ]
[ 2009-08-18 Alisdair adds: ]
Looking at the proposed table in this issue, it really needs two rows:
Table 33: DefaultConstructible
requirements [defaultconstructible]expression post-condition T t;
t
is default-initialized.T{}
Object of type T
is value-initialized.Note I am using the new brace-initialization syntax that is unambiguous in all use cases (no most vexing parse.)
[ 2009-10-03 Daniel adds: ]
The suggested definition
T{}
describing it as value-initialization is wrong, because it belongs to list-initialization which would - as the current rules are - always prefer a initializer-list constructor over a default-constructor. I don't consider this as an appropriate definition ofDefaultConstructible
. My primary suggestion is to ask core, whether the special caseT{}
(which also easily leads to ambiguity situations for more than one initializer-list in a class) would always prefer a default-constructor - if any - before considering an initializer-list constructor or to provide another syntax form to prefer value-initialization over list-initialization. If that fails I would fall back to suggest to use the expressionT()
instead ofT{}
with all it's disadvantages for the meaning of the expressionT t();
[ 2009-10 Santa Cruz: ]
Leave Open. Core is looking to make Alisdair's proposed resolution correct.
[ 2010-01-24 At Alisdair's request, moved his proposal into the proposed wording section. The old wording is preserved here: ]
In section 16.4.4.2 [utility.arg.requirements], before table 33, add the following table:
Table 33:
DefaultConstructible
requirements [defaultconstructible]
expression
post-condition
T t;
T()
T
is default constructed.
[ 2010-02-04: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Rationale:
[ San Francisco: ]
We believe concepts will solve this problem (N2774).
[ Rationale is obsolete. ]
Proposed resolution:
In section 16.4.4.2 [utility.arg.requirements], before table 33, add the following table:
Table 33: DefaultConstructible
requirements [defaultconstructible]expression post-condition T t;
Object t
is default-initialized.T u{};
Object u
is value-initialized.T()
T{}A temporary object of type T
is value-initialized.
regex_replace()
doesn't accept basic_string
s with custom traits and allocatorsSection: 28.6.10.4 [re.alg.replace] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
regex_match()
and regex_search()
take const basic_string<charT, ST,
SA>&
. regex_replace()
takes const basic_string<charT>&
. This prevents
regex_replace()
from accepting basic_string
s with custom traits and
allocators.
Overloads of regex_replace()
taking basic_string
should be additionally
templated on class ST, class SA
and take const basic_string<charT, ST,
SA>&
. Consistency with regex_match()
and regex_search()
would place
class ST, class SA
as the first template arguments; compatibility with
existing code using TR1 and giving explicit template arguments to
regex_replace()
would place class ST, class SA
as the last template
arguments.
[ Batavia (2009-05): ]
Bill comments, "We need to look at the depth of this change."
Pete remarks that we are here dealing with a convenience function that saves a user from calling the iterato-based overload.
Move to Open.
[ 2009-07 Frankfurt: ]
Howard to ask Stephan Lavavej to provide wording.
[ 2009-07-17 Stephan provided wording. ]
[ 2009-07-25 Daniel tweaks both this issue and 726(i). ]
One relevant part of the proposed resolution below suggests to add a new overload of the format member function in the
match_results
class template that accepts two character pointers defining thebegin
andend
of a format range. A more general approach could have proposed a pair of iterators instead, but the used pair of char pointers reflects existing practice. If the committee strongly favors an iterator-based signature, this could be simply changed. I think that the minimum requirement should be aBidirectionalIterator
, but current implementations take advantage (at least partially) of theRandomAccessIterator
sub interface of the char pointers.Suggested Resolution:
[Moved into the proposed resloution]
[ 2009-07-30 Stephan agrees with Daniel's wording. Howard places Daniel's wording in the Proposed Resolution. ]
[ 2009-10 Santa Cruz: ]
Move to Review. Chair is anxious to move this to Ready in Pittsburgh.
[ 2010-01-27 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 28.6.3 [re.syn] as indicated:
// 28.11.4, function template regex_replace: template <class OutputIterator, class BidirectionalIterator, class traits, class charT, class ST, class SA> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT, class ST, class SA, class FST, class FSA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const basic_string<charT, FST, FSA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT, class ST, class SA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT, class ST, class SA> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);
Change 28.6.9 [re.results]/3, class template match_results
as
indicated:
template <class OutputIter> OutputIter format(OutputIter out, const char_type* fmt_first, const char_type* fmt_last, regex_constants::match_flag_type flags = regex_constants::format_default) const; template <class OutputIter, class ST, class SA> OutputIter format(OutputIter out, conststring_typebasic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; template <class ST, class SA>string_typebasic_string<char_type, ST, SA> format(conststring_typebasic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; string_type format(const char_type* fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;
Insert at the very beginning of 28.6.9.6 [re.results.form] the following:
template <class OutputIter> OutputIter format(OutputIter out, const char_type* fmt_first, const char_type* fmt_last, regex_constants::match_flag_type flags = regex_constants::format_default) const;1 Requires: The type
OutputIter
shall satisfy the requirements for an Output Iterator (24.3.5.4 [output.iterators]).2 Effects: Copies the character sequence
[fmt_first,fmt_last)
toOutputIter out
. Replaces each format specifier or escape sequence in the copied range with either the character(s) it represents or the sequence of characters within*this
to which it refers. The bitmasks specified inflags
determine which format specifiers and escape sequences are recognized.3 Returns:
out
.
Change 28.6.9.6 [re.results.form], before p. 1 until p. 3 as indicated:
template <class OutputIter, class ST, class SA> OutputIter format(OutputIter out, conststring_typebasic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;
1 Requires: The typeOutputIter
shall satisfy the requirements for an Output Iterator (24.2.3).2 Effects:
Copies the character sequenceEquivalent to[fmt.begin(),fmt.end())
toOutputIter out
. Replaces each format specifier or escape sequence infmt
with either the character(s) it represents or the sequence of characters within*this
to which it refers. The bitmasks specified inflags
determines what format specifiers and escape sequences are recognizedreturn format(out, fmt.data(), fmt.data() + fmt.size(), flags)
.
3 Returns:out
.
Change 28.6.9.6 [re.results.form], before p. 4 until p. 4 as indicated:
template <class ST, class SA>string_typebasic_string<char_type, ST, SA> format(conststring_typebasic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;Effects:
Returns a copy of the stringConstructs an empty stringfmt
. Replaces each format specifier or escape sequence infmt
with either the character(s) it represents or the sequence of characters within*this
to which it refers. The bitmasks specified in flags determines what format specifiers and escape sequences are recognized.result
of typebasic_string<char_type, ST, SA>
, and callsformat(back_inserter(result), fmt, flags)
.Returns:
result
At the end of 28.6.9.6 [re.results.form] insert as indicated:
string_type format(const char_type* fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;Effects: Constructs an empty string
result
of typestring_type
, and callsformat(back_inserter(result), fmt, fmt + char_traits<char_type>::length(fmt), flags)
.Returns:
result
Change 28.6.10.4 [re.alg.replace] before p. 1 as indicated:
template <class OutputIterator, class BidirectionalIterator, class traits, class charT, class ST, class SA> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);Effects: [..]. If any matches are found then, for each such match, if
!(flags & regex_constants::format_no_copy)
callsstd::copy(m.prefix().first, m.prefix().second, out)
, and then callsm.format(out, fmt, flags)
for the first form of the function andm.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags)
for the second form. [..].
Change 28.6.10.4 [re.alg.replace] before p. 3 as indicated:
template <class traits, class charT, class ST, class SA, class FST, class FSA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const basic_string<charT, FST, FSA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT, class ST, class SA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);Effects: Constructs an empty string
result
of typebasic_string<charT, ST, SA>
, callsregex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags)
, and then returnsresult
.
At the end of 28.6.10.4 [re.alg.replace] add the following new prototype description:
template <class traits, class charT, class ST, class SA> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);Effects: Constructs an empty string
result
of typebasic_string<charT>
, callsregex_replace(back_inserter(result), s, s + char_traits<charT>::length(s), e, fmt, flags)
, and then returnsresult
.
Section: 29.5.4.3 [rand.eng.mers] Status: CD1 Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.eng.mers].
View all issues with CD1 status.
Discussion:
The mersenne_twister_engine
is required to use a seeding method that is given
as an algorithm parameterized over the number of bits W
. I doubt whether the given generalization
of an algorithm that was originally developed only for unsigned 32-bit integers is appropriate
for other bit widths. For instance, W
could be theoretically 16 and UIntType
a 16-bit integer, in
which case the given multiplier would not fit into the UIntType
. Moreover, T. Nishimura and M.
Matsumoto have chosen a dif ferent multiplier for their 64 bit Mersenne Twister
[reference].
I see two possible resolutions:
W
of the mersenne_twister_template
to values of 32 or 64 and use the
multiplier from [the above reference] for the 64-bit case (my preference)W
as a 32-bit array of appropriate length (and a specified byte
order) and always employ the 32-bit algorithm for seeding
See N2424 for further discussion.
[ Bellevue: ]
Stephan Tolksdorf has additional comments on N2424. He comments: "there is a typo in the required behaviour for mt19937_64: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025)." These values need checking.
Take the proposed recommendation in N2424 and move to REVIEW.
Proposed resolution:
See N2424 for the proposed resolution.
[ Stephan Tolksdorf adds pre-Bellevue: ]
I support the proposed resolution in N2424, but there is a typo in the required behaviour for
mt19937_64
: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025). The change to para. 8 proposed by Charles Karney should also be included in the proposed wording.
[ Sophia Antipolis: ]
Note the main part of the issue is resolved by N2424.
Section: 99 [rand.dist.samp.genpdf] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.genpdf].
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Duplicate of: 795
Discussion:
99 [rand.dist.samp.genpdf] describes the interface for a distribution template that is meant to simulate random numbers from any general distribution given only the density and the support of the distribution. I'm not aware of any general purpose algorithm that would be capable of correctly and efficiently implementing the described functionality. From what I know, this is essentially an unsolved research problem. Existing algorithms either require more knowledge about the distribution and the problem domain or work only under very limited circumstances. Even the state of the art special purpose library UNU.RAN does not solve the problem in full generality, and in any case, testing and customer support for such a library feature would be a nightmare.
Possible resolution: For these reasons, I propose to delete section 99 [rand.dist.samp.genpdf].
[ Bellevue: ]
Disagreement persists.
Objection to this issue is that this function takes a general functor. The general approach would be to normalize this function, integrate it, and take the inverse of the integral, which is not possible in general. An example function is sin(1+n*x) — for any spatial frequency that the implementor chooses, there is a value of n that renders that choice arbitrarily erroneous.
Correction: The formula above should instead read 1+sin(n*x).
Objector proposes the following possible compromise positions:
- rand.dist.samp.genpdf takes an number of points so that implementor need not guess.
- replace rand.disk.samp.genpdf with an extension to either or both of the discrete functions to take arguments that take a functor and number of points in place of the list of probabilities. Reference issues 793 and 794.
Proposed resolution:
See N2813 for the proposed resolution.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
Section: 29.5.9.5.3 [rand.dist.norm.chisq] Status: CD1 Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
chi_squared_distribution
, fisher_f_distribution
and student_t_distribution
have parameters for the "degrees of freedom" n
and m
that are specified as integers. For the
following two reasons this is an unnecessary restriction: First, in many applications such as
Bayesian inference or Monte Carlo simulations it is more convenient to treat the respective param-
eters as continuous variables. Second, the standard non-naive algorithms (i.e.
O(1) algorithms) for simulating from these distributions work with floating-point parameters anyway (all
three distributions could be easily implemented using the Gamma distribution, for instance).
Similar arguments could in principle be made for the parameters t
and k
of the discrete
binomial_distribution
and negative_binomial_distribution
, though in both cases continuous
parameters are less frequently used in practice and in case of the binomial_distribution
the implementation would be significantly complicated by a non-discrete parameter (in most
implementations one would need an approximation of the log-gamma function instead of just the
log-factorial function).
Possible resolution: For these reasons, I propose to change the type of the respective parameters to double.
[ Bellevue: ]
In N2424. Not wildly enthusiastic, not really felt necessary. Less frequently used in practice. Not terribly bad either. Move to OPEN.
[ Sophia Antipolis: ]
Marc Paterno: The generalizations were explicitly left out when designing the facility. It's harder to test.
Marc Paterno: Ask implementers whether floating-point is a significant burden.
Alisdair: It's neater to do it now, do ask Bill Plauger.
Disposition: move to review with the option for "NAD" if it's not straightforward to implement; unanimous consent.
Proposed resolution:
See N2424 for the proposed resolution.
[ Stephan Tolksdorf adds pre-Bellevue: ]
In 29.5.9.5.3 [rand.dist.norm.chisq]:
Delete ", where
n
is a positive integer" in the first paragraph.Replace both occurrences of "
explicit chi_squared_distribution(int n = 1);
" with "explicit chi_squared_distribution(RealType n = 1);
".Replace both occurrences of "
int n() const;
" with "RealType n() const;
".In 29.5.9.5.5 [rand.dist.norm.f]:
Delete ", where
m
andn
are positive integers" in the first paragraph.Replace both occurrences of
explicit fisher_f_distribution(int m = 1, int n = 1);with
explicit fisher_f_distribution(RealType m = 1, RealType n = 1);Replace both occurrences of "
int m() const;" with "RealType m() const;
".Replace both occurrences of "
int n() const;" with "RealType n() const;
".In 29.5.9.5.6 [rand.dist.norm.t]:
Delete ", where
n
is a positive integer" in the first paragraph.Replace both occurrences of "
explicit student_t_distribution(int n = 1);
" with "explicit student_t_distribution(RealType n = 1);
".Replace both occurrences of "
int n() const;
" with "RealType n() const;
".
*_ptr<T[N]>
Section: 20.3.1 [unique.ptr] Status: CD1 Submitter: Herb Sutter Opened: 2007-10-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr].
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Discussion:
Please don't provide *_ptr<T[N]>
. It doesn't enable any useful
bounds-checking (e.g., you could imagine that doing op++
on a
shared_ptr<T[N]>
yields a shared_ptr<T[N-1]>
, but that promising path
immediately falters on op--
which can't reliably dereference because we
don't know the lower bound). Also, most buffers you'd want to point to
don't have a compile-time known size.
To enable any bounds-checking would require run-time information, with
the usual triplet: base (lower bound), current offset, and max offset
(upper bound). And I can sympathize with the point of view that you
wouldn't want to require this on *_ptr
itself. But please let's not
follow the <T[N]>
path, especially not with additional functions to
query the bounds etc., because this sets wrong user expectations by
embarking on a path that doesn't go all the way to bounds checking as it
seems to imply.
If bounds checking is desired, consider a checked_*_ptr
instead (e.g.,
checked_shared_ptr
). And make the interfaces otherwise identical so that
user code could easily #define/typedef
between prepending checked_
on
debug builds and not doing so on release builds (for example).
Note that some may object that checked_*_ptr
may seem to make the smart
pointer more like vector
, and we don't want two ways to spell vector
. I
don't agree, but if that were true that would be another reason to
remove *_ptr<T[N]>
which equally makes the smart pointer more like
std::array.
:-)
[ Bellevue: ]
Suggestion that fixed-size array instantiations are going to fail at compile time anyway (if we remove specialization) due to pointer decay, at least that appears to be result from available compilers.
So concerns about about requiring static_assert seem unfounded.
After a little more experimentation with compiler, it appears that fixed size arrays would only work at all if we supply these explicit specialization. So removing them appears less breaking than originally thought.
straw poll unanimous move to Ready.
Proposed resolution:
Change the synopsis under 20.3.1 [unique.ptr] p2:
... template<class T> struct default_delete; template<class T> struct default_delete<T[]>;template<class T, size_t N> struct default_delete<T[N]>;template<class T, class D = default_delete<T>> class unique_ptr; template<class T, class D> class unique_ptr<T[], D>;template<class T, class D, size_t N> class unique_ptr<T[N], D>;...
Remove the entire section [unique.ptr.dltr.dflt2] default_delete<T[N]>
.
Remove the entire section [unique.ptr.compiletime] unique_ptr
for array objects with a compile time length
and its subsections: [unique.ptr.compiletime.dtor], [unique.ptr.compiletime.observers],
[unique.ptr.compiletime.modifiers].
swap
for proxy iteratorsSection: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
View all issues with Resolved status.
Discussion:
This issue was split from 672(i). 672(i) now just
deals with changing the requirements of T
in the Swappable
requirement from CopyConstructible
and CopyAssignable
to
MoveConstructible
and MoveAssignable
.
This issue seeks to widen the Swappable
requirement to support proxy iterators. Here
is example code:
namespace Mine { template <class T> struct proxy {...}; template <class T> struct proxied_iterator { typedef T value_type; typedef proxy<T> reference; reference operator*() const; ... }; struct A { // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable void swap(A&); ... }; void swap(A&, A&); void swap(proxy<A>, A&); void swap(A&, proxy<A>); void swap(proxy<A>, proxy<A>); } // Mine ... Mine::proxied_iterator<Mine::A> i(...) Mine::A a; swap(*i1, a);
The key point to note in the above code is that in the call to swap
, *i1
and a
are different types (currently types can only be Swappable
with the
same type). A secondary point is that to support proxies, one must be able to pass rvalues
to swap
. But note that I am not stating that the general purpose std::swap
should accept rvalues! Only that overloaded swap
s, as in the example above, be allowed
to take rvalues.
That is, no standard library code needs to change. We simply need to have a more flexible
definition of Swappable
.
[ Bellevue: ]
While we believe Concepts work will define a swappable concept, we should still resolve this issue if possible to give guidance to the Concepts work.
Would an ambiguous swap function in two namespaces found by ADL break this wording? Suggest that the phrase "valid expression" means such a pair of types would still not be swappable.
Motivation is proxy-iterators, but facility is considerably more general. Are we happy going so far?
We think this wording is probably correct and probably an improvement on what's there in the WP. On the other hand, what's already there in the WP is awfully complicated. Why do we need the two bullet points? They're too implementation-centric. They don't add anything to the semantics of what swap() means, which is there in the post-condition. What's wrong with saying that types are swappable if you can call swap() and it satisfies the semantics of swapping?
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Leave as Open. Dave to provide wording.
[ 2009-11-08 Howard adds: ]
Updated wording to sync with N3000. Also this issue is very closely related to 594(i).
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added.
Rationale:
Solved by N3048.
Proposed resolution:
Change 16.4.4.2 [utility.arg.requirements]:
-1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables,
T
andV
areis atypes to be supplied by a C++ program instantiating a template;a
,b
, andc
are values of typeconst T
;s
andt
are modifiable lvalues of typeT
;u
is a value of type (possiblyconst
)T
;andrv
is a non-const
rvalue of typeT
;w
is a value of typeT
; andv
is a value of typeV
.
Table 37: Swappable
requirements [swappable]expression Return type Post-condition swap(
sw,tv)void
t
w
has the value originally held byu
v
, andu
v
has the value originally held byt
w
The
Swappable
requirement is met by satisfying one or more of the following conditions:
T
isSwappable
ifT
andV
are the same type andT
satisfies theMoveConstructible
requirements (Table 33) and theMoveAssignable
requirements (Table 35);T
isSwappable
withV
if a namespace scope function namedswap
exists in the same namespace as the definition ofT
orV
, such that the expressionswap(
is valid and has the semantics described in this table.sw,tv)T
isSwappable
ifT
is an array type whose element type isSwappable
.
Rationale:
[ post San Francisco: ]
Solved by N2758.
swap
for shared_ptr
Section: 20.3.2.2.9 [util.smartptr.shared.spec] Status: CD1 Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
When the LWG looked at 674(i) in Kona the following note was made:
We may need to open an issue to deal with the question of whether
shared_ptr
needs an rvalueswap
.
This issue was opened in response to that note.
I believe allowing rvalue shared_ptr
s to swap
is both
appropriate, and consistent with how other library components are currently specified.
[ Bellevue: ]
Concern that the three signatures for swap is needlessly complicated, but this issue merely brings shared_ptr into equal complexity with the rest of the library. Will open a new issue for concern about triplicate signatures.
Adopt issue as written.
Proposed resolution:
Change the synopsis in 20.3.2.2 [util.smartptr.shared]:
void swap(shared_ptr&& r); ... template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);
Change 20.3.2.2.5 [util.smartptr.shared.mod]:
void swap(shared_ptr&& r);
Change 20.3.2.2.9 [util.smartptr.shared.spec]:
template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);
exception_ptr
?Section: 17.9.7 [propagation] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
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Discussion:
Without some lifetime guarantee, it is hard to know how this type can be
used. Very specifically, I don't see how the current wording would
guarantee and exception_ptr
caught at the end of one thread could be safely
stored and rethrown in another thread - the original motivation for this
API.
(Peter Dimov agreed it should be clearer, maybe a non-normative note to explain?)
[ Bellevue: ]
Agree the issue is real.
Intent is lifetime is similar to a shared_ptr (and we might even want to consider explicitly saying that it is a shared_ptr< unspecified type >).
We expect that most implementations will use shared_ptr, and the standard should be clear that the exception_ptr type is intended to be something whose semantics are smart-pointer-like so that the user does not need to worry about lifetime management. We still need someone to draught those words - suggest emailing Peter Dimov.
Move to Open.
Proposed resolution:
Change 17.9.7 [propagation]/7:
-7- Returns: An
exception_ptr
object that refers to the currently handled exception or a copy of the currently handled exception, or a nullexception_ptr
object if no exception is being handled. The referenced object remains valid at least as long as there is anexception_ptr
that refers to it. If the function needs to allocate memory and the attempt fails, it returns anexception_ptr
object that refers to an instance ofbad_alloc
. It is unspecified whether the return values of two successive calls tocurrent_exception
refer to the same exception object. [Note: that is, it is unspecified whethercurrent_exception
creates a new copy each time it is called. --end note]
current_exception
may fail with bad_alloc
Section: 17.9.7 [propagation] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I understand that the attempt to copy an exception may run out of memory,
but I believe this is the only part of the standard that mandates failure
with specifically bad_alloc
, as opposed to allowing an
implementation-defined type derived from bad_alloc
. For instance, the Core
language for a failed new expression is:
Any other allocation function that fails to allocate storage shall indicate failure only by throwing an exception of a type that would match a handler (15.3) of type
std::bad_alloc
(18.5.2.1).
I think we should allow similar freedom here (or add a blanket compatible-exception freedom paragraph in 17)
I prefer the clause 17 approach myself, and maybe clean up any outstanding wording that could also rely on it.
Although filed against a specific case, this issue is a problem throughout the library.
[ Bellevue: ]
Is issue bigger than library?
No - Core are already very clear about their wording, which is inspiration for the issue.
While not sold on the original 18.7.5 use case, the generalised 17.4.4.8 wording is the real issue.
Accept the broad view and move to ready
Proposed resolution:
Add the following exemption clause to 16.4.6.14 [res.on.exception.handling]:
A function may throw a type not listed in its Throws clause so long as it is derived from a class named in the Throws clause, and would be caught by an exception handler for the base type.
has_nothrow_copy_constructor<T>::value
is true if T
has 'a' nothrow copy constructor.Section: 21.3.5.4 [meta.unary.prop] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
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Discussion:
Unfortunately a class can have multiple copy constructors, and I believe to be useful this trait should only return true is ALL copy constructors are no-throw.
For instance:
struct awkward { awkward( const awkward & ) throw() {} awkward( awkward & ) { throw "oops"; } };
Proposed resolution:
Change 21.3.5.4 [meta.unary.prop]:
has_trivial_copy_constructor
T
is a trivial type (3.9) or a reference type or a class typewith a trivial copy constructorwhere all copy constructors are trivial (12.8).
has_trivial_assign
T
is neitherconst
nor a reference type, andT
is a trivial type (3.9) or a class typewith a trivial copy assignment operatorwhere all copy assignment operators are trivial (12.8).
has_nothrow_copy_constructor
has_trivial_copy_constructor<T>::value
istrue
orT
is a class typewith awhere all copy constructorsthat isare known not to throw any exceptions orT
is an array of such a class type
has_nothrow_assign
T
is neitherconst
nor a reference type, andhas_trivial_assign<T>::value
istrue
orT
is a class typewith awhere all copy assignment operators takeingan lvalue of typeT
that is known not to throw any exceptions orT
is an array of such a class type.
Section: 16.4.4.6 [allocator.requirements] Status: C++11 Submitter: Hans Boehm Opened: 2007-10-11 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
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Discussion:
Did LWG recently discuss 16.4.4.6 [allocator.requirements]-2, which states that "All the operations on the allocators are expected to be amortized constant time."?
As I think I pointed out earlier, this is currently fiction for
allocate()
if it has to obtain memory from the OS, and it's unclear to
me how to interpret this for construct()
and destroy()
if they deal with
large objects. Would it be controversial to officially let these take
time linear in the size of the object, as they already do in real life?
Allocate()
more blatantly takes time proportional to the size of the
object if you mix in GC. But it's not really a new problem, and I think
we'd be confusing things by leaving the bogus requirements there. The
current requirement on allocate()
is generally not important anyway,
since it takes O(size) to construct objects in the resulting space.
There are real performance issues here, but they're all concerned with
the constants, not the asymptotic complexity.
Proposed resolution:
Change 16.4.4.6 [allocator.requirements]/2:
-2- Table 39 describes the requirements on types manipulated through allocators.
All the operations on the allocators are expected to be amortized constant time.Table 40 describes the requirements on allocator types.
Section: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Yechezkel Mett Opened: 2007-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The draft standard n2369 uses the term move constructor in a few places, but doesn't seem to define it.
MoveConstructible
requirements are defined in Table 33 in 16.4.4.2 [utility.arg.requirements] as
follows:
MoveConstructible
requirementsexpression post-condition T t = rv
t
is equivalent to the value ofrv
before the construction[Note: There is no requirement on the value of rv
after the construction. -- end note]
(where rv
is a non-const rvalue of type T
).
So I assume the move constructor is the constructor that would be used in filling the above requirement.
For vector::reserve
, vector::resize
and the vector
modifiers given in
23.3.13.5 [vector.modifiers] we have
Requires: If
value_type
has a move constructor, that constructor shall not throw any exceptions.
Firstly "If value_type
has a move constructor" is superfluous; every
type which can be put into a vector
has a move constructor (a copy
constructor is also a move constructor). Secondly it means that for
any value_type
which has a throwing copy constructor and no other move
constructor these functions cannot be used -- which I think will come
as a shock to people who have been using such types in vector
until
now!
I can see two ways to correct this. The simpler, which is presumably
what was intended, is to say "If value_type
has a move constructor and
no copy constructor, the move constructor shall not throw any
exceptions" or "If value_type
has a move constructor which changes the
value of its parameter,".
The other alternative is add to MoveConstructible
the requirement that
the expression does not throw. This would mean that not every type
that satisfies the CopyConstructible
requirements also satisfies the
MoveConstructible
requirements. It would mean changing requirements in
various places in the draft to allow either MoveConstructible
or
CopyConstructible
, but I think the result would be clearer and
possibly more concise too.
Proposed resolution:
Add new defintions to [definitions]:
move constructor
a constructor which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the construction.
move assignment operator
an assignment operator which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the assignment.
move assignment
use of the move assignment operator.
[ Howard adds post-Bellevue: ]
Unfortunately I believe the wording recommended by the LWG in Bellevue is incorrect.
reserve
et. al. will use a move constructor if one is available, else it will use a copy constructor. A type may have both. If the move constructor is used, it must not throw. If the copy constructor is used, it can throw. The sentence in the proposed wording is correct without the recommended insertion. The Bellevue LWG recommended moving this issue to Ready. I am unfortunately pulling it back to Open. But I'm drafting wording to atone for this egregious action. :-)
std::vector
and std:string
lack explicit shrink-to-fit operationsSection: 23.3.13.3 [vector.capacity], 27.4.3.5 [string.capacity] Status: CD1 Submitter: Beman Dawes Opened: 2007-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [vector.capacity].
View all other issues in [vector.capacity].
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Discussion:
A std::vector
can be shrunk-to-fit via the swap idiom:
vector<int> v; ... v.swap(vector<int>(v)); // shrink to fitor:
vector<int>(v).swap(v); // shrink to fitor:
swap(v, vector<int>(v)); // shrink to fit
A non-binding request for shrink-to-fit can be made to a std::string
via:
string s; ... s.reserve(0);
Neither of these is at all obvious to beginners, and even some experienced C++ programmers are not aware that shrink-to-fit is trivially available.
Lack of explicit functions to perform these commonly requested operations makes vector and string less usable for non-experts. Because the idioms are somewhat obscure, code readability is impaired. It is also unfortunate that two similar vector-like containers use different syntax for the same operation.
The proposed resolution addresses these concerns. The proposed function takes no arguments to keep the solution simple and focused.
Proposed resolution:
To Class template basic_string 27.4.3 [basic.string] synopsis, Class template vector 23.3.13 [vector] synopsis, and Class vector<bool> 23.3.14 [vector.bool] synopsis, add:
void shrink_to_fit();
To basic_string capacity 27.4.3.5 [string.capacity] and vector capacity 23.3.13.3 [vector.capacity], add:
void shrink_to_fit();Remarks:
shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]
[
850(i) has been added to deal with this issue with respect to deque
.
]
Section: 23.6 [container.adaptors] Status: Resolved Submitter: Paolo Carlini Opened: 2007-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
After n2369 we have a single push_back
overload in the sequence containers,
of the "emplace" type. At variance with that, still in n2461, we have
two separate overloads, the C++03 one + one taking an rvalue reference
in the container adaptors. Therefore, simply from a consistency point of
view, I was wondering whether the container adaptors should be aligned
with the specifications of the sequence container themselves: thus have
a single push
along the lines:
template<typename... _Args> void push(_Args&&... __args) { c.push_back(std::forward<_Args>(__args)...); }
Proposed resolution:
Change 23.6.3.1 [queue.defn]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Change 23.6.4 [priority.queue]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Change 23.6.4.4 [priqueue.members]:
void push(const value_type& x);
Effects:c.push_back(x);push_heap(c.begin(), c.end(), comp);template<class... Args> void push(value_typeArgs&&...xargs);Effects:
c.push_back(std::moveforward<Args>(xargs)...); push_heap(c.begin(), c.end(), comp);
Change 23.6.6.2 [stack.defn]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Rationale:
Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).
shared_ptr
and nullptr
Section: 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Joe Gottman Opened: 2007-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
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Discussion:
Consider the following program:
int main() { shared_ptr<int> p(nullptr); return 0; }
This program will fail to compile because shared_ptr
uses the following
template constructor to construct itself from pointers:
template <class Y> shared_ptr(Y *);
According
to N2431,
the conversion from nullptr_t
to Y *
is not
deducible, so the above constructor will not be found. There are similar problems with the
constructors that take a pointer and a deleter
or a
pointer, a deleter
and an allocator, as well as the
corresponding forms of reset()
. Note that N2435
will solve this problem for constructing from just nullptr
, but not for constructors that use
deleters
or allocators or for reset()
.
In the case of the functions that take deleters, there is the additional
question of what argument should be passed to the deleter when it is
eventually called. There are two reasonable possibilities: nullptr
or
static_cast<T *>(0)
, where T
is the template argument of the
shared_ptr
. It is not immediately clear which of these is better. If
D::operator()
is a template function similar to shared_ptr
's
constructor, then d(static_cast<T*>(0))
will compile and d(nullptr)
will not. On the other hand, if D::operator()()
takes a parameter that
is a pointer to some type other that T
(for instance U*
where U
derives
from T
) then d(nullptr)
will compile and d(static_cast<T *>(0))
may not.
[ Bellevue: ]
The general idea is right, we need to be able to pass a nullptr to a shared_ptr, but there are a few borderline editorial issues here. (For example, the single-argument nullptr_t constructor in the class synopsis isn't marked explicit, but it is marked explicit in the proposed wording for 20.6.6.2.1. There is a missing empty parenthesis in the form that takes a nullptr_t, a deleter, and an allocator.)
More seriously: this issue says that a shared_ptr constructed from a nullptr is empty. Since "empty" is undefined, it's hard to know whether that's right. This issue is pending on handling that term better.
Peter suggests definition of empty should be "does not own anything"
Is there an editorial issue that post-conditions should refer to get() = nullptr, rather than get() = 0?
No strong feeling towards accept or NAD, but prefer to make a decision than leave it open.
Seems there are no technical merits between NAD and Ready, comes down to "Do we intentially want to allow/disallow null pointers with these functions". Staw Poll - support null pointers 5 - No null pointers 0
Move to Ready, modulo editorial comments
[ post Bellevue Peter adds: ]
The following wording changes are less intrusive:
In 20.3.2.2.2 [util.smartptr.shared.const], add:
shared_ptr(nullptr_t);after:
shared_ptr();(Absence of explicit intentional.)
px.reset( nullptr )
seems a somewhat contrived way to writepx.reset()
, so I'm not convinced of its utility.It's similarly not clear to me whether the deleter constructors need to be extended to take
nullptr
, but if they need to:Add
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);Note that this changes the semantics of the new constructors such that they consistently call
d(p)
instead ofd((T*)0)
whenp
isnullptr
.The ability to be able to pass
0/NULL
to a function that takes ashared_ptr
has repeatedly been requested by users, but the other additions that the proposed resolution makes are not supported by real world demand or motivating examples.It might be useful to split the obvious and non-controversial
nullptr_t
constructor into a separate issue. Waiting for "empty" to be clarified is unnecessary; this is effectively an alias for the default constructor.
[ Sophia Antipolis: ]
We want to remove the reset functions from the proposed resolution.
The remaining proposed resolution text (addressing the constructors) are wanted.
Disposition: move to review. The review should check the wording in the then-current working draft.
Proposed resolution:
In 20.3.2.2 [util.smartptr.shared] p4, add to the definition/synopsis
of shared_ptr
:
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
In 20.3.2.2.2 [util.smartptr.shared.const] add:
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
(reusing the following paragraphs 20.3.2.2.2 [util.smartptr.shared.const]/9-13 that speak of p.)
In 20.3.2.2.2 [util.smartptr.shared.const]/10, change
Effects: Constructs a
shared_ptr
object that owns thepointerobjectp
and the deleterd
. The second constructor shall use a copy ofa
to allocate memory for internal use.
Rationale:
[ San Francisco: ]
"pointer" is changed to "object" to handle the fact that
nullptr_t
isn't a pointer.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Jens Maurer Opened: 2007-11-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
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Discussion:
23.2 [container.requirements] says:
-12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.
A reference is not an object, but this sentence appears to claim so.
What is probably meant here:
An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.
Proposed resolution:
Change 23.2 [container.requirements]:
-12-
Objects passed to member functions of a container as rvalue references shall not be elementsAn object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container.;Nno diagnostic required.
unordered_map
needs an at()
member functionSection: 23.5.3.3 [unord.map.elem] Status: CD1 Submitter: Joe Gottman Opened: 2007-11-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The new member function at()
was recently added to std::map()
. It acts
like operator[]()
, except it throws an exception when the input key is
not found. It is useful when the map
is const
, the value_type
of the
key doesn't have a default constructor, it is an error if the key is
not found, or the user wants to avoid accidentally adding an element to
the map. For exactly these same reasons, at()
would be equally useful
in std::unordered_map
.
Proposed resolution:
Add the following functions to the definition of unordered_map
under "lookup" (23.5.3 [unord.map]):
mapped_type& at(const key_type& k); const mapped_type &at(const key_type &k) const;
Add the following definitions to 23.5.3.3 [unord.map.elem]:
mapped_type& at(const key_type& k); const mapped_type &at(const key_type &k) const;Returns: A reference to
x.second
, wherex
is the (unique) element whose key is equivalent tok
.Throws: An exception object of type
out_of_range
if no such element is present.
[ Bellevue: Editorial note: the "(unique)" differs from map. ]
std::unique_ptr
requires complete type?Section: 20.3.1 [unique.ptr] Status: CD1 Submitter: Daniel Krügler Opened: 2007-11-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr].
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Discussion:
In contrast to the proposed std::shared_ptr
, std::unique_ptr
does currently not support incomplete types, because it
gives no explicit grant - thus instantiating unique_ptr
with
an incomplete pointee type T
automatically belongs to
undefined behaviour according to 16.4.5.8 [res.on.functions]/2, last
bullet. This is an unnecessary restriction and prevents
many well-established patterns - like the bridge pattern - for std::unique_ptr
.
[ Bellevue: ]
Move to open. The LWG is comfortable with the intent of allowing incomplete types and making
unique_ptr
more likeshared_ptr
, but we are not comfortable with the wording. The specification forunique_ptr
should be more like that ofshared_ptr
. We need to know, for individual member functions, which ones require their types to be complete. Theshared_ptr
specification is careful to say that for each function, and we need the same level of care here. We also aren't comfortable with the "part of the operational semantic" language; it's not used elsewhere in the standard, and it's not clear what it means. We need a volunteer to produce new wording.
Proposed resolution:
The proposed changes in the following revision refers to the current state of N2521 including the assumption that [unique.ptr.compiletime] will be removed according to the current state of 740(i).
The specialization unique_ptr<T[]>
has some more restrictive constraints on
type-completeness on T
than unique_ptr<T>
. The following proposed wordings
try to cope with that. If the committee sees less usefulness on relaxed
constraints on unique_ptr<T[]>
, the alternative would be to stop this relaxation
e.g. by adding one further bullet to 20.3.1.4 [unique.ptr.runtime]/1:
"T
shall be a complete type, if used as template argument of
unique_ptr<T[], D>
This issue has some overlap with 673(i), but it seems not to cause any
problems with this one,
because 673(i) adds only optional requirements on D
that do not conflict
with the here discussed
ones, provided that D::pointer
's operations (including default
construction, copy construction/assignment,
and pointer conversion) are specified not to throw, otherwise this
would have impact on the
current specification of unique_ptr
.
In 20.3.1 [unique.ptr]/2 add as the last sentence to the existing para:
The
unique_ptr
provides a semantics of strict ownership. Aunique_ptr
owns the object it holds a pointer to. Aunique_ptr
is notCopyConstructible
, norCopyAssignable
, however it isMoveConstructible
andMoveAssignable
. The template parameterT
ofunique_ptr
may be an incomplete type. [ Note: The uses ofunique_ptr
include providing exception safety for dynamically allcoated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function. -- end note ]
20.3.1.3.2 [unique.ptr.single.ctor]/1: No changes necessary.
[ N.B.: We only need the requirement that
D
isDefaultConstructible
. The current wording says just this. ]
In 20.3.1.3.2 [unique.ptr.single.ctor]/5 change the requires clause to say:
Requires:
The expressionD()(p)
shall be well formed. The default constructor ofD
shall not throw an exception.D
must not be a reference type.D
shall be default constructible, and that construction shall not throw an exception.[ N.B.: There is no need that the expression
D()(p)
is well-formed at this point. I assume that the current wording is based on the correspondingshared_ptr
wording. In case ofshared_ptr
this requirement is necessary, because the corresponding c'tor *can* fail and must invoke deletep/d(p)
in this case.Unique_ptr
is simpler in this regard. The *only* functions that must insist on well-formedness and well-definedness of the expressionget_deleter()(get())
are (1) the destructor and (2)reset
. The reasoning for the wording change to explicitly requireDefaultConstructible
ofD
is to guarantee that invocation ofD
's default c'tor is both well-formed and well-defined. Note also that we do *not* need the requirement thatT
must be complete, also in contrast toshared_ptr
.Shared_ptr
needs this, because it's c'tor is a template c'tor which potentially requiresConvertible<Y*, X*>
, which again requires Completeness ofY
, if!SameType<X, Y>
]
Merge 20.3.1.3.2 [unique.ptr.single.ctor]/12+13 thereby removing the sentence of 12, but transferring the "requires" to 13:
Requires: If
D
is not an lvalue-reference type then[..][ N.B.: For the same reasons as for (3), there is no need that
d(p)
is well-formed/well-defined at this point. The current wording guarantees all what we need, namely that the initialization of both theT*
pointer and theD
deleter are well-formed and well-defined. ]
20.3.1.3.2 [unique.ptr.single.ctor]/21:
Requires: If
D
is not a reference type, construction of the deleterD
from an rvalue of typeE
shall be well formed and shall not throw an exception. IfD
is a reference type, thenE
shall be the same type asD
(diagnostic required).U*
shall be implicitly convertible toT*
. [Note: These requirements imply thatT
andU
be complete types. -- end note]
[
N.B.: The current wording of 21 already implicitly guarantees that U
is completely defined, because it requires that Convertible<U*, T*>
is
true. If the committee wishes this explicit requirement can be added,
e.g. "U
shall be a complete type."
]
20.3.1.3.3 [unique.ptr.single.dtor]: Just before p1 add a new paragraph:
Requires: The expression
get_deleter()(get())
shall be well-formed, shall have well-defined behavior, and shall not throw exceptions. [Note: The use ofdefault_delete
requiresT
to be a complete type. -- end note][ N.B.: This requirement ensures that the whole responsibility on type-completeness of
T
is delegated to this expression. ]
20.3.1.3.4 [unique.ptr.single.asgn]/1: No changes necessary, except the current editorial issue, that "must shall" has to be changed to "shall", but this change is not a special part of this resolution.
[ N.B. The current wording is sufficient, because we can delegate all further requirements on the requirements of the effects clause ]
20.3.1.3.4 [unique.ptr.single.asgn]/6:
Requires: Assignment of the deleter
D
from an rvalueD
shall not throw an exception.U*
shall be implicitly convertible toT*
. [Note: These requirements imply thatT
andU
be complete types. -- end note]
[
N.B.: The current wording of p. 6 already implicitly guarantees that
U
is completely defined, because it requires that Convertible<U*, T*>
is true, see (6)+(8).
]
20.3.1.3.4 [unique.ptr.single.asgn]/11: No changes necessary.
[ N.B.: Delegation to requirements of effects clause is sufficient. ]
20.3.1.3.5 [unique.ptr.single.observers]/1+4+7+9+11:
T* operator->() const;Note: Use typically requires
T
shall be complete. — end note]
20.3.1.3.6 [unique.ptr.single.modifiers]/4: Just before p. 4 add a new paragraph:
Requires: The expression
get_deleter()(get())
shall be well-formed, shall have well-defined behavior, and shall not throw exceptions.
20.3.1.4 [unique.ptr.runtime]: Add one additional bullet on paragraph 1:
A specialization for array types is provided with a slightly altered interface.
- ...
T
shall be a complete type.
[ post Bellevue: Daniel provided revised wording. ]
Section: 24.3.4 [iterator.concepts] Status: C++11 Submitter: Martin Sebor Opened: 2007-12-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.concepts].
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Discussion:
Issue 278(i) defines the meaning of the term "invalid iterator" as one that may be singular.
Consider the following code:
std::deque<int> x, y; std::deque<int>::iterator i = x.end(), j = y.end(); x.swap(y);
Given that swap()
is required not to invalidate iterators
and using the definition above, what should be the expected result of
comparing i
and j
to x.end()
and y.end()
, respectively, after the swap()
?
I.e., is the expression below required to evaluate
to true
?
i == y.end() && j == x.end()
(There are at least two implementations where the expression
returns false
.)
More generally, is the definition introduced in issue 278(i) meant to make any guarantees about whether iterators actually point to the same elements or be associated with the same containers after a non-invalidating operation as they did before?
Here's a motivating example intended to demonstrate the importance of the question:
Container x, y ({ 1, 2}); // pseudocode to initialize y with { 1, 2 } Container::iterator i = y.begin() + 1; Container::iterator j = y.end(); std::swap(x, y); std::find(i, j, 3);
swap()
guarantees that i
and j
continue to be valid. Unless the spec says that even though they are
valid they may no longer denote a valid range the code above must be
well-defined. Expert opinions on this differ as does the behavior of
popular implementations for some standard Containers
.
[ San Francisco: ]
Pablo: add a note to the last bullet of paragraph 11 of 23.1.1 clarifying that the
end()
iterator doesn't refer to an element and that it can therefore be invalidated.Proposed wording:
[Note: The
end()
iterator does not refer to any element and can therefore be invalidated. -- end note]Howard will add this proposed wording to the issue and then move it to Review.
[ Post Summit: ]
Lawrence: suggestion: "Note: The
end()
iterator does not refer to any element"Walter: "Note: The
end()
iterator can nevertheless be invalidated, because it does not refer to any element."Nick: "The
end()
iterator does not refer to any element. It is therefore subject to being invalidated."Consensus: go with Nick
With that update, Recommend Tentatively Ready.
Proposed resolution:
Add to 23.2.2 [container.requirements.general], p11:
Unless otherwise specified (see 23.1.4.1, 23.1.5.1, 23.2.2.3, and 23.2.6.4) all container types defined in this Clause meet the following additional requirements:
- ...
- no
swap()
function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: Theend()
iterator does not refer to any element. It is therefore subject to being invalidated. — end note]
Section: 23.2 [container.requirements], 23.2.8.2 [unord.req.except] Status: CD1 Submitter: Ion Gaztañaga Opened: 2007-12-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
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Discussion:
23.2 [container.requirements]p10 states:
Unless otherwise specified (see 23.2.2.3 and 23.2.5.4) all container types defined in this clause meet the following additional requirements:
- [...]
- no
erase()
,pop_back()
orpop_front()
function throws an exception.
23.3.5.4 [deque.modifiers] and 23.3.13.5 [vector.modifiers] offer
additional guarantees for deque
/vector insert()
and
erase()
members. However, 23.2 [container.requirements] p10
does not mention 23.2.8.2 [unord.req.except] that specifies exception
safety guarantees for unordered containers. In addition,
23.2.8.2 [unord.req.except] p1 offers the following guaratee for
erase()
:
No
erase()
function throws an exception unless that exception is thrown by the container's Hash or Pred object (if any).
Summary:
According to 23.2 [container.requirements] p10 no
erase()
function should throw an exception unless otherwise
specified. Although does not explicitly mention 23.2.8.2 [unord.req.except], this section offers additional guarantees
for unordered containers, allowing erase()
to throw if
predicate or hash function throws.
In contrast, associative containers have no exception safety guarantees
section so no erase()
function should throw, including
erase(k)
that needs to use the predicate function to
perform its work. This means that the predicate of an associative
container is not allowed to throw.
So:
erase(k)
for associative containers is not allowed to throw. On
the other hand, erase(k)
for unordered associative containers
is allowed to throw.
erase(q)
for associative containers is not allowed to throw. On
the other hand, erase(q)
for unordered associative containers
is allowed to throw if it uses the hash or predicate.
erase(q)
is
allowed to throw, the destructor of the object would need to rethrow the
exception or swallow it, leaving the object registered.
Proposed resolution:
Create a new sub-section of 23.2.7 [associative.reqmts] (perhaps [associative.req.except]) titled "Exception safety guarantees".
1 For associative containers, no
clear()
function throws an exception.erase(k)
does not throw an exception unless that exception is thrown by the container's Pred object (if any).2 For associative containers, if an exception is thrown by any operation from within an
insert()
function inserting a single element, theinsert()
function has no effect.3 For associative containers, no
swap
function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Pred object (if any).
Change 23.2.8.2 [unord.req.except]p1:
For unordered associative containers, no
clear()
function throws an exception.Noerase(k)
functiondoes not throwsan exception unless that exception is thrown by the container's Hash or Pred object (if any).
Change 23.2 [container.requirements] p10 to add references to new sections:
Unless otherwise specified (see [deque.modifiers],
and[vector.modifiers], [associative.req.except], [unord.req.except]) all container types defined in this clause meet the following additional requirements:
Change 23.2 [container.requirements] p10 referring to swap
:
- no
swap()
function throws an exceptionunless that exception is thrown by the copy constructor or assignment operator of the container's Compare object (if any; see [associative.reqmts]).
Section: 23 [containers] Status: Resolved Submitter: Sylvain Pion Opened: 2007-12-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Playing with g++'s C++0X mode, I noticed that the following code, which used to compile:
#include <vector> int main() { std::vector<char *> v; v.push_back(0); }
now fails with the following error message:
.../include/c++/4.3.0/ext/new_allocator.h: In member function 'void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, _Args&& ...) [with _Args = int, _Tp = char*]': .../include/c++/4.3.0/bits/stl_vector.h:707: instantiated from 'void std::vector<_Tp, _Alloc>::push_back(_Args&& ...) [with _Args = int, _Tp = char*, _Alloc = std::allocator<char*>]' test.cpp:6: instantiated from here .../include/c++/4.3.0/ext/new_allocator.h:114: error: invalid conversion from 'int' to 'char*'
As far as I know, g++ follows the current draft here.
Does the committee really intend to break compatibility for such cases?
[ Sylvain adds: ]
I just noticed that
std::pair
has the same issue. The following now fails with GCC's -std=c++0x mode:#include <utility> int main() { std::pair<char *, char *> p (0,0); }I have not made any general audit for such problems elsewhere.
[ Bellevue: ]
Motivation is to handle the old-style int-zero-valued
NULL
pointers. Problem: this solution requires concepts in some cases, which some users will be slow to adopt. Some discussion of alternatives involving prohibiting variadic forms and additional library-implementation complexity.Discussion of "perfect world" solutions, the only such solution put forward being to retroactively prohibit use of the integer zero for a
NULL
pointer. This approach was deemed unacceptable given the large bodies of pre-existing code that do use integer zero for aNULL
pointer.Another approach is to change the member names. Yet another approach is to forbid the extension in absence of concepts.
Resolution: These issues (756(i), 767(i), 760(i), 763(i)) will be subsumed into a paper to be produced by Alan Talbot in time for review at the 2008 meeting in France. Once this paper is produced, these issues will be moved to
NADResolved.
Proposed resolution:
Add the following rows to Table 90 "Optional sequence container operations", 23.2.4 [sequence.reqmts]:
expression return type assertion/note
pre-/post-conditioncontainer a.push_front(t)
void
a.insert(a.begin(), t)
Requires:T
shall beCopyConstructible
.list, deque
a.push_front(rv)
void
a.insert(a.begin(), rv)
Requires:T
shall beMoveConstructible
.list, deque
a.push_back(t)
void
a.insert(a.end(), t)
Requires:T
shall beCopyConstructible
.list, deque, vector, basic_string
a.push_back(rv)
void
a.insert(a.end(), rv)
Requires:T
shall beMoveConstructible
.list, deque, vector, basic_string
Change the synopsis in 23.3.5 [deque]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.5.4 [deque.modifiers]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change the synopsis in 23.3.11 [list]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.11.4 [list.modifiers]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change the synopsis in 23.3.13 [vector]:
void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.13.5 [vector.modifiers]:
void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Rationale:
Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).
If there is still an issue with pair, Howard should submit another issue.
Section: 32.5.8 [atomics.types.generic] Status: CD1 Submitter: Alberto Ganesh Barbati Opened: 2007-12-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.types.generic].
View all issues with CD1 status.
Discussion:
in the latest publicly available draft, paper
N2641,
in section 32.5.8 [atomics.types.generic], the following specialization of the template
atomic<>
is provided for pointers:
template <class T> struct atomic<T*> : atomic_address { T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile; T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile; atomic() = default; constexpr explicit atomic(T); atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; T* operator=(T*) volatile; T* operator++(int) volatile; T* operator--(int) volatile; T* operator++() volatile; T* operator--() volatile; T* operator+=(ptrdiff_t) volatile; T* operator-=(ptrdiff_t) volatile; };
First of all, there is a typo in the non-default constructor which
should take a T*
rather than a T
.
As you can see, the specialization redefine and therefore hide a few
methods from the base class atomic_address
, namely fetch_add
, fetch_sub
,
operator=
, operator+=
and operator-=
. That's good, but... what happened
to the other methods, in particular these ones:
void store(T*, memory_order = memory_order_seq_cst) volatile; T* load( memory_order = memory_order_seq_cst ) volatile; T* swap( T*, memory_order = memory_order_seq_cst ) volatile; bool compare_swap( T*&, T*, memory_order, memory_order ) volatile; bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile;
By reading paper
N2427 "C++ Atomic Types and Operations",
I see that the
definition of the specialization atomic<T*>
matches the one in the
draft, but in the example implementation the methods load()
, swap()
and compare_swap()
are indeed present.
Strangely, the example implementation does not redefine the method
store()
. It's true that a T*
is always convertible to void*
, but not
hiding the void*
signature from the base class makes the class
error-prone to say the least: it lets you assign pointers of any type to
a T*
, without any hint from the compiler.
Is there a true intent to remove them from the specialization or are they just missing from the definition because of a mistake?
[ Bellevue: ]
The proposed revisions are accepted.
Further discussion: why is the ctor labeled "constexpr"? Lawrence said this permits the object to be statically initialized, and that's important because otherwise there would be a race condition on initialization.
Proposed resolution:
Change the synopsis in 32.5.8 [atomics.types.generic]:
template <class T> struct atomic<T*> : atomic_address { void store(T*, memory_order = memory_order_seq_cst) volatile; T* load( memory_order = memory_order_seq_cst ) volatile; T* swap( T*, memory_order = memory_order_seq_cst ) volatile; bool compare_swap( T*&, T*, memory_order, memory_order ) volatile; bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile; T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile; T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile; atomic() = default; constexpr explicit atomic(T*); atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; T* operator=(T*) volatile; T* operator++(int) volatile; T* operator--(int) volatile; T* operator++() volatile; T* operator--() volatile; T* operator+=(ptrdiff_t) volatile; T* operator-=(ptrdiff_t) volatile; };
Section: 22.10.17.3 [func.wrap.func] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func].
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Discussion:
N2461 already replaced in 22.10.17.3 [func.wrap.func] it's originally proposed
(implicit) conversion operator to "unspecified-bool-type" by the new
explicit bool conversion, but the inverse conversion should also
use the new std::nullptr_t
type instead of "unspecified-null-pointer-
type".
Proposed resolution:
In 22.10 [function.objects], header <functional>
synopsis replace:
template<class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template<class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&); template<class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template<class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&);
In the class function synopsis of 22.10.17.3 [func.wrap.func] replace
function(unspecified-null-pointer-typenullptr_t); ... function& operator=(unspecified-null-pointer-typenullptr_t);
In 22.10.17.3 [func.wrap.func], "Null pointer comparisons" replace:
template <class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&); template <class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&);
In 22.10.17.3.2 [func.wrap.func.con], replace
function(unspecified-null-pointer-typenullptr_t); ... function& operator=(unspecified-null-pointer-typenullptr_t);
In 22.10.17.3.7 [func.wrap.func.nullptr], replace
template <class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>& f,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>& f);
and replace
template <class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>& f,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>& f);
Section: 22.10.17 [func.wrap] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
It is expected that typical implementations of std::function
will
use dynamic memory allocations at least under given conditions,
so it seems appropriate to change the current lvalue swappabilty of
this class to rvalue swappability.
Proposed resolution:
In 22.10 [function.objects], header <functional>
synopsis, just below of
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);
In 22.10.17.3 [func.wrap.func] class function
definition, change
void swap(function&&);
In 22.10.17.3 [func.wrap.func], just below of
template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&); template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&); template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);
In 22.10.17.3.3 [func.wrap.func.mod] change
void swap(function&& other);
In 22.10.17.3.8 [func.wrap.func.alg] add the two overloads
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&& f1, function<R(ArgTypes...)>& f2); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>&& f2);
Section: 27.4.5 [string.conversions] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.conversions].
View all issues with CD1 status.
Discussion:
The new to_string
and to_wstring
functions described in 27.4.5 [string.conversions]
have throws clauses (paragraphs 8 and 16) which say:
Throws: nothing
Since all overloads return either a std::string
or a std::wstring
by value
this throws clause is impossible to realize in general, since the basic_string
constructors can fail due to out-of-memory conditions. Either these throws
clauses should be removed or should be more detailled like:
Throws: Nothing if the string construction throws nothing
Further there is an editorial issue in p. 14: All three to_wstring
overloads return a string
, which should be wstring
instead (The
header <string>
synopsis of 27.4 [string.classes] is correct in this
regard).
Proposed resolution:
In 27.4.5 [string.conversions], remove the paragraphs 8 and 16.
string to_string(long long val); string to_string(unsigned long long val); string to_string(long double val);
Throws: nothing
wstring to_wstring(long long val); wstring to_wstring(unsigned long long val); wstring to_wstring(long double val);
Throws: nothing
Section: 27.4.5 [string.conversions] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.conversions].
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Discussion:
The return clause 27.4.5 [string.conversions] paragraph 15 of the new to_wstring
overloads says:
Returns: each function returns a
wstring
object holding the character representation of the value of its argument that would be generated by callingwsprintf(buf, fmt, val)
with a format specifier ofL"%lld"
,L"%ulld"
, orL"%f"
, respectively.
Problem is: There does not exist any wsprintf
function in C99 (I checked
the 2nd edition of ISO 9899, and the first and the second corrigenda from
2001-09-01 and 2004-11-15). What probably meant here is the function
swprintf
from <wchar.h>/<cwchar>
, but this has the non-equivalent
declaration:
int swprintf(wchar_t * restrict s, size_t n, const wchar_t * restrict format, ...);
therefore the paragraph needs to mention the size_t
parameter n
.
Proposed resolution:
Change the current wording of 27.4.5 [string.conversions] p. 15 to:
Returns:
eEach function returns awstring
object holding the character representation of the value of its argument that would be generated by callingwith a format specifier
wsswprintf(buf, bufsz, fmt, val)fmt
ofL"%lld"
,L"%ulld"
, orL"%f"
, respectively, wherebuf
designates an internal character buffer of sufficient sizebufsz
.
[Hint to the editor: The resolution also adds to mention the name of the format specifier "fmt"]
I also would like to remark that the current wording of it's equivalent
paragraph 7 should also mention the meaning of buf
and fmt
.
Change the current wording of 27.4.5 [string.conversions] p. 7 to:
Returns:
eEach function returns a string object holding the character representation of the value of its argument that would be generated by callingsprintf(buf, fmt, val)
with a format specifierfmt
of"%lld"
,"%ulld"
, or"%f"
, respectively, wherebuf
designates an internal character buffer of sufficient size.
swap
undefined for most containersSection: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-01-14 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [containers].
View all other issues in [containers].
View all issues with C++11 status.
Discussion:
It appears most containers declare but do not define a member-swap function.
This is unfortunate, as all overload the swap
algorithm to call the
member-swap function!
(required for swappable
guarantees [Table 37] and Container Requirements
[Table 87])
Note in particular that Table 87 gives semantics of a.swap(b)
as swap(a,b)
,
yet for all containers we define swap(a,b)
to call a.swap(b)
- a circular
definition.
A quick survey of clause 23 shows that the following containers provide a definition for member-swap:
array queue stack vector
Whereas the following declare it, but do not define the semantics:
deque list map multimap multiset priority_queue set unordered_map unordered_multi_map unordered_multi_set unordered_set
Suggested resolution:
Provide a definition for each of the affected containers...
[ Bellevue: ]
Move to Open and ask Alisdair to provide wording.
[ 2009-07 Frankfurt: ]
Daniel to provide wording. N2590 is no longer applicable.
[ 2009-07-28 Daniel provided wording. ]
- It assumes that the proposed resolution for 883(i) is applied, which breaks the circularity of definition between member
swap
and freeswap
.- It uses the notation of the pre-concept allocator trait
allocator_propagation_map
, which might be renamed after the next refactoring phase of generalized allocators.- It requires that compare objects, key equal functions and hash functions in containers are swapped via unqualified free
swap
according to 594(i).
[ 2009-09-30 Daniel adds: ]
The outcome of this issue should be considered with the outcome of 1198(i) both in style and in content (e.g. bullet 9 suggests to define the semantic of
void priority_queue::swap(priority_queue&)
in terms of the memberswap
of the container).
[ 2009-10 Santa Cruz: ]
Looked at, but took no action on as it overlaps too much with N2982. Waiting for a new draft WP.
[ 2009-10 Santa Cruz: ]
Leave as open. Pablo to provide wording.
[ 2009-10-26 Pablo updated wording. Here is the wording he replaced: ]
Add a new Throws clause just after 99 [allocator.propagation.map]/5:
static void swap(Alloc& a, Alloc& b);Effects: [..]
Throws: Nothing.
[ This exception requirement is added, such that it's combination with the general container requirements of N2723 [container.requirements.general]/9 make it unambiguously clear that the following descriptions of "swaps the allocators" have the following meaning: (a) This swap is done by calling
allocator_propagation_map<allocator_type>::swap
and (b) This allocator swap does never propagate an exception ]Change 23.2.7.2 [associative.reqmts.except]/3 as indicated:
For associative containers, no
swap
function throws an exception unless that exception is thrown by thecopy constructor or copy assignment operatorswap
of the container'sPred
objects(if any).Change 23.2.8.2 [unord.req.except]/3 as indicated:
For unordered associative containers, no
swap
function throws an exception unless that exception is thrown by thecopy constructor or copy assignment operatorswap
of the container'sHash
orPred
objects, respectively(if any).Insert a new paragraph just after 23.3 [sequences]/1:
In addition to being available via inclusion of the
<algorithm>
header, theswap
function templates in 26.7.3 [alg.swap] are also available when the header<queue>
is included.[ There is a new issue in process that will suggest a minimum header for
swap
andmove
. If this one is provided, this text can be removed and the header dependency should be added to<queue>
]Add one further clause at the end of 23.3.3.4 [array.special]:
[This part is added, because otherwise
array::swap
would otherwise contradict the general contract of 23.2.2 [container.requirements.general] p. 10 b. 5]Throws: Nothing, unless one of the element-wise
swap
calls throws an exception.
In 23.3.5 [deque], class template
deque
synopsis change as indicated:void swap(deque<T,Alloc>&);At the end of 23.3.5.4 [deque.modifiers] add as indicated:
void swap(deque& x);Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
.Complexity: Constant time.
In [forwardlist], class template
forward_list
synopsis change as indicated:void swap(forward_list<T,Allocator>&);At the end of [forwardlist.modifiers] add as indicated:
void swap(forward_list& x);Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
.Complexity: Constant time.
In 23.3.11 [list], class template
list
synopsis change as indicated:void swap(list<T,Allocator>&);At the end of 23.3.11.4 [list.modifiers] add as indicated:
void swap(list& x);Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
.Complexity: Constant time.
At the end of 23.6.4.4 [priqueue.members] add a new prototype description:
void swap(priority_queue& q);Requires:
Compare
shall satisfy theSwappable
requirements ( [swappable]).[ This requirement is added to ensure that even a user defined
swap
which is found by ADL forCompare
satisfies theSwappable
requirements ]Effects:
this->c.swap(q.c); swap(this->comp, q.comp);
Throws: What and if
c.swap(q.c)
andswap(comp, q.comp)
throws.[ This part is added, because otherwise
priority_queue::swap
would otherwise contradict the general contract of 23.2.2 [container.requirements.general] p. 10 b. 5 ]
In 23.3.13 [vector], class template
vector
synopsis change as indicated:void swap(vector<T,Allocator>&);Change 23.3.13.3 [vector.capacity] p. 8 as indicated:
void swap(vector<T,Allocator>& x);Effects: Exchanges the contents and
capacity()
and swaps the allocators of*this
with that ofx
.Insert a new paragraph just before 23.4 [associative]/1:
In addition to being available via inclusion of the
<algorithm>
header, theswap
function templates in 26.7.3 [alg.swap] are also available when any of the headers<map>
or<set>
are included.
In 23.4.3 [map], class template
map
synopsis change as indicated:void swap(map<Key,T,Compare,Allocator>&);At the end of 23.4.3.4 [map.modifiers] add as indicated:
void swap(map& x);Requires: Compare shall satisfy the
Swappable
requirements ( [swappable]).[ This requirement is added to ensure that even a user defined
swap
which is found by ADL forCompare
satisfies theSwappable
requirements ]Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of the comparison objects of*this
andx
.Complexity: Constant time
In 23.4.4 [multimap], class template
multimap
synopsis change as indicated:void swap(multimap<Key,T,Compare,Allocator>&);At the end of 23.4.4.3 [multimap.modifiers] add as indicated:
void swap(multimap& x);Requires: Compare shall satisfy the
Swappable
requirements ( [swappable]).Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of the comparison objects of*this
andx
.Complexity: Constant time
In 23.4.6 [set], class template
set
synopsis change as indicated:void swap(set<Key,Compare,Allocator>&);After section 23.4.6.2 [set.cons] add a new section
set
modifiers 23.4.6.4 [set.modifiers] and add the following paragraphs:void swap(set& x);Requires: Compare shall satisfy the
Swappable
requirements ( [swappable]).Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of the comparison objects of*this
andx
.Complexity: Constant time
In 23.4.7 [multiset], class template
multiset
synosis, change as indicated:void swap(multiset<Key,Compare,Allocator>&);After section 23.4.7.2 [multiset.cons] add a new section
multiset
modifiers [multiset.modifiers] and add the following paragraphs:void swap(multiset& x);Requires: Compare shall satisfy the
Swappable
requirements ( [swappable]).Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of the comparison objects of*this
andx
.Complexity: Constant time
Insert a new paragraph just before 23.5 [unord] p. 1:
In addition to being available via inclusion of the
<algorithm>
header, theswap
function templates in 26.7.3 [alg.swap] are also available when any of the headers<unordered_map>
or<unordered_set>
are included.After section 23.5.3.3 [unord.map.elem] add a new section unordered_map modifiers 23.5.3.4 [unord.map.modifiers] and add the following paragraphs:
void swap(unordered_map& x);Requires:
Hash
andPred
shall satisfy theSwappable
requirements ( [swappable]).[ This requirement is added to ensure that even a user defined
swap
which is found by ADL forHash
andPred
satisfies theSwappable
requirements ]Effects: Exchanges the contents and hash policy and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of thePred
objects and an unqualifiedswap
of theHash
objects of*this
andx
.Complexity: Constant time
After section 23.5.4.2 [unord.multimap.cnstr] add a new section unordered_multimap modifiers 23.5.4.3 [unord.multimap.modifiers] and add the following paragraphs:
void swap(unordered_multimap& x);Requires:
Hash
andPred
shall satisfy theSwappable
requirements ( [swappable]).Effects: Exchanges the contents and hash policy and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of thePred
objects and an unqualifiedswap
of theHash
objects of*this
andx
Complexity: Constant time
After section 23.5.6.2 [unord.set.cnstr] add a new section unordered_set modifiers 23.5.6.4 [unord.set.modifiers] and add the following paragraphs:
void swap(unordered_set& x);Requires:
Hash
andPred
shall satisfy theSwappable
requirements ( [swappable]).Effects: Exchanges the contents and hash policy and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of thePred
objects and an unqualifiedswap
of theHash
objects of*this
andx
Complexity: Constant time
After section 23.5.7.2 [unord.multiset.cnstr] add a new section unordered_multiset modifiers [unord.multiset.modifiers] and add the following paragraphs:
void swap(unordered_multiset& x);Requires:
Hash
andPred
shall satisfy theSwappable
requirements ( [swappable]).Effects: Exchanges the contents and hash policy and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of thePred
objects and an unqualifiedswap
of theHash
objects of*this
andx
Complexity: Constant time
[ 2009-10-30 Pablo and Daniel updated wording. ]
[ 2010 Pittsburgh: Ready for Pittsburgh. ]
Proposed resolution:
[ This resolution is based on the September 2009 WP, N2960, except that it assumes that N2982 and issues 883(i) and 1232(i) have already been applied. Note in particular that Table 91 in N2960 is refered to as Table 90 because N2982 removed the old Table 90. This resolution also addresses issue 431(i). ]
In 23.2.2 [container.requirements.general], replace the a.swap(b) row in table 90, "container requirements" (was table 91 before the application of N2982 to the WP):
a.swap(b)
void
swap(a,b)Exchange the contents ofa
andb
.(Note A) swap(a,b)
void
a.swap(b)
(Note A)
Modify the notes immediately following Table 90 in 23.2.2 [container.requirements.general] as follows (The wording below is after the application of N2982 to N2960. The editor might also want to combine Notes A and B into one.):
Notes: the algorithms
swap(),equal() and lexicographical_compare() are defined in Clause 25. Those entries marked "(Note A)" or "(Note B)"shouldhave linear complexity for array and constant complexity for all other standard containers.
In 23.2.2 [container.requirements.general], before paragraph 8, add:
The expression
a.swap(b)
, for containersa
andb
of a standard container type other thanarray
, exchanges the values ofa
andb
without invoking any move, copy, or swap operations on the individual container elements. AnyCompare
,Pred
, orHash
function objects belonging toa
andb
shall beswappable
and are exchanged by unqualified calls to non-memberswap
. Ifallocator_traits<allocator_type>::propagate_on_container_swap::value == true
, then the allocators ofa
andb
are also exchanged using an unqualified call to non-memberswap
. Otherwise, the behavior is undefined unlessa.get_allocator() == b.get_allocator()
. Each iterator refering to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with valuea.end()
before the swap will have valueb.end()
after the swap. In addition to being available via inclusion of the<utility>
header, theswap
function template in 26.7.3 [alg.swap] is also available within the definition of every standard container'sswap
function.
[ Note to the editor: Paragraph 2 starts with a sentence fragment, clearly from an editing or source-control error. ]
Modify 23.2.7.2 [associative.reqmts.except] as follows:
23.2.4.1 Exception safety guarantees 23.2.7.2 [associative.reqmts.except]
For associative containers, no
clear()
function throws an exception.erase(k)
does not throw an exception unless that exception is thrown by the container'sobject (if any).
PredCompareFor associative containers, if an exception is thrown by any operation from within an
insert()
function inserting a single element, theinsert()
function has no effect.For associative containers, no
swap
function throws an exception unless that exception is thrown by thecopy constructor or copy assignment operatorswap of the container'sobject (if any).
PredCompare
Modify 23.2.8.2 [unord.req.except], paragraph 3 as follows:
For unordered associative containers, no
swap
function throws an exception unless that exception is thrown by thecopy constructor or copy assignment operatorswap of the container'sHash
orPred
object (if any).
Modify section 23.3.3.4 [array.special]:
array specialized algorithms 23.3.3.4 [array.special]
template <class T, size_t N> void swap(array<T,N>& x,array<T,N>& y);
Effects:
swap_ranges(x.begin(), x.end(), y.begin() );x.swap(y);
Add a new section after [array.fill] (Note to the editor: array::fill make use of a concept requirement that must be removed or changed to text.):
array::swap [array.swap]
void swap(array& y);
Effects:
swap_ranges(this->begin(), this->end(), y.begin() );
Throws: Nothing unless one of the element-wise swap calls throws an exception.
[Note: Unlike other containers'
swap
functions,array::swap
takes linear, not constant, time, may exit via an exception, and does not cause iterators to become associated with the other container. — end note]
Insert a new paragraph just after 23.6 [container.adaptors]/1:
For container adaptors, no
swap
function throws an exception unless that exception is thrown by the swap of the adaptor'sContainer
orCompare
object (if any).
Section: 22.4.7 [tuple.helper] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-01-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple.helper].
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Discussion:
The tuple element access API identifies the element in the sequence
using signed integers, and then goes on to enforce the requirement that
I be >= 0. There is a much easier way to do this - declare I as
unsigned
.
In fact the proposal is to use std::size_t
, matching the
type used in the tuple_size
API.
A second suggestion is that it is hard to imagine an API that deduces and index at compile time and returns a reference throwing an exception. Add a specific Throws: Nothing paragraph to each element access API.
In addition to tuple
, update the API applies to
pair
and array
, and should be updated
accordingly.
A third observation is that the return type of the get
functions for std::pair
is pseudo-code, but it is not
clearly marked as such. There is actually no need for pseudo-code as
the return type can be specified precisely with a call to
tuple_element
. This is already done for
std::tuple
, and std::array
does not have a
problem as all elements are of type T
.
Proposed resolution:
Update header <utility> synopsis in 22.2 [utility]
// 20.2.3, tuple-like access to pair: template <class T> class tuple_size; template <intsize_t I, class T> class tuple_element; template <class T1, class T2> struct tuple_size<std::pair<T1, T2> >; template <class T1, class T2> struct tuple_element<0, std::pair<T1, T2> >; template <class T1, class T2> struct tuple_element<1, std::pair<T1, T2> >; template<intsize_t I, class T1, class T2>Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(std::pair<T1, T2>&); template<intsize_t I, class T1, class T2> constPtypename tuple_element<I, std::pair<T1, T2> >::type & get(const std::pair<T1, T2>&);
Update 22.3 [pairs] Pairs
template<intsize_t I, class T1, class T2>Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(pair<T1, T2>&); template<intsize_t I, class T1, class T2> constPtypename tuple_element<I, std::pair<T1, T2> >::type & get(const pair<T1, T2>&);
24 Return type: If
I == 0
then P
is T1
, if I == 1
then P
is T2
, and otherwise the program is ill-formed.
25 Returns: If I == 0
returns p.first
, otherwise if I == 1
returns p.second
, and otherwise the program is ill-formed.
Throws: Nothing.
Update header <tuple> synopsis in 22.4 [tuple] with a APIs as below:
template <intsize_t I, class T> class tuple_element; // undefined template <intsize_t I, class... Types> class tuple_element<I, tuple<Types...> >; // 20.3.1.4, element access: template <intsize_t I, class... Types> typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>&); template <intsize_t I, class ... types> typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>&);
Update 22.4.7 [tuple.helper] Tuple helper classes
template <intsize_t I, class... Types> class tuple_element<I, tuple<Types...> > { public: typedef TI type; };
1 Requires:
. The program is ill-formed if 0 <= I and I < sizeof...(Types)I
is out of bounds.
2 Type: TI
is the type of the I
th element of Types
, where indexing is zero-based.
Update 22.4.8 [tuple.elem] Element access
template <intsize_t I, class... types > typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>& t);
1 Requires:
. The program is ill-formed if 0 <= I and I < sizeof...(Types)I
is out of bounds.
I
th element of t
, where indexing is zero-based.
Throws: Nothing.
template <intsize_t I, class... types> typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>& t);
3 Requires:
. The program is ill-formed if 0 <= I and I < sizeof...(Types)I
is out of bounds.
4 Returns: A const reference to the I
th element of t
, where indexing is zero-based.
Throws: Nothing.
Update header <array> synopsis in 22.2 [utility]
template <class T> class tuple_size; // forward declaration template <intsize_t I, class T> class tuple_element; // forward declaration template <class T, size_t N> struct tuple_size<array<T, N> >; template <intsize_t I, class T, size_t N> struct tuple_element<I, array<T, N> >; template <intsize_t I, class T, size_t N> T& get(array<T, N>&); template <intsize_t I, class T, size_t N> const T& get(const array<T, N>&);
Update 23.3.3.7 [array.tuple] Tuple interface to class template array
tuple_element<size_t I, array<T, N> >::type
3 Requires:
The program is ill-formed if 0 <= I < N.I
is out of bounds.
4 Value: The type T
.
template <intsize_t I, class T, size_t N> T& get(array<T, N>& a);
5 Requires:
. The program is ill-formed if 0 <= I < NI
is out of bounds.
Returns: A reference to the I
th element of a
, where indexing is zero-based.
Throws: Nothing.
template <intsize_t I, class T, size_t N> const T& get(const array<T, N>& a);
6 Requires:
. The program is ill-formed if 0 <= I < NI
is out of bounds.
7 Returns: A const reference to the I
th element of a
, where indexing is zero-based.
Throws: Nothing.
[ Bellevue: Note also that the phrase "The program is ill-formed if I is out of bounds" in the requires clauses are probably unnecessary, and could be removed at the editor's discretion. Also std:: qualification for pair is also unnecessary. ]
assign
function of std::array
Section: 23.3.3 [array] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class template array synopsis in 23.3.3 [array] p. 3 declares a member function
void assign(const T& u);
which's semantic is no-where described. Since this signature is not part of the container requirements, such a semantic cannot be derived by those.
I found only one reference to this function in the issue list, 588(i) where the question is raised:
what's the effect of calling
assign(T&)
on a zero-sized array?
which does not answer the basic question of this issue.
If this function shall be part of the std::array
, it's probable
semantic should correspond to that of boost::array
, but of
course such wording must be added.
Proposed resolution:
Just after the section [array.data] add the following new section:
23.2.1.5 array::fill [array.fill]
void fill(const T& u);1: Effects:
fill_n(begin(), N, u)
[N.B: I wonder, why class array
does not have a "modifiers"
section. If it had, then assign
would naturally belong to it]
Change the synopsis in 23.3.3 [array]/3:
template <class T, size_t N> struct array { ... voidassignfill(const T& u); ...
[ Bellevue: ]
Suggest substituting "fill" instead of "assign".
Set state to Review given substitution of "fill" for "assign".
Section: 32.5.8.2 [atomics.types.operations] Status: CD1 Submitter: Lawrence Crowl Opened: 2008-01-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The load functions are defined as
C atomic_load(volatile A* object); C atomic_load_explicit(volatile A* object, memory_order); C A::load(memory_order order = memory_order_seq_cst) volatile;
which prevents their use in const
contexts.
[ post Bellevue Peter adds: ]
Issue 777(i) suggests making
atomic_load
operate onconst
objects. There is a subtle point here. Atomic loads do not generally write to the object, except potentially for thememory_order_seq_cst
constraint. Depending on the architecture, a dummy write with the same value may be required to be issued by the atomic load to maintain sequential consistency. This, in turn, may make the following code:const atomic_int x{}; int main() { x.load(); }dump core under a straightforward implementation that puts const objects in a read-only section.
There are ways to sidestep the problem, but it needs to be considered.
The tradeoff is between making the data member of the atomic types mutable and requiring the user to explicitly mark atomic members as mutable, as is already the case with mutexes.
Proposed resolution:
Add the const
qualifier to *object
and *this
.
C atomic_load(const volatile A* object); C atomic_load_explicit(const volatile A* object, memory_order); C A::load(memory_order order = memory_order_seq_cst) const volatile;
Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Thorsten Ottosen Opened: 2008-01-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 116
Discussion:
A small issue with std::bitset
: it does not have any constructor
taking a string literal, which is clumsy and looks like an oversigt when
we tried to enable uniform use of string
and const char*
in the library.
Suggestion: Add
explicit bitset( const char* str );
to std::bitset.
Proposed resolution:
Add to synopsis in 22.9.2 [template.bitset]
explicit bitset( const char* str );
Add to synopsis in 22.9.2.2 [bitset.cons]
explicit bitset( const char* str );Effects: Constructs a
bitset
as ifbitset(string(str))
.
Section: 26.7.8 [alg.remove] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The resolution of 283(i) did not resolve similar necessary changes for algorithm
remove_copy[_if]
, which seems to be an oversight.
Proposed resolution:
In 26.7.8 [alg.remove] p.6, replace the N2461 requires clause with:
Requires:
TypeThe rangesT
isEqualityComparable
(31).[first,last)
and[result,result + (last - first))
shall not overlap. The expression*result = *first
shall be valid.
std::merge()
specification incorrect/insufficientSection: 26.8.6 [alg.merge] Status: C++11 Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Though issue 283(i) has fixed many open issues, it seems that some are still open:
Both 25.3.4 [lib.alg.merge] in 14882:2003 and 26.8.6 [alg.merge] in N2461 have no Requires element and the Effects element contains some requirements, which is probably editorial. Worse is that:
[first, last)
, which is not defined by the
function arguments or otherwise.
[ Post Summit Alisdair adds: ]
Suggest:
(where
last
is equal tonext(result, distance(first1, last1) + distance(first2, last2))
, such that resulting range will be sorted in non-decreasing order; that is, for every iteratori
in[result,last)
other thanresult
, the condition*i < *prev(i)
or, respectively,comp(*i, *prev(i))
will befalse
.Note that this might still not be technically accurate in the case of
InputIterators
, depending on other resolutions working their way through the system (1011(i)).
[ Post Summit Daniel adds: ]
If we want to use
prev
andnext
here (Note:merge
is sufficiently satisfied withInputIterator
) we should instead add more to 26 [algorithms] p. 6, but I can currently not propose any good wording for this.
[ Batavia (2009-05): ]
Pete points out the existing wording in [algorithms] p. 4 that permits the use of + in algorithm specifications.
Alisdair points out that that wording may not apply to input iterators.
Move to Review.
[ 2009-07 Frankfurt: ]
Move to Ready.
[ 2009-08-23 Daniel reopens: ]
The proposed wording must be rephrased, because the part
for every iterator
i
in[result,last)
other thanresult
, the condition*i < *(i - 1)
or, respectively,comp(*i, *(i - 1))
will befalse
"isn't meaningful, because the range
[result,last)
is that of a pureOutputIterator
, which is not readable in general.[Howard: Proposed wording updated by Daniel, status moved from Ready to Review.]
[ 2009-10 Santa Cruz: ]
Matt has some different words to propose. Those words have been moved into the proposed wording section, and the original proposed wording now appears here:
In 26.8.6 [alg.merge] replace p.1+ 2:
Effects:
MergesCopies all the elements of the two sorted ranges[first1,last1)
and[first2,last2)
into the range[result,result + (last1 - first1) + (last2 - first2))
, such that resulting range will be sorted in non-decreasing order; that is for every pair of iteratorsi
andj
of either input ranges, where*i
was copied to the output range before*j
was copied to the output range, the condition*j < *i
or, respectively,comp(*j, *i)
will befalse
.Requires:The resulting range shall not overlap with either of the original ranges.
The list will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
in[first,last)
other thanfirst
, the condition*i < *(i - 1)
orcomp(*i, *(i - 1))
will befalse
.
[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 26.8.6 [alg.merge] 1 and 2:
1
Effects: Merges two sorted ranges[first1,last1)
and[first2,last2)
into the range[result, result + (last1 - first1) + (last2 - first2))
.Effects: Copies all the elements of the two ranges
[first1,last1)
and[first2,last2)
into the range[result, result_last)
, whereresult_last
isresult + (last1 - first1) + (last2 - first2)
, such that the resulting range satisfiesis_sorted(result, result_last)
oris_sorted(result, result_last, comp)
, respectively.2 Requires: The ranges
[first1,last1)
and[first2,last2)
shall be sorted with respect tooperator<
orcomp
. The resulting range shall not overlap with either of the original ranges.The list will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
in[first,last)
other thanfirst
, the condition*i < *(i - 1)
orcomp(*i, *(i - 1))
will befalse
.
Change 26.8.6 [alg.merge] p. 6+7 as indicated [This ensures harmonization
between inplace_merge
and merge
]
6 Effects: Merges two
sortedconsecutive ranges[first,middle)
and[middle,last)
, putting the result of the merge into the range[first,last)
. The resulting range will be in non-decreasing order; that is, for every iteratori
in[first,last)
other thanfirst
, the condition*i < *(i - 1)
or, respectively,comp(*i, *(i - 1))
will be false.7 Requires: The ranges
[first,middle)
and[middle,last)
shall be sorted with respect tooperator<
orcomp
. The type of*first
shall satisfy theSwappable
requirements (37), theMoveConstructible
requirements (Table 33), and the theMoveAssignable
requirements (Table 35).
std::complex
should add missing C99 functionsSection: 29.4.7 [complex.value.ops] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A comparision of the N2461 header <complex>
synopsis ([complex.syn])
with the C99 standard (ISO 9899, 2nd edition and the two corrigenda) show
some complex functions that are missing in C++. These are:
cproj
functions
cerf cerfc cexp2 cexpm1 clog10 clog1p clog2 clgamma ctgamma
I propose that at least the required cproj
overloads are provided as equivalent
C++ functions. This addition is easy to do in one sentence (delegation to C99
function).
Please note also that the current entry polar
in 29.4.10 [cmplx.over] p. 1
should be removed from the mentioned overload list. It does not make sense to require that a
function already expecting scalar arguments
should cast these arguments into corresponding
complex<T>
arguments, which are not accepted by
this function.
Proposed resolution:
In 29.4.2 [complex.syn] add just between the declaration of conj
and fabs
:
template<class T> complex<T> conj(const complex<T>&); template<class T> complex<T> proj(const complex<T>&); template<class T> complex<T> fabs(const complex<T>&);
In 29.4.7 [complex.value.ops] just after p.6 (return clause of conj
) add:
template<class T> complex<T> proj(const complex<T>& x);Effects: Behaves the same as C99 function
cproj
, defined in subclause 7.3.9.4."
In 29.4.10 [cmplx.over] p. 1, add one further entry proj
to
the overload list.
The following function templates shall have additional overloads:
arg norm conjpolarproj imag real
seed_seq
constructor is uselessSection: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-27 Last modified: 2015-10-20
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
Part of the resolution of n2423, issue 8 was the proposal to
extend the seed_seq
constructor accepting an input range
as follows (which is now part of N2461):
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end);
First, the expression iterator_traits<InputIterator>::value_type
is invalid due to missing typename
keyword, which is easy to
fix.
Second (and worse), while the language now supports default
template arguments of function templates, this customization
point via the second size_t
template parameter is of no advantage,
because u
can never be deduced, and worse - because it is a
constructor function template - it can also never be explicitly
provided (13.10.2 [temp.arg.explicit]/7).
The question arises, which advantages result from a compile-time
knowledge of u
versus a run time knowledge? If run time knowledge
suffices, this parameter should be provided as normal function
default argument [Resolution marked (A)], if compile-time knowledge
is important, this could be done via a tagging template or more
user-friendly via a standardized helper generator function
(make_seed_seq
), which allows this [Resolution marked (B)].
[ Bellevue: ]
Fermilab does not have a strong opinion. Would prefer to go with solution A. Bill agrees that solution A is a lot simpler and does the job.
Proposed Resolution: Accept Solution A.
Issue 803(i) claims to make this issue moot.
Proposed resolution:
In 29.5.8.1 [rand.util.seedseq]/2, class seed_seq
synopsis replace:
class seed_seq { public: ... template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end, size_t u = numeric_limits<typename iterator_traits<InputIterator>::value_type>::digits); ... };
and do a similar replacement in the member description between p.3 and p.4.
In 29.5.8.1 [rand.util.seedseq]/2, class seed_seq
synopsis and in the
member description between p.3 and p.4 replace:
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end); template<class InputIterator, size_t u> seed_seq(InputIterator begin, InputIterator end, implementation-defined s);
In 29.5.2 [rand.synopsis], header <random>
synopsis, immediately after the
class seed_seq
declaration and in 29.5.8.1 [rand.util.seedseq]/2, immediately
after the class seed_seq
definition add:
template<size_t u, class InputIterator> seed_seq make_seed_seq(InputIterator begin, InputIterator end);
In 29.5.8.1 [rand.util.seedseq], just before p.5 insert two paragraphs:
The first constructor behaves as if it would provide an integral constant expression
u
of typesize_t
of valuenumeric_limits<typename iterator_traits<InputIterator>::value_type>::digits
.The second constructor uses an implementation-defined mechanism to provide an integral constant expression
u
of typesize_t
and is called by the functionmake_seed_seq
.
In 29.5.8.1 [rand.util.seedseq], just after the last paragraph add:
template<size_t u, class InputIterator> seed_seq make_seed_seq(InputIterator begin, InputIterator end);where
u
is used to construct an objects
of implementation-defined type.Returns:
seed_seq(begin, end, s)
;
thread::id
reuseSection: 32.4.3.2 [thread.thread.id] Status: CD1 Submitter: Hans Boehm Opened: 2008-02-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.id].
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Discussion:
The current working paper
(N2497,
integrated just before Bellevue) is
not completely clear whether a given thread::id
value may be reused once
a thread has exited and has been joined or detached. Posix allows
thread ids (pthread_t
values) to be reused in this case. Although it is
not completely clear whether this originally was the right decision, it
is clearly the established practice, and we believe it was always the
intent of the C++ threads API to follow Posix and allow this. Howard
Hinnant's example implementation implicitly relies on allowing reuse
of ids, since it uses Posix thread ids directly.
It is important to be clear on this point, since it the reuse of thread ids often requires extra care in client code, which would not be necessary if thread ids were unique across all time. For example, a hash table indexed by thread id may have to be careful not to associate data values from an old thread with a new one that happens to reuse the id. Simply removing the old entry after joining a thread may not be sufficient, if it creates a visible window between the join and removal during which a new thread with the same id could have been created and added to the table.
[ post Bellevue Peter adds: ]
There is a real issue with
thread::id
reuse, but I urge the LWG to reconsider fixing this by disallowing reuse, rather than explicitly allowing it. Dealing with thread id reuse is an incredibly painful exercise that would just force the world to reimplement a non-conflictingthread::id
over and over.In addition, it would be nice if a
thread::id
could be manipulated atomically in a lock-free manner, as motivated by the recursive lock example:http://www.decadent.org.uk/pipermail/cpp-threads/2006-August/001091.html
Proposed resolution:
Add a sentence to 32.4.3.2 [thread.thread.id]/p1:
An object of type
thread::id
provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.threads.class]). Each thread of execution has athread::id
that is not equal to thethread::id
of other threads of execution and that is not equal to thethread::id
ofstd::thread
objects that do not represent threads of execution. The library may reuse the value of athread::id
of a terminated thread that can no longer be joined.
Section: 30 [time] Status: Resolved Submitter: Christopher Kohlhoff, Jeff Garland Opened: 2008-02-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The draft C++0x thread library requires that the time points of type
system_time
and returned by get_system_time()
represent Coordinated
Universal Time (UTC) (section [datetime.system]). This can lead to
surprising behavior when a library user performs a duration-based wait,
such as condition_variable::timed_wait()
. A complete explanation of the
problem may be found in the
Rationale for the Monotonic Clock
section in POSIX, but in summary:
condition_variable::timed_wait()
(and its POSIX
equivalent, pthread_cond_timedwait()
) are specified using absolute times
to address the problem of spurious wakeups.
condition_variable::timed_wait()
that behave as if by calling the
corresponding absolute time overload with a time point value of
get_system_time() + rel_time
.
POSIX solves the problem by introducing a new monotonic clock, which is
unaffected by changes to the system time. When a condition variable is
initialized, the user may specify whether the monotonic clock is to be
used. (It is worth noting that on POSIX systems it is not possible to
use condition_variable::native_handle()
to access this facility, since
the desired clock type must be specified during construction of the
condition variable object.)
In the context of the C++0x thread library, there are added dimensions to the problem due to the need to support platforms other than POSIX:
One possible minimal solution:
system_time
and get_system_time()
implementation-defined (i.e standard library implementors may choose the
appropriate underlying clock based on the capabilities of the target
platform).
system_time::seconds_since_epoch()
.
explicit system_time(time_t secs, nanoseconds ns
= 0)
to explicit system_time(nanoseconds ns)
.
Proposed resolution:
Rationale:
Addressed by N2661: A Foundation to Sleep On.
binary_search
Section: 26.8.4.5 [binary.search] Status: CD1 Submitter: Daniel Krügler Opened: 2007-09-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
In 26.8.4.5 [binary.search] p. 3 the complexity of binary_search
is described as
At most
log(last - first) + 2
comparisons.
This should be precised and brought in line with the nomenclature used for
lower_bound
, upper_bound
, and equal_range
.
All existing libraries I'm aware of, delegate to
lower_bound
(+ one further comparison). Since
issue 384(i) has now WP status, the resolution of #787 should
be brought in-line with 384(i) by changing the + 2
to + O(1)
.
[ Sophia Antipolis: ]
Alisdair prefers to apply an upper bound instead of O(1), but that would require fixing for
lower_bound
,upper_bound
etc. as well. If he really cares about it, he'll send an issue to Howard.
Proposed resolution:
Change 26.8.4.5 [binary.search]/3
Complexity: At most
log2(last - first) +
comparisons.2O(1)
Section: 24.6.2 [istream.iterator] Status: C++11 Submitter: Martin Sebor Opened: 2008-02-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.iterator].
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Discussion:
Addresses UK 287
It is not clear what the initial state of an
istream_iterator
should be. Is _value_ initialized by reading the stream, or default/value initialized? If it is initialized by reading the stream, what happens if the initialization is deferred until first dereference, when ideally the iterator value should have been that of an end-of-stream iterator which is not safely dereferencable?Recommendation: Specify _value_ is initialized by reading the stream, or the iterator takes on the end-of-stream value if the stream is empty.
The description of how an istream_iterator object becomes an end-of-stream iterator is a) ambiguous and b) out of date WRT issue 468(i):
istream_iterator
reads (usingoperator>>
) successive elements from the input stream for which it was constructed. After it is constructed, and every time++
is used, the iterator reads and stores a value ofT
. If the end of stream is reached (operator void*()
on the stream returnsfalse
), the iterator becomes equal to the end-of-stream iterator value. The constructor with no argumentsistream_iterator()
always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result ofoperator*
on an end of stream is not defined. For any other iterator value aconst T&
is returned. The result ofoperator->
on an end of stream is not defined. For any other iterator value aconst T*
is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that++
operators are not equality preserving, that is,i == j
does not guarantee at all that++i == ++j
. Every time++
is used a new value is read.
istream::operator void*()
returns null if istream::fail()
is true
,
otherwise non-null. istream::fail()
returns true
if failbit
or
badbit
is set in rdstate()
. Reaching the end of stream doesn't
necessarily imply that failbit
or badbit
is set (e.g., after
extracting an int
from stringstream("123")
the stream object will
have reached the end of stream but fail()
is false
and operator
void*()
will return a non-null value).
Also I would prefer to be explicit about calling fail()
here
(there is no operator void*()
anymore.)
[ Summit: ]
Moved from Ready to Open for the purposes of using this issue to address NB UK 287. Martin to handle.
[ 2009-07 Frankfurt: ]
This improves the wording.
Move to Ready.
Proposed resolution:
Change 24.6.2 [istream.iterator]/1:
istream_iterator
reads (usingoperator>>
) successive elements from the input stream for which it was constructed. After it is constructed, and every time++
is used, the iterator reads and stores a value ofT
. Ifthe end of stream is reachedthe iterator fails to read and store a value ofT
(on the stream returns
operator void*()fail()), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments
falsetrueistream_iterator()
always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result ofoperator*
on an end of stream is not defined. For any other iterator value aconst T&
is returned. The result ofoperator->
on an end of stream is not defined. For any other iterator value aconst T*
is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that++
operators are not equality preserving, that is,i == j
does not guarantee at all that++i == ++j
. Every time++
is used a new value is read.
xor_combine_engine(result_type)
should be explicitSection: 99 [rand.adapt.xor] Status: CD1 Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.adapt.xor].
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Discussion:
xor_combine_engine(result_type)
should be explicit
. (Obvious oversight.)
[ Bellevue: ]
Non-controversial. Bill is right, but Fermilab believes that this is easy to use badly and hard to use right, and so it should be removed entirely. Got into TR1 by well defined route, do we have permission to remove stuff? Should probably check with Jens, as it is believed he is the originator. Broad consensus that this is not a robust engine adapter.
Proposed resolution:
Remove xor_combine_engine from synopsis of 29.5.2 [rand.synopsis].
Remove 99 [rand.adapt.xor] xor_combine_engine
.
piecewise_constant_distribution
is undefined for a range with just one endpointSection: 29.5.9.6.2 [rand.dist.samp.pconst] Status: CD1 Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.pconst].
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Discussion:
piecewise_constant_distribution
is undefined for a range with just one
endpoint. (Probably should be the same as an empty range.)
Proposed resolution:
Change 29.5.9.6.2 [rand.dist.samp.pconst] paragraph 3b:
b) If
firstB == lastB
or the sequencew
has the length zero,
discrete_distribution
missing constructorSection: 29.5.9.6.1 [rand.dist.samp.discrete] Status: Resolved Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.discrete].
View all issues with Resolved status.
Discussion:
discrete_distribution
should have a constructor like:
template<class _Fn> discrete_distribution(result_type _Count, double _Low, double _High, _Fn& _Func);
(Makes it easier to fill a histogram with function values over a range.)
[ Bellevue: ]
How do you specify the function so that it does not return negative values? If you do it is a bad construction. This requirement is already there. Where in each bin does one evaluate the function? In the middle. Need to revisit tomorrow.
[ Sophia Antipolis: ]
Bill is not requesting this.
Marc Paterno:
_Fn
cannot return negative values at the points where the function is sampled. It is sampled in the middle of each bin._Fn
cannot return 0 everywhere it is sampled.Jens: lambda expressions are rvalues
Add a library issue to provide an
initializer_list<double>
constructor fordiscrete_distribution
.Marc Paterno: dislikes reference for
_Fn
parameter. Make it pass-by-value (to use lambda), usestd::ref
to wrap giant-state function objects.Daniel: See
random_shuffle
, pass-by-rvalue-reference.Daniel to draft wording.
[ Pre San Francisco, Daniel provided wording: ]
The here proposed changes of the WP refer to the current state of N2691. During the Sophia Antipolis meeting two different proposals came up regarding the functor argument type, either by value or by rvalue-reference. For consistence with existing conventions (state-free algorithms and the
general_pdf_distribution
c'tor signature) the author decided to propose a function argument that is provided by value. If severe concerns exists that stateful functions would be of dominant relevance, it should be possible to replace the two occurrences ofFunc
byFunc&&
in this proposal as part of an editorial process.
Proposed resolution:
Non-concept version of the proposed resolution
In 29.5.9.6.1 [rand.dist.samp.discrete]/1, class discrete_distribution
, just
before the member declaration
explicit discrete_distribution(const param_type& parm);
insert:
template<typename Func> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);
Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::
template<typename Func> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);Complexity: Exactly nf invocations of fw.
Requires:
- fw shall be callable with one argument of type double, and shall return values of a type convertible to double;
- If nf > 0, the relation
xmin
<xmax
shall hold, and for all sample valuesxk
, fw(xk
) shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- The following relations shall hold: nf ≥ 0, and 0 < S =
w0
+. . .+wn-1
.Effects:
- If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value
w0
= 1.Otherwise, sets n = nf, deltax = (
xmax
-xmin
)/n andxcent
=xmin
+ 0.5 * deltax.For each k = 0, . . . ,n-1, calculates:
xk
=xcent
+ k * deltaxwk
= fw(xk
)Constructs a discrete_distribution object with probabilities:
pk
=wk
/S for k = 0, . . . , n-1.
Concept version of the proposed resolution
In 29.5.9.6.1 [rand.dist.samp.discrete]/1, class discrete_distribution
, just
before the member declaration
explicit discrete_distribution(const param_type& parm);
insert:
template<Callable<auto, double> Func> requires Convertible<Func::result_type, double> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);
Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::
template<Callable<auto, double> Func> requires Convertible<Func::result_type, double> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);Complexity: Exactly nf invocations of fw.
Requires:
- If nf > 0, the relation
xmin
<xmax
shall hold, and for all sample valuesxk
, fw(xk
) shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- The following relations shall hold: nf ≥ 0, and 0 < S =
w0
+. . .+wn-1
.Effects:
- If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value
w0
= 1.Otherwise, sets n = nf, deltax = (
For each k = 0, . . . ,n-1, calculates:xmax
-xmin
)/n andxcent
=xmin
+ 0.5 * deltax.xk
=xcent
+ k * deltaxwk
= fw(xk
)Constructs a discrete_distribution object with probabilities:
pk
=wk
/S for k = 0, . . . , n-1.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
piecewise_constant_distribution
missing constructorSection: 29.5.9.6.2 [rand.dist.samp.pconst] Status: Resolved Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.pconst].
View all issues with Resolved status.
Discussion:
piecewise_constant_distribution
should have a constructor like:
template<class _Fn> piecewise_constant_distribution(size_t _Count, _Ty _Low, _Ty _High, _Fn& _Func);
(Makes it easier to fill a histogram with function values over a range.
The two (reference 793(i)) make a sensible replacement for
general_pdf_distribution
.)
[ Sophia Antipolis: ]
Marc: uses variable width of bins and weight for each bin. This is not giving enough flexibility to control both variables.
Add a library issue to provide an constructor taking an
initializer_list<double>
and_Fn
forpiecewise_constant_distribution
.Daniel to draft wording.
[ Pre San Francisco, Daniel provided wording. ]
The here proposed changes of the WP refer to the current state of N2691. For reasons explained in 793(i), the author decided to propose a function argument that is provided by value. The issue proposes a c'tor signature, that does not take advantage of the full flexibility of
piecewise_constant_distribution
, because it restricts on a constant bin width, but the use-case seems to be popular enough to justify it's introduction.
Proposed resolution:
Non-concept version of the proposed resolution
In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution
,
just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert:
template<typename Func> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);
Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:
template<typename Func> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);[p5_1] Complexity: Exactly
nf
invocations offw
.[p5_2] Requires:
fw
shall be callable with one argument of typeRealType
, and shall return values of a type convertible to double;- For all sample values
xk
defined below, fw(xk
) shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- The following relations shall hold:
xmin
<xmax
, and 0 < S =w0
+. . .+wn-1
.[p5_3] Effects:
If nf == 0,
- sets deltax =
xmax
-xmin
, and- lets the sequence
w
have lengthn = 1
and consist of the single valuew0
= 1, and- lets the sequence
b
have lengthn+1
withb0
=xmin
andb1
=xmax
Otherwise,
- sets
n = nf
,deltax =
(xmax
-xmin
)/n,xcent
=xmin
+ 0.5 * deltax, andlets the sequences
for each k = 0, . . . ,n-1, calculates:w
andb
have lengthn
andn+1
, resp. anddxk
= k * deltaxbk
=xmin
+dxk
xk
=xcent
+dxk
wk
= fw(xk
),and
- sets
bn
=xmax
Constructs a
piecewise_constant_distribution
object with the above computed sequenceb
as the interval boundaries and with the probability densities:
ρk
=wk
/(S * deltax) for k = 0, . . . , n-1.[p5_4] [Note: In this context, the subintervals [
bk
,bk+1
) are commonly known as the bins of a histogram. -- end note]
Concept version of the proposed resolution
In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution
,
just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);
Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);[p5_1] Complexity: Exactly
nf
invocations offw
.[p5_2] Requires:
- For all sample values
xk
defined below, fw(xk
) shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- The following relations shall hold:
xmin
<xmax
, and 0 < S =w0
+. . .+wn-1
.[p5_3] Effects:
If nf == 0,
- sets deltax =
xmax
-xmin
, and- lets the sequence
w
have lengthn = 1
and consist of the single valuew0
= 1, and- lets the sequence
b
have lengthn+1
withb0
=xmin
andb1
=xmax
Otherwise,
- sets
n = nf
,deltax =
(xmax
-xmin
)/n,xcent
=xmin
+ 0.5 * deltax, andlets the sequences
for each k = 0, . . . ,n-1, calculates:w
andb
have lengthn
andn+1
, resp. anddxk
= k * deltaxbk
=xmin
+dxk
xk
=xcent
+dxk
wk
= fw(xk
),and
- sets
bn
=xmax
Constructs a
piecewise_constant_distribution
object with the above computed sequenceb
as the interval boundaries and with the probability densities:
ρk
=wk
/(S * deltax) for k = 0, . . . , n-1.[p5_4] [Note: In this context, the subintervals [
bk
,bk+1
) are commonly known as the bins of a histogram. -- end note]
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
Section: 99 [depr.lib.binders] Status: CD1 Submitter: Daniel Krügler Opened: 2008-02-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [depr.lib.binders].
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Discussion:
N2521
and its earlier predecessors have moved the old binders from
[lib.binders] to 99 [depr.lib.binders] thereby introducing some renaming
of the template parameter names (Operation -> Fn
). During this
renaming process the protected data member op
was also renamed to
fn
, which seems as an unnecessary interface breakage to me - even if
this user access point is probably rarely used.
Proposed resolution:
Change [depr.lib.binder.1st]:
template <class Fn> class binder1st : public unary_function<typename Fn::second_argument_type, typename Fn::result_type> { protected: Fnfnop; typename Fn::first_argument_type value; public: binder1st(const Fn& x, const typename Fn::first_argument_type& y); typename Fn::result_type operator()(const typename Fn::second_argument_type& x) const; typename Fn::result_type operator()(typename Fn::second_argument_type& x) const; };-1- The constructor initializes
fn
op
withx
andvalue
withy
.-2-
operator()
returns.
fnop(value,x)
Change [depr.lib.binder.2nd]:
template <class Fn> class binder2nd : public unary_function<typename Fn::first_argument_type, typename Fn::result_type> { protected: Fnfnop; typename Fn::second_argument_type value; public: binder2nd(const Fn& x, const typename Fn::second_argument_type& y); typename Fn::result_type operator()(const typename Fn::first_argument_type& x) const; typename Fn::result_type operator()(typename Fn::first_argument_type& x) const; };-1- The constructor initializes
fn
op
withx
andvalue
withy
.-2-
operator()
returns.
fnop(value,x)
Section: 29.5.8.1 [rand.util.seedseq] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2008-02-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
The for-loop in the algorithm specification has n
iterations, where n
is
defined to be end - begin
, i.e. the number of supplied w-bit quantities.
Previous versions of this algorithm and the general logic behind it
suggest that this is an oversight and that in the context of the
for-loop n
should be the number of full 32-bit quantities in b
(rounded
upwards). If w
is 64, the current algorithm throws away half of all bits
in b
. If w
is 16, the current algorithm sets half of all elements in v
to 0.
There are two more minor issues:
end - begin
is not defined since
InputIterator
is not required to be a random access iterator.
seed_seq
constructor, including bool
. IMHO allowing bool
s unnecessarily
complicates the implementation without any real benefit to the user.
I'd suggest to exclude bool
s as input.
[ Bellevue: ]
Move to Open: Bill will try to propose a resolution by the next meeting.
[ post Bellevue: Bill provided wording. ]
This issue is made moot if 803(i) is accepted.
Proposed resolution:
Replace 29.5.8.1 [rand.util.seedseq] paragraph 6 with:
Effects: Constructs a
seed_seq
object by effectively concatenating the low-orderu
bits of each of the elements of the supplied sequence[begin, end)
in ascending order of significance to make a (possibly very large) unsigned binary numberb
having a total ofn
bits, and then carrying out the following algorithm:for( v.clear(); n > 0; n -= 32 ) v.push_back(b mod 232), b /= 232;
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
tuple
and pair
trivial membersSection: 22.4 [tuple] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-02-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple].
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Discussion:
Classes with trivial special member functions are inherently more
efficient than classes without such functions. This efficiency is
particularly pronounced on modern ABIs that can pass small classes
in registers. Examples include value classes such as complex numbers
and floating-point intervals. Perhaps more important, though, are
classes that are simple collections, like pair
and tuple
. When the
parameter types of these classes are trivial, the pair
s and tuple
s
themselves can be trivial, leading to substantial performance wins.
The current working draft make specification of trivial functions
(where possible) much easer through default
ed and delete
d functions.
As long as the semantics of defaulted and deleted functions match
the intended semantics, specification of defaulted and deleted
functions will yield more efficient programs.
There are at least two cases where specification of an explicitly defaulted function may be desirable.
First, the std::pair
template has a non-trivial default constructor,
which prevents static initialization of the pair even when the
types are statically initializable. Changing the definition to
pair() = default;
would enable such initialization. Unfortunately, the change is not semantically neutral in that the current definition effectively forces value initialization whereas the change would not value initialize in some contexts.
** Does the committee confirm that forced value initialization
was the intent? If not, does the committee wish to change the
behavior of std::pair
in C++0x?
Second, the same default constructor issue applies to std::tuple
.
Furthermore, the tuple
copy constructor is current non-trivial,
which effectively prevents passing it in registers. To enable
passing tuples
in registers, the copy constructor should be
make explicitly default
ed. The new declarations are:
tuple() = default; tuple(const tuple&) = default;
This changes is not implementation neutral. In particular, it prevents implementations based on pointers to the parameter types. It does however, permit implementations using the parameter types as bases.
** How does the committee wish to trade implementation efficiency versus implementation flexibility?
[ Bellevue: ]
General agreement; the first half of the issue is NAD.
Before voting on the second half, it was agreed that a "Strongly Favor" vote meant support for trivial tuples (assuming usual requirements met), even at the expense of other desired qualities. A "Weakly Favor" vote meant support only if not at the expense of other desired qualities.
Concensus: Go forward, but not at expense of other desired qualities.
It was agreed to Alisdair should fold this work in with his other pair/tuple action items, above, and that issue 801 should be "open", but tabled until Alisdair's proposals are disposed of.
[ 2009-05-27 Daniel adds: ]
[ 2009-07 Frankfurt: ]
Wait for dust to settle from fixing exception safety problem with rvalue refs.
[ 2009-07-20 Alisdair adds: ]
Basically, this issue is what should we do with the default constructor for pairs and tuples of trivial types. The motivation of the issue was to force static initialization rather than dynamic initialization, and was rejected in the case of pair as it would change the meaning of existing programs. The advice was "do the best we can" for tuple without changing existing meaning.
Frankfurt seems to simply wait and see the resolution on no-throw move constructors, which (I believe) is only tangentially related to this issue, but as good as any to defer until Santa Cruz.
Looking again now, I think constant (static) initialization for pair can be salvaged by making the default construct constexpr. I have a clarification from Core that this is intended to work, even if the constructor is not trivial/constexpr, so long as no temporaries are implied in the process (even if elided).
[ 2009-10 Santa Cruz: ]
Leave as open. Alisdair to provide wording.
[ 2010 Pittsburgh: ]
We believe this may be NAD Editorial since both pair and tuple now have constexpr default constructors, but we're not sure.
[ 2010 Rapperswil: ]
Daniel believes his pair/tuple paper will resolve this issue.
constexpr
will allow static initialization, and he is already changing the move and copy constructors to be defaulted.
[ 2010-10-24 Daniel adds: ]
The proposed resolution of n3140 should resolve this issue.
Proposed resolution:
See n3140.
seed_seq::seq_seq
Section: 29.5.8.1 [rand.util.seedseq] Status: Resolved Submitter: Charles Karney Opened: 2008-02-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
seed_seq(InputIterator begin, InputIterator end);
constructs a seed_seq
object repacking the bits of supplied sequence [begin, end)
into a
32-bit vector.
This repacking triggers several problems:
seed_seq::generate
required the
introduction of the initial "if (w < 32) v.push_back(n);
" (Otherwise
the unsigned short vectors [1, 0] and [1] generate the same sequence.)
u
.
(Otherwise some sequences could not be obtained on computers where no
integer types are exactly 32-bits wide.)
I propose simplifying this seed_seq
constructor to be "32-bit only".
Despite it's being simpler, there is NO loss of functionality (see
below).
Here's how the description would read
29.5.8.1 [rand.util.seedseq] Class
seed_seq
template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);5 Requires: NO CHANGE
6 Effects: Constructs a
seed_seq
object byfor (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);
Discussion:
The chief virtues here are simplicity, portability, and generality.
iterator_traits<InputIterator>::value_type =
uint_least32_t
the user is guaranteed to get the same behavior across
platforms.
Arguments (and counter-arguments) against making this change (and retaining the n2461 behavior) are:
The user can pass an array of unsigned char
and seed_seq
will nicely
repack it.
Response: So what? Consider the seed string "ABC". The n2461 proposal results in
v = { 0x3, 0x434241 };
while the simplified proposal yields
v = { 0x41, 0x42, 0x43 };
The results produced by seed_seq::generate
with the two inputs are
different but nevertheless equivalently "mixed up" and this remains
true even if the seed string is long.
With long strings (e.g., with bit-length comparable to the number of
bits in the state), v
is longer (by a factor of 4) with the simplified
proposal and seed_seq::generate
will be slower.
Response: It's unlikely that the efficiency of seed_seq::generate
will
be a big issue. If it is, the user is free to repack the seed vector
before constructing seed_seq
.
A user can pass an array of 64-bit integers and all the bits will be used.
Response: Indeed. However, there are many instances in the n2461 where integers are silently coerced to a narrower width and this should just be a case of the user needing to read the documentation. The user can of course get equivalent behavior by repacking his seed into 32-bit pieces. Furthermore, the unportability of the n2461 proposal with
unsigned long s[] = {1, 2, 3, 4}; seed_seq q(s, s+4);
which typically results in v = {1, 2, 3, 4}
on 32-bit machines and in
v = {1, 0, 2, 0, 3, 0, 4, 0}
on 64-bit machines is a major pitfall for
unsuspecting users.
Note: this proposal renders moot issues 782(i) and 800(i).
[ Bellevue: ]
Walter needs to ask Fermilab for guidance. Defer till tomorrow. Bill likes the proposed resolution.
[ Sophia Antipolis: ]
Marc Paterno wants portable behavior between 32bit and 64bit machines; we've gone to significant trouble to support portability of engines and their values.
Jens: the new algorithm looks perfectly portable
Marc Paterno to review off-line.
Modify the proposed resolution to read "Constructs a seed_seq object by the following algorithm ..."
Disposition: move to review; unanimous consent.
Proposed resolution:
Change 29.5.8.1 [rand.util.seedseq]:
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end);-5- Requires:
InputIterator
shall satisfy the requirements of an input iterator (24.1.1) such thatiterator_traits<InputIterator>::value_type
shall denote an integral type.-6- Constructs a
seed_seq
object by the following algorithmrearranging some or all of the bits of the supplied sequence[begin,end)
of w-bit quantities into 32-bit units, as if by the following:
First extract the rightmostu
bits from each of then = end - begin
elements of the supplied sequence and concatenate all the extracted bits to initialize a single (possibly very large) unsigned binary number,b = ∑n-1i=0 (begin[i] mod 2u) · 2w·i
(in which the bits of eachbegin[i]
are treated as denoting an unsigned quantity). Then carry out the following algorithm:
v.clear(); if ($w$ < 32) v.push_back($n$); for( ; $n$ > 0; --$n$) v.push_back(b mod 232), b /= 232;for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
error_code
/error_condition
Section: 19.5 [syserr] Status: CD1 Submitter: Daniel Krügler Opened: 2008-02-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
19.5.4.1 [syserr.errcode.overview]/1, class error_code
and
19.5.5.1 [syserr.errcondition.overview]/, class error_condition
synopses
declare an expository data member cat_
:
const error_category& cat_; // exposition only
which is used to define the semantics of several members. The decision to use a member of reference type lead to several problems:
(Copy)Assignable
, which is probably not the intent.
The simple fix would be to replace the reference by a pointer member.
operator=
overload (template with ErrorCodeEnum
argument) makes in invalid
usage of std::enable_if
: By using the default value for the second enable_if
parameter the return type would be defined to be void&
even in otherwise
valid circumstances - this return type must be explicitly provided (In
error_condition
the first declaration uses an explicit value, but of wrong
type).
message
throws clauses (
19.5.3.2 [syserr.errcat.virtuals]/10, 19.5.4.4 [syserr.errcode.observers]/8, and
19.5.5.4 [syserr.errcondition.observers]/6) guarantee "throws nothing",
although
they return a std::string
by value, which might throw in out-of-memory
conditions (see related issue 771(i)).
[ Sophia Antipolis: ]
Part A: NAD (editorial), cleared by the resolution of issue 832(i).
Part B: Technically correct, save for typo. Rendered moot by the concept proposal (N2620) NAD (editorial).
Part C: We agree; this is consistent with the resolution of issue 721(i).
Howard: please ping Beman, asking him to clear away parts A and B from the wording in the proposed resolution, so it is clear to the editor what needs to be applied to the working paper.
Beman provided updated wording. Since issue 832(i) is not going forward, the provided wording includes resolution of part A.
Proposed resolution:
Resolution of part A:
Change 19.5.4.1 [syserr.errcode.overview] Class error_code overview synopsis as indicated:
private: int val_; // exposition only const error_category&* cat_; // exposition onlyChange 19.5.4.2 [syserr.errcode.constructors] Class error_code constructors as indicated:
error_code();Effects: Constructs an object of type
error_code
.Postconditions:
val_ == 0
andcat_ == &system_category
.Throws: Nothing.
error_code(int val, const error_category& cat);Effects: Constructs an object of type
error_code
.Postconditions:
val_ == val
andcat_ == &cat
.Throws: Nothing.
Change 19.5.4.3 [syserr.errcode.modifiers] Class error_code modifiers as indicated:
void assign(int val, const error_category& cat);Postconditions:
val_ == val
andcat_ == &cat
.Throws: Nothing.
Change 19.5.4.4 [syserr.errcode.observers] Class error_code observers as indicated:
const error_category& category() const;Returns:
*cat_
.Throws: Nothing.
Change 19.5.5.1 [syserr.errcondition.overview] Class error_condition overview synopsis as indicated:
private: int val_; // exposition only const error_category&* cat_; // exposition onlyChange 19.5.5.2 [syserr.errcondition.constructors] Class error_condition constructors as indicated:
[ (If the proposed resolution of issue 805(i) has already been applied, the name
posix_category
will have been changed togeneric_category
. That has no effect on this resolution.) ]error_condition();Effects: Constructs an object of type
error_condition
.Postconditions:
val_ == 0
andcat_ == &posix_category
.Throws: Nothing.
error_condition(int val, const error_category& cat);Effects: Constructs an object of type
error_condition
.Postconditions:
val_ == val
andcat_ == &cat
.Throws: Nothing.
Change 19.5.5.3 [syserr.errcondition.modifiers] Class error_condition modifiers as indicated:
void assign(int val, const error_category& cat);Postconditions:
val_ == val
andcat_ == &cat
.Throws: Nothing.
Change 19.5.5.4 [syserr.errcondition.observers] Class error_condition observers as indicated:
const error_category& category() const;Returns:
*cat_
.Throws: Nothing.
Resolution of part C:
In 19.5.3.2 [syserr.errcat.virtuals], remove the throws clause p. 10.
virtual string message(int ev) const = 0;Returns: A string that describes the error condition denoted by
ev
.
Throws: Nothing.In 19.5.4.4 [syserr.errcode.observers], remove the throws clause p. 8.
string message() const;Returns:
category().message(value())
.
Throws: Nothing.In 19.5.5.4 [syserr.errcondition.observers], remove the throws clause p. 6.
string message() const;Returns:
category().message(value())
.
Throws: Nothing.
posix_error::posix_errno
concernsSection: 19.5 [syserr] Status: CD1 Submitter: Jens Maurer Opened: 2008-02-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
19.5 [syserr]
namespace posix_error { enum posix_errno { address_family_not_supported, // EAFNOSUPPORT ...
should rather use the new scoped-enum facility (9.8.1 [dcl.enum]),
which would avoid the necessity for a new posix_error
namespace, if I understand correctly.
[ Further discussion: ]
See N2347, Strongly Typed Enums, since renamed Scoped Enums.
Alberto Ganesh Barbati also raised this issue in private email, and also proposed the scoped-enum solution.
Nick Stoughton asked in Bellevue that
posix_error
andposix_errno
not be used as names. The LWG agreed.The wording for the Proposed resolution was provided by Beman Dawes.
Proposed resolution:
Change System error support 19.5 [syserr] as indicated:
namespace posix_error {enumposix_errnoclass errc { address_family_not_supported, // EAFNOSUPPORT ... wrong_protocol_type, // EPROTOTYPE };} // namespace posix_errortemplate <> struct is_error_condition_enum<posix_error::posix_errnoerrc> : public true_type {}namespace posix_error {error_code make_error_code(posix_errnoerrc e); error_condition make_error_condition(posix_errnoerrc e);} // namespace posix_error
Change System error support 19.5 [syserr] :
Theis_error_code_enum
andis_error_condition_enum
templates may be specialized for user-defined types to indicate that such a type is eligible for classerror_code
and classerror_condition
automatic conversions, respectively.
Change System error support 19.5 [syserr] and its subsections:
- remove all occurrences of
posix_error::
- change all instances of
posix_errno
toerrc
- change all instances of
posix_category
togeneric_category
- change all instances of
get_posix_category
toget_generic_category
Change Error category objects 19.5.3.5 [syserr.errcat.objects], paragraph 2:
Remarks: The object's
default_error_condition
and equivalent virtual functions shall behave as specified for the classerror_category
. The object's name virtual function shall return a pointer to the string"POSIX""generic".
Change 19.5.4.5 [syserr.errcode.nonmembers] Class error_code
non-member functions as indicated:
error_code make_error_code(posix_errnoerrc e);Returns:
error_code(static_cast<int>(e),
.posixgeneric_category)
Change 19.5.5.5 [syserr.errcondition.nonmembers] Class error_condition
non-member functions as indicated:
error_condition make_error_condition(posix_errnoerrc e);Returns:
error_condition(static_cast<int>(e),
.posixgeneric_category)
Rationale:
Names Considered | |
---|---|
portable |
Too non-specific. Did not wish to reserve such a common word in namespace std. Not quite the right meaning, either. |
portable_error |
Too long. Explicit qualification is always required for scoped enums, so
a short name is desirable. Not quite the right meaning, either. May be
misleading because *_error in the std lib is usually an exception class
name.
|
std_error |
Fairly short, yet explicit. But in fully qualified names like
std::std_error::not_enough_memory , the std_ would be unfortunate. Not
quite the right meaning, either. May be misleading because *_error in
the std lib is usually an exception class name.
|
generic |
Short enough. The category could be generic_category . Fully qualified
names like std::generic::not_enough_memory read well. Reserving in
namespace std seems dicey.
|
generic_error |
Longish. The category could be generic_category . Fully qualified names
like std::generic_error::not_enough_memory read well. Misleading because
*_error in the std lib is usually an exception class name.
|
generic_err |
A bit less longish. The category could be generic_category . Fully
qualified names like std::generic_err::not_enough_memory read well.
|
gen_err |
Shorter still. The category could be generic_category . Fully qualified
names like std::gen_err::not_enough_memory read well.
|
generr |
Shorter still. The category could be generic_category . Fully qualified
names like std::generr::not_enough_memory read well.
|
error |
Shorter still. The category could be generic_category . Fully qualified
names like std::error::not_enough_memory read well. Do we want to use
this general a name?
|
err |
Shorter still. The category could be generic_category . Fully qualified
names like std::err::not_enough_memory read well. Although alone it
looks odd as a name, given the existing errno and namespace std names,
it seems fairly intuitive.
Problem: err is used throughout the standard library as an argument name
and in examples as a variable name; it seems too confusing to add yet
another use of the name.
|
errc |
Short enough. The "c" stands for "constant". The category could be
generic_category . Fully qualified names like
std::errc::not_enough_memory read well. Although alone it looks odd as a
name, given the existing errno and namespace std names, it seems fairly
intuitive. There are no uses of errc in the current C++ standard.
|
unique_ptr::reset
effects incorrect, too permissiveSection: 20.3.1.3.6 [unique.ptr.single.modifiers] Status: CD1 Submitter: Peter Dimov Opened: 2008-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
void unique_ptr::reset(T* p = 0)
is currently specified as:
Effects: If
p == get()
there are no effects. Otherwiseget_deleter()(get())
.
There are two problems with this. One, if get() == 0
and p != 0
, the
deleter is called with a NULL
pointer, and this is probably not what's
intended (the destructor avoids calling the deleter with 0.)
Two, the special check for get() == p
is generally not needed and such a
situation usually indicates an error in the client code, which is being
masked. As a data point, boost::shared_ptr
was changed to assert on such
self-resets in 2001 and there were no complaints.
One might think that self-resets are necessary for operator=
to work; it's specified to perform
reset( u.release() );
and the self-assignment
p = move(p);
might appear to result in a self-reset. But it doesn't; the release()
is
performed first, zeroing the stored pointer. In other words, p.reset(
q.release() )
works even when p
and q
are the same unique_ptr
, and there
is no need to special-case p.reset( q.get() )
to work in a similar
scenario, as it definitely doesn't when p
and q
are separate.
Proposed resolution:
Change 20.3.1.3.6 [unique.ptr.single.modifiers]:
void reset(T* p = 0);-4- Effects: If
there are no effects. Otherwise
p ==get() == 0get_deleter()(get())
.
Change 20.3.1.4.5 [unique.ptr.runtime.modifiers]:
void reset(T* p = 0);...
-2- Effects: If
there are no effects. Otherwise
p ==get() == 0get_deleter()(get())
.
tuple
construction should not fail unless its element's construction failsSection: 22.4.4.2 [tuple.cnstr] Status: CD1 Submitter: Howard Hinnant Opened: 2008-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
527(i) Added a throws clause to bind
constructors. I believe the same throws clause
should be added to tuple
except it ought to take into account move constructors as well.
Proposed resolution:
Add to 22.4.4.2 [tuple.cnstr]:
For each
tuple
constructor and assignment operator, an exception is thrown only if the construction or assignment of one of the types inTypes
throws an exception.
Section: 22.2.4 [forward] Status: CD1 Submitter: Jens Maurer Opened: 2008-03-13 Last modified: 2016-02-01
Priority: Not Prioritized
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Discussion:
p4 (forward) says:
Return type: If
T
is an lvalue-reference type, an lvalue; otherwise, an rvalue.
First of all, lvalue-ness and rvalue-ness are properties of an expression, not of a type (see 7.2.1 [basic.lval]). Thus, the phrasing "Return type" is wrong. Second, the phrase says exactly what the core language wording says for folding references in 13.4.2 [temp.arg.type]/p4 and for function return values in 7.6.1.3 [expr.call]/p10. (If we feel the wording should be retained, it should at most be a note with cross-references to those sections.)
The prose after the example talks about "forwarding as an int&
(an lvalue)" etc.
In my opinion, this is a category error: "int&
" is a type, "lvalue" is a
property of an expression, orthogonal to its type. (Btw, expressions cannot
have reference type, ever.)
Similar with move:
Return type: an rvalue.
is just wrong and also redundant.
Proposed resolution:
Change 22.2.4 [forward] as indicated:
template <class T> T&& forward(typename identity<T>::type&& t);...
Return type: IfT
is an lvalue-reference type, an lvalue; otherwise, an rvalue....
-7- In the first call to
factory
,A1
is deduced asint
, so 2 is forwarded toA
's constructor asanan rvalueint&&
(). In the second call to factory,A1
is deduced asint&
, soi
is forwarded toA
's constructor asanan lvalueint&
(). In both cases,A2
is deduced as double, so 1.414 is forwarded toA
's constructor asan rvaluedouble&&
().template <class T> typename remove_reference<T>::type&& move(T&& t);...
Return type: an rvalue.
std::swap
should be overloaded for array typesSection: 26.7.3 [alg.swap] Status: CD1 Submitter: Niels Dekker Opened: 2008-02-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
For the sake of generic programming, the header <algorithm>
should provide an
overload of std::swap
for array types:
template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);
It became apparent to me that this overload is missing, when I considered how to write a swap
function for a generic wrapper class template.
(Actually I was thinking of Boost's value_initialized.)
Please look at the following template, W
, and suppose that is intended to be a very
generic wrapper:
template<class T> class W { public: T data; };
Clearly W<T>
is CopyConstructible and CopyAssignable, and therefore
Swappable, whenever T
is CopyConstructible and CopyAssignable.
Moreover, W<T>
is also Swappable when T
is an array type
whose element type is CopyConstructible and CopyAssignable.
Still it is recommended to add a custom swap function template to such a class template,
for the sake of efficiency and exception safety.
(E.g., Scott Meyers, Effective C++, Third Edition, item 25: Consider support for a non-throwing
swap.)
This function template is typically written as follows:
template<class T> void swap(W<T>& x, W<T>& y) { using std::swap; swap(x.data, y.data); }
Unfortunately, this will introduce an undesirable inconsistency, when T
is an array.
For instance, W<std::string[8]>
is Swappable, but the current Standard does not
allow calling the custom swap function that was especially written for W
!
W<std::string[8]> w1, w2; // Two objects of a Swappable type. std::swap(w1, w2); // Well-defined, but inefficient. using std::swap; swap(w1, w2); // Ill-formed, just because ADL finds W's swap function!!!
W
's swap
function would try to call std::swap
for an array,
std::string[8]
, which is not supported by the Standard Library.
This issue is easily solved by providing an overload of std::swap
for array types.
This swap function should be implemented in terms of swapping the elements of the arrays, so that
it would be non-throwing for arrays whose element types have a non-throwing swap.
Note that such an overload of std::swap
should also support multi-dimensional
arrays. Fortunately that isn't really an issue, because it would do so automatically, by
means of recursion.
For your information, there was a discussion on this issue at comp.lang.c++.moderated: [Standard Library] Shouldn't std::swap be overloaded for C-style arrays?
Proposed resolution:
Add an extra condition to the definition of Swappable requirements [swappable] in 16.4.4.2 [utility.arg.requirements]:
-
T
isSwappable
ifT
is an array type whose element type isSwappable
.
Add the following to 26.7.3 [alg.swap]:
template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);Requires: Type
T
shall beSwappable
.Effects:
swap_ranges(a, a + N, b);
Section: 31.7.8 [ext.manip] Status: C++11 Submitter: Daniel Krügler Opened: 2008-03-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The recent draft (as well as the original proposal n2072) uses an
operational semantic
for get_money
([ext.manip]/4) and put_money
([ext.manip]/6), which uses
istreambuf_iterator<charT>
and
ostreambuf_iterator<charT>
resp, instead of the iterator instances, with explicitly provided
traits argument (The operational semantic defined by f
is also traits
dependent). This is an obvious oversight because both *stream_buf
c'tors expect a basic_streambuf<charT,traits>
as argument.
The same problem occurs within the get_time
and put_time
semantic
where additional to the problem we
have an editorial issue in get_time
(streambuf_iterator
instead of
istreambuf_iterator
).
[ Batavia (2009-05): ]
This appears to be an issue of presentation.
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 31.7.8 [ext.manip]/4 within function f
replace the first line
template <class charT, class traits, class moneyT> void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) { typedef istreambuf_iterator<charT, traits> Iter; ...
In 31.7.8 [ext.manip]/5 remove the first template charT
parameter:
template <class charT,class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);
In 31.7.8 [ext.manip]/6 within function f
replace the first line
template <class charT, class traits, class moneyT> void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) { typedef ostreambuf_iterator<charT, traits> Iter; ...
In 31.7.8 [ext.manip]/8 within function f
replace the first line
template <class charT, class traits> void f(basic_ios<charT, traits>& str, struct tm *tmb, const charT *fmt) { typedef istreambuf_iterator<charT, traits> Iter; ...
In 31.7.8 [ext.manip]/10 within function f
replace the first line
template <class charT, class traits> void f(basic_ios<charT, traits>& str, const struct tm *tmb, const charT *fmt) { typedef ostreambuf_iterator<charT, traits> Iter; ...
In 31.7 [iostream.format], Header <iomanip>
synopsis change:
template <class charT,class moneyT> T8 put_money(const moneyT& mon, bool intl = false);
pair
of pointers no longer works with literal 0Section: 22.3 [pairs] Status: C++11 Submitter: Doug Gregor Opened: 2008-03-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
#include <utility> int main() { std::pair<char *, char *> p (0,0); }
I just got a bug report about that, because it's valid C++03, but not
C++0x. The important realization, for me, is that the emplace
proposal---which made push_back
variadic, causing the push_back(0)
issue---didn't cause this break in backward compatibility. The break
actually happened when we added this pair constructor as part of adding
rvalue references into the language, long before variadic templates or
emplace came along:
template<class U, class V> pair(U&& x, V&& y);
Now, concepts will address this issue by constraining that pair
constructor to only U
's and V
's that can properly construct "first" and
"second", e.g. (from
N2322):
template<class U , class V > requires Constructible<T1, U&&> && Constructible<T2, V&&> pair(U&& x , V&& y );
[ San Francisco: ]
Suggested to resolve using pass-by-value for that case.
Side question: Should pair interoperate with tuples? Can construct a tuple of a pair, but not a pair from a two-element tuple.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Leave as open. Howard to provide wording.
[ 2010-02-06 Howard provided wording. ]
[ 2010-02-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Rationale:
[ San Francisco: ]
Solved by N2770.
[ The rationale is obsolete. ]
Proposed resolution:
Add a paragraph to 22.3 [pairs]:
template<class U, class V> pair(U&& x, V&& y);6 Effects: The constructor initializes
first
withstd::forward<U>(x)
and second withstd::forward<V>(y)
.Remarks:
U
shall be implicitly convertible tofirst_type
andV
shall be implicitly convertible tosecond_type
, else this constructor shall not participate in overload resolution.
shared_ptr
Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Matt Austern Opened: 2008-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Several places in 20.3.2.2 [util.smartptr.shared] refer to an "empty" shared_ptr
.
However, that term is nowhere defined. The closest thing we have to a
definition is that the default constructor creates an empty shared_ptr
and that a copy of a default-constructed shared_ptr
is empty. Are any
other shared_ptr
s empty? For example, is shared_ptr((T*) 0)
empty? What
are the properties of an empty shared_ptr
? We should either clarify this
term or stop using it.
One reason it's not good enough to leave this term up to the reader's
intuition is that, in light of
N2351
and issue 711(i), most readers'
intuitive understanding is likely to be wrong. Intuitively one might
expect that an empty shared_ptr
is one that doesn't store a pointer,
but, whatever the definition is, that isn't it.
[ Peter adds: ]
Or, what is an "empty"
shared_ptr
?
Are any other
shared_ptrs
empty?Yes. Whether a given
shared_ptr
instance is empty or not is (*) completely specified by the last mutating operation on that instance. Give me an example and I'll tell you whether theshared_ptr
is empty or not.(*) If it isn't, this is a legitimate defect.
For example, is
shared_ptr((T*) 0)
empty?No. If it were empty, it would have a
use_count()
of 0, whereas it is specified to have anuse_count()
of 1.What are the properties of an empty
shared_ptr
?The properties of an empty
shared_ptr
can be derived from the specification. One example is that its destructor is a no-op. Another is that itsuse_count()
returns 0. I can enumerate the full list if you really like.We should either clarify this term or stop using it.
I don't agree with the imperative tone
A clarification would be either a no-op - if it doesn't contradict the existing wording - or a big mistake if it does.
I agree that a clarification that is formally a no-op may add value.
However, that term is nowhere defined.
Terms can be useful without a definition. Consider the following simplistic example. We have a type
X
with the following operations defined:X x; X x2(x); X f(X x); X g(X x1, X x2);A default-constructed value is green.
A copy has the same color as the original.
f(x)
returns a red value if the argument is green, a green value otherwise.
g(x1,x2)
returns a green value if the arguments are of the same color, a red value otherwise.Given these definitions, you can determine the color of every instance of type
X
, even if you have absolutely no idea what green and red mean.Green and red are "nowhere defined" and completely defined at the same time.
Alisdair's wording is fine.
Proposed resolution:
Append the following sentance to 20.3.2.2 [util.smartptr.shared]
The
shared_ptr
class template stores a pointer, usually obtained vianew
.shared_ptr
implements semantics of shared ownership; the last remaining owner of the pointer is responsible for destroying the object, or otherwise releasing the resources associated with the stored pointer. Ashared_ptr
object that does not own a pointer is said to be empty.
vector<bool>::swap(reference, reference)
not definedSection: 23.3.14 [vector.bool] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
vector<bool>::swap(reference, reference)
has no definition.
[ San Francisco: ]
Move to Open. Alisdair to provide a resolution.
[ Post Summit Daniel provided wording. ]
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Just after 23.3.14 [vector.bool]/5 add the following prototype and description:
static void swap(reference x, reference y);
-6- Effects: Exchanges the contents of
x
andy
as-if by:bool b = x; x = y; y = b;
std::function
and reference_closure
do not use perfect forwardingSection: 22.10.17.3.5 [func.wrap.func.inv] Status: Resolved Submitter: Alisdair Meredith Opened: 2008-03-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.inv].
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Discussion:
std::function
and reference_closure
should use "perfect forwarding" as
described in the rvalue core proposal.
[ Sophia Antipolis: ]
According to Doug Gregor, as far as
std::function
is concerned, perfect forwarding can not be obtained because of type erasure. Not everyone agreed with this diagnosis of forwarding.
[ 2009-05-01 Howard adds: ]
Sebastian Gesemann brought to my attention that the
CopyConstructible
requirement onfunction
'sArgTypes...
is an unnecessary restriction.template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)> ...On further investigation, this complaint seemed to be the same issue as this one. I believe the reason
CopyConstructible
was put onArgTypes
in the first place was because of the nature of the invoke member:template<class R, class ...ArgTypes> R function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(arg...); }However now with rvalue-refs, "by value" no longer implies
CopyConstructible
(as Sebastian correctly points out). If rvalue arguments are supplied,MoveConstructible
is sufficient. Furthermore, the constraint need not be applied infunction
if I understand correctly. Rather the client must apply the proper constraints at the call site. Therefore, at the very least, I recommend thatCopyConstructible
be removed from the template classfunction
.Furthermore we need to mandate that the invoker is coded as:
template<class R, class ...ArgTypes> R function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(std::forward<ArgTypes>(arg)...); }Note that
ArgTypes&&
(the "perfect forwarding signature") is not appropriate here as this is not a deduced context forArgTypes
. Instead the client's arguments must implicitly convert to the non-deducedArgType
type. Catching these arguments by value makes sense to enable decay.Next
forward
is used to move theArgTypes
as efficiently as possible, and also with minimum requirements (notCopyConstructible
) to the type-erased functor. For object types, this will be amove
. For reference typeArgTypes
, this will be a copy. The end result must be that the following is a valid program:#include <functional> #include <memory> #include <cassert> std::unique_ptr<int> f(std::unique_ptr<int> p, int& i) { ++i; return std::move(p); } int main() { int i = 2; std::function<std::unique_ptr<int>(std::unique_ptr<int>, int&> g(f); std::unique_ptr<int> p = g(std::unique_ptr<int>(new int(1)), i); assert(*p == 1); assert(i == 3); }[ Tested in pre-concepts rvalue-ref-enabled compiler. ]
In the example above, the first
ArgType
isunique_ptr<int>
and the secondArgType
isint&
. Both must work!
[ 2009-05-27 Daniel adds: ]
in the 2009-05-01 comment of above mentioned issue Howard
- Recommends to replace the
CopyConstructible
requirement by aMoveConstructible
requirement- Says: "Furthermore, the constraint need not be applied in
function
if I understand correctly. Rather the client must apply the proper constraints at the call site"I'm fine with (a), but I think comment (b) is incorrect, at least in the sense I read these sentences. Let's look at Howard's example code:
function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(std::forward<ArgTypes>(arg)...); }In the constrained scope of this
operator()
overload the expression "(*f_)(std::forward<ArgTypes>(arg)...)
" must be valid. How can it do so, ifArgTypes
aren't at leastMoveConstructible
?
[ 2009-07 Frankfurt: ]
Leave this open and wait until concepts are removed from the Working Draft so that we know how to write the proposed resolution in terms of diffs to otherwise stable text.
[ 2009-10 Santa Cruz: ]
Leave as open. Howard to provide wording. Howard welcomes any help.
[ 2009-12-12 Jonathan Wakely adds: ]
22.10.17.3 [func.wrap.func] says
2 A function object
f
of typeF
is Callable for argument typesT1, T2, ..., TN
inArgTypes
and a return typeR
, if, given lvaluest1, t2, ..., tN
of typesT1, T2, ..., TN
, respectively,INVOKE (f, t1, t2, ..., tN)
is well formed (20.7.2) and, ifR
is notvoid
, convertible toR
.N.B. lvalues, which means you can't use
function<R(T&&)>
orfunction<R(unique_ptr<T>)>
I recently implemented rvalue arguments in GCC's
std::function
, all that was needed was to usestd::forward<ArgTypes>
in a few places. The example in issue 815 works.I think 815 could be resolved by removing the requirement that the target function be callable with lvalues. Saying
ArgTypes
need to beCopyConstructible
is wrong, and IMHO sayingMoveConstructible
is unnecessary, since the by-value signature implies that already, but if it is needed it should only be onoperator()
, not the whole class (you could in theory instantiatestd::function<R(noncopyable)>
as long as you don't invoke the call operator.)I think defining invocation in terms of
INVOKE
already implies perfect forwarding, so we don't need to say explicitly thatstd::forward
should be used (N.B. the types that are forwarded are those inArgTypes
, which can differ from the actual parameter types of the target function. The actual parameter types have gone via type erasure, but that's not a problem - IMHO forwarding the arguments asArgTypes
is the right thing to do anyway.)Is it sufficient to simply replace "lvalues" with "values"? or do we need to say something like "lvalues when
Ti
is an lvalue-reference and rvalues otherwise"? I prefer the former, so I propose the following resolution for 815:Edit 22.10.17.3 [func.wrap.func] paragraph 2:
2 A function object
f
of typeF
is Callable for argument typesT1, T2, ..., TN
inArgTypes
and a return typeR
, if, givenlvaluest1, t2, ..., tN
of typesT1, T2, ..., TN
, respectively,INVOKE (f, t1, t2, ..., tN)
is well formed (20.7.2) and, ifR
is notvoid
, convertible toR
.
[ 2009-12-12 Daniel adds: ]
I don't like the reduction to "values" and prefer the alternative solution suggested using "lvalues when Ti is an lvalue-reference and rvalues otherwise". The reason why I dislike the shorter version is based on different usages of "values" as part of defining the semantics of requirement tables via expressions. E.g. 16.4.4.2 [utility.arg.requirements]/1 says "
a
,b
, andc
are values of typeconst T;
" or similar in 23.2.2 [container.requirements.general]/4 or /14 etc. My current reading of all these parts is that both rvalues and lvalues are required to be supported, but this interpretation would violate the intention of the suggested fix of #815, if I correctly understand Jonathan's rationale.
[ 2009-12-12 Howard adds: ]
"lvalues when Ti is an lvalue-reference and rvalues otherwise"
doesn't quite work here because the
Ti
aren't deduced. They are specified by thefunction
type.Ti
might beconst int&
(an lvalue reference) and a validti
might be2
(a non-const rvalue). I've taken another stab at the wording using "expressions" and "bindable to".
[ 2010-02-09 Wording updated by Jonathan, Ganesh and Daniel. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Daniel opens to improve wording. ]
[ 2010-02-11 This issue is now addressed by 870(i). ]
[ 2010-02-12 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
Proposed resolution:
Edit 22.10.17.3 [func.wrap.func] paragraph 2:
2 A function object
f
of typeF
is Callable for argument typesT1, T2, ..., TN
inArgTypes
andareturn typeR
,if, given lvaluesthe expressiont1, t2, ..., tN
of typesT1, T2, ..., TN
, respectively,INVOKE(f, declval<ArgTypes>()..., R
, considered as an unevaluated operand (7 [expr]), is well formed (20.7.2)t1, t2, ..., tN)and, if.R
is notvoid
, convertible toR
bind()
's returned functor have a nofail copy ctor when bind()
is nofail?Section: 22.10.15.4 [func.bind.bind] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2008-02-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.bind.bind].
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Discussion:
Library Issue 527(i) notes that bind(f, t1, ..., tN)
should be nofail when f, t1, ..., tN
have nofail copy ctors.
However, no guarantees are provided for the copy ctor of the functor
returned by bind()
. (It's guaranteed to have a copy ctor, which can
throw implementation-defined exceptions: bind()
returns a forwarding
call wrapper, TR1 3.6.3/2. A forwarding call wrapper is a call wrapper,
TR1 3.3/4. Every call wrapper shall be CopyConstructible, TR1 3.3/4.
Everything without an exception-specification may throw
implementation-defined exceptions unless otherwise specified, C++03
17.4.4.8/3.)
Should the nofail guarantee requested by Library Issue 527(i) be extended
to cover both calling bind()
and copying the returned functor?
[ Howard adds: ]
tuple
construction should probably have a similar guarantee.
[ San Francisco: ]
Howard to provide wording.
[ Post Summit, Anthony provided wording. ]
[ Batavia (2009-05): ]
Part of all of this issue appears to be rendered moot by the proposed resolution to issue 817(i) (q.v.). We recommend the issues be considered simultaneously (or possibly even merged) to ensure there is no overlap. Move to Open, and likewise for issue 817(i).
[ 2009-07 Frankfurt: ]
[ 2009-10 Santa Cruz: ]
[ 2010-02-11 Moved from Ready to Tentatively NAD Editorial, rationale added below. ]
Rationale:
This issue is solved as proposed by 817(i).
Proposed resolution:
Add a new sentence to the end of paragraphs 2 and 4 of 22.10.15.4 [func.bind.bind]:
-2- Returns: A forwarding call wrapper
g
with a weak result type (20.6.2). The effect ofg(u1, u2, ..., uM)
shall beINVOKE(f, v1, v2, ..., vN, Callable<F cv,V1, V2, ..., VN>::result_type)
, where cv represents the cv-qualifiers ofg
and the values and types of the bound argumentsv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofF
or any of the types inBoundArgs...
throw an exception....
-5- Returns: A forwarding call wrapper
g
with a nested typeresult_type
defined as a synonym forR
. The effect ofg(u1, u2, ..., uM)
shall beINVOKE(f, v1, v2, ..., vN, R)
, where the values and types of the bound argumentsv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofF
or any of the types inBoundArgs...
throw an exception.
bind
needs to be movedSection: 22.10.15.4 [func.bind.bind] Status: C++11 Submitter: Howard Hinnant Opened: 2008-03-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.bind.bind].
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Discussion:
Addresses US 72, JP 38 and DE 21
The functor returned by bind()
should have a move constructor that
requires only move construction of its contained functor and bound arguments.
That way move-only functors can be passed to objects such as thread
.
This issue is related to issue 816(i).
US 72:
bind
should support move-only functors and bound arguments.
JP 38:
add the move requirement for bind's return type.
For example, assume following
th1
andth2
,void f(vector<int> v) { } vector<int> v{ ... }; thread th1([v]{ f(v); }); thread th2(bind(f, v));When function object are set to thread,
v
is moved toth1
's lambda expression in a Move Constructor of lambda expression becauseth1
's lambda expression has a Move Constructor. Butbind
ofth2
's return type doesn't have the requirement of Move, so it may not moved but copied.Add the requirement of move to get rid of this useless copy.
And also, add the
MoveConstructible
as well asCopyConstructible
.
DE 21
The specification for bind claims twice that "the values and types for the bound arguments v1, v2, ..., vN are determined as specified below". No such specification appears to exist.
[ San Francisco: ]
Howard to provide wording.
[ Post Summit Alisdair and Howard provided wording. ]
Several issues are being combined in this resolution. They are all touching the same words so this is an attempt to keep one issue from stepping on another, and a place to see the complete solution in one place.
bind
needs to be "moved".- 22.10.15.4 [func.bind.bind]/p3, p6 and p7 were accidently removed from N2798.
- Issue 929(i) argues for a way to pass by && for efficiency but retain the decaying behavior of pass by value for the
thread
constructor. That same solution is applicable here.
[ Batavia (2009-05): ]
We were going to recommend moving this issue to Tentatively Ready until we noticed potential overlap with issue 816 (q.v.).
Move to Open, and recommend both issues be considered together (and possibly merged).
[ 2009-07 Frankfurt: ]
The proposed resolution uses concepts. Leave Open.
[ 2009-10 Santa Cruz: ]
Leave as Open. Howard to provide deconceptified wording.
[ 2009-11-07 Howard updates wording. ]
[ 2009-11-15 Further updates by Peter, Chris and Daniel. ]
[ Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
Change 22.10 [function.objects] p2:
template<class Fn, class...TypesBoundArgs> unspecified bind(Fn&&,TypesBoundArgs&&...); template<class R, class Fn, class...TypesBoundArgs> unspecified bind(Fn&&,TypesBoundArgs&&...);
Change 22.10.4 [func.require]:
4 Every call wrapper (22.10.3 [func.def]) shall be
. A simple call wrapper is a call wrapper that is
CopyMoveConstructibleCopyConstructible
andCopyAssignable
and whose copy constructor, move constructor and assignment operator do not throw exceptions. A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the formtemplate<class...ArgTypesUnBoundsArgs> R operator()(ArgTypesUnBoundsArgs&&... unbound_args) cv-qual;— end note]
Change 22.10.15.4 [func.bind.bind]:
Within this clause:
- Let
FD
be a synonym for the typedecay<F>::type
.- Let
fd
be an lvalue of typeFD
constructed fromstd::forward<F>(f)
.- Let
Ti
be a synonym for the ith type in the template parameter packBoundArgs
.- Let
TiD
be a synonym for the typedecay<Ti>::type
.- Let
ti
be the ith argument in the function parameter packbound_args
.- Let
tid
be an lvalue of typeTiD
constructed fromstd::forward<Ti>(ti)
.- Let
Uj
be the jth deduced type of theUnBoundArgs&&...
parameter of theoperator()
of the forwarding call wrapper.- Let
uj
be the jth argument associated withUj
.template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-1- Requires:
is_constructible<FD, F>::value
shall betrue
. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall betrue
.F
and eachTi
inBoundArgs
shall be CopyConstructible.INVOKE(fd, w1, w2, ..., wN)
(22.10.4 [func.require]) shall be a valid expression for some values w1, w2, ..., wN, whereN == sizeof...(bound_args)
.-2- Returns: A forwarding call wrapper
g
with a weak result type (22.10.4 [func.require]). The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type)
, where cv represents the cv-qualifiers ofg
and the values and types of the bound argumentsv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofFD
or of any of the typesTiD
throws an exception.-3- Throws: Nothing unless the
copyconstructionorofor of one of the values
Ffdtid
types in thethrows an exception.BoundArgs...
pack expansionRemarks: The unspecified return type shall satisfy the requirements of
MoveConstructible
. If all ofFD
andTiD
satisfy the requirements ofCopyConstructible
then the unspecified return type shall satisfy the requirements ofCopyConstructible
. [Note: This implies that all ofFD
andTiD
shall beMoveConstructible
— end note]template<class R, class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-4- Requires:
is_constructible<FD, F>::value
shall betrue
. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall betrue
.F
and eachTi
inBoundArgs
shall be CopyConstructible.INVOKE(fd, w1, w2, ..., wN)
shall be a valid expression for some values w1, w2, ..., wN, whereN == sizeof...(bound_args)
.-5- Returns: A forwarding call wrapper
g
with a nested typeresult_type
defined as a synonym forR
. The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, v1, v2, ..., vN, R)
, where the values and types of the bound argumentsv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofFD
or of any of the typesTiD
throws an exception.-6- Throws: Nothing unless the
copyconstructionorofor of one of the values
Ffdtid
types in thethrows an exception.BoundArgs...
pack expansionRemarks: The unspecified return type shall satisfy the requirements of
MoveConstructible
. If all ofFD
andTiD
satisfy the requirements ofCopyConstructible
then the unspecified return type shall satisfy the requirements ofCopyConstructible
. [Note: This implies that all ofFD
andTiD
shall beMoveConstructible
— end note]-7- The values of the bound arguments
v1, v2, ..., vN
and their corresponding typesV1, V2, ..., VN
depend on the typesTiD
derived fromof the corresponding argumentthe call toti
inbound_args
of typeTi
inBoundArgs
inbind
and the cv-qualifiers cv of the call wrapperg
as follows:
- if
is
tiTiDof typereference_wrapper<T>
the argument istid.get()
and its typeVi
isT&
;- if the value of
is
std::is_bind_expression<TiD>::valuetrue
the argument istid(std::forward<Uj>(uj)...
and its typeu1, u2, ..., uM)Vi
isresult_of<TiD cv (Uj...
;U1&, U2&, ..., UM&)>::type- if the value
j
ofis not zero the argument is
std::is_placeholder<TiD>::valuestd::forward<Uj>(uj)
and its typeVi
isUj&&
;- otherwise the value is
tid
and its typeVi
isTiD cv &
.
Section: 32.5.4 [atomics.order] Status: CD1 Submitter: Jens Maurer Opened: 2008-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [atomics.order].
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Discussion:
32.5.4 [atomics.order] p1 says in the table that
Element Meaning memory_order_acq_rel
the operation has both acquire and release semantics
To my naked eye, that seems to imply that even an atomic read has both acquire and release semantics.
Then, p1 says in the table:
Element Meaning memory_order_seq_cst
the operation has both acquire and release semantics, and, in addition, has sequentially-consistent operation ordering
So that seems to be "the same thing" as memory_order_acq_rel
, with additional
constraints.
I'm then reading p2, where it says:
The
memory_order_seq_cst
operations that load a value are acquire operations on the affected locations. Thememory_order_seq_cst
operations that store a value are release operations on the affected locations.
That seems to imply that atomic reads only have acquire semantics. If that
is intended, does this also apply to memory_order_acq_rel
and the individual
load/store operations as well?
Also, the table in p1 contains phrases with "thus" that seem to indicate consequences of normative wording in 6.9.2 [intro.multithread]. That shouldn't be in normative text, for the fear of redundant or inconsistent specification with the other normative text.
Double-check 32.5.8.2 [atomics.types.operations] that each operation clearly says whether it's a load or a store operation, or both. (It could be clearer, IMO. Solution not in current proposed resolution.)
32.5.4 [atomics.order] p2: What's a "consistent execution"? It's not defined in 6.9.2 [intro.multithread], it's just used in notes there.
And why does 32.5.8.2 [atomics.types.operations] p9 for "load" say:
Requires: The order argument shall not be
memory_order_acquire
normemory_order_acq_rel
.
(Since this is exactly the same restriction as for "store", it seems to be a typo.)
And then: 32.5.8.2 [atomics.types.operations] p12:
These operations are read-modify-write operations in the sense of the "synchronizes with" definition (6.9.2 [intro.multithread]), so both such an operation and the evaluation that produced the input value synchronize with any evaluation that reads the updated value.
This is redundant with 6.9.2 [intro.multithread], see above for the reasoning.
[ San Francisco: ]
Boehm: "I don't think that this changes anything terribly substantive, but it improves the text."
Note that "Rephrase the table in as [sic] follows..." should read "Replace the table in [atomics.order] with the following...."
The proposed resolution needs more work. Crowl volunteered to address all of the atomics issues in one paper.
This issue is addressed in N2783.
Proposed resolution:
edit 32.5.4 [atomics.order], paragraph 1 as follows.
The enumeration
memory_order
specifies the detailed regular (non-atomic) memory synchronization order as defined inClause 1.7section 1.10 and may provide for operation ordering. Its enumerated values and their meanings are as follows:
- For
memory_order_relaxed
,- no operation orders memory.
- For
memory_order_release
,memory_order_acq_rel
, andmemory_order_seq_cst
,- a store operation performs a release operation on the affected memory location.
- For
memory_order_consume
,- a load operation performs a consume operation on the affected memory location.
- For
memory_order_acquire
,memory_order_acq_rel
, andmemory_order_seq_cst
,- a load operation performs an acquire operation on the affected memory location.
remove table 136 in 32.5.4 [atomics.order].
Table 136 — memory_order effectsElementMeaningmemory_order_relaxed
the operation does not order memorymemory_order_release
the operation performs a release operation on the affected memory location, thus making regular memory writes visible to other threads through the atomic variable to which it is appliedmemory_order_acquire
the operation performs an acquire operation on the affected memory location, thus making regular memory writes in other threads released through the atomic variable to which it is applied visible to the current threadmemory_order_consume
the operation performs a consume operation on the affected memory location, thus making regular memory writes in other threads released through the atomic variable to which it is applied visible to the regular memory reads that are dependencies of this consume operation.memory_order_acq_rel
the operation has both acquire and release semanticsmemory_order_seq_cst
the operation has both acquire and release semantics, and, in addition, has sequentially-consistent operation ordering
edit 32.5.4 [atomics.order], paragraph 2 as follows.
TheTherememory_order_seq_cst
operations that load a value are acquire operations on the affected locations. Thememory_order_seq_cst
operations that store a value are release operations on the affected locations. In addition, in a consistent execution, theremust beis a single total order S on allmemory_order_seq_cst
operations, consistent with the happens before order and modification orders for all affected locations, such that eachmemory_order_seq_cst
operation observes either the last preceding modification according to this order S, or the result of an operation that is notmemory_order_seq_cst
. [Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the happens before ordering. —end note]
Section: 17.9.8 [except.nested] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Looking at the wording I submitted for rethrow_if_nested
, I don't think I
got it quite right.
The current wording says:
template <class E> void rethrow_if_nested(const E& e);Effects: Calls
e.rethrow_nested()
only ife
is publicly derived fromnested_exception
.
This is trying to be a bit subtle, by requiring e
(not E
) to be publicly
derived from nested_exception
the idea is that a dynamic_cast
would be
required to be sure. Unfortunately, if e
is dynamically but not statically
derived from nested_exception
, e.rethrow_nested()
is ill-formed.
[ San Francisco: ]
Alisdair was volunteered to provide wording.
[ 2009-10 Santa Cruz: ]
Leave as Open. Alisdair to provide wording.
[ 2009-11-09 Alisdair provided wording. ]
[ 2010-03-10 Dietmar updated wording. ]
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Change 17.9.8 [except.nested], p8:
template <class E> void rethrow_if_nested(const E& e);-8- Effects:
CallsOnly if the dynamic type ofe.rethrow_nested()
oe
is publicly and unambiguously derived fromnested_exception
this callsdynamic_cast<const nested_exception&>(e).rethrow_nested()
.
current_exception()
's interaction with throwing copy ctorsSection: 17.9.7 [propagation] Status: CD1 Submitter: Stephan T. Lavavej Opened: 2008-03-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
View all issues with CD1 status.
Discussion:
As of N2521, the Working Paper appears to be silent about what
current_exception()
should do if it tries to copy the currently handled
exception and its copy constructor throws. 17.9.7 [propagation]/7 says "If the
function needs to allocate memory and the attempt fails, it returns an
exception_ptr
object that refers to an instance of bad_alloc
.", but
doesn't say anything about what should happen if memory allocation
succeeds but the actual copying fails.
I see three alternatives: (1) return an exception_ptr
object that refers
to an instance of some fixed exception type, (2) return an exception_ptr
object that refers to an instance of the copy ctor's thrown exception
(but if that has a throwing copy ctor, an infinite loop can occur), or
(3) call terminate()
.
I believe that terminate()
is the most reasonable course of action, but
before we go implement that, I wanted to raise this issue.
[ Peter's summary: ]
The current practice is to not have throwing copy constructors in exception classes, because this can lead to
terminate()
as described in 14.6.2 [except.terminate]. Thus callingterminate()
in this situation seems consistent and does not introduce any new problems.However, the resolution of core issue 475 may relax this requirement:
The CWG agreed with the position that
std::uncaught_exception()
should returnfalse
during the copy to the exception object and thatstd::terminate()
should not be called if that constructor exits with an exception.Since throwing copy constructors will no longer call
terminate()
, option (3) doesn't seem reasonable as it is deemed too drastic a response in a recoverable situation.Option (2) cannot be adopted by itself, because a potential infinite recursion will need to be terminated by one of the other options.
Proposed resolution:
Add the following paragraph after 17.9.7 [propagation]/7:
Returns (continued): If the attempt to copy the current exception object throws an exception, the function returns an
exception_ptr
that refers to the thrown exception or, if this is not possible, to an instance ofbad_exception
.[Note: The copy constructor of the thrown exception may also fail, so the implementation is allowed to substitute a
bad_exception
to avoid infinite recursion. -- end note.]
Rationale:
[ San Francisco: ]
Pete: there may be an implied assumption in the proposed wording that current_exception() copies the existing exception object; the implementation may not actually do that.
Pete will make the required editorial tweaks to rectify this.
unique_ptr
Section: 20.3.1.4.5 [unique.ptr.runtime.modifiers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.runtime.modifiers].
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Discussion:
Reading resolution of LWG issue 673(i) I noticed the following:
void reset(T*pointer p =0pointer());-1- Requires: Does not accept pointer types which are convertible to
T*
pointer
(diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]
This could be cleaned up by mandating the overload as a public deleted
function. In addition, we should probably overload reset
on nullptr_t
to be a stronger match than the deleted overload. Words...
Proposed resolution:
Add to class template definition in 20.3.1.4 [unique.ptr.runtime]
// modifiers pointer release(); void reset(pointer p = pointer()); void reset( nullptr_t ); template< typename U > void reset( U ) = delete; void swap(unique_ptr&& u);
Update 20.3.1.4.5 [unique.ptr.runtime.modifiers]
void reset(pointer p = pointer()); void reset(nullptr_t);
-1- Requires: Does not accept pointer types which are convertible topointer
(diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]Effects: If
get() == nullptr
there are no effects. Otherwiseget_deleter()(get())
....
[ Note this wording incorporates resolutions for 806(i) (New) and 673(i) (Ready). ]
identity<void>
seems brokenSection: 22.2.4 [forward] Status: Resolved Submitter: Walter Brown Opened: 2008-04-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [forward].
View all issues with Resolved status.
Discussion:
N2588 seems to have added an operator()
member function to the
identity<>
helper in 22.2.4 [forward]. I believe this change makes it no
longer possible to instantiate identity<void>
, as it would require
forming a reference-to-void
type as this operator()
's parameter type.
Suggested resolution: Specialize identity<void>
so as not to require
the member function's presence.
[ Sophia Antipolis: ]
Jens: suggests to add a requires clause to avoid specializing on
void
.Alisdair: also consider cv-qualified
void
.Alberto provided proposed wording.
[ 2009-07-30 Daniel reopens: ]
This issue became closed, because the
ReferentType
requirement fixed the problem - this is no longer the case. In retrospective it seems to be that the root of current issues aroundstd::identity
(823, 700(i), 939(i)) is that it was standardized as something very different (an unconditional type mapper) than traditional usage indicated (a function object that should derive fromstd::unary_function)
, as the SGI definition does. This issue could be solved, ifstd::identity
is removed (one proposal of 939(i)), but until this has been decided, this issue should remain open. An alternative for removing it, would be, to do the following:
Let
identity
stay as a real function object, which would now properly derive fromunary_function
:template <class T> struct identity : unary_function<T, T> { const T& operator()(const T&) const; };Invent (if needed) a generic type wrapper (corresponding to concept
IdentityOf
), e.g.identity_of
, and move it's prototype description back to 22.2.4 [forward]:template <class T> struct identity_of { typedef T type; };and adapt the
std::forward
signature to useidentity_of
instead ofidentity
.
[ 2009-10 Santa Cruz: ]
Proposed resolution:
Change definition of identity
in 22.2.4 [forward], paragraph 2, to:
template <class T> struct identity { typedef T type; requires ReferentType<T> const T& operator()(const T& x) const; };
...
requires ReferentType<T> const T& operator()(const T& x) const;
Rationale:
The point here is to able to write T&
given T
and ReferentType
is
precisely the concept that guarantees so, according to N2677
(Foundational concepts). Because of this, it seems preferable than an
explicit check for cv void
using SameType/remove_cv
as it was suggested
in Sophia. In particular, Daniel remarked that there may be types other
than cv void
which aren't referent types (int[]
, perhaps?).
basic_string
inserterSection: 27.4.4.4 [string.io] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.io].
View all issues with CD1 status.
Discussion:
In the current working paper, the <string>
header synopsis at the end of
27.4 [string.classes] lists a single operator<<
overload
for basic_string
.
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const basic_string<charT,traits,Allocator>& str);
The definition in 27.4.4.4 [string.io] lists two:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str); template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const basic_string<charT,traits,Allocator>& str);
I believe the synopsis in 27.4 [string.classes] is correct, and the first of the two signatures in 27.4.4.4 [string.io] should be deleted.
Proposed resolution:
Delete the first of the two signatures in 27.4.4.4 [string.io]:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const basic_string<charT,traits,Allocator>& str);
Section: 19.5.4.1 [syserr.errcode.overview], 20.3.2.2.12 [util.smartptr.shared.io], 99 [facets.examples], 22.9.4 [bitset.operators], 29.4.6 [complex.ops], 31.6 [stream.buffers], 28.6.8 [re.submatch] Status: Resolved Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2017-03-21
Priority: Not Prioritized
View all other issues in [syserr.errcode.overview].
View all issues with Resolved status.
Discussion:
Addresses UK 220
Should the following use rvalues references to stream in insert/extract operators?
[ Sophia Antipolis ]
Agree with the idea in the issue, Alisdair to provide wording.
[ Daniel adds 2009-02-14: ]
The proposal given in the paper N2831 apparently resolves this issue.
[ Batavia (2009-05): ]
The cited paper is an earlier version of N2844, which changed the rvalue reference binding rules. That paper includes generic templates
operator<<
andoperator>>
that adapt rvalue streams.We therefore agree with Daniel's observation. Move to
NAD EditorialResolved.
Proposed resolution:
constexpr shared_ptr::shared_ptr()?
Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: Peter Dimov Opened: 2008-04-11 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with Resolved status.
Discussion:
Would anyone object to making the default constructor of shared_ptr
(and
weak_ptr
and enable_shared_from_this
) constexpr
? This would enable
static initialization for shared_ptr
variables, eliminating another
unfair advantage of raw pointers.
[ San Francisco: ]
It's not clear to us that you can initialize a pointer with the literal 0 in a constant expression. We need to ask CWG to make sure this works. Bjarne has been appointed to do this.
Core got back to us and assured as that
nullptr
would do the job nicely here.
[ 2009-05-01 Alisdair adds: ]
I don't believe that
constexpr
will buy anything in this case.shared_ptr/weak_ptr/enable_shared_from_this
cannot be literal types as they have a non-trivial copy constructor. As they do not produce literal types, then theconstexpr
default constructor will not guarantee constant initialization, and so not buy the hoped for optimization.I recommend referring this back to Core to see if we can get static initialization for types with
constexpr
constructors, even if they are not literal types. Otherwise this should be closed as NAD.
[ 2009-05-26 Daniel adds: ]
If Alisdair's 2009-05-01 comment is correct, wouldn't that also make
constexpr mutex()
useless, because this class has a non-trivial destructor? (828(i))
[ 2009-07-21 Alisdair adds: ]
The feedback from core is that this and similar uses of
constexpr
constructors to force static initialization should be supported. If there are any problems with this in the working draught, we should file core issues.Recommend we declare the default constructor
constexpr
as the issue suggests (proposed wording added).
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2994.
Proposed resolution:
Change 20.3.2.2 [util.smartptr.shared] and 20.3.2.2.2 [util.smartptr.shared.const]:
constexpr shared_ptr();
Change 20.3.2.3 [util.smartptr.weak] and 20.3.2.3.2 [util.smartptr.weak.const]:
constexpr weak_ptr();
Change 20.3.2.7 [util.smartptr.enab] (2 places):
constexpr enable_shared_from_this();
std::mutex
?Section: 32.6.4.2.2 [thread.mutex.class] Status: Resolved Submitter: Peter Dimov Opened: 2008-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.mutex.class].
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Discussion:
[Note: I'm assuming here that [basic.start.init]/1 will be fixed.]
Currently std::mutex
doesn't support static initialization. This is a
regression with respect to pthread_mutex_t
, which does. I believe that
we should strive to eliminate such regressions in expressive power where
possible, both to ease migration and to not provide incentives to (or
force) people to forego the C++ primitives in favor of pthreads.
[ Sophia Antipolis: ]
We believe this is implementable on POSIX, because the initializer-list feature and the constexpr feature make this work. Double-check core language about static initialization for this case. Ask core for a core issue about order of destruction of statically-initialized objects wrt. dynamically-initialized objects (should come afterwards). Check non-POSIX systems for implementability.
If ubiquitous implementability cannot be assured, plan B is to introduce another constructor, make this constexpr, which is conditionally-supported. To avoid ambiguities, this new constructor needs to have an additional parameter.
[ Post Summit: ]
Jens: constant initialization seems to be ok core-language wise
Consensus: Defer to threading experts, in particular a Microsoft platform expert.
Lawrence to send e-mail to Herb Sutter, Jonathan Caves, Anthony Wiliams, Paul McKenney, Martin Tasker, Hans Boehm, Bill Plauger, Pete Becker, Peter Dimov to alert them of this issue.
Lawrence: What about header file shared with C? The initialization syntax is different in C and C++.
Recommend Keep in Review
[ Batavia (2009-05): ]
Keep in Review status pending feedback from members of the Concurrency subgroup.
[ See related comments from Alisdair and Daniel in 827(i). ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2994.
Proposed resolution:
Change 32.6.4.2.2 [thread.mutex.class]:
class mutex { public: constexpr mutex(); ...
Section: 17.9.7 [propagation] Status: CD1 Submitter: Beman Dawes Opened: 2008-04-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
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Discussion:
Consider this code:
exception_ptr xp;try {do_something(); } catch (const runtime_error& ) {xp = current_exception();} ... rethrow_exception(xp);
Say do_something()
throws an exception object of type
range_error
. What is the type of the exception object thrown by
rethrow_exception(xp)
above? It must have a type of range_error
;
if it were of type runtime_error
it still isn't possible to
propagate an exception and the exception_ptr/current_exception/rethrow_exception
machinery serves no useful purpose.
Unfortunately, the current wording does not explicitly say that. Different people read the current wording and come to different conclusions. While it may be possible to deduce the correct type from the current wording, it would be much clearer to come right out and explicitly say what the type of the referred to exception is.
[ Peter adds: ]
I don't like the proposed resolution of 829. The normative text is unambiguous that the
exception_ptr
refers to the currently handled exception. This term has a standard meaning, see 14.4 [except.handle]/8; this is the exception thatthrow;
would rethrow, see 14.2 [except.throw]/7.A better way to address this is to simply add the non-normative example in question as a clarification. The term currently handled exception should be italicized and cross-referenced. A [Note: the currently handled exception is the exception that a throw expression without an operand (14.2 [except.throw]/7) would rethrow. --end note] is also an option.
Proposed resolution:
After 17.9.7 [propagation] , paragraph 7, add the indicated text:
exception_ptr current_exception();Returns:
exception_ptr
object that refers to the currently handled exception (14.4 [except.handle]) or a copy of the currently handled exception, or a nullexception_ptr
object if no exception is being handled. If the function needs to allocate memory and the attempt fails, it returns anexception_ptr
object that refers to an instance ofbad_alloc
. It is unspecified whether the return values of two successive calls tocurrent_exception
refer to the same exception object. [Note: that is, it is unspecified whethercurrent_exception
creates a new copy each time it is called. -- end note]Throws: nothing.
unique_ptr::pointer
requirements underspecifiedSection: 20.3.1.3 [unique.ptr.single] Status: Resolved Submitter: Daniel Krügler Opened: 2008-05-14 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [unique.ptr.single].
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Discussion:
Issue 673(i) (including recent updates by 821(i)) proposes a useful
extension point for unique_ptr
by granting support for an optional
deleter_type::pointer
to act as pointer-like replacement for element_type*
(In the following: pointer
).
Unfortunately no requirements are specified for the type pointer
which has
impact on at least two key features of unique_ptr
:
The unique_ptr
specification makes great efforts to require that essentially all
operations cannot throw and therefore adds proper wording to the affected
operations of the deleter as well. If user-provided pointer
-emulating types
("smart pointers") will be allowed, either all throw-nothing clauses have to
be replaced by weaker "An exception is thrown only if pointer
's {op} throws
an exception"-clauses or it has to be said explicitly that all used
operations of pointer
are required not to throw. I understand the main
focus of unique_ptr
to be as near as possible to the advantages of native pointers which cannot
fail and thus strongly favor the second choice. Also, the alternative position
would make it much harder to write safe and simple template code for
unique_ptr
. Additionally, I assume that a general statement need to be given
that all of the expressions of pointer
used to define semantics are required to
be well-formed and well-defined (also as back-end for 762(i)).
[ Sophia Antipolis: ]
Howard: We maybe need a core concept
PointerLike
, but we don't need the arithmetic (seeshared_ptr
vs.vector<T>::iterator
.Howard will go through and enumerate the individual requirements wrt.
pointer
for each member function.
[ 2009-07 Frankfurt: ]
Move to Ready.
[ 2009-10-15 Alisdair pulls from Ready: ]
I hate to pull an issue out of Ready status, but I don't think 834 is fully baked yet.
For reference the proposed resolution is to add the following words:
unique_ptr<T, D>::pointer
's operations shall be well-formed, shall have well defined behavior, and shall not throw exceptions.This leaves me with a big question : which operations?
Are all pointer operations required to be nothrow, including operations that have nothing to do with interactions with
unique_ptr
? This was much simpler with concepts where we could point to operations within a certain concept, and so nail down the interactions.
[ 2009-10-15 Daniel adds: ]
I volunteer to prepare a more fine-grained solution, but I would like to ask for feedback that helps me doing so. If this question is asked early in the meeting I might be able to fix it within the week, but I cannot promise that now.
[ 2009-10 Santa Cruz: ]
Leave in open. Daniel to provide wording as already suggested.
[ 2009-12-22 Daniel provided wording and rationale. ]
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
The here proposed resolution has considerable overlap with the requirements that are used in the allocator requirements.
This might be a convincing argument to isolate the common subset into one requirement. The reason I did not do that is basically because we might find out that they are either over-constraining or under-constraining at this late point of specification. Note also that as a result of the idea of a general requirement set I added the requirement:
A default-initialized object may have a singular value
even though this does not play a relevant role for unique_ptr
.
One further characteristics of the resolution is that availability of relational
operators of unique_ptr<T, D>::pointer
is not part of the basic
requirements, which is in sync with the allocator requirements on pointer-like
(this means that unique_ptr
can hold a void_pointer
or
const_void_pointer
).
Solved by N3073.
Proposed resolution:
Change 20.3.1.3 [unique.ptr.single] p. 1 as indicated: [The intent is to
replace the coupling between T*
and the deleter's operator()
by a coupling between unique_ptr<T, D>::pointer
and this
operator()
]
1 - The default type for the template parameter
D
isdefault_delete
. A client-supplied template argumentD
shall be a function pointer or functor for which, given a valued
of typeD
and apointervalueptr
of type, the expression
T*unique_ptr<T, D>::pointerd(ptr)
is valid and has the effect of deallocating the pointer as appropriate for that deleter.D
may also be an lvalue-reference to a deleter.
Change 20.3.1.3 [unique.ptr.single] p. 3 as indicated:
3 - If the type
remove_reference<D>::type::pointer
exists, thenunique_ptr<T, D>::pointer
shall be a synonym forremove_reference<D>::type::pointer
. Otherwiseunique_ptr<T, D>::pointer
shall be a synonym forT*
. The typeunique_ptr<T, D>::pointer
shallbesatisfy the requirements ofEqualityComparable
,DefaultConstructible
,CopyConstructible
(Table 34) and,CopyAssignable
(Table 36),Swappable
, andDestructible
(16.4.4.2 [utility.arg.requirements]). A default-initialized object may have a singular value. A value-initialized object produces the null value of the type. The null value shall be equivalent only to itself. An object of this type can be copy-initialized with a value of typenullptr_t
, compared for equality with a value of typenullptr_t
, and assigned a value of typenullptr_t
. The effect shall be as if a value-initialized object had been used in place of the null pointer constant. An objectp
of this type can be contextually converted tobool
. The effect shall be as ifp != nullptr
had been evaluated in place ofp
. No operation on this type which is part of the above mentioned requirements shall exit via an exception.[Note: Given an allocator type
X
(16.4.4.6 [allocator.requirements]), the typesX::pointer
,X::const_pointer
,X::void_pointer
, andX::const_void_pointer
may be used asunique_ptr<T, D>::pointer
— end note]In addition to being available via inclusion of the
<utility>
header, theswap
function template in 22.2.2 [utility.swap] is also available within the definition ofunique_ptr
'sswap
function.
Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 2+3 as indicated: [The first
change ensures that we explicitly say, how the stored pointer is initialized.
This is important for a constexpr
function, because this may make a
difference for user-defined pointer-like types]
constexpr unique_ptr();...
2 - Effects: Constructs a
unique_ptr
which owns nothing, value-initializing the stored pointer.3 - Postconditions:
get() ==
.0nullptr
Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 6+7 as indicated: [This is a step-by-fix to ensure consistency to the changes of N2976]
unique_ptr(pointer p);...
6 - Effects: Constructs a
unique_ptr
which ownsp
, initializing the stored pointer withp
.7 - Postconditions:
get() == p
.get_deleter()
returns a reference to adefault constructedvalue-initialized deleterD
.
Insert a new effects clause in 20.3.1.3.2 [unique.ptr.single.ctor] just before p. 14: [The intent is to fix the current lack of specification in which way the stored pointer is initialized]
unique_ptr(pointer p,implementation-definedsee below d1); unique_ptr(pointer p,implementation-definedsee below d2);...
Effects: Constructs a
unique_ptr
which ownsp
, initializing the stored pointer withp
and the initializing the deleter as described above.14 - Postconditions:
get() == p
.get_deleter()
returns a reference to the internally stored deleter. IfD
is a reference type thenget_deleter()
returns a reference to the lvalued
.
Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 18+22 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]
unique_ptr(unique_ptr&& u);[..]
18 - Postconditions:
get() == value u.get()
had before the construction andu.get() == nullptr
.get_deleter()
returns a reference to the internally stored deleter which was constructed fromu.get_deleter()
. IfD
is a reference type thenget_deleter()
andu.get_deleter()
both reference the same lvalue deleter.template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);[..]
22 - Postconditions:
get() == value u.get()
had before the construction, modulo any required offset adjustments resulting from the cast fromunique_ptr<U, E>::pointer
topointer
andu.get() == nullptr
.get_deleter()
returns a reference to the internally stored deleter which was constructed fromu.get_deleter()
.
Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 20 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950(i)]
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);20 - Requires: If
D
is not a reference type, construction of the deleterD
from an rvalue of typeE
shall be well formed and shall not throw an exception. IfD
is a reference type, thenE
shall be the same type asD
(diagnostic required).unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
.[Note: These requirements imply thatT
andU
are complete types. — end note]
Change 20.3.1.3.3 [unique.ptr.single.dtor] p. 2 as indicated:
~unique_ptr();...
2 - Effects: If
get() ==
there are no effects. Otherwise0nullptrget_deleter()(get())
.
Change 20.3.1.3.4 [unique.ptr.single.asgn] p. 3+8 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]
unique_ptr& operator=(unique_ptr&& u);[..]
3 - Postconditions: This
unique_ptr
now owns the pointer whichu
owned, andu
no longer owns it,u.get() == nullptr
. [Note: IfD
is a reference type, then the referenced lvalue deleters are move assigned. — end note]template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);[..]
8 - Postconditions: This
unique_ptr
now owns the pointer whichu
owned, andu
no longer owns it,u.get() == nullptr
.
Change 20.3.1.3.4 [unique.ptr.single.asgn] p. 6 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950(i)]
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);[..]
6 - Requires: Assignment of the deleter
D
from an rvalueD
shall not throw an exception.unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
.[Note: These requirements imply thatT
andU
are complete types. — end note]
Change 20.3.1.3.4 [unique.ptr.single.asgn] before p. 11 and p. 12 as indicated: [The first change is a simple typo fix]
unique_ptr& operator=(nullptr_t});11 - Effects:
reset()
.12 - Postcondition:
get() ==
0nullptr
Change 20.3.1.3.5 [unique.ptr.single.observers] p. 1+4+12 as indicated:
typename add_lvalue_reference<T>::type operator*() const;1 - Requires:
get() !=
. The variable definition0nullptradd_lvalue_reference<T>::type t = *get()
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.[..]
pointer operator->() const;4 - Requires:
get() !=
.0nullptr[..]
explicit operator bool() const;12 - Returns:
get() !=
.0nullptr
Change 20.3.1.3.6 [unique.ptr.single.modifiers] p. 1 as indicated:
pointer release();1 - Postcondition:
get() ==
.0nullptr
Change 20.3.1.3.6 [unique.ptr.single.modifiers] p. 9 as indicated: [The
intent is to ensure that potentially user-defined swaps are used. A side-step
fix and harmonization with the specification of the the deleter is realized.
Please note the additional requirement in bullet 2 of this proposed resolution
regarding the availability of the generic swap
templates within the
member swap
function.]
void swap(unique_ptr& u);8 - Requires: The deleter
D
shall beSwappable
and shall not throw an exception underswap
.9 - Effects: The stored pointers of
*this
andu
are exchanged by an unqualified call to non-memberswap
. The stored deleters areexchanged by an unqualified call to non-memberswap
'd (unqualified)swap
.
Change 20.3.1.4.4 [unique.ptr.runtime.observers] p. 1 as indicated:
T& operator[](size_t i) const;Requires:
i <
the size of the array to which the stored pointer points. The variable definitionT& t = get()[i]
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
Change 20.3.1.4.5 [unique.ptr.runtime.modifiers] p. 1 as indicated:
void reset(pointer p = pointer()); void reset(nullptr_t p);1 - Effects: If
get() ==
there are no effects. Otherwise0nullptrget_deleter()(get())
.
Change 20.3.1.6 [unique.ptr.special] as indicated: [We don't add the relational operators to the basic requirement set, therefore we need special handling here]
template <class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() == y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.2 - Returns:
x.get() == y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() != y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.3 - Returns:
x.get() != y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() < y.get()
; shall be well formed, shall have well-defined behavior, and shall not exit via an exception.4 - Returns:
x.get() < y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() <= y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.5 - Returns:
x.get() <= y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() > y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.6 - Returns:
x.get() > y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() >= y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.7 - Returns:
x.get() >= y.get()
.Throws: nothing.
Section: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [basic.ios.members].
View all issues with C++11 status.
Discussion:
The fix for issue 581(i), now integrated into the working paper, overlooks a couple of minor problems.
First, being an unformatted function once again, flush()
is required to create a sentry object whose constructor must, among
other things, flush the tied stream. When two streams are tied
together, either directly or through another intermediate stream
object, flushing one will also cause a call to flush()
on
the other tied stream(s) and vice versa, ad infinitum. The program
below demonstrates the problem.
Second, as Bo Persson notes in his
comp.lang.c++.moderated post,
for streams with the unitbuf
flag set such
as std::stderr
, the destructor of the sentry object will
again call flush()
. This seems to create an infinite
recursion for std::cerr << std::flush;
#include <iostream> int main () { std::cout.tie (&std::cerr); std::cerr.tie (&std::cout); std::cout << "cout\n"; std::cerr << "cerr\n"; }
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Review.
[ 2009-05-26 Daniel adds: ]
I think that the most recently suggested change in [ostream::sentry] need some further word-smithing. As written, it would make the behavior undefined, if under conditions when
pubsync()
should be called, but when in this scenarioos.rdbuf()
returns 0.This case is explicitly handled in
flush()
and needs to be taken care of. My suggested fix is:If
((os.flags() & ios_base::unitbuf) && !uncaught_exception()
&& os.rdbuf() != 0
) is true, callsos.flush()
os.rdbuf()->pubsync()
.Two secondary questions are:
- Should
pubsync()
be invoked in any case or shouldn't a base requirement for this trial be thatos.good() == true
as required in the originalflush()
case?- Since
uncaught_exception()
is explicitly tested, shouldn't a return value of -1 ofpubsync()
producesetstate(badbit)
(which may throwios_base::failure
)?
[ 2009-07 Frankfurt: ]
Daniel volunteered to modify the proposed resolution to address his two questions.
Move back to Open.
[ 2009-07-26 Daniel provided wording. Moved to Review. ]
[ 2009-10-13 Daniel adds: ]
This proposed wording is written to match the outcome of 397(i).
[ 2009 Santa Cruz: ]
Move to Open. Martin to propose updated wording that will also resolve issue 397(i) consistently.
[ 2010-02-15 Martin provided wording. ]
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Just before 31.5.4.3 [basic.ios.members] p. 2 insert a new paragraph:
Requires: If
(tiestr != 0)
istrue
,tiestr
must not be reachable by traversing the linked list of tied stream objects starting fromtiestr->tie()
.
Change [ostream::sentry] p. 4 as indicated:
If
((os.flags() & ios_base::unitbuf) && !uncaught_exception() && os.good())
istrue
, callsos.flush()
os.rdbuf()->pubsync()
. If that function returns -1 setsbadbit
inos.rdstate()
without propagating an exception.
Add after [ostream::sentry] p17, the following paragraph:
Throws: Nothing.
money_base::space
and
money_base::none
on money_get
Section: 28.3.4.7.2.3 [locale.money.get.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.money.get.virtuals].
View all issues with C++11 status.
Duplicate of: 670
Discussion:
In paragraph 2, 28.3.4.7.2.3 [locale.money.get.virtuals] specifies the following:
Where
space
ornone
appears in the format pattern, except at the end, optional white space (as recognized byct.is
) is consumed after any required space.
This requirement can be (and has been) interpreted two mutually exclusive ways by different readers. One possible interpretation is that:
- where
money_base::space
appears in the format, at least one space is required, and- where
money_base::none
appears in the format, space is allowed but not required.
The other is that:
where either
money_base::space
ormoney_base::none
appears in the format, white space is optional.
[ San Francisco: ]
Martin will revise the proposed resolution.
[ 2009-07 Frankfurt: ]
There is a noun missing from the proposed resolution. It's not clear that the last sentence would be helpful, even if the word were not missing:
In either case, any required MISSINGWORD followed by all optional whitespace (as recognized by
ct.is()
) is consumed.Strike this sentence and move to Review.
[ Howard: done. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
I propose to change the text to make it clear that the first interpretation is intended, that is, to make following change to 28.3.4.7.2.3 [locale.money.get.virtuals], p. 2:
When
money_base::space
ormoney_base::none
appears as the last element in the format pattern,except at the end, optional white space (as recognized byno white space is consumed. Otherwise, wherect.is
) is consumed after any required space.money_base::space
appears in any of the initial elements of the format pattern, at least one white space character is required. Wheremoney_base::none
appears in any of the initial elements of the format pattern, white space is allowed but not required. If(str.flags() & str.showbase)
isfalse
, ...
Section: 24.6.2 [istream.iterator] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.iterator].
View all issues with C++11 status.
Discussion:
From message c++std-lib-20003...
The description of istream_iterator
in
24.6.2 [istream.iterator], p. 1 specifies that objects of the
class become the end-of-stream (EOS) iterators under the
following condition (see also issue 788(i) another problem
with this paragraph):
If the end of stream is reached (
operator void*()
on the stream returnsfalse
), the iterator becomes equal to the end-of-stream iterator value.
One possible implementation approach that has been used in practice is
for the iterator to set its in_stream
pointer to 0 when
it reaches the end of the stream, just like the default ctor does on
initialization. The problem with this approach is that
the Effects clause for operator++()
says the
iterator unconditionally extracts the next value from the stream by
evaluating *in_stream >> value
, without checking
for (in_stream == 0)
.
Conformance to the requirement outlined in the Effects clause
can easily be verified in programs by setting eofbit
or failbit
in exceptions()
of the associated
stream and attempting to iterate past the end of the stream: each
past-the-end access should trigger an exception. This suggests that
some other, more elaborate technique might be intended.
Another approach, one that allows operator++()
to attempt
to extract the value even for EOS iterators (just as long
as in_stream
is non-0) is for the iterator to maintain a
flag indicating whether it has reached the end of the stream. This
technique would satisfy the presumed requirement implied by
the Effects clause mentioned above, but it isn't supported by
the exposition-only members of the class (no such flag is shown). This
approach is also found in existing practice.
The inconsistency between existing implementations raises the question
of whether the intent of the specification is that a non-EOS iterator
that has reached the EOS become a non-EOS one again after the
stream's eofbit
flag has been cleared? That is, are the
assertions in the program below expected to pass?
sstream strm ("1 "); istream_iterator eos; istream_iterator it (strm); int i; i = *it++ assert (it == eos); strm.clear (); strm << "2 3 "; assert (it != eos); i = *++it; assert (3 == i);
Or is it intended that once an iterator becomes EOS it stays EOS until the end of its lifetime?
[ San Francisco: ]
We like the direction of the proposed resolution. We're not sure about the wording, and we need more time to reflect on it,
Move to Open. Detlef to rewrite the proposed resolution in such a way that no reference is made to exposition only members of
istream_iterator
.
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
The discussion of this issue on the reflector suggests that the intent
of the standard is for an istreambuf_iterator
that has
reached the EOS to remain in the EOS state until the end of its
lifetime. Implementations that permit EOS iterators to return to a
non-EOS state may only do so as an extension, and only as a result of
calling istream_iterator
member functions on EOS
iterators whose behavior is in this case undefined.
To this end we propose to change 24.6.2 [istream.iterator], p1, as follows:
The result of operator-> on an end-of-stream is not defined. For any other iterator value a
const T*
is returned. Invokingoperator++()
on an end-of-stream iterator is undefined. It is impossible to store things into istream iterators...
Add pre/postconditions to the member function descriptions of istream_iterator
like so:
istream_iterator();Effects: Constructs the end-of-stream iterator.
Postcondition:in_stream == 0
.istream_iterator(istream_type &s);Effects: Initializes
in_stream
with &s. value may be initialized during construction or the first time it is referenced.
Postcondition:in_stream == &s
.istream_iterator(const istream_iterator &x);Effects: Constructs a copy of
x
.
Postcondition:in_stream == x.in_stream
.istream_iterator& operator++();Requires:
in_stream != 0
.
Effects:*in_stream >> value
.istream_iterator& operator++(int);Requires:
in_stream != 0
.
Effects:istream_iterator tmp (*this); *in_stream >> value; return tmp;
Section: 23.4 [associative], 23.5 [unord] Status: Resolved Submitter: Alan Talbot Opened: 2008-05-18 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [associative].
View all issues with Resolved status.
Discussion:
Splice is a very useful feature of list
. This functionality is also very
useful for any other node based container, and I frequently wish it were
available for maps and sets. It seems like an omission that these
containers lack this capability. Although the complexity for a splice is
the same as for an insert, the actual time can be much less since the
objects need not be reallocated and copied. When the element objects are
heavy and the compare operations are fast (say a map<int, huge_thingy>
)
this can be a big win.
Suggested resolution:
Add the following signatures to map, set, multimap, multiset, and the unordered associative containers:
void splice(list<T,Allocator>&& x); void splice(list<T,Allocator>&& x, const_iterator i); void splice(list<T,Allocator>&& x, const_iterator first, const_iterator last);
Hint versions of these are also useful to the extent hint is useful. (I'm looking for guidance about whether hints are in fact useful.)
void splice(const_iterator position, list<T,Allocator>&& x); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last);
[ Sophia Antipolis: ]
Don't try to
splice "list"
into the other containers, it should be container-type.
forward_list
already hassplice_after
.Would "
splice
" make sense for anunordered_map
?Jens, Robert: "
splice
" is not the right term, it implies maintaining ordering inlist
s.Howard:
adopt
?Jens:
absorb
?Alan:
subsume
?Robert:
recycle
?Howard:
transfer
? (but no direction)Jens:
transfer_from
. No.Alisdair: Can we give a nothrow guarantee? If your
compare()
andhash()
doesn't throw, yes.Daniel: For
unordered_map
, we can't guarantee nothrow.
[ San Francisco: ]
Martin: this would possibly outlaw an implementation technique that is currently in use; caching nodes in containers.
Alan: if you cache in the allocator, rather than the individual container, this proposal doesn't interfere with that.
Martin: I'm not opposed to this, but I'd like to see an implementation that demonstrates that it works.
[ 2009-07 Frankfurt: ]
NAD Future.
[ 2009-09-19 Howard adds: ]
I'm not disagreeing with the NAD Future resolution. But when the future gets here, here is a possibility worth exploring:
Add to the "unique" associative containers:
typedef details node_ptr; node_ptr remove(const_iterator p); pair<iterator, bool> insert(node_ptr&& nd); iterator insert(const_iterator p, node_ptr&& nd);And add to the "multi" associative containers:
typedef details node_ptr; node_ptr remove(const_iterator p); iterator insert(node_ptr&& nd); iterator insert(const_iterator p, node_ptr&& nd);
Container::node_ptr
is a smart pointer much likeunique_ptr
. It owns a node obtained from the container it was removed from. It maintains a reference to the allocator in the container so that it can properly deallocate the node if asked to, even if the allocator is stateful. This being said, thenode_ptr
can not outlive the container for this reason.The
node_ptr
offers "const
-free" access to the node'svalue_type
.With this interface, clients have a great deal of flexibility:
- A client can remove a node from one container, and insert it into another (without any heap allocation). This is the splice functionality this issue asks for.
- A client can remove a node from a container, change its key or value, and insert it back into the same container, or another container, all without the cost of allocating a node.
- If the Compare function is nothrow (which is very common), then this functionality is nothrow unless modifying the value throws. And if this does throw, it does so outside of the containers involved.
- If the Compare function does throw, the
insert
function will have the argumentnd
retain ownership of the node.- The
node_ptr
should be independent of theCompare
parameter so that a node can be transferred fromset<T, C1, A>
toset<T, C2, A>
(for example).Here is how the customer might use this functionality:
Splice a node from one container to another:
m2.insert(m1.remove(i));Change the "key" in a
std::map
without the cost of node reallocation:auto p = m.remove(i); p->first = new_key; m.insert(std::move(p));Change the "value" in a
std::set
without the cost of node reallocation:auto p = s.remove(i); *p = new_value; s.insert(std::move(p));Move a move-only or heavy object out of an associative container (as opposed to the proposal in 1041(i)):
MoveOnly x = std::move(*s.remove(i));
remove(i)
transfers ownership of the node from the set to a temporarynode_ptr
.- The
node_ptr
is dereferenced, and that non-const reference is sent tomove
to cast it to an rvalue.- The rvalue
MoveOnly
is move constructed intox
from thenode_ptr
.~node_ptr()
destructs the moved-fromMoveOnly
and deallocates the node.Contrast this with the 1041(i) solution:
MoveOnly x = std::move(s.extract(i).first);The former requires one move construction for
x
while the latter requires two (one into thepair
and then one intox
). Either of these constructions can throw (say if there is only a copy constructor forx
). With the former, the point of throw is outside of the containers
, after the element has been removed from the container. With the latter, one throwing construction takes place prior to the removal of the element, and the second takes place after the element is removed.The "node insertion" API maintains the API associated with inserting
value_type
s so the customer can use familiar techniques for getting an iterator to the inserted node, or finding out whether it was inserted or not for the "unique" containers.Lightly prototyped. No implementation problems. Appears to work great for the client.
[08-2016, Post-Chicago]
Move to Tentatively Resolved
Proposed resolution:
This functionality is provided by P0083R3
ConstructibleAsElement
and bit containersSection: 23.2 [container.requirements], 23.3.14 [vector.bool], 22.9.2 [template.bitset] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
View all issues with CD1 status.
Discussion:
23.2 [container.requirements] p. 3 says:
Objects stored in these components shall be constructed using
construct_element
(20.6.9). For each operation that inserts an element of typeT
into a container (insert
,push_back
,push_front
,emplace
, etc.) with argumentsargs... T
shall beConstructibleAsElement
, as described in table 88. [Note: If the component is instantiated with a scoped allocator of typeA
(i.e., an allocator for whichis_scoped_allocator<A>::value
istrue
), thenconstruct_element
may pass an inner allocator argument toT
's constructor. -- end note]
However vector<bool, A>
(23.3.14 [vector.bool]) and bitset<N>
(22.9.2 [template.bitset]) store bits, not bool
s, and bitset<N>
does not even have an allocator. But these containers are governed by this clause. Clearly this
is not implementable.
Proposed resolution:
Change 23.2 [container.requirements] p. 3:
Objects stored in these components shall be constructed using
construct_element
(20.6.9), unless otherwise specified. For each operation that inserts an element of typeT
into a container (insert
,push_back
,push_front
,emplace
, etc.) with argumentsargs... T
shall beConstructibleAsElement
, as described in table 88. [Note: If the component is instantiated with a scoped allocator of typeA
(i.e., an allocator for whichis_scoped_allocator<A>::value
istrue
), thenconstruct_element
may pass an inner allocator argument toT
's constructor. -- end note]
Change 23.3.14 [vector.bool]/p2:
Unless described below, all operations have the same requirements and semantics as the primary
vector
template, except that operations dealing with thebool
value type map to bit values in the container storage, andconstruct_element
(23.2 [container.requirements]) is not used to construct these values.
Move 22.9.2 [template.bitset] to clause 20.
Section: 99 [func.referenceclosure.cons] Status: CD1 Submitter: Lawrence Crowl Opened: 2008-06-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The std::reference_closure
type has a deleted copy assignment operator
under the theory that references cannot be assigned, and hence the
assignment of its reference member must necessarily be ill-formed.
However, other types, notably std::reference_wrapper
and std::function
provide for the "copying of references", and thus the current definition
of std::reference_closure
seems unnecessarily restrictive. In particular,
it should be possible to write generic functions using both std::function
and std::reference_closure
, but this generality is much harder when
one such type does not support assignment.
The definition of reference_closure
does not necessarily imply direct
implementation via reference types. Indeed, the reference_closure
is
best implemented via a frame pointer, for which there is no standard
type.
The semantics of assignment are effectively obtained by use of the default destructor and default copy assignment operator via
x.~reference_closure(); new (x) reference_closure(y);
So the copy assignment operator generates no significant real burden to the implementation.
Proposed resolution:
In [func.referenceclosure] Class template reference_closure,
replace the =delete
in the copy assignment operator in the synopsis
with =default
.
template<class R , class... ArgTypes > class reference_closure<R (ArgTypes...)> { public: ... reference_closure& operator=(const reference_closure&) =deletedefault; ...
In 99 [func.referenceclosure.cons] Construct, copy, destroy, add the member function description
reference_closure& operator=(const reference_closure& f)Postcondition:
*this
is a copy off
.Returns:
*this
.
complex pow
return type is ambiguousSection: 29.4.10 [cmplx.over] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [cmplx.over].
View all issues with CD1 status.
Discussion:
The current working draft is in an inconsistent state.
29.4.8 [complex.transcendentals] says that:
pow(complex<float>(), int())
returns acomplex<float>
.
29.4.10 [cmplx.over] says that:
pow(complex<float>(), int())
returns acomplex<double>
.
[ Sophia Antipolis: ]
Since
int
promotes todouble
, and C99 doesn't have anint
-based overload forpow
, the C99 result iscomplex<double>
, see also C99 7.22, see also library issue 550(i).Special note: ask P.J. Plauger.
Looks fine.
Proposed resolution:
Strike this pow
overload in 29.4.2 [complex.syn] and in 29.4.8 [complex.transcendentals]:
template<class T> complex<T> pow(const complex<T>& x, int y);
Section: 32.5.8 [atomics.types.generic] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.types.generic].
View all issues with CD1 status.
Discussion:
The atomic classes (and class templates) are required to support aggregate initialization (99 [atomics.types.integral] p. 2 / 99 [atomics.types.address] p. 1) yet also have user declared constructors, so cannot be aggregates.
This problem might be solved with the introduction of the proposed initialization syntax at Antipolis, but the wording above should be altered. Either strike the sentence as redundant with new syntax, or refer to 'brace initialization'.
[ Jens adds: ]
Note that
atomic_itype a1 = { 5 };would be aggregate-initialization syntax (now coming under the disguise of brace initialization), but would be ill-formed, because the corresponding constructor for atomic_itype is explicit. This works, though:
atomic_itype a2 { 6 };
[ San Francisco: ]
The preferred approach to resolving this is to remove the explicit specifiers from the atomic integral type constructors.
Lawrence will provide wording.
This issue is addressed in N2783.
Proposed resolution:
within the synopsis in 99 [atomics.types.integral] edit as follows.
.... typedef struct atomic_bool { .... constexpr
explicitatomic_bool(bool); .... typedef struct atomic_itype { .... constexprexplicitatomic_itype(integral); ....
edit 99 [atomics.types.integral] paragraph 2 as follows.
The atomic integral types shall have standard layout. They shall each have a trivial default constructor, a constexpr
explicitvalue constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor. They shall each support aggregate initialization syntax.
within the synopsis of 99 [atomics.types.address] edit as follows.
.... typedef struct atomic_address { .... constexpr
explicitatomic_address(void*); ....
edit 99 [atomics.types.address] paragraph 1 as follows.
The type
atomic_address
shall have standard layout. It shall have a trivial default constructor, a constexprexplicitvalue constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor. It shall support aggregate initialization syntax.
within the synopsis of 32.5.8 [atomics.types.generic] edit as follows.
.... template <class T> struct atomic { .... constexpr
explicitatomic(T); .... template <> struct atomic<integral> : atomic_itype { .... constexprexplicitatomic(integral); .... template <> struct atomic<T*> : atomic_address { .... constexprexplicitatomic(T*); ....
edit 32.5.8 [atomics.types.generic] paragraph 2 as follows.
Specializations of the
atomic
template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexprexplicitvalue constructor.
Section: 32.5.8.2 [atomics.types.operations] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.types.operations].
View all issues with CD1 status.
Discussion:
The atomic classes and class templates (99 [atomics.types.integral] / 99 [atomics.types.address]) have a constexpr constructor taking a value of the appropriate type for that atomic. However, neither clause provides semantics or a definition for this constructor. I'm not sure if the initialization is implied by use of constexpr keyword (which restricts the form of a constructor) but even if that is the case, I think it is worth spelling out explicitly as the inference would be far too subtle in that case.
[ San Francisco: ]
Lawrence will provide wording.
This issue is addressed in N2783.
Proposed resolution:
before the description of ...is_lock_free
,
that is before 32.5.8.2 [atomics.types.operations] paragraph 4,
add the following description.
constexpr A::A(C desired);
- Effects:
- Initializes the object with the value
desired
. [Note: Construction is not atomic. —end note]
Section: 27.4.3.2 [string.require] Status: C++11 Submitter: Hervé Brönnimann Opened: 2008-06-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.require].
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Discussion:
In March, on comp.lang.c++.moderated, I asked what were the string exception safety guarantees are, because I cannot see *any* in the working paper, and any implementation I know offers the strong exception safety guarantee (string unchanged if a member throws exception). The closest the current draft comes to offering any guarantees is 27.4.3 [basic.string], para 3:
The class template
basic_string
conforms to the requirements for a Sequence Container (23.1.1), for a Reversible Container (23.1), and for an Allocator-aware container (91). The iterators supported bybasic_string
are random access iterators (24.1.5).
However, the chapter 23 only says, on the topic of exceptions: 23.2 [container.requirements], para 10:
Unless otherwise specified (see 23.2.2.3 and 23.2.6.4) all container types defined in this clause meet the following additional requirements:
- if an exception is thrown by...
I take it as saying that this paragraph has *no* implication on
std::basic_string
, as basic_string
isn't defined in Clause 23 and
this paragraph does not define a *requirement* of Sequence
nor Reversible Container, just of the models defined in Clause 23.
In addition, LWG Issue 718(i) proposes to remove 23.2 [container.requirements], para 3.
Finally, the fact that no operation on Traits should throw
exceptions has no bearing, except to suggest (since the only
other throws should be allocation, out_of_range
, or length_error
)
that the strong exception guarantee can be achieved.
The reaction in that group by Niels Dekker, Martin Sebor, and Bo Persson, was all that this would be worth an LWG issue.
A related issue is that erase()
does not throw. This should be
stated somewhere (and again, I don't think that the 23.2 [container.requirements], para 1
applies here).
[ San Francisco: ]
Implementors will study this to confirm that it is actually possible.
[ Daniel adds 2009-02-14: ]
The proposed resolution of paper N2815 interacts with this issue (the paper does not refer to this issue).
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
Add a blanket statement in 27.4.3.2 [string.require]:
- if any member function or operator of
basic_string<charT, traits, Allocator>
throws, that function or operator has no effect.- no
erase()
orpop_back()
function throws.
As far as I can tell, this is achieved by any implementation. If I made a
mistake and it is not possible to offer this guarantee, then
either state all the functions for which this is possible
(certainly at least operator+=
, append
, assign
, and insert
),
or add paragraphs to Effects clauses wherever appropriate.
std::hash
specializations for std::bitset/std::vector<bool>
Section: 22.10.19 [unord.hash] Status: CD1 Submitter: Thorsten Ottosen Opened: 2008-06-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the current working draft, std::hash<T>
is specialized for builtin
types and a few other types. Bitsets seems like one that is missing from
the list, not because it cannot not be done by the user, but because it
is hard or impossible to write an efficient implementation that works on
32bit/64bit chunks at a time. For example, std::bitset
is too much
encapsulated in this respect.
Proposed resolution:
Add the following to the synopsis in 22.10 [function.objects]/2:
template<class Allocator> struct hash<std::vector<bool,Allocator>>; template<size_t N> struct hash<std::bitset<N>>;
Modify the last sentence of 22.10.19 [unord.hash]/1 to end with:
... and
std::string
,std::u16string
,std::u32string
,std::wstring
,std::error_code
,std::thread::id
,std::bitset
,and std::vector<bool>
.
shrink_to_fit
apply to std::deque
?Section: 23.3.5.3 [deque.capacity] Status: CD1 Submitter: Niels Dekker Opened: 2008-06-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 755(i) added a shrink_to_fit
function to std::vector
and std::string
.
It did not yet deal with std::deque
, because of the fundamental
difference between std::deque
and the other two container types. The
need for std::deque
may seem less evident, because one might think that
for this container, the overhead is a small map, and some number of
blocks that's bounded by a small constant.
The container overhead can in fact be arbitrarily large (i.e. is not
necessarily O(N) where N is the number of elements currently held by the
deque
). As Bill Plauger noted in a reflector message, unless the map of
block pointers is shrunk, it must hold at least maxN⁄B pointers where
maxN is the maximum of N over the lifetime of the deque
since its
creation. This is independent of how the map is implemented
(vector
-like circular buffer and all), and maxN bears no relation to N,
the number of elements it currently holds.
Hervé Brönnimann reports a situation where a deque
of requests grew very
large due to some temporary backup (the front request hanging), and the
map of the deque
grew quite large before getting back to normal. Just
to put some color on it, assuming a deque
with 1K pointer elements in
steady regime, that held, at some point in its lifetime, maxN=10M
pointers, with one block holding 128 elements, the spine must be at
least (maxN ⁄ 128), in that case 100K. In that case, shrink-to-fit
would allow to reuse about 100K which would otherwise never be reclaimed
in the lifetime of the deque
.
An added bonus would be that it allows implementations to hang on to
empty blocks at the end (but does not care if they do or not). A
shrink_to_fit
would take care of both shrinks, and guarantee that at
most O(B) space is used in addition to the storage to hold the N
elements and the N⁄B block pointers.
Proposed resolution:
To class template deque
23.3.5 [deque] synopsis, add:
void shrink_to_fit();
To deque capacity 23.3.5.3 [deque.capacity], add:
void shrink_to_fit();Remarks:
shrink_to_fit
is a non-binding request to reduce memory use. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]
begin(n)
mistakenly const
Section: 23.5 [unord] Status: CD1 Submitter: Robert Klarer Opened: 2008-06-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 3 of the four unordered containers the local begin
member is mistakenly declared const
:
local_iterator begin(size_type n) const;
Proposed resolution:
Change the synopsis in 23.5.3 [unord.map], 23.5.4 [unord.multimap], and 23.5.7 [unord.multiset]:
local_iterator begin(size_type n)const;
to_string
needs updating with zero
and one
Section: 22.9.2 [template.bitset] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [template.bitset].
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Discussion:
Issue 396(i) adds defaulted arguments to the to_string
member, but neglects to update
the three newer to_string
overloads.
[ post San Francisco: ]
Daniel found problems with the wording and provided fixes. Moved from Ready to Review.
[ Post Summit: ]
Alisdair: suggest to not repeat the default arguments in B, C, D (definition of to_string members)
Walter: This is not really a definition.
Consensus: Add note to the editor: Please apply editor's judgement whether default arguments should be repeated for B, C, D changes.
Recommend Tentatively Ready.
[ 2009-05-09: See alternative solution in issue 1113(i). ]
Proposed resolution:
replace in 22.9.2 [template.bitset]/1 (class bitset
)
template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const; template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const; basic_string<char, char_traits<char>, allocator<char> > to_string(char zero = '0', char one = '1') const;
replace in 22.9.2.3 [bitset.members]/37
template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const;37 Returns:
to_string<charT, traits, allocator<charT> >(zero, one)
.
replace in 22.9.2.3 [bitset.members]/38
template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const;38 Returns:
to_string<charT, char_traits<charT>, allocator<charT> >(zero, one)
.
replace in 22.9.2.3 [bitset.members]/39
basic_string<char, char_traits<char>, allocator<char> > to_string(char zero = '0', char one = '1') const;39 Returns:
to_string<char, char_traits<char>, allocator<char> >(zero, one)
.
default_delete
converting constructor underspecifiedSection: 20.3.1.2.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.dltr.dflt].
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Discussion:
No relationship between U
and T
in the converting constructor for default_delete
template.
Requirements: U*
is convertible to T*
and has_virtual_destructor<T>
;
the latter should also become a concept.
Rules out cross-casting.
The requirements for unique_ptr
conversions should be the same as those on the deleter.
[ Howard adds 2008-11-26: ]
I believe we need to be careful to not outlaw the following use case, and I believe the current proposed wording (
requires Convertible<U*, T*> && HasVirtualDestructor<T>
) does so:#include <memory> int main() { std::unique_ptr<int> p1(new int(1)); std::unique_ptr<const int> p2(move(p1)); int i = *p2; // *p2 = i; // should not compile }
I've removed "
&& HasVirtualDestructor<T>
" from therequires
clause in the proposed wording.
[ Post Summit: ]
Alisdair: This issue has to stay in review pending a paper constraining
unique_ptr
.Consensus: We agree with the resolution, but
unique_ptr
needs to be constrained, too.Recommend Keep in Review.
[ Batavia (2009-05): ]
Keep in Review status for the reasons cited.
[ 2009-07 Frankfurt: ]
The proposed resolution uses concepts. Howard needs to rewrite the proposed resolution.
Move back to Open.
[ 2009-07-26 Howard provided rewritten proposed wording and moved to Review. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add after 20.3.1.2.2 [unique.ptr.dltr.dflt], p1:
template <class U> default_delete(const default_delete<U>& other);-1- Effects: ...
Remarks: This constructor shall participate in overload resolution if and only if
U*
is implicitly convertible toT*
.
aligned_union
Section: 21.3.8.7 [meta.trans.other] Status: CD1 Submitter: Jens Maurer Opened: 2008-06-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
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Discussion:
With the arrival of extended unions
(N2544),
there is no
known use of aligned_union
that couldn't be handled by
the "extended unions" core-language facility.
Proposed resolution:
Remove the following signature from 21.3.3 [meta.type.synop]:
template <std::size_t Len, class... Types> struct aligned_union;
Remove the second row from table 51 in 21.3.8.7 [meta.trans.other], starting with:
template <std::size_t Len, class... Types> struct aligned_union;
condition_variable::time_wait
return bool
error proneSection: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Beman Dawes Opened: 2008-06-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
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Discussion:
The meaning of the bool
returned by condition_variable::timed_wait
is so
obscure that even the class' designer can't deduce it correctly. Several
people have independently stumbled on this issue.
It might be simpler to change the return type to a scoped enum:
enum class timeout { not_reached, reached };
That's the same cost as returning a bool
, but not subject to mistakes. Your example below would be:
if (cv.wait_until(lk, time_limit) == timeout::reached ) throw time_out();
[ Beman to supply exact wording. ]
[ San Francisco: ]
There is concern that the enumeration names are just as confusing, if not more so, as the bool. You might have awoken because of a signal or a spurious wakeup, for example.
Group feels that this is a defect that needs fixing.
Group prefers returning an enum over a void return.
Howard to provide wording.
[ 2009-06-14 Beman provided wording. ]
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
Change Condition variables 32.7 [thread.condition], Header condition_variable synopsis, as indicated:
namespace std { class condition_variable; class condition_variable_any; enum class cv_status { no_timeout, timeout }; }
Change class condition_variable
32.7.4 [thread.condition.condvar] as indicated:
class condition_variable { public: ... template <class Clock, class Duration>boolcv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time); template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred); template <class Rep, class Period>boolcv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time); template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred); ... }; ... template <class Clock, class Duration>boolcv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time);-15- Precondition:
lock
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting threads (viawait
,wait_for
orwait_until
.).-16- Effects:
- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
,by the current time exceedingifabs_time
Clock::now() >= abs_time
, or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.-17- Postcondition:
lock
is locked by the calling thread.-18- Returns:
Clock::now() < abs_time
cv_status::timeout
if the function unblocked becauseabs_time
was reached, otherwisecv_status::no_timeout
.-19- Throws:
std::system_error
when the effects or postcondition cannot be achieved.-20- Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.template <class Rep, class Period>boolcv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);-21-
EffectsReturns:wait_until(lock, chrono::monotonic_clock::now() + rel_time)
-22- Returns:false
if the call is returning because the time duration specified byrel_time
has elapsed, otherwisetrue
.[ This part of the wording may conflict with 859(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]
template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);-23- Effects:
while (!pred()) if (!wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;-24- Returns:
pred()
.-25- [Note: The returned value indicates whether the predicate evaluates to
true
regardless of whether the timeout was triggered. — end note].
Change Class condition_variable_any 32.7.5 [thread.condition.condvarany] as indicated:
class condition_variable_any { public: ... template <class Lock, class Clock, class Duration>boolcv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time); template <class Lock, class Clock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred); template <class Lock, class Rep, class Period>boolcv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time); template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred); ... }; ... template <class Lock, class Clock, class Duration>boolcv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);-13- Effects:
- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
,by the current time exceedingifabs_time
Clock::now() >= abs_time
, or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.-14- Postcondition:
lock
is locked by the calling thread.-15- Returns:
Clock::now() < abs_time
cv_status::timeout
if the function unblocked becauseabs_time
was reached, otherwisecv_status::no_timeout
.-16- Throws:
std::system_error
when the effects or postcondition cannot be achieved.-17- Error conditions:
- equivalent error condition from
lock.lock()
orlock.unlock()
.template <class Lock, class Rep, class Period>boolcv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);-18-
EffectsReturns:wait_until(lock, chrono::monotonic_clock::now() + rel_time)
-19- Returns:false
if the call is returning because the time duration specified byrel_time
has elapsed, otherwisetrue
.[ This part of the wording may conflict with 859(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]
template <class Lock, class Clock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>&rel_timeabs_time, Predicate pred);-20- Effects:
while (!pred()) if (!wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;-21- Returns:
pred()
.-22- [Note: The returned value indicates whether the predicate evaluates to
true
regardless of whether the timeout was triggered. — end note].
Section: 99 [util.dynamic.safety] Status: CD1 Submitter: Pete Becker Opened: 2008-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The first sentence of the Effects clause for undeclare_reachable
seems
to be missing some words. I can't parse
... for all non-null
p
referencing the argument is no longer declared reachable...
I take it the intent is that undeclare_reachable
should be called only
when there has been a corresponding call to declare_reachable
. In
particular, although the wording seems to allow it, I assume that code
shouldn't call declare_reachable
once then call undeclare_reachable
twice.
I don't know what "shall be live" in the Requires clause means.
In the final Note for undeclare_reachable
, what does "cannot be
deallocated" mean? Is this different from "will not be able to collect"?
For the wording on nesting of declare_reachable
and
undeclare_reachable
, the words for locking and unlocking recursive
mutexes probably are a good model.
[ San Francisco: ]
Nick: what does "shall be live" mean?
Hans: I can provide an appropriate cross-reference to the Project Editor to clarify the intent.
Proposed resolution:
In 99 [util.dynamic.safety] (N2670, Minimal Support for Garbage Collection)
Add at the beginning, before paragraph 39
A complete object is declared reachable while the number of calls to
declare_reachable
with an argument referencing the object exceeds the number ofundeclare_reachable
calls with pointers to the same complete object.
Change paragraph 42 (Requires clause for undeclare_reachable
)
If
p
is not null,the complete object referenced bydeclare_reachable(p)
was previously calledp
shall have been previously declared reachable, and shall be live (6.7.4 [basic.life]) from the time of the call until the lastundeclare_reachable(p)
call on the object.
Change the first sentence in paragraph 44 (Effects clause for
undeclare_reachable
):
Effects:
Once the number of calls toAfter a call toundeclare_reachable(p)
equals the number of calls todeclare_reachable(p)
for all non-nullp
referencing the argument is no longer declared reachable. When this happens, pointers to the object referenced by p may not be subsequently dereferenced.undeclare_reachable(p)
, ifp
is not null and the objectq
referenced byp
is no longer declared reachable, then dereferencing any pointer toq
that is not safely derived results in undefined behavior. ...
Change the final note:
[Note: It is expected that calls to declare_reachable(p)
will consume a small amount of memory, in addition to that occupied
by the referenced object, until the matching call to
undeclare_reachable(p)
is encountered. In addition, the
referenced object cannot be deallocated during this period, and garbage
collecting implementations will not be able to collect the object while
it is declared reachable. Long running programs should arrange
that calls for short-lived objects are matched. --end
note]
Section: 32.7 [thread.condition] Status: C++11 Submitter: Pete Becker Opened: 2008-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
N2661
says that there is a class named monotonic_clock
. It also says that this
name may be a synonym for system_clock
, and that it's conditionally
supported. So the actual requirement is that it can be monotonic or not,
and you can tell by looking at is_monotonic
, or it might not exist at
all (since it's conditionally supported). Okay, maybe too much
flexibility, but so be it.
A problem comes up in the threading specification, where several
variants of wait_for
explicitly use monotonic_clock::now()
. What is the
meaning of an effects clause that says
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
when monotonic_clock
is not required to exist?
[ San Francisco: ]
Nick: maybe instead of saying that
chrono::monotonic_clock
is conditionally supported, we could say that it's always there, but not necessarily supported..Beman: I'd prefer a typedef that identifies the best clock to use for
wait_for
locks.Tom: combine the two concepts; create a duration clock type, but keep the is_monotonic test.
Howard: if we create a
duration_clock
type, is it a typedef or an entirely true type?There was broad preference for a typedef.
Move to Open. Howard to provide wording to add a typedef for duration_clock and to replace all uses of monotonic_clock in function calls and signatures with duration_clock.
[ Howard notes post-San Francisco: ]
After further thought I do not believe that creating a
duration_clock typedef
is the best way to proceed. An implementation may not need to use atime_point
to implement thewait_for
functions.For example, on POSIX systems
sleep_for
can be implemented in terms ofnanosleep
which takes only a duration in terms of nanoseconds. The current working paper does not describesleep_for
in terms ofsleep_until
. And paragraph 2 of 32.2.4 [thread.req.timing] has the words strongly encouraging implementations to use monotonic clocks forsleep_for
:2 The member functions whose names end in
_for
take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions.I believe the approach taken in describing the effects of
sleep_for
andtry_lock_for
is also appropriate forwait_for
. I.e. these are not described in terms of their_until
variants.
[ 2009-07 Frankfurt: ]
Beman will send some suggested wording changes to Howard.
Move to Ready.
[ 2009-07-21 Beman added the requested wording changes to 962(i). ]
Proposed resolution:
Change 32.7.4 [thread.condition.condvar], p21-22:
template <class Rep, class Period> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);Precondition:
lock
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting threads (viawait
,wait_for
orwait_until
).21 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, by the elapsed timerel_time
passing (32.2.4 [thread.req.timing]), or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.Postcondition:
lock
is locked by the calling thread.22 Returns:
false
if the call is returning because the time duration specified byrel_time
has elapsed, otherwisetrue
.[ This part of the wording may conflict with 857(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]
Throws:
std::system_error
when the effects or postcondition cannot be achieved.Error conditions:
operation_not_permitted
-- if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
Change 32.7.4 [thread.condition.condvar], p26-p29:
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);Precondition:
lock
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting threads (viawait
,wait_for
orwait_until
).26 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))
- Executes a loop: Within the loop the function first evaluates
pred()
and exits the loop if the result ofpred()
istrue
.- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock).- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, by the elapsed timerel_time
passing (30.1.4 [thread.req.timing]), or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.- The loop terminates when
pred()
returnstrue
or when the time duration specified byrel_time
has elapsed.27 [Note: There is no blocking if
pred()
is initiallytrue
, even if the timeout has already expired. -- end note]Postcondition:
lock
is locked by the calling thread.28 Returns:
pred()
29 [Note: The returned value indicates whether the predicate evaluates to
true
regardless of whether the timeout was triggered. -- end note]Throws:
std::system_error
when the effects or postcondition cannot be achieved.Error conditions:
operation_not_permitted
-- if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
Change 32.7.5 [thread.condition.condvarany], p18-19:
template <class Lock, class Rep, class Period> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);18 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, by the elapsed timerel_time
passing (32.2.4 [thread.req.timing]), or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.Postcondition:
lock
is locked by the calling thread.19 Returns:
false
if the call is returning because the time duration specified byrel_time
has elapsed, otherwisetrue
.Throws:
std::system_error
when the returned value, effects, or postcondition cannot be achieved.Error conditions:
- equivalent error condition from
lock.lock()
orlock.unlock()
.
Change 32.7.5 [thread.condition.condvarany], p23-p26:
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);Precondition:
lock
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting threads (viawait
,wait_for
orwait_until
).23 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))
- Executes a loop: Within the loop the function first evaluates
pred()
and exits the loop if the result ofpred()
istrue
.- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock).- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, by the elapsed timerel_time
passing (30.1.4 [thread.req.timing]), or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.- The loop terminates when
pred()
returnstrue
or when the time duration specified byrel_time
has elapsed.24 [Note: There is no blocking if
pred()
is initiallytrue
, even if the timeout has already expired. -- end note]Postcondition:
lock
is locked by the calling thread.25 Returns:
pred()
26 [Note: The returned value indicates whether the predicate evaluates to
true
regardless of whether the timeout was triggered. -- end note]Throws:
std::system_error
when the effects or postcondition cannot be achieved.Error conditions:
operation_not_permitted
-- if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
Section: 29 [numerics] Status: C++11 Submitter: Lawrence Crowl Opened: 2008-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are a number of functions that affect the floating point state. These function need to be thread-safe, but I'm unsure of the right approach in the standard, as we inherit them from C.
[ San Francisco: ]
Nick: I think we already say that these functions do not introduce data races; see 17.6.5.6/20
Pete: there's more to it than not introducing data races; are these states maintained per thread?
Howard: 21.5/14 says that strtok and strerror are not required to avoid data races, and 20.9/2 says the same about asctime, gmtime, ctime, and gmtime.
Nick: POSIX has a list of not-safe functions. All other functions are implicitly thread safe.
Lawrence is to form a group between meetings to attack this issue. Nick and Tom volunteered to work with Lawrence.
Move to Open.
[ Post Summit: ]
Hans: Sane oses seem ok. Sensible thing is implementable and makes sense.
Nick: Default wording seems to cover this? Hole in POSIX, these functions need to be added to list of thread-unsafe functions.
Lawrence: Not sufficient, not "thread-safe" per our definition, but think of state as a thread-local variable. Need something like "these functions only affect state in the current thread."
Hans: Suggest the following wording: "The floating point environment is maintained per-thread."
Walter: Any other examples of state being thread safe that are not already covered elsewhere?
Have thread unsafe functions paper which needs to be updated. Should just fold in 29.3 [cfenv] functions.
Recommend Open. Lawrence instead suggests leaving it open until we have suitable wording that may or may not include the thread local commentary.
[ 2009-09-23 Hans provided wording. ]
If I understand the history correctly, Nick, as the Posix liaison, should probably get a veto on this, since I think it came from Posix (?) via WG14 and should probably really be addressed there (?). But I think we are basically in agreement that there is no other sane way to do this, and hence we don't have to worry too much about stepping on toes. As far as I can tell, this same issue also exists in the latest Posix standard (?).
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Add at the end of 29.3.1 [cfenv.syn]:
2 The header defines all functions, types, and macros the same as C99 7.6.
A separate floating point environment shall be maintained for each thread. Each function accesses the environment corresponding to its calling thread.
EqualityComparable
for std::forward_list
Section: 23.2 [container.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2008-06-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 89, Container requirements, defines operator==
in terms of the container
member function size()
and the algorithm std::equal
:
==
is an equivalence relation.a.size() == b.size() && equal(a.begin(), a.end(), b.begin()
The new container forward_list
does not provide a size
member function
by design but does provide operator==
and operator!=
without specifying it's semantic.
Other parts of the (sequence) container requirements do also depend on
size()
, e.g. empty()
or clear()
, but this issue explicitly attempts to solve the missing
EqualityComparable
specification,
because of the special design choices of forward_list
.
I propose to apply one of the following resolutions, which are described as:
O(N)
calls of std::distance()
with the corresponding container ranges and instead uses a special
equals
implementation which takes two container ranges instead of 1 1/2.
size()
is replaced
by distance
with corresponding performance disadvantages.
Both proposal choices are discussed, the preferred choice of the author is to apply (A).
[ San Francisco: ]
There's an Option C: change the requirements table to use distance().
LWG found Option C acceptable.
Martin will draft the wording for Option C.
[ post San Francisco: ]
Martin provided wording for Option C.
[ 2009-07 Frankfurt ]
Other operational semantics (see, for example, Tables 82 and 83) are written in terms of a container's size() member. Daniel to update proposed resolution C.
[ Howard: Commented out options A and B. ]
[ 2009-07-26 Daniel updated proposed resolution C. ]
[ 2009-10 Santa Cruz: ]
Mark NAD Editorial. Addressed by N2986.
[ 2009-10 Santa Cruz: ]
Reopened. N2986 was rejected in full committee on procedural grounds.
[ 2010-01-30 Howard updated Table numbers. ]
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Option (C):
In 23.2.2 [container.requirements.general] change Table 90 -- Container requirements as indicated:
Change the text in the Assertion/note column in the row for "
X u
;" as follows:post:
u.
size() == 0empty() == trueChange the text in the Assertion/note column in the row for "
X();
" as follows:
X().
size() == 0empty() == trueChange the text in the Operational Semantics column in the row for "
a == b
" as follows:
==
is an equivalence relation.
a.size()distance(a.begin(), a.end()) ==b.size()distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin())Add text in the Ass./Note/pre-/post-condition column in the row for "
a == b
" as follows:Requires:
T
isEqualityComparable
Change the text in the Operational Semantics column in the row for "
a.size()
" as follows:
a.end() - a.begin()distance(a.begin(), a.end())Change the text in the Operational Semantics column in the row for "
a.max_size()
" as follows:
of the largest possible container
size()distance(begin(), end())Change the text in the Operational Semantics column in the row for "
a.empty()
" as follows:
a.size() == 0a.begin() == a.end()In 23.2.2 [container.requirements.general] change Table 93 — Allocator-aware container requirements as indicated:
Change the text in the Assertion/note column in the row for "
X() / X u;
" as follows:Requires:
A
isDefaultConstructible
post:,
u.size() == 0u.empty() == trueget_allocator() == A()
Change the text in the Assertion/note column in the row for "
X(m) / X u(m);
" as follows:post:
u.size() == 0u.empty() == true, get_allocator() == mIn 23.2.4 [sequence.reqmts] change Table 94 — Sequence container requirements as indicated:
Change the text in the Assertion/note column in the row for "
X(n, t) / X a(n, t)
" as follows:post:
size()distance(begin(), end()) == n [..]Change the text in the Assertion/note column in the row for "
X(i, j) / X a(i, j)
" as follows:[..] post:
size() ==
distance betweeni
andj
distance(begin(), end()) == distance(i, j)
[..]Change the text in the Assertion/note column in the row for "
a.clear()
" as follows:
a.erase(a.begin(), a.end())
post:
size() == 0a.empty() == trueIn 23.2.7 [associative.reqmts] change Table 96 — Associative container requirements as indicated:
[ Not every occurrence of
size()
was replaced, because all current associative containers have asize
. The following changes ensure consistency regarding the semantics of "erase
" for all tables and adds some missing objects ]
Change the text in the Complexity column in the row for
X(i,j,c)/Xa(i,j,c);
as follows:
N
logN
in general (N
== distance(i, j)
is the distance from); ...i
toj
Change the text in the Complexity column in the row for "
a.insert(i, j)
" as follows:
N log(a.size() + N)
(whereN
is the distance fromi
toj
)N == distance(i, j)
Change the text in the Complexity column in the row for "
a.erase(k)
" as follows:
log(a.size()) + a.count(k)
Change the text in the Complexity column in the row for "
a.erase(q1, q2)
" as follows:
log(a.size()) + N
whereN
is the distance fromq1
toq2
== distance(q1, q2)
.Change the text in the Assertion/note column in the row for "
a.clear()
" as follows:
a.erase(a.begin(),a.end())
post:
size() == 0a.empty() == trueChange the text in the Complexity column in the row for "
a.clear()
" as follows:linear in
a.size()
Change the text in the Complexity column in the row for "
a.count(k)
" as follows:
log(a.size()) + a.count(k)
In 23.2.8 [unord.req] change Table 98 — Unordered associative container requirements as indicated:
[ The same rational as for Table 96 applies here ]
Change the text in the Assertion/note column in the row for "
a.clear()
" as follows:[..] Post:
a.
size() == 0empty() == true
Section: 26.7.6 [alg.fill], 26.7.7 [alg.generate] Status: C++11 Submitter: Daniel Krügler Opened: 2008-07-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In regard to library defect 488(i) I found some more algorithms which
unnecessarily throw away information. These are typically algorithms,
which sequentially write into an OutputIterator
, but do not return the
final value of this output iterator. These cases are:
template<class OutputIterator, class Size, class T> void fill_n(OutputIterator first, Size n, const T& value);
template<class OutputIterator, class Size, class Generator> void generate_n(OutputIterator first, Size n, Generator gen);
In both cases the minimum requirements on the iterator are
OutputIterator
, which means according to the requirements of
24.3.5.4 [output.iterators] p. 2 that only single-pass iterations are guaranteed.
So, if users of fill_n
and generate_n
have only an OutputIterator
available, they have no chance to continue pushing further values
into it, which seems to be a severe limitation to me.
[ Post Summit Daniel "conceptualized" the wording. ]
[ Batavia (2009-05): ]
Alisdair likes the idea, but has concerns about the specific wording about the returns clauses.
Alan notes this is a feature request.
Bill notes we have made similar changes to other algorithms.
Move to Open.
[ 2009-07 Frankfurt ]
We have a consensus for moving forward on this issue, but Daniel needs to deconceptify it.
[ 2009-07-25 Daniel provided non-concepts wording. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Replace the current declaration of fill_n
in 26 [algorithms]/2, header
<algorithm>
synopsis and in 26.7.6 [alg.fill] by
template<class OutputIterator, class Size, class T>voidOutputIterator fill_n(OutputIterator first, Size n, const T& value);
Just after the effects clause add a new returns clause saying:
Returns: For
fill_n
and positiven
, returnsfirst + n
. Otherwise returnsfirst
forfill_n
.
Replace the current declaration of generate_n
in 26 [algorithms]/2,
header <algorithm>
synopsis and in 26.7.7 [alg.generate] by
template<class OutputIterator, class Size, class Generator>voidOutputIterator generate_n(OutputIterator first, Size n, Generator gen);
Just after the effects clause add a new returns clause saying:
For
generate_n
and positiven
, returnsfirst + n
. Otherwise returnsfirst
forgenerate_n
.
Section: 26.11 [specialized.algorithms], 20.3.2.2.7 [util.smartptr.shared.create] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-07-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
LWG issue 402(i) replaced "new
" with "::new
" in the placement
new-expression in 20.2.10.2 [allocator.members]. I believe the rationale
given in 402(i) applies also to the following other contexts:
in 26.11 [specialized.algorithms], all four algorithms unitialized_copy
,
unitialized_copy_n
, unitialized_fill
and unitialized_fill_n
use
the unqualified placement new-expression in some variation of the form:
new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);
in 20.3.2.2.7 [util.smartptr.shared.create] there is a reference to the unqualified placement new-expression:
new (pv) T(std::forward<Args>(args)...),
I suggest to add qualification in all those places. As far as I know, these are all the remaining places in the whole library that explicitly use a placement new-expression. Should other uses come out, they should be qualified as well.
As an aside, a qualified placement new-expression does not need
additional requirements to be compiled in a constrained context. By
adding qualification, the HasPlacementNew
concept introduced recently in
N2677 (Foundational Concepts)
would no longer be needed by library and
should therefore be removed.
[ San Francisco: ]
Detlef: If we move this to Ready, it's likely that we'll forget about the side comment about the
HasPlacementNew
concept.
[ post San Francisco: ]
Daniel:
HasPlacementNew
has been removed from N2774 (Foundational Concepts).
Proposed resolution:
Replace "new
" with "::new
" in:
Section: 23 [containers] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-07-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The term "default constructed" is often used in wording that predates
the introduction of the concept of value-initialization. In a few such
places the concept of value-initialization is more correct than the
current wording (for example when the type involved can be a built-in)
so a replacement is in order. Two of such places are already covered by
issue 867(i). This issue deliberately addresses the hopefully
non-controversial changes in the attempt of being approved more quickly.
A few other occurrences (for example in std::tuple
,
std::reverse_iterator
and std::move_iterator
) are left to separate
issues. For std::reverse_iterator
, see also issue 408(i). This issue is
related with issue 724(i).
[ San Francisco: ]
The list provided in the proposed resolution is not complete. James Dennett will review the library and provide a complete list and will double-check the vocabulary.
[ 2009-07 Frankfurt ]
The proposed resolution is incomplete.
Move to Tentatively NAD Future. Howard will contact Ganesh for wording. If wording is forthcoming, Howard will move it back to Review.
[ 2009-07-18 Ganesh updated the proposed wording. ]
Howard: Moved back to Review. Note that 16.4.4.2 [utility.arg.requirements] refers to a section that is not in the current working paper, but does refer to a section that we expect to reappear after the de-concepts merge. This was a point of confusion we did not recognize when we reviewed this issue in Frankfurt.
Howard: Ganesh also includes a survey of places in the WP surveyed for changes of this nature and purposefully not treated:
Places where changes are not being proposed
In the following paragraphs, we are not proposing changes because it's not clear whether we actually prefer value-initialization over default-initialization (now partially covered by 1012(i)):
20.3.1.3.2 [unique.ptr.single.ctor] para 3 e 7
24.5.1.4 [reverse.iter.cons] para 1
[move.iter.op.const] para 1
In the following paragraphs, the expression "default constructed" need not be changed, because the relevant type does not depend on a template parameter and has a user-provided constructor:
[func.referenceclosure.invoke] para 12, type: reference_closure
32.4.3.3 [thread.thread.constr] para 30, type: thread
32.4.3.6 [thread.thread.member] para 52, type: thread_id
32.4.5 [thread.thread.this], para 1, type: thread_id
[ 2009-08-18 Daniel adds: ]
I have no objections against the currently suggested changes, but I also cross-checked with the list regarding intentionally excluded changes, and from this I miss the discussion of
27.4.3.2 [string.require] p. 2:
"[..] The
Allocator
object used shall be a copy of theAllocator>
object passed to thebasic_string
object's constructor or, if the constructor does not take anAllocator
argument, a copy of a default-constructedAllocator
object."N2723, 29.5.3.4 [rand.req.eng], Table 109, expression "
T()
":Pre-/post-condition: "Creates an engine with the same initial state as all other default-constructed engines of type
X
."as well as in 29.5.6 [rand.predef]/1-9 (N2914), 29.5.8.1 [rand.util.seedseq]/3, 31.7.5.2.2 [istream.cons]/3, 31.7.6.2.2 [ostream.cons]/9 (N2914), 28.6.12 [re.grammar]/2, 32.4.3.5 [thread.thread.assign]/1 (N2914),
[ Candidates for the "the expression "default constructed" need not be changed" list ]
I'm fine, if these would be added to the intentionally exclusion list, but mentioning them makes it easier for other potential reviewers to decide on the relevance or not-relevance of them for this issue.
I suggest to remove the reference of [func.referenceclosure.invoke] in the "it's not clear" list, because this component does no longer exist.
I also suggest to add a short comment that all paragraphs in the resolution whether they refer to N2723 or to N2914 numbering, because e.g. "Change 23.3.5.2 [deque.cons] para 5" is an N2723 coordinate, while "Change 23.3.5.3 [deque.capacity] para 1" is an N2914 coordinate. Even better would be to use one default document for the numbering (probably N2914) and mention special cases (e.g. "Change 16.4.4.2 [utility.arg.requirements] para 2" as referring to N2723 numbering).
[ 2009-08-18 Alisdair adds: ]
I strongly believe the term "default constructed" should not appear in the library clauses unless we very clearly define a meaning for it, and I am not sure what that would be.
In those cases where we do not want to replace "default constructed" with "vale initialized" we should be using "default initialized". If we have a term that could mean either, we reduce portability of programs.
I have not done an exhaustive review to clarify if that is a vendor freedom we have reason to support (e.g. value-init in debug, default-init in release) so I may yet be convinced that LWG has reason to define this new term of art, but generally C++ initialization is confusing enough without supporting further ill-defined terms.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010 Pittsburgh: ]
Moved to review in order to enable conflict resolution with 704(i).
[ 2010-03-26 Daniel harmonized the wording with the upcoming FCD. ]
[ 2010 Rapperswil: ]
Move to Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 16.4.4.2 [utility.arg.requirements] para 2:
2 In general, a default constructor is not required. Certain container class member function signatures specify
the default constructorT()
as a default argument.T()
shall be a well-defined expression (8.5) if one of those signatures is called using the default argument (8.3.6).
Change 23.3.5.2 [deque.cons] para 3:
3 Effects: Constructs a deque with
n
default constructedvalue-initialized elements.
Change 23.3.5.3 [deque.capacity] para 1:
1 Effects: If
sz < size()
, equivalent toerase(begin() + sz, end());
. Ifsize() < sz
, appendssz - size()
default constructedvalue-initialized elements to the sequence.
Change [forwardlist.cons] para 3:
3 Effects: Constructs a
forward_list
object withn
default constructedvalue-initialized elements.
Change [forwardlist.modifiers] para 22:
22 Effects: [...] For the first signature the inserted elements are
default constructedvalue-initialized, and for the second signature they are copies ofc
.
Change 23.3.11.2 [list.cons] para 3:
3 Effects: Constructs a
list
withn
default constructedvalue-initialized elements.
Change 23.3.11.3 [list.capacity] para 1:
1 Effects: If
sz < size()
, equivalent tolist<T>::iterator it = begin(); advance(it, sz); erase(it, end());
. Ifsize() < sz
,appends sz - size()
default constructedvalue-initialized elements to the sequence.
Change 23.3.13.2 [vector.cons] para 3:
3 Effects: Constructs a
vector
withn
default constructedvalue-initialized elements.
Change 23.3.13.3 [vector.capacity] para 9:
9 Effects: If
sz < size()
, equivalent toerase(begin() + sz, end());
. Ifsize() < sz
, appendssz - size()
default constructedvalue-initialized elements to the sequence.
Section: 23.2.8 [unord.req] Status: C++11 Submitter: Sohail Somani Opened: 2008-07-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is there any language in the current draft specifying the behaviour of the following snippet?
unordered_set<int> s; unordered_set<int>::local_iterator it = s.end(0); // Iterate past end - the unspecified part it++;
I don't think there is anything about s.end(n)
being considered an
iterator for the past-the-end value though (I think) it should be.
[ San Francisco: ]
We believe that this is not a substantive change, but the proposed change to the wording is clearer than what we have now.
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
Change Table 97 "Unordered associative container requirements" in 23.2.8 [unord.req]:
Table 97: Unordered associative container requirements expression return type assertion/note pre/post-condition complexity b.begin(n)
local_iterator
const_local_iterator
for constb
.Pre: n shall be in the range [0,b.bucket_count()). Note: [b.begin(n), b.end(n)) is a valid range containing all of the elements in the nth bucket.b.begin(n)
returns an iterator referring to the first element in the bucket. If the bucket is empty, thenb.begin(n) == b.end(n)
.Constant b.end(n)
local_iterator
const_local_iterator
for constb
.Pre: n shall be in the range [0, b.bucket_count())
.b.end(n)
returns an iterator which is the past-the-end value for the bucket.Constant
Section: 23.2.8 [unord.req] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Good ol' associative containers allow both function pointers and function objects as feasible comparators, as described in 23.2.7 [associative.reqmts]/2:
Each associative container is parameterized on
Key
and an ordering relationCompare
that induces a strict weak ordering (25.3) on elements of Key. [..]. The object of typeCompare
is called the comparison object of a container. This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.[..]
The corresponding wording for unordered containers is not so clear, but I read it to disallow function pointers for the hasher and I miss a clear statement for the equality predicate, see 23.2.8 [unord.req]/3+4+5:
Each unordered associative container is parameterized by
Key
, by a function objectHash
that acts as a hash function for values of typeKey
, and by a binary predicatePred
that induces an equivalence relation on values of typeKey
.[..]A hash function is a function object that takes a single argument of type
Key
and returns a value of typestd::size_t
.Two values
k1
andk2
of typeKey
are considered equal if the container's equality function object returnstrue
when passed those values.[..]
and table 97 says in the column "assertion...post-condition" for the
expression X::hasher
:
Hash
shall be a unary function object type such that the expressionhf(k)
has typestd::size_t
.
Note that 22.10 [function.objects]/1 defines as "Function objects are
objects with an operator()
defined.[..]"
Does this restriction exist by design or is it an oversight? If an oversight, I suggest that to apply the following
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Ask Daniel to provide proposed wording that: makes it explicit that function pointers are function objects at the beginning of 22.10 [function.objects]; fixes the "requirements" for typedefs in 22.10.6 [refwrap] to instead state that the function objects defined in that clause have these typedefs, but not that these typedefs are requirements on function objects; remove the wording that explicitly calls out that associative container comparators may be function pointers.
[ 2009-12-19 Daniel updates wording and rationale. ]
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Rationale:
The below provided wording also affects some part of the library which is involved with callable types (22.10.3 [func.def]/3). Reason for this is that callable objects do have a lot in common with function objects. A simple formula seems to be:
callable objects = function objects + pointers to member
The latter group is excluded from function objects because of the expression-based usage of function objects in the algorithm clause, which is incompatible with the notation to dereference pointers to member without a concept map available in the language.
This analysis showed some currently existing normative definition differences between the above subset of callable objects and function objects which seem to be unintended: Backed by the Santa Cruz outcome function objects should include both function pointers and "object[s] with an operator() defined". This clearly excludes class types with a conversion function to a function pointer or all similar conversion function situations described in 12.2 [over.match]/2 b. 2. In contrast to this, the wording for callable types seems to be less constrained (22.10.3 [func.def]/3):
A callable type is a [..] class type whose objects can appear immediately to the left of a function call operator.
The rationale given in N1673 and a recent private communication with Peter Dimov revealed that the intention of this wording was to cover the above mentioned class types with conversion functions as well. To me the current wording of callable types can be read either way and I suggest to make the intention more explicit by replacing
[..] class type whose objects can appear immediately to the left of a function call operator
by
[..] class type whose objects can appear as the leftmost subexpression of a function call expression 7.6.1.3 [expr.call].
and to use the same definition for the class type part of function objects, because there is no reason to exclude class types with a conversion function to e.g. pointer to function from being used in algorithms.
Now this last term "function objects" itself brings us to a third unsatisfactory
state: The term is used both for objects (e.g. "Function objects are
objects[..]" in 22.10 [function.objects]/1) and for types (e.g. "Each
unordered associative container is parameterized [..] by a function object Hash
that acts as a hash function [..]" in 23.2.8 [unord.req]/3). This
impreciseness should be fixed and I suggest to introduce the term function
object type as the counter part to callable type. This word seems
to be a quite natural choice, because the library already uses it here and there
(e.g. "Hash shall be a unary function object type such that the expression
hf(k)
has type std::size_t
." in Table 98, "X::hasher
"
or "Requires: T
shall be a function object type [..]" in
22.10.17.3.6 [func.wrap.func.targ]/3).
Finally I would like to add that part of the issue 870 discussion related to the requirements for typedefs in 22.10.6 [refwrap] during the Santa Cruz meeting is now handled by the new issue 1290(i).
Obsolete rationale:
[ San Francisco: ]
This is fixed by N2776.
Proposed resolution:
Change 22.10 [function.objects]/1 as indicated:
1
Function objects are objects with anAn object type (6.8 [basic.types]) that can be the type of the postfix-expression in a function call (7.6.1.3 [expr.call], 12.2.2.2 [over.match.call]) is called a function object type*. A function object is an object of a function object type. In the places where one would expect to pass a pointer to a function to an algorithmic template (Clause 26 [algorithms]), the interface is specified to acceptoperator()
defined.an object with ana function object. This not only makes algorithmic templates work with pointers to functions, but also enables them to work with arbitrary function objects.operator()
defined* Such a type is either a function pointer or a class type which often has a member
operator()
, but in some cases it can omit that member and provide a conversion to a pointer to function.
Change 22.10.3 [func.def]/3 as indicated: [The intent is to make the commonality of callable types and function object types more explicit and to get rid of wording redundancies]
3 A callable type is
a pointer to function,a pointer to memberfunction, a pointer to member data,or aclass type whose objects can appear immediately to the left of a function call operatorfunction object type (22.10 [function.objects]).
Change [bind]/1 as indicated:
1 The function template
bind
returns an object that binds afunctioncallable object passed as an argument to additional arguments.
Change 22.10.15 [func.bind]/1 as indicated:
1 This subclause describes a uniform mechanism for binding arguments of
functioncallable objects.
Change 22.10.17 [func.wrap]/1 as indicated:
1 This subclause describes a polymorphic wrapper class that encapsulates arbitrary
functioncallable objects.
Change 22.10.17.3 [func.wrap.func]/2 as indicated [The reason for this
change is that 22.10.17.3 [func.wrap.func]/1 clearly says that all callable
types may be wrapped by std::function
and current implementations
indeed do provide support for pointer to members as well. One further suggested
improvement is to set the below definition of Callable in italics]:
2 A
functioncallable objectf
of typeF
isCallableCallable for argument typesT1, T2, ..., TN
inArgTypes
andareturn typeR
,if, given lvaluesthe expressiont1, t2, ..., tN
of typesT1, T2, ..., TN
, respectively,INVOKE(f, declval<ArgTypes>()..., R
, considered as an unevaluated operand (7 [expr]), is well formed (20.7.2)t1, t2, ..., tN)and, if.R
is notvoid
, convertible toR
Change 22.10.17.3.2 [func.wrap.func.con]/7 as indicated:
function(const function& f); template <class A> function(allocator_arg_t, const A& a, const function& f);...
7 Throws: shall not throw exceptions if
f
's target is a function pointer or afunctioncallable object passed viareference_wrapper
. Otherwise, may throwbad_alloc
or any exception thrown by the copy constructor of the storedfunctioncallable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for smallfunctioncallable objects, e.g., wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]
Change 22.10.17.3.2 [func.wrap.func.con]/11 as indicated:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);...
11 [..] [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small
functioncallable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]
Change 22.10.17.3.5 [func.wrap.func.inv]/3 as indicated:
R operator()(ArgTypes... args) const...
3 Throws:
bad_function_call
if!*this
; otherwise, any exception thrown by the wrappedfunctioncallable object.
Change 22.10.17.3.6 [func.wrap.func.targ]/3 as indicated:
template<typename T> T* target(); template<typename T> const T* target() const;...
3 Requires:
T
shall be afunction objecttype that is Callable (22.10.17.3 [func.wrap.func]) for parameter typesArgTypes
and return typeR
.
Change 23.2.7 [associative.reqmts]/2 as indicated: [The suggested
removal seems harmless, because 26.8 [alg.sorting]1 already clarifies
that Compare
is a function object type. Nevertheless it is recommended,
because the explicit naming of "pointer to function" is misleading]
2 Each associative container is parameterized on
Key
and an ordering relationCompare
that induces a strict weak ordering (26.8 [alg.sorting]) on elements ofKey
. In addition,map
andmultimap
associate an arbitrary typeT
with theKey
. The object of typeCompare
is called the comparison object of a container.This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.
Change 23.2.8 [unord.req]/3 as indicated:
3 Each unordered associative container is parameterized by
Key
, by a function object typeHash
that acts as a hash function for values of typeKey
, and by a binary predicatePred
that induces an equivalence relation on values of type Key. [..]
Change 26.1 [algorithms.general]/7 as indicated: [The intent is to bring this part in sync with 22.10 [function.objects]]
7 The
Predicate
parameter is used whenever an algorithm expects a function object (22.10 [function.objects]) that when applied to the result of dereferencing the corresponding iterator returns a value testable astrue
. In other words, if an algorithm takesPredicate pred
as its argument andfirst
as its iterator argument, it should work correctly in the constructif (pred(*first)){...}
. The function objectpred
shall not apply any nonconstant function through the dereferenced iterator.This function object may be a pointer to function, or an object of a type with an appropriate function call operator.
Change 20.3.1.3 [unique.ptr.single]/1 as indicated:
1 The default type for the template parameter
D
isdefault_delete
. A client-supplied template argumentD
shall be a functionpointer or functorobject type for which, given a valued
of typeD
and a pointerptr
of typeT*
, the expressiond(ptr)
is valid and has the effect of deallocating the pointer as appropriate for that deleter.D
may also be an lvalue-reference to a deleter.
T
are too strongSection: 26.10.13 [numeric.iota] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
According to the recent WP
N2691,
26.10.13 [numeric.iota]/1, the requires clause
of std::iota
says:
T
shall meet the requirements ofCopyConstructible
andAssignable
types, and shall be convertible toForwardIterator
's value type.[..]
Neither CopyConstructible
nor Assignable
is needed, instead MoveConstructible
seems to be the correct choice. I guess the current wording resulted as an
artifact from comparing it with similar numerical algorithms like accumulate
.
Note: If this function will be conceptualized, the here proposed
MoveConstructible
requirement can be removed, because this
is an implied requirement of function arguments, see
N2710/[temp.req.impl]/3, last bullet.
[ post San Francisco: ]
Issue pulled by author prior to review.
[ 2009-07-30 Daniel reopened: ]
with the absence of concepts, this issue (closed) is valid again and I suggest to reopen it. I also revised by proposed resolution based on N2723 wording:
[ 2009-10 Santa Cruz: ]
Change 'convertible' to 'assignable', Move To Ready.
Proposed resolution:
Change the first sentence of 26.10.13 [numeric.iota]/1:
Requires:
T
shallmeet the requirements ofbe assignable toCopyConstructible
andAssignable
types, and shallForwardIterator
's value type. [..]
move_iterator::operator[]
has wrong return typeSection: 24.5.4.6 [move.iter.elem] Status: C++11 Submitter: Doug Gregor Opened: 2008-08-21 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [move.iter.elem].
View all issues with C++11 status.
Discussion:
move_iterator
's operator[]
is declared as:
reference operator[](difference_type n) const;
This has the same problem that reverse_iterator
's operator[]
used to
have: if the underlying iterator's operator[]
returns a proxy, the
implicit conversion to value_type&&
could end up referencing a temporary
that has already been destroyed. This is essentially the same issue that
we dealt with for reverse_iterator
in DR 386(i).
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-08-15 Howard adds: ]
I recommend closing this as a duplicate of 1051(i) which addresses this issue for both
move_iterator
andreverse_iterator
.
[ 2009-10 Santa Cruz: ]
Move to Ready. Note that if 1051(i) is reopened, it may yield a better resolution, but 1051(i) is currently marked NAD.
Proposed resolution:
In 24.5.4.2 [move.iterator] and [move.iter.op.index], change the declaration of
move_iterator
's operator[]
to:
referenceunspecified operator[](difference_type n) const;
Rationale:
[ San Francisco: ]
NAD Editorial, see N2777.
initializer_list
constructor for discrete_distribution
Section: 29.5.9.6.1 [rand.dist.samp.discrete] Status: Resolved Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.discrete].
View all issues with Resolved status.
Discussion:
During the Sophia Antipolis meeting it was decided to separate from 793(i) a
subrequest that adds initializer list support to discrete_distribution
, specifically,
the issue proposed to add a c'tor taking a initializer_list<double>
.
Proposed resolution:
In 29.5.9.6.1 [rand.dist.samp.discrete] p. 1, class discrete_distribution
,
just before the member declaration
explicit discrete_distribution(const param_type& parm);
insert
discrete_distribution(initializer_list<double> wl);
Between p.4 and p.5 of the same section insert a new paragraph as part of the new member description:
discrete_distribution(initializer_list<double> wl);Effects: Same as
discrete_distribution(wl.begin(), wl.end())
.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
initializer_list
constructor for piecewise_constant_distribution
Section: 29.5.9.6.2 [rand.dist.samp.pconst] Status: Resolved Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.pconst].
View all issues with Resolved status.
Discussion:
During the Sophia Antipolis meeting it was decided to separate from
794(i) a subrequest that adds initializer list support to
piecewise_constant_distribution
, specifically, the issue proposed
to add a c'tor taking a initializer_list<double>
and a Callable
to evaluate
weight values. For consistency with the remainder of this class and
the remainder of the initializer_list
-aware library the author decided to
change the list argument type to the template parameter RealType
instead. For the reasoning to use Func
instead of Func&&
as c'tor
function argument see issue 793(i).
Proposed resolution:
Non-concept version of the proposed resolution
In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution
,
just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert
template<typename Func> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);
Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:
template<typename Func> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);[p5_1] Complexity: Exactly
nf = max(bl.size(), 1) - 1
invocations offw
.[p5_2] Requires:
fw
shall be callable with one argument of typeRealType
, and shall return values of a type convertible todouble
;- The relation
0 < S = w0+. . .+wn-1
shall hold. For all sampled valuesxk
defined below,fw(xk)
shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- If
nf > 0
letbk = *(bl.begin() + k), k = 0, . . . , bl.size()-1
and the following relations shall hold fork = 0, . . . , nf-1: bk < bk+1
.[p5_3] Effects:
If
nf == 0
,
- lets the sequence
w
have lengthn = 1
and consist of the single valuew0 = 1
, and- lets the sequence
b
have lengthn+1
withb0 = 0
andb1 = 1
.Otherwise,
- sets
n = nf
, and[bl.begin(), bl.end())
shall form the sequenceb
of lengthn+1
, andlets the sequences
w
have lengthn
and for eachk = 0, . . . ,n-1
, calculates:xk = 0.5*(bk+1 + bk) wk = fw(xk)
Constructs a
piecewise_constant_distribution
object with the above computed sequenceb
as the interval boundaries and with the probability densities:ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.
Concept version of the proposed resolution
In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution
,
just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);
Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);[p5_1] Complexity: Exactly
nf = max(bl.size(), 1) - 1
invocations offw
.[p5_2] Requires:
- The relation
0 < S = w0+. . .+wn-1
shall hold. For all sampled valuesxk
defined below,fw(xk)
shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- If
nf > 0
letbk = *(bl.begin() + k), k = 0, . . . , bl.size()-1
and the following relations shall hold fork = 0, . . . , nf-1: bk < bk+1
.[p5_3] Effects:
If
nf == 0
,
- lets the sequence
w
have lengthn = 1
and consist of the single valuew0 = 1
, and- lets the sequence
b
have lengthn+1
withb0 = 0
andb1 = 1
.Otherwise,
- sets
n = nf
, and[bl.begin(), bl.end())
shall form the sequenceb
of lengthn+1
, andlets the sequences
w
have lengthn
and for eachk = 0, . . . ,n-1
, calculates:xk = 0.5*(bk+1 + bk) wk = fw(xk)
Constructs a
piecewise_constant_distribution
object with the above computed sequenceb
as the interval boundaries and with the probability densities:ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
basic_string
access operations should give stronger guaranteesSection: 27.4.3 [basic.string] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [basic.string].
View all other issues in [basic.string].
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Discussion:
During the Sophia Antipolis meeting it was decided to split-off some
parts of the
n2647
("Concurrency modifications for basic_string
")
proposal into a separate issue, because these weren't actually
concurrency-related. The here proposed changes refer to the recent
update document
n2668
and attempt to take advantage of the
stricter structural requirements.
Indeed there exists some leeway for more guarantees that would be very useful for programmers, especially if interaction with transactionary or exception-unaware C API code is important. This would also allow compilers to take advantage of more performance optimizations, because more functions can have throw() specifications. This proposal uses the form of "Throws: Nothing" clauses to reach the same effect, because there already exists a different issue in progress to clean-up the current existing "schizophrenia" of the standard in this regard.
Due to earlier support for copy-on-write, we find the following unnecessary limitations for C++0x:
data()
and c_str()
simply return
a pointer to their guts, which is a non-failure operation. This should
be spelled out. It is also noteworthy to mention that the same
guarantees should also be given by the size query functions,
because the combination of pointer to content and the length is
typically needed during interaction with low-level API.
data()
and c_str()
simply return
a pointer to their guts, which is guaranteed O(1). This should be
spelled out.
operator[]
allows reading access to the terminator
char. For more intuitive usage of strings, reading access to this
position should be extended to the non-const case. In contrast
to C++03 this reading access should now be homogeneously
an lvalue access.
The proposed resolution is split into a main part (A) and a secondary part (B) (earlier called "Adjunct Adjunct Proposal"). (B) extends (A) by also making access to index position size() of the at() overloads a no-throw operation. This was separated, because this part is theoretically observable in specifically designed test programs.
[ San Francisco: ]
We oppose part 1 of the issue but hope to address
size()
in issue 877(i).We do not support part B. 4 of the issue because of the breaking API change.
We support part A. 2 of the issue.
On support part A. 3 of the issue:
Pete's broader comment: now that we know that
basic_string
will be a block of contiguous memory, we should just rewrite its specification with that in mind. The expression of the specification will be simpler and probably more correct as a result.
[ 2009-07 Frankfurt ]
Move proposed resolution A to Ready.
[ Howard: Commented out part B. ]
Proposed resolution:
In 27.4.3.5 [string.capacity], just after p. 1 add a new paragraph:
Throws: Nothing.
In 27.4.3.6 [string.access] replace p. 1 by the following 4 paragraghs:
Requires:
pos ≤ size()
.Returns: If
pos < size()
, returns*(begin() + pos)
. Otherwise, returns a reference to acharT()
that shall not be modified.Throws: Nothing.
Complexity: Constant time.
In 27.4.3.8.1 [string.accessors] replace the now common returns
clause of c_str()
and data()
by the following three paragraphs:
Returns: A pointer
p
such thatp+i == &operator[](i)
for eachi
in[0, size()]
.Throws: Nothing.
Complexity: Constant time.
forward_list
preconditionsSection: 23.3.7 [forward.list] Status: C++11 Submitter: Martin Sebor Opened: 2008-08-23 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list].
View all issues with C++11 status.
Discussion:
forward_list
member functions that take
a forward_list::iterator
(denoted position
in the
function signatures) argument have the following precondition:
Requires:
position
is dereferenceable or equal tobefore_begin()
.
I believe what's actually intended is this:
Requires:
position
is in the range [before_begin()
,end()
).
That is, when it's dereferenceable, position
must point
into *this
, not just any forward_list
object.
[ San Francisco: ]
Robert suggested alternate proposed wording which had large support.
[ Post Summit: ]
Walter: "position is before_begin() or a dereferenceable": add "is" after the "or"
With that minor update, Recommend Tentatively Ready.
Proposed resolution:
Change the Requires clauses [forwardlist], p21, p24, p26, p29, and, [forwardlist.ops], p39, p43, p47 as follows:
Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range[begin(), end())
or equal to. ...before_begin()
Section: 32.5 [atomics] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-08-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics].
View all issues with Resolved status.
Duplicate of: 942
Discussion:
The atomic_exchange
and atomic_exchange_explicit
functions seem to
be inconsistently missing parameters.
[ Post Summit: ]
Lawrence: Need to write up a list for Pete with details.
Detlef: Should not be New, we already talked about in Concurrency group.
Recommend Open.
[ 2009-07 Frankfurt ]
Lawrence will handle all issues relating to atomics in a single paper.
LWG will defer discussion on atomics until that paper appears.
Move to Open.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2992.
Proposed resolution:
Add the appropriate parameters. For example,
bool atomic_exchange(volatile atomic_bool*, bool); bool atomic_exchange_explicit(volatile atomic_bool*, bool, memory_order);
shared_ptr
conversion issueSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++11 Submitter: Peter Dimov Opened: 2008-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
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Discussion:
We've changed shared_ptr<Y>
to not convert to shared_ptr<T>
when Y*
doesn't convert to T*
by resolving issue 687(i). This only fixed the
converting copy constructor though.
N2351
later added move support, and
the converting move constructor is not constrained.
[ San Francisco: ]
We might be able to move this to NAD, Editorial once shared_ptr is conceptualized, but we want to revisit this issue to make sure.
[ 2009-07 Frankfurt ]
Moved to Ready.
This issue now represents the favored format for specifying constrained templates.
Proposed resolution:
We need to change the Requires clause of the move constructor:
shared_ptr(shared_ptr&& r); template<class Y> shared_ptr(shared_ptr<Y>&& r);
RequiresRemarks:For the second constructorThe second constructor shall not participate in overload resolution unlessY*
shall be convertible toT*
.Y*
is convertible toT*
.
in order to actually make the example in 687(i) compile (it now resolves to the move constructor).
duration
non-member arithmetic requirementsSection: 30.5.6 [time.duration.nonmember] Status: CD1 Submitter: Howard Hinnant Opened: 2008-09-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
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Discussion:
N2661
specified the following requirements for the non-member duration
arithmetic:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);Requires: Let
CR
represent thecommon_type
ofRep1
andRep2
. BothRep1
andRep2
shall be implicitly convertible to CR, diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);Requires: Let
CR
represent thecommon_type
ofRep1
andRep2
. BothRep1
andRep2
shall be implicitly convertible toCR
, diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);Requires: Let
CR
represent thecommon_type
ofRep1
andRep2
. BothRep1
andRep2
shall be implicitly convertible toCR
, andRep2
shall not be an instantiation ofduration
, diagnostic required.
During transcription into the working paper, the requirements clauses on these three functions was changed to:
Requires:
CR(Rep1, Rep2)
shall exist. Diagnostic required.
This is a non editorial change with respect to
N2661
as user written representations which are used in duration
need not be
implicitly convertible to or from arithmetic types in order to interoperate with
duration
s based on arithmetic types. An explicit conversion will do
fine for most expressions as long as there exists a common_type
specialization
relating the user written representation and the arithmetic type. For example:
class saturate { public: explicit saturate(long long i); ... }; namespace std { template <> struct common_type<saturate, long long> { typedef saturate type; }; template <> struct common_type<long long, saturate> { typedef saturate type; }; } // std millisecond ms(3); // integral-based duration duration<saturate, milli> my_ms = ms; // ok, even with explicit conversions my_ms = my_ms + ms; // ok, even with explicit conversions
However, when dealing with multiplication of a duration and its representation,
implicit convertibility is required between the rhs and the lhs's representation
for the member *=
operator:
template <class Rep, class Period = ratio<1>> class duration { public: ... duration& operator*=(const rep& rhs); ... }; ... ms *= 2; // ok, 2 is implicitly convertible to long long my_ms *= saturate(2); // ok, rhs is lhs's representation my_ms *= 2; // error, 2 is not implicitly convertible to saturate
The last line does not (and should not) compile. And we want non-member multiplication to have the same behavior as member arithmetic:
my_ms = my_ms * saturate(2); // ok, rhs is lhs's representation my_ms = my_ms * 2; // should be error, 2 is not implicitly convertible to saturate
The requirements clauses of N2661 make the last line an error as expected. However the latest working draft at this time (N2723) allows the last line to compile.
All that being said, there does appear to be an error in these requirements clauses as specified by N2661.
Requires: ... Both
Rep1
andRep2
shall be implicitly convertible to CR, diagnostic required.
It is not necessary for both Rep
s to be implicitly convertible to
the CR
. It is only necessary for the rhs Rep
to be implicitly
convertible to the CR
. The Rep
within the duration
should be allowed to only be explicitly convertible to the CR
. The
explicit-conversion-requirement is covered under 30.5.8 [time.duration.cast].
Proposed resolution:
Change the requirements clauses under 30.5.6 [time.duration.nonmember]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);Requires:
CR(Rep1, Rep2)
shall exist.Rep2
shall be implicitly convertible toCR(Rep1, Rep2)
. Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);Requires
d behavior:CR(Rep1, Rep2)
shall exist.Rep1
shall be implicitly convertible toCR(Rep1, Rep2)
. Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);Requires:
CR(Rep1, Rep2)
shall existRep2
shall be implicitly convertible toCR(Rep1, Rep2)
andRep2
shall not be an instantiation ofduration
. Diagnostic required.
Section: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Note in particular that Table 90 "Container Requirements" gives
semantics of a.swap(b)
as swap(a,b)
, yet for all
containers we define swap(a,b)
to call a.swap(b)
- a
circular definition.
[ San Francisco: ]
Robert to propose a resolution along the lines of "Postcondition: "a = b, b = a" This will be a little tricky for the hash containers, since they don't have
operator==
.
[ Post Summit Anthony Williams provided proposed wording. ]
[ 2009-07 Frankfurt ]
Moved to Ready with minor edits (which have been made).
Proposed resolution:
In table 80 in section 23.2.2 [container.requirements.general],
replace the postcondition of a.swap(b)
with the following:
Table 80 -- Container requirements Expression Return type Operational semantics Assertion/note pre-/post-conidtion Complexity ... ... ... ... ... a.swap(b);
void
Exchange the contents ofswap(a,b)
a
andb
.(Note A)
Remove the reference to swap from the paragraph following the table.
Notes: the algorithms
swap()
,equal()
andlexicographical_compare()
are defined in Clause 25. ...
shared_ptr
swapSection: 20.3.2.2.5 [util.smartptr.shared.mod] Status: Resolved Submitter: Jonathan Wakely Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
#include <memory> #include <cassert> struct A { }; struct B : A { }; int main() { std::shared_ptr<A> pa(new A); std::shared_ptr<B> pb(new B); std::swap<A>(pa, pb); // N.B. no argument deduction assert( pa.get() == pb.get() ); return 0; }
Is this behaviour correct (I believe it is) and if so, is it unavoidable, or not worth worrying about?
This calls the lvalue/rvalue swap overload for shared_ptr
:
template<class T> void swap( shared_ptr<T> & a, shared_ptr<T> && b );
silently converting the second argument from shared_ptr<B>
to
shared_ptr<A>
and binding the rvalue ref to the produced temporary.
This is not, in my opinion, a shared_ptr
problem; it is a general issue
with the rvalue swap overloads. Do we want to prevent this code from
compiling? If so, how?
Perhaps we should limit rvalue args to swap to those types that would
benefit from the "swap trick". Or, since we now have shrink_to_fit()
, just
eliminate the rvalue swap overloads altogether. The original motivation
was:
vector<A> v = ...; ... swap(v, vector<A>(v));
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to
NAD EditorialResolved.
Proposed resolution:
Recommend NAD EditorialResolved, fixed by
N2844.
pair
assignmentSection: 22.3 [pairs] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.2.3 pairs Missing assignemnt operator: template<class U , class V> requires CopyAssignable<T1, U> && CopyAssignable<T2, V> pair& operator=(pair<U , V> const & p );
Well, that's interesting. This assignment operator isn't in the
current working paper, either. Perhaps we deemed it acceptable to
build a temporary of type pair
from pair<U, V>
, then move-assign
from that temporary?
It sounds more like an issue waiting to be opened, unless you want to plug it now. As written we risk moving from lvalues.
[ San Francisco: ]
Would be NAD if better ctors fixed it.
[ post San Francisco: ]
Possibly NAD Editorial, solved by N2770.
[ 2009-05-25 Alisdair adds: ]
Issue 885(i) was something I reported while reviewing the library concepts documents ahead of San Francisco. The missing operator was added as part of the paper adopted at that meeting (N2770) and I can confirm this operator is present in the current working paper. I recommend NAD.
[ 2009-07 Frankfurt ]
We agree with the intent, but we need to wait for the dust to settle on concepts.
[ 2010-03-11 Stefanus provided wording. ]
[ 2010 Pittsburgh: Moved to Ready for Pittsburgh. ]
Proposed resolution:
Add the following declaration 22.3.2 [pairs.pair], before the
declaration of pair& operator=(pair&& p);
:
template<class U, class V> pair& operator=(const pair<U, V>& p);
Add the following description to 22.3.2 [pairs.pair] after paragraph 11 (before
the description of pair& operator=(pair&& p);)
:
template<class U, class V> pair& operator=(const pair<U, V>& p);Requires:
T1
shall satisfy the requirements ofCopyAssignable
fromU
.T2
shall satisfy the requirements ofCopyAssignable
fromV
.Effects: Assigns
p.first
tofirst
andp.second
tosecond
.Returns:
*this
.
tuple
constructionSection: 22.4.4.2 [tuple.cnstr] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
22.4.4.2 [tuple.cnstr]:
Effects: Default initializes each element.
Could be clarified to state each "non-trivial" element. Otherwise we have a conflict with Core deinfition of default initialization - trivial types do not get initialized (rather than initialization having no effect)
I'm going to punt on this one, because it's not an issue that's related to concepts. I suggest bringing it to Howard's attention on the reflector.
[ San Francisco: ]
Text in draft doesn't mean anything, changing to "non-trivial" makes it meaningful.
We prefer "value initializes". Present implementations use value-initialization. Users who don't want value initialization have alternatives.
Request resolution text from Alisdair.
This issue relates to Issue 868(i) default construction and value-initialization.
[ 2009-05-04 Alisdair provided wording and adds: ]
Note: This IS a change of semantic from TR1, although one the room agreed with during the discussion. To preserve TR1 semantics, this would have been worded:
requires DefaultConstructible<Types>... tuple();-2- Effects: Default-initializes each non-trivial element.
[ 2009-07 Frankfurt ]
Move to Ready.
Proposed resolution:
Change p2 in Construction 22.4.4.2 [tuple.cnstr]:
requires DefaultConstructible<Types>... tuple();-2- Effects:
DefaultValue-initializes each element.
this_thread::yield
too strongSection: 32.4.5 [thread.thread.this] Status: C++11 Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I never thought I'd say this, but this_thread::yield
seems to be too
strong in specification. The issue is that some systems distinguish
between yielding to another thread in the same process and yielding
to another process. Given that the C++ standard only talks about
a single program, one can infer that the specification allows yielding
only to another thread within the same program. Posix has no
facility for that behavior. Can you please file an issue to weaken
the wording. Perhaps "Offers the operating system the opportunity
to reschedule."
[ Post Summit: ]
Recommend move to Tentatively Ready.
Proposed resolution:
Change 32.4.5 [thread.thread.this]/3:
void this_thread::yield();Effects: Offers the
operating systemimplementation the opportunity to reschedule.another thread.
thread::id
comparisonsSection: 32.4.3.2 [thread.thread.id] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 324
The thread::id
type supports the full set of comparison operators. This
is substantially more than is required for the associative containers that
justified them. Please place an issue against the threads library.
[ San Francisco: ]
Would depend on proposed extension to POSIX, or non-standard extension. What about hash? POSIX discussing op. POSIX not known to be considering support needed for hash, op.
Group expresses support for putting ids in both unordered and ordered containers.
[ post San Francisco: ]
Howard: It turns out the current working paper N2723 already has
hash<thread::id>
(22.10 [function.objects], 22.10.19 [unord.hash]). We simply overlooked it in the meeting. It is a good thing we voted in favor of it (again). :-)Recommend NAD.
[ Post Summit: ]
Recommend to close as NAD. For POSIX, see if we need to add a function to convert
pthread_t
to integer.
[ Post Summit, Alisdair adds: ]
The recommendation for LWG-889/UK-324 is NAD, already specified.
It is not clear to me that the specification is complete.
In particular, the synopsis of
<functional>
in 22.10 [function.objects] does not mentionhash< thread::id>
norhash< error_code >
, although their existence is implied by 22.10.19 [unord.hash], p1.I am fairly uncomfortable putting the declaration for the
thread_id
specialization into<functional>
asid
is a nested class insidestd::thread
, so it implies that<functional>
would require the definition of thethread
class template in order to forward declaredthread::id
and form this specialization.It seems better to me that the dependency goes the other way around (
<thread>
will more typically make use of<functional>
than vice-versa) and thehash<thread::id>
specialization be declared in the<thread>
header.I think
hash<error_code>
could go into either<system_error>
or<functional>
and have no immediate preference either way. However, it should clearly appear in the synopsis of one of these two.Recommend moving 889 back to open, and tying in a reference to UK-324.
[ Batavia (2009-05): ]
Howard observes that
thread::id
need not be a nested class; it could be atypedef
for a more visible type.
[ 2009-05-24 Alisdair adds: ]
I do not believe this is correct.
thread::id
is explicitly documents as a nested class, rather than as an unspecified typedef analogous to an iterator. If the intent is that this is not implemented as a nested class (under the as-if freedoms) then this is a novel form of standardese.
[ 2009-07 Frankfurt ]
Decided we want to move hash specialization for thread_id to the thread header. Alisdair to provide wording.
[ 2009-07-28 Alisdair provided wording, moved to Review. ]
[ 2009-10 Santa Cruz: ]
Add a strike for
hash<thread::id>
. Move to Ready
[ 2009-11-13 The proposed wording of 1182(i) is a superset of the wording in this issue. ]
[ 2010-02-09 Moved from Ready to Open: ]
Issue 1182(i) is not quite a superset of this issue and it is controversial whether or not the note:
hash template specialization allows
thread::id
objects to be used as keys in unordered containers.should be added to the WP.
[
2010-02-09 Objections to moving this to NAD EditorialResolved, addressed by 1182(i) have been removed. Set to Tentatively NAD EditorialResolved.
]
Rationale:
Proposed resolution:
Remove the following prototype from the synopsis in 22.10 [function.objects]:
template <> struct hash<std::thread::id>;
Add to 32.4 [thread.threads], p1 Header <thread>
synopsis:
template <class T> struct hash; template <> struct hash<thread::id>;
Add template specialization below class definition in 32.4.3.2 [thread.thread.id]
template <> struct hash<thread::id> : public unary_function<thread::id, size_t> { size_t operator()(thread::id val) const; };
Extend note in p2 32.4.3.2 [thread.thread.id] with second sentence:
[Note: Relational operators allow
thread::id
objects to be used as keys in associative containers.hash
template specialization allowsthread::id
objects to be used as keys in unordered containers. — end note]
Add new paragraph to end of 32.4.3.2 [thread.thread.id]
template <> struct hash<thread::id>;An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type
thread::id
.
<system_error>
initializationSection: 19.5.3 [syserr.errcat] Status: C++11 Submitter: Beman Dawes Opened: 2008-09-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The static const error_category
objects generic_category
and
system_category
in header <system_error>
are currently declared:
const error_category& get_generic_category(); const error_category& get_system_category(); static const error_category& generic_category = get_generic_category(); static const error_category& system_category = get_system_category();
This formulation has several problems:
IO streams uses a somewhat different formulation for iostream_category, but still suffer much the same problems.
The original plan was to eliminate these problems by applying the C++0x
constexpr
feature. See LWG issue 832(i). However, that approach turned out
to be unimplementable, since it would require a constexpr
object of a
class with virtual functions, and that is not allowed by the core
language.
The proposed resolution was developed as an alternative. It mitigates the above problems by removing initialization from the visible interface, allowing implementations flexibility.
Implementation experience:
Prototype implementations of the current WP interface and proposed
resolution interface were tested with recent Codegear, GCC, Intel, and Microsoft
compilers on Windows. The code generated by the Microsoft compiler was studied
at length; the WP and proposal versions generated very similar code. For both versions
the compiler did make use of static
initialization; apparently the compiler applied an implicit constexpr
where useful, even in cases where constexpr
would not be permitted by
the language!
Acknowledgements:
Martin Sebor, Chris Kohlhoff, and John Lakos provided useful ideas and comments on initialization issues.
[ San Francisco: ]
Martin: prefers not to create more file-scope static objects, and would like to see
get_*
functions instead.
[Pre-Summit:]
Beman: The proposed resolution has been reworked to remove the file-scope static objects, per Martin's suggestions. The
get_
prefix has been eliminated from the function names as no longer necessary and to conform with standard library naming practice.
[ Post Summit: ]
Agreement that this is wise and essential, text provided works and has been implemented. Seems to be widespread consensus. Move to Tentative Ready.
Proposed resolution:
Change 16.4.6.16 [value.error.codes] Value of error codes as indicated:
Certain functions in the C++ standard library report errors via a
std::error_code
(19.4.2.2) object. That object'scategory()
member shall returna reference tostd::system_category
for errors originating from the operating system, or a reference to an implementation-defined error_category object for errors originating elsewhere. The implementation shall define the possible values of value() for each of these error categories. [Example: For operating systems that are based on POSIX, implementations are encouraged to define the
()
std::system_category
values as identical to the POSIX
()
errno
values, with additional values as defined by the operating system's documentation. Implementations for operating systems that are not based on POSIX are encouraged to define values identical to the operating system's values. For errors that do not originate from the operating system, the implementation may provide enums for the associated values --end example]
Change 19.5.3.1 [syserr.errcat.overview] Class error_category
overview
error_category
synopsis as indicated:
const error_category&get_generic_category(); const error_category&get_system_category();static storage-class-specifier const error_category& generic_category = get_generic_category(); static storage-class-specifier const error_category& system_category = get_system_category();
Change 19.5.3.5 [syserr.errcat.objects] Error category objects as indicated:
const error_category&get_generic_category();Returns: A reference to an object of a type derived from class
error_category
.Remarks: The object's
default_error_condition
andequivalent
virtual functions shall behave as specified for the classerror_category
. The object'sname
virtual function shall return a pointer to the string"GENERIC"
.const error_category&get_system_category();Returns: A reference to an object of a type derived from class
error_category
.Remarks: The object's
equivalent
virtual functions shall behave as specified for classerror_category
. The object'sname
virtual function shall return a pointer to the string"system"
. The object'sdefault_error_condition
virtual function shall behave as follows:If the argument
ev
corresponds to a POSIXerrno
valueposv
, the function shall returnerror_condition(posv, generic_category())
. Otherwise, the function shall returnerror_condition(ev, system_category())
. What constitutes correspondence for any given operating system is unspecified. [Note: The number of potential system error codes is large and unbounded, and some may not correspond to any POSIXerrno
value. Thus implementations are given latitude in determining correspondence. — end note]
Change 19.5.4.2 [syserr.errcode.constructors] Class error_code
constructors
as indicated:
error_code();Effects: Constructs an object of type error_code.
Postconditions:
val_ == 0
andcat_ == &system_category
()
.
Change 19.5.4.3 [syserr.errcode.modifiers] Class error_code
modifiers as
indicated:
void clear();Postconditions:
value() == 0
andcategory() == system_category
()
.
Change 19.5.4.5 [syserr.errcode.nonmembers] Class error_code
non-member
functions as indicated:
error_code make_error_code(errc e);Returns:
error_code(static_cast<int>(e), generic_category
()
)
.
Change 19.5.5.2 [syserr.errcondition.constructors] Class error_condition
constructors as indicated:
error_condition();Effects: Constructs an object of type
error_condition
.Postconditions:
val_ == 0
andcat_ == &generic_category
()
.
Change 19.5.5.3 [syserr.errcondition.modifiers] Class error_condition
modifiers as indicated:
void clear();Postconditions:
value() == 0
andcategory() == generic_category
()
.
Change 19.5.5.5 [syserr.errcondition.nonmembers] Class error_condition
non-member functions as indicated:
error_condition make_error_condition(errc e);Returns:
error_condition(static_cast<int>(e), generic_category())
.
Change 31.5 [iostreams.base] Iostreams base classes, Header <ios>
synopsis as indicated:
concept_map ErrorCodeEnum<io_errc> { }; error_code make_error_code(io_errc e); error_condition make_error_condition(io_errc e);storage-class-specifierconst error_category& iostream_category();
Change [ios::failure] Class ios_base::failure, paragraph 2 as indicated:
When throwing
ios_base::failure
exceptions, implementations should provide values ofec
that identify the specific reason for the failure. [ Note: Errors arising from the operating system would typically be reported assystem_category
()
errors with an error value of the error number reported by the operating system. Errors arising from within the stream library would typically be reported aserror_code(io_errc::stream, iostream_category())
. — end note ]
Change 31.5.6 [error.reporting] Error reporting as indicated:
error_code make_error_code(io_errc e);Returns:
error_code(static_cast<int>(e), iostream_category
())
.error_condition make_error_condition(io_errc e);Returns:
error_condition(static_cast<int>(e), iostream_category
())
.storage-class-specifierconst error_category& iostream_category();The implementation shall initialize iostream_category. Its storage-class-specifier may be static or extern. It is unspecified whether initialization is static or dynamic (3.6.2). If initialization is dynamic, it shall occur before completion of the dynamic initialization of the first translation unit dynamically initialized that includes header <system_error>.
Returns: A reference to an object of a type derived from class
error_category
.Remarks: The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "iostream".
std::thread
, std::call_once
issueSection: 32.4.3.3 [thread.thread.constr], 32.6.7.2 [thread.once.callonce] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I notice that the vararg overloads of std::thread
and std::call_once
(N2723 32.4.3.3 [thread.thread.constr] and 32.6.7.2 [thread.once.callonce]) are no longer specified in terms of
std::bind
; instead, some of the std::bind
wording has been inlined into
the specification.
There are two problems with this.
First, the specification (and implementation) in terms of std::bind
allows, for example:
std::thread th( f, 1, std::bind( g ) );
which executes f( 1, g() )
in a thread. This can be useful. The
"inlined" formulation changes it to execute f( 1, bind(g) )
in a thread.
Second, assuming that we don't want the above, the specification has copied the wording
INVOKE(func, w1, w2, ..., wN)
(20.6.2) shall be a valid expression for some valuesw1, w2, ..., wN
but this is not needed since we know that our argument list is args; it should simply be
INVOKE(func, args...)
(20.6.2) shall be a valid expression
[ Summit: ]
Move to open.
[ Post Summit Anthony provided proposed wording. ]
[ 2009-07 Frankfurt ]
Leave Open. Await decision for thread variadic constructor.
[ 2009-10 Santa Cruz: ]
See proposed wording for 929(i) for this, for the formulation on how to solve this. 929(i) modifies the thread constructor to have "pass by value" behavior with pass by reference efficiency through the use of the
decay
trait. This same formula would be useful forcall_once
.
[ 2010-02-11 Anthony updates wording. ]
[ 2010-02-12 Moved to Tentatively Ready after 5 postive votes on c++std-lib. ]
Proposed resolution:
Modify 32.6.7.2 [thread.once.callonce] p1-p2 with the following:
template<class Callable, class ...Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);Given a function as follows:
template<typename T> typename decay<T>::type decay_copy(T&& v) { return std::forward<T>(v); }1 Requires:
The template parametersCallable
and eachTi
inArgs
shallbesatisfy theCopyConstructible
if an lvalue and otherwiseMoveConstructible
requirements.INVOKE(decay_copy(std::forward<Callable>(func),
(22.10.4 [func.require]) shall be a valid expressionw1, w2, ..., wNdecay_copy(std::forward<Args>(args))...)for some values.w1, w2, ..., wN
, whereN == sizeof...(Args)
2 Effects: Calls to
call_once
on the sameonce_flag
object are serialized. If there has been a prior effective call tocall_once
on the sameonce_flag
object, the call tocall_once
returns without invokingfunc
. If there has been no prior effective call tocall_once
on the sameonce_flag
object,the argumentfunc
(or a copy thereof) is called as if by invokingfunc(args)
INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
is executed. The call tocall_once
is effective if and only iffunc(args)
INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
returns without throwing an exception. If an exception is thrown it is propagated to the caller.
std::mutex
issueSection: 32.6.4.2.2 [thread.mutex.class] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 905
Discussion:
32.6.4.2.2 [thread.mutex.class]/27 (in N2723) says that the behavior is undefined if:
mutex
object calls lock()
or
try_lock()
on that object
I don't believe that this is right. Calling lock()
or try_lock()
on a
locked mutex
is well defined in the general case. try_lock()
is required
to fail and return false
. lock()
is required to either throw an
exception (and is allowed to do so if it detects deadlock) or to block
until the mutex
is free. These general requirements apply regardless of
the current owner of the mutex
; they should apply even if it's owned by
the current thread.
Making double lock()
undefined behavior probably can be justified (even
though I'd still disagree with the justification), but try_lock()
on a
locked mutex
must fail.
[ Summit: ]
Move to open. Proposed resolution:
- In 32.6.4 [thread.mutex.requirements] paragraph 12, change the error condition for
resource_deadlock_would_occur
to: "if the implementation detects that a deadlock would occur"- Strike 32.6.4.2.2 [thread.mutex.class] paragraph 3 bullet 2 "a thread that owns a mutex object calls
lock()
ortry_lock()
on that object, or"
[ 2009-07 Frankfurt ]
Move to Review. Alisdair to provide note.
[ 2009-07-31 Alisdair provided note. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2009-11-18 Peter Opens: ]
I don't believe that the proposed note:
[Note: a program may deadlock if the thread that owns a
mutex
object callslock()
ortry_lock()
on that object. If the program can detect the deadlock, aresource_deadlock_would_occur
error condition may be observed. — end note]is entirely correct. "or
try_lock()
" should be removed, becausetry_lock
is non-blocking and doesn't deadlock; it just returnsfalse
when it fails to lock the mutex.[ Howard: I've set to Open and updated the wording per Peter's suggestion. ]
[ 2009-11-18 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In 32.6.4 [thread.mutex.requirements] paragraph 12 change:
- ...
resource_deadlock_would_occur
-- if thecurrent thread already owns the mutex and is able to detect itimplementation detects that a deadlock would occur.- ...
Strike 32.6.4.2.2 [thread.mutex.class] paragraph 3 bullet 2:
-3- The behavior of a program is undefined if:
- ...
a thread that owns amutex
object callslock()
ortry_lock()
on that object, or- ...
Add the following note after p3 32.6.4.2.2 [thread.mutex.class]
[Note: a program may deadlock if the thread that owns a
mutex
object callslock()
on that object. If the implementation can detect the deadlock, aresource_deadlock_would_occur
error condition may be observed. — end note]
Section: 17.14 [support.runtime] Status: C++11 Submitter: Lawrence Crowl, Alisdair Meredith Opened: 2008-09-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The interaction between longjmp
and exceptions seems unnecessarily
restrictive and not in keeping with existing practice.
Proposed resolution:
Edit paragraph 4 of 17.14 [support.runtime] as follows:
The function signature
longjmp(jmp_buf jbuf, int val)
has more restricted behavior in this International Standard. Asetjmp/longjmp
call pair has undefined behavior if replacing thesetjmp
andlongjmp
bycatch
andthrow
woulddestroyinvoke any non-trivial destructors for any automatic objects.
Section: 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Hans Boehm Opened: 2008-09-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
View all issues with C++11 status.
Discussion:
It is unclear whether shared_ptr
is thread-safe in the sense that
multiple threads may simultaneously copy a shared_ptr
. However this
is a critical piece of information for the client, and it has significant
impact on usability for many applications. (Detlef Vollman thinks it
is currently clear that it is not thread-safe. Hans Boehm thinks
it currently requires thread safety, since the use_count
is not an
explicit field, and constructors and assignment take a const reference
to an existing shared_ptr
.)
Pro thread-safety:
Many multi-threaded usages are impossible. A thread-safe version can be used to destroy an object when the last thread drops it, something that is often required, and for which we have no other easy mechanism.
Against thread-safety:
The thread-safe version is well-known to be far more expensive, even if used by a single thread. Many applications, including all single-threaded ones, do not care.
[ San Francisco: ]
Beman: this is a complicated issue, and would like to move this to Open and await comment from Peter Dimov; we need very careful and complete rationale for any decision we make; let's go slow
Detlef: I think that
shared_ptr
should not be thread-safe.Hans: When you create a thread with a lambda, it in some cases makes it very difficult for the lambda to reference anything in the heap. It's currently ambiguous as to whether you can use a
shared_ptr
to get at an object.Leave in Open. Detlef will submit an alternative proposed resolution that makes
shared_ptr
explicitly unsafe.A third option is to support both threadsafe and non-safe share_ptrs, and to let the programmer decide which behavior they want.
Beman: Peter, do you support the PR?
Peter:
Yes, I support the proposed resolution, and I certainly oppose any attempts to
make shared_ptr
thread-unsafe.I'd mildly prefer if
[Note: This is true in spite of that fact that such functions often modify
use_count()
--end note]is changed to
[Note: This is true in spite of that fact that such functions often cause a change in
use_count()
--end note](or something along these lines) to emphasise that
use_count()
is not, conceptually, a variable, but a return value.
[ 2009-07 Frankfurt ]
Vote: Do we want one thread-safe shared pointer or two? If two, one would allow concurrent construction and destruction of shared pointers, and one would not be thread-safe. If one, then it would be thread-safe.
No concensus on that vote.
Hans to improve wording in consultation with Pete. Leave Open.
[ 2009-10 Santa Cruz: ]
Move to Ready. Ask Editor to clear up wording a little when integrating to make it clear that the portion after the first comma only applies for the presence of data races.
[ 2009-10-24 Hans adds: ]
I think we need to pull 896 back from ready, unfortunately. My wording doesn't say the right thing.
I suspect we really want to say something along the lines of:
For purposes of determining the presence of a data race, member functions access and modify only the
shared_ptr
andweak_ptr
objects themselves and not objects they refer to. Changes inuse_count()
do not reflect modifications that can introduce data races.But I think this needs further discussion by experts to make sure this is right.
Detlef and I agree continue to disagree on the resolution, but I think we agree that it would be good to try to expedite this so that it can be in CD2, since it's likely to generate NB comments no matter what we do. And lack of clarity of intent is probably the worst option. I think it would be good to look at this between meetings.
[ 2010-01-20 Howard: ]
I've moved Hans' suggested wording above into the proposed resolution section and preserved the previous wording here:
Make it explicitly thread-safe, in this weak sense, as I believe was intended:
Insert in 20.3.2.2 [util.smartptr.shared], before p5:
For purposes of determining the presence of a data race, member functions do not modify
const shared_ptr
and constweak_ptr
arguments, nor any objects they refer to. [Note: This is true in spite of that fact that such functions often cause a change inuse_count()
--end note]On looking at the text, I'm not sure we need a similar disclaimer anywhere else, since nothing else has the problem with the modified
use_count()
. I think Howard arrived at a similar conclusion.
[ 2010 Pittsburgh: Moved to Ready for Pittsburgh ]
Proposed resolution:
Insert a new paragraph at the end of 20.3.2.2 [util.smartptr.shared]:
For purposes of determining the presence of a data race, member functions access and modify only the
shared_ptr
andweak_ptr
objects themselves and not objects they refer to. Changes inuse_count()
do not reflect modifications that can introduce data races.
Section: 23.3.7.5 [forward.list.modifiers] Status: Resolved Submitter: Howard Hinnant Opened: 2008-09-22 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.modifiers].
View all issues with Resolved status.
Discussion:
This issue was split off from 892(i) at the request of the LWG.
[ San Francisco: ]
This issue is more complicated than it looks.
paragraph 47: replace each
(first, last) with (first, last]
add a statement after paragraph 48 that complexity is O(1)
remove the complexity statement from the first overload of splice_after
We may have the same problems with other modifiers, like erase_after. Should it require that all iterators in the range (position, last] be dereferenceable?
There are actually 3 issues here:
What value should erase_after
return? With list
, code often
looks like:
for (auto i = l.begin(); i != l.end();) { // inspect *i and decide if you want to erase it // ... if (I want to erase *i) i = l.erase(i); else ++i; }
I.e. the iterator returned from erase
is useful for setting up the
logic for operating on the next element. For forward_list
this might
look something like:
auto i = fl.before_begin(); auto ip1 = i; for (++ip1; ip1 != fl.end(); ++ip1) { // inspect *(i+1) and decide if you want to erase it // ... if (I want to erase *(i+1)) i = fl.erase_after(i); else ++i; ip1 = i; }
In the above example code, it is convenient if erase_after
returns
the element prior to the erased element (range) instead of the element
after the erase element (range).
Existing practice:
There is not a strong technical argument for either solution over the other.
With all other containers, operations always work on the range
[first, last)
and/or prior to the given position
.
With forward_list
, operations sometimes work on the range
(first, last]
and/or after the given position
.
This is simply due to the fact that in order to operate on
*first
(with forward_list
) one needs access to
*(first-1)
. And that's not practical with
forward_list
. So the operating range needs to start with (first
,
not [first
(as the current working paper says).
Additionally, if one is interested in splicing the range (first, last)
,
then (with forward_list
), one needs practical (constant time) access to
*(last-1)
so that one can set the next field in this node to
the proper value. As this is not possible with forward_list
, one must
specify the last element of interest instead of one past the last element of
interest. The syntax for doing this is to pass (first, last]
instead
of (first, last)
.
With erase_after
we have a choice of either erasing the range
(first, last]
or (first, last)
. Choosing the latter
enables:
x.erase_after(pos, x.end());
With the former, the above statement is inconvenient or expensive due to the lack
of constant time access to x.end()-1
. However we could introduce:
iterator erase_to_end(const_iterator position);
to compensate.
The advantage of the former ((first, last]
) for erase_after
is a consistency with splice_after
which uses (first, last]
as the specified range. But this either requires the addition of erase_to_end
or giving up such functionality.
splice_after
should work on the source range (first, last]
if the operation is to be Ο(1). When splicing an entire list x
the
algorithm needs (x.before_begin(), x.end()-1]
. Unfortunately x.end()-1
is not available in constant time unless we specify that it must be. In order to
make x.end()-1
available in constant time, the implementation would have
to dedicate a pointer to it. I believe the design of
N2543
intended a nominal overhead of foward_list
of 1 pointer. Thus splicing
one entire forward_list
into another can not be Ο(1).
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Review.
[ 2009-07 Frankfurt ]
We may need a new issue to correct splice_after, because it may no longer be correct to accept an rvalues as an argument. Merge may be affected, too. This might be issue 1133(i). (Howard: confirmed)
Move this to Ready, but the Requires clause of the second form of splice_after should say "(first, last)," not "(first, last]" (there are three occurrences). There was considerable discussion on this. (Howard: fixed)
Alan suggested removing the "foward_last<T. Alloc>&& x" parameter from the second form of splice_after, because it is redundant. PJP wanted to keep it, because it allows him to check for bad ranges (i.e. "Granny knots").
We prefer to keep
x
.Beman. Whenever we deviate from the customary half-open range in the specification, we should add a non-normative comment to the standard explaining the deviation. This clarifies the intention and spares the committee much confusion in the future.
Alan to write a non-normative comment to explain the use of fully-closed ranges.
Move to Ready, with the changes described above. (Howard: awaiting note from Alan)
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved, addressed by N2988.
Proposed resolution:
Wording below assumes issue 878(i) is accepted, but this issue is independent of that issue.
Change [forwardlist.modifiers]:
iterator erase_after(const_iterator position);Requires: The iterator following
position
is dereferenceable.Effects: Erases the element pointed to by the iterator following
position
.Returns:
An iterator pointing to the element following the one that was erased, orAn iterator equal toend()
if no such element existsposition
.iterator erase_after(const_iterator position, const_iterator last);Requires: All iterators in the range
are dereferenceable.
[(position,last)Effects: Erases the elements in the range
.
[(position,last)Returns: An iterator equal to
position
last
Change [forwardlist.ops]:
void splice_after(const_iterator position, forward_list<T,Allocator>&& x);Requires:
position
isbefore_begin()
or a dereferenceable iterator in the range[begin(), end))
.&x != this
.Effects: Inserts the contents of
x
afterposition
, andx
becomes empty. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.Throws: Nothing.
Complexity:
Ο(1)Ο(distance(x.begin(), x.end())
)...
void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator first, const_iterator last);Requires:
position
isbefore_begin()
or a dereferenceable iterator in the range[begin(), end))
.(first,last)
is a valid range inx
, and all iterators in the range(first,last)
are dereferenceable.position
is not an iterator in the range(first,last)
.Effects: Inserts elements in the range
(first,last)
afterposition
and removes the elements fromx
. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.Complexity: Ο(1).
Section: 23.3.7.6 [forward.list.ops] Status: C++11 Submitter: Arch Robison Opened: 2008-09-08 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
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Discussion:
I ran across a small contradiction in working draft n2723.
[forwardlist]p2: A
forward_list
satisfies all of the requirements of a container (table 90), except that thesize()
member function is not provided.[forwardlist.ops]p57: Complexity: At most
size() + x.size() - 1
comparisons.
Presumably [forwardlist.ops]p57 needs to be rephrased to not use
size()
, or note that it is used there only for sake of notational convenience.
[ 2009-03-29 Beman provided proposed wording. ]
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
Change [forwardlist.ops], forward_list operations, paragraph 19, merge complexity as indicated:
Complexity: At most
comparisons.
size() + x.size()distance(begin(), end()) + distance(x.begin(), x.end()) - 1
shared_ptr
for nullptr_t
Section: 20.3.2.2.3 [util.smartptr.shared.dest] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.dest].
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Discussion:
James Dennett, message c++std-lib-22442:
The wording below addresses one case of this, but opening an issue to address the need to sanity check uses of the term "pointer" in 20.3.2.2 [util.smartptr.shared] would be a good thing.
There's one more reference, in ~shared_ptr;
we can apply your suggested change to it, too. That is:
Change 20.3.2.2.3 [util.smartptr.shared.dest]/1 second bullet from:
Otherwise, if *this owns a pointer p and a deleter d, d(p) is called.
to:
Otherwise, if *this owns an object p and a deleter d, d(p) is called.
[ Post Summit: ]
Recommend Review.
[ Batavia (2009-05): ]
Peter Dimov notes the analogous change has already been made to "the new
nullptr_t
taking constructors in 20.3.2.2.2 [util.smartptr.shared.const] p9-13."We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 20.3.2.2.3 [util.smartptr.shared.dest] p1 second bullet:
- ...
- Otherwise, if
*this
ownsa pointeran objectp
and a deleterd
,d(p)
is called.
Section: 31.10.4 [ifstream] Status: C++11 Submitter: Niels Dekker Opened: 2008-09-20 Last modified: 2023-02-07
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
It appears that we have an issue similar to issue 675(i) regarding the move-assignment of
stream types. For example, when assigning to an std::ifstream
, ifstream1
, it
seems preferable to close the file originally held by ifstream1
:
ifstream1 = std::move(ifstream2);
The current Draft
(N2723)
specifies that the move-assignment of stream types like ifstream
has the
same effect as a swap:
Assign and swap [ifstream.assign]
basic_ifstream& operator=(basic_ifstream&& rhs);Effects:
swap(rhs)
.
[ Batavia (2009-05): ]
Howard agrees with the analysis and the direction proposed.
Move to Open pending specific wording to be supplied by Howard.
[ 2009-07 Frankfurt: ]
Howard is going to write wording.
[ 2009-07-26 Howard provided wording. ]
[ 2009-09-13 Niels adds: ]
Note: The proposed change of 31.10.3.3 [filebuf.assign] p1 depends on the resolution of LWG 1204(i), which allows implementations to assume that
*this
andrhs
refer to different objects.
[ 2009 Santa Cruz: ]
Leave as Open. Too closely related to 911(i) to move on at this time.
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Change 31.8.2.3 [stringbuf.assign]/1:
basic_stringbuf& operator=(basic_stringbuf&& rhs);-1- Effects:
After the move assignmentswap(rhs)
.*this
reflects the same observable state it would have if it had been move constructed fromrhs
(31.8.2.2 [stringbuf.cons]).
Change [istringstream.assign]/1:
basic_istringstream& operator=(basic_istringstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change [ostringstream.assign]/1:
basic_ostringstream& operator=(basic_ostringstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change [stringstream.assign]/1:
basic_stringstream& operator=(basic_stringstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change 31.10.3.3 [filebuf.assign]/1:
basic_filebuf& operator=(basic_filebuf&& rhs);-1- Effects:
Begins by callingswap(rhs)
.this->close()
. After the move assignment*this
reflects the same observable state it would have if it had been move constructed fromrhs
(31.10.3.2 [filebuf.cons]).
Change [ifstream.assign]/1:
basic_ifstream& operator=(basic_ifstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change [ofstream.assign]/1:
basic_ofstream& operator=(basic_ofstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change [fstream.assign]/1:
basic_fstream& operator=(basic_fstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
result_of
argument typesSection: 99 [func.ret] Status: C++11 Submitter: Jonathan Wakely Opened: 2008-09-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.ret].
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Discussion:
The WP and TR1 have the same text regarding the argument types of a
result_of
expression:
The values
ti
are lvalues when the corresponding typeTi
is a reference type, and rvalues otherwise.
I read this to mean that this compiles:
typedef int (*func)(int&); result_of<func(int&&)>::type i = 0;
even though this doesn't:
int f(int&); f( std::move(0) );
Should the text be updated to say "when Ti
is an lvalue-reference
type" or am I missing something?
I later came up with this self-contained example which won't compile, but I think it should:
struct X { void operator()(int&); int operator()(int&&); } x; std::result_of< X(int&&) >::type i = x(std::move(0));
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
Change 99 [func.ret], p1:
... The values
ti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.
Section: 22.9.2.3 [bitset.members] Status: C++11 Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.members].
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Discussion:
The current standard 14882::2003(E) as well as the current draft
N2723
have in common a contradiction of the operational semantics of member function
test
22.9.2.3 [bitset.members] p.56-58 and the immutable
member operator[]
overload 22.9.2.3 [bitset.members] p.64-66 (all references
are defined in terms of
N2723):
bool test(size_t pos) const;
Requires:
pos
is validThrows:
out_of_range
ifpos
does not correspond to a valid bit position.Returns:
true
if the bit at positionpos
in*this
has the value one.
constexpr bool operator[](size_t pos) const;
Requires:
pos
shall be valid.Throws: nothing.
Returns:
test(pos)
.
Three interpretations:
operator[]
overload is indeed allowed to throw an exception
(via test()
, if pos
corresponds to an invalid bit position) which does
not leave the call frame. In this case this function cannot be a
constexpr
function, because test()
is not, due to
7.7 [expr.const]/2, last bullet.
test
in case of an
invalid bit position. There is only little evidence for this interpretation.
operator[]
should not throw any exception,
but that test
has the contract to do so, if the provided bit position
is invalid.
The problem became worse, because issue 720(i)
recently voted into WP argued that member test
logically must be
a constexpr
function, because it was used to define the semantics
of another constexpr
function (the operator[]
overload).
Three alternatives are proposed, corresponding to the three bullets (A), (B), and (C), the author suggests to follow proposal (C).
Proposed alternatives:
Remove the constexpr
specifier in front of operator[]
overload and
undo that of member test
(assuming 720(i) is accepted) in both the
class declaration 22.9.2 [template.bitset]/1 and in the member description
before 22.9.2.3 [bitset.members]/56 and before /64 to read:
constexprbool test(size_t pos) const; ..constexprbool operator[](size_t pos) const;
Change the throws clause of p. 65 to read:
Throws:
nothingout_of_range
ifpos
does not correspond to a valid bit position.
Replace the throws clause p. 57 to read:
Throws:
nothing.out_of_range
ifpos
does not correspond to a valid bit position
Undo the addition of the constexpr
specifier to the test
member
function in both class declaration 22.9.2 [template.bitset]/1 and in the
member description before 22.9.2.3 [bitset.members]/56, assuming that 720(i)
was applied.
constexprbool test(size_t pos) const;
Change the returns clause p. 66 to read:
Returns:
test(pos)
true
if the bit at positionpos
in*this
has the value one, otherwisefalse
.
[ Post Summit: ]
Lawrence: proposed resolutions A, B, C are mutually exclusive.
Recommend Review with option C.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Undo the addition of the constexpr
specifier to the test
member
function in both class declaration 22.9.2 [template.bitset] p.1 and in the
member description before 22.9.2.3 [bitset.members] p.56, assuming that 720(i)
was applied.
constexprbool test(size_t pos) const;
Change the returns clause p. 66 to read:
Returns:
test(pos)
true
if the bit at positionpos
in*this
has the value one, otherwisefalse
.
Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Anthony Williams Opened: 2008-09-26 Last modified: 2017-11-29
Priority: Not Prioritized
View all other issues in [atomics.types.generic].
View all issues with Resolved status.
Discussion:
Addresses US 90
The deleted copy-assignment operators for the atomic types are not marked as volatile in N2723, whereas the assignment operators from the associated non-atomic types are. e.g.
atomic_bool& operator=(atomic_bool const&) = delete; atomic_bool& operator=(bool) volatile;
This leads to ambiguity when assigning a non-atomic value to a non-volatile instance of an atomic type:
atomic_bool b; b=false;
Both assignment operators require a standard conversions: the
copy-assignment operator can use the implicit atomic_bool(bool)
conversion constructor to convert false
to an instance of
atomic_bool
, or b
can undergo a qualification conversion in order to
use the assignment from a plain bool
.
This is only a problem once issue 845(i) is applied.
[ Summit: ]
Move to open. Assign to Lawrence. Related to US 90 comment.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Add volatile qualification to the deleted copy-assignment operator of all the atomic types:
atomic_bool& operator=(atomic_bool const&) volatile = delete; atomic_itype& operator=(atomic_itype const&) volatile = delete;
etc.
This will mean that the deleted copy-assignment operator will require
two conversions in the above example, and thus be a worse match than
the assignment from plain bool
.
regex_token_iterator
should use initializer_list
Section: 28.6.11.2 [re.tokiter] Status: C++11 Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.tokiter].
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Discussion:
Addresses UK 319
Construction of a regex_token_iterator
(28.6.11.2 [re.tokiter]/6+) usually
requires the provision of a sequence of integer values, which
can currently be done via a std::vector<int>
or
a C array of int
. Since the introduction of initializer_list
in the
standard it seems much more reasonable to provide a
corresponding constructor that accepts an initializer_list<int>
instead. This could be done as a pure addition or one could
even consider replacement. The author suggests the
replacement strategy (A), but provides an alternative additive
proposal (B) as a fall-back, because of the handiness of this
range type:
[ Batavia (2009-05): ]
We strongly recommend alternative B of the proposed resolution in order that existing code not be broken. With that understanding, move to Tentatively Ready.
Original proposed wording:
In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 change the constructor declaration:
template <std::size_t N>regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re,const int (&submatches)[N]initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default);
In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence
The third constructor initializes the member
subs
to hold a copy of the sequence of integer values pointed to by the iterator range[
.&submatches.begin(),&submatches.end()+ N)
In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 insert the
following constructor declaration between the already existing ones
accepting a std::vector
and a C array of int
, resp.:
regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default);
In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence
The third and fourth constructor initialize
sthe membersubs
to hold a copy of the sequence of integer values pointed to by the iterator range[&submatches,&submatches + N)
and[submatches.begin(),submatches.end())
, respectively.
Proposed resolution:
In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 insert the
following constructor declaration between the already existing ones
accepting a std::vector
and a C array of int
, resp.:
regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default);
In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence
The third and fourth constructor initialize
sthe membersubs
to hold a copy of the sequence of integer values pointed to by the iterator range[&submatches,&submatches + N)
and[submatches.begin(),submatches.end())
, respectively.
move/swap
semanticSection: 31.7.5 [input.streams], 31.7.6 [output.streams] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Class template basic_istream
, basic_ostream
and basic_iostream
implements public move constructors, move assignment operators and swap
method and free functions. This might induce both the user and the
compiler to think that those types are MoveConstructible
, MoveAssignable
and Swappable
. However, those class templates fail to fulfill the user
expectations. For example:
std::ostream os(std::ofstream("file.txt")); assert(os.rdbuf() == 0); // buffer object is not moved to os, file.txt has been closed std::vector<std::ostream> v; v.push_back(std::ofstream("file.txt")); v.reserve(100); // causes reallocation assert(v[0].rdbuf() == 0); // file.txt has been closed! std::ostream&& os1 = std::ofstream("file1.txt"); os1 = std::ofstream("file2.txt"); os1 << "hello, world"; // still writes to file1.txt, not to file2.txt! std::ostream&& os1 = std::ofstream("file1.txt"); std::ostream&& os2 = std::ofstream("file2.txt"); std::swap(os1, os2); os1 << "hello, world"; // writes to file1.txt, not to file2.txt!
This is because the move constructor, the move assignment operator and
swap
are all implemented through calls to std::basic_ios
member
functions move()
and swap()
that do not move nor swap the controlled
stream buffers. That can't happen because the stream buffers may have
different types.
Notice that for basic_streambuf
, the member function swap()
is
protected. I believe that is correct and all of basic_istream
,
basic_ostream
, basic_iostream
should do the same as the move ctor, move
assignment operator and swap member function are needed by the derived
fstream
s and stringstream
s template. The free swap functions for
basic_(i|o|io)stream
templates should be removed for the same reason.
[ Batavia (2009-05): ]
We note that the rvalue swap functions have already been removed.
Bill is unsure about making the affected functions protected; he believes they may need to be public.
We are also unsure about removing the lvalue swap functions as proposed.
Move to Open.
[ 2009-07 Frankfurt: ]
It's not clear that the use case is compelling.
Howard: This needs to be implemented and tested.
[ 2009-07-26 Howard adds: ]
I started out thinking I would recommend NAD for this one. I've turned around to agree with the proposed resolution (which I've updated to the current draft). I did not fully understand Ganesh's rationale, and attempt to describe my improved understanding below.
The move constructor, move assignment operator, and swap function are different for
basic_istream
,basic_ostream
andbasic_iostream
than other classes. A timely conversation with Daniel reminded me of this long forgotten fact. These members are sufficiently different that they would be extremely confusing to use in general, but they are very much needed for derived clients.
- The move constructor moves everything but the
rdbuf
pointer.- The move assignment operator moves everything but the
rdbuf
pointer.- The swap function swaps everything but the
rdbuf
pointer.The reason for this behavior is that for the std-derived classes (stringstreams, filestreams), the
rdbuf
pointer points back into the class itself (self referencing). It can't be swapped or moved. But this fact isn't born out at thestream
level. Rather it is born out at thefstream
/sstream
level. And the lower levels just need to deal with that fact by not messing around with therdbuf
pointer which is stored down at the lower levels.In a nutshell, it is very confusing for all of those who are not so intimately related with streams that they've implemented them. And it is even fairly confusing for some of those who have (including myself). I do not think it is safe to swap or move
istreams
orostreams
because this will (by necessary design) separate stream state from streambuffer state. Derived classes (such asfstream
andstringstream
must be used to keep the stream state and stream buffer consistently packaged as one unit during a move or swap.I've implemented this proposal and am living with it day to day.
[ 2009 Santa Cruz: ]
Leave Open. Pablo expected to propose alternative wording which would rename move construction, move assignment and swap, and may or may not make them protected. This will impact issue 900(i).
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
31.7.5.2 [istream]: make the following member functions protected:
basic_istream(basic_istream&& rhs); basic_istream& operator=(basic_istream&& rhs); void swap(basic_istream& rhs);
Ditto: remove the swap free function signature
// swap: template <class charT, class traits> void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y);
31.7.5.2.3 [istream.assign]: remove paragraph 4
template <class charT, class traits> void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y);
Effects:x.swap(y)
.
31.7.5.7 [iostreamclass]: make the following member function protected:
basic_iostream(basic_iostream&& rhs); basic_iostream& operator=(basic_iostream&& rhs); void swap(basic_iostream& rhs);
Ditto: remove the swap free function signature
template <class charT, class traits> void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y);
31.7.5.7.4 [iostream.assign]: remove paragraph 3
template <class charT, class traits> void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y);
Effects:x.swap(y)
.
31.7.6.2 [ostream]: make the following member function protected:
basic_ostream(basic_ostream&& rhs); basic_ostream& operator=(basic_ostream&& rhs); void swap(basic_ostream& rhs);
Ditto: remove the swap free function signature
// swap: template <class charT, class traits> void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);
31.7.6.2.3 [ostream.assign]: remove paragraph 4
template <class charT, class traits> void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);
Effects:x.swap(y)
.
Section: 22.10.16 [func.memfn] Status: C++11 Submitter: Bronek Kozicki Opened: 2008-10-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.memfn].
View all issues with C++11 status.
Duplicate of: 1230
Discussion:
Daniel Krügler wrote:
Shouldn't above list be completed for &- and &&-qualified member functions This would cause to add:
template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);
yes, absolutely. Thanks for spotting this. Without this change mem_fn
cannot be initialized from pointer to ref-qualified member function. I
believe semantics of such function pointer is well defined.
[ Post Summit Daniel provided wording. ]
[ Batavia (2009-05): ]
We need to think about whether we really want to go down the proposed path of combinatorial explosion. Perhaps a Note would suffice.
We would really like to have an implementation before proceeding.
Move to Open, and recommend this be deferred until after the next Committee Draft has been issued.
[ 2009-10-10 Daniel updated wording to post-concepts. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change 22.10 [function.objects]/2, header
<functional>
synopsis as follows:
// 20.7.14, member function adaptors: template<class R, class T> unspecified mem_fn(R T::*); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...)); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);
Change the prototype list of 22.10.16 [func.memfn] as follows [NB: The following text, most notably p.2 and p.3 which discuss influence of the cv-qualification on the definition of the base class's first template parameter remains unchanged. ]:
template<class R, class T> unspecified mem_fn(R T::* pm); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...)); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);
Remove 22.10.16 [func.memfn]/5:
Remarks: Implementations may implementmem_fn
as a set of overloaded function templates.
Section: 21.4.3 [ratio.ratio] Status: C++11 Submitter: Pablo Halpern Opened: 2008-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ratio.ratio].
View all issues with C++11 status.
Discussion:
The compile-time functions that operate on ratio<N,D>
require the
cumbersome and error-prone "evaluation" of a type
member using a
meta-programming style that predates the invention of template aliases.
Thus, multiplying three ratios a
, b
, and c
requires the expression:
ratio_multiply<a, ratio_multiply<b, c>::type>::type
The simpler expression:
ratio_multiply<a, ratio_multiply<b, c>>
Could be used by if template aliases were employed in the definitions.
[ Post Summit: ]
Jens: not a complete proposed resolution: "would need to make similar change"
Consensus: We agree with the direction of the issue.
Recommend Open.
[ 2009-05-11 Daniel adds: ]
Personally I'm not in favor for the addition of:
typedef ratio type;For a reader of the standard it's usage or purpose is unclear. I haven't seen similar examples of attempts to satisfy non-feature complete compilers.
[ 2009-05-11 Pablo adds: ]
The addition of type to the
ratio
template allows the previous style (i.e., in the prototype implementations) to remain valid and permits the use of transitional library implementations for C++03 compilers. I do not feel strongly about its inclusion, however, and leave it up to the reviewers to decide.
[ Batavia (2009-05): ]
Bill asks for additional discussion in the issue that spells out more details of the implementation. Howard points us to issue 948(i) which has at least most of the requested details. Tom is strongly in favor of overflow-checking at compile time. Pete points out that there is no change of functionality implied. We agree with the proposed resolution, but recommend moving the issue to Review to allow time to improve the discussion if needed.
[ 2009-07-21 Alisdair adds: ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
In 21.4 [ratio] p.3 change as indicated:
// ratio arithmetic template <class R1, class R2>structusing ratio_add = see below; template <class R1, class R2>structusing ratio_subtract = see below; template <class R1, class R2>structusing ratio_multiply = see below; template <class R1, class R2>structusing ratio_divide = see below;
In 21.4.3 [ratio.ratio], change as indicated:
namespace std { template <intmax_t N, intmax_t D = 1> class ratio { public: typedef ratio type; static const intmax_t num; static const intmax_t den; }; }
In 21.4.4 [ratio.arithmetic] change as indicated:
template <class R1, class R2>structusing ratio_add = see below{ typedef see below type; };1 The
nested typedeftyperatio_add<R1, R2>
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::den + R2::num * R1::den
andT2
has the valueR1::den * R2::den
.
template <class R1, class R2>structusing ratio_subtract = see below{ typedef see below type; };2 The
nested typedeftyperatio_subtract<R1, R2>
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::den - R2::num * R1::den
andT2
has the valueR1::den * R2::den
.
template <class R1, class R2>structusing ratio_multiply = see below{ typedef see below type; };3 The
nested typedeftyperatio_multiply<R1, R2>
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::num
andT2
has the valueR1::den * R2::den
.
template <class R1, class R2>structusing ratio_divide = see below{ typedef see below type; };4 The
nested typedeftyperatio_divide<R1, R2>
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::den
andT2
has the valueR1::den * R2::num
.
In 30.5.2 [time.duration.cons] p.4 change as indicated:
Requires:
treat_as_floating_point<rep>::value
shall be true orratio_divide<Period2, period>::
shall be 1.[..]type::den
In 30.5.8 [time.duration.cast] p.2 change as indicated:
Returns: Let CF be
ratio_divide<Period, typename ToDuration::period>
, and [..]::type
Section: B [implimits] Status: C++11 Submitter: Sohail Somani Opened: 2008-10-11 Last modified: 2016-02-01
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses DE 24
With respect to the section 22.10.15.5 [func.bind.place]:
TR1 dropped some suggested implementation quantities for the number of placeholders. The purpose of this defect is to put these back for C++0x.
[ Post Summit: ]
see DE 24
Recommend applying the proposed resolution from DE 24, with that Tentatively Ready.
Original proposed resolution:
Add 22.10.15.5 [func.bind.place] p.2:
While the exact number of placeholders (
_M
) is implementation defined, this number shall be at least 10.
Proposed resolution:
Add to B [implimits]:
Section: 32.5 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics].
View all issues with Resolved status.
Discussion:
Right now, C++0x doesn't have atomic<float>
. We're thinking of adding
the words to support it for TR2 (note: that would be slightly post-C++0x). If we
need it, we could probably add the words.
Proposed resolutions: Using atomic<FP>::compare_exchange
(weak or
strong) should be either:
I propose Option 1 for C++0x for expediency. If someone wants to argue
for Option 2, they need to say what exactly they want compare_exchange
to mean in this case (IIRC, C++0x doesn't even assume IEEE 754).
[ Summit: ]
Move to open. Blocked until concepts for atomics are addressed.
[ Post Summit Anthony adds: ]
Recommend NAD. C++0x does have
std::atomic<float>
, and bothcompare_exchange_weak
andcompare_exchange_strong
are well-defined in this case. Maybe change the note in 32.5.8.2 [atomics.types.operations] paragraph 20 to:[Note: The effect of the compare-and-exchange operations is
if (!memcmp(object,expected,sizeof(*object))) *object = desired; else *expected = *object;This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Change the note in 32.5.8.2 [atomics.types.operations] paragraph 20 to:
[Note: The effect of the compare-and-exchange operations is
if (*object == *expected!memcmp(object,expected,sizeof(*object))) *object = desired; else *expected = *object;This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]
Section: 32.5 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics].
View all issues with Resolved status.
Discussion:
Right now, the compare_exchange_weak
loop should rapidly converge on the
padding contents. But compare_exchange_strong
will require a bit more
compiler work to ignore padding for comparison purposes.
Note that this isn't a problem for structs with no padding, and we do
already have one portable way to ensure that there is no padding that
covers the key use cases: Have elements be the same type. I suspect that
the greatest need is for a structure of two pointers, which has no
padding problem. I suspect the second need is a structure of a pointer
and some form of an integer. If that integer is intptr_t
, there will be
no padding.
Related but separable issue: For unused bitfields, or other unused
fields for that matter, we should probably say it's the programmer's
responsibility to set them to zero or otherwise ensure they'll be
ignored by memcmp
.
Proposed resolution: Using
atomic<struct-with-padding>::compare_exchange_strong
should be either:
I propose Option 1 for C++0x for expediency, though I'm not sure how to
say it. I would be happy with Option 2, which I believe would mean that
compare_exchange_strong
would be implemented to avoid comparing padding
bytes, or something equivalent such as always zeroing out padding when
loading/storing/comparing. (Either implementation might require compiler
support.)
[ Summit: ]
Move to open. Blocked until concepts for atomics are addressed.
[ Post Summit Anthony adds: ]
The resolution of LWG 923(i) should resolve this issue as well.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
shared_ptr
's explicit conversion from unique_ptr
Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++11 Submitter: Rodolfo Lima Opened: 2008-10-12 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
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Discussion:
The current working draft
(N2798),
section 20.3.2.2.2 [util.smartptr.shared.const] declares
shared_ptr
's constructor that takes a rvalue reference to unique_ptr
and
auto_ptr
as being explicit, affecting several valid smart pointer use
cases that would take advantage of this conversion being implicit, for
example:
class A; std::unique_ptr<A> create(); void process(std::shared_ptr<A> obj); int main() { process(create()); // use case #1 std::unique_ptr<A> uobj = create(); process(std::move(uobj)); // use case #2 return 0; }
If unique_ptr
to shared_ptr
conversions are explicit, the above lines
should be written:
process(std::shared_ptr<A>(create())); // use case #1 process(std::shared_ptr<A>(std::move(uobj))); // use case #2
The extra cast required doesn't seems to give any benefits to the user, nor protects him of any unintended conversions, this being the raison d'etre of explicit constructors.
It seems that this constructor was made explicit to mimic the conversion
from auto_ptr
in pre-rvalue reference days, which accepts both lvalue and
rvalue references. Although this decision was valid back then, C++0x
allows the user to express in a clear and non verbose manner when he wants
move semantics to be employed, be it implicitly (use case 1) or explicitly
(use case 2).
[ Batavia (2009-05): ]
Howard and Alisdair like the motivating use cases and the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
In both 20.3.2.2 [util.smartptr.shared] paragraph 1 and 20.3.2.2.2 [util.smartptr.shared.const] change:
template <class Y>explicitshared_ptr(auto_ptr<Y> &&r); template <class Y, class D>explicitshared_ptr(unique_ptr<Y, D> &&r);
Section: 32.4.3.3 [thread.thread.constr] Status: C++11 Submitter: Anthony Williams Opened: 2008-10-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.constr].
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Discussion:
Addresses UK 323
The thread
constructor for starting a new thread with a function and
arguments is overly constrained by the signature requiring rvalue
references for func
and args
and the CopyConstructible
requirements
for the elements of args
. The use of an rvalue reference for the
function restricts the potential use of a plain function name, since
the type of the bound parameter will be deduced to be a function
reference and decay to pointer-to-function will not happen. This
therefore complicates the implementation in order to handle a simple
case. Furthermore, the use of rvalue references for args prevents the
array to pointer decay. Since arrays are not CopyConstructible
or even
MoveConstructible
, this essentially prevents the passing of arrays as
parameters. In particular it prevents the passing of string literals.
Consequently a simple case such as
void f(const char*); std::thread t(f,"hello");
is ill-formed since the type of the string literal is const char[6]
.
By changing the signature to take all parameters by value we can
eliminate the CopyConstructible
requirement and permit the use of
arrays, as the parameter passing semantics will cause the necessary
array-to-pointer decay. They will also cause the function name to
decay to a pointer to function and allow the implementation to handle
functions and function objects identically.
The new signature of the thread
constructor for a function and
arguments is thus:
template<typename F,typename... Args> thread(F,Args... args);
Since the parameter pack Args
can be empty, the single-parameter
constructor that takes just a function by value is now redundant.
[ Howard adds: ]
I agree with everything Anthony says in this issue. However I believe we can optimize in such a way as to get the pass-by-value behavior with the pass-by-rvalue-ref performance. The performance difference is that the latter removes a
move
when passing in an lvalue.This circumstance is very analogous to
make_pair
(22.3 [pairs]) where we started with passing by const reference, changed to pass by value to get pointer decay, and then changed to pass by rvalue reference, but modified withdecay<T>
to retain the pass-by-value behavior. If we were to apply the same solution here it would look like:template <class F> explicit thread(F f);template <class F, class ...Args> thread(F&& f, Args&&... args);-4- Requires:
F
and eachTi
inArgs
shall beCopyConstructible
if an lvalue and otherwiseMoveConstructible
.INVOKE(f, w1, w2, ..., wN)
(22.10.4 [func.require]) shall be a valid expression for some valuesw1, w2, ... , wN,
whereN == sizeof...(Args)
.-5- Effects: Constructs an object of type
thread
and executes. Constructs the following objects in memory which is accessible to a new thread of execution as if:INVOKE(f, t1, t2, ..., tN)
in a new thread of execution, wheret1, t2, ..., tN
are the values inargs...
typename decay<F>::type g(std::forward<F>(f)); tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);The new thread of execution executes
INVOKE(g, wi...)
where thewi...
refers to the elements stored in thetuple w
. Any return value fromg
is ignored.IfIf the evaluation off
terminates with an uncaught exception,std::terminate()
shall be called.INVOKE(g, wi...)
terminates with an uncaught exception,std::terminate()
shall be called [Note:std::terminate()
could be called before enteringg
. -- end note]. Any exception thrown before the evaluation ofINVOKE
has started shall be catchable in the calling thread.Text referring to when
terminate()
is called was contributed by Ganesh.
[ Batavia (2009-05): ]
We agree with the proposed resolution, but would like the final sentence to be reworded since "catchable" is not a term of art (and is used nowhere else).
[ 2009-07 Frankfurt: ]
This is linked to N2901.
Howard to open a separate issue to remove (1176(i)).
In Frankfurt there is no consensus for removing the variadic constructor.
[ 2009-10 Santa Cruz: ]
We want to move forward with this issue. If we later take it out via 1176(i) then that's ok too. Needs small group to improve wording.
[ 2009-10 Santa Cruz: ]
Stefanus provided revised wording. Moved to Review Here is the original wording:
Modify the class definition of
std::thread
in 32.4.3 [thread.thread.class] to remove the following signature:template<class F> explicit thread(F f);template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);Modify 32.4.3.3 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:
-4- Requires:
F
and eachTi
inArgs
shall beCopyConstructible
if an lvalue and otherwiseMoveConstructible
.INVOKE(f, w1, w2, ..., wN)
(22.10.4 [func.require]) shall be a valid expression for some valuesw1, w2, ... , wN,
whereN == sizeof...(Args)
.-5- Effects: Constructs an object of type
thread
and executes. Constructs the following objects:INVOKE(f, t1, t2, ..., tN)
in a new thread of execution, wheret1, t2, ..., tN
are the values inargs...
typename decay<F>::type g(std::forward<F>(f)); tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);and executes
INVOKE(g, wi...)
in a new thread of execution. These objects shall be destroyed when the new thread of execution completes. Any return value fromg
is ignored.IfIf the evaluation off
terminates with an uncaught exception,std::terminate()
shall be called.INVOKE(g, wi...)
terminates with an uncaught exception,std::terminate()
shall be called [Note:std::terminate()
could be called before enteringg
. -- end note]. Any exception thrown before the evaluation ofINVOKE
has started shall be catchable in the calling thread.-6- Synchronization: The invocation of the constructor happens before the invocation of
f
g
.
[ 2010-01-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Modify the class definition of std::thread
in 32.4.3 [thread.thread.class] to remove the
following signature:
template<class F> explicit thread(F f);template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);
Modify 32.4.3.3 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:
Given a function as follows:
template<typename T> typename decay<T>::type decay_copy(T&& v) { return std::forward<T>(v); }-4- Requires:
F
and eachTi
inArgs
shallbesatisfy theCopyConstructible
if an lvalue and otherwiseMoveConstructible
requirements.INVOKE(f, w1, w2, ..., wN)
(22.10.4 [func.require]) shall be a valid expression for some valuesw1, w2, ... , wN,
whereN == sizeof...(Args)
.INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)
(22.10.4 [func.require]) shall be a valid expression.-5- Effects: Constructs an object of type
thread
and executesThe new thread of execution executesINVOKE(f, t1, t2, ..., tN)
in a new thread of execution, wheret1, t2, ..., tN
are the values inargs...
. Any return value fromf
is ignored. Iff
terminates with an uncaught exception,std::terminate()
shall be called.INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)
with the calls todecay_copy()
being evaluated in the constructing thread. Any return value from this invocation is ignored. [Note: this implies any exceptions not thrown from the invocation of the copy off
will be thrown in the constructing thread, not the new thread. — end note]. If the invocation ofINVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)
terminates with an uncaught exception,std::terminate
shall be called.-6- Synchronization: The invocation of the constructor happens before the invocation of the copy of
f
.
extent<T, I>
Section: 21.3.5.4 [meta.unary.prop] Status: C++11 Submitter: Yechezkel Mett Opened: 2008-11-04 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
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Discussion:
The draft (N2798) says in 21.3.5.4 [meta.unary.prop] Table 44:
Table 44 -- Type property queries Template Value template <class T, unsigned I = 0> struct extent;
If T
is not an array type (8.3.4), or if it has rank less thanI
, or ifI
is 0 andT
has type "array of unknown bound ofU
", then 0; otherwise, the size of theI
'th dimension ofT
Firstly it isn't clear from the wording if I
is 0-based or 1-based
("the I
'th dimension" sort of implies 1-based). From the following
example it is clear that the intent is 0-based, in which case it
should say "or if it has rank less than or equal to I
".
Sanity check:
The example says assert((extent<int[2], 1>::value) == 0);
Here the rank is 1 and I
is 1, but the desired result is 0.
[ Post Summit: ]
Do not use "size" or "value", use "bound". Also, move the cross-reference to 8.3.4 to just after "bound".
Recommend Tentatively Ready.
Proposed resolution:
In Table 44 of 21.3.5.4 [meta.unary.prop], third row, column "Value", change the cell content:
Table 44 -- Type property queries Template Value template <class T, unsigned I = 0> struct extent;
If T
is not an array type(8.3.4), or if it has rank less than or equal toI
, or ifI
is 0 andT
has type "array of unknown bound ofU
", then 0; otherwise, thesizebound (8.3.4) of theI
'th dimension ofT
, where indexing ofI
is zero-based.
[ Wording supplied by Daniel. ]
unique_ptr(pointer p)
for pointer deleter typesSection: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2008-11-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.ctor].
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Discussion:
Addresses US 79
20.3.1.3.2 [unique.ptr.single.ctor]/5 no longer requires for D
not to be a pointer type. I believe this restriction was accidently removed
when we relaxed the completeness reuqirements on T
. The restriction
needs to be put back in. Otherwise we have a run time failure that could
have been caught at compile time:
{ unique_ptr<int, void(*)(void*)> p1(malloc(sizeof(int))); // should not compile } // p1.~unique_ptr() dereferences a null function pointer unique_ptr<int, void(*)(void*)> p2(malloc(sizeof(int)), free); // ok
[ Post Summit: ]
Recommend Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for
enable_if
type constraining, possibly following Robert's formula.
[ 2009-07 Frankfurt: ]
We need to consider whether some requirements in the Requires paragraphs of [unique.ptr] should instead be Remarks.
Leave Open. Howard to provide wording, and possibly demonstrate how this can be implemented using enable_if.
[ 2009-07-27 Howard adds: ]
The two constructors to which this issue applies are not easily constrained with
enable_if
as they are not templated:unique_ptr(); explicit unique_ptr(pointer p);To "SFINAE" these constructors away would take heroic effort such as specializing the entire
unique_ptr
class template on pointer deleter types. There is insufficient motivation for such heroics. Here is the expected and reasonable implementation for these constructors:unique_ptr() : ptr_(pointer()) { static_assert(!is_pointer<deleter_type>::value, "unique_ptr constructed with null function pointer deleter"); } explicit unique_ptr(pointer p) : ptr_(p) { static_assert(!is_pointer<deleter_type>::value, "unique_ptr constructed with null function pointer deleter"); }I.e. just use
static_assert
to verify that the constructor is not instantiated with a function pointer for a deleter. The compiler will automatically take care of issuing a diagnostic if the deleter is a reference type (uninitialized reference error).In keeping with our discussions in Frankfurt, I'm moving this requirement on the implementation from the Requires paragraph to a Remarks paragraph.
[ 2009-08-17 Daniel adds: ]
It is insufficient to require a diagnostic. This doesn't imply an ill-formed program as of 3.18 [defns.diagnostic] (a typical alternative would be a compiler warning), but exactly that seems to be the intend. I suggest to use the following remark instead:
Remarks: The program shall be ill-formed if this constructor is instantiated when
D
is a pointer type or reference type.Via the general standard rules of 4.1 [intro.compliance] the "diagnostic required" is implied.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Change the description of the default constructor in 20.3.1.3.2 [unique.ptr.single.ctor]:
unique_ptr();-1- Requires:
D
shall be default constructible, and that construction shall not throw an exception.D
shall not be a reference type or pointer type (diagnostic required)....
Remarks: The program shall be ill-formed if this constructor is instantiated when
D
is a pointer type or reference type.
Add after 20.3.1.3.2 [unique.ptr.single.ctor]/8:
unique_ptr(pointer p);...
Remarks: The program shall be ill-formed if this constructor is instantiated when
D
is a pointer type or reference type.
duration
is missing operator%
Section: 30.5 [time.duration] Status: C++11 Submitter: Terry Golubiewski Opened: 2008-11-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration].
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Discussion:
Addresses US 81
duration
is missing operator%
. This operator is convenient
for computing where in a time frame a given duration
lies. A
motivating example is converting a duration
into a "broken-down"
time duration such as hours::minutes::seconds:
class ClockTime { typedef std::chrono::hours hours; typedef std::chrono::minutes minutes; typedef std::chrono::seconds seconds; public: hours hours_; minutes minutes_; seconds seconds_; template <class Rep, class Period> explicit ClockTime(const std::chrono::duration<Rep, Period>& d) : hours_ (std::chrono::duration_cast<hours> (d)), minutes_(std::chrono::duration_cast<minutes>(d % hours(1))), seconds_(std::chrono::duration_cast<seconds>(d % minutes(1))) {} };
[ Summit: ]
Agree except that there is a typo in the proposed resolution. The member operators should be
operator%=
.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.
[ 2009-07 Frankfurt: ]
Howard to open a separate issue (1177(i)) to handle the removal of member functions from overload sets, provide wording, and possibly demonstrate how this can be implemented using enable_if (see 947(i)).
Move to Ready.
Proposed resolution:
Add to the synopsis in 30 [time]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s); template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
Add to the synopsis of duration
in 30.5 [time.duration]:
template <class Rep, class Period = ratio<1>> class duration { public: ... duration& operator%=(const rep& rhs); duration& operator%=(const duration& d); ... };
Add to 30.5.4 [time.duration.arithmetic]:
duration& operator%=(const rep& rhs);Effects:
rep_ %= rhs
.Returns:
*this
.duration& operator%=(const duration& d);Effects:
rep_ %= d.count()
.Returns:
*this
.
Add to 30.5.6 [time.duration.nonmember]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);Requires:
Rep2
shall be implicitly convertible toCR(Rep1, Rep2)
andRep2
shall not be an instantiation ofduration
. Diagnostic required.Returns:
duration<CR, Period>(d) %= s
.template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);Returns:
common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type(lhs) %= rhs
.
default_delete<T[]>::operator()
should only accept T*
Section: 20.3.1.2.3 [unique.ptr.dltr.dflt1] Status: C++11 Submitter: Howard Hinnant Opened: 2008-12-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Consider:
derived* p = new derived[3];
std::default_delete<base[]> d;
d(p); // should fail
Currently the marked line is a run time failure. We can make it a compile
time failure by "poisoning" op(U*)
.
[ Post Summit: ]
Recommend Review.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Add to 20.3.1.2.3 [unique.ptr.dltr.dflt1]:
namespace std { template <class T> struct default_delete<T[]> { void operator()(T*) const; template <class U> void operator()(U*) const = delete; }; }
std::identity
and reference-to-temporariesSection: 22.2.4 [forward] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-12-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
std::identity
takes an argument of type T const &
and returns a result of T const &
.
Unfortunately, this signature will accept a value of type other than T
that
is convertible-to-T
, and then return a reference to the dead temporary. The
constraint in the concepts version simply protects against returning
reference-to-void
.
Solutions:
i/ Return-by-value, potentially slicing bases and rejecting non-copyable types
ii/ Provide an additional overload:
template< typename T > template operator( U & ) = delete;This seems closer on intent, but moves beyond the original motivation for the operator, which is compatibility with existing (non-standard) implementations.
iii/ Remove the
operator()
overload. This restores the original definition of theidentity
, although now effectively a type_trait rather than part of the perfect forwarding protocol.iv/ Remove
std::identity
completely; its original reason to exist is replaced with theIdentityOf
concept.
My own preference is somewhere between (ii) and (iii) - although I stumbled over the issue with a specific application hoping for resolution (i)!
[ Batavia (2009-05): ]
We dislike options i and iii, and option ii seems like overkill. If we remove it (option iv), implementers can still provide it under a different name.
Move to Open pending wording (from Alisdair) for option iv.
[ 2009-05-23 Alisdair provided wording for option iv. ]
[ 2009-07-20 Alisdair adds: ]
I'm not sure why this issue was not discussed at Frankfurt (or I missed the discussion) but the rationale is now fundamentally flawed. With the removal of concepts,
std::identity
again becomes an important library type so we cannot simply remove it.At that point, we need to pick one of the other suggested resolutions, but have no guidance at the moment.
[ 2009-07-20 Howard adds: ]
I believe the rationale for not addressing this issue in Frankfurt was that it did not address a national body comment.
I also believe that removal of
identity
is still a practical option as my latest reformulation offorward
, which is due to comments suggested at Summit, no longer usesidentity
. :-)template <class T, class U, class = typename enable_if < !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value >::type, class = typename enable_if < is_same<typename remove_all<T>::type, typename remove_all<U>::type>::value >::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }[ The above code assumes acceptance of 1120(i) for the definition of
remove_all
. This is just to make the syntax a little more palatable. Without this trait the above is still very implementable. ]Paper with rationale is on the way ... really, I promise this time! ;-)
[ 2009-07-30 Daniel adds: See 823(i) for an alternative resolution. ]
[ 2009-10 Santa Cruz: ]
Move to Ready. Howard will update proposed wording to reflect current draft.
Proposed resolution:
Strike from 22.2 [utility]:
template <class T> struct identity;
Remove from 22.2.4 [forward]:
template <class T> struct identity { typedef T type; const T& operator()(const T& x) const; };const T& operator()(const T& x) const;
-2- Returns:x
std::distance
Section: 24.4.3 [iterator.operations] Status: Resolved Submitter: Thomas Opened: 2008-12-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 270
Regarding the std::distance
- function, 24.4.3 [iterator.operations]
p.4 says:
Returns the number of increments or decrements needed to get from first to last.
This sentence is completely silent about the sign of the return value. 24.4.3 [iterator.operations] p.1 gives more information about the underlying operations, but again no inferences about the sign can be made. Strictly speaking, that is taking that sentence literally, I think this sentence even implies a positive return value in all cases, as the number of increments or decrements is clearly a ratio scale variable, with a natural zero bound.
Practically speaking, my implementations did what common sense and
knowledge based on pointer arithmetic forecasts, namely a positive sign
for increments (that is, going from first
to last
by operator++
), and a
negative sign for decrements (going from first
to last
by operator--
).
Here are my two questions:
First, is that paragraph supposed to be interpreted in the way what I called 'common sense', that is negative sign for decrements ? I am fairly sure that's the supposed behavior, but a double-check here in this group can't hurt.
Second, is the present wording (2003 standard version - no idea about the draft for the upcoming standard) worth an edit to make it a bit more sensible, to mention the sign of the return value explicitly ?
[ Daniel adds: ]
My first thought was that resolution 204(i) would already cover the issue report, but it seems that current normative wording is in contradiction to that resolution:
Referring to N2798, 24.4.3 [iterator.operations]/ p.4 says:
Effects: Returns the number of increments or decrements needed to get from
first
tolast
.IMO the part " or decrements" is in contradiction to p. 5 which says
Requires:
last
shall be reachable fromfirst
.because "reachable" is defined in 24.3.4 [iterator.concepts]/7 as
An iterator
j
is called reachable from an iteratori
if and only if there is a finite sequence of applications of the expression++i
that makesi == j
.[..]Here is wording that would be consistent with this definition of "reachable":
Change 24.4.3 [iterator.operations] p4 as follows:
Effects: Returns the number of increments
or decrementsneeded to get fromfirst
tolast
.
Thomas adds more discussion and an alternative view point here.
[ Summit: ]
The proposed wording below was verbally agreed to. Howard provided.
[ Batavia (2009-05): ]
Pete reports that a recent similar change has been made for the
advance()
function.We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-07 Frankfurt: ]
Leave Open pending arrival of a post-Concepts WD.
[ 2009-10-14 Daniel provided de-conceptified wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready, replacing the Effects clause in the proposed wording with "If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise returns the number of increments needed to get from first to list.".
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3066.
Proposed resolution:
Change 24.3.5.7 [random.access.iterators], Table 105 as indicated [This change is not
essential but it simplifies the specification] for the row with expression "b - a
"
and the column Operational semantics:
(a < b) ?distance(a,b): -distance(b,a)
Change 24.4.3 [iterator.operations]/4+5 as indicated:
template<class InputIterator> typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);4 Effects: If
InputIterator
meets the requirements of random access iterator then returns(last - first)
, otherwiseRreturns the number of incrementsor decrementsneeded to get fromfirst
tolast
.5 Requires: If
InputIterator
meets the requirements of random access iterator thenlast
shall be reachable fromfirst
orfirst
shall be reachable fromlast
, otherwiselast
shall be reachable fromfirst
.
ssize_t
undefinedSection: 99 [atomics.types.address] Status: C++11 Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.types.address].
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Discussion:
There is a row in "Table 122 - Atomics for standard typedef types"
in 99 [atomics.types.integral] with atomic_ssize_t
and ssize_t
. Unless, I'm missing something ssize_t
is not defined by the standard.
[ Summit: ]
Move to review. Proposed resolution: Remove the typedef. Note:
ssize_t
is a POSIX type.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Remove the row containing ssize_t
from Table 119
"Atomics for standard typedef types" in 99 [atomics.types.address].
atomic<bool>
derive from atomic_bool
?Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I think it's fairly obvious that atomic<bool>
is supposed to be derived
from atomic_bool
(and otherwise follow the atomic<integral>
interface),
though I think the current wording doesn't support this. I raised this
point along with atomic<floating-point>
privately with Herb and I seem
to recall it came up in the resulting discussion on this list. However,
I don't see anything on the current libs issue list mentioning this
problem.
32.5.8 [atomics.types.generic]/3 reads
There are full specializations over the integral types on the atomic class template. For each integral type integral in the second column of table 121 or table 122, the specialization
atomic<integral>
shall be publicly derived from the corresponding atomic integral type in the first column of the table. These specializations shall have trivial default constructors and trivial destructors.
Table 121 does not include (atomic_bool
, bool
),
so that this should probably be mentioned explicitly in the quoted paragraph.
[ Summit: ]
Move to open. Lawrence will draft a proposed resolution. Also, ask Howard to fix the title.
[ Post Summit Anthony provided proposed wording. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Replace paragraph 3 in 32.5.8 [atomics.types.generic] with
-3- There are full specializations over the integral types on the
atomic
class template. For each integral typeintegral
in the second column of table 121 or table 122, the specializationatomic<integral>
shall be publicly derived from the corresponding atomic integral type in the first column of the table. In addition, the specializationatomic<bool>
shall be publicly derived fromatomic_bool
. These specializations shall have trivial default constructors and trivial destructors.
duration
arithmetic: contradictory requirementsSection: 30.5.6 [time.duration.nonmember] Status: Resolved Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 30.5.6 [time.duration.nonmember], paragraph 8 says that calling
dur / rep
when rep
is an instantiation of duration
requires a diagnostic. That's followed by an operator/
that takes
two durations. So dur1 / dur2
is legal under the second version,
but requires a diagnostic under the first.
[ Howard adds: ]
Please see the thread starting with c++std-lib-22980 for more information.
[ Batavia (2009-05): ]
Move to Open, pending proposed wording (and preferably an implementation).
[ 2009-07-27 Howard adds: ]
I've addressed this issue under the proposed wording for 1177(i) which cleans up several places under 30.5 [time.duration] which used the phrase "diagnostic required".
For clarity's sake, here is an example implementation of the constrained
operator/
:template <class _Duration, class _Rep, bool = __is_duration<_Rep>::value> struct __duration_divide_result { }; template <class _Duration, class _Rep2, bool = is_convertible<_Rep2, typename common_type<typename _Duration::rep, _Rep2>::type>::value> struct __duration_divide_imp { }; template <class _Rep1, class _Period, class _Rep2> struct __duration_divide_imp<duration<_Rep1, _Period>, _Rep2, true> { typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period> type; }; template <class _Rep1, class _Period, class _Rep2> struct __duration_divide_result<duration<_Rep1, _Period>, _Rep2, false> : __duration_divide_imp<duration<_Rep1, _Period>, _Rep2> { }; template <class _Rep1, class _Period, class _Rep2> inline typename __duration_divide_result<duration<_Rep1, _Period>, _Rep2>::type operator/(const duration<_Rep1, _Period>& __d, const _Rep2& __s) { typedef typename common_type<_Rep1, _Rep2>::type _Cr; duration<_Cr, _Period> __r = __d; __r /= static_cast<_Cr>(__s); return __r; }
__duration_divide_result
is basically a custom-builtenable_if
that will containtype
only ifRep2
is not aduration
and ifRep2
is implicitly convertible tocommon_type<typename Duration::rep, Rep2>::type
.__is_duration
is simply a private trait that answersfalse
, but is specialized forduration
to answertrue
.The constrained
operator%
works identically.
[ 2009-10 Santa Cruz: ]
Proposed resolution:
ratio
arithmetic tweakSection: 21.4.4 [ratio.arithmetic] Status: C++11 Submitter: Howard Hinnant Opened: 2008-12-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ratio.arithmetic].
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Discussion:
N2800, 21.4.4 [ratio.arithmetic] lacks a paragraph from the proposal N2661:
ratio arithmetic [ratio.arithmetic]
... If the implementation is unable to form the indicated
ratio
due to overflow, a diagnostic shall be issued.
The lack of a diagnostic on compile-time overflow is a significant lack of functionality. This paragraph could be put back into the WP simply editorially. However in forming this issue I realized that we can do better than that. This paragraph should also allow alternative formulations which go to extra lengths to avoid overflow when possible. I.e. we should not mandate overflow when the implementation can avoid it.
For example:
template <class R1, class R2> struct ratio_multiply { typedef see below} type;The nested typedef type shall be a synonym for
ratio<T1, T2>
whereT1
has the valueR1::num * R2::num
andT2
has the valueR1::den * R2::den
.
Consider the case where intmax_t
is a 64 bit 2's complement signed integer,
and we have:
typedef std::ratio<0x7FFFFFFFFFFFFFFF, 0x7FFFFFFFFFFFFFF0> R1; typedef std::ratio<8, 7> R2; typedef std::ratio_multiply<R1, R2>::type RT;
According to the present formulation the implementaiton will multiply
0x7FFFFFFFFFFFFFFF * 8
which will result in an overflow and subsequently
require a diagnostic.
However if the implementation is first allowed to divde 0x7FFFFFFFFFFFFFFF
by 7
obtaining 0x1249249249249249 / 1
and divide
8
by 0x7FFFFFFFFFFFFFF0
obtaining 1 / 0x0FFFFFFFFFFFFFFE
,
then the exact result can then be computed without overflow:
[0x7FFFFFFFFFFFFFFF/0x7FFFFFFFFFFFFFF0] * [8/7] = [0x1249249249249249/0x0FFFFFFFFFFFFFFE]
Example implmentation which accomplishes this:
template <class R1, class R2> struct ratio_multiply { private: typedef ratio<R1::num, R2::den> _R3; typedef ratio<R2::num, R1::den> _R4; public: typedef ratio<__ll_mul<_R3::num, _R4::num>::value, __ll_mul<_R3::den, _R4::den>::value> type; };
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
Add a paragraph prior to p1 in 21.4.4 [ratio.arithmetic]:
Implementations may use other algorithms to compute the indicated ratios to avoid overflow. If overflow occurs, a diagnostic shall be issued.
owner_less
Section: 20.3.2.4 [util.smartptr.ownerless] Status: C++11 Submitter: Thomas Plum Opened: 2008-12-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
20.3.2.4 [util.smartptr.ownerless] (class template owner_less
) says that
operator()(x,y)
shall return x.before(y)
.
However, shared_ptr
and weak_ptr
have an owner_before()
but not a
before()
, and there's no base class to provide a missing before()
.
Being that the class is named owner_less
, I'm guessing that
"before()
" should be "owner_before()
", right?
[ Herve adds: ]
Agreed with the typo, it should be "shall return
x.owner_before(y)
".
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
Change 20.3.2.4 [util.smartptr.ownerless] p2:
-2-
operator()(x,y)
shall returnx.owner_before(y)
. [Note: ...
unique_ptr
converting ctor shouldn't accept array formSection: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
unique_ptr
's of array type should not convert to
unique_ptr
's which do not have an array type.
struct Deleter
{
void operator()(void*) {}
};
int main()
{
unique_ptr<int[], Deleter> s;
unique_ptr<int, Deleter> s2(std::move(s)); // should not compile
}
[ Post Summit: ]
Walter: Does the "diagnostic required" apply to both arms of the "and"?
Tom Plum: suggest to break into several sentences
Walter: suggest "comma" before the "and" in both places
Recommend Review.
[ Batavia (2009-05): ]
The post-Summit comments have been applied to the proposed resolution. We now agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for
enable_if
type constraining, possibly following Robert's formula.
[ 2009-08-01 Howard updates wording and sets to Review. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-02-27 Pete Opens: ]
The proposed replacement text doesn't make sense.
If
D
is a reference type, thenE
shall be the same type asD
, else this constructor shall not participate in overload resolution.This imposes two requirements. 1. If
D
is a reference type,E
has to beD
. 2. IfD
is not a reference type, the constructor shall not participate in overload resolution. If the latter apples, the language in the preceding paragraph that this constructor shall not throw an exception ifD
is not a reference type is superfluous. I suspect that's not the intention, but I can't parse this text any other way.
U
shall not be an array type, else this constructor shall not participate in overload resolution.I don't know what this means.
[ 2010-02-27 Peter adds: ]
I think that the intent is (proposed text):
Remarks: this constructor shall only participate in overload resolution if:
unique_ptr<U, E>::pointer
is implicitly convertible topointer
,U
is not an array type, and- if
D
is a reference type,E
is the same type asD
.
[ 2010-02-28 Howard adds: ]
I like Peter's proposal. Here is a tweak of it made after looking at my implementation. I believe this fixes a further defect not addressed by the current proposed wording:
Remarks: this constructor shall only participate in overload resolution if:
unique_ptr<U, E>::pointer
is implicitly convertible topointer
, andU
is not an array type, and- if
D
is a reference type,E
is the same type asD
, elseE
shall be implicitly convertible toD
.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3073.
Proposed resolution:
Change 20.3.1.3.2 [unique.ptr.single.ctor]:
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);-20- Requires: If
D
is not a reference type, construction of the deleterD
from an rvalue of typeE
shall be well formed and shall not throw an exception.IfD
is a reference type, thenE
shall be the same type asD
(diagnostic required).unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
. [Note: These requirements imply thatT
andU
are complete types. — end note]Remarks: If
D
is a reference type, thenE
shall be the same type asD
, else this constructor shall not participate in overload resolution.unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
, else this constructor shall not participate in overload resolution.U
shall not be an array type, else this constructor shall not participate in overload resolution. [Note: These requirements imply thatT
andU
are complete types. — end note]
Change 20.3.1.3.4 [unique.ptr.single.asgn]:
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);-6- Requires: Assignment of the deleter
D
from an rvalueD
shall not throw an exception.unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
. [Note: These requirements imply thatT
andU
are complete types. — end note]Remarks:
unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
, else this operator shall not participate in overload resolution.U
shall not be an array type, else this operator shall not participate in overload resolution. [Note: These requirements imply thatT
andU
are complete types. — end note]
Section: 30.4.1 [time.traits.is.fp] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
[time.traits.is_fp] says that the type Rep
"is
assumed to be ... a class emulating an integral type." What are the
requirements for such a type?
[ 2009-05-10 Howard adds: ]
IntegralLike
.
[ Batavia (2009-05): ]
As with issue 953(i), we recommend this issue be addressed in the context of providing concepts for the entire
thread
header.We look forward to proposed wording.
Move to Open.
[ 2009-08-01 Howard adds: ]
I have surveyed all clauses of [time.traits.duration_values], 30.4.3 [time.traits.specializations] and 30.5 [time.duration]. I can not find any clause which involves the use of a
duration::rep
type where the requirements on therep
type are not clearly spelled out. These requirements were carefully crafted to allow any arithmetic type, or any user-defined type emulating an arithmetic type.Indeed,
treat_as_floating_point
becomes completely superfluous ifduration::rep
can never be a class type.There will be some
Rep
types which will not meet the requirements of everyduration
operation. This is no different than the fact thatvector<T>
can easily be used for typesT
which are notDefaultConstructible
, even though some members ofvector<T>
requireT
to beDefaultConstructible
. This is why the requirements onRep
are specified for each operation individually.In [time.traits.is_fp] p1:
template <class Rep> struct treat_as_floating_point : is_floating_point<Rep> { };The
duration
template uses thetreat_as_floating_point
trait to help determine if aduration
object can be converted to anotherduration
with a different tick period. Iftreat_as_floating_point<Rep>::value
istrue
, thenRep
is a floating-point type and implicit conversions are allowed amongduration
s. Otherwise, the implicit convertibility depends on the tick periods of theduration
s. IfRep
is a class type which emulates a floating-point type, the author ofRep
can specializetreat_as_floating_point
so thatduration
will treat thisRep
as if it were a floating-point type. OtherwiseRep
is assumed to be an integral type or a class emulating an integral type.The phrases "a class type which emulates a floating-point type" and "a class emulating an integral type" are clarifying phrases which refer to the summation of all the requirements on the
Rep
type specified in detail elsewhere (and should not be repeated here).This specification has been implemented, now multiple times, and the experience has been favorable. The current specification clearly specifies the requirements at each point of use (though I'd be happy to fix any place I may have missed, but none has been pointed out).
I am amenable to improved wording of this paragraph (and any others), but do not have any suggestions for improved wording at this time. I am strongly opposed to changes which would significantly alter the semantics of the specification under 30 [time] without firmly grounded and documented rationale, example implementation, testing, and user experience which relates a positive experience.
I recommend NAD unless someone wants to produce some clarifying wording.
[ 2009-10 Santa Cruz: ]
Stefanus to provide wording to turn this into a note.
[ 2010-02-11 Stefanus provided wording. ]
[ 2010 Rapperswil: ]
Move to Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change [time.traits.is_fp]/1:
1 The
duration
template uses thetreat_as_floating_point
trait to help determine if aduration
object can be converted to anotherduration
with a different tick period. Iftreat_as_floating_point<Rep>::value
istrue
, thenimplicit conversions are allowed amongRep
is a floating-point type andduration
s. Otherwise, the implicit convertibility depends on the tick periods of theduration
s.If[Note: The intention of this trait is to indicate whether a given class behaves like a floating point type, and thus allows division of one value by another with acceptable loss of precision. IfRep
is a class type which emulates a floating-point type, the author ofRep
can specializetreat_as_floating_point
so that duration will treat thisRep
as if it were a floating-point type. OtherwiseRep
is assumed to be an integral type or a class emulating an integral type.treat_as_floating_point<Rep>::value
isfalse
,Rep
will be treated as if it behaved like an integral type for the purpose of these conversions. — end note]
Section: 30.3 [time.clock.req] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.req].
View all issues with Resolved status.
Discussion:
30.3 [time.clock.req] says that a clock's rep
member is "an
arithmetic type or a class emulating an arithmetic type." What are the
requirements for such a type?
[ 2009-05-10 Howard adds: ]
This wording was aimed directly at the
ArithmeticLike
concept.
[ Batavia (2009-05): ]
We recommend this issue be addressed in the context of providing concepts for the entire
thread
header.May resolve for now by specifying arithmetic types, and in future change to
ArithmeticLike
. However, Alisdair believes this is not feasible.Bill disagrees.
We look forward to proposed wording. Move to Open.
[ 2009-08-01 Howard adds: ]
[ 2009-10 Santa Cruz: ]
Stefanus to provide wording to turn this into a note.
[ 2010-02-11 Stephanus provided wording for 951(i) which addresses this issue as well. ]
[ 2010 Rapperswil: ]
Proposed resolution:
Section: 30.3 [time.clock.req] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.req].
View all issues with C++11 status.
Discussion:
Table 55 — Clock Requirements (in 30.3 [time.clock.req])
C1::time_point
require C1
and C2
to "refer to the same epoch", but "epoch" is not defined.
time_point
definition if it is
valid to compare their time_point
s by comparing their
respective duration
s." What does "valid" mean here? And, since
C1::rep
is "THE representation type of the native
duration
and time_point
" (emphasis added), there
doesn't seem to be much room for some other representation.
C1::is_monotonic
has type "const bool
". The
"const
" should be removed.
C1::period
has type ratio
. ratio
isn't a type,
it's a template. What is the required type?
[ 2009-05-10 Howard adds: ]
"epoch" is purposefully not defined beyond the common English
definition. The C standard
also chose not to define epoch, though POSIX did. I believe it is a strength
of the C standard that epoch is not defined. When it is known that two time_point
s
refer to the same epoch, then a definition of the epoch is not needed to compare
the two time_point
s, or subtract them.
A time_point
and a Clock
implicitly refer to an (unspecified) epoch.
The time_point
represents an offset (duration
) from an epoch.
The sentence:
Different clocks may share a
time_point
definition if it is valid to compare theirtime_point
s by comparing their respectiveduration
s.
is redundant and could be removed. I believe the sentence which follows the above:
C1
andC2
shall refer to the same epoch.
is sufficient. If two clocks share the same epoch, then by definition, comparing
their time_point
s is valid.
is_monotonic
is meant to never change (be const
). It is also
desired that this value be usable in compile-time computation and branching.
This should probably instead be worded:
An instantiation of
ratio
.
[ Batavia (2009-05): ]
Re (a): It is not clear to us whether "epoch" is a term of art.
Re (b), (c), and (d): We agree with Howard's comments, and would consider adding to (c) a
static constexpr
requirement.Move to Open pending proposed wording.
[ 2009-05-25 Daniel adds: ]
In regards to (d) I suggest to say "a specialization of ratio" instead of "An instantiation of ratio". This seems to be the better matching standard core language term for this kind of entity.
[ 2009-05-25 Ganesh adds: ]
Regarding (a), I found this paper on the ISO website using the term "epoch" consistently with the current wording:
which is part of ISO/IEC 18026 "Information technology -- Spatial Reference Model (SRM)".
[ 2009-08-01 Howard: Moved to Reivew as the wording requested in Batavia has been provided. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change 30.3 [time.clock.req] p1:
-1- A clock is a bundle consisting of a native
duration
, a nativetime_point
, and a functionnow()
to get the currenttime_point
. The origin of the clock'stime_point
is referred to as the clock's epoch as defined in section 6.3 of ISO/IEC 18026. A clock shall meet the requirements in Table 45.
Remove the sentence from the time_point
row of the table "Clock Requirements":
C1::time_point
|
chrono::time_point<C1> or chrono::time_point<C2, C1::duration>
|
The native time_point type of the clock.
time_point definition if it is valid to compare their time_point s by comparing their respective duration s.C1 and C2 shall refer to the same epoch.
|
Change the row starting with C1::period
of the table "Clock Requirements":
C1::period
|
a specialization of ratio
|
The tick period of the clock in seconds. |
Section: 30.3 [time.clock.req] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.req].
View all issues with C++11 status.
Discussion:
30.3 [time.clock.req] uses the word "native" in several places,
but doesn't define it. What is a "native duration
"?
[ 2009-05-10 Howard adds: ]
The standard uses "native" in several places without defining it (e.g. 5.13.3 [lex.ccon]). It is meant to mean "that which is defined by the facility", or something along those lines. In this case it refers to the nested
time_point
andduration
types of the clock. Better wording is welcome.
[ Batavia (2009-05): ]
Move to Open pending proposed wording from Pete.
[ 2009-10-23 Pete provides wording: ]
[ 2009-11-18 Daniel adds: ]
I see that 32.6.4.3 [thread.timedmutex.requirements]/3 says:
Precondition: If the tick
period
ofrel_time
is not exactly convertible to the native tickperiod
, theduration
shall be rounded up to the nearest native tickperiod
.I would prefer to see that adapted as well. Following the same style as the proposed resolution I come up with
Precondition: If the tick
period
ofrel_time
is not exactly convertible to thenativetickperiod
of the execution environment, theduration
shall be rounded up to the nearestnativetickperiod
of the execution environment.
[ 2010-03-28 Daniel synced wording with N3092 ]
[ Post-Rapperswil, Howard provides wording: ]
Moved to Tentatively Ready with revised wording from Howard Hinnant after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 30.3 [time.clock.req]:
1 A clock is a bundle consisting of a
nativeduration
, anativetime_point
, and a functionnow()
to get the currenttime_point
. The origin of the clock'stime_point
is referred to as the clock's epoch. A clock shall meet the requirements in Table 56.2 ...
Table 56 — Clock requirements Expression Return type Operational semantics C1::rep
An arithmetic type or a class emulating an arithmetic type The representation type of the nativeC1::duration
.andtime_point
.C1::period
... ... C1::duration
chrono::duration<C1::rep, C1::period>
The nativeduration
type of the clock.C1::time_point
chrono::time_point<C1>
orchrono::time_point<C2, C1::duration>
The nativetime_point
type of the clock.C1
andC2
shall refer to the same epoch....
Section: 30.7.2 [time.clock.system] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.system].
View all issues with C++11 status.
Discussion:
30.7.2 [time.clock.system]: to_time_t
is overspecified. It
requires truncation, but should allow rounding. For example, suppose a
system has a clock that gives times in milliseconds, but time()
rounds
those times to the nearest second. Then system_clock
can't use any
resolution finer than one second, because if it did, truncating times
between half a second and a full second would produce the wrong time_t
value.
[ Post Summit Anthony Williams provided proposed wording. ]
[ Batavia (2009-05): ]
Move to Review pending input from Howard. and other stakeholders.
[ 2009-05-23 Howard adds: ]
I am in favor of the wording provided by Anthony.
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
In 30.7.2 [time.clock.system] replace paragraphs 3 and 4 with:
time_t to_time_t(const time_point& t);-3- Returns: A
time_t
object that represents the same point in time ast
when both values aretruncatedrestricted to the coarser of the precisions oftime_t
andtime_point
. It is implementation defined whether values are rounded or truncated to the required precision.time_point from_time_t(time_t t);-4- Returns: A
time_point
object that represents the same point in time ast
when both values aretruncatedrestricted to the coarser of the precisions oftime_t
andtime_point
. It is implementation defined whether values are rounded or truncated to the required precision.
Section: 32.7.4 [thread.condition.condvar] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
View all issues with Resolved status.
Discussion:
32.7.4 [thread.condition.condvar]: the specification for wait_for
with no predicate has an effects clause that says it calls wait_until
,
and a returns clause that sets out in words how to determine the return
value. Is this description of the return value subtly different from the
description of the value returned by wait_until
? Or should the effects
clause and the returns clause be merged?
[ Summit: ]
Move to open. Associate with LWG 859(i) and any other monotonic-clock related issues.
[ 2009-08-01 Howard adds: ]
I believe that 859(i) (currently Ready) addresses this issue, and that this issue should be marked NAD, solved by 859(i) (assuming it moves to WP).
[ 2009-10 Santa Cruz: ]
Mark as
NAD EditorialResolved, addressed by resolution of Issue 859(i).
Proposed resolution:
Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [thread.mutex.requirements].
View all other issues in [thread.mutex.requirements].
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Discussion:
32.6.4 [thread.mutex.requirements]: paragraph 4 is entitled "Error conditions", but according to 16.3.2.4 [structure.specifications], "Error conditions:" specifies "the error conditions for error codes reported by the function." It's not clear what this should mean when there is no function in sight.
[ Summit: ]
Move to open.
[ Beman provided proposed wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready. Fix the proposed wording with "functions of type Mutex" -> "functions of Mutex type"
Proposed resolution:
Change 32.6.4 [thread.mutex.requirements] Mutex requirements, paragraph 4 as indicated:
-4-
Error conditions:The error conditions for error codes, if any, reported by member functions of Mutex type shall be:
not_enough_memory
— if there is not enough memory to construct the mutex object.resource_unavailable_try_again
— if any native handle type manipulated is not available.operation_not_permitted
— if the thread does not have the necessary permission to change the state of the mutex object.device_or_resource_busy
— if any native handle type manipulated is already locked.invalid_argument
— if any native handle type manipulated as part of mutex construction is incorrect.
Section: 32.6.5.4.3 [thread.lock.unique.locking] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.lock.unique.locking].
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Discussion:
32.6.5.4.3 [thread.lock.unique.locking]: unique_lock::lock
is
required to throw an object of type std::system_error
"when the
postcondition cannot be achieved." The postcondition is owns == true
,
and this is trivial to achieve. Presumably, the requirement is intended
to mean something more than that.
[ Summit: ]
Move to open.
[ Beman has volunteered to provide proposed wording. ]
[ 2009-07-21 Beman added wording to address 32.2.2 [thread.req.exception] in response to the Frankfurt notes in 859(i). ]
[ 2009-09-25 Beman: minor update to wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change Exceptions 32.2.2 [thread.req.exception] as indicated:
Some functions described in this Clause are specified to throw exceptions of type
system_error
(19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function fromsatisfying its postconditions or from returning a meaningful valuemeeting its specifications. Failure to allocate storage shall be reported as described in 16.4.6.14 [res.on.exception.handling].
Change thread assignment 32.4.3.6 [thread.thread.member], join(), paragraph 8 as indicated:
Throws:
std::system_error
whenthe postconditions cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change thread assignment 32.4.3.6 [thread.thread.member], detach(), paragraph 13 as indicated:
Throws:
std::system_error
whenthe effects or postconditions cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 11, as indicated:
Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 3, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 8, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 13, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 18, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 22, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Function call_once 32.6.7.2 [thread.once.callonce], paragraph 4, as indicated
Throws:
std::system_error
whenthe effects cannot be achievedan exception is required (32.2.2 [thread.req.exception]), or any exception thrown byfunc
.
Change Class condition_variable 32.7.4 [thread.condition.condvar], paragraph 12, as indicated:
Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Class condition_variable 32.7.4 [thread.condition.condvar], paragraph 19, as indicated:
Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Class condition_variable_any 32.7.5 [thread.condition.condvarany], paragraph 10, as indicated:
Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Class condition_variable_any 32.7.5 [thread.condition.condvarany], paragraph 16, as indicated:
Throws:
std::system_error
whenthe returned value, effects, or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 32.7.4 [thread.condition.condvar] as indicated:
template <class Rep, class Period> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);...Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required ([thread.req.exception]).
Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 32.7.4 [thread.condition.condvar] as indicated:
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);...Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 32.7.5 [thread.condition.condvarany] as indicated:
template <class Lock, class Rep, class Period> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);...Throws:
std::system_error
whenthe returned value, effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 32.7.5 [thread.condition.condvarany] as indicated:
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);...Throws:
std::system_error
whenthe returned value, effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Section: 32.4.3.6 [thread.thread.member] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.member].
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Discussion:
32.4.3.6 [thread.thread.member]: thread::detach
is required to
throw an exception if the thread is "not a detachable thread".
"Detachable" is never defined.
[ Howard adds: ]
Due to a mistake on my part, 3 proposed resolutions appeared at approximately the same time. They are all three noted below in the discussion.
[ Summit, proposed resolution: ]
In 32.4.3.6 [thread.thread.member] change:
void detach();...
-14- Error conditions:
no_such_process
-- if the thread is notavalidthread.invalid_argument
-- if the thread is nota detachablejoinablethread.
[ Post Summit, Jonathan Wakely adds: ]
A
thread
is detachable if it is joinable. As we've defined joinable, we can just use that.This corresponds to the pthreads specification, where pthread_detach fails if the thread is not joinable:
EINVAL: The implementation has detected that the value specified by thread does not refer to a joinable thread.
Jonathan recommends this proposed wording:
In 32.4.3.6 [thread.thread.member] change:
void detach();...
-14- Error conditions:
- ...
invalid_argument
-- not adetachablejoinable thread.
[ Post Summit, Anthony Williams adds: ]
This is covered by the precondition that
joinable()
betrue
.Anthony recommends this proposed wording:
In 32.4.3.6 [thread.thread.member] change:
void detach();...
-14- Error conditions:
- ...
invalid_argument
-- not a detachable thread.
[ 2009-10 Santa Cruz: ]
Mark as Ready with proposed resolution from Summit.
Proposed resolution:
In 32.4.3.6 [thread.thread.member] change:
void detach();...
-14- Error conditions:
no_such_process
-- if the thread is notavalidthread.invalid_argument
-- if the thread is nota detachablejoinablethread.
Section: 32.7.5 [thread.condition.condvarany] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvarany].
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Discussion:
The requirements for the constructor for condition_variable
has several
error conditions, but the requirements for the constructor for
condition_variable_any
has none. Is this difference intentional?
[ Summit: ]
Move to open, pass to Howard. If this is intentional, a note may be helpful. If the error conditions are to be copied from
condition_variable
, this depends on LWG 965(i).
[ Post Summit Howard adds: ]
The original intention (N2447) was to let the OS return whatever errors it was going to return, and for those to be translated into exceptions, for both
condition_variable
andcondition_variable_any
. I have not received any complaints about specific error conditions from vendors on non-POSIX platforms, but such complaints would not surprise me if they surfaced.
[ 2009-10 Santa Cruz: ]
Leave open. Benjamin to provide wording.
[ 2010 Pittsburgh: ]
We don't have throw clauses for condition variables.
This issue may be dependent on LWG 1268(i).
Leave open. Detlef will coordinate with Benjamin.
Consider merging LWG 964, 966(i), and 1268(i) into a single paper.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
Section: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.7.4 [thread.condition.condvar]: the constructor for
condition_variable
throws an exception with error code
device_or_resource_busy
"if attempting to initialize a
previously-initialized but as of yet undestroyed condition_variable
."
How can this occur?
[ Summit: ]
Move to review. Proposed resolution: strike the
device_or_resource_busy
error condition from the constructor ofcondition_variable
.
- This is a POSIX error that cannot occur in this interface because the C++ interface does not separate declaration from initialization.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 32.7.4 [thread.condition.condvar] p3:
- ...
device_or_resource_busy
-- if attempting to initialize a previously-initialized but as of yet undestroyedcondition_variable
.
Section: 32.7.4 [thread.condition.condvar] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.7.4 [thread.condition.condvar]:
condition_variable::wait
and
condition_variable::wait_until
both have a postcondition that
lock
is locked by the calling thread, and a throws clause that
requires throwing an exception if this postcondition cannot be achieved.
How can the implementation detect that this lock
can never be
obtained?
[ Summit: ]
Move to open. Requires wording. Agreed this is an issue, and the specification should not require detecting deadlocks.
[ 2009-08-01 Howard provides wording. ]
The proposed wording is inspired by the POSIX spec which says:
- [EINVAL]
- The value specified by cond or mutex is invalid.
- [EPERM]
- The mutex was not owned by the current thread at the time of the call.
I do not believe [EINVAL] is possible without memory corruption (which we don't specify). [EPERM] is possible if this thread doesn't own the mutex, which is listed as a precondition. "May" is used instead of "Shall" because not all OS's are POSIX.
[ 2009-10 Santa Cruz: ]
Leave open, Detlef to provide improved wording.
[ 2009-10-23 Detlef Provided wording. ]
Detlef's wording put in Proposed resolution. Original wording here:
Change 32.7.4 [thread.condition.condvar] p12, p19 and 32.7.5 [thread.condition.condvarany] p10, p16:
Throws: May throw
std::system_error
if a precondition is not met.when the effects or postcondition cannot be achieved.
[ 2009-10 Santa Cruz: ]
Leave open, Detlef to provide improved wording.
[ 2009-11-18 Anthony adds: ]
condition_variable::wait
takes aunique_lock<mutex>
. We know whether or not aunique_lock
owns a lock, through use of itsowns_lock()
member.I would like to propose the following resolution:
Modify the first sentence of 32.7.4 [thread.condition.condvar] p9:
void wait(unique_lock<mutex>& lock);9 Precondition:
lock
is locked by the calling threadlock.owns_lock()
istrue
, and either...
Replace 32.7.4 [thread.condition.condvar] p11-13 with:
void wait(unique_lock<mutex>& lock);...
11 Postcondition:
lock
is locked by the calling threadlock.owns_lock()
istrue
.12 Throws:
std::system_error
when the effects or postcondition cannot be achievedif the implementation detects that the preconditions are not met or the effects cannot be achieved. Any exception thrown bylock.lock()
orlock.unlock()
.13 Error Conditions: The error conditions are implementation defined.
equivalent error condition fromlock.lock()
orlock.unlock()
.
[ 2010 Pittsburgh: ]
There are heavy conflicts with adopted papers.
This issue is dependent on LWG 1268(i).
Leave open pending outstanding edits to the working draft. Detlef will provide wording.
[2011-03-24 Madrid]
Rationale:
This has been resolved since filing, with the introduction of system_error
to the thread specification.
Proposed resolution:
Replace 32.7.4 [thread.condition.condvar] p12, p19 and 32.7.5 [thread.condition.condvarany] p10, p16:
Throws:std::system_error
when the effects or postcondition cannot be achieved.
Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.Throws: It is implementation-defined whether a
std::system_error
with implementation-defined error condition is thrown if the precondition is not met.
Section: 32.4.3.3 [thread.thread.constr] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
the error handling for the constructor for condition_variable
distinguishes lack of memory from lack of other resources, but the error
handling for the thread constructor does not. Is this difference
intentional?
[ Beman has volunteered to provide proposed wording. ]
[ 2009-09-25 Beman provided proposed wording. ]
The proposed resolution assumes 962(i) has been accepted and its proposed resolution applied to the working paper.
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 4, as indicated:
Error conditions:
not_enough_memory
— if there is not enough memory to construct the mutex object.resource_unavailable_try_again
— if any native handle type manipulated is not available.operation_not_permitted
— if the thread does not have the necessary permission to change the state of the mutex object.device_or_resource_busy
— if any native handle type manipulated is already locked.invalid_argument
— if any native handle type manipulated as part of mutex construction is incorrect.
Change Class condition_variable
32.7.4 [thread.condition.condvar],
default constructor, as indicated:
condition_variable();
Effects: Constructs an object of type
condition_variable
.Throws:
std::system_error
when an exception is required (32.2.2 [thread.req.exception]).Error conditions:
not_enough_memory
— if a memory limitation prevents initialization.resource_unavailable_try_again
— if some non-memory resource limitation prevents initialization.device_or_resource_busy
— if attempting to initialize a previously-initialized but as of yet undestroyedcondition_variable
.
Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.6.4 [thread.mutex.requirements]: several functions are required to throw exceptions "if the thread does not have the necessary permission ...". "The necessary permission" is not defined.
[ Summit: ]
Move to open.
[ Beman has volunteered to provide proposed wording. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready with minor word-smithing in the example.
Proposed resolution:
Change Exceptions 32.2.2 [thread.req.exception] as indicated:
Some functions described in this Clause are specified to throw exceptions of type
system_error
(19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function from meeting its specifications. [Note: See 16.4.6.14 [res.on.exception.handling] for exceptions thrown to report storage allocation failures. —end note][Example:
Consider a function in this clause that is specified to throw exceptions of type
system_error
and specifies Error conditions that includeoperation_not_permitted
for a thread that does not have the privilege to perform the operation. Assume that, during the execution of this function, anerrno
ofEPERM
is reported by a POSIX API call used by the implementation. Since POSIX specifies anerrno
ofEPERM
when "the caller does not have the privilege to perform the operation", the implementation mapsEPERM
to anerror_condition
ofoperation_not_permitted
(19.5 [syserr]) and an exception of typesystem_error
is thrown.—end example]
Editorial note: For the sake of exposition, the existing text above is shown with the changes proposed in issues 962 and 967. The proposed additional example is independent of whether or not the 962 and 967 proposed resolutions are accepted.
Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 4, as indicated:
—
operation_not_permitted
— if the thread does not have thenecessary permission to change the state of the mutex objectprivilege to perform the operation.
Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 12, as indicated:
—
operation_not_permitted
— if the thread does not have thenecessary permission to change the state of the mutexprivilege to perform the operation.
addressof
overload unneededSection: 20.2.11 [specialized.addressof] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.2.11 [specialized.addressof] specifies:
template <ObjectType T> T* addressof(T& r); template <ObjectType T> T* addressof(T&& r);
The two signatures are ambiguous when the argument is an lvalue. The second signature seems not useful: what does it mean to take the address of an rvalue?
[ Post Summit: ]
Recommend Review.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-11-18 Moved from Pending WP to WP. Confirmed in N3000. ]
Proposed resolution:
Change 20.2.11 [specialized.addressof]:
template <ObjectType T> T* addressof(T& r);template <ObjectType T> T* addressof(T&& r);
duration<double>
should not implicitly convert to duration<int>
Section: 30.5.2 [time.duration.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The following code should not compile because it involves implicit truncation
errors (against the design philosophy of the duration
library).
duration<double> d(3.5);
duration<int> i = d; // implicit truncation, should not compile
This intent was codified in the example implementation which drove this proposal but I failed to accurately translate the code into the specification in this regard.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for
enable_if
type constraining, possibly following Robert's formula.
[ 2009-08-01 Howard adds: ]
[ 2009-10 Santa Cruz: ]
Proposed resolution:
Change 30.5.2 [time.duration.cons], p4:
template <class Rep2, class Period2> duration(const duration<Rep2, Period2>& d);-4- Requires:
treat_as_floating_point<rep>::value
shall betrue
or bothratio_divide<Period2, period>::type::den
shall be 1 andtreat_as_floating_point<Rep2>::value
shall befalse
. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-basedduration
types. Such a construction could easily lead to confusion about the value of theduration
. — end note]
is_convertible
cannot be instantiated for non-convertible typesSection: 21.3.7 [meta.rel] Status: C++11 Submitter: Daniel Krügler Opened: 2009-01-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 206
The current specification of std::is_convertible
(reference is draft
N2798)
is basically defined by 21.3.7 [meta.rel] p.4:
In order to instantiate the template
is_convertible<From, To>
, the following code shall be well formed:template <class T> typename add_rvalue_reference<T>::type create(); To test() { return create<From>(); }[Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]
The first sentence can be interpreted, that e.g. the expression
std::is_convertible<double, int*>::value
is ill-formed because std::is_convertible<double, int*>
could not be
instantiated, or in more general terms: The wording requires that
std::is_convertible<X, Y>
cannot be instantiated for otherwise valid
argument types X
and Y
if X
is not convertible to Y
.
This semantic is both unpractical and in contradiction to what the last type traits paper N2255 proposed:
If the following
test
function is well formed codeb
istrue
, else it isfalse
.template <class T> typename add_rvalue_reference<T>::type create(); To test() { return create<From>(); }[Note: This definition gives well defined results for
reference
types,void
types, array types, and function types. — end note]
[ Post Summit: ]
Jens: Checking that code is well-formed and then returning
true
/false
sounds like speculative compilation. John Spicer would really dislike this. Please find another wording suggesting speculative compilation.Recommend Open.
[ Post Summit, Howard adds: ]
John finds the following wording clearer:
Template Condition Comments template <class From, class To>
struct is_convertible;see below From
andTo
shall be complete types, arrays of unknown bound, or (possibly cv-qualified)void
types.Given the following function prototype:
template <class T> typename add_rvalue_reference<T>::type create();
is_convertible<From, To>::value
shall betrue
if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, elseis_convertible<From, To>::value
shall befalse
.To test() { return create<From>(); }
Original proposed wording:
In 21.3.7 [meta.rel]/4 change:
In order to instantiate the templateIf the following code is well formedis_convertible<From, To>
, the following code shall be well formedis_convertible<From, To>::value
istrue
, otherwisefalse
:[..]
Revision 2
In 21.3.7 [meta.rel] change:
Template Condition Comments ... ... ... template <class From, class To>
struct is_convertible;The code set out below shall be well formed.see belowFrom
andTo
shall be complete types, arrays of unknown bound, or (possibly cv-qualified)void
types.-4-
In order to instantiate the templateGiven the following function prototype:is_convertible<From, To>
, the following code shall be well formed:template <class T> typename add_rvalue_reference<T>::type create();
is_convertible<From, To>::value
inherits either directly or indirectly fromtrue_type
if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, elseis_convertible<From, To>::value
inherits either directly or indirectly fromfalse_type
.To test() { return create<From>(); }[Note: This requirement gives well defined results for reference types, void types, array types, and function types. -- end note]
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 21.3.7 [meta.rel] change:
Template Condition Comments ... ... ... template <class From, class To>
struct is_convertible;The code set out below shall be well formed.see belowFrom
andTo
shall be complete types, arrays of unknown bound, or (possibly cv-qualified)void
types.-4-
In order to instantiate the templateGiven the following function prototype:is_convertible<From, To>
, the following code shall be well formed:template <class T> typename add_rvalue_reference<T>::type create();the predicate condition for a template specialization
is_convertible<From, To>
shall be satisfied, if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function.To test() { return create<From>(); }[Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]
std::stack
should be movableSection: 23.6.6.2 [stack.defn] Status: Resolved Submitter: Daniel Krügler Opened: 2009-02-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis given in 23.6.6.2 [stack.defn] does not show up
requires MoveConstructible<Cont> stack(stack&&); requires MoveAssignable<Cont> stack& operator=(stack&&);
although the other container adaptors do provide corresponding members.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-08-18 Daniel updates the wording and Howard sets to Review. ]
[ 2009-08-23 Howard adds: ]
1194(i) also adds these move members using an editorially different style.
[ 2009-10 Santa Cruz: ]
Proposed resolution:
In the class stack synopsis of 23.6.6.2 [stack.defn] insert:
template <class T, class Container = deque<T> > class stack { [..] explicit stack(const Container&); explicit stack(Container&& = Container()); stack(stack&& s) : c(std::move(s.c)) {} stack& operator=(stack&& s) { c = std::move(s.c); return *this; } [..] };
Section: 22.10.19 [unord.hash] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-02-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 208
I don't see an open issue on supporting std::hash
for smart pointers
(unique_ptr
and shared_ptr
at least).
It seems reasonable to at least expect support for the smart pointers, especially as they support comparison for use in ordered associative containers.
[ Batavia (2009-05): ]
Howard points out that the client can always supply a custom hash function.
Alisdair replies that the smart pointer classes are highly likely to be frequently used as hash keys.
Bill would prefer to be conservative.
Alisdair mentions that this issue may also be viewed as a subissue or duplicate of issue 1025(i).
Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.
[ 2009-05-31 Peter adds: ]
Howard points out that the client can always supply a custom hash function.
Not entirely true. The client cannot supply the function that hashes the address of the control block (the equivalent of the old
operator<
, now proudly carrying the awkward name of 'owner_before
'). Only the implementation can do that, not necessarily via specializinghash<>
, of course.This hash function makes sense in certain situations for
shared_ptr
(when one needs to switch fromset/map
using ownership ordering tounordered_set/map
) and is the only hash function that makes sense forweak_ptr
.
[ 2009-07-28 Alisdair provides wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2009-11-16 Moved from Ready to Open: ]
Pete writes:
As far as I can see, "...suitable for using this type as key in unordered associative containers..." doesn't define any semantics. It's advice to the reader, and if it's present at all it should be in a note. But we have far too much of this sort of editorial commentary as it is.
And in the resolution of 978 it's clearly wrong: it says that if there is no hash specialization available for
D::pointer
, the implementation may providehash<unique_ptr<T,D>>
if the result is not suitable for use in unordered containers.Howard writes:
Is this a request to pull 978 from Ready?
Barry writes:
I read this as more than a request. The PE says it's wrong, so it can't be Ready.
[ 2010-01-31 Alisdair: related to 1245(i) and 1182(i). ]
[ 2010-02-08 Beman updates wording. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add the following declarations to the synopsis of <memory>
in
20.2 [memory]
// [util.smartptr.hash] hash support template <class T> struct hash; template <class T, class D> struct hash<unique_ptr<T,D>>; template <class T> struct hash<shared_ptr<T>>;
Add a new subclause under [util.smartptr] called hash support
hash support [util.smartptr.hash]
template <class T, class D> struct hash<unique_ptr<T,D>>;Specialization meeting the requirements of class template
hash
(22.10.19 [unord.hash]). For an objectp
of typeUP
, whereUP
is a typeunique_ptr<T,D>
,hash<UP>()(p)
shall evaluate to the same value ashash<typename UP::pointer>()(p.get())
. The specializationhash<typename UP::pointer>
is required to be well-formed.template <class T> struct hash<shared_ptr<T>>;Specialization meeting the requirements of class template
hash
(22.10.19 [unord.hash]). For an objectp
of typeshared_ptr<T>
,hash<shared_ptr<T>>()(p)
shall evaluate to the same value ashash<T*>()(p.get())
.
initializer_list
supportSection: 23.2.8 [unord.req] Status: C++11 Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Refering to
N2800
all container requirements tables (including those for
associative containers) provide useful member function overloads
accepting std::initializer_list
as argument, the only exception is
Table 87. There seems to be no reason for not providing them, because 23.5 [unord]
is already initializer_list
-aware. For the sake of
library interface consistency and user-expectations corresponding
overloads should be added to the table requirements of unordered
containers as well.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
In 23.2.8 [unord.req]/9 insert:
...
[q1, q2)
is a valid range ina
,il
designates an object of typeinitializer_list<value_type>
,t
is a value of typeX::value_type
, ...
In 23.2.8 [unord.req], Table 87 insert:
Table 87 - Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity X(i, j)
X a(i, j)X
... ... X(il)
X
Same as X(il.begin(), il.end())
.Same as X(il.begin(), il.end())
.... ... ... ... a = b
X
... ... a = il
X&
a = X(il); return *this;
Same as a = X(il)
.... ... ... ... a.insert(i, j)
void
... ... a.insert(il)
void
Same as a.insert(il.begin(), il.end())
.Same as a.insert(il.begin(), il.end())
.
Section: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to
N2800,
the associative container requirements table 85 says
that assigning an initializer_list
to such a container is of
constant complexity, which is obviously wrong.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
In 23.2.7 [associative.reqmts], Table 85 change:
Table 85 - Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity a = il
X&
a = X(il);
return *this;constantSame asa = X(il)
.
unique_ptr
reference deleters should not be moved fromSection: 20.3.1.3 [unique.ptr.single] Status: Resolved Submitter: Howard Hinnant Opened: 2009-02-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Dave brought to my attention that when a unique_ptr
has a non-const reference
type deleter, move constructing from it, even when the unique_ptr
containing
the reference is an rvalue, could have surprising results:
D d(some-state);
unique_ptr<A, D&> p(new A, d);
unique_ptr<A, D> p2 = std::move(p);
// has d's state changed here?
I agree with him. It is the unique_ptr
that is the rvalue, not the
deleter. When the deleter is a reference type, the unique_ptr
should
respect the "lvalueness" of the deleter.
Thanks Dave.
[ Batavia (2009-05): ]
Seems correct, but complicated enough that we recommend moving to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Change 20.3.1.3.2 [unique.ptr.single.ctor], p20-21
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);-20- Requires: If
is not a reference type, construction of the deleter
DED
from an rvalue of typeE
shall be well formed and shall not throw an exception. OtherwiseE
is a reference type and construction of the deleterD
from an lvalue of typeE
shall be well formed and shall not throw an exception. IfD
is a reference type, thenE
shall be the same type asD
(diagnostic required).unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
. [Note:
These requirements imply thatT
andU
are complete types. — end note]-21- Effects: Constructs a
unique_ptr
which owns the pointer whichu
owns (if any). If the deleterE
is not a reference type,itthis deleter is move constructed fromu
's deleter, otherwisethe referencethis deleter is copy constructed fromu
.'s deleter. After the construction,u
no longer owns a pointer. [Note: The deleter constructor can be implemented withstd::forward<
. — end note]DE>
Change 20.3.1.3.4 [unique.ptr.single.asgn], p1-3
unique_ptr& operator=(unique_ptr&& u);-1- Requires: If the deleter
D
is not a reference type,Aassignment of the deleterD
from an rvalueD
shall not throw an exception. Otherwise the deleterD
is a reference type, and assignment of the deleterD
from an lvalueD
shall not throw an exception.-2- Effects: reset(u.release()) followed by an
moveassignment fromu
's deleter to this deleterstd::forward<D>(u.get_deleter())
.-3- Postconditions: This
unique_ptr
now owns the pointer whichu
owned, andu
no longer owns it.[Note: IfD
is a reference type, then the referenced lvalue deleters are move assigned. — end note]
Change 20.3.1.3.4 [unique.ptr.single.asgn], p6-7
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);Requires: If the deleter
E
is not a reference type,Aassignment of the deleterD
from an rvalueshall not throw an exception. Otherwise the deleter
DEE
is a reference type, and assignment of the deleterD
from an lvalueE
shall not throw an exception.unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
. [Note: These requirements imply thatT
andU>
are complete types. — end note]Effects:
reset(u.release())
followed by anmoveassignment fromu
's deleter to this deleterstd::forward<E>(u.get_deleter())
.If eitherD
orE
is a reference type, then the referenced lvalue deleter participates in the move assignment.
<cinttypes>
have macro guards?Section: 31.13 [c.files] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The C standard says about <inttypes.h>
:
C++ implementations should define these macros only when
__STDC_FORMAT_MACROS
is defined before<inttypes.h>
is included.
The C standard has a similar note about <stdint.h>
. For <cstdint>
we adopted a "thanks but no thanks" policy and documented that fact in 17.4.1 [cstdint.syn]:
... [Note: The macros defined by
<stdint>
are provided unconditionally. In particular, the symbols__STDC_LIMIT_MACROS
and__STDC_CONSTANT_MACROS
(mentioned in C99 footnotes 219, 220, and 222) play no role in C++. — end note]
I recommend we put a similar note in 31.13 [c.files] regarding <cinttypes>
.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Add to 31.13 [c.files]:
Table 112 describes header
<cinttypes>
. [Note: The macros defined by<cintypes>
are provided unconditionally. In particular, the symbol__STDC_FORMAT_MACROS
(mentioned in C99 footnote 182) plays no role in C++. — end note]
Section: 23.2.2 [container.requirements.general] Status: Resolved Submitter: Rani Sharoni Opened: 2009-02-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Introduction
This proposal is meant to resolve potential regression of the N2800 draft, see next section, and to relax the requirements for containers of types with throwing move constructors.
The basic problem is that some containers operations, like push_back
,
have a strong exception safety
guarantee (i.e. no side effects upon exception) that are not achievable when
throwing move constructors are used since there is no way to guarantee revert
after partial move. For such operations the implementation can at most provide
the basic guarantee (i.e. valid but unpredictable) as it does with multi
copying operations (e.g. range insert).
For example, vector<T>::push_back()
(where T
has a move
constructor) might resize the vector
and move the objects to the new underlying
buffer. If move constructor throws it might
not be possible to recover the throwing object or to move the old objects back to
the original buffer.
The current draft is explicit by disallowing throwing move
for some operations (e.g. vector<>::reserve
) and not clear about other
operations mentioned in 23.2.2 [container.requirements.general]/10
(e.g. single element insert
): it guarantees strong exception
safety without explicitly disallowing a throwing move constructor.
Regression
This section only refers to cases in which the contained object is by itself a standard container.
Move constructors of standard containers are allowed to throw and therefore existing operations are broken, compared with C++03, due to move optimization. (In fact existing implementations like Dinkumware are actually throwing).
For example, vector< list<int> >::reserve
yields
undefined behavior since list<int>
's move constructor is allowed to throw.
On the other hand, the same operation has strong exception safety guarantee in
C++03.
There are few options to solve this regression:
Option 1 is suggested by proposal N2815 but it might not be applicable for existing implementations for which containers default constructors are throwing.
Option 2 limits the usage significantly and it's error prone
by allowing zombie objects that are nothing but destructible (e.g. no clear()
is allowed after move). It also potentially complicates the implementation by
introducing special state.
Option 3 is possible, for example, using default
construction and swap
instead of move for standard containers case. The
implementation is also free to provide special hidden operation for non
throwing move without forcing the user the cope with the limitation of option-2
when using the public move.
Option 4 impact the efficiency in all use cases due to rare throwing move.
The proposed wording will imply option 1 or 3 though option 2 is also achievable using more wording. I personally oppose to option 2 that has impact on usability.
Relaxation for user types
Disallowing throwing move constructors in general seems very restrictive
since, for example, common implementation of move will be default construction
+ swap
so move will throw if the
default constructor will throw. This is currently the case with the Dinkumware
implementation of node based containers (e.g. std::list
)
though this section doesn't refer to standard types.
For throwing move constructors it seem that the implementation should have no problems to provide the basic guarantee instead of the strong one. It's better to allow throwing move constructors with basic guarantee than to disallow it silently (compile and run), via undefined behavior.
There might still be cases in which the relaxation will break existing generic code that assumes the strong guarantee but it's broken either way given a throwing move constructor since this is not a preserving optimization.
[ Batavia (2009-05): ]
Bjarne comments (referring to his draft paper): "I believe that my suggestion simply solves that. Thus, we don't need a throwing move."
Move to Open and recommend it be deferred until after the next Committee Draft is issued.
[ 2009-10 Santa Cruz: ]
Should wait to get direction from Dave/Rani (N2983).
[ 2010-03-28 Daniel updated wording to sync with N3092. ]
The suggested change of 23.3.5.4 [deque.modifiers]/2 should be removed, because the current wording does say more general things:
2 Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
there are no effects. If an exception is thrown by the move constructor of a non-CopyConstructibleT
, the effects are unspecified.The suggested change of 23.3.13.3 [vector.capacity]/2 should be removed, because the current wording does say more general things:
2 Effects: A directive that informs a
vector
of a planned change in size, so that it can manage the storage allocation accordingly. Afterreserve()
,capacity()
is greater or equal to the argument ofreserve
if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
. If an exception is thrown other than by the move constructor of a non-CopyConstructible
type, there are no effects.
[2011-03-15: Daniel updates wording to sync with N3242 and comments]
The issue has nearly been resolved by previous changes to the working paper, in particular all suggested changes for
deque
andvector
are no longer necessary. The still remaining parts involve the unordered associative containers.
[2011-03-24 Madrid meeting]
It looks like this issue has been resolved already by noexcept
paper N3050
Rationale:
Resolved by N3050
Proposed resolution:
23.2.2 [container.requirements.general] paragraph 10 add footnote:
-10- Unless otherwise specified (see 23.2.7.2 [associative.reqmts.except], 23.2.8.2 [unord.req.except], 23.3.5.4 [deque.modifiers], and 23.3.13.5 [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:
- …
[Note: for compatibility with C++ 2003, when "no effect" is required, standard containers should not use the
value_type
's throwing move constructor when the contained object is by itself a standard container. — end note]
23.2.8.2 [unord.req.except] change paragraph 2+4 to say:
-2- For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an
[…] -4- For unordered associative containers, if an exception is thrown from within ainsert()
function inserting a single element, theinsert()
function has no effect unless the exception is thrown by the contained object move constructor.rehash()
function other than by the container's hash function or comparison function, therehash()
function has no effect unless the exception is thrown by the contained object move constructor.
Keep 23.3.5.4 [deque.modifiers] paragraph 2 unchanged [Drafting note: The originally proposed wording did suggest to add a last sentence as follows:
If an exception is thrown by
push_back()
oremplace_back()
function, that function has no effects unless the exception is thrown by the move constructor ofT
.
— end drafting note ]
-2- Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertableT
, the effects are unspecified.
Keep 23.3.13.3 [vector.capacity] paragraph 2 unchanged [Drafting note: The originally proposed wording did suggest to change the last sentence as follows:
If an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.
— end drafting note ]
-2- Effects: A directive that informs a
vector
of a planned change in size, so that it can manage the storage allocation accordingly. Afterreserve()
,capacity()
is greater or equal to the argument ofreserve
if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
. If an exception is thrown other than by the move constructor of a non-CopyInsertable
type, there are no effects.
Keep 23.3.13.3 [vector.capacity] paragraph 12 unchanged [Drafting note: The originally proposed wording did suggest to change the old paragraph as follows:
-12- Requires:
IfIf an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.value_type
has a move constructor, that constructor shall not throw any exceptions.
— end drafting note ]
-12- Requires: If an exception is thrown other than by the move constructor of a non-
CopyInsertable
T
there are no effects.
Keep 23.3.13.5 [vector.modifiers] paragraph 1 unchanged [Drafting note: The originally proposed wording did suggest to change the old paragraph as follows:
-1-
Requires: IfRemarks: If an exception is thrown byvalue_type
has a move constructor, that constructor shall not throw any exceptions.push_back()
oremplace_back()
function, that function has no effect unless the exception is thrown by the move constructor ofT
.
— end drafting note ]
-1- Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
or by anyInputIterator
operation there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertable
T
, the effects are unspecified.
try_lock
contradictionSection: 32.6.6 [thread.lock.algorithm] Status: C++11 Submitter: Chris Fairles Opened: 2009-02-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 32.6.6 [thread.lock.algorithm], the generic try_lock
effects (p2) say that a failed
try_lock
is when it either returns false
or throws an exception. In
the event a call to try_lock
does fail, by either returning false
or
throwing an exception, it states that unlock
shall be called for all
prior arguments. Then the returns clause (p3) goes on to state
in a note that after returning, either all locks are locked or none
will be. So what happens if multiple locks fail on try_lock
?
Example:
#include <mutex> int main() { std::mutex m0, m1, m2; std::unique_lock<std::mutex> l0(m0, std::defer_lock); std::unique_lock<std::mutex> l1(m1); //throws on try_lock std::unique_lock<std::mutex> l2(m2); //throws on try_lock int result = std::try_lock(l0, l1, l2); assert( !l0.owns_lock() ); assert( l1.owns_lock() ); //?? assert( l2.owns_lock() ); //?? }
The first lock's try_lock
succeeded but, being a prior argument to a
lock whose try_lock
failed, it gets unlocked as per the effects clause
of 32.6.6 [thread.lock.algorithm]. However, 2 locks remain locked in this case but the return
clause states that either all arguments shall be locked or none will
be. This seems to be a contradiction unless the intent is for
implementations to make an effort to unlock not only prior arguments,
but the one that failed and those that come after as well. Shouldn't
the note only apply to the arguments that were successfully locked?
Further discussion and possible resolutions in c++std-lib-23049.
[ Summit: ]
Move to review. Agree with proposed resolution.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 32.6.6 [thread.lock.algorithm], p2:
-2- Effects: Calls
try_lock()
for each argument in order beginning with the first until all arguments have been processed or a call totry_lock()
fails, either by returningfalse
or by throwing an exception. If a call totry_lock()
fails,unlock()
shall be called for all prior arguments and there shall be no further calls totry_lock()
.
Delete the note from 32.6.6 [thread.lock.algorithm], p3
-3- Returns: -1 if all calls to
try_lock()
returnedtrue
, otherwise a 0-based index value that indicates the argument for whichtry_lock()
returnedfalse
.[Note: On return, either all arguments will be locked or none will be locked. -- end note]
reference_wrapper
and function typesSection: 22.10.6 [refwrap] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis in 22.10.6 [refwrap] says:
template <ObjectType T> class reference_wrapper ...
And then paragraph 3 says:
The template instantiation
reference_wrapper<T>
shall be derived fromstd::unary_function<T1, R>
only if the typeT
is any of the following:
- a function type or a pointer to function type taking one argument of type
T1
and returningR
But function types are not ObjectType
s.
Paragraph 4 contains the same contradiction.
[ Post Summit: ]
Jens: restricted reference to ObjectType
Recommend Review.
[ Post Summit, Peter adds: ]
In https://svn.boost.org/trac/boost/ticket/1846 however Eric Niebler makes the very reasonable point that
reference_wrapper<F>
, whereF
is a function type, represents a reference to a function, a legitimate entity. Soboost::ref
was changed to allow it.http://svn.boost.org/svn/boost/trunk/libs/bind/test/ref_fn_test.cpp
Therefore, I believe an alternative proposed resolution for issue 987 could simply allow
reference_wrapper
to be used with function types.
[ Post Summit, Howard adds: ]
I agree with Peter (and Eric). I got this one wrong on my first try. Here is code that demonstrates how easy (and useful) it is to instantiate
reference_wrapper
with a function type:#include <functional> template <class F> void test(F f); void f() {} int main() { test(std::ref(f)); }Output (link time error shows type of
reference_wrapper
instantiated with function type):Undefined symbols: "void test<std::reference_wrapper<void ()()> >(std::reference_wrapper<void ()()>)",...I've taken the liberty of changing the proposed wording to allow function types and set to Open. I'll also freely admit that I'm not positive
ReferentType
is the correct concept.
[ Batavia (2009-05): ]
Howard observed that
FunctionType
, a concept not (yet?) in the Working Paper, is likely the correct constraint to be applied. However, the proposed resolution provides an adequate approximation.Move to Review.
[ 2009-05-23 Alisdair adds: ]
By constraining to
PointeeType
we rule out the ability forT
to be a reference, and call in reference-collapsing. I'm not sure if this is correct and intended, but would like to be sure the case was considered.Is dis-allowing reference types and the implied reference collapsing the intended result?
[ 2009-07 Frankfurt ]
Moved from Review to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-10-14 Daniel provided de-conceptified wording. ]
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
Change 22.10.6 [refwrap]/1 as indicated:
reference_wrapper<T>
is aCopyConstructible
andCopyAssignable
wrapper around a reference to an object or function of typeT
.
monotonic_clock::is_monotonic
must be true
Section: 99 [time.clock.monotonic] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is some confusion over what the value of monotonic_clock::is_monotonic
when monotonic_clock
is a synonym for system_clock
. The
intent is that if monotonic_clock
exists, then monotonic_clock::is_monotonic
is true
.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
Change 99 [time.clock.monotonic], p1:
-1- Objects of class
monotonic_clock
represent clocks for which values oftime_point
never decrease as physical time advances.monotonic_clock
may be a synonym forsystem_clock
if and only ifsystem_clock::is_monotonic
istrue
.
wstring_convert
Section: 99 [depr.conversions.string] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
Addresses JP-50 [CD1]
Add custom allocator parameter to wstring_convert
, since we cannot
allocate memory for strings from a custom allocator.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change [conversions.string]:
template<class Codecvt, class Elem = wchar_t, class Wide_alloc = std::allocator<Elem>, class Byte_alloc = std::allocator<char> > class wstring_convert { public: typedef std::basic_string<char, char_traits<char>, Byte_alloc> byte_string; typedef std::basic_string<Elem, char_traits<Elem>, Wide_alloc> wide_string; ...
Change [conversions.string], p3:
-3- The class template describes an ob ject that controls conversions between wide string ob jects of class
std::basic_string<Elem, char_traits<Elem>, Wide_alloc>
and byte string objects of classstd::basic_string<char, char_traits<char>, Byte_alloc>
(also known as.std::string
)
_Exit
needs better specificationSection: 17.5 [support.start.term] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK-188 [CD1]
The function _Exit
does not appear to be defined in this standard.
Should it be added to the table of functions included-by-reference to
the C standard?
[ 2009-05-09 Alisdair fixed some minor issues in the wording. ]
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Add to 17.5 [support.start.term] Table 20 (Header
<cstdlib>
synopsis) Functions:
_Exit
Add before the description of abort(void)
:
void _Exit [[noreturn]] (int status)The function
_Exit(int status)
has additional behavior in this International Standard:
- The program is terminated without executing destructors for objects of automatic, thread, or static storage duration and without calling the functions passed to
atexit()
(6.9.3.4 [basic.start.term]).
quick_exit
should terminate well-definedSection: 17.6.4.3 [new.handler] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK-193 [CD1]
quick_exit
has been added as a new valid way to terminate a program in a
well defined way.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 17.6.4.3 [new.handler], p2:
-2- Required behavior: ...
- ...
call eitherterminate execution of the program without returning to the callerabort()
orexit();
Section: 16.3.2.4 [structure.specifications] Status: C++11 Submitter: Thomas Plum Opened: 2009-03-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK-163 [CD1]
Many functions are defined as "Effects: Equivalent to a...", which seems to also define the preconditions, effects, etc. But this is not made clear.
After studying the occurrences of "Effects: Equivalent to", I agree with the diagnosis but disagree with the solution. In 27.4.3.3 [string.cons] we find
14 Effects: If
InputIterator
is an integral type, equivalent tobasic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)
15 Otherwise constructs a string from the values in the range
[begin, end)
, as indicated in the Sequence Requirements table (see 23.1.3).
This would be devishly difficult to re-write with an explicit "Equivalent to:" clause. Instead, I propose the following, which will result in much less editorial re-work.
[ 2009-05-09 Alisdair adds: ]
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Add a new paragraph after 16.3.2.4 [structure.specifications], p3:
-3- Descriptions of function semantics contain the following elements (as appropriate):154
- Requires: the preconditions for calling the function
- Effects: the actions performed by the function
- Postconditions: the observable results established by the function
- Returns: a description of the value(s) returned by the function
- Throws: any exceptions thrown by the function, and the conditions that would cause the exception
- Complexity: the time and/or space complexity of the function
- Remarks: additional semantic constraints on the function
- Error conditions: the error conditions for error codes reported by the function.
- Notes: non-normative comments about the function
Whenever the Effects element specifies that the semantics of some function
F
are Equivalent to some code-sequence, then the various elements are interpreted as follows. IfF
's semantics specifies a Requires element, then that requirement is logically imposed prior to the equivalent-to semantics. Then, the semantics of the code-sequence are determined by the Requires, Effects, Postconditions, Returns, Throws, Complexity, Remarks, Error Conditions and Notes specified for the (one or more) function invocations contained in the code-sequence. The value returned fromF
is specified byF
's Returns element, or ifF
has no Returns element, a non-void
return fromF
is specified by the Returns elements in code-sequence. IfF
's semantics contains a Throws (or Postconditions, or Complexity) element, then that supersedes any occurrences of that element in the code-sequence.
Section: 20.3.1.3.6 [unique.ptr.single.modifiers] Status: C++11 Submitter: Pavel Minaev Opened: 2009-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Consider the following (simplified) implementation of
std::auto_ptr<T>::reset()
:
void reset(T* newptr = 0) { if (this->ptr && this->ptr != newptr) { delete this->ptr; } this->ptr = newptr; }
Now consider the following code which uses the above implementation:
struct foo { std::auto_ptr<foo> ap; foo() : ap(this) {} void reset() { ap.reset(); } }; int main() { (new foo)->reset(); }
With the above implementation of auto_ptr, this results in U.B. at the point of auto_ptr::reset(). If this isn't obvious yet, let me explain how this goes step by step:
foo::reset()
entered
auto_ptr::reset()
entered
auto_ptr::reset()
tries to delete foo
foo::~foo()
entered, tries to destruct its members
auto_ptr::~auto_ptr()
executed - auto_ptr
is no longer a valid object!
foo::~foo()
left
auto_ptr::reset()
sets its "ptr" field to 0 <- U.B.! auto_ptr
is not a valid object here already!
[
Thanks to Peter Dimov who recognized the connection to unique_ptr
and
brought this to the attention of the LWG, and helped with the solution.
]
[ Howard adds: ]
To fix this behavior
reset
must be specified such that deleting the pointer is the last action to be taken withinreset
.
[ Alisdair adds: ]
The example providing the rationale for LWG 998(i) is poor, as it relies on broken semantics of having two object believing they are unique owners of a single resource. It should not be surprising that UB results from such code, and I feel no need to go out of our way to support such behaviour.
If an example is presented that does not imply multiple ownership of a unique resource, I would be much more ready to accept the proposed resolution.
[ Batavia (2009-05): ]
Howard summarizes:
This issue has to do with circular ownership, and affects
auto_ptr
, too (but we don't really care about that). It is intended to spell out the order in which operations must be performed so as to avoid the possibility of undefined behavior in the self-referential case.Howard points to message c++std-lib-23175 for another example, requested by Alisdair.
We agree with the issue and with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 20.3.1.3.6 [unique.ptr.single.modifiers], p5 (Effects clause for reset
), and p6:
-5- Effects:
IfAssignsget() == nullptr
there are no effects. Otherwiseget_deleter()(get())
.p
to the storedpointer
, and then if the old value of thepointer
is not equal tonullptr
, callsget_deleter()(
the old value of thepointer)
. [Note: The order of these operations is significant because the call toget_deleter()
may destroy*this
. -- end note]-6- Postconditions:
get() == p
. [Note: The postcondition does not hold if the call toget_deleter()
destroys*this
sincethis->get()
is no longer a valid expression. -- end note]
Section: 26.11 [specialized.algorithms] Status: C++11 Submitter: Peter Dimov Opened: 2009-03-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The same fix (reference 987(i)) may be applied to addressof
, which is also constrained to
ObjectType
. (That was why boost::ref
didn't work with functions - it
tried to apply boost::addressof
and the reinterpret_cast<char&>
implementation of addressof
failed.)
[ Batavia (2009-05): ]
We agree.
Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-10-10 Daniel updates wording to concept-free. ]
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
[
The resolution assumes that addressof
is reintroduced as described in
n2946
]
In 26.11 [specialized.algorithms] change as described:
template <class T> T* addressof(T& r);Returns: The actual address of the object or function referenced by
r
, even in the presence of an overloadedoperator&
.
Section: 16.4.5.8 [res.on.functions] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 179
According to the 4th bullet there is a problem if "if any replacement function or handler function or destructor operation throws an exception". There should be no problem throwing exceptions so long as they are caught within the function.
[ Batavia (2009-05): ]
The phrasing "throws an exception" is commonly used elsewhere to mean "throws or propagates an exception." Move to Open pending a possible more general resolution.
[ 2009-07 Frankfurt: ]
Replace "propagates" in the proposed resolution with the phrase "exits via" and move to Ready.
Proposed resolution:
Change the 4th bullet of 16.4.5.8 [res.on.functions], p2:
- if any replacement function or handler function or destructor operation
throwsexits via an exception, unless specifically allowed in the applicable Required behavior: paragraph.
operator delete
in garbage collected implementationSection: 17.6.3 [new.delete] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 190
It is not entirely clear how the current specification acts in the presence of a garbage collected implementation.
[ Summit: ]
Agreed.
[ 2009-05-09 Alisdair adds: ]
Proposed wording is too strict for implementations that do not support garbage collection. Updated wording supplied.
[ Batavia (2009-05): ]
We recommend advancing this to Tentatively Ready with the understanding that it will not be moved for adoption unless and until the proposed resolution to Core issue #853 is adopted.
Proposed resolution:
(Editorial note: This wording ties into the proposed resolution for Core #853)
Add paragraphs to 17.6.3.2 [new.delete.single]:
void operator delete(void* ptr) throw();void operator delete(void* ptr, const std::nothrow_t&) throw();[ The second signature deletion above is editorial. ]
Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-10- ...
void operator delete(void* ptr, const std::nothrow_t&) throw();Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-15- ...
Add paragraphs to 17.6.3.3 [new.delete.array]:
void operator delete[](void* ptr) throw();void operator delete[](void* ptr, const std::nothrow_t&) throw();[ The second signature deletion above is editorial. ]
Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-9- ...
void operator delete[](void* ptr, const std::nothrow_t&) throw();Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-13- ...
Add paragraphs to 17.6.3.4 [new.delete.placement]:
void operator delete(void* ptr, void*) throw();Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-7- ...
void operator delete[](void* ptr, void*) throw();Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-9- ...
next/prev
wrong iterator typeSection: 24.4.3 [iterator.operations] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 271
next/prev
return an incremented iterator without changing the value of
the original iterator. However, even this may invalidate an
InputIterator
. A ForwardIterator
is required to guarantee the
'multipass' property.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-10-14 Daniel provided de-conceptified wording. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Change header <iterator>
synopsis 24.2 [iterator.synopsis] as indicated:
// 24.4.4, iterator operations: ... template <classInputForwardIterator>InputForwardIterator next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);
Change 24.4.3 [iterator.operations] before p.6 as indicated:
template <classInputForwardIterator>InputForwardIterator next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);
reverse_iterator
default ctor should value initializeSection: 24.5.1.4 [reverse.iter.cons] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 277
The default constructor default-initializes current, rather than value-initializes. This means that when Iterator corresponds to a trivial type, the current member is left un-initialized, even when the user explictly requests value intialization! At this point, it is not safe to perform any operations on the reverse_iterator other than assign it a new value or destroy it. Note that this does correspond to the basic definition of a singular iterator.
[ Summit: ]
Agree with option i.
[ Batavia (2009-05): ]
We believe this should be revisited in conjunction with issue 408(i), which nearly duplicates this issue. Move to Open.
[ 2009-07 post-Frankfurt: ]
Change "constructed" to "initialized" in two places in the proposed resolution.
Move to Tentatively Ready.
[ 2009 Santa Cruz: ]
Moved to Ready for this meeting.
Proposed resolution:
Change [reverse.iter.con]:
reverse_iterator();-1- Effects:
DefaultValue initializescurrent
. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on adefault constructedvalue initialized iterator of typeIterator
.
Change [move.iter.op.const]:
move_iterator();-1- Effects: Constructs a
move_iterator
,defaultvalue initializingcurrent
. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value initialized iterator of typeIterator
.
basic_regex
should be created/assigned from initializer listsSection: 28.6.7.2 [re.regex.construct] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 317 and JP 74
UK 317:
basic_string
has both a constructor and an assignment operator that accepts an initializer list,basic_regex
should have the same.
JP 74:
basic_regex & operator= (initializer_list<T>);
is not defined.
[ Batavia (2009-05): ]
UK 317 asks for both assignment and constructor, but the requested constructor is already present in the current Working Paper. We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 28.6.7 [re.regex]:
template <class charT, class traits = regex_traits<charT> > class basic_regex { ... basic_regex& operator=(const charT* ptr); basic_regex& operator=(initializer_list<charT> il); template <class ST, class SA> basic_regex& operator=(const basic_string<charT, ST, SA>& p); ... };
Add in 28.6.7.2 [re.regex.construct]:
-20- ...
basic_regex& operator=(initializer_list<charT> il);-21- Effects: returns
assign(il.begin(), il.end());
integral_constant
objects useable in integral-constant-expressionsSection: 21.3.4 [meta.help] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 205 [CD1]
integral_constant
objects should be usable in integral-constant-expressions.
The addition to the language of literal types and the enhanced rules for
constant expressions make this possible.
[ Batavia (2009-05): ]
We agree that the
static
data member ought be declaredconstexpr
, but do not see a need for the proposedoperator value_type()
. (A use case would be helpful.) Move to Open.
[ 2009-05-23 Alisdair adds: ]
The motivating case in my mind is that we can then use
true_type
andfalse_type
as integral Boolean expressions, for example inside astatic_assert
declaration. In that sense it is purely a matter of style.Note that Boost has applied the non-explicit conversion operator for many years as it has valuable properties for extension into other metaprogramming libraries, such as MPL. If additional rationale is desired I will poll the Boost lists for why this extension was originally applied. I would argue that explicit conversion is more appropriate for 0x though.
[ 2009-07-04 Howard adds: ]
Here's a use case which demonstrates the syntactic niceness which Alisdair describes:
#define requires(...) class = typename std::enable_if<(__VA_ARGS__)>::type template <class T, class U, requires(!is_lvalue_reference<T>() || is_lvalue_reference<T>() && is_lvalue_reference<U>()), requires(is_same<typename base_type<T>::type, typename base_type<U>::type>)> inline T&& forward(U&& t) { return static_cast<T&&>(t); }
[ 2009-07 post-Frankfurt: ]
Move to Tentatively Ready.
[ 2009 Santa Cruz: ]
Moved to Ready for this meeting.
Proposed resolution:
Add to the integral_constant
struct definition in 21.3.4 [meta.help]:
template <class T, T v> struct integral_constant { static constexpr T value = v; typedef T value_type; typedef integral_constant<T,v> type; constexpr operator value_type() { return value; } };
nullptr_t
assignments to unique_ptr
Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 211 [CD1]
The nullptr_t
type was introduced to resolve the null pointer literal
problem. It should be used for the assignment operator, as with the
constructor and elsewhere through the library.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change the synopsis in 20.3.1.3 [unique.ptr.single]:
unique_ptr& operator=(unspecified-pointer-typenullptr_t);
Change 20.3.1.3.4 [unique.ptr.single.asgn]:
unique_ptr& operator=(unspecified-pointer-typenullptr_t);
Assigns from the literal 0 orNULL
. [Note: The unspecified-pointer-type is often implemented as a pointer to a private data member, avoiding many of the implicit conversion pitfalls. — end note]
Section: 99 [depr.util.smartptr.shared.atomic] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2017-11-29
Priority: Not Prioritized
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Discussion:
Addresses JP 44 [CD1]
The 1st parameter p
and 2nd parameter v
is now
shared_ptr<T>*
.
It should be shared_ptr<T>&
, or if these are
shared_ptr<T>*
then add the "p
shall not be a
null pointer" at the requires.
[ Summit: ]
Agree. All of the functions need a requirement that
p
(orv
) is a pointer to a valid object.
[ 2009-07 post-Frankfurt: ]
Lawrence explained that these signatures match the regular atomics. The regular atomics must not use references because these signatures are shared with C. The decision to pass shared_ptrs by pointer rather than by reference was deliberate and was motivated by the principle of least surprise.
Lawrence to write wording that requires that the pointers not be null.
[ 2009-09-20 Lawrence provided wording: ]
The parameter types for atomic shared pointer access were deliberately chosen to be pointers to match the corresponding parameters of the atomics chapter. Those in turn were deliberately chosen to match C functions, which do not have reference parameters.
We adopt the second suggestion, to require that such pointers not be null.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
In section "shared_ptr
atomic access"
[util.smartptr.shared.atomic], add to each function the
following clause.
Requires:
p
shall not be null.
thread::join()
effects?Section: 32.4.3.6 [thread.thread.member] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
While looking at thread::join()
I think I spotted a couple of
possible defects in the specifications. I could not find a previous
issue or NB comment about that, but I might have missed it.
The postconditions clause for thread::join()
is:
Postconditions: If
join()
throws an exception, the value returned byget_id()
is unchanged. Otherwise,get_id() == id()
.
and the throws clause is:
Throws:
std::system_error
when the postconditions cannot be achieved.
Now... how could the postconditions not be achieved?
It's just a matter of resetting the value of get_id()
or leave it
unchanged! I bet we can always do that. Moreover, it's a chicken-and-egg
problem: in order to decide whether to throw or not I depend on the
postconditions, but the postconditions are different in the two cases.
I believe the throws clause should be:
Throws:
std::system_error
when the effects or postconditions cannot be achieved.
as it is in detach()
, or, even better, as the postcondition is
trivially satisfiable and to remove the circular dependency:
Throws:
std::system_error
if the effects cannot be achieved.
Problem is that... ehm... join()
has no "Effects" clause. Is that intentional?
[ See the thread starting at c++std-lib-23204 for more discussion. ]
[ Batavia (2009-05): ]
Pete believes there may be some more general language (in frontmatter) that can address this and related issues such as 962(i).
Move to Open.
[ 2009-11-18 Anthony provides wording. ]
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Edit 32.4.3.6 [thread.thread.member] as indicated:
void join();5 Precondition:
joinable()
istrue
.Effects: Blocks until the thread represented by
*this
has completed.6 Synchronization: The completion of the thread represented by
*this
happens before (6.9.2 [intro.multithread])join()
returns. [Note: Operations on*this
are not synchronized. — end note]7 Postconditions:
IfThe thread represented byjoin()
throws an exception, the value returned byget_id()
is unchanged. Otherwise,*this
has completed.get_id() == id()
.8 ...
Section: 23.2.2 [container.requirements.general] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 222 [CD1]
It is not clear what purpose the Requirement tables serve in the
Containers clause. Are they the definition of a library Container? Or
simply a conventient shorthand to factor common semantics into a single
place, simplifying the description of each subsequent container? This
becomes an issue for 'containers' like array
, which does not meet the
default-construct-to-empty requirement, or forward_list
which does not
support the size operation. Are these components no longer containers?
Does that mean the remaining requirements don't apply? Or are these
contradictions that need fixing, despite being a clear design decision?
Recommend:
Clarify all the tables in 23.2 [container.requirements] are
there as a convenience for documentation, rather than a strict set of
requirements. Containers should be allowed to relax specific
requirements if they call attention to them in their documentation. The
introductory text for array
should be expanded to mention a
default constructed array
is not empty, and
forward_list
introduction should mention it does not provide
the required size
operation as it cannot be implemented
efficiently.
[ Summit: ]
Agree in principle.
[ 2009-07 post-Frankfurt: ]
We agree in principle, but we have a timetable. This group feels that the issue should be closed as NAD unless a proposed resolution is submitted prior to the March 2010 meeting.
[ 2009-10 Santa Cruz: ]
Looked at this and still intend to close as NAD in March 2010 unless there is proposed wording that we like.
[ 2010-02-02 Nicolai M. Josuttis updates proposed wording and adds: ]
I just came across issue #1034 (response to UK 222), which covers the role of container requirements. The reason I found this issue was that I am wondering why
array<>
is specified to be a sequence container. For me, currently, this follows from Sequence containers 23.2.4 [sequence.reqmts] saying:The library provides five basic kinds of sequence containers:
array
,vector
,forward_list
,list
, anddeque
. while later on in Table 94 "Sequence container requirements" are defined.IMO, you can hardly argue that this is NAD. We MUST say somewhere that either array is not a sequence container or does not provide all operations of a sequence container (even not all requirements of a container in general).
Here is the number of requirements
array<>
does not meet (AFAIK):general container requirements:
- a default constructed
array
is not emptyswap
has no constant complexityNote also that
swap
not only has linear complexity it also invalidates iterators (or to be more precise, assigns other values to the elements), which is different from the effect swap has for other containers. For this reason, I must say that i tend to propose to removeswap()
forarrays
.sequence container requirements:
- There is no constructor and assignment for a range
- There is no constructor and assignment for
n
copies oft
- There are no
emplace
,insert
,erase
,clear
,assign
operationsIn fact, out of all sequence container requirements
array<>
only provides the following operations: from sequence requirements (Table 94):X(il); a = il;and from optional requirements (Table 95):
[], at(), front(), back()This is almost nothing!
Note in addition, that due to the fact that
array
is an aggregate and not a container withinitializer_lists
a construction or assignment with an initializer list is valid for all sequence containers but not valid for array:vector<int> v({1,2,3}); // OK v = {4,5,6}; // OK array<int,3> a({1,2,3}); // Error array<int,3> a = {1,2,3}; // OK a = {4,5,6}; // ErrorBTW, for this reason, I am wondering, why
<array>
includes<initializer_list>
.IMO, we can't really say that
array
is a sequence container.array
is special. As the solution to this issue seemed to miss some proposed wording where all could live with, let me try to suggest some.
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: Ok with move to Ready except for "OPEN:" part. ]
Proposed resolution:
In Sequence containers 23.2.4 [sequence.reqmts] modify paragraph 1 as indicated:
1 A sequence container organizes a finite set of objects, all of the same type, into a strictly linear arrangement. The library provides
fivefour basic kinds of sequence containers:array
,vector
,forward_list
,list
, anddeque
. In addition,array
is provided as a sequence container that only provides limited sequence operations because it has a fixed number of elements.ItThe library also provides container adaptors that make it easy to construct abstract data types, such asstack
s orqueue
s, out of the basic sequence container kinds (or out of other kinds of sequence containers that the user might define).
Modify paragraph 2 as follows (just editorial):
2 The
five basicsequence containers offer the programmer different complexity trade-offs and should be used accordingly.vector
orarray
is the type of sequence container that should be used by default.list
orforward_list
should be used when there are frequent insertions and deletions from the middle of the sequence.deque
is the data structure of choice when most insertions and deletions take place at the beginning or at the end of the sequence.
In Class template array 23.3.3 [array] modify paragraph 3 as indicated:
3
Unless otherwise specified, allAn array satisfies all of the requirements of a container and of a reversible container (given in two tables in 23.2 [container.requirements]) except that a default constructedarray
operations are as described in 23.2.array
is not empty,swap
does not have constant complexity, andswap
may throw exceptions. Anarray
satisfies some of the requirements of a sequence container (given in 23.2.4 [sequence.reqmts]). Descriptions are provided here only for operations onarray
that are not describedin that Clausein one of these tables or for operations where there is additional semantic information.
In array specialized algorithms 23.3.3.4 [array.special] add to the
specification of swap()
:
template <class T, size_t N> void swap(array<T,N>& x, array<T,N>& y);1 Effects: ...
Complexity: Linear in
N
.
match_results
as library containerSection: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 232 [CD1]
match_results
may follow the requirements but is not listed a general
purpose library container.
Remove reference to match_results
against a[n]
operation.
[ Summit: ]
Agree.
operator[]
is defined elsewhere.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 23.2.4 [sequence.reqmts] Table 84, remove reference to
match_results
in the row describing the a[n]
operation.
Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 233 [CD1]
Table 84 is missing references to several new container types.
[ Summit: ]
Agree.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 23.2.4 [sequence.reqmts] Table 84, Add reference to listed containers to the following rows:
Table 84 -- Optional sequence container operations Expression Return type Operational semantics Container a.front()
... ... vector, list, deque, basic_string, array, forward_list
a.back()
... ... vector, list, deque, basic_string, array
a.emplace_front(args)
... ... list, deque, forward_list
a.push_front(t)
... ... list, deque, forward_list
a.push_front(rv)
... ... list, deque, forward_list
a.pop_front()
... ... list, deque, forward_list
a[n]
... ... vector, deque, basic_string, array
a.at(n)
... ... vector, deque, basic_string, array
back
function should also support const_iterator
Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 234 [CD1]
The reference to iterator
in semantics for back
should
also allow for const_iterator
when called on a const-qualified
container. This would be ugly to specify in the 03 standard, but is
quite easy with the addition of auto
in this new standard.
[ Summit: ]
Agree.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 23.2.4 [sequence.reqmts] Table 84, replace iterator with auto in semantics for back:
Table 84 — Optional sequence container operations Expression Return type Operational semantics Container a.back()
reference; const_reference
for constanta
{
iteratorauto tmp = a.end();
--tmp;
return *tmp; }vector, list, deque, basic_string
iterator
and const_iterator
Section: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 238 [CD1]
Leaving it unspecified whether or not iterator
and const_iterator
are the
same type is dangerous, as user code may or may not violate the One
Definition Rule by providing overloads for
both types. It is probably too late to specify a single behaviour, but
implementors should document what to expect. Observing that problems can be
avoided by users restricting themselves to using const_iterator
, add a note to that effect.
Suggest Change 'unspecified' to 'implementation defined'.
[ Summit: ]
Agree with issue. Agree with adding the note but not with changing the normative text. We believe the note provides sufficient guidance.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 23.2.7 [associative.reqmts] p6, add:
-6-
iterator
of an associative container meets the requirements of theBidirectionalIterator
concept. For associative containers where the value type is the same as the key type, bothiterator
andconst_iterator
are constant iterators. It is unspecified whether or notiterator
andconst_iterator
are the same type. [Note:iterator
andconst_iterator
have identical semantics in this case, anditerator
is convertible toconst_iterator
. Users can avoid violating the One Definition Rule by always usingconst_iterator
in their function parameter lists -- end note]
Section: 23.2.7 [associative.reqmts] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
Addresses UK 239 [CD1]
It is not possible to take a move-only key out of an unordered
container, such as (multi
)set
or
(multi
)map
, or the new unordered containers.
Add below a.erase(q)
, a.extract(q)
, with the following notation:
a.extract(q)>
, Return type pair<key, iterator>
Extracts the element pointed to by q
and erases it from the
set
. Returns a pair
containing the value pointed to by
q
and an iterator
pointing to the element immediately
following q
prior to the element being erased. If no such
element exists,returns a.end()
.
[ Summit: ]
We look forward to a paper on this topic. We recommend no action until a paper is available. The paper would need to address exception safety.
[ Post Summit Alisdair adds: ]
Would
value_type
be a better return type thankey_type
?
[ 2009-07 post-Frankfurt: ]
Leave Open. Alisdair to contact Chris Jefferson about this.
[ 2009-09-20 Howard adds: ]
See the 2009-09-19 comment of 839(i) for an API which accomplishes this functionality and also addresses several other use cases which this proposal does not.
[ 2009-10 Santa Cruz: ]
Mark as NAD Future. No consensus to make the change at this time.
Original resolution [SUPERSEDED]:
In 23.2.7 [associative.reqmts] Table 85, add:
Table 85 -- Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity a.erase(q)
... ... ... a.extract(q)
pair<key_type, iterator>
Extracts the element pointed to by q
and erases it from theset
. Returns apair
containing the value pointed to byq
and aniterator
pointing to the element immediately followingq
prior to the element being erased. If no such element exists, returnsa.end()
.amortized constant In 23.2.8 [unord.req] Table 87, add:
Table 87 -- Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity a.erase(q)
... ... ... a.extract(q)
pair<key_type, iterator>
Extracts the element pointed to by q
and erases it from theset
. Returns apair
containing the value pointed to byq
and aniterator
pointing to the element immediately followingq
prior to the element being erased. If no such element exists, returnsa.end()
.amortized constant
[08-2016, Post-Chicago]
Move to Tentatively Resolved
Proposed resolution:
This functionality is provided by P0083R3
compare_exchange
is not a read-modify-write operationSection: 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US 91 [CD1]
It is unclear whether or not a failed compare_exchange
is a RMW operation
(as used in 6.9.2 [intro.multithread]).
Suggested solution:
Make failing compare_exchange
operations not be RMW.
[ Anthony Williams adds: ]
In 32.5.8.2 [atomics.types.operations] p18 it says that "These operations are atomic read-modify-write operations" (final sentence). This is overly restrictive on the implementations of
compare_exchange_weak
andcompare_exchange_strong
on platforms without a native CAS instruction.
[ Summit: ]
Group agrees with the resolution as proposed by Anthony Williams in the attached note.
[ Batavia (2009-05): ]
We recommend the proposed resolution be reviewed by members of the Concurrency Subgroup.
[ 2009-07 post-Frankfurt: ]
This is likely to be addressed by Lawrence's upcoming paper. He will adopt the proposed resolution.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Change 32.5.8.2 [atomics.types.operations] p18:
-18- Effects: Atomically, compares the value pointed to by
object
or bythis
for equality with that inexpected
, and if true, replaces the value pointed to byobject
or bythis
with desired, and if false, updates the value inexpected
with the value pointed to byobject
or bythis
. Further, if the comparison is true, memory is affected according to the value ofsuccess
, and if the comparison is false, memory is affected according to the value offailure
. When only onememory_order
argument is supplied, the value ofsuccess
isorder
, and the value offailure
isorder
except that a value ofmemory_order_acq_rel
shall be replaced by the valuememory_order_acquire
and a value ofmemory_order_release
shall be replaced by the valuememory_order_relaxed
. If the comparison istrue
,Tthese operations are atomic read-modify-write operations (1.10). If the comparison isfalse
, these operations are atomic load operations.
constexpr
literalsSection: 32.6 [thread.mutex] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.mutex].
View all issues with C++11 status.
Discussion:
Addresses UK 325 [CD1]
We believe constexpr
literal values should be a more natural expression
of empty tag types than extern objects as it should improve the
compiler's ability to optimize the empty object away completely.
[ Summit: ]
Move to review. The current specification is a "hack", and the proposed specification is a better "hack".
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change the synopsis in 32.6 [thread.mutex]:
struct defer_lock_t {}; struct try_to_lock_t {}; struct adopt_lock_t {};externconstexpr defer_lock_t defer_lock {};externconstexpr try_to_lock_t try_to_lock {};externconstexpr adopt_lock_t adopt_lock {};
unique_lock
constructorSection: 32.6.5.4.2 [thread.lock.unique.cons] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.lock.unique.cons].
View all issues with C++11 status.
Discussion:
Addresses UK 326 [CD1]
The precondition that the mutex is not owned by this thread offers introduces the risk of unnecessary undefined behaviour into the program. The only time it matters whether the current thread owns the mutex is in the lock operation, and that will happen subsequent to construction in this case. The lock operation has the identical pre-condition, so there is nothing gained by asserting that precondition earlier and denying the program the right to get into a valid state before calling lock.
[ Summit: ]
Agree, move to review.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Strike 32.6.5.4.2 [thread.lock.unique.cons] p7:
unique_lock(mutex_type& m, defer_lock_t);
-7- Precondition: Ifmutex_type
is not a recursive mutex the calling thread does not own the mutex.
Section: 32.10 [futures] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures].
View all issues with Resolved status.
Discussion:
Addresses UK 329 [CD1]
future
, promise
and packaged_task
provide a
framework for creating future values, but a simple function to tie all
three components together is missing. Note that we only need a simple
facility for C++0x. Advanced thread pools are to be left for TR2.
Simple Proposal:
Provide a simple function along the lines of:
template< typename F, typename ... Args > requires Callable< F, Args... > future< Callable::result_type > async( F&& f, Args && ... );
Semantics are similar to creating a thread
object with a packaged_task
invoking f
with forward<Args>(args...)
but details are left unspecified to allow different scheduling and thread
spawning implementations.
It is unspecified whether a task submitted to async is run on its own thread
or a thread previously used for another async task. If a call to async
succeeds, it shall be safe to wait for it from any thread.
The state of thread_local
variables shall be preserved during async
calls.
No two incomplete async tasks shall see the same value of
this_thread::get_id()
.
[Note: this effectively forces new tasks to be run on a new thread, or a
fixed-size pool with no queue. If the
library is unable to spawn a new thread or there are no free worker threads
then the async
call should fail. --end note]
[ Summit: ]
The concurrency subgroup has revisited this issue and decided that it could be considered a defect according to the Kona compromise. A task group was formed lead by Lawrence Crowl and Bjarne Stroustrup to write a paper for Frankfort proposing a simple asynchronous launch facility returning a
future
. It was agreed that the callable must be run on a separate thread from the caller, but not necessarily a brand-new thread. The proposal might or might not allow for an implementation that uses fixed-size or unlimited thread pools.Bjarne in c++std-lib-23121: I think that what we agreed was that to avoid deadlock
async()
would almost certainly be specified to launch in a different thread from the thread that executedasync()
, but I don't think it was a specific design constraint.
[ 2009-10 Santa Cruz: ]
Proposed resolution: see N2996 (Herb's and Lawrence's paper on Async). Move state to
NAD editorialResolved.
Proposed resolution:
get()
blocks when not readySection: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with Resolved status.
Discussion:
Addresses UK 334 [CD1]
Behaviour of get()
is undefined if calling get()
while
not is_ready()
. The intent is that get()
is a blocking
call, and will wait for the future to become ready.
[ Summit: ]
Agree, move to Review.
[ 2009-04-03 Thomas J. Gritzan adds: ]
This issue also applies to
shared_future::get()
.Suggested wording:
Add a paragraph to [futures.shared_future]:
void shared_future<void>::get() const;Effects: If
is_ready()
would returnfalse
, block on the asynchronous result associated with*this
.
[ Batavia (2009-05): ]
It is not clear to us that this is an issue, because the proposed resolution's Effects clause seems to duplicate information already present in the Synchronization clause. Keep in Review status.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
Add a paragraph to [futures.unique_future]:
R&& unique_future::get(); R& unique_future<R&>::get(); void unique_future<void>::get();Note:...
Effects: If
is_ready()
would returnfalse
, block on the asynchronous result associated with*this
.Synchronization: if
*this
is associated with apromise
object, the completion ofset_value()
orset_exception()
to thatpromise
happens before (1.10)get()
returns.
std::unique_future
Section: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with Resolved status.
Discussion:
Addresses UK 335 [CD1]
std::unique_future
is MoveConstructible
, so you can transfer the
association with an asynchronous result from one instance to another.
However, there is no way to determine whether or not an instance has
been moved from, and therefore whether or not it is safe to wait for it.
std::promise<int> p;
std::unique_future<int> uf(p.get_future());
std::unique_future<int> uf2(std::move(uf));
uf.wait(); // oops, uf has no result to wait for.
Suggest we add a waitable()
function to unique_future
(and shared_future
) akin to std::thread::joinable()
,
which returns true
if there is an associated result to wait for
(whether or not it is ready).
Then we can say:
if(uf.waitable()) uf.wait();
[ Summit: ]
Create an issue. Requires input from Howard. Probably NAD.
[ Post Summit, Howard throws in his two cents: ]
Here is a copy/paste of my last prototype of
unique_future
which was several years ago. At that time I was callingunique_future
future
:template <class R> class future { public: typedef R result_type; private: future(const future&);// = delete; future& operator=(const future&);// = delete; template <class R1, class F1> friend class prommise; public: future(); ~future(); future(future&& f); future& operator=(future&& f); void swap(future&& f); bool joinable() const; bool is_normal() const; bool is_exceptional() const; bool is_ready() const; R get(); void join(); template <class ElapsedTime> bool timed_join(const ElapsedTime&); };
shared_future
had a similar interface. I intentionally reused thethread
interface where possible to lessen the learning curve std::lib clients will be faced with.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
promise
invertedSection: 32.10.6 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
View all issues with Resolved status.
Discussion:
Addresses UK 339 [CD1]
Move assignment is going in the wrong direction, assigning from
*this
to the passed rvalue, and then returning a reference to
an unusable *this
.
[ Summit: ]
Agree, move to Review.
[ Batavia (2009-05): ]
We recommend deferring this issue until after Detlef's paper (on futures) has been issued.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
Strike 32.10.6 [futures.promise] p6 and change p7:
promise& operator=(promise&& rhs);
-6- Effects: move assigns its associated state torhs
.-7- Postcondition:
associated state of*this
has no associated state.*this
is the same as the associated state ofrhs
before the call.rhs
has no associated state.
get_future()
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
View all issues with Resolved status.
Discussion:
Addresses UK 340 [CD1]
There is an implied postcondition for get_future()
that the state of the
promise
is transferred into the future
leaving the promise
with no
associated state. It should be spelled out.
[ Summit: ]
Agree, move to Review.
[ 2009-04-03 Thomas J. Gritzan adds: ]
promise::get_future()
must not invalidate the state of the promise object.A promise is used like this:
promise<int> p; unique_future<int> f = p.get_future(); // post 'p' to a thread that calculates a value // use 'f' to retrieve the value.So
get_future()
must return an object that shares the same associated state with*this
.But still, this function should throw an
future_already_retrieved
error when it is called twice.
packaged_task::get_future()
throwsstd::bad_function_call
if itsfuture
was already retrieved. It should throwfuture_error(future_already_retrieved)
, too.Suggested resolution:
Replace p12/p13 32.10.6 [futures.promise]:
-12- Throws:
future_error
ifthe*this
has no associated statefuture
has already been retrieved.-13- Error conditions:
future_already_retrieved
ifthe*this
has no associated statefuture
associated with the associated state has already been retrieved.Postcondition: The returned object and
*this
share the associated state.Replace p14 32.10.10 [futures.task]:
-14- Throws:
if the future
std::bad_function_callfuture_errorassociated with the taskhas already been retrieved.Error conditions:
future_already_retrieved
if thefuture
associated with the task has already been retrieved.Postcondition: The returned object and
*this
share the associated task.
[ Batavia (2009-05): ]
Keep in Review status pending Detlef's forthcoming paper on futures.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
Add after p13 32.10.6 [futures.promise]:
unique_future<R> get_future();-13- ...
Postcondition:
*this
has no associated state.
reverse_iterator::operator->
should also support smart pointersSection: 24.5.1.6 [reverse.iter.elem] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [reverse.iter.elem].
View all issues with Resolved status.
Duplicate of: 2775
Discussion:
Addresses UK 281 [CD1]
The current specification for return value for reverse_iterator::operator->
will always be a true pointer type, but reverse_iterator
supports proxy
iterators where the pointer type may be some kind of 'smart pointer'.
[ Summit: ]
move_iterator
avoids this problem by returning a value of the wrapped Iterator type. study group formed to come up with a suggested resolution.
move_iterator
solution shown in proposed wording.
[ 2009-07 post-Frankfurt: ]
Howard to deconceptize. Move to Review after that happens.
[ 2009-08-01 Howard deconceptized: ]
[ 2009-10 Santa Cruz: ]
We can't think of any reason we can't just define reverse iterator's pointer types to be the same as the underlying iterator's pointer type, and get it by calling the right arrow directly.
Here is the proposed wording that was replaced:
template <class Iterator> class reverse_iterator { […] typedeftypename iterator_traits<Iterator>::pointerpointer;Change [reverse.iter.opref]:
pointer operator->() const;Returns:
&(operator*());this->tmp = current; --this->tmp; return this->tmp;
[ 2010-03-03 Daniel opens: ]
- There is a minor problem with the exposition-only declaration of the private member
deref_tmp
which is modified in a const member function (and the same problem occurs in the specification ofoperator*
). The fix is to make it a mutable member.The more severe problem is that the resolution for some reasons does not explain in the rationale why it was decided to differ from the suggested fix (using
deref_tmp
instead oftmp
) in the [ 2009-10 Santa Cruz] comment:this->deref_tmp = current; --this->deref_tmp; return this->deref_tmp;combined with the change of
typedef typename iterator_traits<Iterator>::pointer pointer;to
typedef Iterator pointer;The problem of the agreed on wording is that the following rather typical example, that compiled with the wording before 1052 had been applied, won't compile anymore:
#include <iterator> #include <utility> int main() { typedef std::pair<int, double> P; P op; std::reverse_iterator<P*> ri(&op + 1); ri->first; // Error }Comeau online returns (if a correspondingly changed
reverse_iterator
is used):"error: expression must have class type return deref_tmp.operator->(); ^ detected during instantiation of "Iterator reverse_iterator<Iterator>::operator->() const [with Iterator=std::pair<int, double> *]""Thus the change will break valid, existing code based on
std::reverse_iterator
.IMO the suggestion proposed in the comment is a necessary fix, which harmonizes with the similar specification of
std::move_iterator
and properly reflects the recursive nature of the evaluation ofoperator->
overloads.Suggested resolution:
In the class template
reverse_iterator
synopsis of 24.5.1.2 [reverse.iterator] change as indicated:namespace std { template <class Iterator> class reverse_iterator : public iterator<typename iterator_traits<Iterator>::iterator_category, typename iterator_traits<Iterator>::value_type, typename iterator_traits<Iterator>::difference_type,typename iterator_traits<Iterator>::pointer, typename iterator_traits<Iterator>::reference> { public: [..] typedeftypename iterator_traits<Iterator>::pointerpointer; [..] protected: Iterator current; private: mutable Iterator deref_tmp; // exposition only };- Change [reverse.iter.opref]/1 as indicated:
pointer operator->() const;1
ReturnsEffects:&(operator*())
.deref_tmp = current; --deref_tmp; return deref_tmp;
[ 2010 Pittsburgh: ]
We prefer to make to use a local variable instead of
deref_tmp
withinoperator->()
. And although this means that themutable
change is no longer needed, we prefer to keep it because it is needed foroperator*()
anyway.Here is the proposed wording that was replaced:
Change [reverse.iter.opref]:
pointer operator->() const;Returns:
&(operator*());deref_tmp = current; --deref_tmp; return deref_tmp::operator->();
[ 2010-03-10 Howard adds: ]
Here are three tests that the current proposed wording passes, and no other solution I've seen passes all three:
Proxy pointer support:
#include <iterator> #include <cassert> struct X { int m; }; X x; struct IterX { typedef std::bidirectional_iterator_tag iterator_category; typedef X& reference; struct pointer { pointer(X& v) : value(v) {} X& value; X* operator->() const {return &value;} }; typedef std::ptrdiff_t difference_type; typedef X value_type; // additional iterator requirements not important for this issue reference operator*() const { return x; } pointer operator->() const { return pointer(x); } IterX& operator--() {return *this;} }; int main() { std::reverse_iterator<IterX> ix; assert(&ix->m == &(*ix).m); }Raw pointer support:
#include <iterator> #include <utility> int main() { typedef std::pair<int, double> P; P op; std::reverse_iterator<P*> ri(&op + 1); ri->first; // Error }Caching iterator support:
#include <iterator> #include <cassert> struct X { int m; }; struct IterX { typedef std::bidirectional_iterator_tag iterator_category; typedef X& reference; typedef X* pointer; typedef std::ptrdiff_t difference_type; typedef X value_type; // additional iterator requirements not important for this issue reference operator*() const { return value; } pointer operator->() const { return &value; } IterX& operator--() {return *this;} private: mutable X value; }; int main() { std::reverse_iterator<IterX> ix; assert(&ix->m == &(*ix).m); }
[ 2010 Pittsburgh: ]
Moved to NAD Future, rationale added.
[LEWG Kona 2017]
Recommend that we accept the "Alternate Proposed Resolution" from 2775(i).
[ Original rationale: ]
The LWG did not reach a consensus for a change to the WP.
Previous resolution [SUPERSEDED]:
In the class template
reverse_iterator
synopsis of 24.5.1.2 [reverse.iterator] change as indicated:namespace std { template <class Iterator> class reverse_iterator : public iterator<typename iterator_traits<Iterator>::iterator_category, typename iterator_traits<Iterator>::value_type, typename iterator_traits<Iterator>::difference_type,typename iterator_traits<Iterator&>::pointer, typename iterator_traits<Iterator>::reference> { public: [..] typedeftypename iterator_traits<Iterator&>::pointerpointer; [..] protected: Iterator current; private: mutable Iterator deref_tmp; // exposition only };- Change [reverse.iter.opref]/1 as indicated:
pointer operator->() const;1
ReturnsEffects:&(operator*())
.deref_tmp = current; --deref_tmp; return deref_tmp;[Alternate Proposed Resolution from 2775(i), which was closed as a dup of this issue]
This wording is relative to N4606.
Modify [reverse.iter.opref] as indicated:
constexpr pointer operator->() const;-1-
Returns:Effects: Ifaddressof(operator*())
.Iterator
is a pointer type, as if by:Iterator tmp = current; return --tmp;Otherwise, as if by:
Iterator tmp = current; --tmp; return tmp.operator->();
[2018-08-06; Daniel rebases to current working draft and reduces the proposed wording to that preferred by LEWG]
[2018-11-13; Casey Carter comments]
The acceptance of P0896R4 during the San Diego meeting resolves this
issue: The wording in [reverse.iter.elem] for operator->
is equivalent to the PR for LWG 1052.
Proposed resolution:
This wording is relative to N4762.
Modify 24.5.1.6 [reverse.iter.elem] as indicated:
constexpr pointer operator->() const;-2-
Returns:Effects: Ifaddressof(operator*())
.Iterator
is a pointer type, as if by:Iterator tmp = current; return --tmp;Otherwise, as if by:
Iterator tmp = current; --tmp; return tmp.operator->();
forward
brokenSection: 22.2.4 [forward] Status: Resolved Submitter: Howard Hinnant Opened: 2009-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [forward].
View all issues with Resolved status.
Discussion:
This is a placeholder issue to track the fact that we (well I) put the standard into an inconsistent state by requesting that we accept N2844 except for the proposed changes to [forward].
There will exist in the post meeting mailing N2835 which in its current state reflects the state of affairs prior to the Summit meeting. I hope to update it in time for the post Summit mailing, but as I write this issue I have not done so yet.
[ Batavia (2009-05): ]
Move to Open, awaiting the promised paper.
[ 2009-08-02 Howard adds: ]
My current preferred solution is:
template <class T> struct __base_type { typedef typename remove_cv<typename remove_reference<T>::type>::type type; }; template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type, class = typename enable_if< is_same<typename __base_type<T>::type, typename __base_type<U>::type>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }This has been tested by Bill, Jason and myself.
It allows the following lvalue/rvalue casts:
- Cast an lvalue
t
to an lvalueT
(identity).- Cast an lvalue
t
to an rvalueT
.- Cast an rvalue
t
to an rvalueT
(identity).It disallows:
- Cast an rvalue
t
to an lvalueT
.- Cast one type
t
to another typeT
(such asint
todouble
)."a." is disallowed as it can easily lead to dangling references. "b." is disallowed as this function is meant to only change the lvalue/rvalue characteristic of an expression.
Jason has expressed concern that "b." is not dangerous and is useful in contexts where you want to "forward" a derived type as a base type. I find this use case neither dangerous, nor compelling. I.e. I could live with or without the "b." constraint. Without it, forward would look like:
template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }Or possibly:
template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type, class = typename enable_if< is_base_of<typename __base_type<U>::type, typename __base_type<T>::type>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }The "promised paper" is not in the post-Frankfurt mailing only because I'm waiting for the non-concepts draft. But I'm hoping that by adding this information here I can keep people up to date.
[ 2009-08-02 David adds: ]
forward
was originally designed to do one thing: perfect forwarding. That is, inside a function template whose actual argument can be a const or non-const lvalue or rvalue, restore the original "rvalue-ness" of the actual argument:template <class T> void f(T&& x) { // x is an lvalue here. If the actual argument to f was an // rvalue, pass static_cast<T&&>(x) to g; otherwise, pass x. g( forward<T>(x) ); }Attempting to engineer
forward
to accomodate uses other than perfect forwarding dilutes its idiomatic meaning. The solution proposed here declares thatforward<T>(x)
means nothing more thanstatic_cast<T&&>(x)
, with a patchwork of restrictions on whatT
andx
can be that can't be expressed in simple English.I would be happy with either of two approaches, whose code I hope (but can't guarantee) I got right.
Use a simple definition of
forward
that accomplishes its original purpose without complications to accomodate other uses:template <class T, class U> T&& forward(U& x) { return static_cast<T&&>(x); }Use a definition of
forward
that protects the user from as many potential mistakes as possible, by actively preventing all other uses:template <class T, class U> boost::enable_if_c< // in forward<T>(x), x is a parameter of the caller, thus an lvalue is_lvalue_reference<U>::value // in caller's deduced T&& argument, T can only be non-ref or lvalue ref && !is_rvalue_reference<T>::value // Must not cast cv-qualifications or do any type conversions && is_same<T&,U&>::value , T&&>::type forward(U&& a) { return static_cast<T&&>(a); }
[ 2009-09-27 Howard adds: ]
A paper, N2951, is available which compares several implementations (including David's) with respect to several use cases (including Jason's) and provides wording for one implementation.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2951.
Proposed resolution:
Section: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
View all issues with Resolved status.
Discussion:
Addresses UK 98 [CD1]
It would be useful to be able to determine the underlying
type of an arbitrary enumeration type. This would allow
safe casting to an integral type (especially needed for
scoped enums, which do not promote), and would allow
use of numeric_limits
. In general it makes generic
programming with enumerations easier.
[ Batavia (2009-05): ]
Pete observes (and Tom concurs) that the proposed resolution seems to require compiler support for its implementation, as it seems necessary to look at the range of values of the enumerated type. To a first approximation, a library solution could give an answer based on the size of the type. If the user has specialized
numeric_limits
for the enumerated type, then the library might be able to do better, but there is no such requirement. Keep status as Open and solicit input from CWG.
[ 2009-05-23 Alisdair adds: ]
Just to confirm that the BSI originator of this comment assumed it did indeed imply a compiler intrinsic. Rather than request a Core extension, it seemed in keeping with that the type traits interface provides a library API to unspecified compiler features - where we require several other traits (e.g.
has_trivial_*
) to get the 'right' answer now, unlike in TR1.
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2984.
Proposed resolution:
Add a new row to the table in 21.3.8.7 [meta.trans.other]:
Table 41 -- Other transformations Template Condition Comments template< class T > struct enum_base;
T
shall be an enumeration type (9.8.1 [dcl.enum])The member typedef type
shall name the underlying type of the enumT
.
std
for implementationsSection: 16.4.2.2 [contents] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [contents].
View all issues with C++11 status.
Discussion:
Addresses UK 168 [CD1]
We should make it clear (either by note or normatively) that namespace
std
may contain inline namespaces, and that entities specified to be
defined in std may in fact be defined in one of these inline namespaces.
(If we're going to use them for versioning, eg when TR2 comes along,
we're going to need that.)
Replace "namespace std or namespaces nested within namespace std" with "namespace std or namespaces nested within namespace std or inline namespaces nested directly or indirectly within namespace std"
[ Summit: ]
adopt UK words (some have reservations whether it is correct)
[ 2009-05-09 Alisdair improves the wording. ]
[ Batavia (2009-05): ]
Bill believes there is strictly speaking no need to say that because no portable test can detect the difference. However he agrees that it doesn't hurt to say this.
Move to Tentatively Ready.
Proposed resolution:
Change 16.4.2.2 [contents] p2:
All library entities except macros,
operator new
andoperator delete
are defined within the namespacestd
or namespaces nested within namespacestd
. It is unspecified whether names declared in a specific namespace are declared directly in that namespace, or in an inline namespace inside that namespace. [Footnote: This gives implementers freedom to support multiple configurations of the library.]
[[noreturn]]
attribute in the librarySection: 17 [support] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses UK 189 and JP 27 [CD1]
The addition of the [[noreturn]]
attribute to the language will be an
important aid for static analysis tools.
The following functions should be declared in C++ with the
[[noreturn]]
attribute: abort
exit
quick_exit
terminate
unexpected
rethrow_exception
throw_with_nested
.
[ Summit: ]
Agreed.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 17.5 [support.start.term] p3:
-2- ...
void abort [[noreturn]] (void)-3- ...
-6- ...
void exit [[noreturn]] (int status)-7- ...
-11- ...
void quick_exit [[noreturn]] (int status)-12- ...
Change the <exception>
synopsis in 17.9 [support.exception]:
void unexpected [[noreturn]] (); ... void terminate [[noreturn]] (); ... void rethrow_exception [[noreturn]] (exception_ptr p); ... template <class T> void throw_with_nested [[noreturn]] (T&& t);// [[noreturn]]
Change 99 [unexpected]:
void unexpected [[noreturn]] ();
Change 17.9.5.4 [terminate]:
void terminate [[noreturn]] ();
Change 17.9.7 [propagation]:
void rethrow_exception [[noreturn]] (exception_ptr p);
In the synopsis of 17.9.8 [except.nested] and the definition area change:
template <class T> void throw_with_nested [[noreturn]] (T&& t);// [[noreturn]]
Section: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func].
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Discussion:
The synopsis in 22.10.17.3 [func.wrap.func] says:
template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)> { ... template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function(F); template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function(F&&); ... template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F); template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&); ... template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes..> && Convertible<Callable<F, ArgTypes...>::result_type function& operator=(F); template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function& operator=(F&&); ... };
Each of the 3 pairs above are ambiguous. We need only one of each pair, and we
could do it with either one. If we choose the F&&
version we
need to bring decay
into the definition to get the pass-by-value behavior.
In the proposed wording I've gotten lazy and just used the pass-by-value signature.
[ 2009-05-01 Daniel adds: ]
[ Batavia (2009-05): ]
We briefly discussed whether we ought support moveable function objects, but decided that should be a separate issue if someone cares to propose it.
Move to Tentatively Ready.
Proposed resolution:
Change the synopsis of 22.10.17.3 [func.wrap.func], and remove the associated definitions in 22.10.17.3.2 [func.wrap.func.con]:
template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)> { ... template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function(F);template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function(F&&);... template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F);template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&);... template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes..> && Convertible<Callable<F, ArgTypes...>::result_type function& operator=(F);template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function& operator=(F&&);... };
is_bind_expression
should derive from integral_constant<bool>
Section: 22.10.15.2 [func.bind.isbind] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.bind.isbind].
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Discussion:
Class template is_bind_expression 22.10.15.2 [func.bind.isbind]:
namespace std { template<class T> struct is_bind_expression { static const bool value = see below; }; }
is_bind_expression
should derive from std::integral_constant<bool>
like
other similar trait types.
[ Daniel adds: ]
We need the same thing for the trait
is_placeholder
as well.
[ 2009-03-22 Daniel provided wording. ]
[ Batavia (2009-05): ]
We recommend this be deferred until after the next Committee Draft is issued.
Move to Open.
[ 2009-05-31 Peter adds: ]
I am opposed to the proposed resolution and to the premise of the issue in general. The traits's default definitions should NOT derive from
integral_constant
, because this is harmful, as it misleads people into thinking thatis_bind_expression<E>
always derives fromintegral_constant
, whereas it may not.
is_bind_expression
andis_placeholder
allow user specializations, and in fact, this is their primary purpose. Such user specializations may not derive fromintegral_constant
, and the places whereis_bind_expression
andis_placeholder
are used intentionally do not require such derivation.The long-term approach here is to switch to
BindExpression<E>
andPlaceholder<P>
explicit concepts, of course, but until that happens, I say leave them alone.
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready. We are comfortable with requiring user specializations to derive from
integral_constant
.
Proposed resolution:
In 22.10.15.2 [func.bind.isbind] change as indicated:
namespace std { template<class T> struct is_bind_expression : integral_constant<bool, see below> { };{ static const bool value = see below; };}
In 22.10.15.2 [func.bind.isbind]/2 change as indicated:
static const bool value;-2-
Iftrue
ifT
is a type returned frombind
,false
otherwise.T
is a type returned frombind
,is_bind_expression<T>
shall be publicly derived fromintegral_constant<bool, true>
, otherwise it shall be publicly derived fromintegral_constant<bool, false>
.
In 22.10.15.3 [func.bind.isplace] change as indicated:
namespace std { template<class T> struct is_placeholder : integral_constant<int, see below> { };{ static const int value = see below; };}
In 22.10.15.3 [func.bind.isplace]/2 change as indicated:
static const int value;-2-
value isIfJ
ifT
is the type ofstd::placeholders::_J
, 0 otherwise.T
is the type ofstd::placeholders::_J
,is_placeholder<T>
shall be publicly derived fromintegral_constant<int, J>
otherwise it shall be publicly derived fromintegral_constant<int, 0>
.
allocator_arg
should be constexpr
Section: 20.2 [memory] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [memory].
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Discussion:
Declaration of allocator_arg
should be constexpr
to ensure constant
initialization.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 20.2 [memory] p2:
// 20.8.1, allocator argument tag struct allocator_arg_t { }; constexpr allocator_arg_t allocator_arg = allocator_arg_t();
Section: 22 [utilities], 23 [containers] Status: Resolved Submitter: Alan Talbot Opened: 2009-03-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utilities].
View all issues with Resolved status.
Discussion:
Addresses US 65 and US 74.1 [CD1]
US 65:
Scoped allocators and allocator propagation traits add a small amount of utility at the cost of a great deal of machinery. The machinery is user visible, and it extends to library components that don't have any obvious connection to allocators, including basic concepts and simple components like
pair
andtuple
.Suggested resolution:
Sketch of proposed resolution: Eliminate scoped allocators, replace allocator propagation traits with a simple uniform rule (e.g. always propagate on copy and move), remove all mention of allocators from components that don't explicitly allocate memory (e.g. pair), and adjust container interfaces to reflect this simplification.
Components that I propose eliminating include
HasAllocatorType
,is_scoped_allocator
,allocator_propagation_map
,scoped_allocator_adaptor
, andConstructibleAsElement
.
US 74.1:
Scoped allocators represent a poor trade-off for standardization, since (1) scoped-allocator--aware containers can be implemented outside the C++ standard library but used with its algorithms, (2) scoped allocators only benefit a tiny proportion of the C++ community (since few C++ programmers even use today's allocators), and (3) all C++ users, especially the vast majority of the C++ community that won't ever use scoped allocators are forced to cope with the interface complexity introduced by scoped allocators.
In essence, the larger community will suffer to support a very small subset of the community who can already implement their own data structures outside of the standard library. Therefore, scoped allocators should be removed from the working paper.
Some evidence of the complexity introduced by scoped allocators:
22.3 [pairs], 22.4 [tuple]: Large increase in the number of pair and tuple constructors.
23 [containers]: Confusing "AllocatableElement" requirements throughout.
Suggested resolution:
Remove support for scoped allocators from the working paper. This includes at least the following changes:
Remove 99 [allocator.element.concepts]
Remove 20.6 [allocator.adaptor]
Remove [construct.element]
In Clause 23 [containers]: replace requirements naming the
AllocatableElement
concept with requirements namingCopyConstructible
,MoveConstructible
,DefaultConstructible
, orConstructible
, as appropriate.
[ Post Summit Alan moved from NAD to Open. ]
[ 2009-05-15 Ganesh adds: ]
The requirement
AllocatableElement
should not be replaced withConstructible
on theemplace_xxx()
functions as suggested. In the one-parameter case theConstructible
requirement is not satisfied when the constructor is explicit (as per [concept.map.fct], twelfth bullet) but we do want to allow explicit constructors in emplace, as the following example shows:vector<shared_ptr<int>> v; v.emplace_back(new int); // should be allowed
If the issue is accepted and scoped allocators are removed, I suggest to add a new pair of concepts to [concept.construct], namely:
auto concept HasExplicitConstructor<typename T, typename... Args> { explicit T::T(Args...); } auto concept ExplicitConstructible<typename T, typename... Args> : HasExplicitConstructor<T, Args...>, NothrowDestructible<T> { }We should then use
ExplicitConstructible
as the requirement for allemplace_xxx()
member functions.For coherence and consistency with the similar concepts
Convertible/ExplicitlyConvertible
, we might also consider changingConstructible
to:auto concept Constructible<typename T, typename... Args> : HasConstructor<T, Args...>, ExplicitConstructible<T, Args...> { }Moreover, all emplace-related concepts in [container.concepts] should also use
ExplicitConstructible
instead ofConstructible
in the definitions of their axioms. In fact the concepts in [container.concepts] should be corrected even if the issue is not accepted.On the other hand, if the issue is not accepted, the scoped allocator adaptors should be fixed because the following code:
template <typename T> using scoped_allocator = scoped_allocator_adaptor<allocator<T>>; vector<shared_ptr<int>, scoped_allocator<shared_ptr<int>>> v; v.emplace_back(new int); // ops! doesn't compile
doesn't compile, as the member function
construct()
of the scoped allocator requires non-explicit constructors through conceptConstructibleWithAllocator
. Fixing that is not difficult but probably more work than it's worth and is therefore, IMHO, one more reason in support of the complete removal of scoped allocators.
[ 2009-06-09 Alan adds: ]
I reopened this issue because I did not think that these National Body comments were adequately addressed by marking them NAD. My understanding is that something can be marked NAD if it is clearly a misunderstanding or trivial, but a substantive issue that has any technical merit requires a disposition that addresses the concerns.
The notes in the NB comment list (US 65 & US 74.1) say that:
- this issue has not introduced any new arguments not previously discussed,
- the vote (4-9-3) was not a consensus for removing scoped allocators,
- the issue is resolved by N2840.
My opinion is:
- there are new arguments in both comments regarding concepts (which were not present in the library when the scoped allocator proposal was voted in),
- the vote was clearly not a consensus for removal, but just saying there was a vote does not provide a rationale,
- I do not believe that N2840 addresses these comments (although it does many other things and was voted in with strong approval).
My motivation to open the issue was to ensure that the NB comments were adequately addressed in a way that would not risk a "no" vote on our FCD. If there are responses to the technical concerns raised, then perhaps they should be recorded. If the members of the NB who authored the comments are satisfied with N2840 and the other disposition remarks in the comment list, then I am sure they will say so. In either case, this issue can be closed very quickly in Frankfurt, and hopefully will have helped make us more confident of approval with little effort. If in fact there is controversy, my thought is that it is better to know now rather than later so there is more time to deal with it.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
Rationale:
Scoped allocators have been revised significantly.
RandomAccessIterator
's operator-
has nonsensical effects clauseSection: 24.3.5.7 [random.access.iterators] Status: C++11 Submitter: Doug Gregor Opened: 2009-03-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [random.access.iterators].
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Discussion:
Addresses UK 265
UK-265:
This effects clause is nonesense. It looks more like an axiom stating equivalence, and certainly an effects clause cannot change the state of two arguments passed by const reference
[ 2009-09-18 Alisdair adds: ]
For random access iterators, the definitions of
(b-a)
and(a<b)
are circular:From table Table 104 -- Random access iterator requirements:
b - a :==> (a < b) ? distance(a,b) : -distance(b,a) a < b :==> b - a > 0
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2010-02-13 Alisdair opens. ]
Looking again at LWG 1079(i), the wording in the issue no longer exists, and appears to be entirely an artefact of the concepts wording.
This issue is currently on our Ready list (not even Tentative!) but I think it has to be pulled as there is no way to apply the resolution.
Looking at the current paper, I think this issue is now "NAD, solved by the removal of concepts". Unfortunately it is too late to poll again, so we will have to perform that review in Pittsburgh.
[ 2010-02-13 Daniel updates the wording to address the circularity problem. ]
[ The previous wording is preserved here: ]
Modify 24.3.5.7 [random.access.iterators]p7-9 as follows:
difference_type operator-(const X& a, const X& b);-7- Precondition: there exists a value
-8-n
ofdifference_type
such thata == b + n
.Effects:-9- Returns:b == a + (b - a)
(a < b) ? distance(a,b) : -distance(b,a)
n
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Modify Table 105 in 24.3.5.7 [random.access.iterators]:
Table 105 — Random access iterator requirements (in addition to bidirectional iterator) Expression Return type Operational semantics Assertion/note
pre-/post-conditionb - a
Distance
return
distance(a,b)n
pre: there exists a value n
ofDistance
such thata + n == b
.b == a + (b - a)
.
std::promise
should provide non-member swap
overloadSection: 32.10.6 [futures.promise] Status: Resolved Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
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Discussion:
Addresses UK 342 [CD1]
std::promise
is missing a non-member overload of swap
. This is
inconsistent with other types that provide a swap
member function.
Add a non-member overload void swap(promise&& x,promise&& y){ x.swap(y); }
[ Summit: ]
Create an issue. Move to review, attention: Howard. Detlef will also look into it.
[ Post Summit Daniel provided wording. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
In 32.10.6 [futures.promise], before p.1, immediately after class template promise add:
template <class R> void swap(promise<R>& x, promise<R>& y);
Change 32.10.6 [futures.promise]/10 as indicated (to fix a circular definition):
-10- Effects:
swap(*this, other)Swaps the associated state of*this
andother
Throws: Nothing.
After the last paragraph in 32.10.6 [futures.promise] add the following prototype description:
template <class R> void swap(promise<R>& x, promise<R>& y);Effects:
x.swap(y)
Throws: Nothing.
Section: 32 [thread] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread].
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Discussion:
Addresses JP 76 [CD1]
A description for "Throws: Nothing." are not unified.
At the part without throw, "Throws: Nothing." should be described.
Add "Throws: Nothing." to the following.
[ Summit: ]
Pass on to editor.
[ Post Summit: Editor declares this non-editorial. ]
[ 2009-08-01 Howard provided wording: ]
The definition of "Throws: Nothing." that I added is probably going to be controversial, but I beg you to consider it seriously.
In C++ there are three "flow control" options for a function:
- It can return, either with a value, or with
void
.- It can call a function which never returns, such as
std::exit
orstd::terminate
.- It can throw an exception.
The above list can be abbreviated with:
- Returns.
- Ends program.
- Throws exception.
In general a function can have the behavior of any of these 3, or any combination of any of these three, depending upon run time data.
- R
- E
- T
- RE
- RT
- ET
- RET
A function with no throw spec, and no documentation, is in general a RET function. It may return, it may end the program, or it may throw. When we specify a function with an empty throw spec:
void f() throw();We are saying that
f()
is an RE function: It may return or end the program, but it will not throw.I posit that there are very few places in the library half of the standard where we intend for functions to be able to end the program (call
terminate
). And none of those places where we do sayterminate
could be called, do we currently say "Throws: Nothing.".I believe that if we define "Throws: Nothing." to mean R, we will both clarify many, many places in the standard, and give us a good rationale for choosing between "Throws: Nothing." (R) and
throw()
(RE) in the future. Indeed, this may give us motivation to change severalthrow()
s to "Throws: Nothing.".I did not add the following changes as JP 76 requested as I believe we want to allow these functions to throw:
Add a paragraph under 32.6.5.2 [thread.lock.guard] p4:
explicit lock_guard(mutex_type& m);Throws: Nothing.
Add a paragraph under 32.6.5.4.2 [thread.lock.unique.cons] p6:
explicit unique_lock(mutex_type& m);Throws: Nothing.
Add a paragraph under 32.7.5 [thread.condition.condvarany] p19, p21 and p25:
template <class Lock, class Rep, class Period> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);Throws: Nothing.
template <class Lock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& rel_time, Predicate pred);Throws: Nothing.
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);Throws: Nothing.
[ 2009-10 Santa Cruz: ]
Defer pending further developments with exception restriction annotations.
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-24 Pete moved to Open: ]
A "Throws: Nothing" specification is not the place to say that a function is not allowed to call
exit()
. While I agree with the thrust of the proposed resolution, "doesn't throw exceptions" is a subset of "always returns normally". If it's important to say that most library functions don't callexit()
, say so.
[ 2010 Pittsburgh: ]
Move to Ready except for the added paragraph to 16.3.2.4 [structure.specifications].
Proposed resolution:
Add a paragraph under 32.4.3.7 [thread.thread.static] p1:
unsigned hardware_concurrency();-1- Returns: ...
Throws: Nothing.
Add a paragraph under 32.7.4 [thread.condition.condvar] p7 and p8:
[Informational, not to be incluced in the WP: The POSIX spec allows only:
- [EINVAL]
- The value
cond
does not refer to an initialized condition variable. — end informational]void notify_one();-7- Effects: ...
Throws: Nothing.
void notify_all();-8- Effects: ...
Throws: Nothing.
Add a paragraph under 32.7.5 [thread.condition.condvarany] p6 and p7:
void notify_one();-6- Effects: ...
Throws: Nothing.
void notify_all();-7- Effects: ...
Throws: Nothing.
packaged_task
member swap
, missing non-member swap
Section: 32.10.10 [futures.task] Status: Resolved Submitter: Daniel Krügler Opened: 2009-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task].
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Discussion:
Class template packaged_task
in 32.10.10 [futures.task] shows a member swap
declaration, but misses to document it's effects (No prototype provided). Further on this class
misses to provide a non-member swap.
[ Batavia (2009-05): ]
Alisdair notes that paragraph 2 of the proposed resolution has already been applied in the current Working Draft.
We note a pending
future
-related paper by Detlef; we would like to wait for this paper before proceeding.Move to Open.
[ 2009-05-24 Daniel removed part 2 of the proposed resolution. ]
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready, removing bullet 3 from the proposed resolution but keeping the other two bullets.
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
In 32.10.10 [futures.task], immediately after the definition of class template packaged_task add:
template<class R, class... Argtypes> void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&);
At the end of 32.10.10 [futures.task] (after p. 20), add the following prototype description:
template<class R, class... Argtypes> void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y);Effects:
x.swap(y)
Throws: Nothing.
random_shuffle
algorithmSection: 26.7.13 [alg.random.shuffle] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.random.shuffle].
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Discussion:
There are a couple of issues with the declaration of the random_shuffle
algorithm accepting a random number engine.
RandomNumberEngine
concept is now provided by the random number
library
(n2836)
and the placeholder should be removed.
[ 2009-05-02 Daniel adds: ]
this issue completes adding necessary requirement to the third new
random_shuffle
overload. The current suggestion is:template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand> requires ShuffleIterator<Iter> void random_shuffle(Iter first, Iter last, Rand&& g);IMO this is still insufficient and I suggest to add the requirement
Convertible<Rand::result_type, Iter::difference_type>to the list (as the two other overloads already have).
Rationale:
Its true that this third overload is somewhat different from the remaining two. Nevertheless we know from
UniformRandomNumberGenerator
, that it'sresult_type
is an integral type and that it satisfiesUnsignedIntegralLike<result_type>
.To realize it's designated task, the algorithm has to invoke the
Callable
aspect ofg
and needs to perform some algebra involving it'smin()/max()
limits to compute another index value that at this point is converted intoIter::difference_type
. This is so, because 24.3.5.7 [random.access.iterators] uses this type as argument of it's algebraic operators. Alternatively consider the equivalent iterator algorithms in 24.4.3 [iterator.operations] with the same result.This argument leads us to the conclusion that we also need
Convertible<Rand::result_type, Iter::difference_type>
here.
[ Batavia (2009-05): ]
Alisdair notes that point (ii) has already been addressed.
We agree with the proposed resolution to point (i) with Daniel's added requirement.
Move to Review.
[ 2009-06-05 Daniel updated proposed wording as recommended in Batavia. ]
[ 2009-07-28 Alisdair adds: ]
Revert to Open, with a note there is consensus on direction but the wording needs updating to reflect removal of concepts.
[ 2009-10 post-Santa Cruz: ]
Leave Open, Walter to work on it.
[
2010 Pittsburgh: Moved to NAD EditorialResolved, addressed by
N3056.
]
Rationale:
Solved by N3056.
Proposed resolution:
Change in [algorithms.syn] and 26.7.13 [alg.random.shuffle]:
concept UniformRandomNumberGenerator<typename Rand> { }template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand> requires ShuffleIterator<Iter> && Convertible<Rand::result_type, Iter::difference_type> void random_shuffle(Iter first, Iter last, Rand&& g);
explicit operator bool() const
" in I/O librarySection: 31.5.4.4 [iostate.flags] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP 65 and JP 66 [CD1]
Switch from "unspecified-bool-type" to "explicit operator bool() const".
Replace operator unspecified-bool-type() const;
" with explicit operator bool() const;
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Review.
[ 2009 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Change the synopis in 31.5.4 [ios]:
explicit operatorunspecified-bool-typebool() const;
Change 31.5.4.4 [iostate.flags]:
explicit operatorunspecified-bool-typebool() const;-1- Returns:
!fail()
Iffail()
then a value that will evaluate false in a boolean context; otherwise a value that will evaluate true in a boolean context. The value type returned shall not be convertible to int.
[Note: This conversion can be used in contexts where a bool is expected (e.g., anif
condition); however, implicit conversions (e.g., toint
) that can occur withbool
are not allowed, eliminating some sources of user error. One possible implementation choice for this type is pointer-to-member. -- end note]
Section: 16.4.5.10 [res.on.objects] Status: C++11 Submitter: Beman Dawes Opened: 2009-03-27 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [res.on.objects].
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Discussion:
N2775, Small library thread-safety revisions, among other changes, removed a note from 16.4.5.10 [res.on.objects] that read:
[Note: This prohibition against concurrent non-const access means that modifying an object of a standard library type shared between threads without using a locking mechanism may result in a data race. --end note.]
That resulted in wording which is technically correct but can only be understood by reading the lengthy and complex 16.4.6.10 [res.on.data.races] Data race avoidance. This has the effect of making 16.4.5.10 [res.on.objects] unclear, and has already resulted in a query to the LWG reflector. See c++std-lib-23194.
[ Batavia (2009-05): ]
The proposed wording seems to need a bit of tweaking ("really bad idea" isn't quite up to standardese). We would like feedback as to whether the original Note's removal was intentional.
Change the phrase "is a really bad idea" to "risks undefined behavior" and move to Review status.
[ 2009-10 Santa Cruz: ]
Note: Change to read: "Modifying...", Delete 'thus', move to Ready
Proposed resolution:
Change 16.4.5.10 [res.on.objects] as indicated:
The behavior of a program is undefined if calls to standard library functions from different threads may introduce a data race. The conditions under which this may occur are specified in 17.6.4.7.
[Note: Modifying an object of a standard library type shared between threads risks undefined behavior unless objects of the type are explicitly specified as being sharable without data races or the user supplies a locking mechanism. --end note]
Section: 17.2 [support.types] Status: C++11 Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE 18
Freestanding implementations do not (necessarily) have support for multiple threads (see 6.9.2 [intro.multithread]). Applications and libraries may want to optimize for the absence of threads. I therefore propose a preprocessor macro to indicate whether multiple threads can occur.
There is ample prior implementation experience for this
feature with various spellings of the macro name. For
example, gcc implicitly defines _REENTRANT
if multi-threading support is selected on the compiler
command-line.
While this is submitted as a library issue, it may be more appropriate to add the macro in 16.8 cpp.predefined in the core language.
See also N2693.
[ Batavia (2009-05): ]
We agree with the issue, and believe it is properly a library issue.
We prefer that the macro be conditionally defined as part of the
<thread>
header.Move to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-02-25 Pete moved to Open: ]
The proposed resolution adds a feature-test macro named
__STDCPP_THREADS
, described after the following new text:The standard library defines the following macros; no explicit prior inclusion of any header file is necessary.
The correct term here is "header", not "header file". But that's minor. The real problem is that library entities are always defined in headers. If
__STDCPP_THREADS
is defined without including any header it's part of the language and belongs with the other predefined macros in the Preprocessor clause.Oddly enough, the comments from Batavia say "We prefer that the macro be conditionally defined as part of the
<thread>
header." There's no mention of a decision to change this.
[ 2010-02-26 Ganesh updates wording. ]
[ 2010 Pittsburgh: Adopt Ganesh's wording and move to Review. ]
[ 2010-03-08 Pete adds: ]
Most macros we have begin and end with with double underbars, this one only begins with double underbars.
[ 2010 Pittsburgh: Ganesh's wording adopted and moved to Ready for Pittsburgh. ]
Proposed resolution:
Change 16.4.2.5 [compliance]/3:
3 The supplied version of the header
<cstdlib>
shall declare at least the functionsabort()
,atexit()
, andexit()
(18.5). The supplied version of the header<thread>
either shall meet the same requirements as for a hosted implementation or including it shall have no effect. The other headers listed in this table shall meet the same requirements as for a hosted implementation.
Add the following line to table 15:
Table 15 — C++ headers for freestanding implementations Subclause Header(s) ... 32.4 [thread.threads] Threads <thread>
Add to the <thread>
synopsis in 32.4 [thread.threads]/1 the line:
namespace std { #define __STDCPP_THREADS __cplusplus class thread; ...
get_pointer_safety()
Section: 99 [util.dynamic.safety] Status: C++11 Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE 18
In 99 [util.dynamic.safety], get_pointer_safety()
purports
to define behavior for
non-safely derived pointers ( [basic.stc.dynamic.safety]). However,
the cited core-language section in paragraph 4 specifies undefined behavior
for the use of such pointer values. This seems an unfortunate near-contradiction.
I suggest to specify the term relaxed pointer safety in
the core language section and refer to it from the library description.
This issue deals with the library part, the corresponding core issue (c++std-core-13940)
deals with the core modifications.
See also N2693.
[ Batavia (2009-05): ]
We recommend if this issue is to be moved, the issue be moved concurrently with the cited Core issue.
We agree with the intent of the proposed resolution. We would like input from garbage collection specialists.
Move to Open.
[ 2009-10 Santa Cruz: ]
The core issue is 853 and is in Ready status.
Proposed resolution:
In 99 [util.dynamic.safety] p16, replace the description of
get_pointer_safety()
with:
pointer_safety get_pointer_safety();
Returns: an enumeration value indicating the implementation's treatment of pointers that are not safely derived (3.7.4.3). Returnspointer_safety::relaxed
if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program. Returnspointer_safety::preferred
if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program but allows the implementation to hint that it could be desirable to avoid dereferencing pointers that are not safely derived as described. [Example:pointer_safety::preferred
might be returned to detect if a leak detector is running to avoid spurious leak reports. -- end note] Returnspointer_safety::strict
if pointers that are not safely derived might be treated differently than pointers that are safely derived.Returns: Returns
pointer_safety::strict
if the implementation has strict pointer safety ( [basic.stc.dynamic.safety]). It is implementation-defined whetherget_pointer_safety
returnspointer_safety::relaxed
orpointer_safety::preferred
if the implementation has relaxed pointer safety ( [basic.stc.dynamic.safety]).FootnoteThrows: nothing
Footnote)
pointer_safety::preferred
might be returned to indicate to the program that a leak detector is running so that the program can avoid spurious leak reports.
auto_ptr
to unique_ptr
conversionSection: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Message c++std-lib-23182 led to a discussion in which several people
expressed interest in being able to convert an auto_ptr
to a
unique_ptr
without the need to call release
. Below is
wording to accomplish this.
[ Batavia (2009-05): ]
Pete believes it not a good idea to separate parts of a class's definition. Therefore, if we do this, it should be part of
unique-ptr
's specification.Alisdair believes the lvalue overload may be not necessary.
Marc believes it is more than just sugar, as it does ease the transition to
unique-ptr
.We agree with the resolution as presented. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-08-01 Howard deconceptifies wording: ]
I also moved the change from 99 [depr.auto.ptr] to 20.3.1.3 [unique.ptr.single] per the Editor's request in Batavia (as long as I was making changes anyway). Set back to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Add to 20.3.1.3 [unique.ptr.single]:
template <class T, class D> class unique_ptr { public: template <class U> unique_ptr(auto_ptr<U>& u); template <class U> unique_ptr(auto_ptr<U>&& u); };
Add to 20.3.1.3.2 [unique.ptr.single.ctor]:
template <class U> unique_ptr(auto_ptr<U>& u); template <class U> unique_ptr(auto_ptr<U>&& u);Effects: Constructs a
unique_ptr
withu.release()
.Postconditions:
get() ==
the valueu.get()
had before the construciton, modulo any required offset adjustments resulting from the cast fromU*
toT*
.u.get() == nullptr
.Throws: nothing.
Remarks:
U*
shall be implicitly convertible toT*
andD
shall be the same type asdefault_delete<T>
, else these constructors shall not participate in overload resolution.
system_error
constructor postcondition overly strictSection: 19.5.8.2 [syserr.syserr.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
19.5.8.2 [syserr.syserr.members] says:
system_error(error_code ec, const string& what_arg);Effects: Constructs an object of class
system_error
.Postconditions:
code() == ec
andstrcmp(runtime_error::what(), what_arg.c_str()) == 0
.
However the intent is for:
std::system_error se(std::errc::not_a_directory, "In FooBar"); ... se.what(); // returns something along the lines of: // "In FooBar: Not a directory"
The way the constructor postconditions are set up now, to achieve both
conformance, and the desired intent in the what()
string, the
system_error
constructor must store "In FooBar" in the base class,
and then form the desired output each time what()
is called. Or
alternatively, store "In FooBar" in the base class, and store the desired
what()
string in the derived system_error
, and override
what()
to return the string in the derived part.
Both of the above implementations seem suboptimal to me. In one I'm computing
a new string every time what()
is called. And since what()
can't propagate exceptions, the client may get a different string on different
calls.
The second solution requires storing two strings instead of one.
What I would like to be able to do is form the desired what()
string
once in the system_error
constructor, and store that in the
base class. Now I'm:
what()
only once.what()
definition is sufficient and nothrow.This is smaller code, smaller data, and faster.
ios_base::failure
has the same issue.
[
Comments about this change received favorable comments from the system_error
designers.
]
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
In 19.5.8.2 [syserr.syserr.members], change the following constructor postconditions:
system_error(error_code ec, const string& what_arg);-2- Postconditions:
code() == ec
and.
strcmp(runtime_error::what(), what_arg.c_str()) == 0string(what()).find(what_arg) != string::npossystem_error(error_code ec, const char* what_arg);-4- Postconditions:
code() == ec
and.
strcmp(runtime_error::what(), what_arg) == 0string(what()).find(what_arg) != string::npossystem_error(error_code ec);-6- Postconditions:
code() == ec
and.strcmp(runtime_error::what(), ""
system_error(int ev, const error_category& ecat, const string& what_arg);-8- Postconditions:
code() == error_code(ev, ecat)
and.
strcmp(runtime_error::what(), what_arg.c_str()) == 0string(what()).find(what_arg) != string::npossystem_error(int ev, const error_category& ecat, const char* what_arg);-10- Postconditions:
code() == error_code(ev, ecat)
and.
strcmp(runtime_error::what(), what_arg) == 0string(what()).find(what_arg) != string::npossystem_error(int ev, const error_category& ecat);-12- Postconditions:
code() == error_code(ev, ecat)
and.strcmp(runtime_error::what(), "") == 0
In 19.5.8.2 [syserr.syserr.members], change the description of what()
:
const char *what() const throw();-14- Returns: An NTBS incorporating
the arguments supplied in the constructor.runtime_error::what()
andcode().message()
[Note:
One possible implementation would be:The return NTBS might take the form:what_arg + ": " + code().message()
if (msg.empty()) { try { string tmp = runtime_error::what(); if (code()) { if (!tmp.empty()) tmp += ": "; tmp += code().message(); } swap(msg, tmp); } catch(...) { return runtime_error::what(); } return msg.c_str();— end note]
In [ios::failure], change the synopsis:
namespace std { class ios_base::failure : public system_error { public: explicit failure(const string& msg, const error_code& ec = io_errc::stream); explicit failure(const char* msg, const error_code& ec = io_errc::stream);virtual const char* what() const throw();}; }
In [ios::failure], change the description of the constructors:
explicit failure(const string& msg, , const error_code& ec = io_errc::stream);-3- Effects: Constructs an object of class
failure
by constructing the base class withmsg
andec
.
-4- Postcondition:code() == ec
andstrcmp(what(), msg.c_str()) == 0
explicit failure(const char* msg, const error_code& ec = io_errc::stream);-5- Effects: Constructs an object of class
failure
by constructing the base class withmsg
andec
.
-6- Postcondition:code() == ec and strcmp(what(), msg) == 0
In [ios::failure], remove what
(the base class definition
need not be repeated here).
const char* what() const;
-7- Returns: The messagemsg
with which the exception was created.
basic_ios::move
should accept lvaluesSection: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
With the rvalue reference changes in
N2844
basic_ios::move
no longer has the most convenient signature:
void move(basic_ios&& rhs);
This signature should be changed to accept lvalues. It does not need to be
overloaded to accept rvalues. This is a special case that only derived clients
will see. The generic move
still needs to accept rvalues.
[ Batavia (2009-05): ]
Tom prefers, on general principles, to provide both overloads. Alisdair agrees.
Howard points out that there is no backward compatibility issue as this is new to C++0X.
We agree that both overloads should be provided, and Howard will provide the additional wording. Move to Open.
[ 2009-05-23 Howard adds: ]
Added overload, moved to Review.
[ 2009 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add a signature to the existing prototype in the synopsis of 31.5.4 [ios] and in 31.5.4.3 [basic.ios.members]:
void move(basic_ios& rhs); void move(basic_ios&& rhs);
shared_future::get()
?Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Thomas J. Gritzan Opened: 2009-04-03 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
It is not clear, if multiple threads are waiting in a
shared_future::get()
call, if each will rethrow the stored exception.
Paragraph 9 reads:
Throws: the stored exception, if an exception was stored and not retrieved before.
The "not retrieved before" suggests that only one exception is thrown,
but one exception for each call to get()
is needed, and multiple calls
to get()
even on the same shared_future
object seem to be allowed.
I suggest removing "and not retrieved before" from the Throws paragraph.
I recommend adding a note that explains that multiple calls on get()
are
allowed, and each call would result in an exception if an exception was
stored.
[ Batavia (2009-05): ]
We note there is a pending paper by Detlef on such
future
-related issues; we would like to wait for his paper before proceeding.Alisdair suggests we may want language to clarify that this
get()
function can be called from several threads with no need for explicit locking.Move to Open.
[ 2010-01-23 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. ]
Rationale:
Resolved by paper N2997.
Proposed resolution:
Change [futures.shared_future]:
const R& shared_future::get() const; R& shared_future<R&>::get() const; void shared_future<void>::get() const;...
-9- Throws: the stored exception, if an exception was stored
and not retrieved before. [Note: Multiple calls onget()
are allowed, and each call would result in an exception if an exception was stored. — end note]
Section: 32.2.2 [thread.req.exception] Status: C++11 Submitter: Christopher Kohlhoff Opened: 2009-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current formulation of 32.2.2 [thread.req.exception]/2 reads:
The
error_category
of theerror_code
reported by such an exception'scode()
member function is as specified in the error condition Clause.
This constraint on the code's associated error_categor
means an
implementation must perform a mapping from the system-generated
error to a generic_category()
error code. The problems with this
include:
The original error produced by the operating system is lost.
The latter was one of Peter Dimov's main objections (in a private
email discussion) to the original error_code
-only design, and led to
the creation of error_condition
in the first place. Specifically,
error_code
and error_condition
are intended to perform
the following roles:
error_code
holds the original error produced by the operating
system.
error_condition
and the generic category provide a set of well
known error constants that error codes may be tested against.
Any mapping determining correspondence of the returned error code to the conditions listed in the error condition clause falls under the "latitude" granted to implementors in 19.5.3.5 [syserr.errcat.objects]. (Although obviously their latitude is restricted a little by the need to match the right error condition when returning an error code from a library function.)
It is important that this error_code/error_condition
usage is done
correctly for the thread library since it is likely to set the pattern for future
TR libraries that interact with the operating system.
[ Batavia (2009-05): ]
Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
Change 32.2.2 [thread.req.exception] p.2:
-2-
TheTheerror_category
(19.5.1.1) of theerror_code
reported by such an exception'scode()
member function is as specified in the error condition Clause.error_code
reported by such an exception'scode()
member function shall compare equal to one of the conditions specified in the function's error condition Clause. [Example: When the thread constructor fails:ec.category() == implementation-defined // probably system_category ec == errc::resource_unavailable_try_again // holds true— end example]
for_each
overconstrained?Section: 26.6.5 [alg.foreach] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-04-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Quoting working paper for reference (26.6.5 [alg.foreach]):
template<InputIterator Iter, Callable<auto, Iter::reference> Function> requires CopyConstructible<Function> Function for_each(Iter first, Iter last, Function f);1 Effects: Applies f to the result of dereferencing every iterator in the range [first,last), starting from first and proceeding to last - 1.
2 Returns: f.
3 Complexity: Applies f exactly last - first times.
P2 implies the passed object f
should be invoked at each stage, rather than
some copy of f
. This is important if the return value is to usefully
accumulate changes. So the requirements are an object of type Function
can
be passed-by-value, invoked multiple times, and then return by value. In
this case, MoveConstructible
is sufficient. This would open support for
move-only functors, which might become important in concurrent code as you
can assume there are no other references (copies) of a move-only type and so
freely use them concurrently without additional locks.
[ See further discussion starting with c++std-lib-23686. ]
[ Batavia (2009-05): ]
Pete suggests we may want to look at this in a broader context involving other algorithms. We should also consider the implications of parallelism.
Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.
[ 2009-10-14 Daniel de-conceptified the proposed resolution. ]
The note in 26.1 [algorithms.general]/9 already says the right thing:
Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.
So we only need to ensure that the wording for
for_each
is sufficiently clear, which is the intend of the following rewording.
[ 2009-10-15 Daniel proposes: ]
Add a new Requires clause just after the prototype declaration (26.6.5 [alg.foreach]):
Requires:
Function
shall beMoveConstructible
( [moveconstructible]),CopyConstructible
is not required.Change 26.6.5 [alg.foreach]/2 as indicated:
Returns: std::move(f).
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready, using Daniel's wording without the portion saying "CopyConstructible is not required".
[ 2009-10-27 Daniel adds: ]
I see that during the Santa Cruz meeting the originally proposed addition
,
CopyConstructible
is not required.was removed. I don't think that this removal was a good idea. The combination of 26.1 [algorithms.general] p.9
[Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.[..]
with the fact that
CopyConstructible
is a refinementMoveConstructible
makes it necessary that such an explicit statement is given. Even the existence of the usage ofstd::move
in the Returns clause doesn't help much, because this would still be well-formed for aCopyConstructible
without move constructor. Let me add that the originally proposed addition reflects current practice in the standard, e.g. 26.7.9 [alg.unique] p.5 usages a similar terminology.For similar wording need in case for auto_ptr see 973(i).
[ Howard: Moved from Tentatively Ready to Open. ]
[
2009-11-20 Howard restores "not CopyConstructible
" to the spec.
]
[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add a new Requires clause just after the prototype declaration (26.6.5 [alg.foreach]):
Requires:
Function
shall meet the requirements ofMoveConstructible
( [moveconstructible]).Function
need not meet the requirements ofCopyConstructible
( [copyconstructible]).
Change 26.6.5 [alg.foreach]/2 as indicated:
Returns: std::move(f).
bitset::to_string
could be simplifiedSection: 22.9.2 [template.bitset] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 853(i) our resolution is changing the signature by adding two defaulting arguments to 3 calls. In principle, this means that ABI breakage is not an issue, while API is preserved.
With that observation, it would be very nice to use the new ability to supply default template parameters to function templates to collapse all 3 signatures into 1. In that spirit, this issue offers an alternative resolution than that of 853(i).
[ Batavia (2009-05): ]
Move to Open, and look at the issue again after 853(i) has been accepted. We further recommend this be deferred until after the next Committee Draft.
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
In 22.9.2 [template.bitset]/1 (class bitset) ammend:
template <class charT = char, class traits = char_traits<charT>, class Allocator = allocator<charT>> basic_string<charT, traits, Allocator> to_string(charT zero = charT('0'), charT one = charT('1')) const;template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string() const; template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string() const; basic_string<char, char_traits<char>, allocator<char> > to_string() const;
In 22.9.2.3 [bitset.members] prior to p35 ammend:
template <class charT = char, class traits = char_traits<charT>, class Allocator = allocator<charT>> basic_string<charT, traits, Allocator> to_string(charT zero = charT('0'), charT one = charT('1')) const;
Section: 21 [meta] Status: C++11 Submitter: Daniel Krügler Opened: 2009-05-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Related to 975(i) and 1023(i).
The current wording in 21.3.2 [meta.rqmts] is still unclear concerning it's requirements on the type traits classes regarding ambiguities. Specifically it's unclear
true_type
/false_type
.
integral_constant
types making the contained names ambiguous
[ Batavia (2009-05): ]
Alisdair would prefer to factor some of the repeated text, but modulo a corner case or two, he believes the proposed wording is otherwise substantially correct.
Move to Open.
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
[ The usage of the notion of a BaseCharacteristic below might be useful in other places - e.g. to define the base class relation in 22.10.6 [refwrap], 22.10.16 [func.memfn], or 22.10.17.3 [func.wrap.func]. In this case it's definition should probably be moved to Clause 17 ]
Change 21.3.2 [meta.rqmts] p.1 as indicated:
[..] It shall be
DefaultConstructible
,CopyConstructible
, and publicly and unambiguously derived, directly or indirectly, from its BaseCharacteristic, which is a specialization of the templateintegral_constant
(20.6.3), with the arguments to the templateintegral_constant
determined by the requirements for the particular property being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the UnaryTypeTrait.
Change 21.3.2 [meta.rqmts] p.2 as indicated:
[..] It shall be
DefaultConstructible
,CopyConstructible
, and publicly and unambiguously derived, directly or indirectly, froman instanceits BaseCharacteristic, which is a specialization of the templateintegral_constant
(20.6.3), with the arguments to the templateintegral_constant
determined by the requirements for the particular relationship being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the BinaryTypeTrait.
Change 21.3.5 [meta.unary] p.2 as indicated:
Each of these templates shall be a UnaryTypeTrait (20.6.1),
publicly derived directly or indirectly fromwhere its BaseCharacteristic shall betrue_type
if the corresponding condition is true, otherwise fromfalse_type
true_type
if the corresponding condition is true, otherwisefalse_type
.
Change 21.3.7 [meta.rel] p.2 as indicated:
Each of these templates shall be a BinaryTypeTrait (20.6.1),
publicly derived directly or indirectly fromwhere its BaseCharacteristic shall betrue_type
if the corresponding condition is true, otherwise fromfalse_type
true_type
if the corresponding condition is true, otherwisefalse_type
.
Section: 22.4.4 [tuple.tuple] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It is not currently possible to construct tuple
literal values,
even if the elements are all literal types. This is because parameters
are passed to constructor by reference.
An alternative would be to pass all constructor arguments by value, where it is known that *all* elements are literal types. This can be determined with concepts, although note that the negative constraint really requires factoring out a separate concept, as there is no way to provide an 'any of these fails' constraint inline.
Note that we will have similar issues with pair
(and
tuple
constructors from pair
) although I am steering
clear of that class while other constructor-related issues settle.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2994.
Proposed resolution:
Ammend the tuple class template declaration in 22.4.4 [tuple.tuple] as follows
Add the following concept:
auto concept AllLiteral< typename ... Types > { requires LiteralType<Types>...; }ammend the constructor
template <class... UTypes> requires AllLiteral<Types...> && Constructible<Types, UTypes>... explicit tuple(UTypes...); template <class... UTypes> requires !AllLiteral<Types...> && Constructible<Types, UTypes&&>... explicit tuple(UTypes&&...);ammend the constructor
template <class... UTypes> requires AllLiteral<Types...> && Constructible<Types, UTypes>... tuple(tuple<UTypes...>); template <class... UTypes> requires !AllLiteral<Types...> && Constructible<Types, const UTypes&>... tuple(const tuple<UTypes...>&);
Update the same signatures in 22.4.4.2 [tuple.cnstr], paras 3 and 5.
tuple
copy constructorSection: 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [tuple.cnstr].
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Discussion:
The copy constructor for the tuple
template is constrained. This seems an
unusual strategy, as the copy constructor will be implicitly deleted if the
constraints are not met. This is exactly the same effect as requesting an
=default;
constructor. The advantage of the latter is that it retains
triviality, and provides support for tuple
s as literal types if issue
1116(i) is also accepted.
Actually, it might be worth checking with core if a constrained copy constructor is treated as a constructor template, and as such does not suppress the implicit generation of the copy constructor which would hide the template in this case.
[ 2009-05-27 Daniel adds: ]
This would solve one half of the suggested changes in 801(i).
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2994.
Proposed resolution:
Change 22.4.4 [tuple.tuple] and 22.4.4.2 [tuple.cnstr] p4:
requires CopyConstructible<Types>...tuple(const tuple&) = default;
tuple
query APIs do not support cv-qualificationSection: 22.4.7 [tuple.helper] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple.helper].
View all issues with C++11 status.
Discussion:
The APIs tuple_size
and tuple_element
do not support
cv-qualified tuple
s, pair
s or array
s.
The most generic solution would be to supply partial specializations once
for each cv-type in the tuple
header. However, requiring this header for
cv-qualified pair
s/array
s seems unhelpful. The BSI editorial
suggestion (UK-198/US-69,
N2533)
to merge tuple
into <utility>
would help with pair
,
but not array
. That might be resolved by making a dependency between the
<array>
header and <utility>
, or simply recognising
the dependency be fulfilled in a Remark.
[ 2009-05-24 Daniel adds: ]
All
tuple_size
templates with a base class need to derive publicly, e.g.template <IdentityOf T> class tuple_size< const T > : public tuple_size<T> {};The same applies to the tuple_element class hierarchies.
What is actually meant with the comment
this solution relies on 'metafunction forwarding' to inherit the nested typename type
?
I ask, because all base classes are currently unconstrained and their instantiation is invalid in the constrained context of the
tuple_element
partial template specializations.
[ 2009-05-24 Alisdair adds: ]
I think a better solution might be to ask Pete editorially to change all declarations of tupling APIs to use the struct specifier instead of class.
"metafunction forwarding" refers to the MPL metafunction protocol, where a metafunction result is declared as a nested typedef with the name "type", allowing metafunctions to be chained by means of inheritance. It is a neater syntax than repeatedly declaring a typedef, and inheritance syntax is slightly nicer when it comes to additional typename keywords.
The constrained template with an unconstrained base is a good observation though.
[ 2009-10 post-Santa Cruz: ]
Move to Open, Alisdair to provide wording. Once wording is provided, Howard will move to Review.
[ 2010-03-28 Daniel deconceptified wording. ]
[ Post-Rapperswil - Daniel provides wording: ]
The below given P/R reflects the discussion from the Rapperswil meeting that the wording should not constrain implementation freedom to realize the actual issue target. Thus the original code form was replaced by normative words.
While preparing this wording it turned out that several tuple_size
specializations as
that of pair
and array
are underspecified, because the underlying type of the member
value is not specified except that it is an integral type. For the specializations we could introduce a
canonical one - like size_t
- or we could use the same type as the specialization of the
unqualified type uses. The following wording follows the second approach.
The wording refers to N3126.
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
<tuple>
synopsis, as indicated:
// 20.4.2.5, tuple helper classes: template <class T> class tuple_size; // undefined template <class T> class tuple_size<const T>; template <class T> class tuple_size<volatile T>; template <class T> class tuple_size<const volatile T>; template <class... Types> class tuple_size<tuple<Types...> >; template <size_t I, class T> class tuple_element; // undefined template <size_t I, class T> class tuple_element<I, const T>; template <size_t I, class T> class tuple_element<I, volatile T>; template <size_t I, class T> class tuple_element<I, const volatile T>; template <size_t I, class... Types> class tuple_element<I, tuple<Types...> >;
template <class T> class tuple_size<const T>; template <class T> class tuple_size<volatile T>; template <class T> class tuple_size<const volatile T>;Let TS denote
tuple_size<T>
of the cv-unqualified typeT
. Then each of the three templates shall meet the UnaryTypeTrait requirements (20.7.1) with a BaseCharacteristic ofintegral_constant<remove_cv<decltype(TS::value)>::type, TS::value>
.
template <size_t I, class T> class tuple_element<I, const T>; template <size_t I, class T> class tuple_element<I, volatile T>; template <size_t I, class T> class tuple_element<I, const volatile T>;Let TE denote
tuple_element<I, T>
of the cv-unqualified typeT
. Then each of the three templates shall meet the TransformationTrait requirements (20.7.1) with a member typedeftype
that shall name the same type as the following type:
- for the first specialization, the type
add_const<TE::type>::type
,- for the second specialization, the type
add_volatile<TE::type>::type
, and- for the third specialization, the type
add_cv<TE::type>::type
constexpr
Section: 21.4.3 [ratio.ratio] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ratio.ratio].
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Discussion:
The values num
and den
in the ratio
template
should be declared constexpr
.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2994.
Proposed resolution:
21.4.3 [ratio.ratio]
namespace std { template <intmax_t N, intmax_t D = 1> class ratio { public: static constexpr intmax_t num; static constexpr intmax_t den; }; }
Section: 31.5.2.2.6 [ios.init] Status: C++11 Submitter: James Kanze Opened: 2009-05-14 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [ios.init].
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Discussion:
As currently formulated, the standard doesn't require that there
is ever a flush of cout
, etc. (This implies, for example, that
the classical hello, world program may have no output.) In the
current draft
(N2798),
there is a requirement that the objects
be constructed before main
, and before the dynamic
initialization of any non-local objects defined after the
inclusion of <iostream>
in the same translation unit. The only
requirement that I can find concerning flushing, however, is in
[ios::Init], where the destructor of the last
std::ios_base::Init
object flushes. But there is, as far as I
can see, no guarantee that such an object ever exists.
Also, the wording in [iostreams.objects] says that:
The objects are constructed and the associations are established at some time prior to or during the first time an object of class
ios_base::Init
is constructed, and in any case before the body of main begins execution.
In [ios::Init], however, as an effect of the constructor, it says that
If
init_cnt
is zero, the function stores the value one ininit_cnt
, then constructs and initializes the objectscin
,cout
,cerr
,clog
wcin
,wcout
,wcerr
, andwclog
"
which seems to forbid earlier construction.
(Note that with these changes, the exposition only "static
int init_cnt
" in ios_base::Init
can be dropped.)
Of course, a determined programmer can still inhibit the flush with things like:
new std::ios_base::Init ; // never deleted
or (in a function):
std::ios_base::Init ensureConstruction ; // ... exit( EXIT_SUCCESS ) ;
Perhaps some words somewhere to the effect that all
std::ios_base::Init
objects should have static lifetime
would be in order.
[ 2009 Santa Cruz: ]
Moved to Ready. Some editorial changes are expected (in addition to the proposed wording) to remove
init_cnt
fromInit
.
Proposed resolution:
Change 31.4 [iostream.objects]/2:
-2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class
ios_base::Init
is constructed, and in any case before the body of main begins execution.292 The objects are not destroyed during program execution.293If a translation unit includesThe results of including<iostream>
or explicitly constructs anios_base::Init
object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit.<iostream>
in a translation unit shall be as if<iostream>
defined an instance ofios_base::Init
with static lifetime. Similarly, the entire program shall behave as if there were at least one instance ofios_base::Init
with static lifetime.
Change [ios::Init]/3:
Init();-3- Effects: Constructs an object of class
Init
.IfConstructs and initializes the objectsinit_cnt
is zero, the function stores the value one ininit_cnt
, then constructs and initializes the objectscin
,cout
,cerr
,clog
(27.4.1),wcin
,wcout
,wcerr
, andwclog
(27.4.2). In any case, the function then adds one to the value stored ininit_cnt
.cin
,cout
,cerr
,clog
,wcin
,wcout
,wcerr
andwclog
if they have not already been constructed and initialized.
Change [ios::Init]/4:
~Init();-4- Effects: Destroys an object of class
Init
.The function subtracts one from the value stored inIf there are no other instances of the class still in existance, callsinit_cnt
and, if the resulting stored value is one,cout.flush()
,cerr.flush()
,clog.flush()
,wcout.flush()
,wcerr.flush()
,wclog.flush()
.
istreambuff_iterator::equal
needs a const & parameterSection: 24.6.4.4 [istreambuf.iterator.ops] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2017-11-29
Priority: Not Prioritized
View all other issues in [istreambuf.iterator.ops].
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Discussion:
The equal
member function of istreambuf_iterator
is
declared const
, but takes its argument by non-const reference.
This is not compatible with the operator==
free function overload, which is
defined in terms of calling equal
yet takes both arguments by reference to
const.
[ The proposed wording is consistent with 110(i) with status TC1. ]
[ 2009-11-02 Howard adds: ]
Set to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Ammend in both:
24.6.4 [istreambuf.iterator]
[istreambuf.iterator::equal]
bool equal(const istreambuf_iterator& b) const;
istream(buf)_iterator
should support literal sentinel valueSection: 24.6.2.2 [istream.iterator.cons], 24.6.4 [istreambuf.iterator] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.iterator.cons].
View all issues with Resolved status.
Discussion:
istream_iterator
and istreambuf_iterator
should support literal sentinel
values. The default constructor is frequently used to terminate ranges, and
could easily be a literal value for istreambuf_iterator
, and
istream_iterator
when iterating value types. A little more work using a
suitably sized/aligned char-array for storage (or an updated component like
boost::optional
proposed for TR2) would allow istream_iterator
to support
constexpr
default constructor in all cases, although we might leave this
tweak as a QoI issue. Note that requiring constexpr
be supported also
allows us to place no-throw guarantees on this constructor too.
[ 2009-06-02 Daniel adds: ]
I agree with the usefulness of the issue suggestion, but we need to ensure that
istream_iterator
can satisfy be literal if needed. Currently this is not clear, because 24.6.2 [istream.iterator]/3 declares a copy constructor and a destructor and explains their semantic in 24.6.2.2 [istream.iterator.cons]/3+4.The prototype semantic specification is ok (although it seems somewhat redundant to me, because the semantic doesn't say anything interesting in both cases), but for support of trivial class types we also need a trivial copy constructor and destructor as of 11 [class]/6. The current non-informative specification of these two special members suggests to remove their explicit declaration in the class and add explicit wording that says that if
T
is trivial a default constructed iterator is also literal, alternatively it would be possible to mark both as defaulted and add explicit (memberwise) wording that guarantees that they are trivial.Btw.: I'm quite sure that the
istreambuf_iterator
additions to ensure triviality are not sufficient as suggested, because the library does not yet give general guarantees that a defaulted special member declaration makes this member also trivial. Note that e.g. the atomic types do give a general statement!Finally there is a wording issue: There does not exist something like a "literal constructor". The core language uses the term "constexpr constructor" for this.
Suggestion:
Change 24.6.2 [istream.iterator]/3 as indicated:
constexpr istream_iterator(); istream_iterator(istream_type& s); istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default; ~istream_iterator() = default;Change 24.6.2.2 [istream.iterator.cons]/1 as indicated:
constexpr istream_iterator();-1- Effects: Constructs the end-of-stream iterator. If
T
is a literal type, then this constructor shall be a constexpr constructor.Change 24.6.2.2 [istream.iterator.cons]/3 as indicated:
istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;-3- Effects: Constructs a copy of
x
. IfT
is a literal type, then this constructor shall be a trivial copy constructor.Change 24.6.2.2 [istream.iterator.cons]/4 as indicated:
~istream_iterator() = default;-4- Effects: The iterator is destroyed. If
T
is a literal type, then this destructor shall be a trivial destructor.Change 24.6.4 [istreambuf.iterator] before p. 1 as indicated:
constexpr istreambuf_iterator() throw(); istreambuf_iterator(const istreambuf_iterator&) throw() = default; ~istreambuf_iterator() throw() = default;Change 24.6.4 [istreambuf.iterator]/1 as indicated:
[..] The default constructor
istreambuf_iterator()
and the constructoristreambuf_iterator(0)
both construct an end of stream iterator object suitable for use as an end-of-range. All specializations ofistreambuf_iterator
shall have a trivial copy constructor, a constexpr default constructor and a trivial destructor.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2994.
Proposed resolution:
24.6.2 [istream.iterator] para 3
constexpr istream_iterator();
24.6.2.2 [istream.iterator.cons]
constexpr istream_iterator();-1- Effects: Constructs the end-of-stream iterator. If
T
is a literal type, then this constructor shall be a literal constructor.
24.6.4 [istreambuf.iterator]
constexpr istreambuf_iterator() throw();
copy_exception
name misleadingSection: 17.9.7 [propagation] Status: C++11 Submitter: Peter Dimov Opened: 2009-05-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The naming of std::copy_exception
misleads almost everyone
(experts included!) to think that it is the function that copies an
exception_ptr
:
exception_ptr p1 = current_exception(); exception_ptr p2 = copy_exception( p1 );
But this is not at all what it does. The above actually creates an
exception_ptr p2
that contains a copy of p1
, not of
the exception to which p1
refers!
This is, of course, all my fault; in my defence, I used copy_exception
because I was unable to think of a better name.
But I believe that, based on what we've seen so far, any other name would be better.
Therefore, I propose copy_exception
to be renamed to
create_exception
:
template<class E> exception_ptr create_exception(E e);
with the following explanatory paragraph after it:
Creates an
exception_ptr
that refers to a copy ofe
.
[ 2009-05-13 Daniel adds: ]
What about
make_exception_ptrin similarity to
make_pair
andmake_tuple
,make_error_code
andmake_error_condition
, ormake_shared
? Or, if a stronger symmetry tocurrent_exception
is preferred:make_exceptionWe have not a single
create_*
function in the library, it was alwaysmake_*
used.
[ 2009-05-13 Peter adds: ]
make_exception_ptr
works for me.
[ 2009-06-02 Thomas J. Gritzan adds: ]
To avoid surprises and unwanted recursion, how about making a call to
std::make_exception_ptr
with anexception_ptr
illegal?It might work like this:
template<class E> exception_ptr make_exception_ptr(E e); template<> exception_ptr make_exception_ptr<exception_ptr>(exception_ptr e) = delete;
[ 2009 Santa Cruz: ]
Move to Review for the time being. The subgroup thinks this is a good idea, but doesn't want to break compatibility unnecessarily if someone is already shipping this. Let's talk to Benjamin and PJP tomorrow to make sure neither objects.
[ 2009-11-16 Jonathan Wakely adds: ]
GCC 4.4 shipped with
copy_exception
but we could certainly keep that symbol in the library (but not the headers) so it can still be found by any apps foolishly relying on the experimental C++0x mode being ABI stable.
[ 2009-11-16 Peter adopts wording supplied by Daniel. ]
[ 2009-11-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 17.9 [support.exception]/1, header <exception>
synopsis as indicated:
exception_ptr current_exception(); void rethrow_exception [[noreturn]] (exception_ptr p); template<class E> exception_ptrcopy_exceptionmake_exception_ptr(E e);
Change 17.9.7 [propagation]:
template<class E> exception_ptrcopy_exceptionmake_exception_ptr(E e);-11- Effects: Creates an
exception_ptr
that refers to a copy ofe
, as iftry { throw e; } catch(...) { return current_exception(); }...
Change 32.10.6 [futures.promise]/7 as indicated:
Effects: if the associated state of
*this
is not ready, stores an exception object of typefuture_error
with an error code ofbroken_promise
as if bythis->set_exception(
. Destroys ...copy_exceptionmake_exception_ptr( future_error(future_errc::broken_promise))
alignment_of
Section: 21.3.5.4 [meta.unary.prop] Status: C++11 Submitter: Niels Dekker Opened: 2009-06-01 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
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Discussion:
The alignment_of
template is no longer necessary, now that the
core language will provide alignof
. Scott Meyers raised this
issue at comp.std.c++,
C++0x: alignof vs. alignment_of,
May 21, 2009. In a reply, Daniel Krügler pointed out that
alignof
was added to the working paper after
alignment_of
. So it appears that alignment_of
is only
part of the current Working Draft
(N2857)
because it is in TR1.
Having both alignof
and alignment_of
would cause
unwanted confusion. In general, I think TR1 functionality should not be
brought into C++0x if it is entirely redundant with other C++0x language
or library features.
[ 2009-11-16 Chris adds: ]
I would like to suggest the following new wording for this issue, based on recent discussions. Basically this doesn't delete
alignment_of
, it just makes it clear that it is just a wrapper foralignof
. This deletes the first part of the proposed resolution, changes the second part by leaving inalignof(T)
but changing the precondition and leaves the 3rd part unchanged.Suggested Resolution:
Change the first row of Table 44 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:
Table 44 — Type property queries template <class T> struct alignment_of;
alignof(T)
.
Precondition:T
shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified)void
.alignof(T)
shall be definedChange text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:
Table 51 — Other transformations … aligned_storage;
Len
shall not be zero.Align
shall be equal tofor some type
alignment_of<T>::valuealignof(T)T
or to default-alignment.…
[ 2010-01-30 Alisdair proposes that Chris' wording be moved into the proposed wording section and tweaks it on the way. ]
Original proposed wording saved here:
Remove from Header <type_traits> synopsis 21.3.3 [meta.type.synop]:
template <class T> struct alignment_of;Remove the first row of Table 44 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:
Table 44 — Type property queries template <class T> struct alignment_of;
alignof(T)
.
Precondition:T
shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified)void
.Change text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:
Table 51 — Other transformations … aligned_storage;
Len
shall not be zero. Align shall be equal tofor some type
alignment_of<T>::valuealignof(T)T
or to default-alignment.…
[ 2010-01-30 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change the first row of Table 43 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:
Table 43 — Type property queries template <class T> struct alignment_of;
alignof(T)
.
Precondition:T
shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified)void
.alignof(T)
is a valid expression (7.6.2.6 [expr.alignof])
Change text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:
Table 51 — Other transformations … aligned_storage;
Len
shall not be zero.Align
shall be equal tofor some type
alignment_of<T>::valuealignof(T)T
or to default-alignment.…
Section: 23.3.7.6 [forward.list.ops], 23.3.11.5 [list.ops] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
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Discussion:
IIUC,
N2844
means that lvalues will no longer bind to rvalue references.
Therefore, the current specification of list::splice
(list
operations 23.3.11.5 [list.ops]) will be a breaking change of behaviour for existing
programs. That is because we changed the signature to swallow via an rvalue
reference rather than the lvalue reference used in 03.
Retaining this form would be safer, requiring an explicit move when splicing
from lvalues. However, this will break existing programs.
We have the same problem with forward_list
, although without the risk of
breaking programs so here it might be viewed as a positive feature.
The problem signatures:
void splice_after(const_iterator position, forward_list<T,Alloc>&& x); void splice_after(const_iterator position, forward_list<T,Alloc>&& x, const_iterator i); void splice_after(const_iterator position, forward_list<T,Alloc>&& x, const_iterator first, const_iterator last); void splice(const_iterator position, list<T,Alloc>&& x); void splice(const_iterator position, list<T,Alloc>&& x, const_iterator i); void splice(const_iterator position, list<T,Alloc>&& x, const_iterator first, const_iterator last);
Possible resolutions:
Option A. Add an additional (non-const) lvalue-reference overload in each case
Option B. Change rvalue reference back to (non-const) lvalue-reference overload in each case
Option C. Add an additional (non-const) lvalue-reference
overload in just the std::list
cases
I think (B) would be very unfortunate, I really like the forward_list
behaviour in (C) but feel (A) is needed for consistency.
My actual preference would be NAD, ship with this as a breaking change as it is a more explicit interface. I don't think that will fly though!
See the thread starting with c++std-lib-23725 for more discussion.
[ 2009-10-27 Christopher Jefferson provides proposed wording for Option C. ]
[ 2009-12-08 Jonathan Wakely adds: ]
As Bill Plauger pointed out,
list::merge
needs similar treatment.[ Wording updated. ]
[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In 23.3.11 [list]
Add lvalue overloads before rvalue ones:
void splice(const_iterator position, list<T,Allocator>& x); void splice(const_iterator position, list<T,Allocator>&& x); void splice(const_iterator position, list<T,Allocator>& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>& x, const_iterator first, const_iterator last); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last); void merge(list<T,Allocator>& x); template <class Compare> void merge(list<T,Allocator>& x, Compare comp); void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);
In 23.3.11.5 [list.ops], similarly add lvalue overload before each rvalue one:
(After paragraph 2)
void splice(const_iterator position, list<T,Allocator>& x); void splice(const_iterator position, list<T,Allocator>&& x);
(After paragraph 6)
void splice(const_iterator position, list<T,Allocator>& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i);
(After paragraph 10)
void splice(const_iterator position, list<T,Allocator>& x, const_iterator first, const_iterator last); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last);
In 23.3.11.5 [list.ops], after paragraph 21
void merge(list<T,Allocator>& x); template <class Compare> void merge(list<T,Allocator>& x, Compare comp); void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);
<stdint.h>
, <fenv.h>
, <tgmath.h>
,
and maybe <complex.h>
Section: 29.7 [c.math], 99 [stdinth], 99 [fenv], 99 [cmplxh] Status: C++11 Submitter: Robert Klarer Opened: 2009-05-26 Last modified: 2017-06-15
Priority: Not Prioritized
View all other issues in [c.math].
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Discussion:
This is probably editorial.
The following items should be removed from the draft, because they're redundant with Annex D, and they arguably make some *.h headers non-deprecated:
99 [stdinth] (regarding <stdint.h>
)
99 [fenv] (regarding <fenv.h>
Line 3 of 29.7 [c.math] (regarding <tgmath.h>
)
99 [cmplxh] (regarding <complex.h>
, though the note in this subclause is not redundant)
[ 2009-06-10 Ganesh adds: ]
While searching for
stdint
in the CD, I found that<stdint.h>
is also mentioned in 6.8.2 [basic.fundamental] p.5. I guess it should refer to<cstdint>
instead.
[ 2009 Santa Cruz: ]
Real issue. Maybe just editorial, maybe not. Move to Ready.
Proposed resolution:
Remove the section 99 [stdinth].
Remove the section 99 [fenv].
Remove 29.7 [c.math], p3:
-3- The header<tgmath.h>
effectively includes the headers<complex.h>
and<math.h>
.
Remove the section 99 [cmplxh].
exception_ptr
should support contextual conversion to bool
Section: 17.9.7 [propagation] Status: Resolved Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
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Discussion:
As of
N2857
17.9.7 [propagation] p.5, the implementation-defined type
exception_ptr
does provide the following ways to check whether
it is a null value:
void f(std::exception_ptr p) { p == nullptr; p == 0; p == exception_ptr(); }
This is rather cumbersome way of checking for the null value and I suggest to require support for evaluation in a boolean context like so:
void g(std::exception_ptr p) { if (p) {} !p; }
[ 2009 Santa Cruz: ]
Move to Ready. Note to editor: considering putting in a cross-reference to 7.3 [conv], paragraph 3, which defines the phrase "contextually converted to
bool
".
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
In section 17.9.7 [propagation] insert a new paragraph between p.5 and p.6:
An object
e
of typeexception_ptr
can be contextually converted tobool
. The effect shall be as ife != exception_ptr()
had been evaluated in place ofe
. There shall be no implicit conversion to arithmetic type, to enumeration type or to pointer type.
nested_exception::rethrow_nested()
Section: 17.9.8 [except.nested] Status: C++11 Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [except.nested].
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Discussion:
It was recently mentioned in a newsgroup article
http://groups.google.de/group/comp.std.c++/msg/f82022aff68edf3d
that the specification of the member function rethrow_nested()
of the
class nested_exception
is incomplete, specifically it remains unclear
what happens, if member nested_ptr()
returns a null value. In
17.9.8 [except.nested] we find only the following paragraph related to that:
void rethrow_nested() const; // [[noreturn]]-4- Throws: the stored exception captured by this
nested_exception
object.
This is a problem, because it is possible to create an object of
nested_exception
with exactly such a state, e.g.
#include <exception> #include <iostream> int main() try { std::nested_exception e; // OK, calls current_exception() and stores it's null value e.rethrow_nested(); // ? std::cout << "A" << std::endl; } catch(...) { std::cout << "B" << std::endl; }
I suggest to follow the proposal of the reporter, namely to invoke
terminate()
if nested_ptr()
return a null value of exception_ptr
instead
of relying on the fallback position of undefined behavior. This would
be consistent to the behavior of a throw;
statement when no
exception is being handled.
[ 2009 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change around 17.9.8 [except.nested] p.4 as indicated:
-4- Throws: the stored exception captured by this
nested_exception
object, ifnested_ptr() != nullptr
- Remarks: If
nested_ptr() == nullptr
,terminate()
shall be called.
conj
and proj
Section: 29.4.10 [cmplx.over] Status: C++11 Submitter: Marc Steinbach Opened: 2009-06-11 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [cmplx.over].
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Discussion:
In clause 1, the Working Draft (N2857) specifies overloads of the functions
arg, conj, imag, norm, proj, real
for non-complex arithmetic types (float
, double
,
long double
, and integers).
The only requirement (clause 2) specifies effective type promotion of arguments.
I strongly suggest to add the following requirement on the return types:
All the specified overloads must return real (i.e., non-complex) values, specifically, the nested
value_type
of effectively promoted arguments.
(This has no effect on arg
, imag
, norm
, real
:
they are real-valued anyway.)
Rationale:
Mathematically, conj()
and proj()
, like the transcendental functions, are
complex-valued in general but map the (extended) real line to itself.
In fact, both functions act as identity on the reals.
A typical user will expect conj()
and proj()
to preserve this essential
mathematical property in the same way as exp()
, sin()
, etc.
A typical use of conj()
, e.g., is the generic scalar product of n-vectors:
template<typename T> inline T scalar_product(size_t n, T const* x, T const* y) { T result = 0; for (size_t i = 0; i < n; ++i) result += x[i] * std::conj(y[i]); return result; }
This will work equally well for real and complex floating-point types T
if
conj()
returns T
. It will not work with real types if conj()
returns complex values.
Instead, the implementation of scalar_product
becomes either less efficient
and less useful (if a complex result is always returned), or unnecessarily
complicated (if overloaded versions with proper return types are defined).
In the second case, the real-argument overload of conj()
cannot be used.
In fact, it must be avoided.
Overloaded conj()
and proj()
are principally needed in generic programming.
All such use cases will benefit from the proposed return type requirement,
in a similar way as the scalar_product
example.
The requirement will not harm use cases where a complex return value
is expected, because of implicit conversion to complex.
Without the proposed return type guarantee, I find overloaded versions
of conj()
and proj()
not only useless but actually troublesome.
[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Insert a new paragraph after 29.4.10 [cmplx.over]/2:
All of the specified overloads shall have a return type which is the nested
value_type
of the effectively cast arguments.
operator+
Section: 27.4.4.1 [string.op.plus] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-06-12 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Many of the basic_string operator+
overloads return an rvalue-reference. Is
that really intended?
I'm considering it might be a mild performance tweak to avoid making un-necessary copies of a cheaply movable type, but it opens risk to dangling references in code like:
auto && s = string{"x"} + string{y};
and I'm not sure about:
auto s = string{"x"} + string{y};
[ 2009-10-11 Howard updated Returns: clause for each of these. ]
[ 2009-11-05 Howard adds: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Strike the &&
from the return type in the following function
signatures:
27.4 [string.classes] p2 Header Synopsis
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, const basic_string<charT,traits,Allocator>& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(const basic_string<charT,traits,Allocator>& lhs, basic_string<charT,traits,Allocator>&& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, basic_string<charT,traits,Allocator>&& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(const charT* lhs, basic_string<charT,traits,Allocator>&& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, const charT* rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);[string.op+]
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, const basic_string<charT,traits,Allocator>& rhs);Returns:
std::move(lhs.append(rhs))
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(const basic_string<charT,traits,Allocator>& lhs, basic_string<charT,traits,Allocator>&& rhs);Returns:
std::move(rhs.insert(0, lhs))
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, basic_string<charT,traits,Allocator>&& rhs);Returns:
std::move(lhs.append(rhs))
[Note: Or equivalentlystd::move(rhs.insert(0, lhs))
— end note]template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(const charT* lhs, basic_string<charT,traits,Allocator>&& rhs);Returns:
std::move(rhs.insert(0, lhs))
.template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs);Returns:
std::move(rhs.insert(0, 1, lhs))
.template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, const charT* rhs);Returns:
std::move(lhs.append(rhs))
.template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);Returns:
std::move(lhs.append(1, rhs))
.
Section: 32.5 [atomics] Status: Resolved Submitter: LWG Opened: 2009-06-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics].
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Discussion:
Addresses US 87, UK 311
The atomics chapter is not concept enabled.
Needs to also consider issues 923(i) and 924(i).
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Section: 17.5 [support.start.term] Status: C++11 Submitter: LWG Opened: 2009-06-16 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [support.start.term].
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Discussion:
Addresses UK 187
The term "thread safe" is not defined nor used in this context anywhere else in the standard.
Suggested action:
Clarify the meaning of "thread safe".
[ 2009 Santa Cruz: ]
The "thread safe" language has already been change in the WP. It was changed to "happen before", but the current WP text is still a little incomplete: "happen before" is binary, but the current WP text only mentions one thing.
Move to Ready.
Proposed resolution:
For the following functions in 17.5 [support.start.term].
extern "C" int at_quick_exit(void (*f)(void)); extern "C++" int at_quick_exit(void (*f)(void));
Edit paragraph 10 as follows. The intent is to provide the other half of the happens before relation; to note indeterminate ordering; and to clean up some formatting.
Effects: The
at_quick_exit()
functions register the function pointed to byf
to be called without arguments whenquick_exit
is called. It is unspecified whether a call toat_quick_exit()
that does nothappen-beforehappen before (1.10) all calls toquick_exit
will succeed. [Note: theat_quick_exit()
functions shall not introduce a data race (17.6.4.7).exitnote—end note] [Note: The order of registration may be indeterminate ifat_quick_exit
was called from more than one thread. —end note] [Note: Theat_quick_exit
registrations are distinct from theatexit
registrations, and applications may need to call both registration functions with the same argument. —end note]
For the following function.
void quick_exit [[noreturn]] (int status)
Edit paragraph 13 as follows. The intent is to note that thread-local variables may be different.
Effects: Functions registered by calls to
at_quick_exit
are called in the reverse order of their registration, except that a function shall be called after any previously registered functions that had already been called at the time it was registered. Objects shall not be destroyed as a result of callingquick_exit
. If control leaves a registered function called byquick_exit
because the function does not provide a handler for a thrown exception,terminate()
shall be called. [Note: Functions registered by one thread may be called by any thread, and hence should not rely on the identity of thread-storage-duration objects. —end note] After calling registered functions,quick_exit
shall call_Exit(status)
. [Note: The standard file buffers are not flushed. See: ISO C 7.20.4.4. —end note]
Section: 32.5 [atomics] Status: Resolved Submitter: LWG Opened: 2009-06-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 312
The contents of the <stdatomic.h>
header are not listed anywhere,
and <cstdatomic>
is listed as a C99 header in chapter 17.
If we intend to use these for compatibility with a future C standard,
we should not use them now.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2992.
Proposed resolution:
Remove <cstdatomic>
from the C99 headers in table 14.
Add a new header <atomic>
to the headers in table 13.
Update chapter 29 to remove reference to <stdatomic.h>
and replace the use of <cstdatomic>
with <atomic>
.
[ If and when WG14 adds atomic operations to C we can add corresponding headers to table 14 with a TR. ]
Section: 32.5.5 [atomics.lockfree] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2016-02-25
Priority: Not Prioritized
View all other issues in [atomics.lockfree].
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Discussion:
Addresses US 88
The "lockfree" facilities do not tell the programmer enough.
There are 2 problems here.
First, at least on x86,
it's less important to me whether some integral types are lock free
than what is the largest type I can pass to atomic and have it be lock-free.
For example, if long long
s are not lock-free,
ATOMIC_INTEGRAL_LOCK_FREE
is probably 1,
but I'd still be interested in knowing whether longs are always lock-free.
Or if long longs at any address are lock-free,
I'd expect ATOMIC_INTEGRAL_LOCK_FREE
to be 2,
but I may actually care whether I have access to
the cmpxchg16b
instruction.
None of the support here helps with that question.
(There are really 2 related questions here:
what alignment requirements are there for lock-free access;
and what processor is the program actually running on,
as opposed to what it was compiled for?)
Second, having atomic_is_lock_free
only apply to individual objects
is pretty useless
(except, as Lawrence Crowl points out,
for throwing an exception when an object is unexpectedly not lock-free).
I'm likely to want to use its result to decide what algorithm to use,
and that algorithm is probably going to allocate new memory
containing atomic objects and then try to act on them.
If I can't predict the lock-freedom of the new object
by checking the lock-freedom of an existing object,
I may discover after starting the algorithm that I can't continue.
[ 2009-06-16 Jeffrey Yasskin adds: ]
To solve the first problem, I think 2 macros would help:
MAX_POSSIBLE_LOCK_FREE_SIZE
andMAX_GUARANTEED_LOCK_FREE_SIZE
, which expand to the maximum value ofsizeof(T)
for which atomic may (or will, respectively) use lock-free operations. Lawrence points out that this "relies heavily on implementations using word-size compare-swap on sub-word-size types, which in turn requires address modulation." He expects that to be the end state anyway, so it doesn't bother him much.To solve the second, I think one could specify that equally aligned objects of the same type will return the same value from
atomic_is_lock_free()
. I don't know how to specify "equal alignment". Lawrence suggests an additional function,atomic_is_always_lock_free()
.
[ 2009-10-22 Benjamin Kosnik: ]
In the evolution discussion of N2925, "More Collected Issues with Atomics," there is an action item with respect to LWG 1146(i), US 88
This is stated in the paper as:
Relatedly, Mike Sperts will create an issue to propose adding a traits mechanism to check the compile-time properties through a template mechanism rather than macros
Here is my attempt to do this. I don't believe that a separate trait is necessary for this, and that instead
atomic_integral::is_lock_free
can be re-purposed with minimal work as follows.[ Howard: Put Benjamin's wording in the proposed wording section. ]
[ 2009-10-22 Alberto Ganesh Barbati: ]
Just a thought... wouldn't it be better to use a scoped enum instead of plain integers? For example:
enum class is_lock_free { never = 0, sometimes = 1, always = 2; };if compatibility with C is deemed important, we could use an unscoped enum with suitably chosen names. It would still be more descriptive than 0, 1 and 2.
Previous resolution [SUPERSEDED]:
Header
<cstdatomic>
synopsis [atomics.synopsis]Edit as follows:
namespace std { ... // 29.4, lock-free property#define ATOMIC_INTEGRAL_LOCK_FREE unspecified#define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_ADDRESS_LOCK_FREE unspecifiedLock-free Property 32.5.5 [atomics.lockfree]
Edit the synopsis as follows.
namespace std {#define ATOMIC_INTEGRAL_LOCK_FREE unspecified#define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_ADDRESS_LOCK_FREE unspecified }Edit paragraph 1 as follows.
The ATOMIC_...._LOCK_FREE macros
ATOMIC_INTEGRAL_LOCK_FREE and ATOMIC_ADDRESS_LOCK_FREEindicate the general lock-free property ofintegral and address atomicthe corresponding atomic integral types, with the signed and unsigned variants grouped together.The properties also apply to the corresponding specializations of the atomic template.A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.Operations on Atomic Types 32.5.8.2 [atomics.types.operations]
Edit as follows.
voidstatic constexpr bool A::is_lock_free() const volatile;Returns: True if the
object'stypes's operations are lock-free, false otherwise. [Note: In the same way that<limits>
std::numeric_limits<short>::max()
is related to<limits.h>
__LONG_LONG_MAX__
,<atomic> std::atomic_short::is_lock_free
is related to<stdatomic.h>
andATOMIC_SHORT_LOCK_FREE
— end note]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2992.
Proposed resolution:
Resolved by N2992.Section: 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US 90
The C++0X draft declares all of the functions dealing with atomics (section 32.5.8.2 [atomics.types.operations]) to take volatile arguments. Yet it also says (29.4-3),
[ Note: Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. Thus, volatile qualified operations on non-volatile objects may be merged under some conditions. — end note ]
I was thinking about how to implement this in gcc, and I believe that we'll want to overload most of the functions on volatile and non-volatile. Here's why:
To let the compiler take advantage of the permission
to merge non-volatile atomic operations and reorder atomics in certain,
we'll need to tell the compiler backend
about exactly which atomic operation was used.
So I expect most of the functions of the form atomic_<op>_explicit()
(e.g. atomic_load_explicit
, atomic_exchange_explicit
,
atomic_fetch_add_explicit
, etc.)
to become compiler builtins.
A builtin can tell whether its argument was volatile or not,
so those functions don't really need extra explicit overloads.
However, I don't expect that we'll want to add builtins
for every function in chapter 29,
since most can be implemented in terms of the _explicit
free functions:
class atomic_int {
__atomic_int_storage value;
public:
int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
// &value has type "volatile __atomic_int_storage*".
atomic_fetch_add_explicit(&value, increment, order);
}
...
};
But now this always calls
the volatile builtin version of atomic_fetch_add_explicit()
,
even if the atomic_int
wasn't declared volatile.
To preserve volatility and the compiler's permission to optimize,
I'd need to write:
class atomic_int {
__atomic_int_storage value;
public:
int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
atomic_fetch_add_explicit(&value, increment, order);
}
int fetch_add(int increment, memory_order order = memory_order_seq_cst) {
atomic_fetch_add_explicit(&value, increment, order);
}
...
};
But this is visibly different from the declarations in the standard
because it's now overloaded.
(Consider passing &atomic_int::fetch_add
as a template parameter.)
The implementation may already have permission to add overloads to the member functions:
16.4.6.5 [member.functions] An implementation may declare additional non-virtual member function signatures within a class:
...
- by adding a member function signature for a member function name.
but I don't see an equivalent permission to add overloads to the free functions.
[ 2009-06-16 Lawrence adds: ]
I recommend allowing non-volatile overloads.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Section: 31.10.6 [fstream] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2018-07-02
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses JP 73
Description
It is a problem
from C++98, fstream
cannot appoint a filename of wide
character string(const wchar_t
and const wstring&
).
Suggestion
Add
interface corresponding to wchar_t
, char16_t
and char32_t
.
[ 2009-07-01 Alisdair notes that this is a duplicate of 454(i) which has more in-depth rationale. ]
[ 2009-09-21 Daniel adds: ]
I suggest to mark this issue as NAD Future with the intend to solve the issue with a single file path c'tor template assuming a provision of a TR2 filesystem library.
[ 2009 Santa Cruz: ]
[LEWG Kona 2017]
Recommend NAD: Needs a paper. Recommend providing overload that takes filesystem::path. Note that there are other similar issues elsewhere in the library, for example basic_filebuf. Handled by another issue.
Proposed resolution:
Resolved by the adoption of 2676(i).
Section: 16 [library] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US 63
Description
The behavior of the library in the presence of threads is incompletely specified.
For example, if thread 1 assigns to X
, then writes data
to file f
, which is read by thread 2, and then accesses
variable X
, is thread 2 guaranteed to be able to see the
value assigned to X
by thread 1? In other words, does the
write of the data "happen before" the read?
Another example: does simultaneous access using operator
at()
to different characters in the same non-const string
really introduce a data race?
Suggestion
Notes
17 SG: should go to threads group; misclassified in document
Concurrency SG: Create an issue. Hans will look into it.
[ 2009 Santa Cruz: ]
Move to "Open". Hans and the rest of the concurrency working group will study this. We can't make progress without a thorough review and a paper.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3069.
Proposed resolution:
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++11 Submitter: Seungbeom Kim Opened: 2009-06-27 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [facet.num.put.virtuals].
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Discussion:
In Table 73 — Floating-point conversions, 28.3.4.3.3.3 [facet.num.put.virtuals], in N2914, we have the following entries:
State | stdio equivalent |
---|---|
floatfield == ios_base::fixed | ios_base::scientific && !uppercase |
%a |
floatfield == ios_base::fixed | ios_base::scientific |
%A |
These expressions are supposed to mean:
floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase floatfield == (ios_base::fixed | ios_base::scientific)
but technically parsed as:
((floatfield == ios_base::fixed) | ios_base::scientific) && (!uppercase) ((floatfield == ios_base::fixed) | ios_base::scientific)
and should be corrected with additional parentheses, as shown above.
[ 2009-10-28 Howard: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Change Table 83 — Floating-point conversions in 28.3.4.3.3.3 [facet.num.put.virtuals]:
State | stdio equivalent |
---|---|
floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase |
%a |
floatfield == (ios_base::fixed | ios_base::scientific) |
%A |
Section: 16.4.5.2.1 [namespace.std] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 175
Description
Local types can now be used to instantiate templates, but don't have external linkage.
Suggestion
Remove the reference to external linkage.
Notes
We accept the proposed solution. Martin will draft an issue.
[ 2009-07-28 Alisdair provided wording. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
16.4.5.2.1 [namespace.std]
Strike "of external linkage" in p1 and p2:
-1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace
std
or to a namespace within namespacestd
unless otherwise specified. A program may add a concept map for any standard library concept or a template specialization for any standard library template to namespacestd
only if the declaration depends on a user-defined typeof external linkageand the specialization meets the standard library requirements for the original template and is not explicitly prohibited.179-2- The behavior of a C++ program is undefined if it declares
- an explicit specialization of any member function of a standard library class template, or
- an explicit specialization of any member function template of a standard library class or class template, or
- an explicit or partial specialization of any member class template of a standard library class or class template.
A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type
of external linkageand the instantiation meets the standard library requirements for the original template.
Section: 32.2.4 [thread.req.timing] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.req.timing].
View all issues with C++11 status.
Discussion:
Addresses UK 322, US 96
Description
Not all systems can provide a monotonic clock. How are they expected to treat a _for function?
Suggestion
Add at least a note explaining the intent for systems that do not support a monotonic clock.
Notes
Create an issue, together with UK 96. Note that the specification as is already allows a non-monotonic clock due to the word “should” rather than “shall”. If this wording is kept, a footnote should be added to make the meaning clear.
[ 2009-06-29 Beman provided a proposed resolution. ]
[ 2009-10-31 Howard adds: ]
Set to Tentatively Ready after 5 positive votes on c++std-lib.
[ 2010-02-24 Pete moved to Open: ]
LWG 1158's proposed resolution replaces the ISO-specified normative term "should" with "are encouraged but not required to", which presumably means the same thing, but has no ISO normative status. The WD used the latter formulation in quite a few non-normative places, but only three normative ones. I've changed all the normative uses to "should".
[ 2010-03-06 Beman updates wording. ]
[ 2010 Pittsburgh: Moved to Ready. ]
Proposed resolution:
Change Timing specifications 32.2.4 [thread.req.timing] as indicated:
The member functions whose names end in _for
take an argument that
specifies a relative time. Implementations should use a monotonic clock to
measure time for these functions. [Note: Implementations are not
required to use a monotonic clock because such a clock may be unavailable.
— end note]
resource_deadlock_would_occur
Section: 32.6.5.4.3 [thread.lock.unique.locking] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.lock.unique.locking].
View all issues with C++11 status.
Duplicate of: 1219
Discussion:
Addresses UK 327, UK 328
UK 327 Description
Not clear what
the specification for error condition
resource_deadlock_would_occur
means. It is perfectly
possible for this thread to own the mutex without setting
owns to true on this specific lock object. It is also
possible for lock operations to succeed even if the thread
does own the mutex, if the mutex is recursive. Likewise, if
the mutex is not recursive and the mutex has been locked
externally, it is not always possible to know that this
error condition should be raised, depending on the host
operating system facilities. It is possible that 'i.e.' was
supposed to be 'e.g.' and that suggests that recursive
locks are not allowed. That makes sense, as the
exposition-only member owns is boolean and not a integer to
count recursive locks.
UK 327 Suggestion
Add a precondition !owns
. Change the 'i.e.'
in the error condition to be 'e.g.' to allow for this
condition to propogate deadlock detection by the host OS.
UK 327 Notes
Create an issue. Assigned to Lawrence Crowl. Note: not sure what try_lock means for recursive locks when you are the owner. POSIX has language on this, which should ideally be followed. Proposed fix is not quite right, for example, try_lock should have different wording from lock.
UK 328 Description
There is a missing precondition that owns
is true, or an if(owns)
test is missing from the effect
clause
UK 328 Suggestion
Add a
precondition that owns == true
. Add an error condition to
detect a violation, rather than yield undefined behaviour.
UK 328 Notes
Handle in same issue as UK 327. Also uncertain that the proposed resolution is the correct one.
[ 2009-11-11 Alisdair notes that this issue is very closely related to 1219(i), if not a dup. ]
[ 2010-02-12 Anthony provided wording. ]
[ 2010 Pittsburgh: ]
Wording updated and moved to Ready for Pittsburgh.
Proposed resolution:
Modify 32.6.5.4.3 [thread.lock.unique.locking] p3 to say:
void lock();...
3 Throws: Any exception thrown by
pm->lock()
.std::system_error
if an exception is required (32.2.2 [thread.req.exception]).std::system_error
with an error condition ofoperation_not_permitted
ifpm
is0
.std::system_error
with an error condition ofresource_deadlock_would_occur
if on entryowns
istrue
.std::system_error
when the postcondition cannot be achieved.
Remove 32.6.5.4.3 [thread.lock.unique.locking] p4 (Error condition clause).
Modify 32.6.5.4.3 [thread.lock.unique.locking] p8 to say:
bool try_lock();...
8 Throws: Any exception thrown by
pm->try_lock()
.std::system_error
if an exception is required (32.2.2 [thread.req.exception]).std::system_error
with an error condition ofoperation_not_permitted
ifpm
is0
.std::system_error
with an error condition ofresource_deadlock_would_occur
if on entryowns
istrue
.std::system_error
when the postcondition cannot be achieved.
Remove 32.6.5.4.3 [thread.lock.unique.locking] p9 (Error condition clause).
Modify 32.6.5.4.3 [thread.lock.unique.locking] p13 to say:
template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);...
13 Throws: Any exception thrown by
pm->try_lock_until()
.std::system_error
if an exception is required (32.2.2 [thread.req.exception]).std::system_error
with an error condition ofoperation_not_permitted
ifpm
is0
.std::system_error
with an error condition ofresource_deadlock_would_occur
if on entryowns
istrue
.std::system_error
when the postcondition cannot be achieved.
Remove 32.6.5.4.3 [thread.lock.unique.locking] p14 (Error condition clause).
Modify 32.6.5.4.3 [thread.lock.unique.locking] p18 to say:
template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);...
18 Throws: Any exception thrown by
pm->try_lock_for()
.std::system_error
if an exception is required (32.2.2 [thread.req.exception]).std::system_error
with an error condition ofoperation_not_permitted
ifpm
is0
.std::system_error
with an error condition ofresource_deadlock_would_occur
if on entryowns
istrue
.std::system_error
when the postcondition cannot be achieved.
Remove 32.6.5.4.3 [thread.lock.unique.locking] p19 (Error condition clause).
future_error
public constructor is 'exposition only'Section: 32.10.4 [futures.future.error] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses UK 331
Description
Not clear what it means for a public constructor to be 'exposition only'. If the intent is purely to support the library calling this constructor then it can be made private and accessed through friendship. Otherwise it should be documented for public consumption.
Suggestion
Declare the constructor as private with a note about intended friendship, or remove the exposition-only comment and document the semantics.
Notes
Create an issue. Assigned to Detlef. Suggested resolution probably makes sense.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2997.
Proposed resolution:
unique_future
limitationsSection: 32.10.7 [futures.unique.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with Resolved status.
Discussion:
Addresses UK 336
Description
It is possible
to transfer ownership of the asynchronous result from one
unique_future instance to another via the move-constructor.
However, it is not possible to transfer it back, and nor is
it possible to create a default-constructed unique_future
instance to use as a later move target. This unduly limits
the use of unique_future
in code. Also, the lack of a
move-assignment operator restricts the use of unique_future
in containers such as std::vector
- vector::insert
requires
move-assignable for example.
Suggestion
Add a default constructor with the
semantics that it creates a unique_future
with no
associated asynchronous result. Add a move-assignment
operator which transfers ownership.
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
shared_future
should support an efficient move constructorSection: 32.10.8 [futures.shared.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.shared.future].
View all issues with Resolved status.
Discussion:
Addresses UK 337
Description
shared_future
should support an efficient move constructor that can avoid
unnecessary manipulation of a reference count, much like
shared_ptr
Suggestion
Add a move constructor
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
shared_future
is inconsistent with shared_ptr
Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.shared.future].
View all issues with Resolved status.
Discussion:
Addresses UK 338
Description
shared_future
is currently
CopyConstructible
, but not CopyAssignable
. This is
inconsistent with shared_ptr
, and will surprise users.
Users will then write work-arounds to provide this
behaviour. We should provide it simply and efficiently as
part of shared_future. Note that since the shared_future
member functions for accessing the state are all declared
const, the original usage of an immutable shared_future
value that can be freely copied by multiple threads can be
retained by declaring such an instance as "const
shared_future
".
Suggestion
Remove "=delete"
from the copy-assignment operator of shared_future. Add a
move-constructor shared_future(shared_future&&
rhs)
, and a move-assignment operator shared_future&
operator=(shared_future&& rhs)
. The postcondition
for the copy-assignment operator is that *this
has the same
associated state as rhs
. The postcondition for the
move-constructor and move assignment is that *this
has the
same associated as rhs
had before the
constructor/assignment call and that rhs
has no associated
state.
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Adressed by N2997.
Proposed resolution:
promise
move constructorSection: 32.10.6 [futures.promise] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
View all issues with Resolved status.
Discussion:
Addresses UK 343
Description
The move constructor of a std::promise
object does not need to allocate any memory, so the
move-construct-with-allocator overload of the constructor
is superfluous.
Suggestion
Remove the constructor with the signature template <class
Allocator> promise(allocator_arg_t, const Allocator&
a, promise& rhs);
Notes
Create an issue. Detlef will look into it. Will solicit feedback from Pablo. Note that "rhs" argument should also be an rvalue reference in any case.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Adressed by N2997.
Proposed resolution:
Section: 99 [allocator.propagation], 99 [allocator.propagation.map], 23 [containers] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses US 77
Description
Allocator-specific move and copy behavior for containers (N2525) complicates a little-used and already-complicated portion of the standard library (allocators), and breaks the conceptual model of move-constructor and move-assignment operations on standard containers being efficient operations. The extensions for allocator-specific move and copy behavior should be removed from the working paper.
With the introduction of rvalue references, we are teaching
programmers that moving from a standard container (e.g., a
vector<string>
) is an efficient, constant-time
operation. The introduction of N2525 removed that
guarantee; depending on the behavior of four different
traits (20.8.4), the complexity of copy and move operations
can be constant or linear time. This level of customization
greatly increases the complexity of standard containers,
and benefits only a tiny fraction of the C++ community.
Suggestion
Remove 20.8.4.
Remove 20.8.5.
Remove all references to the facilities in 20.8.4 and 20.8.5 from clause 23.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
num_get
not fully compatible with strto*
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++17 Submitter: Cosmin Truta Opened: 2009-07-04 Last modified: 2017-07-30
Priority: 3
View other active issues in [facet.num.get.virtuals].
View all other issues in [facet.num.get.virtuals].
View all issues with C++17 status.
Discussion:
As specified in the latest draft,
N2914,
num_get
is still not fully compatible with the following C
functions: strtoul
, strtoull
,
strtof
and
strtod
.
In C, when conversion of a string to an unsigned integer type falls
outside the
representable range, strtoul
and strtoull
return
ULONG_MAX
and ULLONG_MAX
, respectively,
regardless
whether the input field represents a positive or a negative value.
On the other hand, the result of num_get
conversion of
negative
values to unsigned integer types is zero. This raises a compatibility
issue.
Moreover, in C, when conversion of a string to a floating-point type falls
outside the representable range, strtof
, strtod
and
strtold
return ±HUGE_VALF
,
±HUGE_VAL
and ±HUGE_VALL
, respectively.
On the other hand, the result of num_get
conversion of such
out-of-range floating-point values results in the most positive/negative
representable value.
Although many C library implementations do implement HUGE_VAL
(etc.) as the highest representable (which is, usually, the infinity),
this isn't required by the C standard. The C library specification makes no
statement regarding the value of HUGE_VAL
and friends, which
potentially raises the same compatibility issue as in the above case of
unsigned integers.
In addition, neither C nor C++ define symbolic constants for the maximum
representable floating-point values (they only do so only for the maximum
representable finite floating-point values), which raises a
usability
issue (it would be hard for the programmer to check the result of
num_get
against overflow).
As such, we propose to adjust the specification of num_get
to
closely follow the behavior of all of its underlying C functions.
[ 2010 Rapperswil: ]
Some concern that this is changing the specification for an existing C++03 function, but it was pointed out that this was underspecified as resolved by issue 23. This is clean-up for that issue in turn. Some concern that we are trying to solve the same problem in both clause 22 and 27.
Bill: There's a change here as to whether val is stored to in an error case.
Pablo: Don't think this changes whether val is stored to or not, but changes the value that is stored.
Bill: Remembers having skirmishes with customers and testers as to whether val is stored to, and the resolution was not to store in error cases.
Howard: Believes since C++03 we made a change to always store in overflow.
Everyone took some time to review the issue.
Pablo: C++98 definitely did not store any value during an error condition.
Dietmar: Depends on the question of what is considered an error, and whether overflow is an error or not, which was the crux of LWG 23.
Pablo: Yes, but given the "zero, if the conversion function fails to convert the entire field", we are requiring every error condition to store.
Bill: When did this happen?
Alisdair: One of the last two or three meetings.
Dietmar: To store a value in case of failure is a very bad idea.
Move to Open, needs more study.
[2011-03-24 Madrid meeting]
Move to deferred
[ 2011 Bloomington ]
The proposed wording looks good, no-one sure why this was held back before. Move to Review.
[2012,Kona]
Move to Open.
THe issues is what to do with -1
. Should it match 'C' or do the "sane" thing.
A fix here changes behavior, but is probably what we want.
Pablo to provide wording, with help from Howard.
[2015-05-06 Lenexa: Move to Ready]
STL: I like that this uses strtof, which I think is new in C99. that avoids truncation from using atof. I have another issue ...
MC: yes LWG 2403 (stof should call strtof)
PJP: the last line is horrible, you don't assign to err, you call setstate(ios_base::failbit). Ah, no, this is inside num_get so the caller does the setstate.
MC: we need all these words. are they the right words?
JW: I'd like to take a minute to check my impl. Technically this implies a change in behaviour (from always using strtold and checking the extracted floating point value, to using the right function). Oh, we already do exactly this.
MC: Move to Ready
6 in favor, none opposed, 1 abstention
Proposed resolution:
Change 28.3.4.3.2.3 [facet.num.get.virtuals] as follows:
Stage 3: The sequence of
char
s accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header<cstdlib>
:
- For a signed integer value, the function
strtoll
.- For an unsigned integer value, the function
strtoull
.- For a
float
value, the functionstrtof
.- For a
double
value, the functionstrtod
.- For a
floating-pointlong double
value, the functionstrtold
.The numeric value to be stored can be one of:
- zero, if the conversion function fails to convert the entire field.
ios_base::failbit
is assigned toerr
.- the most positive (or negative) representable value, if the field to be converted to a signed integer type represents a value too large positive (or negative) to be represented in
val
.ios_base::failbit
is assigned toerr
.the most negative representable value or zero for an unsigned integer type, if the field represents a value too large negative to be represented inval
.ios_base::failbit
is assigned toerr
.- the most positive representable value, if the field to be converted to an unsigned integer type represents a value that cannot be represented in
val
.- the converted value, otherwise.
The resultant numeric value is stored in
val
. If the conversion function fails to convert the entire field, or if the field represents a value outside the range of representable values,ios_base::failbit
is assigned toerr
.
Section: 27.1 [strings.general] Status: C++11 Submitter: Beman Dawes Opened: 2009-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [strings.general].
View all issues with C++11 status.
Discussion:
Addresses UK 218
Prior to the introduction of constant expressions into the library,
basic_string
elements had to be POD types, and thus had to be both trivially
copyable and standard-layout. This ensured that they could be memcpy'ed and
would be compatible with other libraries and languages, particularly the C
language and its library.
N2349,
Constant Expressions in the Standard Library Revision 2, changed the
requirement in 21/1 from "POD type" to "literal type". That change had the
effect of removing the trivially copyable and standard-layout requirements from
basic_string
elements.
This means that basic_string
elements no longer are guaranteed to be
memcpy'able, and are no longer guaranteed to be standard-layout types:
3.9/p2 and 3.9/p3 both make it clear that a "trivially copyable type" is required for memcpy to be guaranteed to work.
Literal types (3.9p12) may have a non-trivial copy assignment operator, and that violates the trivially copyable requirements given in 9/p 6, bullet item 2.
Literal types (3.9p12) have no standard-layout requirement, either.
This situation probably arose because the wording for "Constant Expressions in the Standard Library" was in process at the same time the C++ POD deconstruction wording was in process.
Since trivially copyable types meet the C++0x requirements for literal types,
and thus work with constant expressions, it seems an easy fix to revert the
basic_string
element wording to its original state.
[ 2009-07-28 Alisdair adds: ]
When looking for any resolution for this issue, consider the definition of "character container type" in 3.10 [defns.character.container]. This does require the character type to be a POD, and this term is used in a number of places through clause 21 and 28. This suggests the PODness constraint remains, but is much more subtle than before. Meanwhile, I suspect the change from POD type to literal type was intentional with the assumption that trivially copyable types with non-trivial-but-constexpr constructors should serve as well. I don't believe the current wording offers the right guarantees for either of the above designs.
[ 2009-11-04 Howard modifies proposed wording to disallow array types as char-like types. ]
[ 2010-01-23 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change General 27.1 [strings.general] as indicated:
This Clause describes components for manipulating sequences of any
literalnon-array POD (3.9) type. In this Clause such types are called char-like types, and objects of char-like types are called char-like objects or simply characters.
Section: 30.5 [time.duration] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-07-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration].
View all issues with C++11 status.
Discussion:
The duration
types in 30.5 [time.duration] are exactly the sort of type
that should be "literal types" in the new standard. Likewise,
arithmetic operations on duration
s should be declared constexpr
.
[ 2009-09-21 Daniel adds: ]
An alternative (and possibly preferable solution for potentially heap-allocating
big_int
representation types) would be to ask the core language to allow references toconst
literal types as feasible arguments forconstexpr
functions.
[ 2009-10-30 Alisdair adds: ]
I suggest this issue moves from New to Open.
Half of this issue was dealt with in paper n2994 on constexpr constructors.
The other half (duration arithmetic) is on hold pending Core support for
const &
inconstexpr
functions.
[ 2010-03-15 Alisdair updated wording to be consistent with N3078. ]
[ 2010 Rapperswil: ]
This issue was the motivation for Core adding the facility for
constexpr
functions to take parameters byconst &
. Move to Tentatively Ready.
[ Adopted at 2010-11 Batavia. ]
Proposed resolution:
Add constexpr
to declaration of following functions and constructors:
Modify p1 30 [time], and the prototype definitions in 30.5.6 [time.duration.nonmember], 30.5.7 [time.duration.comparisons], and 30.5.8 [time.duration.cast]:
Header
<chrono>
synopsis// duration arithmetic template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type constexpr operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type constexpr operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> constexpr operator*(const duration<Rep1, Period>& d, const Rep2& s); template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> constexpr operator*(const Rep1& s, const duration<Rep2, Period>& d); template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> constexpr operator/(const duration<Rep1, Period>& d, const Rep2& s); template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<Rep1, Rep2>::type constexpr operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); // duration comparisons template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator==(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator!=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator< (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator<=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator> (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator>=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); // duration_cast template <class ToDuration, class Rep, class Period> constexpr ToDuration duration_cast(const duration<Rep, Period>& d);
Change 30.5 [time.duration]:
template <class Rep, class Period = ratio<1>> class duration { ... public: ... constexpr duration(const duration&) = default; ... };
[
Note - this edit already seems assumed by definition of the duration static members
zero/min/max
. They cannot meaningfully be constexpr
without
this change.
]
select_on_container_(copy|move)_construction
over-constrainedSection: 99 [allocator.concepts.members] Status: Resolved Submitter: Alberto Ganesh Barbati Opened: 2009-07-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
I believe the two functions
select_on_container_(copy|move)_construction()
are over-constrained. For
example, the return value of the "copy" version is (see
99 [allocator.concepts.members]/21):
Returns:
x
if the allocator should propagate from the existing container to the new container on copy construction, otherwiseX()
.
Consider the case where a user decides to provide an explicit concept
map for Allocator to adapt some legacy allocator class, as he wishes to
provide customizations that the LegacyAllocator
concept map template
does not provide. Now, although it's true that the legacy class is
required to have a default constructor, the user might have reasons to
prefer a different constructor to implement
select_on_container_copy_construction()
. However, the current wording
requires the use of the default constructor.
Moreover, it's not said explicitly that x
is supposed to be the
allocator of the existing container. A clarification would do no harm.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
Replace 99 [allocator.concepts.members]/21 with:
X select_on_container_copy_construction(const X& x);-21- Returns:
an allocator object to be used by the new container on copy construction. [Note:x
if the allocator should propagate from the existing container to the new container on copy construction, otherwiseX()
.x
is the allocator of the existing container that is being copied. The most obvious choices for the return value arex
, if the allocator should propagate from the existing container, andX()
. — end note]
Replace 99 [allocator.concepts.members]/25 with:
X select_on_container_move_construction(X&& x);-25- Returns:
an allocator object to be used by the new container on move construction. [Note:move(x)
if the allocator should propagate from the existing container to the new container on move construction, otherwiseX()
.x
is the allocator of the existing container that is being moved. The most obvious choices for the return value aremove(x)
, if the allocator should propagate from the existing container, andX()
. — end note]
Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: Jason Merrill Opened: 2009-07-16 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with Resolved status.
Discussion:
I've been implementing compiler support for is_standard_layout
, and
noticed a few nits about 21.3.5.4 [meta.unary.prop]:
has_trivial_assign
&&
has_trivial_copy_constructor
&& has_trivial_destructor
is similar, but
not identical, specifically with respect to const types.
has_trivial_copy_constructor
and has_trivial_assign
lack the "or an
array of such a class type" language that most other traits in that
section, including has_nothrow_copy_constructor
and has_nothrow_assign
,
have; this seems like an oversight.
[ See the thread starting with c++std-lib-24420 for further discussion. ]
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2984.
Proposed resolution:
Section: 30.5 [time.duration] Status: C++11 Submitter: Howard Hinnant Opened: 2009-07-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration].
View all issues with C++11 status.
Discussion:
"diagnostic required" has been used (by me) for code words meaning "use
enable_if
to constrain templated functions. This needs to be
improved by referring to the function signature as not participating in
the overload set, and moving this wording to a Remarks paragraph.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2009-11-19 Pete opens: ]
Oh, and speaking of 1177, most of the changes result in rather convoluted prose. Instead of saying
A shall be B, else C
it should be
C if A is not B
That is:
Rep2
shall be implicitly convertible toCR(Rep1, Rep2)
, else this signature shall not participate in overload resolution.should be
This signature shall not participate in overload resolution if
Rep2
is not implicitly convertible toCR(Rep1, Rep2)
.That is clearer, and eliminates the false requirement that
Rep2
"shall be" convertible.
[ 2009-11-19 Howard adds: ]
I've updated the wording to match Pete's suggestion and included bullet 16 from 1195(i).
[ 2009-11-19 Jens adds: ]
Further wording suggestion using "unless":
This signature shall not participate in overload resolution unless
Rep2
is implicitly convertible toCR(Rep1, Rep2)
.
[ 2009-11-20 Howard adds: ]
I've updated the wording to match Jens' suggestion.
[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
[ This proposed resolution addresses 947(i) and 974(i). ]
Change 30.5.2 [time.duration.cons] (and reorder the Remarks paragraphs per 16.3.2.4 [structure.specifications]):
template <class Rep2> explicit duration(const Rep2& r);
Requires:Remarks: This constructor shall not participate in overload resolution unlessRep2
shall beis implicitly convertible torep
and
treat_as_floating_point<rep>::value
shall beistrue
, ortreat_as_floating_point<Rep2>::value
shall beisfalse
.
Diagnostic required[Example:duration<int, milli> d(3); // OK duration<int, milli> d(3.5); // error— end example]
Effects: Constructs an object of type
duration
.Postcondition:
count() == static_cast<rep>(r)
.template <class Rep2, class Period2> duration(const duration<Rep2, Period2>& d);
Requires:Remarks: This constructor shall not participate in overload resolution unlesstreat_as_floating_point<rep>::value
shall beistrue
orratio_divide<Period2, period>::type::den
shall beis 1.Diagnostic required.[Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note] [Example:duration<int, milli> ms(3); duration<int, micro> us = ms; // OK duration<int, milli> ms2 = us; // error— end example]
Effects: Constructs an object of type
duration
, constructingrep_
fromduration_cast<duration>(d).count()
.
Change the following paragraphs in 30.5.6 [time.duration.nonmember]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);
RequiresRemarks: This operator shall not participate in overload resolution unlessRep2
shall beis implicitly convertible toCR(Rep1, Rep2)
.Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);
RequiresRemarks: This operator shall not participate in overload resolution unlessRep1
shall beis implicitly convertible toCR(Rep1, Rep2)
.Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);
RequiresRemarks: This operator shall not participate in overload resolution unlessRep2
shall beis implicitly convertible toCR(Rep1, Rep2)
andRep2
shall not beis not an instantiation ofduration
.Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);
RequiresRemarks: This operator shall not participate in overload resolution unlessRep2
shall beis implicitly convertible toCR(Rep1, Rep2)
andRep2
shall not beis not an instantiation ofduration
.Diagnostic required.
Change the following paragraphs in 30.5.8 [time.duration.cast]:
template <class ToDuration, class Rep, class Period> ToDuration duration_cast(const duration<Rep, Period>& d);
RequiresRemarks: This function shall not participate in overload resolution unlessToDuration
shall beis an instantiation ofduration
.Diagnostic required.
Change 30.6.2 [time.point.cons]/3 as indicated:
Requires:Duration2
shall be implicitly convertible toduration
. Diagnostic required.Remarks: This constructor shall not participate in overload resolution unless
Duration2
is implicitly convertible toduration
.
Change the following paragraphs in 30.6.8 [time.point.cast]:
template <class ToDuration, class Clock, class Duration> time_point<Clock, ToDuration> time_point_cast(const time_point<Clock, Duration>& t);
RequiresRemarks: This function shall not participate in overload resolution unlessToDuration
shall beis an instantiation ofduration
.Diagnostic required.
Section: 16.4.6.2 [res.on.headers] Status: C++11 Submitter: Beman Dawes Opened: 2009-07-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
See Frankfurt notes of 1001(i).
Proposed resolution:
Change 16.4.6.2 [res.on.headers], Headers, paragraph 1, as indicated:
A C++ header may include other C++ headers.
[footnote]A C++ header shall provide the declarations and definitions that appear in its synopsis (6.3 [basic.def.odr]). A C++ header shown in its synopsis as including other C++ headers shall provide the declarations and definitions that appear in the synopses of those other headers.
[footnote] C++ headers must include a C++ header that contains any needed definition (3.2).
string_type
member typedef in class sub_match
Section: 28.6.8.2 [re.submatch.members] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
The definition of class template sub_match
is strongly dependent
on the type basic_string<value_type>
, both in interface and effects,
but does not provide a corresponding typedef string_type
, as e.g.
class match_results
does, which looks like an oversight to me that
should be fixed.
[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In the class template sub_match
synopsis 28.6.8 [re.submatch]/1
change as indicated:
template <class BidirectionalIterator> class sub_match : public std::pair<BidirectionalIterator, BidirectionalIterator> { public: typedef typename iterator_traits<BidirectionalIterator>::value_type value_type; typedef typename iterator_traits<BidirectionalIterator>::difference_type difference_type; typedef BidirectionalIterator iterator; typedef basic_string<value_type> string_type; bool matched; difference_type length() const; operatorbasic_string<value_type>string_type() const;basic_string<value_type>string_type str() const; int compare(const sub_match& s) const; int compare(constbasic_string<value_type>string_type& s) const; int compare(const value_type* s) const; };
In 28.6.8.2 [re.submatch.members]/2 change as indicated:
operatorbasic_string<value_type>string_type() const;Returns:
matched ?
.basic_string<value_type>string_type(first, second) :basic_string<value_type>string_type()
In 28.6.8.2 [re.submatch.members]/3 change as indicated:
basic_string<value_type>string_type str() const;Returns:
matched ?
.basic_string<value_type>string_type(first, second) :basic_string<value_type>string_type()
In 28.6.8.2 [re.submatch.members]/5 change as indicated:
int compare(constbasic_string<value_type>string_type& s) const;
sub_match
comparison operatorsSection: 28.6.8.3 [re.submatch.op] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.submatch.op].
View all issues with C++11 status.
Discussion:
Several heterogeneous comparison operators of class template
sub_match
are specified by return clauses that are not valid
in general. E.g. 28.6.8.3 [re.submatch.op]/7:
template <class BiIter, class ST, class SA> bool operator==( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);Returns:
lhs == rhs.str()
.
The returns clause would be ill-formed for all cases where
ST != std::char_traits<iterator_traits<BiIter>::value_type>
or SA != std::allocator<iterator_traits<BiIter>::value_type>
.
The generic character of the comparison was intended, so
there are basically two approaches to fix the problem: The
first one would define the semantics of the comparison
using the traits class ST
(The semantic of basic_string::compare
is defined in terms of the compare function of the corresponding
traits class), the second one would define the semantics of the
comparison using the traits class
std::char_traits<iterator_traits<BiIter>::value_type>
which is essentially identical to
std::char_traits<sub_match<BiIter>::value_type>
I suggest to follow the second approach, because
this emphasizes the central role of the sub_match
object as part of the comparison and would also
make sure that a sub_match
comparison using some
basic_string<char_t, ..>
always is equivalent to
a corresponding comparison with a string literal
because of the existence of further overloads (beginning
from 28.6.8.3 [re.submatch.op]/19). If users really want to
take advantage of their own traits::compare
, they can
simply write a corresponding compare function that
does so.
[ Post-Rapperswil ]
The following update is a result of the discussion during the Rapperswil meeting, the P/R expresses all comparisons by
delegating to sub_match's compare functions. The processing is rather mechanical: Only ==
and <
where defined by referring to sub_match
's compare function, all remaining ones where replaced by the canonical
definitions in terms of these two.
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
template <class BiIter, class ST, class SA> bool operator==( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);7 Returns:
.
lhs == rhs.str()rhs.compare(lhs.c_str()) == 0
template <class BiIter, class ST, class SA> bool operator!=( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);8 Returns:
.
lhs != rhs.str()!(lhs == rhs)
template <class BiIter, class ST, class SA> bool operator<( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);9 Returns:
.
lhs < rhs.str()rhs.compare(lhs.c_str()) > 0
template <class BiIter, class ST, class SA> bool operator>( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);10 Returns:
.
lhs > rhs.str()rhs < lhs
template <class BiIter, class ST, class SA> bool operator>=( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);11 Returns:
.
lhs >= rhs.str()!(lhs < rhs)
template <class BiIter, class ST, class SA> bool operator<=( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);12 Returns:
.
lhs <= rhs.str()!(rhs < lhs)
template <class BiIter, class ST, class SA> bool operator==(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);13 Returns:
.
lhs.str() == rhslhs.compare(rhs.c_str()) == 0
template <class BiIter, class ST, class SA> bool operator!=(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);14 Returns:
.
lhs.str() != rhs!(lhs == rhs)
template <class BiIter, class ST, class SA> bool operator<(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);15 Returns:
.
lhs.str() < rhslhs.compare(rhs.c_str()) < 0
template <class BiIter, class ST, class SA> bool operator>(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);16 Returns:
.
lhs.str() > rhsrhs < lhs
template <class BiIter, class ST, class SA> bool operator>=(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);17 Returns:
.
lhs.str() >= rhs!(lhs < rhs)
template <class BiIter, class ST, class SA> bool operator<=(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);18 Returns:
.
lhs.str() <= rhs!(rhs < lhs)
template <class BiIter> bool operator==(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);19 Returns:
.
lhs == rhs.str()rhs.compare(lhs) == 0
template <class BiIter> bool operator!=(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);20 Returns:
.
lhs != rhs.str()!(lhs == rhs)
template <class BiIter> bool operator<(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);21 Returns:
.
lhs < rhs.str()rhs.compare(lhs) > 0
template <class BiIter> bool operator>(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);22 Returns:
.
lhs > rhs.str()rhs < lhs
template <class BiIter> bool operator>=(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);23 Returns:
.
lhs >= rhs.str()!(lhs < rhs)
template <class BiIter> bool operator<=(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);24 Returns:
.
lhs <= rhs.str()!(rhs < lhs)
template <class BiIter> bool operator==(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);25 Returns:
.
lhs.str() == rhslhs.compare(rhs) == 0
template <class BiIter> bool operator!=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);26 Returns:
.
lhs.str() != rhs!(lhs == rhs)
template <class BiIter> bool operator<(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);27 Returns:
.
lhs.str() < rhslhs.compare(rhs) < 0
template <class BiIter> bool operator>(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);28 Returns:
.
lhs.str() > rhsrhs < lhs
template <class BiIter> bool operator>=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);29 Returns:
.
lhs.str() >= rhs!(lhs < rhs)
template <class BiIter> bool operator<=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);30 Returns:
.
lhs.str() <= rhs!(rhs < lhs)
template <class BiIter> bool operator==(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);
31 Returns:basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) == rhs.str()
.
31 Returns:rhs.compare(typename sub_match<BiIter>::string_type(1, lhs)) == 0
.
template <class BiIter> bool operator!=(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);32 Returns:
.
basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) != rhs.str()!(lhs == rhs)
template <class BiIter> bool operator<(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);
33 Returns:basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) < rhs.str()
.
33 Returns:rhs.compare(typename sub_match<BiIter>::string_type(1, lhs)) > 0
.
template <class BiIter> bool operator>(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);34 Returns:
.
basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) > rhs.str()rhs < lhs
template <class BiIter> bool operator>=(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);35 Returns:
.
basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) >= rhs.str()!(lhs < rhs)
template <class BiIter> bool operator<=(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);36 Returns:
.
basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) <= rhs.str()!(rhs < lhs)
template <class BiIter> bool operator==(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);
37 Returns:lhs.str() == basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)
.
37 Returns:lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) == 0
.
template <class BiIter> bool operator!=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);38 Returns:
.
lhs.str() != basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(lhs == rhs)
template <class BiIter> bool operator<(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);
39 Returns:lhs.str() < basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)
.
39 Returns:lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) < 0
.
template <class BiIter> bool operator>(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);40 Returns:
.
lhs.str() > basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)rhs < lhs
template <class BiIter> bool operator>=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);41 Returns:
.
lhs.str() >= basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(lhs < rhs)
template <class BiIter> bool operator<=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);42 Returns:
.
lhs.str() <= basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(rhs < lhs)
Section: 22.10.19 [unord.hash] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-07-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord.hash].
View all issues with C++11 status.
Discussion:
Addresses UK 324
The implied library dependencies created by spelling out all the hash
template specializations in the <functional>
synopsis are unfortunate.
The potential coupling is greatly reduced if the hash
specialization is
declared in the appropriate header for each library type, as it is much
simpler to forward declare the primary template and provide a single
specialization than it is to implement a hash
function for a string
or
vector
without providing a definition for the whole string/vector
template in order to access the necessary bits.
Note that the proposed resolution purely involves moving the declarations of a few specializations, it specifically does not make any changes to 22.10.19 [unord.hash].
[ 2009-09-15 Daniel adds: ]
I suggest to add to the current existing proposed resolution the following items.
Add to the very first strike-list of the currently suggested resolution the following lines:
template <> struct hash<std::error_code>;template <> struct hash<std::thread::id>;
Add the following declarations to 19.5 [syserr], header
<system_error>
synopsis after // 19.5.4:
// 19.5.x hash support template <class T> struct hash; template <> struct hash<error_code>;
Add a new clause 19.5.X (probably after 19.5.4):
19.5.X Hash support [syserr.hash]
template <> struct hash<error_code>;An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type
error_code
suitable for using this type as key in unordered associative containers (23.5 [unord]).
Add the following declarations to 32.4.3.2 [thread.thread.id] just after the declaration of the comparison operators:
template <class T> struct hash; template <> struct hash<thread::id>;
Add a new paragraph at the end of 32.4.3.2 [thread.thread.id]:
template <> struct hash<thread::id>;An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type
thread::id
suitable for using this type as key in unordered associative containers (23.5 [unord]).
std::hash<std::thread::id>
to header <thread>
.
[ 2009-11-13 Alisdair adopts Daniel's suggestion and the extended note from 889(i). ]
[ 2010-01-31 Alisdair: related to 1245(i) and 978(i). ]
[ 2010-02-07 Proposed wording updated by Beman, Daniel, Alisdair and Ganesh. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Strike the following specializations declared in the <functional>
synopsis p2 22.10 [function.objects]
template <> struct hash<std::string>;template <> struct hash<std::u16string>;template <> struct hash<std::u32string>;template <> struct hash<std::wstring>;template <> struct hash<std::error_code>;template <> struct hash<std::thread::id>;template <class Allocator> struct hash<std::vector<bool, Allocator> >;template <std::size_t N> struct hash<std::bitset<N> >;
Add the following at the end of 22.10.19 [unord.hash]:
template <> struct hash<bool>; template <> struct hash<char>; template <> struct hash<signed char>; template <> struct hash<unsigned char>; template <> struct hash<char16_t>; template <> struct hash<char32_t>; template <> struct hash<wchar_t>; template <> struct hash<short>; template <> struct hash<unsigned short>; template <> struct hash<int>; template <> struct hash<unsigned int>; template <> struct hash<long>; template <> struct hash<long long>; template <> struct hash<unsigned long>; template <> struct hash<unsigned long long>; template <> struct hash<float>; template <> struct hash<double>; template <> struct hash<long double>; template<class T> struct hash<T*>;Specializations meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to 19.5 [syserr], header <system_error>
synopsis after // 19.5.4:
// [syserr.hash] hash support template <class T> struct hash; template <> struct hash<error_code>;
Add a new clause 19.5.X (probably after 19.5.4):
19.5.X Hash support [syserr.hash]
template <> struct hash<error_code>;Specialization meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to the synopsis of <string>
in 27.4 [string.classes]
// [basic.string.hash] hash support template <class T> struct hash; template <> struct hash<string>; template <> struct hash<u16string>; template <> struct hash<u32string>; template <> struct hash<wstring>;
Add a new clause 21.4.X
21.4.X Hash support [basic.string.hash]>
template <> struct hash<string>; template <> struct hash<u16string>; template <> struct hash<u32string>; template <> struct hash<wstring>;Specializations meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to the synopsis of <vector>
in
23.3 [sequences]
// 21.4.x hash support template <class T> struct hash; template <class Allocator> struct hash<vector<bool, Allocator>>;
Add a new paragraph to the end of 23.3.14 [vector.bool]
template <class Allocator> struct hash<vector<bool, Allocator>>;Specialization meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to the synopsis of <bitset>
in 22.9.2 [template.bitset]
// [bitset.hash] hash support template <class T> struct hash; template <size_t N> struct hash<bitset<N> >;
Add a new subclause 20.3.7.X [bitset.hash]
20.3.7.X bitset hash support [bitset.hash]
template <size_t N> struct hash<bitset<N> >;Specialization meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to 32.4.3.2 [thread.thread.id] synopsis just after the declaration of the comparison operators:
template <class T> struct hash; template <> struct hash<thread::id>;
Add a new paragraph at the end of 32.4.3.2 [thread.thread.id]:
template <> struct hash<thread::id>;Specialization meeting the requirements of class template
hash
22.10.19 [unord.hash].
Change Header <typeindex> synopsis 17.7.6 [type.index.synopsis] as indicated:
namespace std { class type_index; // [type.index.hash] hash support template <class T> struct hash; template<> struct hash<type_index>;: public unary_function<type_index, size_t> { size_t operator()(type_index index) const; }}
Change Template specialization hash<type_index> [type.index.templ] as indicated:
20.11.4
Template specialization hash<type_index> [type.index.templ]Hash support [type.index.hash]size_t operator()(type_index index) const;
Returns:index.hash_code()
template<> struct hash<type_index>;Specialization meeting the requirements of class template
hash
[unord.hash]. For an objectindex
of typetype_index
,hash<type_index>()(index)
shall evaluate to the same value asindex.hash_code()
.
basic_ios::set_rdbuf
may break class invariantsSection: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The protected member function set_rdbuf
had been added during the
process of adding move and swap semantics to IO classes. A relevant
property of this function is described by it's effects in
31.5.4.3 [basic.ios.members]/19:
Effects: Associates the
basic_streambuf
object pointed to by sb with this stream without callingclear()
.
This means that implementors of or those who derive from existing IO classes
could cause an internal state where the stream buffer could be 0, but the
IO class has the state good()
. This would break several currently existing
implementations which rely on the fact that setting a stream buffer via the
currently only ways, i.e. either by calling
void init(basic_streambuf<charT,traits>* sb);
or by calling
basic_streambuf<charT,traits>* rdbuf(basic_streambuf<charT,traits>* sb);
to set rdstate()
to badbit
, if the buffer is 0. This has the effect that many
internal functions can simply check rdstate()
instead of rdbuf()
for being 0.
I therefore suggest that a requirement is added for callers of set_rdbuf
to
set a non-0 value.
[ 2009-10 Santa Cruz: ]
Moved to Open. Martin volunteers to provide new wording, where
set_rdbuf()
sets thebadbit
but does not cause an exception to be thrown like a call toclear()
would.
[ 2009-10-20 Martin provides wording: ]
Change 31.5.4.3 [basic.ios.members] around p. 19 as indicated:
void set_rdbuf(basic_streambuf<charT, traits>* sb);
Effects: Associates thebasic_streambuf
object pointed to bysb
with this stream without callingclear()
. Postconditions:rdbuf() == sb
.Effects: As if:
iostate state = rdstate(); try { rdbuf(sb); } catch(ios_base::failure) { if (0 == (state & ios_base::badbit)) unsetf(badbit); }Throws: Nothing.
Rationale:
We need to be able to call set_rdbuf()
on stream objects
for which (rdbuf() == 0
) holds without causing ios_base::failure
to
be thrown. We also don't want badbit
to be set as a result of
setting rdbuf()
to 0 if it wasn't set before the call. This changed
Effects clause maintains the current behavior (as of N2914) without
requiring that sb
be non-null.
[ Post-Rapperswil ]
Several reviewers and the submitter believe that the best solution would be to add a pre-condition that the buffer shall not be a null pointer value.
Moved to Tentatively Ready with revised wording provided by Daniel after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
void set_rdbuf(basic_streambuf<charT, traits>* sb);?? Requires:
23 Effects: Associates thesb != nullptr
.basic_streambuf
object pointed to bysb
with this stream without callingclear()
.24 Postconditions:
rdbuf() == sb
.25 Throws: Nothing.
Rationale:
We believe that setting a nullptr
stream buffer can be prevented.
Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-07-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
(wording relative to N2723 pending new working paper)
According to p3 24.3 [iterator.requirements], Forward iterators, Bidirectional iterators and Random Access iterators all satisfy the requirements for an Output iterator:
XXX iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified ...
Meanwhile, p4 goes on to contradict this:
Besides its category, a forward, bidirectional, or random access iterator can also be mutable or constant...
... Constant iterators do not satisfy the requirements for output iterators
The latter seems to be the overriding concern, as the iterator tag
hierarchy does not define forward_iterator_tag
as multiply derived from
both input_iterator_tag
and output_iterator_tag
.
The work on concepts for iterators showed us that output iterator really is fundamentally a second dimension to the iterator categories, rather than part of the linear input -> forward -> bidirectional -> random-access sequence. It would be good to clear up these words to reflect that, and separately list output iterator requirements in the requires clauses for the appropriate algorithms and operations.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
std::decay
Section: 21.3.8.7 [meta.trans.other] Status: C++11 Submitter: Jason Merrill Opened: 2009-08-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
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Discussion:
I notice that std::decay
is specified to strip the cv-quals from
anything but an array or pointer. This seems incorrect for values of
class type, since class rvalues can have cv-qualified type (7.2.1 [basic.lval]/9).
[ 2009-08-09 Howard adds: ]
See the thread starting with c++std-lib-24568 for further discussion. And here is a convenience link to the original proposal. Also see the closely related issue 705(i).
[ 2010 Pittsburgh: Moved to Ready. ]
Proposed resolution:
Add a note to decay
in 21.3.8.7 [meta.trans.other]:
[Note: This behavior is similar to the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. — end note]
Section: 23.2.8 [unord.req], 23.5 [unord] Status: C++11 Submitter: Matt Austern Opened: 2009-08-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Consider a typical use case: I create an unordered_map
and then start
adding elements to it one at a time. I know that it will eventually need
to store a few million elements, so, for performance reasons, I would
like to reserve enough capacity that none of the calls to insert
will
trigger a rehash.
Unfortunately, the existing interface makes this awkward. The user
naturally sees the problem in terms of the number of elements, but the
interface presents it as buckets. If m
is the map and n
is the expected
number of elements, this operation is written m.rehash(n /
m.max_load_factor())
— not very novice friendly.
[ 2009-09-30 Daniel adds: ]
I recommend to replace "
resize
" by a different name like "reserve
", because that would better match the intended use-case. Rational: Any existing resize function has the on-success post-condition that the provided size is equal tosize()
, which is not satisfied for the proposal. Reserve seems to fit the purpose of the actual renaming suggestion.
[ 2009-10-28 Ganesh summarizes alternative resolutions and expresses a strong preference for the second (and opposition to the first): ]
In the unordered associative container requirements (23.2.8 [unord.req]), remove the row for rehash and replace it with:
Table 87 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity a.
rehashreserve(n)void
Post: a.bucket_count > max(a.size(), n) / a.max_load_factor()
and.a.bucket_count() >= n
Average case linear in a.size()
, worst case quadratic.Make the corresponding change in the class synopses in 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.6 [unord.set], and 23.5.7 [unord.multiset].
In 23.2.8 [unord.req]/9, table 98, append a new row after the last one:
Table 87 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity a.rehash(n)
void
Post: a.bucket_count > a.size() / a.max_load_factor()
anda.bucket_count() >= n
.Average case linear in a.size()
, worst case quadratic.a.reserve(n)
void
Same as a.rehash(ceil(n / a.max_load_factor()))
Average case linear in a.size()
, worst case quadratic.In 23.5.3 [unord.map]/3 in the definition of class template
unordered_map
, in 23.5.4 [unord.multimap]/3 in the definition of class templateunordered_multimap
, in 23.5.6 [unord.set]/3 in the definition of class templateunordered_set
and in 23.5.7 [unord.multiset]/3 in the definition of class templateunordered_multiset
, add the following line after member functionrehash()
:void reserve(size_type n);
[ 2009-10-28 Howard: ]
Moved to Tentatively Ready after 5 votes in favor of Ganesh's option 2 above. The original proposed wording now appears here:
Informally: instead of providing
rehash(n)
provideresize(n)
, with the semantics "make the container a good size forn
elements".In the unordered associative container requirements (23.2.8 [unord.req]), remove the row for rehash and replace it with:
Table 87 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity a.
rehashresize(n)void
Post: a.bucket_count > max(a.size(), n) / a.max_load_factor()
and.a.bucket_count() >= n
Average case linear in a.size()
, worst case quadratic.Make the corresponding change in the class synopses in 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.6 [unord.set], and 23.5.7 [unord.multiset].
Proposed resolution:
In 23.2.8 [unord.req]/9, table 98, append a new row after the last one:
Table 87 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity a.rehash(n)
void
Post: a.bucket_count > a.size() / a.max_load_factor()
anda.bucket_count() >= n
.Average case linear in a.size()
, worst case quadratic.a.reserve(n)
void
Same as a.rehash(ceil(n / a.max_load_factor()))
Average case linear in a.size()
, worst case quadratic.
In 23.5.3 [unord.map]/3 in the definition of class template unordered_map
, in
23.5.4 [unord.multimap]/3 in the definition of class template unordered_multimap
, in
23.5.6 [unord.set]/3 in the definition of class template unordered_set
and in
23.5.7 [unord.multiset]/3 in the definition of class template unordered_multiset
, add the
following line after member function rehash()
:
void reserve(size_type n);
tuple get
API should respect rvaluesSection: 22.4.8 [tuple.elem] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The tuple get
API should respect rvalues. This would allow for moving a
single element out of a tuple
-like type.
[ 2009-10-30 Alisdair adds: ]
The issue of rvalue overloads of get for tuple-like types was briefly discussed in Santa Cruz.
The feedback was this would be welcome, but we need full wording for the other types (
pair
andarray
) before advancing.I suggest the issue moves to Open from New as it has been considered, feedback given, and it has not (yet) been rejected as NAD.
[ 2010 Rapperswil: ]
Note that wording has been provided, and this issue becomes more important now that we have added a function to support forwarding argument lists as
tuple
s. Move to Tentatively Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Add the following signature to p2 22.4.1 [tuple.general]
template <size_t I, class ... Types> typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...> &&);
And again to 22.4.8 [tuple.elem].
template <size_t I, class ... Types> typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...>&& t);Effects: Equivalent to
return std::forward<typename tuple_element<I, tuple<Types...> >::type&&>(get<I>(t));
[Note: If a
T
inTypes
is some reference typeX&
, the return type isX&
, notX&&
. However, if the element type is non-reference typeT
, the return type isT&&
. — end note]
Add the following signature to p1 22.2 [utility]
template <size_t I, class T1, class T2> typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&&);
And to p5 22.3.4 [pair.astuple]
template <size_t I, class T1, class T2> typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&& p);Returns: If
I == 0
returnsstd::forward<T1&&>(p.first)
; ifI == 1
returnsstd::forward<T2&&>(p.second)
; otherwise the program is ill-formed.Throws: Nothing.
Add the following signature to 23.3 [sequences] <array>
synopsis
template <size_t I, class T, size_t N> T&& get(array<T,N> &&);
And after p8 23.3.3.7 [array.tuple]
template <size_t I, class T, size_t N> T&& get(array<T,N> && a);Effects: Equivalent to
return std::move(get<I>(a));
basic_string
missing definitions for cbegin
/ cend
/ crbegin
/ crend
Section: 27.4.3.4 [string.iterators] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-08-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Unlike the containers in clause 23, basic_string
has definitions for
begin()
and end()
, but these have not been updated to include cbegin
,
cend
, crbegin
and crend
.
[ 2009-10-28 Howard: ]
Moved to Tentatively NAD after 5 positive votes on c++std-lib. Added rationale.
[ 2009-10-28 Alisdair disagrees: ]
I'm going to have to speak up as the dissenting voice.
I agree the issue could be handled editorially, and that would be my preference if Pete feels this is appropriate. Failing that, I really think this issue should be accepted and moved to ready. The other begin/end functions all have a semantic definition for this template, and it is confusing if a small few are missing.
I agree that an alternative would be to strike all the definitions for
begin/end/rbegin/rend
and defer completely to the requirements tables in clause 23. I think that might be confusing without a forward reference though, as those tables are defined in a later clause than thebasic_string
template itself. If someone wants to pursue this I would support it, but recommend it as a separate issue.So my preference is strongly to move Ready over NAD, and a stronger preference for NAD Editorial if Pete is happy to make these changes.
[ 2009-10-29 Howard: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib. Removed rationale to mark it NAD. :-)
Proposed resolution:
Add to 27.4.3.4 [string.iterators]
iterator begin(); const_iterator begin() const; const_iterator cbegin() const;...
iterator end(); const_iterator end() const; const_iterator cend() const;...
reverse_iterator rbegin(); const_reverse_iterator rbegin() const; const_reverse_iterator crbegin() const;...
reverse_iterator rend(); const_reverse_iterator rend() const; const_reverse_iterator crend() const;
default_delete
cannot be instantiated with incomplete typesSection: 20.3.1.2 [unique.ptr.dltr] Status: C++11 Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to the general rules of 16.4.5.8 [res.on.functions] p 2 b 5 the effects are undefined, if an incomplete type is used to instantiate a library template. But neither in 20.3.1.2 [unique.ptr.dltr] nor in any other place of the standard such explicit allowance is given. Since this template is intended to be instantiated with incomplete types, this must be fixed.
[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2009-11-17 Alisdair Opens: ]
LWG 1193 tries to support
unique_ptr
for incomplete types. I believe the proposed wording goes too far:The template parameter
T
ofdefault_delete
may be an incomplete type.Do we really want to support
cv-void
? Suggested ammendment:The template parameter
T
ofdefault_delete
may be an incomplete type other thancv-void
.We might also consider saying something about arrays of incomplete types.
Did we lose support for
unique_ptr<function-type>
when the concept-enabled work was shelved? If so, we might want adefault_delete
partial specialization for function types that does nothing. Alternatively, function types should not be supported by default, but there is no reason a user cannot support them via their own deletion policy.Function-type support might also lead to conditionally supporting a function-call operator in the general case, and that seems way too inventive at this stage to me, even if we could largely steal wording directly from
reference_wrapper
.shared_ptr
would have similar problems too.
[ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add two new paragraphs directly to 20.3.1.2 [unique.ptr.dltr] (before 20.3.1.2.2 [unique.ptr.dltr.dflt]) with the following content:
The class template
default_delete
serves as the default deleter (destruction policy) for the class templateunique_ptr
.The template parameter
T
ofdefault_delete
may be an incomplete type.
queue
constructorSection: 23.6 [container.adaptors] Status: C++11 Submitter: Howard Hinnant Opened: 2009-08-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.6.3.1 [queue.defn] has the following queue
constructor:
template <class Alloc> explicit queue(const Alloc&);
This will be implemented like so:
template <class Alloc> explicit queue(const Alloc& a) : c(a) {}
The issue is that Alloc
can be anything that a container will construct
from, for example an int
. Is this intended to compile?
queue<int> q(5);
Before the addition of this constructor, queue<int>(5)
would not compile.
I ask, not because this crashes, but because it is new and appears to be
unintended. We do not want to be in a position of accidently introducing this
"feature" in C++0X and later attempting to remove it.
I've picked on queue
. priority_queue
and stack
have
the same issue. Is it useful to create a priority_queue
of 5
identical elements?
[ Daniel, Howard and Pablo collaborated on the proposed wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
[ This resolution includes a semi-editorial clean up, giving definitions to members which in some cases weren't defined since C++98. This resolution also offers editorially different wording for 976(i), and it also provides wording for 1196(i). ]
Change [ontainer.adaptor], p1:
The container adaptors each take a
Container
template parameter, and each constructor takes aContainer
reference argument. This container is copied into theContainer
member of each adaptor. If the container takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor. Otherwise, normal copy or move construction is used for the container argument.[Note: it is not necessary for an implementation to distinguish between the one-argument constructor that takes aContainer
and the one- argument constructor that takes an allocator_type. Both forms use their argument to construct an instance of the container. — end note]
Change [ueue.def], p1:
template <class T, class Container = deque<T> > class queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit queue(const Container&); explicit queue(Container&& = Container()); queue(queue&& q);: c(std::move(q.c)) {}template <class Alloc> explicit queue(const Alloc&); template <class Alloc> queue(const Container&, const Alloc&); template <class Alloc> queue(Container&&, const Alloc&); template <class Alloc> queue(queue&&, const Alloc&); queue& operator=(queue&& q);{ c = std::move(q.c); return *this; }bool empty() const { return c.empty(); } ... };
Add a new section after 23.6.3.1 [queue.defn], [queue.cons]:
queue
constructors [queue.cons]explicit queue(const Container& cont);Effects: Initializes
c
withcont
.explicit queue(Container&& cont = Container());Effects: Initializes
c
withstd::move(cont)
.queue(queue&& q)Effects: Initializes
c
withstd::move(q.c)
.For each of the following constructors, if
uses_allocator<container_type, Alloc>::value
isfalse
, then the constructor shall not participate in overload resolution.template <class Alloc> explicit queue(const Alloc& a);Effects: Initializes
c
witha
.template <class Alloc> queue(const container_type& cont, const Alloc& a);Effects: Initializes
c
withcont
as the first argument anda
as the second argument.template <class Alloc> queue(container_type&& cont, const Alloc& a);Effects: Initializes
c
withstd::move(cont)
as the first argument anda
as the second argument.template <class Alloc> queue(queue&& q, const Alloc& a);Effects: Initializes
c
withstd::move(q.c)
as the first argument anda
as the second argument.queue& operator=(queue&& q);Effects: Assigns
c
withstd::move(q.c)
.Returns:
*this
.
Add to 23.6.4.2 [priqueue.cons]:
priority_queue(priority_queue&& q);Effects: Initializes
c
withstd::move(q.c)
and initializescomp
withstd::move(q.comp)
.For each of the following constructors, if
uses_allocator<container_type, Alloc>::value
isfalse
, then the constructor shall not participate in overload resolution.template <class Alloc> explicit priority_queue(const Alloc& a);Effects: Initializes
c
witha
and value-initializescomp
.template <class Alloc> priority_queue(const Compare& compare, const Alloc& a);Effects: Initializes
c
witha
and initializescomp
withcompare
.template <class Alloc> priority_queue(const Compare& compare, const Container& cont, const Alloc& a);Effects: Initializes
c
withcont
as the first argument anda
as the second argument, and initializescomp
withcompare
.template <class Alloc> priority_queue(const Compare& compare, Container&& cont, const Alloc& a);Effects: Initializes
c
withstd::move(cont)
as the first argument anda
as the second argument, and initializescomp
withcompare
.template <class Alloc> priority_queue(priority_queue&& q, const Alloc& a);Effects: Initializes
c
withstd::move(q.c)
as the first argument anda
as the second argument, and initializescomp
withstd::move(q.comp)
.priority_queue& operator=(priority_queue&& q);Effects: Assigns
c
withstd::move(q.c)
and assignscomp
withstd::move(q.comp)
.Returns:
*this
.
Change 23.6.6.2 [stack.defn]:
template <class T, class Container = deque<T> > class stack { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit stack(const Container&); explicit stack(Container&& = Container()); stack(stack&& s); template <class Alloc> explicit stack(const Alloc&); template <class Alloc> stack(const Container&, const Alloc&); template <class Alloc> stack(Container&&, const Alloc&); template <class Alloc> stack(stack&&, const Alloc&); stack& operator=(stack&& s); bool empty() const { return c.empty(); } ... };
Add a new section after 23.6.6.2 [stack.defn], [stack.cons]:
stack
constructors [stack.cons]stack(stack&& s);Effects: Initializes
c
withstd::move(s.c)
.For each of the following constructors, if
uses_allocator<container_type, Alloc>::value
isfalse
, then the constructor shall not participate in overload resolution.template <class Alloc> explicit stack(const Alloc& a);Effects: Initializes
c
witha
.template <class Alloc> stack(const container_type& cont, const Alloc& a);Effects: Initializes
c
withcont
as the first argument anda
as the second argument.template <class Alloc> stack(container_type&& cont, const Alloc& a);Effects: Initializes
c
withstd::move(cont)
as the first argument anda
as the second argument.template <class Alloc> stack(stack&& s, const Alloc& a);Effects: Initializes
c
withstd::move(s.c)
as the first argument anda
as the second argument.stack& operator=(stack&& s);Effects: Assigns
c
withstd::move(s.c)
.Returns:
*this
.
Section: 16 [library] Status: C++11 Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Several parts of the library use the notion of "Diagnostic required" to indicate that in the corresponding situation an error diagnostic should occur, e.g. 20.3.1.2.2 [unique.ptr.dltr.dflt]/2
void operator()(T *ptr) const;Effects: calls
delete
onptr
. A diagnostic is required ifT
is an incomplete type.
The problem with this approach is that such a requirement is insufficient to prevent undefined behavior, if this situation occurs. According to 3.18 [defns.diagnostic] a diagnostic message is defined as
a message belonging to an implementation-defined subset of the implementation's output messages.
which doesn't indicate any relation to an ill-formed program. In fact, "compiler warnings" are a typical expression of such diagnostics. This means that above wording can be interpreted by compiler writers that they satisfy the requirements of the standard if they just produce such a "warning", if the compiler happens to compile code like this:
#include <memory> struct Ukn; // defined somewhere else Ukn* create_ukn(); // defined somewhere else int main() { std::default_delete<Ukn>()(create_ukn()); }
In this and other examples discussed here it was the authors intent to guarantee that the program is ill-formed with a required diagnostic, therefore such wording should be used instead. According to the general rules outlined in 4.1 [intro.compliance] it should be sufficient to require that these situations produce an ill-formed program and the "diagnostic required" part should be implied. The proposed resolution also suggests to remove several redundant wording of "Diagnostics required" to ensure that the absence of such saying does not cause a misleading interpretation.
[ 2009 Santa Cruz: ]
Move to NAD.
It's not clear that there's any important difference between "ill-formed" and "diagnostic required". From 4.1 [intro.compliance], 3.26 [defns.ill.formed], and 3.68 [defns.well.formed] it appears that an ill-formed program is one that is not correctly constructed according to the syntax rules and diagnosable semantic rules, which means that... "a conforming implementation shall issue at least one diagnostic message." The author's intent seems to be that we should be requiring a fatal error instead of a mere warning, but the standard just doesn't have language to express that distinction. The strongest thing we can ever require is a "diagnostic".
The proposed rewording may be a clearer way of expressing the same thing that the WP already says, but such a rewording is editorial.
[ 2009 Santa Cruz: ]
Considered again. Group disagrees that the change is technical, but likes it editorially. Moved to NAD Editorial.
[ 2009-11-19: Moved from NAD Editorial to Open. Please see the thread starting with Message c++std-lib-25916. ]
[ 2009-11-20 Daniel updated wording. ]
The following resolution differs from the previous one by avoiding the unusual and misleading term "shall be ill-formed", which does also not follow the core language style. This resolution has the advantage of a minimum impact on the current wording, but I would like to mention that a more intrusive solution might be preferrable - at least as a long-term solution: Jens Maurer suggested the following approach to get rid of the usage of the term "ill-formed" from the library by introducing a new category to existing elements to the list of 16.3.2.4 [structure.specifications]/3, e.g. "type requirements" or "static constraints" that define conditions that can be checked during compile-time and any violation would make the program ill-formed. As an example, the currently existing phrase 22.4.7 [tuple.helper]/1
Requires:
I < sizeof...(Types)
. The program is ill-formed ifI
is out of bounds.could then be written as
Static constraints:
I < sizeof...(Types)
.
[ 2009-11-21 Daniel updated wording. ]
[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 21.4 [ratio]/2 as indicated:
Throughout this subclause, if the template argument types
R1
andR2
shall beare not specializations of theratio
template, the program is ill-formed.Diagnostic required.
Change 21.4.3 [ratio.ratio]/1 as indicated:
If tThe template argument D
shall not
be is zero, and or the absolute values of
the template arguments N
and D
shall be are
not representable by type intmax_t
, the program is
ill-formed. Diagnostic required. [..]
Change 21.4.4 [ratio.arithmetic]/1 as indicated:
Implementations may use other algorithms to compute these values. If overflow occurs, the program is ill-formed
a diagnostic shall be issued.
Change 21.4.5 [ratio.comparison]/2 as indicated:
[...] Implementations may use other algorithms to compute this relationship to avoid overflow. If overflow occurs, the program is ill-formed
a diagnostic is required.
Change 20.3.1.2.2 [unique.ptr.dltr.dflt]/2 as indicated:
Effects: calls
delete
onptr
.A diagnostic is required ifT
is an incomplete type.Remarks: If
T
is an incomplete type, the program is ill-formed.
Change 20.3.1.2.3 [unique.ptr.dltr.dflt1]/1 as indicated:
void operator()(T* ptr) const;Effects:
callsoperator()
delete[]
onptr
.A diagnostic is required ifT
is an incomplete type.Remarks: If
T
is an incomplete type, the program is ill-formed.
Change 20.3.1.3.2 [unique.ptr.single.ctor] as indicated: [Note: This editorially improves the currently suggested wording of 932(i) by replacing
"shall be ill-formed" by "is ill-formed"]
[If N3025 is accepted this bullet is applied identically in that paper as well.]
-1- Requires:
D
shall be default constructible, and that construction shall not throw an exception.D
shall not be a reference type or pointer type (diagnostic required)....
Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument
D
, the program is ill-formed.
Change 20.3.1.3.2 [unique.ptr.single.ctor]/8 as indicated: [Note: This editorially improves the currently suggested wording of 932(i) by replacing
"shall be ill-formed" by "is ill-formed"]
[If N3025 is accepted this bullet is applied identically in that paper as well.]
unique_ptr(pointer p);...
Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument
D
, the program is ill-formed.
Change 20.3.1.3.2 [unique.ptr.single.ctor]/13 as indicated:
[..] If
d
is an rvalue, it will bind to the second constructor of this pair and the program is ill-formed.That constructor shall emit a diagnostic.[Note: The diagnostic could be implemented using astatic_assert
which assures thatD
is not a reference type. — end note] Elsed
is an lvalue and will bind to the first constructor of this pair. [..]
Change 20.3.1.4 [unique.ptr.runtime]/1 as indicated:
A specialization for array types is provided with a slightly altered interface.
- Conversions among different types of
unique_ptr<T[], D>
or to or from the non-array forms ofunique_ptr
are disallowed (diagnostic required)produce an ill-formed program.- ...
Change 30.5 [time.duration]/2-4 as indicated:
2 Requires:
Rep
shall be an arithmetic type or a class emulating an arithmetic type.If a program instantiatesduration
with aduration
type for the template argumentRep
a diagnostic is required.3 Remarks: If
duration
is instantiated with aduration
type for the template argumentRep
, the program is ill-formed.
34RequiresRemarks: IfPeriod
shall beis not a specialization ofratio
,diagnostic requiredthe program is ill-formed.
45RequiresRemarks: IfPeriod::num
shall beis not positive,diagnostic requiredthe program is ill-formed.
Change 30.6 [time.point]/2 as indicated:
If
Duration
shall beis not an instance ofduration
, the program is ill-formed.Diagnostic required.
Section: 23.6.4.2 [priqueue.cons] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-08-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
The class template priority_queue
declares signatures for a move
constructor and move assignment operator in its class definition.
However, it does not provide a definition (unlike std::queue
, and
proposed resolution for std::stack
.) Nor does it provide a text clause
specifying their behaviour.
[ 2009-08-23 Daniel adds: ]
[ 2009-10 Santa Cruz: ]
Proposed resolution:
bucket_count() == 0
?Section: 23.2.8 [unord.req] Status: C++11 Submitter: Howard Hinnant Opened: 2009-08-24 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [unord.req].
View all other issues in [unord.req].
View all issues with C++11 status.
Discussion:
Table 97 "Unordered associative container requirements" in 23.2.8 [unord.req] says:
Table 97 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity b.bucket(k)
size_type
Returns the index of the bucket in which elements with keys equivalent to k
would be found, if any such element existed. Post: the return value shall be in the range[0, b.bucket_count())
.Constant
What should b.bucket(k)
return if b.bucket_count() == 0
?
I believe allowing b.bucket_count() == 0
is important. It is a
very reasonable post-condition of the default constructor, or of a moved-from
container.
I can think of several reasonable results from b.bucket(k)
when
b.bucket_count() == 0
:
numeric_limits<size_type>::max()
.
domain_error
.
b.bucket_count() != 0
.
[ 2009-08-26 Daniel adds: ]
A forth choice would be to add the pre-condition "
b.bucket_count() != 0
" and thus imply undefined behavior if this is violated.[ Howard: I like this option too, added to the list. ]
Further on here my own favorite solution (rationale see below):
Suggested resolution:
[Rationale: I suggest to follow choice (1). The main reason is that all associative container functions which take a key argument, are basically free of pre-conditions and non-disrupting, therefore excluding choices (3) and (4). Option (2) seems a bit unexpected to me. It would be more natural, if several similar functions would exist which would also justify the existence of a symbolic constant like npos for this situation. The value 0 is both simple and consistent, it has exactly the same role as a past-the-end iterator value. A typical use-case is:
size_type pos = m.bucket(key); if (pos != m.bucket_count()) { ... } else { ... }— end Rationale]
- Change Table 97 in 23.2.8 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):
Table 97 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity b.bucket(k)
size_type
Returns the index of the bucket in which elements with keys equivalent to k
would be found, if any such element existed. Post: if b.bucket_count() != 0, the return value shall be in the range[0, b.bucket_count())
, otherwise 0.Constant
[ 2010-01-25 Choice 4 put into proposed resolution section. ]
[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change Table 97 in 23.2.8 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):
Table 97 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity b.bucket(k)
size_type
Pre: b.bucket_count() > 0
Returns the index of the bucket in which elements with keys equivalent tok
would be found, if any such element existed. Post: the return value shall be in the range[0, b.bucket_count())
.Constant
Section: 23.6 [container.adaptors] Status: C++11 Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.adaptors].
View all issues with C++11 status.
Discussion:
Under 23.6 [container.adaptors] of
N2914
the member function of swap
of queue
and stack
call:
swap(c, q.c);
But under 23.6 [container.adaptors] of N2723 these members are specified to call:
c.swap(q.c);
Neither draft specifies the semantics of member swap
for
priority_queue
though it is declared.
Although the distinction between member swap
and non-member
swap
is not important when these adaptors are adapting standard
containers, it may be important for user-defined containers.
We (Pablo and Howard) feel that
it is more likely for a user-defined container to support a namespace scope
swap
than a member swap
, and therefore these adaptors
should use the container's namespace scope swap
.
[ 2009-09-30 Daniel adds: ]
The outcome of this issue should be considered with the outcome of 774(i) both in style and in content (e.g. 774(i) bullet 9 suggests to define the semantic of
void priority_queue::swap(priority_queue&)
in terms of the memberswap
of the container).
[ 2010-03-28 Daniel update to diff against N3092. ]
[ 2010 Rapperswil: ]
Preference to move the wording into normative text, rather than inline function definitions in the class synopsis. Move to Tenatively Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 23.6.3.1 [queue.defn]:
template <class T, class Container = deque<T> > class queue { ... void swap(queue& q) { using std::swap;c.swap(c, q.c); } ... };
Change 23.6.4 [priority.queue]:
template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue { ... void swap(priority_queue& q);{ using std::swap; swap(c, q.c); swap(comp, q.comp); } ... };
Change 23.6.6.2 [stack.defn]:
template <class T, class Container = deque<T> > class stack { ... void swap(stack& s) { using std::swap;c.swap(c, s.c); } ... };
Section: 23.6 [container.adaptors] Status: C++11 Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2020-11-29
Priority: Not Prioritized
View all other issues in [container.adaptors].
View all issues with C++11 status.
Discussion:
queue
has a constructor:
template <class Alloc> queue(queue&&, const Alloc&);
but it is missing a corresponding constructor:
template <class Alloc> queue(const queue&, const Alloc&);
The same is true of priority_queue
, and stack
. This
"extended copy constructor" is needed for consistency and to ensure that the
user of a container adaptor can always specify the allocator for his adaptor.
[ 2010-02-01 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
[ This resolution has been harmonized with the proposed resolution to issue 1194(i) ]
Change 23.6.3.1 [queue.defn], p1:
template <class T, class Container = deque<T> > class queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit queue(const Container&); explicit queue(Container&& = Container()); queue(queue&& q); template <class Alloc> explicit queue(const Alloc&); template <class Alloc> queue(const Container&, const Alloc&); template <class Alloc> queue(Container&&, const Alloc&); template <class Alloc> queue(const queue&, const Alloc&); template <class Alloc> queue(queue&&, const Alloc&); queue& operator=(queue&& q); bool empty() const { return c.empty(); } ... };
To the new section 23.6.3.2 [queue.cons], introduced in 1194(i), add:
template <class Alloc> queue(const queue& q, const Alloc& a);Effects: Initializes
c
withq.c
as the first argument anda
as the second argument.
Change 23.6.4 [priority.queue] as follows (I've an included an editorial change to move the poorly-placed move-assignment operator):
template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; Compare comp; public: priority_queue(const Compare& x, const Container&); explicit priority_queue(const Compare& x = Compare(), Container&& = Container()); template <class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container&); template <class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& = Container()); priority_queue(priority_queue&&);priority_queue& operator=(priority_queue&&);template <class Alloc> explicit priority_queue(const Alloc&); template <class Alloc> priority_queue(const Compare&, const Alloc&); template <class Alloc> priority_queue(const Compare&, const Container&, const Alloc&); template <class Alloc> priority_queue(const Compare&, Container&&, const Alloc&); template <class Alloc> priority_queue(const priority_queue&, const Alloc&); template <class Alloc> priority_queue(priority_queue&&, const Alloc&); priority_queue& operator=(priority_queue&&); ... };
Add to 23.6.4.2 [priqueue.cons]:
template <class Alloc> priority_queue(const priority_queue& q, const Alloc& a);Effects: Initializes
c
withq.c
as the first argument anda
as the second argument, and initializescomp
withq.comp
.
Change 23.6.6.2 [stack.defn]:
template <class T, class Container = deque<T> > class stack { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit stack(const Container&); explicit stack(Container&& = Container()); stack(stack&& s); template <class Alloc> explicit stack(const Alloc&); template <class Alloc> stack(const Container&, const Alloc&); template <class Alloc> stack(Container&&, const Alloc&); template <class Alloc> stack(const stack&, const Alloc&); template <class Alloc> stack(stack&&, const Alloc&); stack& operator=(stack&& s); bool empty() const { return c.empty(); } ... };
To the new section 23.6.6.3 [stack.cons], introduced in 1194(i), add:
template <class Alloc> stack(const stack& s, const Alloc& a);Effects: Initializes
c
withs.c
as the first argument anda
as the second argument.
ref
-wrappers in make_tuple
Section: 22.4.5 [tuple.creation], 22.3 [pairs] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-05 Last modified: 2018-10-05
Priority: Not Prioritized
View all other issues in [tuple.creation].
View all issues with Resolved status.
Discussion:
Spotting a recent thread on the boost lists regarding collapsing
optional representations in optional<optional<T>>
instances, I wonder if
we have some of the same issues with make_tuple
, and now make_pair
?
Essentially, if my generic code in my own library is handed a
reference_wrapper
by a user, and my library in turn delegates some logic
to make_pair
or make_tuple
, then I am going to end up with a pair
/tuple
holding a real reference rather than the intended reference wrapper.
There are two things as a library author I can do at this point:
std::make_tuple
make_tuple
that does not unwrap rereferences, a lost
opportunity to re-use the standard library.
(There may be some metaprogramming approaches my library can use to wrap
the make_tuple
call, but all will be significantly more complex than
simply implementing a simplified make_tuple
.)
Now I don't propose we lose this library facility, I think unwrapping
references will be the common behaviour. However, we might want to
consider adding another overload that does nothing special with
ref
-wrappers. Note that we already have a second overload of
make_tuple
in the library, called tie
.
[ 2009-09-30 Daniel adds: ]
I suggest to change the currently proposed paragraph for
make_simple_pair
template<typename... Types> pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);
Type requirements:Remarks: The program shall be ill-formed, ifsizeof...(Types) == 2
.sizeof...(Types) != 2
....
or alternatively (but with a slightly different semantic):
Remarks: If
sizeof...(Types) != 2
, this function shall not participate in overload resolution.to follow a currently introduced style and because the library does not have yet a specific "Type requirements" element. If such thing would be considered as useful this should be done as a separate issue. Given the increasing complexity of either of these wordings it might be preferable to use the normal two-argument-declaration style again in either of the following ways:
template<class T1, class T2> pair<typename decay<T1>::type, typename decay<T2>::type> make_simple_pair(T1&& t1, T2&& t2); template<class T1, class T2> pair<V1, V2> make_simple_pair(T1&& t1, T2&& t2);Let
V1
betypename decay<T1>::type
andV2
betypename decay<T2>::type
.
[ 2009-10 post-Santa Cruz: ]
Mark as Tentatively NAD Future.
[2018-10-05 Status to Resolved]
Alisdair notes that this is solved by CTAD that was added in C++17
Rationale:
Does not have sufficient support at this time. May wish to reconsider for a future standard.
Proposed resolution:
Add the following function to 22.3 [pairs] and signature in appropriate synopses:
template<typename... Types> pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);Type requirements:
sizeof...(Types) == 2
.Returns:
pair<typename decay<Types>::type...>(std::forward<Types>(t)...)
.
[
Draughting note: I chose a variadic representation similar to make_tuple
rather than naming both types as it is easier to read through the
clutter of metaprogramming this way. Given there are exactly two
elements, the committee may prefer to draught with two explicit template
type parameters instead
]
Add the following function to 22.4.5 [tuple.creation] and signature in appropriate synopses:
template<typename... Types> tuple<typename decay<Types>::type...> make_simple_tuple(Types&&... t);Returns:
tuple<typename decay<Types>::type...>(std::forward<Types>(t)...)
.
Section: 31.7.6.6 [ostream.rvalue], 31.7.5.6 [istream.rvalue] Status: C++20 Submitter: Howard Hinnant Opened: 2009-09-06 Last modified: 2021-02-25
Priority: 2
View all other issues in [ostream.rvalue].
View all issues with C++20 status.
Discussion:
31.7.6.6 [ostream.rvalue] was created to preserve the ability to insert into (and extract from 31.7.5.6 [istream.rvalue]) rvalue streams:
template <class charT, class traits, class T> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);1 Effects:
os << x
2 Returns:
os
This is good as it allows code that wants to (for example) open, write to, and
close an ofstream
all in one statement:
std::ofstream("log file") << "Some message\n";
However, I think we can easily make this "rvalue stream helper" even easier to use. Consider trying to quickly create a formatted string. With the current spec you have to write:
std::string s = static_cast<std::ostringstream&>(std::ostringstream() << "i = " << i).str();
This will store "i = 10
" (for example) in the string s
. Note
the need to cast the stream back to ostringstream&
prior to using
the member .str()
. This is necessary because the inserter has cast
the ostringstream
down to a more generic ostream
during the
insertion process.
I believe we can re-specify the rvalue-inserter so that this cast is unnecessary. Thus our customer now has to only type:
std::string s = (std::ostringstream() << "i = " << i).str();
This is accomplished by having the rvalue stream inserter return an rvalue of
the same type, instead of casting it down to the base class. This is done by
making the stream generic, and constraining it to be an rvalue of a type derived
from ios_base
.
The same argument and solution also applies to the inserter. This code has been implemented and tested.
[ 2009 Santa Cruz: ]
NAD Future. No concensus for change.
[LEWG Kona 2017]
Recommend Open: Design looks right.
[ 2018-05-25, Billy O'Neal requests this issue be reopened and provides P/R rebased against N4750 ]
Billy O'Neal requests this issue be reopened, as changing operator>> and operator<< to use perfect forwarding as described here is necessary to implement LWG 2534(i) which was accepted. Moreover, this P/R also resolves LWG 2498(i).
Previous resolution [SUPERSEDED]:
Change 31.7.5.6 [istream.rvalue]:
template <classcharT, class traitsIstream, class T>basic_istream<charT, traits>&Istream&& operator>>(basic_istream<charT, traits>Istream&& is, T& x);1 Effects:
is >> x
2 Returns:
std::move(is)
3 Remarks: This signature shall participate in overload resolution if and only if
Istream
is not an lvalue reference type and is derived fromios_base
.Change 31.7.6.6 [ostream.rvalue]:
template <classcharT, class traitsOstream, class T>basic_ostream<charT, traits>&Ostream&& operator<<(basic_ostream<charT, traits>Ostream&& os, const T& x);1 Effects:
os << x
2 Returns:
std::move(os)
3 Remarks: This signature shall participate in overload resolution if and only if
Ostream
is not an lvalue reference type and is derived fromios_base
.
[2018-12-03, Ville comments]
The implementation in libstdc++ doesn't require derivation from ios_base
, it
requires convertibility to basic_istream/basic_ostream
. This has been found to be
important to avoid regressions with existing stream wrappers.
basic_istream/basic_ostream
specialization
the template parameter converts to. This was done in order to try and be closer to the earlier
specification's return type, which specified basic_ostream<charT, traits>&
and basic_istream<charT, traits>&
. So we detected convertibility to
(a type convertible to) those, and returned the result of that conversion. That doesn't seem to
be necessary, and probably bothers certain chaining cases. Based on recent experiments, it seems
that this return-type dance (as opposed to just returning what the p/r suggests) is unnecessary,
and doesn't trigger any regressions.
[2019-01-20 Reflector prioritization]
Set Priority to 2
Previous resolution [SUPERSEDED]:
This resolution is relative to N4750.
Change 31.7.5.6 [istream.rvalue] as follows:
template <classcharT, class traitsIstream, class T>basic_istream<charT, traits>&Istream&& operator>>(basic_istream<charT, traits>Istream&& is, T&& x);-1- Effects: Equivalent to:
is >> std::forward<T>(x)
return std::move(is);-2- Remarks: This function shall not participate in overload resolution unless the expression
is >> std::forward<T>(x)
is well-formed,Istream
is not an lvalue reference type, andIstream
is derived fromios_base
.Change 31.7.6.6 [ostream.rvalue]:
template <classcharT, class traitsOstream, class T>basic_ostream<charT, traits>&Ostream&& operator<<(basic_ostream<charT, traits>Ostream&& os, const T& x);-1- Effects: As if by:
os << x;
-2- Returns:
std::move(os)
-3- Remarks: This signature shall not participate in overload resolution unless the expression
os << x
is well-formed,Ostream
is not an lvalue reference type, andOstream
is derived fromios_base
.
[2019-03-17; Daniel comments and provides updated wording]
After discussion with Ville it turns out that the "derived from ios_base
" approach works fine and
no breakages were found in regression tests. As result of that discussion the wording was rebased to the
most recent working draft and the "overload resolution participation" wording was replaced by a
corresponding Constraints: element.
[2020-02 Status to Immediate on Friday morning in Prague.]
Proposed resolution:
This wording is relative to N4810.
Change 31.7.5.6 [istream.rvalue] as follows:
template <classcharT, class traitsIstream, class T>basic_istream<charT, traits>&Istream&& operator>>(basic_istream<charT, traits>Istream&& is, T&& x);-?- Constraints: The expression
-1- Effects: Equivalent to:is >> std::forward<T>(x)
is well-formed andIstream
is publicly and unambiguously derived fromios_base
.is >> std::forward<T>(x)
; return std::move(is);
-2- Remarks: This function shall not participate in overload resolution unless the expressionis >> std::forward<T>(x)
is well-formed.
Change 31.7.6.6 [ostream.rvalue]:
template <classcharT, class traitsOstream, class T>basic_ostream<charT, traits>&Ostream&& operator<<(basic_ostream<charT, traits>Ostream&& os, const T& x);-?- Constraints: The expression
-1- Effects: As if by:os << x
is well-formed andOstream
is publicly and unambiguously derived fromios_base
.os << x;
-2- Returns:std::move(os)
.-3- Remarks: This signature shall not participate in overload resolution unless the expressionos << x
is well-formed.
Section: 16.4.5.9 [res.on.arguments] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [res.on.arguments].
View all issues with C++11 status.
Discussion:
When a library function binds an rvalue reference parameter to an argument, the library must be able to assume that the bound argument is a temporary, and not a moved-from lvalue. The reason for this is that the library function must be able to modify that argument without concern that such modifications will corrupt the logic of the calling code. For example:
template <class T, class A> void vector<T, A>::push_back(value_type&& v) { // This function should move from v, potentially modifying // the object v is bound to. }
If v
is truly bound to a temporary, then push_back
has the
only reference to this temporary in the entire program. Thus any
modifications will be invisible to the rest of the program.
If the client supplies std::move(x)
to push_back
, the onus is
on the client to ensure that the value of x
is no longer important to
the logic of his program after this statement. I.e. the client is making a statement
that push_back
may treat x
as a temporary.
The above statement is the very foundation upon which move semantics is based.
The standard is currently lacking a global statement to this effect. I propose the following addition to 16.4.5.9 [res.on.arguments]:
Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
- If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer invalid for its intended use), the behavior is undefined.
- If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
- If a function argument binds to an rvalue reference parameter, the C++ standard library may assume that this parameter is a unique reference to this argument. If the parameter is a generic parameter of the form
T&&
, and an lvalue of typeA
is bound, then the binding is considered to be to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus not covered by this clause. [Note: If a program casts an lvalue to an rvalue while passing that lvalue to a library function (e.g.move(x)
), then the program is effectively asking the library to treat that lvalue as a temporary. The library is at liberty to optimize away aliasing checks which might be needed if the argument were an lvalue. — end note]
Such a global statement will eliminate the need for piecemeal statements such as 23.2.2 [container.requirements.general]/13:
An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.
Additionally this clarifies that move assignment operators need not perform the
traditional if (this != &rhs)
test commonly found (and needed) in
copy assignment operators.
[ 2009-09-13 Niels adds: ]
Note: This resolution supports the change of 31.10.3.3 [filebuf.assign]/1, proposed by LWG 900(i).
[ 2009 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add a bullet to 16.4.5.9 [res.on.arguments]:
Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
- If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer invalid for its intended use), the behavior is undefined.
- If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
- If a function argument binds to an rvalue reference parameter, the C++ standard library may assume that this parameter is a unique reference to this argument. If the parameter is a generic parameter of the form
T&&
, and an lvalue of typeA
is bound, then the binding is considered to be to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus not covered by this clause. [Note: If a program casts an lvalue to an rvalue while passing that lvalue to a library function (e.g.move(x)
), then the program is effectively asking the library to treat that lvalue as a temporary. The library is at liberty to optimize away aliasing checks which might be needed if the argument were an lvalue. — end note]
Delete 23.2.2 [container.requirements.general]/13:
An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.
Section: 26 [algorithms] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-09-13 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [algorithms].
View all other issues in [algorithms].
View all issues with C++11 status.
Discussion:
There are a number of algorithms whose result might depend on the handling of an empty range. In some cases the result is not clear, while in others it would help readers to clearly mention the result rather than require some subtle intuition of the supplied wording.
[alg.all_of]
Returns:
true
ifpred(*i)
istrue
for every iteratori
in the range[first,last)
, ...
What does this mean if the range is empty?
I believe that we intend this to be true
and suggest a
non-normative note to clarify:
Add to p1 [alg.all_of]:
[Note: Returns
true
if[first,last)
is empty. — end note]
[alg.none_of]
Returns:
true
ifpred(*i)
isfalse
for every iteratori
in the range[first,last)
, ...
What does this mean if the range empty?
I believe that we intend this to be true
and suggest a
non-normative note to clarify:
Add to p1 [alg.none_of]:
[Note: Returns
true
if[first,last)
is empty. — end note]
[alg.any_of]
The specification for an empty range is actually fairly clear in this
case, but a note wouldn't hurt and would be consistent with proposals
for all_of
/none_of
algorithms.
Add to p1 [alg.any_of]:
[Note: Returns
false
if[first,last)
is empty. — end note]
26.6.8 [alg.find.end]
what does this mean if [first2,last2)
is empty?
I believe the wording suggests the algorithm should return
last1
in this case, but am not 100% sure. Is this in fact the
correct result anyway? Surely an empty range should always match and the
naive expected result would be first1
?
My proposed wording is a note to clarify the current semantic:
Add to p2 26.6.8 [alg.find.end]:
[Note: Returns
last1
if[first2,last2)
is empty. — end note]
I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:
Alternative wording: (NOT a note)
Add to p2 26.6.8 [alg.find.end]:
Returns
first1
if[first2,last2)
is empty.
26.6.9 [alg.find.first.of]
The phrasing seems precise when [first2, last2)
is empty, but a small
note to confirm the reader's understanding might still help.
Add to p2 26.6.9 [alg.find.first.of]
[Note: Returns
last1
if[first2,last2)
is empty. — end note]
26.6.15 [alg.search]
What is the expected result if [first2, last2)
is empty?
I believe the wording suggests the algorithm should return last1
in this
case, but am not 100% sure. Is this in fact the correct result anyway?
Surely an empty range should always match and the naive expected result
would be first1
?
My proposed wording is a note to clarify the current semantic:
Add to p2 26.6.15 [alg.search]:
[Note: Returns
last1
if[first2,last2)
is empty. — end note]
Again, I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:
Alternative wording: (NOT a note)
Add to p2 26.6.15 [alg.search]:
Returns
first1
if[first2,last2)
is empty.
26.8.5 [alg.partitions]
Is an empty range partitioned or not?
Proposed wording:
Add to p1 26.8.5 [alg.partitions]:
[Note: Returns
true
if[first,last)
is empty. — end note]
26.8.7.2 [includes]
Returns:
true
if every element in the range[first2,last2)
is contained in the range[first1,last1)
. ...
I really don't know what this means if [first2,last2)
is empty.
I could loosely guess that this implies empty ranges always match, and
my proposed wording is to clarify exactly that:
Add to p1 26.8.7.2 [includes]:
[Note: Returns
true
if[first2,last2)
is empty. — end note]
26.8.8.3 [pop.heap]
The effects clause is invalid if the range [first,last)
is empty, unlike
all the other heap alogorithms. The should be called out in the
requirements.
Proposed wording:
Revise p2 26.8.8.3 [pop.heap]
Requires: The range
[first,last)
shall be a valid non-empty heap.
[Editorial] Reverse order of 26.8.8.3 [pop.heap] p1 and p2.
26.8.9 [alg.min.max]
minmax_element
does not clearly specify behaviour for an empty
range in the same way that min_element
and max_element
do.
Add to p31 26.8.9 [alg.min.max]:
Returns
make_pair(first, first)
iffirst == last
.
26.8.11 [alg.lex.comparison]
The wording here seems quite clear, especially with the sample algorithm implementation. A note is recommended purely for consistency with the rest of these issue resolutions:
Add to p1 26.8.11 [alg.lex.comparison]:
[Note: An empty sequence is lexicographically less than any other non-empty sequence, but not to another empty sequence. — end note]
[
2009-11-11 Howard changes Notes to Remarks and changed search
to
return first1
instead of last1
.
]
[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add to [alg.all_of]:
Remarks: Returns
true
if[first,last)
is empty.
Add to [alg.any_of]:
Remarks: Returns
false
if[first,last)
is empty.
Add to [alg.none_of]:
Remarks: Returns
true
if[first,last)
is empty.
Add to 26.6.8 [alg.find.end]:
Remarks: Returns
last1
if[first2,last2)
is empty.
Add to 26.6.9 [alg.find.first.of]
Remarks: Returns
last1
if[first2,last2)
is empty.
Add to 26.6.15 [alg.search]:
Remarks: Returns
first1
if[first2,last2)
is empty.
Add to 26.8.5 [alg.partitions]:
Remarks: Returns
true
if[first,last)
is empty.
Add to 26.8.7.2 [includes]:
Remarks: Returns
true
if[first2,last2)
is empty.
Revise p2 26.8.8.3 [pop.heap]
Requires: The range
[first,last)
shall be a valid non-empty heap.
[Editorial]
Reverse order of 26.8.8.3 [pop.heap] p1 and p2.
Add to p35 26.8.9 [alg.min.max]:
template<class ForwardIterator, class Compare> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);Returns:
make_pair(m, M)
, wherem
is the first iterator in[first,last)
such that no iterator in the range refers to a smaller element, and whereM
is the last iterator in[first,last)
such that no iterator in the range refers to a larger element. Returnsmake_pair(first, first)
iffirst == last
.
Add to 26.8.11 [alg.lex.comparison]:
Remarks: An empty sequence is lexicographically less than any other non-empty sequence, but not less than another empty sequence.
move_backward
and copy_backward
Section: 26.7.2 [alg.move] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
26.7.2 [alg.move], p6 says:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 move_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);...
Requires:
result
shall not be in the range[first,last)
.
This is essentially an "off-by-one" error.
When result == last
, which
is allowed by this specification, then the range [first, last)
is being move assigned into the range [first, last)
. The move
(forward) algorithm doesn't allow self move assignment, and neither should
move_backward
. So last
should be included in the range which
result
can not be in.
Conversely, when result == first
, which is not allowed by this
specification, then the range [first, last)
is being move assigned into the range [first - (last-first), first)
.
I.e. into a non-overlapping range. Therefore first
should
not be included in the range which result
can not be in.
The same argument applies to copy_backward
though copy assigning elements
to themselves (result == last
) should be harmless (though is disallowed
by copy
).
[ 2010 Pittsburgh: Moved to Ready. ]
Proposed resolution:
Change 26.7.2 [alg.move], p6:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 move_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);...
Requires:
result
shall not be in the range.
[(first,last])
Change 26.7.1 [alg.copy], p13:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 copy_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);...
Requires:
result
shall not be in the range.
[(first,last])
std::list
operations?Section: 23.3.11.5 [list.ops] Status: C++11 Submitter: Loïc Joly Opened: 2009-09-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [list.ops].
View all issues with C++11 status.
Discussion:
It looks to me like some operations of std::list
(sort
, reverse
, remove
, unique
&
merge
) do not specify the validity of iterators, pointers &
references to elements of the list after those operations. Is it implied
by some other text in the standard?
I believe sort
& reverse
do not invalidating
anything, remove
& unique
only invalidates what
refers to erased elements, merge
does not invalidate anything
(with the same precision as splice
for elements who changed of
container). Are those assumptions correct ?
[ 2009-12-08 Jonathan Wakely adds: ]
23.2.2 [container.requirements.general] paragraph 11 says iterators aren't invalidated unless specified, so I don't think it needs to be repeated on every function that doesn't invalidate iterators.
list::unique
says it "eliminates" elements, that should probably be "erases" because IMHO that term is used elsewhere and so makes it clearer that iterators to the erased elements are invalidated.
list::merge
coud use the same wording aslist::splice
w.r.t iterators and references to moved elements.Suggested resolution:
In 23.3.11.5 [list.ops] change paragraph 19
void unique(); template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);Effects:
EliminatesErases all but the first element from every consecutive group ...Add to the end of paragraph 23
void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);...
Effects: ... that is, for every iterator
i
, in the range other than the first, the conditioncomp(*i, *(i - 1)
will be false. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.
[ 2009-12-12 Loïc adds wording. ]
[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Alisdair opens: ]
I object to the current resolution of #1207. I believe it is overly strict with regard to
list
end iterators, being the only mutating operations to require such stability.More importantly, the same edits need to be applied to
forward_list
, which uses slightly different words to describe some of these operations so may require subtly different edits (not checked.)I am prepared to pick up the
end()
iterator as a separate (new) issue, as part of the FCD ballot review (BSI might tell me 'no' first ;~) but I do want to seeforward_list
adjusted at the same time.
[ 2010-03-28 Daniel adds the first 5 bullets in an attempt to address Alisdair's concerns. ]
[ 2010 Rapperswil: ]
The wording looks good. Move to Tentatively Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change [forwardlist.ops]/12 as indicated:
void remove(const T& value); template <class Predicate> void remove_if(Predicate pred);12 Effects: Erases all the elements in the list referred by a list iterator
i
for which the following conditions hold:*i == value (for remove()), pred(*i)
is true (for remove_if()
). This operation shall be stable: the relative order of the elements that are not removed is the same as their relative order in the original list. Invalidates only the iterators and references to the erased elements.
Change [forwardlist.ops]/15 as indicated:
template <class BinaryPredicate> void unique(BinaryPredicate pred);15 Effects::
EliminatesErases all but the first element from every consecutive group of equal elements referred to by the iteratori
in the range[first + 1,last)
for which*i == *(i-1)
(for the version with no arguments) orpred(*i, *(i - 1))
(for the version with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.
Change [forwardlist.ops]/19 as indicated:
void merge(forward_list<T,Allocator>&& x); template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp)[..]
19 Effects:: Merges
x
into*this
. This operation shall be stable: for equivalent elements in the two lists, the elements from*this
shall always precede the elements fromx
.x
is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.
Change [forwardlist.ops]/22 as indicated:
void sort(); template <class Compare> void sort(Compare comp);[..]
22 Effects:: Sorts the list according to the
operator<
or thecomp
function object. This operation shall be stable: the relative order of the equivalent elements is preserved. If an exception is thrown the order of the elements in*this
is unspecified. Does not affect the validity of iterators and references.
Change [forwardlist.ops]/24 as indicated:
void reverse();24 Effects:: Reverses the order of the elements in the list. Does not affect the validity of iterators and references.
Change 23.3.11.5 [list.ops], p15:
void remove(const T& value); template <class Predicate> void remove_if(Predicate pred);Effects: Erases all the elements in the list referred by a list iterator
i
for which the following conditions hold:*i == value, pred(*i) != false
. Invalidates only the iterators and references to the erased elements.
Change 23.3.11.5 [list.ops], p19:
void unique(); template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);Effects:
EliminatesErases all but the first element from every consecutive group of equal elements referred to by the iteratori
in the range[first + 1,last)
for which*i == *(i-1)
(for the version ofunique
with no arguments) orpred(*i, *(i - 1))
(for the version ofunique
with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.
Change 23.3.11.5 [list.ops], p23:
void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);Effects: If
(&x == this)
does nothing; otherwise, merges the two sorted ranges[begin(), end())
and[x.begin(), x.end())
. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
, in the range other than the first, the conditioncomp(*i, *(i - 1)
will be false. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.
Change 23.3.11.5 [list.ops], p26:
void reverse();Effects: Reverses the order of the elements in the list. Does not affect the validity of iterators and references.
Change 23.3.11.5 [list.ops], p30:
void sort(); template <class Compare> void sort(Compare comp);Effects: Sorts the list according to the
operator<
or aCompare
function object. Does not affect the validity of iterators and references.
valarray initializer_list
constructor has incorrect effectsSection: 29.6.2.2 [valarray.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [valarray.cons].
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Discussion:
29.6.2.2 [valarray.cons] says:
valarray(initializer_list<T> il);Effects: Same as
valarray(il.begin(), il.end())
.
But there is no valarray
constructor taking two const T*
.
[ 2009-10-29 Howard: ]
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
Proposed resolution:
Change 29.6.2.2 [valarray.cons]:
valarray(initializer_list<T> il);Effects: Same as
valarray(il.begin(), il.
.endsize())
match_results
should be moveableSection: 28.6.9.2 [re.results.const] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2009-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.results.const].
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Discussion:
In Working Draft
N2914,
match_results
lacks a move constructor and move
assignment operator. Because it owns dynamically allocated memory, it
should be moveable.
As far as I can tell, this isn't tracked by an active issue yet; Library
Issue 723(i) doesn't talk about match_results
.
[ 2009-09-21 Daniel provided wording. ]
[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add the following member declarations to 28.6.9 [re.results]/3:
// 28.10.1, construct/copy/destroy: explicit match_results(const Allocator& a = Allocator()); match_results(const match_results& m); match_results(match_results&& m); match_results& operator=(const match_results& m); match_results& operator=(match_results&& m); ~match_results();
Add the following new prototype descriptions to 28.6.9.2 [re.results.const]
using the table numbering of
N3000
(referring to the table titled "match_results
assignment operator effects"):
match_results(const match_results& m);4 Effects: Constructs an object of class
match_results
, as a copy ofm
.match_results(match_results&& m);5 Effects: Move-constructs an object of class
match_results
fromm
satisfying the same postconditions as Table 131. Additionally the storedAllocator
value is move constructed fromm.get_allocator()
. After the initialization of*this
setsm
to an unspecified but valid state.6 Throws: Nothing if the allocator's move constructor throws nothing.
match_results& operator=(const match_results& m);7 Effects: Assigns
m
to*this
. The postconditions of this function are indicated in Table 131.match_results& operator=(match_results&& m);8 Effects: Move-assigns
m
to*this
. The postconditions of this function are indicated in Table 131. After the assignment,m
is in a valid but unspecified state.9 Throws: Nothing.
Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.requirements].
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Discussion:
p6 Iterator requirements 24.3 [iterator.requirements]
An iterator
j
is called reachable from an iteratori
if and only if there is a finite sequence of applications of the expression++i
that makesi == j
. Ifj
is reachable fromi
, they refer to the same container.
A good example would be stream iterators, which do not refer to a container. Typically, the end iterator from a range of stream iterators will compare equal for many such ranges. I suggest striking the second sentence.
An alternative wording might be:
If
j
is reachable fromi
, and bothi
andj
are dereferencable iterators, then they refer to the same range.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
Change 24.3 [iterator.requirements], p6:
An iterator
j
is called reachable from an iteratori
if and only if there is a finite sequence of applications of the expression++i
that makesi == j
.Ifj
is reachable fromi
, they refer to the same container.
Section: 24.5.4.2 [move.iterator] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-05-14
Priority: Not Prioritized
View other active issues in [move.iterator].
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Discussion:
I contend that while we can support both bidirectional and random access
traversal, the category of a move iterator should never be better than
input_iterator_tag
.
The contentious point is that you cannot truly have a multipass property when values are moved from a range. This is contentious if you view a moved-from object as still holding a valid value within the range.
The second reason comes from the Forward Iterator requirements table:
Forward iterators 24.3.5.5 [forward.iterators]
Table 102 — Forward iterator requirements
For expression
*a
the return type is: "T&
ifX
is mutable, otherwiseconst T&
"
There is a similar constraint on a->m
.
There is no support for rvalue references, nor do I believe their should
be. Again, opinions may vary but either this table or the definition of
move_iterator
need updating.
Note: this requirement probably need updating anyway if we wish to support proxy iterators but I am waiting to see a new working paper before filing that issue.
[ 2009-10 post-Santa Cruz: ]
Move to Open. Howard to put his rationale mentioned above into the issue as a note.
[ 2009-10-26 Howard adds: ]
vector::insert(pos, iter, iter)
is significantly more effcient wheniter
is a random access iterator, as compared to when it is an input iterator.When
iter
is an input iterator, the best algorithm is to append the inserted range to the end of thevector
usingpush_back
. This may involve several reallocations before the input range is exhausted. After the append, then one can usestd::rotate
to place the inserted range into the correct position in the vector.But when
iter
is a random access iterator, the best algorithm is to first compute the size of the range to be inserted (last - first
), do a buffer reallocation if necessary, scoot existing elements in thevector
down to make the "hole", and then insert the new elements directly to their correct place.The insert-with-random-access-iterators algorithm is considerably more efficient than the insert-with-input-iterators algorithm
Now consider:
vector<A> v; // ... build up a large vector of A ... vector<A> temp; // ... build up a large temporary vector of A to later be inserted ... typedef move_iterator<vector<A>::iterator> MI; // Now insert the temporary elements: v.insert(v.begin() + N, MI(temp.begin()), MI(temp.end()));A major motivation for using
move_iterator
in the above example is the expectation thatA
is cheap to move but expensive to copy. I.e. the customer is looking for high performance. If we allowvector::insert
to subtract twoMI
's to get the distance between them, the customer enjoys substantially better performance, compared to if we say thatvector::insert
can not subtract twoMI
's.I can find no rationale for not giving this performance boost to our customers. Therefore I am strongly against restricting
move_iterator
to theinput_iterator_tag
category.I believe that the requirement that forward iterators have a dereference that returns an lvalue reference to cause unacceptable pessimization. For example
vector<bool>::iterator
also does not return abool&
on dereference. Yet I am not aware of a single vendor that is willing to shipvector<bool>::iterator
as an input iterator. Everyone classifies it as a random access iterator. Not only does this not cause any problems, it prevents significant performance problems.
Previous resolution [SUPERSEDED]:
Class template move_iterator 24.5.4.2 [move.iterator]
namespace std { template <class Iterator> class move_iterator { public: ... typedeftypename iterator_traits<Iterator>::iterator_categoryinput_iterator_tag iterator_category;
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
See N3066.
Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.requirements].
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Discussion:
Forward iterator and bidirectional iterator place different requirements on the result of post-increment/decrement operator. The same form should be used in each case.
Merging row from:
Table 102 -- Forward iterator requirements Table 103 -- Bidirectional iterator requirements r++ : convertible to const X& r-- : convertible to const X& *r++ : T& if X is mutable, otherwise const T& *r-- : convertible to T
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
Section: 23.2.7 [associative.reqmts] Status: C++14 Submitter: Daniel Krügler Opened: 2009-09-20 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [associative.reqmts].
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Discussion:
Scott Meyers' mentions on a recent posting on c.s.c++ some arguments that point to an incomplete resolution of 103(i) and to an inconsistency of requirements on keys in ordered and unordered associative containers:
1) 103(i) introduced the term immutable without defining it in a unique manner in 23.2.7 [associative.reqmts]/5:
[..] Keys in an associative container are immutable.
According to conventional dictionaries immutable is an unconditional way of saying that something cannot be changed. So without any further explicit allowance a user always runs into undefined behavior if (s)he attempts to modify such a key. IMO this was not the intend of the committee to resolve 103(i) in that way because the comments suggest an interpretation that should give any user the freedom to modify the key in an explicit way provided it would not affect the sort order in that container.
2) Another observation was that surprisingly no similar 'safety guards' exists against unintentional key changes for the unordered associative containers, specifically there is no such requirement as in 23.2.7 [associative.reqmts]/6 that "both
iterator
andconst_iterator
are constant iterators". But the need for such protection against unintentional changes as well as the constraints in which manner any explicit changes may be performed are both missing and necessary, because such changes could potentially change the equivalence of keys that is measured by thehasher
andkey_equal
.I suggest to fix the unconditional wording involved with "immutable keys" by at least adding a hint for the reader that users may perform such changes in an explicit manner and to perform similar wording changes as 103(i) did for the ordered associative containers also for the unordered containers.
[ 2010-03-27 Daniel provides wording. ]
This update attempts to provide normative wording that harmonizes the key and function object constraints of associative and unordered containers.
[ 2010 Batavia: ]
We're uncomfortable with the first agenda item, and we can live with the second agenda item being applied before or after Madrid.
[ 2011 Bloomington ]
Further discussion persuades us this issue is Ready (and so moved). We may need a further issue clarifying the notion of key value vs. key object, as object identity appears to be important to this wording.
Proposed resolution:
Change 23.2.7 [associative.reqmts]/2 as indicated: [This ensures that associative containers make better clear what this "arbitrary" type is, as the unordered containers do in 23.2.8 [unord.req]/3]
2 Each associative container is parameterized on
Key
and an ordering relationCompare
that induces a strict weak ordering (25.4) on elements ofKey
. In addition,map
andmultimap
associate an arbitrary mapped typetypeT
with theKey
. The object of typeCompare
is called the comparison object of a container.
Change 23.2.7 [associative.reqmts]/5 as indicated: [This removes the too strong requirement that keys must not be changed at all and brings this line in sync with 23.2.8 [unord.req]/7. We take care about the real constraints by the remaining suggested changes. The rationale provided by LWG 103(i) didn't really argue why that addition is necessary, and I believe the remaining additions make it clear that any user changes have strong restrictions]:
5 For
set
andmultiset
the value type is the same as the key type. Formap
andmultimap
it is equal topair<const Key, T>
.Keys in an associative container are immutable.
Change 23.2.8 [unord.req]/3+4 as indicated: [The current sentence of p.4 has doesn't say something really new and this whole subclause misses to define the concepts of the container-specific hasher object and predicate object. We introduce the term key equality predicate which is already used in the requirements table. This change does not really correct part of this issue, but is recommended to better clarify the nomenclature and the difference between the function objects and the function object types, which is important, because both can potentially be stateful.]
3 Each unordered associative container is parameterized by
Key
, by a function object typeHash
that meets theHash
requirements (20.2.4) and acts as a hash function for argument values of typeKey
, and by a binary predicatePred
that induces an equivalence relation on values of typeKey
. Additionally,unordered_map
andunordered_multimap
associate an arbitrary mapped typeT
with theKey
.4 The container's object of type
Hash
- denoted byhash
- is called the hash function of the container. The container's object of typePred
- denoted bypred
- is called the key equality predicate of the container.A hash function is a function object that takes a single argument of type.Key
and returns a value of typestd::size_t
Change 23.2.8 [unord.req]/5 as indicated: [This adds a similar safe-guard as the last sentence of 23.2.7 [associative.reqmts]/3]
5 Two values
k1
andk2
of typeKey
are considered equivalent if the container's key equality predicatereturnskey_equal
function objecttrue
when passed those values. Ifk1
andk2
are equivalent, the container's hash function shall return the same value for both. [Note: thus, when an unordered associative container is instantiated with a non-defaultPred
parameter it usually needs a non-defaultHash
parameter as well. — end note] For any two keysk1
andk2
in the same container, callingpred(k1, k2)
shall always return the same value. For any keyk
in a container, callinghash(k)
shall always return the same value.
After 23.2.8 [unord.req]/7 add the following new paragraph: [This ensures the same level of compile-time protection that we already require for associative containers. It is necessary for similar reasons, because any change in the stored key which would change it's equality relation to others or would change it's hash value such that it would no longer fall in the same bucket, would break the container invariants]
7 For
unordered_set
andunordered_multiset
the value type is the same as the key type. Forunordered_map
andunordered_multimap
it isstd::pair<const Key, T>
.For unordered containers where the value type is the same as the key type, both
iterator
andconst_iterator
are constant iterators. It is unspecified whether or notiterator
andconst_iterator
are the same type. [Note:iterator
andconst_iterator
have identical semantics in this case, anditerator
is convertible toconst_iterator
. Users can avoid violating the One Definition Rule by always usingconst_iterator
in their function parameter lists. — end note]
list::merge
with unequal allocatorsSection: 23.3.11.5 [list.ops] Status: C++11 Submitter: Pablo Halpern Opened: 2009-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Bellevue (I think), we passed
N2525,
which, among other things, specifies that the behavior of
list::splice
is undefined if the allocators of the two lists
being spliced do not compare equal. The same rationale should apply to
list::merge
. The intent of list::merge
(AFAIK) is to
move nodes from one sorted list
into another sorted
list
without copying the elements. This is possible only if the
allocators compare equal.
Proposed resolution:
Relative to the August 2009 WP, N2857, change 23.3.11.5 [list.ops], paragraphs 22-25 as follows:
void merge(list&& x); template <class Compare> void merge(list&& x, Compare comp);Requires: both the list and the argument list shall be sorted according to operator< or comp.
Effects: If
(&x == this)
does nothing; otherwise, merges the two sorted ranges[begin(), end())
and[x.begin(), x.end())
. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
, in the range other than thefirst
, the conditioncomp(*i, *(i - 1))
will befalse
.Remarks: Stable. If
(&x != this)
the range[x.begin(), x.end())
is empty after the merge. No elements are copied by this operation. The behavior is undefined ifthis->get_allocator() != x.get_allocator()
.Complexity: At most
size() + x.size() - 1
applications ofcomp
if(&x != this)
; otherwise, no applications ofcomp
are performed. If an exception is thrown other than by a comparison there are no effects.
Section: 17.9.8 [except.nested] Status: C++11 Submitter: Pete Becker Opened: 2009-09-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
LWG 1066(i) adds [[noreturn]]
to a bunch of things.
It doesn't add it to rethrow_nested()
, which seems like an obvious
candidate. I've made the changes indicated in the issue, and haven't
changed rethrow_nested()
.
[ 2009 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add [[noreturn]]
to rethrow_nested()
in 17.9.8 [except.nested].
Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
If an object *o
contains a mutex mu
and a
correctly-maintained reference count c
, is the following code
safe?
o->mu.lock(); bool del = (--(o->c) == 0); o->mu.unlock(); if (del) { delete o; }
If the implementation of mutex::unlock()
can touch the mutex's
memory after the moment it becomes free, this wouldn't be safe, and
"Construction and destruction of an object of a Mutex type need not be
thread-safe" 32.6.4 [thread.mutex.requirements] may imply that
it's not safe. Still, it's useful to allow mutexes to guard reference
counts, and if it's not allowed, users are likely to write bugs.
[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add a new paragraph after 32.6.4.2.2 [thread.mutex.class] p1:
1 The class
mutex
provides a non-recursive mutex ...[Note: After a thread
A
has calledunlock()
, releasing the mutex, it is possible for another threadB
to lock the same mutex, observe that it is no longer in use, unlock and destroy it, before threadA
appears to have returned from its unlock call. Implementations are required to handle such scenarios correctly, as long as threadA
doesn't access the mutex after the unlock call returns. These cases typically occur when a reference-counted object contains a mutex that is used to protect the reference count. — end note]
condition_variable
wait on?Section: 32.7 [thread.condition] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
"Class condition_variable
provides a condition variable that can only
wait on an object of type unique_lock
" should say "...object of type
unique_lock<mutex>
"
[ 2009-11-06 Howard adds: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Change 32.7 [thread.condition], p1:
Condition variables provide synchronization primitives used to block a thread until notified by some other thread that some condition is met or until a system time is reached. Class
condition_variable
provides a condition variable that can only wait on an object of typeunique_lock<mutex>
, allowing maximum efficiency on some platforms. Classcondition_variable_any
provides a general condition variable that can wait on objects of user-supplied lock types.
condition_variable
wordingSection: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.7.4 [thread.condition.condvar] says:
~condition_variable();Precondition: There shall be no thread blocked on
*this
. [Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait. Beware that destroying acondition_variable
object while the corresponding predicate isfalse
is likely to lead to undefined behavior. — end note]
The text hasn't introduced the notion of a "corresponding predicate" yet.
[ 2010-02-11 Anthony provided wording. ]
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Modify 32.7.4 [thread.condition.condvar]p4 as follows:
~condition_variable();4 Precondition: There shall be no thread blocked on
*this
. [Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait.Beware that destroying aThe user must take care to ensure that no threads wait oncondition_variable
object while the corresponding predicate is false is likely to lead to undefined behavior.*this
once the destructor has been started, especially when the waiting threads are calling the wait functions in a loop or using the overloads ofwait
,wait_for
orwait_until
that take a predicate. — end note]
Section: 32.7 [thread.condition] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.7.4 [thread.condition.condvar] says:
void wait(unique_lock<mutex>& lock);...
Effects:
- ...
- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.
Should that be lock.lock()
?
[ 2009-11-17 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 32.7.4 [thread.condition.condvar] p10:
void wait(unique_lock<mutex>& lock);...
Effects:
- ...
- If the function exits via an exception,
lock.
shall be called prior to exiting the function scope.unlock()
And make a similar change in p16, and in 32.7.5 [thread.condition.condvarany], p8 and p13.
result_of
issue Section: 99 [func.ret] Status: Resolved Submitter: Sebastian Gesemann Opened: 2009-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I think the text about std::result_of
could be a little more precise.
Quoting from
N2960...
99 [func.ret] Function object return types
template<class> class result_of; template<class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public: typedef see below type; };Given an rvalue
fn
of typeFn
and valuest1, t2, ..., tN
of typesT1, T2, ... TN
inArgTypes
respectivly, thetype
member is the result type of the expressionfn(t1,t2,...,tN)
. the valuesti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.
This text doesn't seem to consider lvalue reference types for Fn
.
Also, it's not clear whether this class template can be used for
"SFINAE" like std::enable_if
. Example:
template<typename Fn, typename... Args> typename std::result_of<Fn(Args...)>::type apply(Fn && fn, Args && ...args) { // Fn may be an lvalue reference, too return std::forward<Fn>(fn)(std::forward<Args>(args)...); }
Either std::result_of<...>
can be instantiated and simply may not have
a typedef "type
" (-->SFINAE) or instantiating the class template for
some type combinations will be a "hard" compile-time error.
[ 2010-02-14 Daniel adds: ]
This issue should be considered resolved by 1255(i) and 1270(i). The wish to change
result_of
into a compiler-support trait was beyond the actual intention of the submitter Sebastian.
[
2010 Pittsburgh: Moved to NAD EditorialResolved, rationale added below.
]
Rationale:
Proposed resolution:
[ These changes will require compiler support ]
Change 99 [func.ret]:
template<class> class result_of; // undefined template<class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public:typedefsee belowtype;};
Given an rvaluefn
of typeFn
and valuest1, t2, ..., tN
of typesT1, T2, ... TN
inArgTypes
respectivly, thetype
member is the result type of the expressionfn(t1,t2,...,tN)
. the valuesti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.The class template
result_of
shall meet the requirements of a TransformationTrait: Given the typesFn
,T1
,T2
, ...,TN
every template specializationresult_of<Fn(T1,T2,...,TN)>
shall define the member typedef type equivalent todecltype(RE)
if and only if the expressionRE
value<Fn>() ( value<T1>(), value<T2>(), ... value<TN>() )would be well-formed. Otherwise, there shall be no member typedef
type
defined.
[
The value<>
helper function is a utility Daniel Krügler
proposed in
N2958.
]
Section: 32.10.3 [futures.errors] Status: Resolved Submitter: Daniel Krügler Opened: 2009-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Defect issue 890(i) overlooked to adapt the future_category
from
32.10.1 [futures.overview] and 32.10.3 [futures.errors]:
extern const error_category* const future_category;
which should be similarly transformed into function form.
[ 2009-10-27 Howard: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ 2009-11-11 Daniel adds: ]
I just observe that the proposed resolution of this issue is incomplete and needs to reworded. The problem is that the corresponding declarations
constexpr error_code make_error_code(future_errc e); constexpr error_condition make_error_condition(future_errc e);as constexpr functions are incompatible to the requirements of constexpr functions given their specified implementation. Note that the incompatibility is not a result of the modifications proposed by the issue resolution, but already existed within the N2960 state where we have
extern const error_category* const future_category;combined with
constexpr error_code make_error_code(future_errc e);3 Returns:
error_code(static_cast<int>(e), *future_category)
.constexpr error_code make_error_condition(future_errc e);4 Returns:
error_condition(static_cast<int>(e), *future_category)
.Neither is any of the constructors of
error_code
anderror_condition
constexpr, nor does the expression*future_category
satisfy the requirements for a constant expression (7.7 [expr.const]/2 bullet 6 in N3000).The simple solution is just to remove the constexpr qualifiers for both functions, which makes sense, because none of the remaining
make_error_*
overloads in the library is constexpr. One might consider to realize that thosemake_*
functions could satisfy the constexpr requirements, but this looks not like an easy task to me, because it would need to rely on a not yet existing language feature. If such a change is wanted, a new issue should be opened after the language extension approval (if at all) [1].If no-one complaints I would like to ask Howard to add the following modifications to this issue, alternatively a new issue could be opened but I don't know what the best solution is that would cause as little overhead as possible.
What-ever the route is, the following is my proposed resolution for this issue interaction part of the story:
In 32.10.1 [futures.overview]/1, Header
<future>
synopsis and in 32.10.3 [futures.errors]/3+4 change as indicated:constexprerror_code make_error_code(future_errc e);constexprerror_condition make_error_condition(future_errc e);[1] Let me add that we have a related NAD issue here: 832(i) so the chances for realization are little IMO.
[ Howard: I've updated the proposed wording as Daniel suggests and set to Review. ]
[ 2009-11-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change in 32.10.1 [futures.overview], header <future>
synopsis:
externconst error_category&* constfuture_category();
In 32.10.1 [futures.overview]/1, Header <future>
synopsis
change as indicated:
constexprerror_code make_error_code(future_errc e);constexprerror_condition make_error_condition(future_errc e);
Change in 32.10.3 [futures.errors]:
externconst error_category&* constfuture_category();
1-future_category
shall point to a statically initialized object of a type derived from classerror_category
.1- Returns: A reference to an object of a type derived from class
error_category
.constexprerror_code make_error_code(future_errc e);3 Returns:
error_code(static_cast<int>(e),
.*future_category())constexprerror_codecondition make_error_condition(future_errc e);4 Returns:
error_condition(static_cast<int>(e),
.*future_category())
<bitset>
synopsis overspecifiedSection: 22.9.2 [template.bitset] Status: C++11 Submitter: Bo Persson Opened: 2009-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The resolutions to some library defect reports, like 1178(i)
requires that #includes
in each synopsis should be taken
literally. This means that the <bitset>
header now
must include <stdexcept>
, even though none of the
exceptions are mentioned in the <bitset>
header.
Many other classes are required to throw exceptions like
invalid_argument
and out_of_range
, without explicitly
including <stdexcept>
in their synopsis. It is totally
possible for implementations to throw the needed exceptions from utility
functions, whose implementations are not visible in the headers.
I propose that <stdexcept>
is removed from the
<bitset>
header.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Change 22.9.2 [template.bitset]:
#include <cstddef> // for size_t #include <string>#include <stdexcept> // for invalid_argument,// out_of_range, overflow_error#include <iosfwd> // for istream, ostream namespace std { ...
error_code operator=
typoSection: 19.5.4.3 [syserr.errcode.modifiers] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2009-10-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
N2960 19.5.4.1 [syserr.errcode.overview] and 19.5.4.3 [syserr.errcode.modifiers] say:
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type& operator=(ErrorCodeEnum e);
They should say:
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code>::type& operator=(ErrorCodeEnum e);
Or (I prefer this form):
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code&>::type operator=(ErrorCodeEnum e);
This is because enable_if
is declared as (21.3.8.7 [meta.trans.other]):
template <bool B, class T = void> struct enable_if;
So, the current wording makes operator=
return
void&
, which is not good.
19.5.4.3 [syserr.errcode.modifiers]/4 says
Returns:
*this
.
which is correct.
Additionally,
19.5.5.1 [syserr.errcondition.overview]/1 says:
template<typename ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::type & operator=( ErrorConditionEnum e );
Which contains several problems (typename
versus class
inconsistency, lack of ::value
, error_code
instead of
error_condition
), while 19.5.5.3 [syserr.errcondition.modifiers] says:
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type& operator=(ErrorConditionEnum e);
Which returns void&
. They should both say:
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition>::type& operator=(ErrorConditionEnum e);
Or (again, I prefer this form):
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition&>::type operator=(ErrorConditionEnum e);
Additionally, 19.5.5.3 [syserr.errcondition.modifiers] lacks a
"Returns: *this
." paragraph, which is presumably
necessary.
[ 2009-10-18 Beman adds: ]
The proposed resolution for issue 1237(i) makes this issue moot, so it should become NAD.
[ 2009-10 Santa Cruz: ]
Proposed resolution:
Change 19.5.4.1 [syserr.errcode.overview] and 19.5.4.3 [syserr.errcode.modifiers]:
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code&>::type&operator=(ErrorCodeEnum e);
Change 19.5.5.1 [syserr.errcondition.overview]:
template<typenameclass ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_conditionde&>::type&operator=( ErrorConditionEnum e );
Change 19.5.5.3 [syserr.errcondition.modifiers]:
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition&>::type&operator=(ErrorConditionEnum e);Postcondition:
*this == make_error_condition(e)
.Returns:
*this
.Throws: Nothing.
weak_ptr
comparisons incompletely resolvedSection: 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The
n2637
paper suggested several updates of the ordering semantics of shared_ptr
and weak_ptr
, among those the explicit comparison operators of weak_ptr
were
removed/deleted, instead a corresponding functor owner_less
was added.
The problem is that
n2637
did not clearly enough specify, how the previous wording parts describing
the comparison semantics of weak_ptr
should be removed.
[ 2009-11-06 Howard adds: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Change 20.3.2.3 [util.smartptr.weak]/2 as described, the intention is to fix
the now no longer valid requirement that weak_ptr
is LessComparable
[Note the deleted comma]:
Specializations of
weak_ptr
shall beCopyConstructible
,andCopyAssignable
,andallowing their use in standard containers.LessThanComparable
,
In 20.3.2.3.6 [util.smartptr.weak.obs] remove the paragraphs 9-11 including prototype:
template<class T, class U> bool operator<(const weak_ptr<T>& a, const weak_ptr<U>& b);Returns: an unspecified value such that
operator<
is a strict weak ordering as described in 25.4;under the equivalence relation defined byoperator<
,!(a < b) && !(b < a)
, twoweak_ptr
instances are equivalent if and only if they share ownership or are both empty.
Throws: nothing.
[Note: Allowsweak_ptr
objects to be used as keys in associative containers. — end note]
NULL
Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Matt Austern Opened: 2009-10-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
On g++ 4.2.4 (x86_64-linux-gnu), the following file gives a compile error:
#include <vector> void foo() { std::vector<int*> v(500L, NULL); }
Is this supposed to work?
The issue: if NULL
happens to be defined as 0L
, this is an invocation of
the constructor with two arguments of the same integral type.
23.2.4 [sequence.reqmts]/14
(N3035)
says that this will behave as if the overloaded constructor
X(size_type, const value_type& = value_type(), const allocator_type& = allocator_type())
were called instead, with the arguments
static_cast<size_type>(first)
, last
and
alloc
, respectively. However, it does not say whether this
actually means invoking that constructor with the exact textual form of
the arguments as supplied by the user, or whether the standard permits
an implementation to invoke that constructor with variables of the same
type and value as what the user passed in. In most cases this is a
distinction without a difference. In this particular case it does make a
difference, since one of those things is a null pointer constant and the
other is not.
Note that an implementation based on forwarding functions will use the latter interpretation.
[ 2010 Pittsburgh: Moved to Open. ]
[ 2010-03-19 Daniel provides wording. ]
- Adapts the numbering used in the discussion to the recent working paper N3035.
- Proposes a resolution that requires implementations to use sfinae-like means to possibly filter away the too generic template c'tor. In fact this resolution is equivalent to that used for the
pair-NULL
problem (811(i)), the only difference is, that issue 1234 was already a C++03 problem.
[ Post-Rapperswil ]
Wording was verified to match with the most recent WP. Jonathan Wakely and Alberto Barbati observed that the current
WP has a defect that should be fixed here as well: The functions signatures fx1
and fx3
are
incorrectly referring to iterator
instead of const_iterator
.
Moved to Tentatively Ready with revised wording after 7 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 23.2.3 [sequence.reqmts]/14+15 as indicated:
14 For every sequence container defined in this Clause and in Clause 21:
If the constructor
template <class InputIterator> X(InputIterator first, InputIterator last, const allocator_type& alloc = allocator_type())is called with a type
InputIterator
that does not qualify as an input iterator, then the constructor shall not participate in overload resolution.will behave as if the overloaded constructor:X(size_type, const value_type& = value_type(), const allocator_type& = allocator_type())
were called instead, with the arguments.static_cast<size_type>(first)
,last
andalloc
, respectivelyIf the member functions of the forms:
template <class InputIterator> // such as insert() rt fx1(const_iterator p, InputIterator first, InputIterator last); template <class InputIterator> // such as append(), assign() rt fx2(InputIterator first, InputIterator last); template <class InputIterator> // such as replace() rt fx3(const_iterator i1, const_iterator i2, InputIterator first, InputIterator last);are called with a type
InputIterator
that does not qualify as an input iterator, then these functions shall not participate in overload resolution.will behave as if the overloaded member functions:rt fx1(iterator, size_type, const value_type&);rt fx2(size_type, const value_type&);rt fx3(iterator, iterator, size_type, const value_type&);
were called instead, with the same arguments.
15 In the previous paragraph the alternative binding will fail iffirst
is not implicitly convertible toX::size_type
or iflast
is not implicitly convertible toX::value_type
.
error_code
/error_condition members
Section: 19.5 [syserr] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [syserr].
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Discussion:
I'm just reflecting on the now SFINAE-constrained constructors
and assignment operators of error_code
and error_condition
:
These are the only library components that are pro-actively
announcing that they are using std::enable_if
as constraining tool,
which has IMO several disadvantages:
With the availability of template default arguments and
decltype, using enable_if
in the C++0x standard library, seems
unnecessary restricting implementation freedom. E.g. there
should be no need for a useless specification of a dummy
default function argument, which only confuses the reader.
A more reasonable implementation could e.g. be
template <class ErrorCodeEnum class = typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type> error_code(ErrorCodeEnum e);
As currently specified, the function signatures are so unreadable, that errors quite easily happen, see e.g. 1229(i).
We have a lot of constrained functions in other places, that now have a standard phrase that is easily understandable:
Remarks: This constructor/function shall participate in overload resolution if and only if X.
where X describes the condition. Why should these components deviate?
If enable_if
would not be explicitly specified, the standard library
is much better prepared for the future. It would also be possible, that
libraries with partial support for not-yet-standard-concepts could provide
a much better diagnostic as is possible with enable_if
. This again
would allow for experimental concept implementations in the wild,
which as a result would make concept standardization a much more
natural thing, similar to the way as templates were standardized
in C++.
In summary: I consider it as a library defect that error_code
and
error_condition
explicitly require a dependency to enable_if
and
do limit implementation freedom and I volunteer to prepare a
corresponding resolution.
[ 2009-10-18 Beman adds: ]
I support this proposed resolution, and thank Daniel for writing it up.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
[ Should this resolution be accepted, I recommend to resolve 1229(i) as NAD ]
In 19.5.4.1 [syserr.errcode.overview]/1, class error_code
,
change as indicated:
// 19.5.2.2 constructors: error_code(); error_code(int val, const error_category& cat); template <class ErrorCodeEnum> error_code(ErrorCodeEnum e, typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type * = 0); // 19.5.2.3 modifiers: void assign(int val, const error_category& cat); template <class ErrorCodeEnum>typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::typeerror_code& operator=(ErrorCodeEnum e); void clear();
Change 19.5.4.2 [syserr.errcode.constructors] around the prototype before p. 7:
template <class ErrorCodeEnum> error_code(ErrorCodeEnum e, typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type * = 0);Remarks: This constructor shall not participate in overload resolution, unless
is_error_code_enum<ErrorCodeEnum>::value == true
.
Change 19.5.4.3 [syserr.errcode.modifiers] around the prototype before p. 3:
template <class ErrorCodeEnum>typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::typeerror_code& operator=(ErrorCodeEnum e);Remarks: This operator shall not participate in overload resolution, unless
is_error_code_enum<ErrorCodeEnum>::value == true
.
In 19.5.5.1 [syserr.errcondition.overview]/1, class error_condition
, change
as indicated:
// 19.5.3.2 constructors: error_condition(); error_condition(int val, const error_category& cat); template <class ErrorConditionEnum> error_condition(ErrorConditionEnum e, typename enable_if<is_error_condition_enum<ErrorConditionEnum>::type* = 0); // 19.5.3.3 modifiers: void assign(int val, const error_category& cat); template<typenameclass ErrorConditionEnum>typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::typeerror_condition & operator=( ErrorConditionEnum e ); void clear();
Change 19.5.5.2 [syserr.errcondition.constructors] around the prototype before p. 7:
template <class ErrorConditionEnum> error_condition(ErrorConditionEnum e, typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type* = 0);Remarks: This constructor shall not participate in overload resolution, unless
is_error_condition_enum<ErrorConditionEnum>::value == true
.
Change 19.5.5.3 [syserr.errcondition.modifiers] around the prototype before p. 3:
template <class ErrorConditionEnum>typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::typeerror_condition& operator=(ErrorConditionEnum e);Remarks: This operator shall not participate in overload resolution, unless
is_error_condition_enum<ErrorConditionEnum>::value == true
.Postcondition:
*this == make_error_condition(e)
.Returns:
*this
std::function
not neededSection: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func].
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Discussion:
The class template std::function
contains the following member
declarations:
// deleted overloads close possible hole in the type system template<class R2, class... ArgTypes2> bool operator==(const function<R2(ArgTypes2...)>&) = delete; template<class R2, class... ArgTypes2> bool operator!=(const function<R2(ArgTypes2...)>&) = delete;
The leading comment here is part of the history of std::function
, which
was introduced with
N1402.
During that time no explicit conversion functions existed, and the
"safe-bool" idiom (based on pointers-to-member) was a popular
technique. The only disadvantage of this idiom was that given two
objects f1
and f2
of type std::function
the expression
f1 == f2;
was well-formed, just because the built-in operator==
for pointer to member
was considered after a single user-defined conversion. To fix this, an
overload set of undefined comparison functions was added,
such that overload resolution would prefer those ending up in a linkage error.
The new language facility of deleted functions provided a much better
diagnostic mechanism to fix this issue.
The central point of this issue is, that with the replacement of the
safe-bool idiom by explicit conversion to bool the original "hole in the
type system" does no longer exist and therefore the comment is wrong and
the superfluous function definitions should be removed as well. An
explicit conversion function is considered in direct-initialization
situations only, which indirectly contain the so-called "contextual
conversion to bool" (7.3 [conv]/3). These conversions are not considered for
==
or !=
as defined by the core language.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
In 22.10.17.3 [func.wrap.func]/1, class function change as indicated:
// 20.7.15.2.3, function capacity: explicit operator bool() const;// deleted overloads close possible hole in the type systemtemplate<class R2, class... ArgTypes2>bool operator==(const function<R2(ArgTypes2...)>&) = delete;template<class R2, class... ArgTypes2>bool operator!=(const function<R2(ArgTypes2...)>&) = delete;
unique_copy
needs to require EquivalenceRelation
Section: 26.7.9 [alg.unique] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A lot of fixes were silently applied during concept-time and we should
not lose them again. The Requires clause of 26.7.9 [alg.unique]/5
doesn't mention that ==
and the predicate need to satisfy an
EquivalenceRelation
, as it is correctly said for unique
.
This was intentionally fixed during conceptification, were we had:
template<InputIterator InIter, class OutIter> requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type> && EqualityComparable<InIter::value_type> && HasAssign<InIter::value_type, InIter::reference> && Constructible<InIter::value_type, InIter::reference> OutIter unique_copy(InIter first, InIter last, OutIter result); template<InputIterator InIter, class OutIter, EquivalenceRelation<auto, InIter::value_type> Pred> requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type> && HasAssign<InIter::value_type, InIter::reference> && Constructible<InIter::value_type, InIter::reference> && CopyConstructible<Pred> OutIter unique_copy(InIter first, InIter last, OutIter result, Pred pred);
Note that EqualityComparable implied an equivalence relation.
[
N.B. adjacent_find
was also specified to require
EquivalenceRelation
, but that was considered as a defect in
concepts, see 1000(i)
]
[ 2009-10-31 Howard adds: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Change 26.7.9 [alg.unique]/5 as indicated:
template<class InputIterator, class OutputIterator> OutputIterator unique_copy(InputIterator first, InputIterator last, OutputIterator result); template<class InputIterator, class OutputIterator, class BinaryPredicate> OutputIterator unique_copy(InputIterator first, InputIterator last, OutputIterator result, BinaryPredicate pred);Requires: The comparison function shall be an equivalence relation. The ranges
[first,last)
and[result,result+(last-first))
shall not overlap. The expression*result = *first
shall be valid. If neitherInputIterator
norOutputIterator
meets the requirements of forward iterator then the value type ofInputIterator
shall beCopyConstructible
(34) andCopyAssignable
(table 36). OtherwiseCopyConstructible
is not required.
wait_*()
in *future
for synchronous functionsSection: 32.10 [futures] Status: Resolved Submitter: Detlef Vollmann Opened: 2009-10-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
With the addition of async()
, a future
might be
associated with a function that is not running in a different thread but
is stored to by run synchronously on the get()
call. It's not
clear what the wait()
functions should do in this case.
Suggested resolution:
Throw an exception.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
std::hash<string>
& coSection: 22.10.19 [unord.hash] Status: C++11 Submitter: Paolo Carlini Opened: 2009-10-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 22.10.19 [unord.hash], operator()
is specified as
taking the argument by value. Moreover, it is said that operator()
shall
not throw exceptions.
However, for the specializations for class types, like string
,
wstring
, etc, the former requirement seems suboptimal from the
performance point of view (a specific PR has been filed about this in the GCC Bugzilla)
and, together with the latter requirement, hard if not impossible to
fulfill. It looks like pass by const reference should be allowed in such
cases.
[ 2009-11-18: Ganesh updates wording. ]
I've removed the list of types for which
hash
shall be instantiated because it's already explicit in the synopsis of header<functional>
in 22.10 [function.objects]/2.
[ 2009-11-18: Original wording here: ]
Add to 22.10.19 [unord.hash]/2:
namespace std { template <class T> struct hash : public std::unary_function<T, std::size_t> { std::size_t operator()(T val) const; }; }The return value of
operator()
is unspecified, except that equal arguments shall yield the same result.operator()
shall not throw exceptions. It is also unspecified whetheroperator()
ofstd::hash
specializations for class types takes its argument by value or const reference.
[ 2009-11-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2009-11-24 Ville Opens: ]
I have received community requests to ask for this issue to be reopened. Some users feel that mandating the inheritance is overly constraining.
[ 2010-01-31 Alisdair: related to 978(i) and 1182(i). ]
[ 2010-02-07 Proposed resolution updated by Beman, Daniel and Ganesh. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Insert a new subclause either before or after the current 16.4.4.6 [allocator.requirements]:
Hash
Requirements [hash.requirements]This subclause defines the named requirement
Hash
, used in several clauses of the C++ standard library. A typeH
meets theHash
requirement if
it is a function object type (22.10 [function.objects]).
it satisfies the requirements of
CopyConstructible
, andDestructible
(16.4.4.2 [utility.arg.requirements]),the expressions shown in the following table are valid and have the indicated semantics, and
it satisfies all other requirements of this subclause.
Given
Key
is an argument type for function objects of typeH
, in the table belowh
is a value of type (possiblyconst
)H
,u
is an lvalue of typeKey
, andk
is a value of a type convertible to (possiblyconst
)Key
:
Table ? — Hash
requirementsExpression Return type Requirement h(k)
size_t
Shall not throw exceptions. The value returned shall depend only on the argument k
. [Note: Thus all evaluations of the expressionh(k)
with the same value fork
yield the same result. — end note] [Note: Fort1
andt2
of different values, the probability thath(t1)
andh(t2)
compare equal should be very small, approaching(1.0/numeric_limits<size_t>::max())
. — end note] Comment (not to go in WP): The wording for the second note is based on a similar note in 28.3.4.5.1.3 [locale.collate.virtuals]/3h(u)
size_t
Shall not modify u
.
Change 22.10.19 [unord.hash] as indicated:
1 The unordered associative containers defined in Clause 23.5 [unord] use specializations of the class template
hash
as the default hash function. For all object typesT
for which there exists a specializationhash<T>
, the instantiationhash<T>
shall:
- satisfy the
Hash
requirements([hash.requirements]), withT
as the function call argument type, theDefaultConstructible
requirements ([defaultconstructible]), theCopyAssignable
requirements ([copyassignable]), and theSwappable
requirements ([swappable]),- provide two nested types
result_type
andargument_type
which shall be synonyms forsize_t
andT
, respectively,- satisfy the requirement that if
k1 == k2
istrue
,h(k1) == h(k2)
istrue
, whereh
is an object of typehash<T>
, andk1
,k2
are objects of typeT
.
This class template is only required to be instantiable for integer types (6.8.2 [basic.fundamental]), floating-point types (6.8.2 [basic.fundamental]), pointer types (9.3.4.2 [dcl.ptr]), andstd::string
,std::u16string
,std::u32string
,std::wstring
,std::error_code
,std::thread::id
,std::bitset
, andstd::vector<bool>
.namespace std { template <class T> struct hash : public std::unary_function<T, std::size_t> { std::size_t operator()(T val) const; }; }
2 The return value ofoperator()
is unspecified, except that equal arguments shall yield the same result.operator()
shall not throw exceptions.
Change Unordered associative containers 23.2.8 [unord.req] as indicated:
Each unordered associative container is parameterized by
Key
, by a function object typeHash
([hash.requirements]) that acts as a hash function for argument values of typeKey
, and by a binary predicatePred
that induces an equivalence relation on values of typeKey
. Additionally,unordered_map
andunordered_multimap
associate an arbitrary mapped typeT
with theKey
.A hash function is a function object that takes a single argument of type
Key
and returns a value of typestd::size_t
.Two values
k1
andk2
of typeKey
are considered equal if the container's equality function object returnstrue
when passed those values. Ifk1
andk2
are equal, the hash function shall return the same value for both. [Note: Thus supplying a non-defaultPred
parameter usually implies the need to supply a non-defaultHash
parameter. — end note]
auto_ptr
is overspecifiedSection: 99 [auto.ptr] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-10-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This issue is extracted as the ongoing point-of-interest from earlier issue 463(i).
auto_ptr
is overspecified as the auto_ptr_ref
implementation detail is formally specified, and the technique is
observable so workarounds for compiler defects can cause a working
implementation of the primary auto_ptr
template become
non-conforming.
auto_ptr_ref
is a documentation aid to describe a possible
mechanism to implement the class. It should be marked exposition only,
as per similar classes, e.g., istreambuf_iterator::proxy
[ 2009-10-25 Daniel adds: ]
I wonder, whether the revised wording shouldn't be as straight as for
istream_buf
by adding one further sentence:An implementation is permitted to provide equivalent functionality without providing a class with this name.
[ 2009-11-06 Alisdair adds Daniel's suggestion to the proposed wording. ]
[ 2009-11-06 Howard moves issue to Review. ]
[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add the term "exposition only" in the following two places:
Ammend 99 [auto.ptr]p2:
The exposition only class
Ttemplateauto_ptr_ref
holds a reference to anauto_ptr
. It is used by theauto_ptr
conversions to allowauto_ptr
objects to be passed to and returned from functions. An implementation is permitted to provide equivalent functionality without providing a class with this name.namespace std { template <class Y> struct auto_ptr_ref { }; // exposition only
Section: 23.5 [unord] Status: Resolved Submitter: Herb Sutter Opened: 2009-10-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
See N2986.
[ 2010-01-22 Alisdair Opens. ]
[ 2010-01-24 Alisdair provides wording. ]
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3068.
Proposed resolution:
Apply paper N2986.
basic_ios
default ctorSection: 31.5.4.2 [basic.ios.cons] Status: C++11 Submitter: Martin Sebor Opened: 2009-10-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The basic_ios
default ctor is required to leave the objects members
uninitialized (see below). The paragraph says the object must be
initialized by calling basic_ios::init()
before it's destroyed but
I can't find a requirement that it be initialized before calling
any of the class other member functions. Am I not looking in the
right place or that an issue?
[ 2009-10-25 Daniel adds: ]
I agree, that your wording makes that clearer, but suggest to write
... calling
basic_ios::init
before ...()Doing so, I recommend to adapt that of
ios_base();
as well, where we have:Effects: Each
ios_base
member has an indeterminate value after construction. These members shall be initialized by callingbasic_ios::init
. If anios_base
object is destroyed before these initializations have taken place, the behavior is undefined.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 31.5.2.8 [ios.base.cons] p1:
ios_base();Effects: Each
ios_base
member has an indeterminate value after construction.TheseThe object's members shall be initialized by callingbasic_ios::init
before the object's first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined..If anios_base
object is destroyed before these initializations have taken place, the behavior is undefined.
Change 31.5.4.2 [basic.ios.cons] p2:
basic_ios();Effects: Constructs an object of class
basic_ios
(27.5.2.7) leaving its member objects uninitialized. The object shall be initialized by callingitsbasic_ios::init
before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.member function. If it is destroyed before it has been initialized the behavior is undefined.
<bitset>
still overspecifiedSection: 22.9.2 [template.bitset] Status: C++11 Submitter: Martin Sebor Opened: 2009-10-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 1227(i) — <bitset>
synopsis overspecified makes the observation
that std::bitset
, and in fact the whole library, may be implemented
without needing to #include <stdexcept>
in any library header. The
proposed resolution removes the #include <stdexcept>
directive from
the header.
I'd like to add that the <bitset>
header (as well as the rest of
the library) has also been implemented without #including the
<cstddef>
header in any library header. In the case of std::bitset
,
the template is fully usable (i.e., it may be instantiated and all
its member functions may be used) without ever mentioning size_t
.
In addition, just like no library header except for <bitset>
#includes <stdexcept>
in its synopsis, no header but <bitset>
#includes <cstddef>
either.
Thus I suggest that the #include <cstddef>
directive be similarly
removed from the synopsis of <bitset>
.
[ 2010-02-08 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
Change 22.9.2 [template.bitset]:
#include <cstddef> // for size_t#include <string> #include <iosfwd> // for istream, ostream namespace std { ...
wbuffer_convert::state_type
inconsistencySection: 99 [depr.conversions.buffer] Status: C++11 Submitter: Bo Persson Opened: 2009-10-21 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
The synopsis for wbuffer_convert
[conversions.buffer]/2 contains
typedef typename Tr::state_type state_type;
making state_type
a synonym for (possibly) some
char_traits<x>::state_type
.
However, in paragraph 9 of the same section, we have
typedef typename Codecvt::state_type state_type;The type shall be a synonym for
Codecvt::state_type
.
From what I can see, it might be hard to implement wbuffer_convert
if
the types were not both std::mbstate_t
, but I cannot find a requirement
that they must be the same type.
[ Batavia 2010: ]
Howard to draft wording, move to Review. Run it by Bill. Need to move this in Madrid.
[2011-03-06: Howard drafts wording]
[2011-03-24 Madrid meeting]
Moved to Immediate
Proposed resolution:
Modify the state_type
typedef in the synopsis of [conversions.buffer] p.2 as shown
[This makes the synopsis consistent with [conversions.buffer] p.9]:
namespace std { template<class Codecvt, class Elem = wchar_t, class Tr = std::char_traits<Elem> > class wbuffer_convert : public std::basic_streambuf<Elem, Tr> { public: typedef typenameTrCodecvt::state_type state_type; […] }; }
emplace
vs. insert
inconsistence in assoc. containersSection: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Boris Dušek Opened: 2009-10-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the latest published draft
N2960,
section 23.2.7 [associative.reqmts], paragraph 8, it is specified
that that insert
does not invalidate any iterators. As per
23.2.2 [container.requirements.general], paragraph 12, this holds
true not only for insert
, but emplace
as well. This
gives the insert
member a special treatment w.r.t.
emplace
member in 23.2.7 [associative.reqmts], par. 8,
since both modify the container. For the sake of consistency, in 23.2.7 [associative.reqmts], par. 8: either reference to insert
should be removed (i.e. count on 23.2.2 [container.requirements.general],
par. 12), or reference to emplace
be added (i.e. mention all
members of assoc. containers that modify it).
[ 2009-11-18 Chris provided wording. ]
This suggested wording covers both the issue discussed, and a number of other identical issues (namely
insert
being discussed withoutemplace
). I'm happy to go back and split and introduce a new issue if appropriate, but I think the changes are fairly mechanical and obvious.
[
2010-01-23 Daniel Krügler and J. Daniel García updated wording to
make the use of hint
consistent with insert
.
]
[
2011-02-23 Daniel Krügler adapts wording to numbering changes to match the N3225 draft. During this
action it was found that 23.2.8 [unord.req] had been changed considerably
due to acceptance of N3068
during the Pittsburgh meeting, such that the suggested wording change to
p. 6 can no longer be applied. The new wording is more general and should
now include insert()
and emplace()
calls as well ("mutating operations").
]
[2011-03-06 Daniel Krügler adapts wording to numbering changes to match the N3242 draft]
Proposed resolution:
Modify bullet 1 of 23.2.2 [container.requirements.general], p. 10:
10 Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:
insert()
or
emplace()
function while inserting a single element, that
function has no effects.
Modify 23.2.7 [associative.reqmts], p. 4:
4 An associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. The
set
andmap
classes support unique keys; themultiset
andmultimap
classes support equivalent keys. Formultiset
andmultimap
,insert
,emplace
, anderase
preserve the relative ordering of equivalent elements.
Modify Table 102 — Associative container requirements in 23.2.7 [associative.reqmts]:
Table 102 — Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity […] a_eq.emplace(args)
iterator
[…] Effects: Inserts a T
objectt
constructed withstd::forward<Args>(args)...
and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent tot
exists ina_eq
,t
is inserted at the end of that range.logarithmic a.emplace_hint(p, args)
iterator
equivalent to a.emplace(std::forward<Args>(args)...)
. Return value is an iterator pointing to the element with the key equivalent to the newly inserted element.TheThe element is inserted as close as possible to the position just prior toconst_iterator p
is a hint pointing to where the search should start.p
.Implementations are permitted to ignore the hint.logarithmic in general, but amortized constant if the element is inserted right afterbeforep
[…]
Modify 23.2.7 [associative.reqmts], p. 9:
9 The
insert
andemplace
members shall not affect the validity of iterators and references to the container, and theerase
members shall invalidate only iterators and references to the erased elements.
Modify 23.2.7.2 [associative.reqmts.except], p. 2:
2 For associative containers, if an exception is thrown by any operation from within an
insert()
oremplace()
function inserting a single element, theinsertion has no effect.insert()
function
Modify 23.2.8 [unord.req], p. 13 and p. 14:
6 An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys.
unordered_set
andunordered_map
support unique keys.unordered_multiset
andunordered_multimap
support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other in the iteration order of the container. Thus, although the absolute order of elements in an unordered container is not specified, its elements are grouped into equivalent-key groups such that all elements of each group have equivalent keys. Mutating operations on unordered containers shall preserve the relative order of elements within each equivalent-key group unless otherwise specified.[…]
13 The
insert
andemplace
members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The erase members shall invalidate only iterators and references to the erased elements.14 The
insert
andemplace
members shall not affect the validity of iterators if(N+n) < z * B
, whereN
is the number of elements in the container prior to the insert operation,n
is the number of elements inserted,B
is the container's bucket count, andz
is the container's maximum load factor.
Modify 23.2.8.2 [unord.req.except], p. 2:
2 For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an
insert()
oremplace()
function inserting a single element, theinsertioninsert()
functionhas no effect.
vector<bool>::flip
Section: 23.3.14 [vector.bool] Status: C++11 Submitter: Christopher Jefferson Opened: 2009-11-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The effects of vector<bool>::flip
has the line:
It is unspecified whether the function has any effect on allocated but unused bits.
While this is technically true, it is misleading, as any member function in any standard container may change unused but allocated memory. Users can never observe such changes as it would also be undefined behaviour to read such memory.
[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Strike second sentence from the definition of vector<bool>::flip()
,
23.3.14 [vector.bool], paragraph 5.
Effects: Replaces each element in the container with its complement.
It is unspecified whether the function has any effect on allocated but unused bits.
declval
should be added to the librarySection: 22.2 [utility] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
During the Santa Cruz meeting it was decided to split off the provision
of the library utility value()
proposed in
N2979
from the concrete request of the
UK 300
comment.
The provision of a new library component that allows the production of
values in unevaluated expressions is considered as important
to realize constrained templates in C++0x where concepts are not
available.
The following proposed resolution is an improvement over that suggested in N2958, because the proposed component can now be defined without loss of general usefulness and any use by user-code will make the program ill-formed. A possible prototype implementation that satisfies the core language requirements can be written as:
template<class T>
struct declval_protector {
static const bool stop = false;
static typename std::add_rvalue_reference<T>::type delegate(); // undefined
};
template<class T>
typename std::add_rvalue_reference<T>::type declval() {
static_assert(declval_protector<T>::stop, "declval() must not be used!");
return declval_protector<T>::delegate();
}
Further-on the earlier suggested name value()
has been changed to declval()
after discussions with committee members.
Finally the suggestion shown below demonstrates that it can simplify
existing standard wording by directly using it in the library
specification, and that it also improves an overlooked corner case for
common_type
by adding support for cv void
.
[ 2009-11-19 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
[ The proposed resolution has been updated to N3000 numbering and wording ]
Change 22.2 [utility], header <utility>
synopsis
as indicated:
// 20.3.3, forward/move: template <class T> struct identity; template <class T, class U> T&& forward(U&&); template <class T> typename remove_reference<T>::type&& move(T&&); // 20.3.4, declval: template <class T> typename add_rvalue_reference<T>::type declval(); // as unevaluated operand
Immediately after the current section 22.2.4 [forward] insert a new section:
20.3.4 Function template declval [declval]
The library provides the function template declval
to simplify
the definition of expressions which occur as
unevaluated operands (7 [expr]). The
template parameter T
of declval
may
be an incomplete type.
template <class T> typename add_rvalue_reference<T>::type declval(); // as unevaluated operand
Remarks: If this function is used according to 6.3 [basic.def.odr], the program is ill-formed.
[Example:
template<class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&);declares a function template
convert
, which only participates in overloading if the typeFrom
can be explicitly cast to typeTo
. For another example see class templatecommon_type
(21.3.8.7 [meta.trans.other]). — end example]
This bullet just makes clear that after applying
N2984,
the changes in 21.3.5.4 [meta.unary.prop], before
table Type property queries should not use declval
,
because the well-formedness requirement of the specification of
is_constructible
would become more complicated, because we
would need to make sure that the expression CE is checked in an
unevaluated context.
Also 21.3.7 [meta.rel]/4 is not modified similar to the previous bullet,
because with the stricter requirements of not using declval()
the well-formedness condition
would be harder to specify. The following changes are only editorial ones (e.g.
the removal of the duplicate declaration of create()
):
Given the following function prototype:
template <class T> typename add_rvalue_reference<T>::type create();the predicate condition for a template specialization
is_convertible<From, To>
shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:template <class T> typename add_rvalue_reference<T>::type create();To test() { return create<From>(); }
Change the entry in column "Comments" for common_type
in Table 51 —
Other transformations (21.3.8.7 [meta.trans.other]):
[
NB: This wording change extends the type domain of common_type
for cv
void => cv void
transformations and thus makes common_type
usable for
all binary type combinations that are supported by is_convertible
]
The member typedef
type
shall be defined as set out below. All types in the parameter packT
shall be complete or (possibly cv-qualified)void
. A program may specialize this trait if at least one template parameter in the specialization is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note]
Change 21.3.8.7 [meta.trans.other]/3 as indicated:
[
NB: This wording change is more than an editorial simplification of
the definition of common_type
: It also extends its usefulness for cv
void
types as outlined above
]
The nested typedef
common_type::type
shall be defined as follows:[..]
template <class T, class U> struct common_type<T, U> {private: static T&& __t(); static U&& __u(); public:typedef decltype(true ?__tdeclval<T>() :__udeclval<U>()) type; };
Change 99 [func.ret]/1 as indicated [This part solves some main aspects of issue 1225(i)]:
namespace std { template <class> class result_of; // undefined template <class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public :// typestypedefsee belowdecltype(declval<Fn>() ( declval<ArgTypes>()... )) type; }; }
1 Given an rvaluefn
of typeFn
and valuest1, t2, ..., tN
of typesT1, T2, ..., TN
inArgTypes
, respectively, thetype
member is the result type of the expressionfn(t1, t2, ...,tN)
. The valuesti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.
weak_ptr
comparison functions should be removedSection: 20.3.2.3 [util.smartptr.weak] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Additional to the necessary cleanup of the description of the the
weak_ptr
component from 20.3.2.3 [util.smartptr.weak]
described in 1231(i) it turns out that the currently deleted
comparison functions of weak_ptr
are not needed at all: There
is no safe-bool conversion from weak_ptr
, and it won't silently
chose a conversion to shared_ptr
.
[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 20.3.2.3 [util.smartptr.weak]/1 as indicated:
namespace std { template<class T> class weak_ptr { public: ...// comparisonstemplate<class Y> bool operator<(weak_ptr<Y> const&) const = delete;template<class Y> bool operator<=(weak_ptr<Y> const&) const = delete;template<class Y> bool operator>(weak_ptr<Y> const&) const = delete;template<class Y> bool operator>=(weak_ptr<Y> const&) const = delete;}; ...
concept_map
Section: 31.5 [iostreams.base] Status: C++11 Submitter: Beman Dawes Opened: 2009-11-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current WP still contains a concept_map
.
[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change Iostreams base classes 31.5 [iostreams.base], Header <ios> synopsis, as indicated:
concept_map ErrorCodeEnum<io_errc> { };template <> struct is_error_code_enum<io_errc> : true_type { } error_code make_error_code(io_errc e); error_condition make_error_condition(io_errc e); const error_category& iostream_category();
Section: 22.10.17.3.3 [func.wrap.func.mod] Status: Resolved Submitter: Daniel Krügler Opened: 2009-11-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
As of 22.10.17.3.3 [func.wrap.func.mod]/2+ we have the following prototype description:
template<class F, Allocator Alloc> void assign(F, const Alloc&);Effects:
function(f, a).swap(*this)
Two things: First the concept debris needs to be removed, second and
much more importantly, the effects clause is now impossible to satisfy,
because there is no constructor that would match the parameter sequence
(FunctionObject
, Allocator
) [plus the fact that no
f
and no a
is part of the signature]. The most
probable candidate is
template<class F, class A> function(allocator_arg_t, const A&, F);
and the effects clause needs to be adapted to use this signature.
[ 2009-11-13 Daniel brought wording up to date. ]
[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-11 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. It was noted that this issue was in partial conflict with 1288(i), and the two issues were merged in 1288(i). ]
Rationale:
Proposed resolution:
Change in 22.10.17.3.3 [func.wrap.func.mod] the complete prototype description as indicated
[ Question to the editor: Shouldn't there a paragraph number in front of the Effects clause? ]
template<class F,Allocator Allocclass A> void assign(F f, const Alloc& a);3 Effects:
function(
f, aallocator_arg, a, f).swap(*this)
is_constructible<int*,void*>
reports trueSection: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: Peter Dimov Opened: 2009-11-07 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
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Discussion:
The specification of is_constructible<T,Args...>
in
N3000
uses
static_cast<T>(create<Args>()...)
for the one-argument case, but static_cast
also permits
unwanted conversions such as void*
to T*
and
Base*
to Derived*
.
[ Post-Rapperswil: ]
Moved to
NAD EditorialResolved, this issue is addressed by paper n3047
Proposed resolution:
Change 21.3.5.4 [meta.unary.prop], p6:
the predicate condition for a template specialization
is_constructible<T, Args>
shall be satisfied, if and only if the followingexpression CEvariable definition would be well-formed:
if
sizeof...(Args) == 0
1, the expression:static_cast<T>(create<Args>()...)T t;otherwise
the expression:T t(create<Args>()...);
to_string
/ to_wstring
Section: 27.4.5 [string.conversions] Status: C++11 Submitter: Christopher Jefferson Opened: 2009-11-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Reported on the gcc mailing list.
The code "
int i; to_string(i);
" fails to compile, as 'int
' is ambiguous between 'long long
' and 'long long unsigned
'. It seems unreasonable to expect users to cast numbers up to a larger type just to useto_string
.
[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
27.4 [string.classes], change to_string
and
to_wstring
to:
string to_string(int val); string to_string(unsigned val); string to_string(long val); string to_string(unsigned long val); string to_string(long long val); string to_string(unsigned long long val); string to_string(float val); string to_string(double val); string to_string(long double val); wstring to_wstring(int val); wstring to_wstring(unsigned val); wstring to_wstring(long val); wstring to_wstring(unsigned long val); wstring to_wstring(long long val); wstring to_wstring(unsigned long long val); wstring to_wstring(float val); wstring to_wstring(double val); wstring to_wstring(long double val);
In 27.4.5 [string.conversions], paragraph 7, change to:
string to_string(int val); string to_string(unsigned val); string to_string(long val); string to_string(unsigned long val); string to_string(long long val); string to_string(unsigned long long val); string to_string(float val); string to_string(double val); string to_string(long double val);7 Returns: each function returns a
string
object holding the character representation of the value of its argument that would be generated by callingsprintf(buf, fmt, val)
with a format specifier of"%d"
,"%u"
,"%ld"
,"%lu"
,"%lld"
,"%llu"
,"%f"
,"%f"
, or"%Lf"
, respectively, wherebuf
designates an internal character buffer of sufficient size.
In 27.4.5 [string.conversions], paragraph 14, change to:
wstring to_wstring(int val); wstring to_wstring(unsigned val); wstring to_wstring(long val); wstring to_wstring(unsigned long val); wstring to_wstring(long long val); wstring to_wstring(unsigned long long val); wstring to_wstring(float val); wstring to_wstring(double val); wstring to_wstring(long double val);14 Returns: Each function returns a
wstring
object holding the character representation of the value of its argument that would be generated by callingswprintf(buf, buffsz, fmt, val)
with a format specifier ofL"%d"
,L"%u"
,L"%ld"
,L"%lu"
,L"%lld"
,L"%llu"
,L"%f"
,L"%f"
, orL"%Lf"
, respectively, wherebuf
designates an internal character buffer of sufficient sizebuffsz
.
std::less<std::shared_ptr<T>>
is underspecifiedSection: 20.3.2.2.8 [util.smartptr.shared.cmp] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-11-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.3.2.2.8 [util.smartptr.shared.cmp]/5 says:
For templates
greater
,less
,greater_equal
, andless_equal
, the partial specializations forshared_ptr
shall yield a total order, even if the built-in operators<
,>
,<=
, and>=
do not. Moreover,less<shared_ptr<T> >::operator()(a, b)
shall returnstd::less<T*>::operator()(a.get(), b.get())
.
This is necessary in order to use shared_ptr
as the key in associate
containers because
n2637
changed operator<
on shared_ptr
s to be
defined in terms of operator<
on the stored pointers (a mistake IMHO
but too late now.) By 7.6.9 [expr.rel]/2 the result of comparing builtin
pointers is unspecified except in special cases which generally do not
apply to shared_ptr
.
Earlier versions of the WP (n2798, n2857) had the following note on that paragraph:
[Editor's note: It's not clear to me whether the first sentence is a requirement or a note. The second sentence seems to be a requirement, but it doesn't really belong here, under
operator<
.]
I agree completely - if partial specializations are needed they should be properly specified.
20.3.2.2.8 [util.smartptr.shared.cmp]/6 has a note saying the comparison operator
allows shared_ptr
objects to be used as keys in associative
containers, which is misleading because something else like a
std::less
partial specialization is needed. If it is not correct that
note should be removed.
20.3.2.2.8 [util.smartptr.shared.cmp]/3 refers to 'x
' and
'y
' but the prototype has parameters 'a
' and
'b
' - that needs to be fixed even if the rest of the issue is
NAD.
I see two ways to fix this, I prefer the first because it removes the
need for any partial specializations and also fixes operator>
and
other comparisons when defined in terms of operator<
.
Replace 20.3.2.2.8 [util.smartptr.shared.cmp]/3 with the following and remove p5:
template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);3 Returns:
x.get() < y.get()
.std::less<V>()(a.get(), b.get())
, whereV
is the composite pointer type (7.6.9 [expr.rel]).4 Throws: nothing.
5 For templatesgreater
,less
,greater_equal
, andless_equal
, the partial specializations forshared_ptr
shall yield a total order, even if the built-in operators<
,>
,<=
, and>=
do not. Moreover,less<shared_ptr<T> >::operator()(a, b)
shall returnstd::less<T*>::operator()(a.get(), b.get())
.6 [Note: Defining a comparison operator allows
shared_ptr
objects to be used as keys in associative containers. — end note]
Add to 20.3.2.2 [util.smartptr.shared]/1 (after the shared_ptr
comparisons)
template<class T> struct greater<shared_ptr<T>>; template<class T> struct less<shared_ptr<T>>; template<class T> struct greater_equal<shared_ptr<T>>; template<class T> struct less_equal<shared_ptr<T>>;
Remove 20.3.2.2.8 [util.smartptr.shared.cmp]/5 and /6 and replace with:
template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);3 Returns:
.
xa.get() <yb.get()4 Throws: nothing.
5 For templatesgreater
,less
,greater_equal
, andless_equal
, the partial specializations forshared_ptr
shall yield a total order, even if the built-in operators<
,>
,<=
, and>=
do not. Moreover,less<shared_ptr<T> >::operator()(a, b)
shall returnstd::less<T*>::operator()(a.get(), b.get())
.
6 [Note: Defining a comparison operator allowsshared_ptr
objects to be used as keys in associative containers. — end note]template<class T> struct greater<shared_ptr<T>> : binary_function<shared_ptr<T>, shared_ptr<T>, bool> { bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const; };
operator()
returnsgreater<T*>()(a.get(), b.get())
.template<class T> struct less<shared_ptr<T>> : binary_function<shared_ptr<T>, shared_ptr<T>, bool> { bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const; };
operator()
returnsless<T*>()(a.get(), b.get())
.template<class T> struct greater_equal<shared_ptr<T>> : binary_function<shared_ptr<T>, shared_ptr<T>, bool> { bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const; };
operator()
returnsgreater_equal<T*>()(a.get(), b.get())
.template<class T> struct less_equal<shared_ptr<T>> : binary_function<shared_ptr<T>, shared_ptr<T>, bool> { bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const; };
operator()
returnsless_equal<T*>()(a.get(), b.get())
.
[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Replace 20.3.2.2.8 [util.smartptr.shared.cmp]/3 with the following and remove p5:
template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);3 Returns:
x.get() < y.get()
.less<V>()(a.get(), b.get())
, whereV
is the composite pointer type (7.6.9 [expr.rel]) ofT*
andU*
.4 Throws: nothing.
5 For templatesgreater
,less
,greater_equal
, andless_equal
, the partial specializations forshared_ptr
shall yield a total order, even if the built-in operators<
,>
,<=
, and>=
do not. Moreover,less<shared_ptr<T> >::operator()(a, b)
shall returnstd::less<T*>::operator()(a.get(), b.get())
.6 [Note: Defining a comparison operator allows
shared_ptr
objects to be used as keys in associative containers. — end note]
quick_exit
support for freestanding implementationsSection: 16.4.2.5 [compliance] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-11-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 172
This issue is a response to NB comment UK-172
The functions quick_exit
and at_quick_exit
should be
added to the required features of <cstdlib>
in a
freestanding implementation.
This comment was rejected in Summit saying neither at_exit
nor
at_quick_exit
should be required. This suggests the comment was
misread, as atexit
is already required to be supported. If the LWG
really did wish to not require the registration functions be supported,
then a separate issue should be opened to change the current standard.
Given both exit
and atexit
are required, the UK panel feels it is
appropriate to require the new quick_exit
facility is similarly
supported.
[ 2009-12-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Ammend p3 Freestanding implementations 16.4.2.5 [compliance]
3 The supplied version of the header
<cstdlib>
shall declare at least the functionsabort
,()atexit
,()at_quick_exit
,andexit
, and()quick_exit
(17.5 [support.start.term]). The other headers listed in this table shall meet the same requirements as for a hosted implementation.
shared_future::get
and deferred async
functionsSection: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
If a shared_future
is constructed with the result of an async
call with a
deferred function, and two or more copies of that shared_future
are created,
with multiple threads calling get()
, it is not clear which thread runs the
deferred function. [futures.shared_future]p22 from
N3000
says (minus editor's note):
Effects: if the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.
In the absence of wording to the contrary, this implies that every thread that
calls wait()
will execute the deferred function.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Replace [futures.shared_future]p22 with the following:
Effects: If the associated state
contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.was created by apromise
orpackaged_task
object, block until the associated state is ready. If the associated state is associated with a thread created for anasync
call (32.10.9 [futures.async]), as ifassociated-thread.join()
.If the associated state contains a deferred function, calls to
wait()
on allshared_future
objects that share the same associated state are serialized. The first call towait()
that shares a given associated state executes the deferred function and stores the return value or exception in the associated state.Synchronization: if the associated state was created by a
promise
object, the completion ofset_value()
orset_exception()
to thatpromise
happens before (6.9.2 [intro.multithread])wait()
returns. If the associated state was created by apackaged_task
object, the completion of the associated task happens beforewait()
returns. If the associated state is associated with a thread created for anasync
call (32.10.9 [futures.async]), the completion of the associated thread happens-beforewait()
returns.If the associated state contained a deferred function, the invocation of the deferred function happens-before any call to
wait()
on afuture
that shares that state returns.
condition_variable_any::wait_for
Section: 32.7.5 [thread.condition.condvarany] Status: C++11 Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvarany].
View all issues with C++11 status.
Discussion:
32.7.5 [thread.condition.condvarany]p18 and 32.7.5 [thread.condition.condvarany]p27 specify incorrect preconditions for
condition_variable_any::wait_for
. The stated preconditions require that
lock
has a mutex()
member function, and that this produces the
same result for all concurrent calls to wait_for()
. This is
inconsistent with wait()
and wait_until()
which do not impose
such a requirement.
[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Remove 32.7.5 [thread.condition.condvarany]p18 and 32.7.5 [thread.condition.condvarany]p27.
template <class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);
18 Precondition: lock is locked by the calling thread, and either
no other thread is waiting on thiscondition_variable
object orlock.mutex()
returns the same value for each of the lock arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads....
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);
27 Precondition: lock is locked by the calling thread, and either
no other thread is waiting on thiscondition_variable
object orlock.mutex()
returns the same value for each of the lock arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.
Section: 32.6 [thread.mutex] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.mutex].
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Discussion:
The Mutex
requirements in 32.6.4 [thread.mutex.requirements] and
32.6.4.3 [thread.timedmutex.requirements] confuse the requirements on the
behaviour of std::mutex
et al with the requirements on
Lockable
types for use with std::unique_lock
,
std::lock_guard
and std::condition_variable_any
.
[ 2010 Pittsburgh: ]
Concepts of threads chapter and issue presentation are: Lockable < Mutex < TimedMutex and Lockable < TimedLockable < TimedMutex.
Typo in failed deletion of Mutex in 30.4.4 p4 edits.
Lockable requirements are too weak for condition_variable_any, but the Mutex requirements are too strong.
Need subset of Lockable requirements for condition_variable_any that does not include try_lock. E.g. CvLockable < Lockable.
Text needs updating to recent draft changes.
Needs to specify exception behavior in Lockable.
The current standard is fine for what it says, but it places requirements that are too strong on authors of mutexes and locks.
Move to open status. Suggest Anthony look at condition_variable_any requirements. Suggest Anthony refine requirements/concepts categories.
[ 2010-03-28 Daniel synced with N3092. ]
[ 2010-10-25 Daniel adds: ]
Accepting n3130 would solve this issue.
[ 2010-11 Batavia: ]
Resolved by adopting n3197.
Proposed resolution:
Add a new section to 32.2 [thread.req] after 32.2.4 [thread.req.timing] as follows:
30.2.5 Requirements for Lockable types
The standard library templates
unique_lock
(32.6.5.4 [thread.lock.unique]),lock_guard
(32.6.5.2 [thread.lock.guard]),lock
,try_lock
(32.6.6 [thread.lock.algorithm]) andcondition_variable_any
(32.7.5 [thread.condition.condvarany]) all operate on user-suppliedLockable
objects. Such an object must support the member functions specified for either theLockable
Requirements or theTimedLockable
requirements as appropriate to acquire or release ownership of alock
by a giventhread
. [Note: the nature of any lock ownership and any synchronization it may entail are not part of these requirements. — end note]30.2.5.1 Lockable Requirements
In order to qualify as a
Lockable
type, the following expressions must be supported, with the specified semantics, wherem
denotes a value of typeL
that supports theLockable
:The expression
m.lock()
shall be well-formed and have the following semantics:
- Effects:
- Block until a lock can be acquired for the current thread.
- Return type:
void
The expression
m.try_lock()
shall be well-formed and have the following semantics:
- Effects:
- Attempt to acquire a lock for the current thread without blocking.
- Return type:
bool
- Returns:
true
if the lock was acquired,false
otherwise.The expression
m.unlock()
shall be well-formed and have the following semantics:
- Effects:
- Release a lock on
m
held by the current thread.- Return type:
void
- Throws:
- Nothing if the current thread holds a lock on
m
.30.2.5.2
TimedLockable
RequirementsFor a type to qualify as
TimedLockable
it must meet theLockable
requirements, and additionally the following expressions must be well-formed, with the specified semantics, wherem
is an instance of a typeTL
that supports theTimedLockable
requirements,rel_time
denotes instantiation ofduration
(30.5 [time.duration]) andabs_time
denotes an instantiation oftime_point
(30.6 [time.point])The expression
m.try_lock_for(rel_time)
shall be well-formed and have the following semantics:
- Effects:
- Attempt to acquire a lock for the current thread within the specified time period.
- Return type:
bool
- Returns:
true
if the lock was acquired,false
otherwise.The expression
m.try_lock_until(abs_time)
shall be well-formed and have the following semantics:
- Effects:
- Attempt to acquire a lock for the current thread before the specified point in time.
- Return type:
bool
- Returns:
true
if the lock was acquired,false
otherwise.
Replace 32.6.4 [thread.mutex.requirements] paragraph 2 with the following:
2 This section describes requirements on
template argument types used to instantiate templates defined inthe mutex types supplied by the C++ standard library.The template definitions in the C++ standard library referThese types shall conform to the namedMutex
requirements whose details are set out below. In this description,m
is an object ofaone of the standard library mutex typesMutex
typestd::mutex
,std::recursive_mutex
,std::timed_mutex
orstd::recursive_timed_mutex
..
Add the following paragraph after 32.6.4 [thread.mutex.requirements] paragraph 2:
A
Mutex
type shall conform to theLockable
requirements (30.2.5.1).
Replace 32.6.4.3 [thread.timedmutex.requirements] paragraph 1 with the following:
The C++ standard library
TimedMutex
typesstd::timed_mutex
andstd::recursive_timed_mutex
Ashall meet the requirements for aTimedMutex
typeMutex
type. In addition,itthey shall meet the requirements set outin this Clause 30.4.2below, whererel_time
denotes an instantiation ofduration
(30.5 [time.duration]) andabs_time
denotes an instantiation oftime_point
(30.6 [time.point]).
Add the following paragraph after 32.6.4.3 [thread.timedmutex.requirements] paragraph 1:
A
TimedMutex
type shall conform to theTimedLockable
requirements (30.2.5.1).
Add the following paragraph following 32.6.5.2 [thread.lock.guard] paragraph 1:
The supplied
Mutex
type shall meet theLockable
requirements (30.2.5.1).
Add the following paragraph following 32.6.5.4 [thread.lock.unique] paragraph 1:
The supplied
Mutex
type shall meet theLockable
requirements (30.2.5.1).unique_lock<Mutex>
meets theLockable
requirements. IfMutex
meets theTimedLockable
requirements (30.2.5.2) thenunique_lock<Mutex>
also meets theTimedLockable
requirements.
Replace the use of "mutex" or "mutex object" with "lockable object" throughout clause 32.6.5 [thread.lock] paragraph 1:
1 A lock is an object that holds a reference to a
mutexlockable object and may unlock themutexlockable object during the lock's destruction (such as when leaving block scope). A thread of execution may use a lock to aid in managingmutexownership of a lockable object in an exception safe manner. A lock is said to own amutexlockable object if it is currently managing the ownership of thatmutexlockable object for a thread of execution. A lock does not manage the lifetime of themutexlockable object it references. [ Note: Locks are intended to ease the burden of unlocking themutexlockable object under both normal and exceptional circumstances. — end note ]
32.6.5 [thread.lock] paragaph 2:
2 Some lock constructors take tag types which describe what should be done with the
mutexlockable object during the lock's constuction.
32.6.5.2 [thread.lock.guard] paragaph 1:
1 An object of type
lock_guard
controls the ownership of amutexlockable object within a scope. Alock_guard
object maintains ownership of amutexlockable object throughout thelock_guard
object's lifetime. The behavior of a program is undefined if themutexlockable object referenced bypm
does not exist for the entire lifetime (3.8) of thelock_guard
object.Mutex
shall meet theLockable
requirements (30.2.5.1).
32.6.5.4 [thread.lock.unique] paragaph 1:
1 An object of type
unique_lock
controls the ownership of amutexlockable object within a scope.MutexoOwnership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to anotherunique_lock
object. Objects of typeunique_lock
are not copyable but are movable. The behavior of a program is undefined if the contained pointerpm
is not null and the mutex pointed to bypm
does not exist for the entire remaining lifetime (3.8) of theunique_lock
object.Mutex
shall meet theLockable
requirements (30.2.5.1).
Add the following to the precondition of unique_lock(mutex_type&
m, const chrono::time_point<Clock, Duration>& abs_time)
in
32.6.5.4.2 [thread.lock.unique.cons] paragraph 18:
template <class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);18 Requires: If
mutex_type
is not a recursive mutex the calling thread does not own the mutex. The suppliedmutex_type
type shall meet theTimedLockable
requirements (30.2.5.2).
Add the following to the precondition of unique_lock(mutex_type&
m,
const chrono::duration<Rep, Period>& rel_time)
in
32.6.5.4.2 [thread.lock.unique.cons]
paragraph 22
22 Requires: If
mutex_type
is not a recursive mutex the calling thread does not own the mutex. The suppliedmutex_type
type shall meet theTimedLockable
requirements (30.2.5.2).
Add the following as a precondition of bool try_lock_until(const
chrono::time_point<Clock, Duration>& abs_time)
before
32.6.5.4.3 [thread.lock.unique.locking] paragraph 8
template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);Requires: The supplied
mutex_type
type shall meet theTimedLockable
requirements (30.2.5.2).
Add the following as a precondition of bool try_lock_for(const
chrono::duration<Rep, Period>& rel_time)
before
32.6.5.4.3 [thread.lock.unique.locking] paragraph 12
template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);Requires: The supplied
mutex_type
type shall meet theTimedLockable
requirements (30.2.5.2).
Replace 32.6.6 [thread.lock.algorithm] p1 with the following:
template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);1 Requires: Each template parameter type shall meet the
requirements (30.2.5.1).
MutexLockable, except that a call to[Note: Thetry_lock()
may throw an exception.unique_lock
class template meets these requirements when suitably instantiated. — end note]
Replace 32.6.6 [thread.lock.algorithm] p4 with the following:
template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);4 Requires: Each template parameter type shall meet the
Mutex
requirements (30.2.5.1).MutexLockable, except that a call to[Note: Thetry_lock()
may throw an exception.unique_lock
class template meets these requirements when suitably instantiated. — end note]
Replace 32.7.5 [thread.condition.condvarany] paragraph 1 with:
1 A
Lock
type shall meet therequirements for aMutex
typeLockable
requirements (30.2.5.1), except thattry_lock
is not required. [Note: All of the standard mutex types meet this requirement. — end note]
async
Section: 32.10.5 [futures.state] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.state].
View all issues with Resolved status.
Discussion:
The current description of the associated state in 32.10.5 [futures.state]
does not allow for futures created by an async
call. The description
therefore needs to be extended to cover that.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Add a new sentence to 32.10.5 [futures.state] p2:
2 This associated state consists of some state information and some (possibly not yet evaluated) result, which can be a (possibly
void
) value or an exception. If the associated state was created by a call toasync
(32.10.9 [futures.async]) then it may also contain a deferred function or an associatedthread
.
Add an extra bullet to 32.10.5 [futures.state] p3:
The result of an associated state can be set by calling:
promise::set_value
,promise::set_exception
,or- packaged_task::operator()
., or- a call to
async
(32.10.9 [futures.async]).
result_of
should be moved to <type_traits>
Section: 99 [func.ret] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-11-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.ret].
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Discussion:
Addresses UK 198
NB Comment: UK-198 makes this request among others. It refers to a more detailed issue that BSI did not manage to submit by the CD1 ballot deadline though.
result_of
is essentially a metafunction to return the type of an
expression, and belongs with the other library metafunctions in
<type_traits>
rather than lurking in <functional>
.
The current definition in <functional>
made sense when
result_of
was nothing more than a protocol to enable several components
in <functional>
to provide their own result types, but it has
become a more general tool. For instance, result_of
is now used in the
threading and futures components.
Now that <type_traits>
is a required header for free-standing
implementations it will be much more noticeable (in such environments) that a
particularly useful trait is missing, unless that implementation also chooses to
offer <functional>
as an extension.
The simplest proposal is to simply move the wording (editorial direction below) although a more consistent form for type_traits would reformat this as a table.
Following the acceptance of 1255(i), result_of
now
depends on the declval
function template, tentatively provided
in <utility>
which is not (yet) required of a
free-standing implementation.
This dependency is less of an issue when result_of
continues to
live in <functional>
.
Personally, I would prefer to clean up the dependencies so both
result_of
and declval
are available in a free-standing
implementation, but that would require slightly more work than suggested
here. A minimal tweak would be to require <utility>
in a
free-standing implementation, although there are a couple of subtle
issues with make_pair
, which uses reference_wrapper
in
its protocol and that is much harder to separate cleanly from
<functional>
.
An alternative would be to enact the other half of
N2979
and create a new minimal header for the new C++0x library facilities to
be added to the freestanding requirements (plus swap
.)
I have a mild preference for the latter, although there are clearly
reasons to consider better library support for free-standing in general,
and adding the whole of <utility>
could be considered a step in that
direction. See NB comment
JP-23
for other suggestions (array
, ratio
)
[ 2010-01-27 Beman updated wording. ]
The original wording is preserved here:
Move 99 [func.ret] to a heading below 21 [meta]. Note that in principle we should not change the tag, although this is a new tag for 0x. If it has been stable since TR1 it is clearly immutable though.
This wording should obviously adopt any other changes currently in (Tentatively) Ready status that touch this wording, such as 1255(i).
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
From Function objects 22.10 [function.objects], Header <functional>
synopsis, remove:
// 20.7.4 result_of: template <class> class result_of; // undefined template <class F, class... Args> class result_of<F(ArgTypes...)>;
Remove Function object return types 99 [func.ret] in its entirety. This sub-section reads:
namespace std { template <class> class result_of; // undefined template <class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public : // types typedef see below type; }; }Given an rvalue
fn
of typeFn
and valuest1, t2, ..., tN
of types T1, T2, ..., TN inArgTypes
, respectively, the type member is the result type of the expressionfn(t1, t2, ...,tN)
. The valuesti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.
To Header <type_traits> synopsis 21.3.3 [meta.type.synop], add at the indicated location:
template <class T> struct underlying_type; template <class T> struct result_of; // not defined template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;
To Other transformations 21.3.8.7 [meta.trans.other], Table 51 — Other transformations, add:
Template Condition Comments template <class T>
struct underlying_type;T
shall be an enumeration type (7.2)The member typedef type
shall name the underlying type ofT
.template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;
Fn
shall be a function object type 22.10 [function.objects], reference to function, or reference to function object type.decltype(declval<Fn>()(declval<ArgTypes>()...))
shall be well formed.The member typedef type
shall name the typedecltype(declval<Fn>()(declval<ArgTypes>()...))
.
At the end of Other transformations 21.3.8.7 [meta.trans.other] add:
[Example: Given these definitions:
typedef bool(&PF1)(); typedef short(*PF2)(long); struct S { operator PF2() const; double operator()(char, int&); };the following assertions will hold:
static_assert(std::is_same<std::result_of<S(int)>::type, short>::value, "Error!"); static_assert(std::is_same<std::result_of<S&(unsigned char, int&)>::type, double>::value, "Error!"); static_assert(std::is_same<std::result_of<PF1()>::type, bool>::value, "Error!");— end example]
CR
undefined in duration operatorsSection: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
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Discussion:
IMO CR
alone is not really defined (it should be CR(Rep1,
Rep2)
).
[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 30.5.6 [time.duration.nonmember] paragraphs 9 and 12:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);9 Returns:
duration<CR(Rep1, Rep2), Period>(d) /= s
.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);12 Returns:
duration<CR(Rep1, Rep2), Period>(d) %= s
.
promise::set_value
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
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Discussion:
The definitions of promise::set_value
need tidying up, the
synopsis says:
// setting the result void set_value(const R& r); void set_value(see below);
Why is the first one there? It implies it is always present for all specialisations of promise, which is not true.
The definition says:
void set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();
The lack of qualification on the first one again implies it's present for all specialisations, again not true.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Change the synopsis in 32.10.6 [futures.promise]:
// setting the resultvoid set_value(const R& r);void set_value(see below);
And the definition be changed by qualifying the first signature:
void promise::set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();
future::valid
should be callable on an invalid futureSection: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
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Discussion:
[futures.unique_future]/3 says:
The effect of calling any member function other than the destructor or the move-assignment operator on a
future
object for whichvalid() == false
is undefined.
This means calling future::valid()
is undefined unless it will
return true
, so you can only use it if you know the answer!
[ 2009-12-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change [futures.unique_future]/3:
The effect of calling any member function other than the destructor, or the move-assignment operator, or
valid
, on afuture
object for whichvalid() == false
is undefined.
atomic_future
constructorSection: 99 [futures.atomic_future] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.atomic_future].
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Discussion:
In 99 [futures.atomic_future] this constructor:
atomic_future(future<R>&&);
is declared in the synopsis, but not defined. Instead n2997 defines:
atomic_future(const future<R>&& rhs);
and n3000 defines
atomic_future(atomic_future<R>&& rhs);
both of which are wrong. The constructor definition should be changed to match the synopsis.
[ 2009-12-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Adjust the signature above 99 [futures.atomic_future]/6 like so:
atomic_future(atomic_future<R>&& rhs);
Section: 32.10 [futures] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures].
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Discussion:
[futures.unique_future]/1 should be updated to mention
async
.
[futures.shared_future]/1 should also be updated for
async
. That paragraph also says
... Its value or exception can be set by use of a
shared_future
,promise
(32.10.6 [futures.promise]), orpackaged_task
(32.10.10 [futures.task]) object that shares the same associated state.
How can the value be set by a shared_future
?
99 [futures.atomic_future]/1 says
An
atomic_future
object can only be created by use of apromise
(32.10.6 [futures.promise]) orpackaged_task
(32.10.10 [futures.task]) object.
which is wrong, it's created from a std::future
, which could
have been default-constructed. That paragraph should be closer to the
text of [futures.shared_future]/1, and should also mention
async
.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
forwardlist
missing allocator constructorsSection: 23.3.7 [forward.list] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-12 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list].
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Discussion:
I found that forward_list has only
forward_list(const forward_list<T,Allocator>& x); forward_list(forward_list<T,Allocator>&& x);
but misses
forward_list(const forward_list& x, const Allocator&); forward_list(forward_list&& x, const Allocator&);
Note to other reviewers: I also checked the container adaptors for similar inconsistencies, but as far as I can see these are already handled by the current active issues 1194(i) and 1199(i).
[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In [forwardlist]/3, class template forward_list synopsis change as indicated:
forward_list(const forward_list<T,Allocator>& x); forward_list(forward_list<T,Allocator>&& x); forward_list(const forward_list&, const Allocator&); forward_list(forward_list&&, const Allocator&);
std::thread::id
should be trivially copyableSection: 32.4.3.2 [thread.thread.id] Status: C++11 Submitter: Anthony Williams Opened: 2009-11-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.id].
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Discussion:
The class definition of std::thread::id
in
N3000
is:
class thread::id { public: id(); };
Typically, I expect that the internal data members will either be
pointers or integers, so that in practice the class will be trivially
copyable. However, I don't think the current wording guarantees it, and
I think it would be useful. In particular, I can see a use for
std::atomic<std::thread::id>
to allow a thread
to claim ownership of a data structure atomicly, and
std::atomic<T>
requires that T
is trivially
copyable.
[ 2010-02-12 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]
Proposed resolution:
Add a new sentence to 32.4.3.2 [thread.thread.id] p1:
1 An object of type
thread::id
provides a unique identifier for each thread of execution and a single distinct value for allthread
objects that do not represent a thread of execution (32.4.3 [thread.thread.class]). Each thread of execution has an associatedthread::id
object that is not equal to thethread::id
object of any other thread of execution and that is not equal to thethread::id
object of anystd::thread
object that does not represent threads of execution. The library may reuse the value of athread::id
of a terminated thread that can no longer be joined.thread::id
shall be a trivially copyable class (11 [class]).
forward_list::insert_after
Section: 23.3.7.5 [forward.list.modifiers] Status: C++11 Submitter: Bo Persson Opened: 2009-11-25 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.modifiers].
View all issues with C++11 status.
Discussion:
After applying LDR149(i), forward_list
now has 5
overloads of insert_after
, all returning an iterator.
However, two of those - inserting a single object - return "An iterator
pointing to a copy of x
[the inserted object]" while the other
three - inserting zero or more objects - return an iterator equivalent
to the position parameter, pointing before any possibly inserted
objects.
Is this the intended change?
I don't really know what insert_after(position, empty_range)
should really return, but always returning position
seems less
than useful.
[ 2010-02-04 Howard adds: ]
I agree this inconsistency will be error prone and needs to be fixed. Additionally
emplace_after
's return value is unspecified.
[ 2010-02-04 Nico provides wording. ]
[ 2010 Pittsburgh: ]
We prefer to return an iterator to the last inserted element. Modify the proposed wording and then set to Ready.
[ 2010-03-15 Howard adds: ]
Wording updated and set to Ready.
Proposed resolution:
In forward_list
modifiers [forwardlist.modifiers]
make the following modifications:
iterator insert_after(const_iterator position, size_type n, const T& x);...
10 Returns:
position.An iterator pointing to the last inserted copy ofx
orposition
ifn == 0
.template <class InputIterator> iterator insert_after(const_iterator position, InputIterator first, InputIterator last);...
13 Returns:
position.An iterator pointing to the last inserted element orposition
iffirst == last
.iterator insert_after(const_iterator position, initializer_list<T> il);...
15 Returns:
position.An iterator pointing to the last inserted element orposition
ifil
is empty.template <class... Args> iterator emplace_after(const_iterator position, Args&&... args);...
17 ...
Returns: An iterator pointing to the new constructed element from args.
[u|bi]nary_function
specializationSection: 99 [depr.base] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2009-11-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [depr.base].
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Discussion:
A program should not be allowed to add specialization of class templates
unary_function
and binary_function
, in force of 16.4.5.2.1 [namespace.std]/1.
If a program were allowed to specialize these templates, the library could no
longer rely on them to provide the intended typedefs or there might be other
undesired interactions.
[ 2010-03-27 Daniel adds: ]
Accepting issue 1290(i) would resolve this issue as NAD editorial.
[ 2010-10-24 Daniel adds: ]
Accepting n3145 would resolve this issue as NAD editorial.
[ 2010 Batavia: ]
Pete: Is this issue actually addressed by N3198, or did deprecating unary/binary_function?
We determined that this issue is NOT resolved and that it must be resolved or else N3198 could break code that does specialize unary/binary function. Matt: don't move to NAD Howard: I suggest we go further and move 1279 to ready for Madrid. Group: Agrees move 1279 to ready for Madrid
Previous proposed resolution:
1 The following
classesclass templates are provided to simplify the typedefs of the argument and result types:. A program shall not declare specializations of these templates.
[2011-03-06 Daniel comments]
This meeting outcome was not properly reflected in the proposed resolution. I also adapted the suggested wording to the N3242 numbering and content state. During this course of action it turned out that the first suggested wording change has already been applied.
Proposed resolution:
Change paragraph 99 [depr.base]/1 as follows:
1 The class templates
unary_function
andbinary_function
are deprecated. A program shall not declare specializations of these templates.
Section: 24.6.2.2 [istream.iterator.cons], 24.6.3.2 [ostream.iterator.cons.des] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.iterator.cons].
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Discussion:
24.6.2.2 [istream.iterator.cons] describes the effects in terms of:
basic_istream<charT,traits>* in_stream; // exposition only3 Effects: Initializes in_stream with
s
.
That should be &s
and similarly for 24.6.3.2 [ostream.iterator.cons.des].
[ 2009-12-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
Change 24.6.2.2 [istream.iterator.cons] like so:
istream_iterator(istream_type& s);3 Effects: Initializes in_stream with
&s
. value ...
And 24.6.3.2 [ostream.iterator.cons.des] like so:
ostream_iterator(ostream_type& s);1 Effects: Initializes out_stream with
&s
and delim with null.ostream_iterator(ostream_type& s, const charT* delimiter);2 Effects: Initializes out_stream with
&s
and delim withdelimiter
.
Section: 21.4.3 [ratio.ratio] Status: Resolved Submitter: Vicente Juan Botet Escribá Opened: 2009-12-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ratio.ratio].
View all issues with Resolved status.
Discussion:
CopyConstruction and Assignment between ratio
s having the same
normalized form. Current
N3000
do not allows to copy-construct or assign ratio
instances of
ratio
classes having the same normalized form.
Two ratio
classes ratio<N1,D1>
and
ratio<N2,D2>
have the same normalized form if
ratio<N1, D1>::num == ratio<N2, D2>::num && ratio<N1, D1>::den == ratio<N2, D2>::den
This simple example
ratio<1,3> r1; ratio<3,9> r2; r1 = r2; // (1)
fails to compile in (1). Other example
ratio<1,3> r1; ratio_subtract<ratio<2,3>, ratio<1,3>>::type r2; r1 = r2;
The nested type of ratio_subtract<ratio<2,3>,
ratio<1,3>>
could be ratio<3,9>
so the compilation
could fail. It could also be ratio<1,3>
and the compilation
succeeds.
In 21.4.4 [ratio.arithmetic] 3 and similar clauses
3 The nested typedef
type
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::den - R2::num * R1::den
andT2
has the valueR1::den * R2::den
.
the meaning of synonym let think that the result shall be a normalized
ratio
equivalent to ratio<T1, T2>
, but there is not an
explicit definition of what synonym means in this context.
Additionally we should add a typedef for accessing the normalized
ratio
, and change 21.4.4 [ratio.arithmetic] to return only this
normalized result.
[ 2010 Pittsburgh: ]
There is no consensus to add the converting copy constructor or converting copy assignment operator. However there was consensus to add the typedef.
Proposed wording modified. Original proposed wording preserved here. Moved to Review.
Make
ratio
default constructible, copy-constructible and assignable from anyratio
which has the same reduced form.Add to 21.4.3 [ratio.ratio] synopsis
template <intmax_t N, intmax_t D = 1> class ratio { public: static constexpr intmax_t num; static constexpr intmax_t den; typedef ratio<num, den> type; ratio() = default; template <intmax_t N2, intmax_t D2> ratio(const ratio<N2, D2>&); template <intmax_t N2, intmax_t D2> ratio& operator=(const ratio<N2, D2>&); };Add to 21.4.3 [ratio.ratio]:
Two ratio classes
ratio<N1,D1>
andratio<N2,D2>
have the same reduced form ifratio<N1,D1>::type
is the same type asratio<N2,D2>::type
Add a new section: [ratio.cons]
Construction and assignment [ratio.cons]
template <intmax_t N2, intmax_t D2> ratio(const ratio<N2, D2>& r);Effects: Constructs a
ratio
object.Remarks: This constructor shall not participate in overload resolution unless
r
has the same reduced form as*this
.template <intmax_t N2, intmax_t D2> ratio& operator=(const ratio<N2, D2>& r);Effects: None.
Returns:
*this
.Remarks: This operator shall not participate in overload resolution unless
r
has the same reduced form as*this
.Change 21.4.4 [ratio.arithmetic]
Implementations may use other algorithms to compute these values. If overflow occurs, a diagnostic shall be issued.
template <class R1, class R2> struct ratio_add { typedef see below type; };The nested typedef
type
shall be a synonym forratio<T1, T2>::type
whereT1
has the valueR1::num * R2::den + R2::num * R1::den
andT2
has the valueR1::den * R2::den
.template <class R1, class R2> struct ratio_subtract { typedef see below type; };The nested typedef
type
shall be a synonym forratio<T1, T2>::type
whereT1
has the valueR1::num * R2::den - R2::num * R1::den
andT2
has the valueR1::den * R2::den
.template <class R1, class R2> struct ratio_multiply { typedef see below type; };The nested typedef
type
shall be a synonym forratio<T1, T2>::type
whereT1
has the valueR1::num * R2::num
andT2
has the valueR1::den * R2::den
.template <class R1, class R2> struct ratio_divide { typedef see below type; };The nested typedef
type
shall be a synonym forratio<T1, T2>::type
whereT1
has the valueR1::num * R2::den
andT2
has the valueR1::den * R2::num
.
[ 2010-03-27 Howard adds: ]
Daniel brought to my attention the recent addition of the typedef
type
to the FCD N3092:typedef ratio type;This issue was discussed in Pittsburgh, and the decision there was to accept the typedef as proposed and move to Review. Unfortunately the issue was accidently applied to the FCD, and incorrectly. The FCD version of the typedef refers to
ratio<N, D>
, but the typedef is intended to refer toratio<num, den>
which in general is not the same type.I've updated the wording to diff against N3092.
[Batavia: NAD EditorialResolved - see rationale below]
Rationale:
Already fixed in working draft
Proposed resolution:
Add to 21.4.3 [ratio.ratio] synopsis
template <intmax_t N, intmax_t D = 1> class ratio { public: static constexpr intmax_t num; static constexpr intmax_t den; typedef ratio<num, den> type; };
MoveConstructible
and MoveAssignable
need clarification
of moved-from stateSection: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2009-12-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
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Discussion:
Addresses UK 150
There is on going confusion over what one can and can not do with a moved-from object (e.g. UK 150, 910(i)). This issue attempts to clarify that moved-from objects are valid objects with an unknown state.
[ 2010-01-22 Wording tweaked by Beman. ]
[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-01-23 Alisdair opens: ]
I'm afraid I must register an objection.
My primary objection is that I have always been opposed to this kind of a resolution as over-constraining. My preferred example is a call implementing the pImpl idiom via
unique_ptr
. Once the pImpl has been moved from, it is no longer safe to call the vast majority of the object's methods, yet I see no reason to make such a type unusable in the standard library. I would prefer a resolution along the lines suggested in the UK comment, which only requires that the object can be safely destroyed, and serve as the target of an assignment operator (if supported.)However, I will not hold the issue up if I am a lone dissenting voice on this (yes, that is a call to hear more support, or I will drop that objection in Pittsburgh)
With the proposed wording, I'm not clear what the term 'valid object' means. In my example above, is a pImpl holding a null pointer 'valid'? What about a float holding a signalling NaN? What determines if an object is valid? Without a definition of a valid/invalid object, I don't think this wording adds anything, and this is an objection that I do want resolved.
[ 2010-01-24 Alisdair removes his objection. ]
[ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Reopened. The wording here has been merged into 1309(i). ]
[
2010-02-10 Moved to Tentatively NAD EditorialResolved after 5 postive votes on
c++std-lib. Rationale added below.
]
Rationale:
This issue is now addressed by 1309(i).
Proposed resolution:
Change the follwing tables in 16.4.4.2 [utility.arg.requirements] as shown:
Table 33 — MoveConstructible
requirements [moveconstructible]Expression Post-condition T t(rv)
t
is equivalent to the value ofrv
before the construction.[Note: There is no requirement on the value ofrv
after the construction.rv
remains a valid object. Its state is unspecified. — end note]
Table 35 — MoveAssignable
requirements [moveassignable]Expression Return type Return value Post-condition t = rv
T&
t
t
is equivalent to the value ofrv
before the assigment.[Note: There is no requirement on the value ofrv
after the assignment.rv
remains a valid object. Its state is unspecified. — end note]
vector<bool> initializer_list
constructor missing an allocator argumentSection: 23.3.14 [vector.bool] Status: C++11 Submitter: Bo Persson Opened: 2009-12-09 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [vector.bool].
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Discussion:
The specialization for vector<bool>
(23.3.14 [vector.bool])
has a constructor
vector(initializer_list<bool>);
which differs from the base template's constructor (and other containers) in
that it has no allocator
parameter.
[ 2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change the signature in the synopsis of 23.3.14 [vector.bool] to
vector(initializer_list<bool>, const Allocator& = Allocator());
allocator_traits
call to new
Section: 20.2.9.3 [allocator.traits.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-12-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.traits.members].
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Discussion:
LWG issue 402(i) added "::
" to the call to new
within allocator::construct
. I suspect we want to retain that fix.
[ 2009-12-13 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]
Proposed resolution:
Change 16.4.4.6 [allocator.requirements], table 40 "Allocator requirements":
Table 40 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault a.construct(c,args)
(not used) Effect: Constructs an object of type C
atc
::new ((void*)c) C(forward<Args>(args)...)
Change 20.2.9.3 [allocator.traits.members], p4:
template <class T, class... Args> static void construct(Alloc& a, T* p, Args&&... args);4 Effects: calls
a.construct(p, std::forward<Args>(args)...)
if that call is well-formed; otherwise, invokes::new (static_cast<void*>(p)) T(std::forward<Args>(args)...)
.
allocator_traits::select_on_container_copy_construction
type-oSection: 20.2.9.3 [allocator.traits.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-12-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.traits.members].
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Discussion:
allocator_traits::select_on_container_copy_construction
refers to an
unknown "a
":
static Alloc select_on_container_copy_construction(const Alloc& rhs);7 Returns:
rhs.select_on_container_copy_construction(a)
if that expression is well-formed; otherwise,rhs
.
[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 20.2.9.3 [allocator.traits.members], p7:
static Alloc select_on_container_copy_construction(const Alloc& rhs);7 Returns:
rhs.select_on_container_copy_construction(
if that expression is well-formed; otherwise,a)rhs
.
std::function
requires CopyConstructible
target objectSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.con].
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Discussion:
I think std::function
should require CopyConstructible
for the
target object.
I initially thought that MoveConstructible
was enough, but it's not. If
F
is move-only then function's copy constructor cannot be called, but
because function uses type erasure, F
is not known and so the copy
constructor cannot be disabled via enable_if
. One option would be to
throw an exception if you try to copy a function with a non-copyable target
type, but I think that would be a terrible idea.
So although the constructors require that the target be initialised by
std::move(f)
, that's only an optimisation, and a copy constructor is
required.
[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add to 22.10.17.3.2 [func.wrap.func.con] paragraph 9:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);9 Requires:
F
shall beCopyConstructible
.f
shall be callable for argument typesArgTypes
and return typeR
. The copy constructor and destructor ofA
shall not throw exceptions.
std::function
assignment from rvaluesSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.con].
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Discussion:
In 22.10.17.3.2 [func.wrap.func.con]
template<class F> function& operator=(F f);20 Effects:
function(f).swap(*this);
21 Returns:
*this
This assignment operator can be called such that F
is an rvalue-reference e.g.
func.operator=<F&&>(f);
There are two issues with this.
f
is passed as an lvalue and so there will be an
unnecessary copy. The argument should be forwarded, so that the copy can be
avoided.
F
is a deduced context it can be made to work with either lvalues or rvalues.
The same issues apply to function::assign
.
N.B. this issue is not related to 1287(i) and applies whether that issue is resolved or not. The wording below assumes the resolution of LWG 1258(i) has been applied.
[ 2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 201002-11 Opened by Alisdair for the purpose of merging 1258(i) into this issue as there is a minor conflict. ]
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In 22.10.17.3.2 [func.wrap.func.con]
template<class F> function& operator=(F&& f);20 Effects:
function(std::forward<F>(f)).swap(*this);
21 Returns:
*this
In 22.10.17.3.3 [func.wrap.func.mod]
template<class F,Allocator Allocclass A> void assign(F&& f, const Alloc& a);3 Effects:
function(
f, aallocator_arg, a, std::forward<F>(f)).swap(*this);
Update member function signature for class template in 22.10.17.3 [func.wrap.func]
template<class F> function& operator=(F&&); template<class F, class A> void assign(F&&, const A&);
[u|bi]nary_function
inheritanceSection: 22.10 [function.objects] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-14 Last modified: 2025-03-13
Priority: Not Prioritized
View all other issues in [function.objects].
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Discussion:
This issue is a follow-up of the discussion on issue 870(i) during the 2009 Santa Cruz meeting.
The class templates unary_function
and binary_function
are
actually very simple typedef providers,
namespace std { template <class Arg, class Result> struct unary_function { typedef Arg argument_type; typedef Result result_type; }; template <class Arg1, class Arg2, class Result> struct binary_function { typedef Arg1 first_argument_type; typedef Arg2 second_argument_type; typedef Result result_type; }; }
which may be used as base classes (similarly to the iterator template),
but were originally not intended as a customization point. The SGI
documentation introduced the concept
Adaptable Unary
Function as function objects "with nested typedefs that define its argument
type and result type" and a similar definition for
Adaptable Binary
Function related to binary_function
. But as of TR1 a protocol was
introduced that relies on inheritance relations based on these types. 22.10.6 [refwrap]/3 b. 3 requires that a specialization of
reference_wrapper<T>
shall derive from unary_function
,
if type T
is "a class type that is derived from
std::unary_function<T1, R>
" and a similar inheritance-based rule
for binary_function
exists as well.
As another disadvantage it has been pointed out in the TR1 issue list, N1837
(see section 10.39), that the requirements of mem_fn
22.10.16 [func.memfn]/2+3 to derive from
std::unary_function/std::binary_function
under circumstances, where the
provision of corresponding typedefs would be sufficient, unnecessarily prevent
implementations that take advantage of empty-base-class optimizations.
Both requirements should be relaxed in the sense that the
reference_wrapper
should provide typedef's argument_type
,
first_argument_type
, and second_argument_type
based on similar
rules as the weak result type rule (22.10.4 [func.require]/3) does
specify the presence of result_type
member types.
For a related issue see also 1279(i).
[ 2010-10-24 Daniel adds: ]
Accepting n3145 would resolve this issue as NAD editorial.
[ 2010-11 Batavia: Solved by N3198 ]
Resolved by adopting n3198.
Previous proposed resolution:
[ The here proposed resolution is an attempt to realize the common denominator of the reflector threads c++std-lib-26011, c++std-lib-26095, and c++std-lib-26124. ]
Change [base]/1 as indicated: [The intend is to provide an alternative fix for issue 1279(i) and some editorial harmonization with existing wording in the library, like 99 [iterator.basic]/1]
1 The following class templates are provided to simplify the definition of typedefs of the argument and result types for function objects. The behavior of a program that adds specializations for any of these templates is undefined.
:namespace std { template <class Arg, class Result> struct unary_function { typedef Arg argument_type; typedef Result result_type; }; } namespace std { template <class Arg1, class Arg2, class Result> struct binary_function { typedef Arg1 first_argument_type; typedef Arg2 second_argument_type; typedef Result result_type; }; }Change 22.10.6 [refwrap], class template
reference_wrapper
synopsis as indicated: [The intent is to remove the requirement thatreference_wrapper
derives fromunary_function
orbinary_function
if the situation requires the definition of the typedefsargument_type
,first_argument_type
, orsecond_argument_type
. This change is suggested, because the new way of definition uses the same strategy as the weak result type specification applied to argument types, which provides the following advantages: It creates less potential conflicts between[u|bi]nary_function
bases and typedefs in a function object and it ensures that user-defined function objects which provide typedefs but no such bases are handled as first class citizens.]namespace std { template <class T> class reference_wrapper: public unary_function<T1, R> // see below: public binary_function<T1, T2, R> // see below{ public : // types typedef T type; typedef see below result_type; // not always defined typedef see below argument_type; // not always defined typedef see below first_argument_type; // not always defined typedef see below second_argument_type; // not always defined // construct/copy/destroy ... };Change 22.10.6 [refwrap]/3 as indicated: [The intent is to remove the requirement that
reference_wrapper
derives fromunary_function
if the situation requires the definition of the typedefargument_type
andresult_type
. Note that this clause does concentrate onargument_type
alone, because theresult_type
is already ruled by p. 2 via the weak result type specification. The new way of specifyingargument_type
is equivalent to the weak result type specification]3 The template instantiation
reference_wrapper<T>
shallbe derived fromdefine a nested type namedstd::unary_function<T1, R>
argument_type
as a synonym forT1
only if the typeT
is any of the following:
- a function type or a pointer to function type taking one argument of type
T1
and returningR
- a pointer to member function
R T0::f
cv (where cv represents the member function's cv-qualifiers); the typeT1
is cvT0*
- a class type
that is derived fromwith a member typestd::unary_function<T1, R>
argument_type
; the typeT1
isT::argument_type
Change 22.10.6 [refwrap]/4 as indicated: [The intent is to remove the requirement that
reference_wrapper
derives frombinary_function
if the situation requires the definition of the typedeffirst_argument_type
,second_argument_type
, andresult_type
. Note that this clause does concentrate onfirst_argument_type
andsecond_argument_type
alone, because theresult_type
is already ruled by p. 2 via the weak result type specification. The new way of specifyingfirst_argument_type
andsecond_argument_type
is equivalent to the weak result type specification]The template instantiation
reference_wrapper<T>
shallbe derived fromdefine two nested types namedstd::binary_function<T1, T2, R>
first_argument_type
andsecond_argument_type
as a synonym forT1
andT2
, respectively, only if the typeT
is any of the following:
- a function type or a pointer to function type taking two arguments of types
T1
andT2
and returningR
- a pointer to member function
R T0::f(T2)
cv (where cv represents the member function's cv-qualifiers); the typeT1
is cvT0*
- a class type
that is derived fromwith member typesstd::binary_function<T1, T2, R>
first_argument_type
andsecond_argument_type
; the typeT1
isT::first_argument_type
and the typeT2
isT::second_argument_type
Change 22.10.16 [func.memfn]/2+3 as indicated: [The intent is to remove the requirement that mem_fn's return type has to derive from
[u|bi]nary_function
. The reason for suggesting the change here is to better support empty-base-class optimization choices as has been pointed out in N1837]2 The simple call wrapper shall
be derived fromdefine two nested types namedstd::unary_function<cv T*, Ret>
argument_type
andresult_type
as a synonym forcv T*
andRet
, respectively, whenpm
is a pointer to member function with cv-qualifier cv and taking no arguments, whereRet
ispm
's return type.3 The simple call wrapper shall
be derived fromdefine three nested types namedstd::binary_function<cv T*, T1, Ret>
first_argument_type
,second_argument_type
, andresult_type
as a synonym forcv T*
,T1
, andRet
, respectively, whenpm
is a pointer to member function with cv-qualifier cv and taking one argument of typeT1
, whereRet
ispm
's return type.
Proposed resolution:
Addressed by paper n3198.
promise::set_value
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-12-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 32.10.6 [futures.promise]
Does promise<R>::set_value
return normally if the copy/move
constructor of R
throws?
The exception could be caught and set using
promise<R>::set_exception
, or it could be allowed to leave the
set_value
call, but it's not clear which is intended. I suggest the
exception should not be caught.
N.B. This doesn't apply to promise<R&>::set_value
or
promise<void>::set_value
because they don't construct a new
object.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Change 32.10.6 [futures.promise]/18:
18 Throws:
future_error
if its associated state is already ready or, for the first version an exception thrown by the copy constructor ofR
, or for the second version an exception thrown by the move constructor ofR
.
std::function
should support all callable typesSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.con].
View all issues with C++11 status.
Discussion:
Some parts of the specification of std::function
is unnecessarily
restricted to a subset of all callable types (as defined in 22.10.3 [func.def]/3), even though the intent clearly is to be usable for
all of them as described in 22.10.17.3 [func.wrap.func]/1. This
argument becomes strengthened by the fact that current C++0x-compatible
compilers work fine with them:
#include <functional> #include <iostream> struct A { int foo(int i) const {return i+1;} }; struct B { int mem; }; int main() { std::function<int(const A&, int)> f(&A::foo); A a; std::cout << f(a, 1) << '\n'; std::cout << f.target_type().name() << '\n'; typedef int (A::* target_t)(int) const; target_t* p = f.target<target_t>(); std::cout << (p != 0) << '\n'; std::function<int(B&)> f2(&B::mem); B b = { 42 }; std::cout << f2(b) << '\n'; std::cout << f2.target_type().name() << '\n'; typedef int (B::* target2_t); target2_t* p2 = f2.target<target2_t>(); std::cout << (p2 != 0) << '\n'; }
The problematic passages are 22.10.17.3.2 [func.wrap.func.con]/10:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);...
10 Postconditions:
!*this
if any of the following hold:
f
is aNULL
function pointer.f
is aNULL
member function pointer.F
is an instance of the function class template, and!f
because it does not consider pointer to data member and all constraints based on function objects which like 22.10.17.3 [func.wrap.func]/2 or 22.10.17.3.6 [func.wrap.func.targ]/3. The latter two will be resolved by the proposed resolution of 870(i) and are therefore not handled here.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 22.10.17.3.2 [func.wrap.func.con]/10+11 as indicated:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);...
10 Postconditions:
!*this
if any of the following hold:
f
is aNULL
function pointer.f
is aNULL
pointer to memberfunction pointer.F
is an instance of the function class template, and!f
11 Otherwise,
*this
targets a copy off
or, initialized withstd::move(f)
if. [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small function objects, for example, wheref
is not a pointer to member function, and targets a copy ofmem_fn(f)
iff
is a pointer to member functionf
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]
unique_ptr<T[], D>
needs to get rid of unspecified-pointer-typeSection: 20.3.1.4 [unique.ptr.runtime] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.runtime].
View all issues with Resolved status.
Discussion:
Addresses UK 211
As a response to UK 211 LWG issue 1021(i) has replaced the
unspecified-pointer-type by nullptr_t
to allow assignment of
type-safe null-pointer literals in the non-array form of
unique_ptr::operator=
, but did not the same for the specialization for
arrays of runtime length. But without this parallel change of the signature we
have a status quo, where unique_ptr<T[], D>
declares a member
function which is completely unspecified.
[ 2009-12-21 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
In 20.3.1.4 [unique.ptr.runtime], class template unique_ptr<T[],
D>
synopsis, change as indicated:
// assignment unique_ptr& operator=(unique_ptr&& u); unique_ptr& operator=(unspecified-pointer-typenullptr_t);
Section: 22.10.4 [func.require] Status: C++11 Submitter: Jens Maurer Opened: 2009-12-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.require].
View all issues with C++11 status.
Discussion:
The current wording in the standard makes it hard to discriminate the difference between a "call wrapper" as defined in 22.10.3 [func.def]/5+6:
5 A call wrapper type is a type that holds a callable object and supports a call operation that forwards to that object.
6 A call wrapper is an object of a call wrapper type.
and a "forwarding call wrapper" as defined in 22.10.4 [func.require]/4:
4 [..] A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form
template<class... ArgTypes> R operator()(ArgTypes&&... args) cv-qual;— end note]
Reason for this lack of clear difference seems to be that the wording adaption to variadics and rvalues that were applied after it's original proposal in N1673:
[..] A forwarding call wrapper is a call wrapper that can be called with an argument list
t1, t2, ..., tN
where eachti
is an lvalue. The effect of calling a forwarding call wrapper with one or more arguments that are rvalues is implementation defined. [Note: in a typical implementation forwarding call wrappers have overloaded function call operators of the formtemplate<class T1, class T2, ..., class TN> R operator()(T1& t1, T2& t2, ..., TN& tN) cv-qual;— end note]
combined with the fact that the word "forward" has two different meanings in this context. This issue attempts to clarify the difference better.
[ 2010-09-14 Daniel provides improved wording and verified that it is correct against N3126. Previous resolution is shown here: ]
4 [..] A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and uses perfect forwarding to deliver the arguments to the wrapped callable object. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form
template<class... ArgTypes> R operator()(ArgTypes&&... args) cv-qual;— end note]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 22.10.4 [func.require]/4 as indicated:
[..] A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments as references to the wrapped callable object. This forwarding step shall ensure that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form
template<class... UnBoundArgs> R operator()(UnBoundArgs&&... unbound_args) cv-qual;— end note ]
Section: 22.10.4 [func.require] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.require].
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Discussion:
22.10.4 [func.require]/3 b 1 says
3 If a call wrapper (22.10.3 [func.def]) has a weak result type the type of its member type
result_type
is based on the typeT
of the wrapper's target object (22.10.3 [func.def]):
- if
T
is a function, reference to function, or pointer to function type,result_type
shall be a synonym for the return type ofT
;- [..]
The first two enumerated types (function and reference to function)
can never be valid types for T
, because
22.10.3 [func.def]/7
7 A target object is the callable object held by a call wrapper.
and 22.10.3 [func.def]/3
3 A callable type is a pointer to function, a pointer to member function, a pointer to member data, or a class type whose objects can appear immediately to the left of a function call operator.
exclude functions and references to function as "target objects".
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 22.10.4 [func.require]/3 b 1 as indicated:
3 If a call wrapper (22.10.3 [func.def]) has a weak result type the type of its member type
result_type
is based on the typeT
of the wrapper's target object (22.10.3 [func.def]):
- if
T
is afunction, reference to function, orpointer to function type,result_type
shall be a synonym for the return type ofT
;- [..]
unique_ptr
's relational operator functions should induce a total orderSection: 20.3.1.6 [unique.ptr.special] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.special].
View all issues with Resolved status.
Discussion:
The comparison functions of unique_ptr
currently directly delegate to
the underlying comparison functions of unique_ptr<T, D>::pointer
.
This is disadvantageous, because this would not guarantee to induce a total
ordering for native pointers and it is hard to define a total order for mixed
types anyway.
The currently suggested resolution for shared_ptr
comparison as of
1262(i) uses a normalization strategy: They perform the comparison on
the composite pointer type (7.6.9 [expr.rel]). This is not
exactly possible for unique_ptr
in the presence of user-defined
pointer-like types but the existing definition of std::duration
comparison as of 30.5.7 [time.duration.comparisons] via
common_type
of both argument types demonstrates a solution of this
problem. The approach can be seen as the general way to define a composite
pointer type and this is the approach which is used for here suggested
wording change.
For consistency reasons I would have preferred the same normalization strategy
for ==
and !=
, but Howard convinced me not to do so (now).
[ 2010-11-03 Daniel comments and adjustes the currently proposed wording changes: ]
Issue 1401(i) is remotely related. Bullet A of its proposed resolution
provides an alternative solution for issue discussed here and addresses NB comment GB-99.
Additionally I updated the below suggested wording in regard to the following:
It is an unncessary requirement that the below defined effective composite pointer-like
type CT
satisfies the LessThanComparable
requirements. All what is
needed is, that the function object type less<CT>
induces a strict
weak ordering on the pointer values.
[2011-03-24 Madrid meeting]
Proposed resolution:
Change 20.3.1.6 [unique.ptr.special]/4-7 as indicated: [The implicit
requirements and remarks imposed on the last three operators are the same as for
the first one due to the normative "equivalent to" usage within a Requires
element, see 16.3.2.4 [structure.specifications]/4. The effects of this
change are that all real pointers wrapped in a unique_ptr
will order
like shared_ptr
does.]
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);? Requires: Let
CT
becommon_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type
. Then the specializationless<CT>
shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.4 Returns:
less<CT>()(x.get(), y.get())
.x.get() < y.get()? Remarks: If
unique_ptr<T1, D1>::pointer
is not implicitly convertible toCT
orunique_ptr<T2, D2>::pointer
is not implicitly convertible toCT
, the program is ill-formed.template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);5 Effects: Equivalent to
return !(y < x)
Returns:.x.get() <= y.get()
template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);6 Effects: Equivalent to
return (y < x)
Returns:.x.get() > y.get()
template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);7 Effects: Equivalent to
return !(x < y)
Returns:.x.get() >= y.get()
ctype_byname<char>
Section: 28.3.2 [locale.syn] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-12-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
The <locale>
synopsis in 28.3.2 [locale.syn] calls out an
explicit specialization for ctype_byname<char>
, however no such
specialization is defined in the standard. The only reference I can find to
ctype_byname<char>
is 28.3.3.1.2.2 [locale.facet]:Table 77
— Required specializations (for facets) which also refers to
ctype_byname<wchar_t>
which has no special consideration.
Is the intent an explicit instantiation which would use a slightly different syntax? Should the explicit specialization simply be struck?
[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
28.3.2 [locale.syn]
Strike the explicit specialization for
ctype_byname<char>
from the<locale>
synopsis... template <class charT> class ctype_byname;template <> class ctype_byname<char>; // specialization...
get_time
Section: 31.7.8 [ext.manip] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-12-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ext.manip].
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Discussion:
Extended Manipulators 31.7.8 [ext.manip] p8 defines the semantics of
get_time
in terms of a function f
.
template <class charT, class traits> void f(basic_ios<charT, traits>& str, struct tm* tmb, const charT* fmt) { typedef istreambuf_iterator<charT, traits> Iter; typedef time_get<charT, Iter> TimeGet; ios_base::iostate err = ios_base::goodbit; const TimeGet& tg = use_facet<TimeGet>(str.getloc()); tm.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt)); if (err != ios_base::goodbit) str.setstate(err): }
Note the call to tm.get
. This is clearly an error, as tm
is a
type and not an object. I believe this should be tg.get
, rather than
tm
, but this is not my area of expertise.
[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 31.7.8 [ext.manip] p8:
template <class charT, class traits> void f(basic_ios<charT, traits>& str, struct tm* tmb, const charT* fmt) { typedef istreambuf_iterator<charT, traits> Iter; typedef time_get<charT, Iter> TimeGet; ios_base::iostate err = ios_base::goodbit; const TimeGet& tg = use_facet<TimeGet>(str.getloc()); tgm.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt)); if (err != ios_base::goodbit) str.setstate(err): }
promise::swap
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-12-26 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
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Discussion:
32.10.6 [futures.promise]/12 defines the effects of
promise::swap(promise&)
as
void swap(promise& other);12 Effects:
swap(*this, other)
and 32.10.6 [futures.promise]/25 defines swap(promise<R&>,
promise<R>&)
as
template <class R> void swap(promise<R>& x, promise<R>& y);25 Effects:
x.swap(y)
.
[ 2010-01-13 Daniel added "Throws: Nothing." ]
[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change 32.10.6 [futures.promise] paragraph 12
void swap(promise& other);12 Effects:
Exchanges the associated states ofswap(*this, other)
*this
andother
.13 ...
Throws: Nothing.
shared_ptr
, unique_ptr
, and rvalue references v2Section: 20.3.1.3 [unique.ptr.single], 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2010-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [unique.ptr.single].
View all other issues in [unique.ptr.single].
View all issues with C++11 status.
Discussion:
N3000 20.3.2.2 [util.smartptr.shared]/1 still says:
template <class Y, class D> explicit shared_ptr(const unique_ptr<Y, D>& r) = delete; template <class Y, class D> shared_ptr& operator=(const unique_ptr<Y, D>& r) = delete;
I believe that this is unnecessary now that "rvalue references v2" prevents rvalue references from binding to lvalues, and I didn't see a Library Issue tracking this.
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Strike from 20.3.1.3 [unique.ptr.single]:
template <class T, class D = default_delete<T>> class unique_ptr { ... unique_ptr(const unique_ptr&) = delete;template <class U, class E> unique_ptr(const unique_ptr<U, E>&) = delete;unique_ptr& operator=(const unique_ptr&) = delete;template <class U, class E> unique_ptr& operator=(const unique_ptr<U, E>&) = delete;};
Strike from 20.3.2.2 [util.smartptr.shared]:
template<class T> class shared_ptr { ...template <class Y, class D> explicit shared_ptr(const unique_ptr<Y, D>& r) = delete;...template <class Y, class D> shared_ptr& operator=(const unique_ptr<Y, D>& r) = delete;... };
shared_future
Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.shared.future].
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Discussion:
The revised futures package in the current working paper simplified the
is_ready/has_exception/has_value
set of APIs, replacing them with a
single 'valid' method. This method is used in many places to signal pre- and
post- conditions, but that edit is not complete. Each method on a
shared_future
that requires an associated state should have a
pre-condition that valid() == true
.
[ 2010-01-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Insert the following extra paragraphs:
In [futures.shared_future]
shared_future();4 Effects: constructs ...
Postcondition:
valid() == false
.Throws: nothing.
void wait() const;Requires:
valid() == true
.22 Effects: if the associated ...
template <class Rep, class Period> bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;Requires:
valid() == true
.23 Effects: if the associated ...
template <class Clock, class Duration> bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;Requires:
valid() == true
.25 Effects: blocks until ...
atomic_future
Section: 99 [futures.atomic_future] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.atomic_future].
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Discussion:
The revised futures package in the current working paper simplified the
is_ready/has_exception/has_value
set of APIs, replacing them with a
single 'valid' method. This method is used in many places to signal pre- and
post- conditions, but that edit is not complete.
Atomic future retains the extended earlier API, and provides defined,
synchronized behaviour for all calls. However, some preconditions and throws
clauses are missing, which can easily be built around the new valid()
api. Note that for consistency, I suggest is_ready/has_exception/has_value
throw
an exception if valid()
is not true
, rather than
return false
. I think this is implied by the existing pre-condition on
is_ready
.
[ 2010-01-23 See discussion starting with Message c++std-lib-26666. ]
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Insert the following extra paragraphs:
In 99 [futures.atomic_future]
bool is_ready() const;17
PreconditionRequires:valid() == true
.18 Returns:
true
only if the associated state is ready.Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
bool has_exception() const;Requires:
valid() == true
.19 Returns:
true
only if the associated state is ready and contains an exception.Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
bool has_value() const;Requires:
valid() == true
.20 Returns:
true
only if the associated state is ready and contains a value.Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
void wait() const;Requires:
valid() == true
.22 Effects: blocks until ...
Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
template <class Rep, class Period> bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;Requires:
valid() == true
.23 Effects: blocks until ...
24 Returns:
true
only if ...Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
template <class Clock, class Duration> bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;Requires:
valid() == true
.25 Effects: blocks until ...
26 Returns:
true
only if ...Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
pointer
and const_pointer
for <array>
Section: 23.3.3 [array] Status: C++11 Submitter: Nicolai Josuttis Opened: 2010-01-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [array].
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Discussion:
Class <array>
is the only sequence container class that has no
types pointer
and const_pointer
defined. You might argue that
this makes no sense because there is no allocator support, but on the other
hand, types reference
and const_reference
are defined for
array
.
[ 2010-02-11 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
Add to Class template array 23.3.3 [array]:
namespace std { template <class T, size_t N > struct array { ... typedef T value_type; typedef T * pointer; typedef const T * const_pointer; ... }; }
exception_ptr
and allocator
pointers don't understand !=Section: 17.9.7 [propagation] Status: Resolved Submitter: Daniel Krügler Opened: 2010-01-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
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Discussion:
The current requirements for a conforming implementation of
std::exception_ptr
(17.9.7 [propagation]/1-6) does not clarify
whether the expression
e1 != e2 e1 != nullptr
with e1
and e2
being two values of type
std::exception_ptr
are supported or not. Reason for this oddity is that
the concept EqualityComparable
does not provide operator !=
.
For the same reason programmers working against the types X::pointer
,
X::const_pointer
, X::void_pointer
, and
X::const_void_pointer
of any allocator concept X
(16.4.4.6 [allocator.requirements]/4 + Table 40) in a generic context can not rely
on the availability of the != operation, which is rather unnatural and
error-prone.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3073.
Proposed resolution:
Move/CopyConstructible
Section: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2010-02-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
View all issues with C++11 status.
Discussion:
Table 33 — MoveConstructible requirements [moveconstructible] and Table 34 — CopyConstructible requirements [copyconstructible] support solely the following expression:
T t(rv)
where rv
is defined to be as "non-const rvalue of type T
" and
t
as a "modifiable lvalue of type T
" in 16.4.4.2 [utility.arg.requirements]/1.
This causes two different defects:
We cannot move/copy-initialize a const lvalue of type T
as in:
int get_i(); const int i1(get_i());
both in Table 33 and in Table 34.
The single support for
T t(rv)
in case of CopyConstructible
means that we cannot provide an
lvalue as a source of a copy as in
const int& get_lri(); int i2(get_lri());
I believe this second defect is due to the fact that this single expression supported both initialization situations according to the old (implicit) lvalue reference -> rvalue reference conversion rules.
Finally [copyconstructible] refers to some name u
which is not part of
the expression, and both [copyconstructible] and [moveconstructible] should
support construction expressions from temporaries - this would be a stylistic
consequence in the light of the new DefaultConstructible
requirements
and compared with existing requirements (see e.g. Container requirements or the
output/forward iterator requirements)..
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Reopened. The proposed wording of 1283(i) has been merged here. ]
[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 16.4.4.2 [utility.arg.requirements]/1 as indicated: [This change
suggestion is motivated to make type descriptions clearer: First, a
,
b
, and c
may also be non-const T
. Second, u
is described in a manner consistent with the container requirements tables.]
1 The template definitions in the C++ standard library refer to various named requirements whose details are set out in tables 31-38. In these tables,
T
is an object or reference type to be supplied by a C++ program instantiating a template;a
,b
, andc
are values of type (possiblyconst) T
;s
andt
are modifiable lvalues of typeT
;u
denotes an identifier;is a value of type (possiblyconst
)T
; andrv
is annon-constrvalue of typeT
; andv
is an lvalue of type (possiblyconst
)T
or an rvalue of typeconst T
.
In 16.4.4.2 [utility.arg.requirements] Table 33 ([moveconstructible])
change as indicated [Note: The symbol u
is defined to be either a
const or a non-const value and is the right one we need here]:
Table 33 — MoveConstructible
requirements [moveconstructible]Expression Post-condition T
tu(rv);is equivalent to the value of
turv
before the constructionT(rv)
T(rv)
is equivalent to the value ofrv
before the construction[Note: There is no requirement on the value ofrv
after the construction.rv
remains a valid object. Its state is unspecified. — end note]
In 16.4.4.2 [utility.arg.requirements] Table 34 ([copyconstructible])
change as indicated [Note: The symbol u
is defined to be either a
const or a non-const value and is the right one we need here. The expressions
using a
are recommended to ensure that lvalues are supported as sources
of the copy expression]:
Table 34 — CopyConstructible
requirements [copyconstructible]
(in addition toMoveConstructible
)Expression Post-condition T
tu(rv);the value of is unchanged and is equivalent to
uv
tuT(v)
the value of v
is unchanged and is equivalent toT(v)
[Note: A type that satisfies theCopyConstructible
requirements also satisfies theMoveConstructible
requirements. — end note]
In Table 35 — MoveAssignable requirements [moveassignable] change as indicated:
Table 35 — MoveAssignable
requirements [moveassignable]Expression Return type Return value Post-condition t = rv
T&
t
t
is equivalent to the value ofrv
before the assigment.[Note: There is no requirement on the value ofrv
after the assignment.rv
remains a valid object. Its state is unspecified. — end note]
In 16.4.4.2 [utility.arg.requirements] change Table 36 as indicated:
Table 36 — CopyAssignable
requirements [copyassignable]
(in addition toMoveAssignable
)Expression Return type Return value Post-condition t =
uvT&
t
t
is equivalent to, the value of
uvis unchanged
uv[Note: A type that satisfies theCopyAssignable
requirements also satisfies theMoveAssignable
requirements. — end note]
forward_list splice_after
from lvaluesSection: 23.3.7.6 [forward.list.ops] Status: C++11 Submitter: Howard Hinnant Opened: 2010-02-05 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
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Discussion:
We've moved 1133(i) to Tentatively Ready and I'm fine with that.
1133(i) adds lvalue-references to the splice
signatures for list
. So now
list
can splice
from lvalue and rvalue lists (which was the intent of the
original move papers btw). During the discussion of this issue it was mentioned
that if we want to give the same treatment to forward_list
, that should be a
separate issue.
This is that separate issue.
Consider the following case where you want to splice elements from one place in
a forward_list
to another. Currently this must be coded like so:
fl.splice_after(to_here, std::move(fl), from1, from2);
This looks pretty shocking to me. I would expect to be able to code instead:
fl.splice_after(to_here, fl, from1, from2);
but we currently don't allow it.
When I say move(fl)
, I consider that as saying that I don't care about
the value of fl
any more (until I assign it a new value). But in the
above example, this simply isn't true. I do care about the value of fl
after the move, and I'm not assigning it a new value. I'm merely permuting its
current value.
I propose adding forward_list&
overloads to the 3
splice_after
members. For consistency's sake (principal of least
surprise) I'm also proposing to overload merge
this way as well.
Proposed resolution:
Add to the synopsis of [forwardlist.overview]:
template <class T, class Allocator = allocator<T> > class forward_list { public: ... // [forwardlist.ops], forward_list operations: void splice_after(const_iterator p, forward_list& x); void splice_after(const_iterator p, forward_list&& x); void splice_after(const_iterator p, forward_list& x, const_iterator i); void splice_after(const_iterator p, forward_list&& x, const_iterator i); void splice_after(const_iterator p, forward_list& x, const_iterator first, const_iterator last); void splice_after(const_iterator p, forward_list&& x, const_iterator first, const_iterator last); ... void merge(forward_list& x); void merge(forward_list&& x); template <class Compare> void merge(forward_list& x, Compare comp); template <class Compare> void merge(forward_list&& x, Compare comp); ... };
Add to the signatures of [forwardlist.ops]:
void splice_after(const_iterator p, forward_list& x); void splice_after(const_iterator p, forward_list&& x);1 ...
void splice_after(const_iterator p, forward_list& x, const_iterator i); void splice_after(const_iterator p, forward_list&& x, const_iterator i);4 ...
void splice_after(const_iterator p, forward_list& x, const_iterator first, const_iterator last); void splice_after(const_iterator p, forward_list&& x, const_iterator first, const_iterator last);7 ...
void merge(forward_list& x); void merge(forward_list&& x); template <class Compare> void merge(forward_list& x, Compare comp); template <class Compare> void merge(forward_list&& x, Compare comp);16 ...
Section: 24.3.5.5 [forward.iterators] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [forward.iterators].
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Discussion:
The following example demonstrates code that would meet the guarantees of a Forward Iterator, but only permits a single traversal of the underlying sequence:
template< typename ForwardIterator> struct bad_iterator { shared_ptr<ForwardIterator> impl; bad_iterator( ForwardIterator iter ) { : impl{new ForwardIterator{iter} } { } auto operator*() const -> decltype(*ForwardIterator{}) { return **impl; } auto operator->() const -> ForwardIterator { return *impl; } auto operator==(bad_iterator const & rhs) const -> bool { return impl == rhs.impl; } auto operator++() -> bad_iterator& { ++(*impl); return *this; } // other operations as necessary... };
Here, we use shared_ptr
to wrap a forward iterator, so all iterators
constructed from the same original iterator share the same 'value', and
incrementing any one copy increments all others.
There is a missing guarantee, expressed by the following code sequence
FwdIter x = seq.begin(); // obtain forward iterator from a sequence FwdIter y = x; // copy the iterator assert(x == y); // iterators must be the same ++x; // increment *just one* iterator assert(x != y); // iterators *must now be different* ++y; // increment the other iterator assert(x == y); // now the iterators must be the same again
That inequality in the middle is an essential guarantee. Note that this list is
simplified, as each assertion should also note that they refer to exactly the
same element (&*x == &*y)
but I am not complicating the issue
with tests to support proxy iterators, or value types overloading unary
operator+
.
I have not yet found a perverse example that can meet this additional constraint, and not meet the multi-pass expectations of a Forward Iterator without also violating other Forward Iterator requirements.
Note that I do not yet have standard-ready wording to resolve the problem, as saying this neatly and succinctly in 'standardese' is more difficult.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
vector::data
no longer returns a raw pointerSection: 23.3.13.4 [vector.data] Status: C++11 Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [vector.data].
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Discussion:
The original intent of vector::data
was to match array::data
in providing a simple API with direct access to the contiguous buffer of
elements that could be passed to a "classic" C API. At some point, the return
type became the 'pointer
' typedef, which is not derived from the
allocator
via allocator traits - it is no longer specified to precisely
T *
. The return type of this function should be corrected to no longer
use the typedef.
[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
23.3.13 [vector]
Update the class definition in p2:
// 23.3.6.3 data accesspointerT * data();const_pointerconst T * data() const;
23.3.13.4 [vector.data]
Adjust signatures:
pointerT * data();const_pointerconst T * data() const;
scoped_allocator_adaptor operator==
has no definitionSection: 20.6 [allocator.adaptor] Status: C++11 Submitter: Pablo Halpern Opened: 2009-02-11 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.adaptor].
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Discussion:
The WP (N3000) contains these declarations:
template <class OuterA1, class OuterA2, class... InnerAllocs> bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b); template <class OuterA1, class OuterA2, class... InnerAllocs> bool operator!=(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);
But does not define what the behavior of these operators are.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Add a new section after 20.6.4 [allocator.adaptor.members]:
Scoped allocator operators [scoped.adaptor.operators]
template <class OuterA1, class OuterA2, class... InnerAllocs> bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);Returns:
a.outer_allocator() == b.outer_allocator()
ifsizeof...(InnerAllocs)
is zero; otherwise,a.outer_allocator() == b.outer_allocator() && a.inner_allocator() == b.inner_allocator()
.template <class OuterA1, class OuterA2, class... InnerAllocs> bool operator!=(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);Returns:
!(a == b)
.
Section: 23.2.2 [container.requirements.general] Status: C++11 Submitter: Alisdair Meredith Opened: 2010-02-16 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
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Discussion:
The requirements on container iterators are spelled out in 23.2.2 [container.requirements.general], table 91.
Table 91 — Container requirements Expression Return type Operational semantics Assertion/note
pre-/post-conditionComplexity ...
X::iterator
iterator type whose value type is T
any iterator category except output iterator. Convertible to X::const_iterator
.compile time X::const_iterator
constant iterator type whose value type is T
any iterator category except output iterator compile time ...
As input iterators do not have the multi-pass guarantee, they are not suitable
for iterating over a container. For example, taking two calls to
begin()
, incrementing either iterator might invalidate the other.
While data structures might be imagined where this behaviour produces
interesting and useful results, it is very unlikely to meet the full set of
requirements for a standard container.
[ Post-Rapperswil: ]
Daniel notes: I changed the currently suggested P/R slightly, because it is not robust in regard to new fundamental iterator catagories. I recommend to say instead that each container::iterator shall satisfy (and thus may refine) the forward iterator requirements.
Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Table 93 — Container requirements Expression Return type Operational
semanticsAssertion/note
pre-/post-conditionComplexity ...
X::iterator
iterator type
whose value
type isT
any iterator category except output iterator
that meets the forward iterator requirements. convertible
toX::const_iterator
compile time X::const_iterator
constant iterator type
whose value
type isT
any iterator category except output iterator
that meets the forward iterator requirements.compile time ...
scoped_allocator_adaptor construct
and destroy
don't
use allocator_traits
Section: 20.6.4 [allocator.adaptor.members] Status: Resolved Submitter: Howard Hinnant Opened: 2010-02-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.adaptor.members].
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Discussion:
20.6.4 [allocator.adaptor.members] p8-9 says:
template <class T, class... Args> void construct(T* p, Args&&... args);8 Effects: let
OUTERMOST(x)
bex
ifx
does not have anouter_allocator()
function andOUTERMOST(x.outer_allocator())
otherwise.
- If
uses_allocator<T, inner_allocator_type>::value
isfalse
andis_constructible<T, Args...>::value
istrue
, callsOUTERMOST(*this).construct(p, std::forward<Args>(args)...)
.- Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value
istrue
, callsOUTERMOST(*this).construct(p, allocator_arg, inner_allocator(),std::forward<Args>(args)...)
.- Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, Args..., inner_allocator_type>::value
istrue
, callsOUTERMOST(*this).construct(p, std::forward<Args>(args)..., inner_allocator())
.- Otherwise, the program is ill-formed. [Note: an error will result if
uses_allocator
evaluates totrue
but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]template <class T> void destroy(T* p);9 Effects: calls
outer_allocator().destroy(p)
.
In all other calls where applicable scoped_allocator_adaptor
does not
call members of an allocator directly, but rather does so indirectly via
allocator_traits
. For example:
size_type max_size() const;7 Returns:
allocator_traits<OuterAlloc>::max_size(outer_allocator())
.
Indeed, without the indirection through allocator_traits
the
definitions for construct
and destroy
are likely to fail at
compile time since the outer_allocator()
may not have the members
construct
and destroy
.
[ The proposed wording is a product of Pablo, Daniel and Howard. ]
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3059.
Proposed resolution:
In 20.6.4 [allocator.adaptor.members] move and change p8 as indicated, and change p9 as indicated:
Let
OUTERMOST(x)
bex
ifx
does not have anouter_allocator()
member function andOUTERMOST(x.outer_allocator())
otherwise. LetOUTERMOST_ALLOC_TRAITS(x)
beallocator_traits<decltype(OUTERMOST(x))>
. [Note:OUTERMOST(x)
andOUTERMOST_ALLOC_TRAITS(x)
are recursive operations. It is incumbent upon the definition ofouter_allocator()
to ensure that the recursion terminates. It will terminate for all instantiations ofscoped_allocator_adaptor
. — end note]template <class T, class... Args> void construct(T* p, Args&&... args);8 Effects:
letOUTERMOST(x)
bex
ifx
does not have anouter_allocator()
function andOUTERMOST(x.outer_allocator())
otherwise.
- If
uses_allocator<T, inner_allocator_type>::value
isfalse
andis_constructible<T, Args...>::value
istrue
, calls.
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)... )- Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value
istrue
, calls.
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, allocator_arg, inner_allocator(), std::forward<Args>(args)... )- Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, Args..., inner_allocator_type>::value
istrue
, calls.
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)..., inner_allocator() )- Otherwise, the program is ill-formed. [Note: an error will result if
uses_allocator
evaluates totrue
but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]template <class T> void destroy(T* p);9 Effects: calls
.
outer_allocator().OUTERMOST_ALLOC_TRAITS(outer_allocator())::destroy( OUTERMOST(outer_allocator()), p)
CopyConstructible
requirements are insufficientSection: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Daniel Krügler Opened: 2010-02-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
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Discussion:
With the acceptance of library defect 822(i) only
direct-initialization is supported, and not copy-initialization in the
requirement sets MoveConstructible
and CopyConstructible
. This
is usually a good thing, if only the library implementation needs to obey these
restrictions, but the Empire strikes back quickly:
Affects user-code: std::exception_ptr
is defined purely via
requirements, among them CopyConstructible
. A strict reading of the
standard would make implementations conforming where std::exception_ptr
has an explicit copy-c'tor and user-code must code defensively. This is a very
unwanted effect for such an important component like
std::exception_ptr
.
Wrong re-use: Recently proposed requirement sets
(NullablePointer
as of
N3025,
Hash) or cleanup of existing requirement sets (e.g. iterator requirements as of
N3046)
tend to reuse existing requirement sets, so reusing CopyConstructible
is attempting, even in cases, where the intend is to support copy-initialization
as well.
Inconsistency: The current iterator requirements set Table 102 (output iterator requirements) and Table 103 (forward iterator requirements) demonstrate quite clearly a strong divergence of copy-semantics: The specified semantics of
X u(a); X u = a;
are underspecified compared to the most recent clarifications of the
CopyConstructible
requirements, c.f. issue 1309(i) which is
very unsatisfactory. This will become worse for each further issue that involves
the CopyConstructible
specification (for possible directions see 1173(i)).
The suggested resolution is to define two further requirements
implicit-MoveConstructible
and implicit-CopyConstructible
(or
any other reasonable name like MoveConvertible
and
CopyConvertible
) each with a very succinct but precise meaning solving
all three problems mentioned above.
[Batavia: Resolved by accepting n3215.]
Proposed resolution:
Add the following new table ?? after Table 34 — MoveConstructible
requirements [moveconstructible]:
Table ?? — Implicit MoveConstructible
requirements [implicit.moveconstructible] (in addition toMoveConstructible
)Expression Operational Semantics T u = rv;
Equivalent to: T u(rv);
Add the following new table ?? after Table 35 — CopyConstructible
requirements [copyconstructible]:
Table ?? — Implicit CopyConstructible
requirements [implicit.copyconstructible] (in addition toCopyConstructible
)Expression Operational Semantics T u = v;
Equivalent to: T u(v);
Change 16.4.4.4 [nullablepointer.requirements]/1 as follows:
A
NullablePointer
type is a pointer-like type that supports null values. A typeP
meets the requirements ofNullablePointer
if:
P
satisfies the requirements ofEqualityComparable
,DefaultConstructible
,implicit CopyConstructible
,CopyAssignable
, andDestructible
,- [..]
Change 16.4.4.5 [hash.requirements]/1 as indicated: [explicit copy-constructible functors could not be provided as arguments to any algorithm that takes these by value. Also a typo is fixed.]
1 A type
H
meets the Hash requirements if:
- it is a function object type (20.8),
- it satisfies
ifesthe requirements ofimplicit CopyConstructible
andDestructible
(20.2.1),- [..]
Change 21.3.2 [meta.rqmts]/1+2 as indicated:
1 A UnaryTypeTrait describes a property of a type. It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the property being described. It shall be
DefaultConstructible
,implicit CopyConstructible
, [..]2 A
BinaryTypeTrait
describes a relationship between two types. It shall be a class template that takes two template type arguments and, optionally, additional arguments that help define the relationship being described. It shall beDefaultConstructible
,implicit CopyConstructible
, and [..]
Change 22.10.4 [func.require]/4 as indicated: [explicit copy-constructible functors could not be provided as arguments to any algorithm that takes these by value]
4 Every call wrapper (20.8.1) shall be
implicit MoveConstructible
. A simple call wrapper is a call wrapper that isimplicit CopyConstructible
andCopyAssignable
and whose copy constructor, move constructor, and assignment operator do not throw exceptions. [..]
Change 22.10.6 [refwrap]/1 as indicated:
1
reference_wrapper<T>
is animplicit
CopyConstructible
andCopyAssignable
wrapper around a reference to an object or function of typeT
.
Change 22.10.15.4 [func.bind.bind]/5+9 as indicated:
5 Remarks: The return type shall satisfy the requirements of
implicit MoveConstructible
. If all ofFD
andTiD
satisfy the requirements ofCopyConstructible
, then the return type shall satisfy the requirements ofimplicit CopyConstructible
. [Note: this implies that all ofFD
andTiD
areMoveConstructible
. — end note][..]
9 Remarks: The return type shall satisfy the requirements of
implicit MoveConstructible
. If all ofFD
andTiD
satisfy the requirements ofCopyConstructible
, then the return type shall satisfy the requirements ofimplicit CopyConstructible
. [Note: this implies that all ofFD
andTiD
areMoveConstructible
. — end note]
Change 22.10.15.5 [func.bind.place] as indicated:
1 All placeholder types shall be
DefaultConstructible
andimplicit CopyConstructible
, and [..]
Change 20.3.1 [unique.ptr]/5 as indicated:
5 Each object of a type
U
instantiated form theunique_ptr
template specified in this subclause has the strict ownership semantics, specified above, of a unique pointer. In partial satisfaction of these semantics, each suchU
isimplicit MoveConstructible
andMoveAssignable
, but is notCopyConstructible
norCopyAssignable
. The template parameterT
ofunique_ptr
may be an incomplete type.
Change 20.3.2.2 [util.smartptr.shared]/2 as indicated:
2 Specializations of
shared_ptr
shall beimplicit CopyConstructible
,CopyAssignable
, andLessThanComparable
, [..]
Change 20.3.2.3 [util.smartptr.weak]/2 as indicated:
2 Specializations of
weak_ptr
shall beimplicit CopyConstructible
andCopyAssignable
, allowing their use in standard containers. The template parameterT
ofweak_ptr
may be an incomplete type.
Change 24.3.5.2 [iterator.iterators]/2 as indicated: [This fixes a defect in the Iterator requirements. None of the usual algorithms accepting iterators would be usable with iterators with explicit copy-constructors]
2 A type
X
satisfies the Iterator requirements if:
X
satisfies theimplicit CopyConstructible
,CopyAssignable
, andDestructible
requirements (20.2.1) and lvalues of typeX
are swappable (20.2.2), and [..]- ...
Change 99 [auto.ptr]/3 as indicated:
3 [..] Instances of
auto_ptr
meet the requirements ofimplicit MoveConstructible
andMoveAssignable
, but do not meet the requirements ofCopyConstructible
andCopyAssignable
. — end note]
basic_string::replace
should use const_iterator
Section: 27.4.3.7.6 [string.replace] Status: C++11 Submitter: Daniel Krügler Opened: 2010-02-19 Last modified: 2016-11-12
Priority: Not Prioritized
View all other issues in [string.replace].
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Discussion:
In contrast to all library usages of purely positional iterator values several
overloads of std::basic_string::replace
still use iterator instead of
const_iterator
arguments. The paper
N3021
quite nicely visualizes the purely positional responsibilities of the function
arguments.
This should be fixed to make the library consistent, the proposed changes are quite mechanic.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
In 27.4.3 [basic.string], class template basic_string
synopsis
change as indicated:
// 21.4.6 modifiers: ... basic_string& replace(const_iterator i1, const_iterator i2, const basic_string& str); basic_string& replace(const_iterator i1, const_iterator i2, const charT* s, size_type n); basic_string& replace(const_iterator i1, const_iterator i2, const charT* s); basic_string& replace(const_iterator i1, const_iterator i2, size_type n, charT c); template<class InputIterator> basic_string& replace(const_iterator i1, const_iterator i2, InputIterator j1, InputIterator j2); basic_string& replace(const_iterator, const_iterator, initializer_list<charT>);
In 27.4.3.7.6 [string.replace] before p.18, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, const basic_string& str);
In 27.4.3.7.6 [string.replace] before p.21, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, const charT* s, size_type n);
In 27.4.3.7.6 [string.replace] before p.24, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, const charT* s);
In 27.4.3.7.6 [string.replace] before p.27, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, size_type n, charT c);
In 27.4.3.7.6 [string.replace] before p.30, change the following signatures as indicated:
template<class InputIterator> basic_string& replace(const_iterator i1, const_iterator i2, InputIterator j1, InputIterator j2);
In 27.4.3.7.6 [string.replace] before p.33, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, initializer_list<charT> il);
pair
and tuple
Section: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Daniel Krügler Opened: 2010-03-20 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [pairs.pair].
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Discussion:
In analogy to library defect 811(i), tuple
's variadic constructor
template <class... UTypes> explicit tuple(UTypes&&... u);
creates the same problem as pair:
#include <tuple> int main() { std::tuple<char*> p(0); }
produces a similar compile error for a recent gcc implementation.
I suggest to follow the same resolution path as has been applied to
pair
's corresponding c'tor, that is require that these c'tors should
not participate in overload resolution, if the arguments are not implicitly
convertible to the element types.
Further-on both pair
and tuple
provide converting constructors
from different pairs
/tuples
that should be not available, if
the corresponding element types are not implicitly convertible. It seems
astonishing that in the following example
struct A { explicit A(int); }; A a = 1; // Error std::tuple<A> ta = std::make_tuple(1); // # OK?
the initialization marked with # could be well-formed.
[ Only constraints on constructors are suggested. Adding similar constraints on assignment operators is considered as QoI, because the assigments wouldn't be well-formed anyway. ]
Following 22.3.2 [pairs.pair]/5 add a new Remarks element:
template<class U, class V> pair(const pair<U, V>& p);5 Effects: Initializes members from the corresponding members of the argument
, performing implicit conversions as needed.Remarks: This constructor shall not participate in overload resolution unless
U
is implicitly convertible tofirst_type
andV
is implicitly convertible tosecond_type
.
Following 22.3.2 [pairs.pair]/6 add a new Remarks element:
template<class U, class V> pair(pair<U, V>&& p);6 Effects: The constructor initializes
first
withstd::move(p.first)
and second withstd::move(p.second)
.Remarks: This constructor shall not participate in overload resolution unless
U
is implicitly convertible tofirst_type
andV
is implicitly convertible tosecond_type
.
Following 22.4.4.2 [tuple.cnstr]/7 add a new Remarks element:
template <class... UTypes> explicit tuple(UTypes&&... u);6 Requires: Each type in
Types
shall satisfy the requirements ofMoveConstructible
(Table 33) from the corresponding type inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.7 Effects: Initializes the elements in the
tuple
with the corresponding value instd::forward<UTypes>(u)
.Remarks: This constructor shall not participate in overload resolution unless each type in
UTypes
is implicitly convertible to its corresponding type inTypes
.
Following 22.4.4.2 [tuple.cnstr]/13 add a new Remarks element:
template <class... UTypes> tuple(const tuple<UTypes...>& u);12 Requires: Each type in
Types
shall be constructible from the corresponding type inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.13 Effects: Constructs each element of
*this
with the corresponding element ofu
.Remarks: This constructor shall not participate in overload resolution unless each type in
UTypes
is implicitly convertible to its corresponding type inTypes
.14 [Note:
enable_if
can be used to make the converting constructor and assignment operator exist only in the cases where the source and target have the same number of elements. — end note]
Following 22.4.4.2 [tuple.cnstr]/16 add a new Remarks element:
template <class... UTypes> tuple(tuple<UTypes...>&& u);15 Requires: Each type in
Types
shall shall satisfy the requirements ofMoveConstructible
(Table 33) from the corresponding type inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.16 Effects: Move-constructs each element of
*this
with the corresponding element ofu
.Remarks: This constructor shall not participate in overload resolution unless each type in
UTypes
is implicitly convertible to its corresponding type inTypes
.[Note:
enable_if
can be used to make the converting constructor and assignment operator exist only in the cases where the source and target have the same number of elements. — end note]
Following 22.4.4.2 [tuple.cnstr]/18 add a new Remarks element:
template <class U1, class U2> tuple(const pair<U1, U2>& u);17 Requires: The first type in
Types
shall be constructible fromU1
and the second type inTypes
shall be constructible fromU2
.sizeof...(Types) == 2
.18 Effects: Constructs the first element with
u.first
and the second element withu.second
.Remarks: This constructor shall not participate in overload resolution unless
U1
is implicitly convertible to the first type inTypes
andU2
is implicitly convertible to the second type inTypes
.
Following 22.4.4.2 [tuple.cnstr]/20 add a new Remarks element:
template <class U1, class U2> tuple(pair<U1, U2>&& u);19 Requires: The first type in
Types
shall shall satisfy the requirements ofMoveConstructible
(Table 33) fromU1
and the second type inTypes
shall be move-constructible fromU2
.sizeof...(Types) == 2
.20 Effects: Constructs the first element with
std::move(u.first)
and the second element withstd::move(u.second)
Remarks: This constructor shall not participate in overload resolution unless
U1
is implicitly convertible to the first type inTypes
andU2
is implicitly convertible to the second type inTypes
.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
Proposed resolution:
See n3140.
bitset
Section: 22.9.2.2 [bitset.cons] Status: C++11 Submitter: Christopher Jefferson Opened: 2010-03-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.cons].
View all issues with C++11 status.
Discussion:
As mentioned on the boost mailing list:
The following code, valid in C++03, is broken in C++0x due to ambiguity
between the "unsigned long long
" and "char*
"
constructors.
#include <bitset> std::bitset<10> b(0);
[ The proposed resolution has been reviewed by Stephan T. Lavavej. ]
[ Post-Rapperswil ]
The proposed resolution has two problems:
it fails to provide support for non-terminated strings, which could be easily added and constitutes an important use-case. For example, the following code would invoke UB with the current P/R:
because it requires the evaluation (under the as-if rule, to be fair, but it doesn't matter) ofchar s[4] = { '0', '1', '0', '1' }; // notice: not null-terminated! bitset<4> b(s, 0, 4);
basic_string<char>(s)
it promotes a consistency between the two bitset
constructors that take a const std::string&
and a
const char*
, respectively, while practice established by
std::basic_string
would recommend a different set of
parameters. In particular, the constructor of
std::basic_string
that takes a const char*
does
not have a pos
parameter
Moved to Tentatively Ready with revised wording provided by Alberto Ganesh Babati after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
<bitset>
in
22.9.2 [template.bitset]/1, replace the fourth bitset constructor:
explicit bitset(const char *str);template <class charT> explicit bitset( const charT *str, typename basic_string<charT>::size_type n = basic_string<charT>::npos, charT zero = charT('0'), charT one = charT('1'));
explicit bitset(const char *str);template <class charT> explicit bitset(const charT *str, typename basic_string<charT>::size_type n = basic_string<charT>::npos, charT zero = charT('0'), charT one = charT('1'));
Effects: Constructs an object of class
bitset<N>
as if by
bitset(string(str)).
bitset( n == basic_string<charT>::npos ? basic_string<charT>(str) : basic_string<charT>(str, n), 0, n, zero, one)
pair
and tuple
functionsSection: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Daniel Krügler Opened: 2010-03-07 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [pairs.pair].
View all other issues in [pairs.pair].
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Discussion:
There are several constructors and creation functions of std::tuple that impose requirements on it's arguments, that are unnecessary restrictive and don't match the intention for the supported argument types. This is related to the fact that tuple is supposed to accept both object types and lvalue-references and the usual MoveConstructible and CopyConstructible requirements are bad descriptions for non-const references. Some examples:
22.4.4.2 [tuple.cnstr] before p.4 and p.8, resp.:
explicit tuple(const Types&...);4 Requires: Each type in
Types
shall be copy constructible.tuple(const tuple& u) = default;8 Requires: Each type in
Types
shall satisfy the requirements ofCopyConstructible
(Table 34).
A tuple that contains lvalue-references to non-const can never
satisfy the CopyConstructible
requirements. CopyConstructible
requirements refine the MoveConstructible
requirements and
this would require that these lvalue-references could bind
rvalues. But the core language does not allow that. Even, if we
would interpret that requirement as referring to the underlying
non-reference type, this requirement would be wrong as well,
because there is no reason to disallow a type such as
struct NoMoveNoCopy { NoMoveNoCopy(NoMoveNoCopy&&) = delete; NoMoveNoCopy(const NoMoveNoCopy&) = delete; ... }:
for the instantiation of std::tuple<NoMoveNoCopy&>
and
that of it's copy constructor.
A more reasonable requirement for this example would be to require that
"is_constructible<Ti, const Ti&>::value
shall
evaluate to true for all Ti
in Types
". In this case
the special reference-folding and const-merging rules of references
would make this well-formed in all cases. We could also add the further
constraint "if Ti
is an object type, it shall satisfy the
CopyConstructible
requirements", but this additional
requirement seems not really to help here. Ignoring it would only mean
that if a user would provide a curious object type C
that
satisfies the std::is_constructible<C, const C&>
test, but not the "C
is CopyConstructible
" test would
produce a tuple<C>
that does not satisfy the
CopyConstructible
requirements as well.
22.4.4.2 [tuple.cnstr] before p.6 and p.10, resp.:
template <class... UTypes> explicit tuple(UTypes&&... u);6 Requires: Each type in
Types
shall satisfy the requirements ofMoveConstructible
(Table 33) from the corresponding type inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.tuple(tuple&& u);10 Requires: Each
type
inTypes
shall shall satisfy the requirements ofMoveConstructible
(Table 33).
We have a similar problem as in (a): Non-const lvalue-references
are intended template arguments for std::tuple
, but cannot satisfy
the MoveConstructible
requirements. In this case the correct
requirements would be
is_constructible<Ti, Ui>::value
shall evaluate to true for allTi
inTypes
and for allUi
inUTypes
and
is_constructible<Ti, Ti>::value
shall evaluate to true for allTi
inTypes
respectively.
Many std::pair
member functions do not add proper requirements, e.g.
the default c'tor does not require anything. This is corrected within the
suggested resolution. Further-on the P/R has been adapted to the FCD numbering.
[ 2010-03-25 Daniel updated wording: ]
The issue became updated to fix some minor inconsistencies and to ensure a similarly required fix for
std::pair
, which has the same specification problem asstd::tuple
, sincepair
became extended to support reference members as well.
[Original proposed resolution:]
Change 22.3.2 [pairs.pair]/1 as indicated [The changes for the effects elements are not normative changes, they just ensure harmonization with existing wording style]:
constexpr pair();Requires:
first_type
andsecond_type
shall satisfy theDefaultConstructible
requirements.1 Effects: Value-initializes
first
andsecond
.Initializes its members as if implemented:pair() : first(), second() { }
.
Change 22.3.2 [pairs.pair]/2 as indicated:
pair(const T1& x, const T2& y);Requires:
is_constructible<T1, const T1&>::value
istrue
andis_constructible<T2, const T2&>::value
istrue
.2 Effects: The constructor initializes
first
withx
andsecond
withy
.
Change 22.3.2 [pairs.pair]/3 as indicated:
template<class U, class V> pair(U&& x, V&& y);Requires:
is_constructible<first_type, U>::value
istrue
andis_constructible<second_type, V>::value
istrue
.3 Effects: The constructor initializes
first
withstd::forward<U>(x)
andsecond
withstd::forward<V>(y)
.4 Remarks: If
U
is not implicitly convertible tofirst_type
orV
is not implicitly convertible tosecond_type
this constructor shall not participate in overload resolution.
Change 22.3.2 [pairs.pair]/5 as indicated [The change in the effects element should be non-normatively and is in compatible to the change suggestion of 1324(i)]:
template<class U, class V> pair(const pair<U, V>& p);Requires:
is_constructible<first_type, const U&>::value
istrue
andis_constructible<second_type, const V&>::value
istrue
.5 Effects: Initializes members from the corresponding members of the argument
, performing implicit conversions as needed.
Change 22.3.2 [pairs.pair]/6 as indicated:
template<class U, class V> pair(pair<U, V>&& p);Requires:
is_constructible<first_type, U>::value
istrue
andis_constructible<second_type, V>::value
istrue
.6 Effects: The constructor initializes
first
withstd::
andmoveforward<U>(p.first)second
withstd::
.moveforward<V>(p.second)
Change 22.3.2 [pairs.pair]/7+8 as indicated [The deletion in the effects element should be non-normatively]:
template<class... Args1, class... Args2> pair(piecewise_construct_t, tuple<Args1...> first_args, tuple<Args2...> second_args);7 Requires:
is_constructible<first_type, Args1...>::value
istrue
andis_constructible<second_type, Args2...>::value
istrue
.All the types inArgs1
andArgs2
shall beCopyConstructible
(Table 35).T1
shall be constructible fromArgs1
.T2
shall be constructible fromArgs2
.8 Effects: The constructor initializes
first
with arguments of typesArgs1...
obtained by forwarding the elements offirst_args
and initializessecond
with arguments of typesArgs2...
obtained by forwarding the elements ofsecond_args
.(Here, forwarding an elementThis form of construction, whereby constructor arguments forx
of typeU
within atuple
object means callingstd::forward<U>(x)
.)first
andsecond
are each provided in a separatetuple
object, is called piecewise construction.
Change 22.3.2 [pairs.pair] before 12 as indicated:
pair& operator=(pair&& p);Requires:
first_type
andsecond_type
shall satisfy theMoveAssignable
requirements.12 Effects: Assigns to
first
withstd::move(p.first)
and tosecond
withstd::move(p.second)
.13 Returns:
*this
.
Change [pairs.pair] before 14 as indicated: [The heterogeneous usage of MoveAssignable is actually not defined, but the library uses it at several places, so we follow this tradition until a better term has been agreed on. One alternative could be to write "first_type shall be assignable from an rvalue of U [..]"]
template<class U, class V> pair& operator=(pair<U, V>&& p);Requires:
first_type
shall beMoveAssignable
fromU
andsecond_type
shall beMoveAssignable
fromV
.14 Effects: Assigns to
first
withstd::move(p.first)
and tosecond
withstd::move(p.second)
.15 Returns:
*this
.
Change 22.4.4.2 [tuple.cnstr]/4+5 as indicated:
explicit tuple(const Types&...);4 Requires:
is_constructible<Ti, const Ti&>::value == true
for eEach typeTi
inTypes
shall be copy constructible.5 Effects:
Copy iInitializes each element with the value of the corresponding parameter.
Change 22.4.4.2 [tuple.cnstr]/6 as indicated:
template <class... UTypes> explicit tuple(UTypes&&... u);6 Requires:
is_constructible<Ti, Ui>::value == true
for eEach typeTi
inTypes
shall satisfy the requirements ofand for the corresponding typeMoveConstructible
(Table 33) fromUi
inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.7 Effects: Initializes the elements in the
tuple
with the corresponding value instd::forward<UTypes>(u)
.
Change 22.4.4.2 [tuple.cnstr]/8+9 as indicated:
tuple(const tuple& u) = default;8 Requires:
is_constructible<Ti, const Ti&>::value == true
for eEach typeTi
inTypes
shall satisfy the requirements of.CopyConstructible
(Table 34)9 Effects: Initializes
Copy constructseach element of*this
with the corresponding element ofu
.
Change 22.4.4.2 [tuple.cnstr]/10+11 as indicated:
tuple(tuple&& u);10 Requires: Let
i
be in[0, sizeof...(Types))
and letTi
be thei
th type inTypes
. Thenis_constructible<Ti, Ti>::value
shall betrue
for alli
.Each type in.Types
shall shall satisfy the requirements ofMoveConstructible
(Table 34)11 Effects: For each
Ti
inTypes
, initializes thei
thMove-constructs eachelement of*this
withthe corresponding element ofstd::forward<Ti>(get<i>(
u
))
.
Change 22.4.4.2 [tuple.cnstr]/15+16 as indicated:
template <class... UTypes> tuple(tuple<UTypes...>&& u);15 Requires: Let
i
be in[0, sizeof...(Types))
,Ti
be thei
th type inTypes
, andUi
be thei
th type inUTypes
. Thenis_constructible<Ti, Ui>::value
shall betrue
for alli
.Each type in.Types
shall shall satisfy the requirements ofMoveConstructible
(Table 34) from the corresponding type inUTypes
sizeof...(Types) == sizeof...(UTypes)
.16 Effects: For each type
Ti
, initializes thei
thMove-constructs eachelement of*this
withthe corresponding element ofstd::forward<Ui>(get<i>(
u
))
.
Change 22.4.4.2 [tuple.cnstr]/19+20 as indicated:
template <class U1, class U2> tuple(pair<U1, U2>&& u);19 Requires:
is_constructible<T1, U1>::value == true
fort
he first typeT
T1
inTypes
shall shall satisfy the requirements ofandMoveConstructible
(Table 33) fromU1
is_constructible<T2, U2>::value == true
for the second typeT2
inTypes
shall be move-constructible from.U2
sizeof...(Types) == 2
.20 Effects: Initializes
Constructsthe first element withstd::forward<U1>
and the second element withmove(u.first)std::forward<U2>
.move(u.second)
Change 22.4.5 [tuple.creation]/9-16 as indicated:
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);9 Requires:
is_constructible<Ti, const Ti&>::value == true
for each typeTi
All the typesinTTypes
shall be.CopyConstructible
(Table 34)is_constructible<Ui, const Ui&>::value == true
for each typeUi
All the typesinUTypes
shall be.CopyConstructible
(Table 34)10 Returns: A
tuple
object constructed by initializingcopy constructingits firstsizeof...(TTypes)
elements from the corresponding elements oft
and initializingcopy constructingits lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);11 Requires: Let
i
be in[0, sizeof...(TTypes))
,Ti
be thei
th type inTypes
,j
be in[0, sizeof...(UTypes))
, andUj
be thej
th type inUTypes
.is_constructible<Ti, Ti>::value
shall betrue
for each typeTi
andis_constructible<Uj, const Uj&>::value
shall betrue
for each typeUj
All the types in.TTypes
shall beMoveConstructible
(Table 34). All the types inUTypes
shall beCopyConstructible
(Table 35)12 Returns: A
tuple
object constructed by initializing thei
th element withstd::forward<Ti>(get<i>(t))
for allTi
inTTypes
and initializing the(j+sizeof...(TTypes))
th element withget<j>(u)
for allUj
inUTypes
.move constructing its first.sizeof...(TTypes)
elements from the corresponding elements oft
and copy constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);13 Requires: Let
i
be in[0, sizeof...(TTypes))
,Ti
be thei
th type inTypes
,j
be in[0, sizeof...(UTypes))
, andUj
be thej
th type inUTypes
.is_constructible<Ti, const Ti&>::value
shall betrue
for each typeTi
andis_constructible<Uj, Uj>::value
shall betrue
for each typeUj
All the types in.TTypes
shall beCopyConstructible
(Table 35). All the types inUTypes
shall beMoveConstructible
(Table 34)14 Returns: A
tuple
object constructed by initializing thei
th element withget<i>(t)
for each typeTi
and initializing the(j+sizeof...(TTypes))
th element withstd::forward<Uj>(get<j>(u))
for each typeUj
copy constructing its first.sizeof...(TTypes)
elements from the corresponding elements oft
and move constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);15 Requires: Let
i
be in[0, sizeof...(TTypes))
,Ti
be thei
th type inTypes
,j
be in[0, sizeof...(UTypes))
, andUj
be thej
th type inUTypes
.is_constructible<Ti, Ti>::value
shall betrue
for each typeTi
andis_constructible<Uj, Uj>::value
shall betrue
for each typeUj
All the types in.TTypes
shall beMoveConstructible
(Table 34). All the types inUTypes
shall beMoveConstructible
(Table 34)16 Returns: A
tuple
object constructed by initializing thei
th element withstd::forward<Ti>(get<i>(t))
for each typeTi
and initializing the(j+sizeof...(TTypes))
th element withstd::forward<Uj>(get<j>(u))
for each typeUj
move constructing its first.sizeof...(TTypes)
elements from the corresponding elements oft
and move constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
Proposed resolution:
See n3140.
<cmath>
replacing C macros with the same nameSection: 29.7 [c.math] Status: Resolved Submitter: Michael Wong Opened: 2010-03-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 29.7 [c.math]p12
The templates defined in <cmath>
replace the C99 macros
with the same names. The templates have the following declarations:
namespace std { template <class T> bool signbit(T x); template <class T> int fpclassify(T x); template <class T> bool isfinite(T x); template <class T> bool isinf(T x); template <class T> bool isnan(T x); template <class T> bool isnormal(T x); template <class T> bool isgreater(T x, T y); template <class T> bool isgreaterequal(T x, T y); template <class T> bool isless(T x, T y); template <class T> bool islessequal(T x, T y); template <class T> bool islessgreater(T x, T y); template <class T> bool isunordered(T x, T y); }
and p13:
13 The templates behave the same as the C99 macros with corresponding names defined in C99 7.12.3, Classification macros, and C99 7.12.14, Comparison macros in the C standard.
The C Std versions look like this:
7.12.14.1/p1:
Synopsis
1
#include <math.h>
int isgreaterequal(real-floating x, real-floating y);
which is not necessarily the same types as is required by C++ since the two parameters may be different. Would it not be better if it were truly aligned with C?
[ 2010 Pittsburgh: Bill to ask WG-14 if heterogeneous support for the two-parameter macros is intended. ]
[ 2010-09-13 Daniel comments: ]
I recommend to resolve this issue as NAD Editorial because the accepted resolution for NB comment US-136 by motion 27 does address this.
[ 2010-09-14 Bill comments: ]
Motion 27 directly addresses LWG 1327 and solves the problem presented there. Moreover, the solution has been aired before WG14 with no dissent. These functions now behave the same for mixed-mode calls in both C and C++
Proposed resolution:
Apply proposed resolution for US-136
failbit
if eofbit
is already setSection: 31.7.5.2.4 [istream.sentry] Status: Resolved Submitter: Paolo Carlini Opened: 2010-02-17 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [istream.sentry].
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Discussion:
Basing on the recent discussion on the library reflector, see c++std-lib-27728 and follow ups, I hereby formally ask for LWG 419 to be re-opened, the rationale being that according to the current specifications, per n3000, it seems actually impossible to seek away from end of file, contrary to the rationale which led 342(i) to its closure as NAD. My request is also supported by Martin Sebor, and I'd like also to add, as tentative proposed resolution for the re-opened issue, the wording suggested by Sebor, thus, change the beginning of [istream::sentry]/2, to:
2 Effects: If
(!noskipws && !is.good())
is, calls
falsetrueis.setstate(failbit)
. Otherwise prepares for formatted or unformatted input. ...
[ 2010-10 Batavia ]
Resolved by adopting n3168.
Previous proposed resolution:
Change [istream::sentry] p.2:2 Effects: If
(!noskipws && !is.good())
is, calls
falsetrueis.setstate(failbit)
. Otherwise prepares for formatted or unformatted input. ...
Proposed resolution:
Addressed by paper n3168.
vector<bool>
Section: 23.2.3 [container.requirements.dataraces] Status: Resolved Submitter: Jeffrey Yaskin Opened: 2010-03-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The common implementation of vector<bool>
is as an
unsynchronized bitfield. The addition of 23.2.3 [container.requirements.dataraces]/2 would require either a
change in representation or a change in access synchronization, both of
which are undesireable with respect to compatibility and performance.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3069.
Proposed resolution:
Container data races 23.2.3 [container.requirements.dataraces]
Paragraph 1 is unchanged as follows:
1 For purposes of avoiding data races (17.6.4.8), implementations shall consider the following functions to be
const
:begin
,end
,rbegin
,rend
,front
,back
,data
,find
,lower_bound
,upper_bound
,equal_range
, and, except in associative containers,operator[]
.
Edit paragraph 2 as follows:
2 Notwithstanding (17.6.4.8), implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting
vector<bool>
, are modified concurrently.
Edit paragraph 3 as follows:
3 [Note: For a
vector<int> x
with a size greater than one,x[1] = 5
and*x.begin() = 10
can be executed concurrently without a data race, butx[0] = 5
and*x.begin() = 10
executed concurrently may result in a data race. As an exception to the general rule, for avector<bool> y
,y[i] = true
may race withy[j] = true
. —end note]
Section: 16.4.4.5 [hash.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-26 Last modified: 2025-03-13
Priority: Not Prioritized
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Discussion:
The currently added Hash
requirements demand in Table 40 — Hash
requirements [hash]:
Table 40 — Hash requirements [hash] Expression Return type Requirement h(k)
size_t
Shall not throw exceptions. [..]
While it surely is a generally accepted idea that hash function objects should not throw exceptions, this basic constraint for such a fundamental requirement set does neither match the current library policy nor real world cases:
The new definition goes beyond the original hash requirements as specified by SGI library in regard to the exception requirement:
noexcept
language facility libraries can still take
advantage of no-throw guarantees of hasher functions with stricter guarantees.
Even though the majority of all known move, swap, and hash functions won't throw and in some cases must not throw, it seems like unnecessary over-constraining the definition of a Hash functor not to propagate exceptions in any case and it contradicts the general principle of C++ to impose such a requirement for this kind of fundamental requirement.
[ 2010-11-11 Daniel asks the working group whether they would prefer a replacement for the second bullet of the proposed resolution (a result of discussing this with Alberto) of the form: ]
Add to 22.10.19 [unord.hash]/1 a new bullet:
1 The unordered associative containers defined in Clause 23.5 use specializations of the class template
hash
as the default hash function. For all object typesKey
for which there exists a specializationhash<Key>
, the instantiationhash<Key>
shall:
- satisfy the
Hash
requirements (20.2.4), withKey
as the function call argument type, theDefaultConstructible
requirements (33), theCopyAssignable
requirements (37),- be swappable (20.2.2) for lvalues,
- provide two nested types
result_type
andargument_type
which shall be synonyms forsize_t
andKey
, respectively,- satisfy the requirement that if
k1 == k2
is true,h(k1) == h(k2)
is also true, whereh
is an object of typehash<Key>
andk1
andk2
are objects of typeKey
,.- satisfy the requirement
noexcept(h(k)) == true
, whereh
is an object of typehash<Key>
andk
is an object of typeKey
, unlesshash<Key>
is a user-defined specialization that depends on at least one user-defined type.
[Batavia: Closed as NAD Future, then reopened. See the wiki for Tuesday.]
Proposed resolution:
Change Table 26 — Hash
requirements [tab:hash] as indicated:
Table 26 — Hash
requirements [tab:hash]Expression Return type Requirement h(k)
size_t
Shall not throw exceptions.[…]
Add to 22.10.19 [unord.hash] p. 1 a new bullet:
1 The unordered associative containers defined in Clause 23.5 [unord] use specializations of the class template
hash
as the default hash function. For all object typesKey
for which there exists a specializationhash<Key>
, the instantiationhash<Key>
shall:
- satisfy the
Hash
requirements ([hash.requirements]), withKey
as the function call argument type, theDefaultConstructible
requirements (Table [defaultconstructible]), theCopyAssignable
requirements (Table [copyassignable]),- be swappable ([swappable.requirements]) for lvalues,
- provide two nested types
result_type
andargument_type
which shall be synonyms forsize_t
andKey
, respectively,- satisfy the requirement that if
k1 == k2
is true,h(k1) == h(k2)
is also true, whereh
is an object of typehash<Key>
andk1
andk2
are objects of typeKey
,.- satisfy the requirement that the expression
h(k)
, whereh
is an object of typehash<Key>
andk
is an object of typeKey
, shall not throw an exception, unlesshash<Key>
is a user-defined specialization that depends on at least one user-defined type.
std::function
invocationSection: 22.10.17.3.5 [func.wrap.func.inv] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.inv].
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Discussion:
The current wording of 22.10.17.3.5 [func.wrap.func.inv]/1:
R operator()(ArgTypes... args) constEffects:
INVOKE(f, t1, t2, ..., tN, R)
(20.8.2), wheref
is the target object (20.8.1) of*this
andt1, t2, ..., tN
are the values inargs...
.
uses an unclear relation between the actual args and the used variables
ti
. It should be made clear, that std::forward
has to be used
to conserve the expression lvalueness.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 22.10.17.3.5 [func.wrap.func.inv]/1+2 as indicated:
R operator()(ArgTypes... args) const1 Effects::
INVOKE(f, std::forward<ArgTypes>(args)...
(20.8.2), wheret1, t2, ..., tN, R)f
is the target object (20.8.1) of*this
and.t1, t2, ..., tN
are the values inargs...
2 Returns: Nothing if
R
isvoid
, otherwise the return value ofINVOKE(f, std::forward<ArgTypes>(args)...
.t1, t2, ..., tN, R)3 Throws:
bad_function_call
if!*this
; otherwise, any exception thrown by the wrapped callable object.
Section: 24.5.2.2.2 [back.insert.iter.ops], 24.5.2.3.2 [front.insert.iter.ops], 24.5.2.4.2 [insert.iter.ops] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-28 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
In C++03 this was valid code:
#include <vector> #include <iterator> int main() { typedef std::vector<bool> Cont; Cont c; std::back_insert_iterator<Cont> it = std::back_inserter(c); *it = true; }
In C++0x this code does no longer compile because of an ambiguity error for this
operator=
overload pair:
back_insert_iterator<Container>& operator=(typename Container::const_reference value); back_insert_iterator<Container>& operator=(typename Container::value_type&& value);
This is so, because for proxy-containers like std::vector<bool>
the const_reference
usually is a non-reference type and in this case
it's identical to Container::value_type
, thus forming the ambiguous
overload pair
back_insert_iterator<Container>& operator=(bool value); back_insert_iterator<Container>& operator=(bool&& value);
The same problem exists for std::back_insert_iterator
,
std::front_insert_iterator
, and std::insert_iterator
.
One possible fix would be to require that const_reference
of a proxy
container must not be the same as the value_type
, but this would break
earlier valid code. The alternative would be to change the first signature to
back_insert_iterator<Container>& operator=(const typename Container::const_reference& value);
This would have the effect that this signature always expects an lvalue or rvalue, but it would not create an ambiguity relative to the second form with rvalue-references. [For all non-proxy containers the signature will be the same as before due to reference-collapsing and const folding rules]
[ Post-Rapperswil ]
This problem is not restricted to the unspeakable vector<bool>
, but is already existing for other proxy
containers like gcc's rope
class. The following code does no longer work ([Bug libstdc++/44963]):
#include <iostream> #include <ext/rope> using namespace std; int main() { __gnu_cxx::crope line("test"); auto ii(back_inserter(line)); *ii++ = 'm'; // #1 *ii++ = 'e'; // #2 cout << line << endl; }
Both lines marked with #1 and #2 issue now an error because the library has properly implemented the current wording state (Thanks to Paolo Calini for making me aware of this real-life example).
The following P/R is a revision of the orignal P/R and was initially suggested by Howard Hinnant. Paolo verified that the approach works in gcc.
Moved to Tentatively Ready with revised wording after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
back_insert_iterator
synopsis as indicated:
template <class Container> class back_insert_iterator : public iterator<output_iterator_tag,void,void,void,void> { protected: Container* container; public: [..] back_insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value); back_insert_iterator<Container>& operator=(typename Container::value_type&& value); [..] };
back_insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value);1 Effects:
container->push_back(value)
;
2 Returns:*this
.
front_insert_iterator
synposis as indicated:
template <class Container> class front_insert_iterator : public iterator<output_iterator_tag,void,void,void,void> { protected: Container* container; public: [..] front_insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value); front_insert_iterator<Container>& operator=(typename Container::value_type&& value); [..] };
front_insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value);1 Effects:
container->push_front(value)
;
2 Returns:*this
.
template <class Container> class insert_iterator : public iterator<output_iterator_tag,void,void,void,void> { protected: Container* container; typename Container::iterator iter; public: [..] insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value); insert_iterator<Container>& operator=(typename Container::value_type&& value); [..] };
insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value);1 Effects:
iter = container->insert(iter, value); ++iter;2 Returns:
*this
.
tuple::operator<()
Section: 22.4.9 [tuple.rel] Status: C++11 Submitter: Joe Gottman Opened: 2010-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The requirements section for std::tuple
says the following:
Requires: For all
i
, where0 <= i and i < sizeof...(Types)
,get<i>(t) < get<i>(u)
is a valid expression returning a type that is convertible tobool
.sizeof...(TTypes) == sizeof...(UTypes)
.
This is necessary but not sufficient, as the algorithm for comparing
tuple
s also computes get<i>(u) < get<i>(t)
(note the order)
[ Post-Rapperswil ]
Moved to Tentatively Ready with updated wording correcting change-bars after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
template<class... TTypes, class... UTypes> bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);Requires: For all
i
, where0 <= i
andi < sizeof...(Types)
,get<i>(t) < get<i>(u)
andget<i>(u) < get<i>(t)
is a valid expression returning a type that isare valid expressions returning types that are convertible tobool
.sizeof...(TTypes) == sizeof...(UTypes)
.
regex_traits::isctype
Section: 28.6.6 [re.traits] Status: C++11 Submitter: Howard Hinnant Opened: 2010-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
28.6.6 [re.traits]/12 describes regex_traits::isctype
in terms
of ctype::is(c, m)
, where c
is a charT
and m
is a ctype_base::mask
. Unfortunately 28.3.4.2.2.2 [locale.ctype.members]
specifies this function as ctype::is(m, c)
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 28.6.6 [re.traits] p.12:
bool isctype(charT c, char_class_type f) const;11 ...
12 Returns: Converts
f
into a valuem
of typestd::ctype_base::mask
in an unspecified manner, and returnstrue
ifuse_facet<ctype<charT> >(getloc()).is(
iscm,mc)true
. Otherwise returnstrue
iff
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence"w"
is not equal to 0 andc == '_'
, or iff
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence"blank"
is not equal to 0 andc
is one of an implementation-defined subset of the characters for whichisspace(c, getloc())
returnstrue
, otherwise returnsfalse
.
Section: 26.6.15 [alg.search] Status: C++11 Submitter: Howard Hinnant Opened: 2010-06-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
LWG 1205(i) (currently in WP) clarified the return value of several algorithms when dealing with empty ranges. In particular it recommended for 26.6.15 [alg.search]:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);1 Effects: ...
2 Returns: ... Returns
last1
if no such iterator is found.3 Remarks: Returns
first1
if[first2,last2)
is empty.
Unfortunately this got translated to an incorrect specification for what gets returned when no such iterator is found (N3092):
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);1 Effects: ...
2 Returns: ... Returns
first1
if[first2,last2)
is empty or if no such iterator is found.
LWG 1205(i) is correct and N3092 is not equivalent nor correct.
I have not reviewed the other 10 recommendations of 1205(i).
[ Post-Rapperswil ]
It was verified that none of the remaining possibly affected algorithms does have any similar problems and a concrete P/R was added that used a similar style as has been applied to the other cases.
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);1 - [...]
2 - Returns: The first iteratori
in the range[first1,last1 - (last2-first2))
such that for any nonnegative integern
less thanlast2 - first2
the following corresponding conditions hold:*(i + n) == *(first2 + n)
,pred(*(i + n), *(first2 + n)) != false
. Returnsfirst1
if[first2,last2)
is emptyor, otherwise returnslast1
if no such iterator is found.
uninitialized_fill_n
should return the end of its rangeSection: 26.11.7 [uninitialized.fill] Status: C++11 Submitter: Jared Hoberock Opened: 2010-07-14 Last modified: 2017-06-15
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
N3092's
specification of uninitialized_fill_n
discards useful information and
is inconsistent with other algorithms such as fill_n
which accept an
iterator and a size. As currently specified, unintialized_fill_n
requires an additional linear traversal to find the end of the range.
Instead of returning void
, unintialized_fill_n
should return
one past the last iterator it dereferenced.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
In section 20.2 [memory] change:,
template <class ForwardIterator, class Size, class T>voidForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);
In section [uninitialized.fill.n] change,
template <class ForwardIterator, class Size, class T>voidForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);1 Effects:
for (; n--; ++first) ::new (static_cast<void*>(&*first)) typename iterator_traits<ForwardIterator>::value_type(x); return first;
forward_list::resize
take the object to be copied by value?Section: 23.3.7.5 [forward.list.modifiers] Status: C++11 Submitter: James McNellis Opened: 2010-07-16 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
In
N3092
[forwardlist.modifiers], the resize()
member function is
declared as:
void resize(size_type sz, value_type c);
The other sequence containers (list
, deque
, and
vector
) take 'c'
by const reference.
Is there a reason for this difference? If not, then resize()
should
be declared as:
void resize(size_type sz, const value_type& c);
The declaration would need to be changed both at its declaration in the class definition at [forwardlist]/3 and where its behavior is specified at [forwardlist.modifiers]/22.
This would make forward_list
consistent with the CD1 issue 679(i).
[ Post-Rapperswil ]
Daniel changed the P/R slightly, because one paragraph number has been changed since the issue
had been submitted. He also added a similar Requires element that exists in all other containers with
a resize
member function. He deliberately did not touch the wrong usage of "default-constructed" because that
will be taken care of by LWG issue 868(i).
Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
forward_list
synopsis, as indicated:
... void resize(size_type sz); void resize(size_type sz, const value_type& c); void clear(); ...
void resize(size_type sz); void resize(size_type sz, const value_type& c);27 Effects: If
28 - Requires:sz < distance(begin(), end())
, erases the lastdistance(begin(), end()) - sz
elements from the list. Otherwise, insertssz - distance(begin(), end())
elements at the end of the list. For the first signature the inserted elements are default constructed, and for the second signature they are copies ofc
.T
shall beDefaultConstructible
for the first form and it shall beCopyConstructible
for the second form.
throw()
with noexcept
Section: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1351
Discussion:
Addresses GB-60, CH-16
Dynamic exception specifications are deprecated; the
library should recognise this by replacing all non-throwing
exception specifications of the form throw()
with the
noexcept
form.
[ Resolution proposed by ballot comment: ]
Replace all non-throwing exception specifications of the form 'throw()' with the 'noexcept' form.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of n3148 would satisfy this request.
[ 2010-Batavia: ]
Resolved by adopting n3148.
Proposed resolution:
See n3148 See n3150 See n3195 See n3155 See n3156
noexcept
move operationsSection: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-61
All library types should have non-throwing move
constructors and move-assignment operators unless
wrapping a type with a potentially throwing move operation.
When such a type is a class-template, these
operations should have a conditional noexcept
specification.
There are many other places where a noexcept
specification may be considered, but the move operations
are a special case that must be called out, to effectively
support the move_if_noexcept
function template.
[ Resolution proposed by ballot comment: ]
Review every class and class template in the library. If noexcept
move constructor/assignment operators can be implicitly declared, then they
should be implicitly declared, or explicitly defaulted. Otherwise, a move
constructor/move assignment operator with a noexcept
exception
specification should be provided.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of n3157 would satisfy this request.
[2011-03-24 Madrid meeting]
Resolved by papers to be listed later
Proposed resolution:
See n3157
noexcept
where library specification does not permit exceptionsSection: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1352
Discussion:
Addresses GB-62, CH-17
Issues with efficiency and unsatisfactory semantics mean many library functions document they do not throw exceptions with a Throws: Nothing clause, but do not advertise it with an exception specification. The semantic issues are largely resolved with the new 'noexcept' specifications, and the noexcept operator means we will want to detect these guarantees programatically in order to construct programs taking advantage of the guarantee.
[ Resolution proposed by ballot comment: ]
Add a noexcept
exception specification on each
libary API that offers an unconditional Throws:
Nothing guarantee. Where the guarantee is
conditional, add the appropriate
noexcept(constant-expression)
if an appropriate
constant expression exists.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of n3149 would satisfy this request.
[ 2010-Batavia: ]
Resolved by adopting n3149.
Proposed resolution:
This issue is resolved by the adoption of n3195
noexcept
judiciously throughout the librarySection: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-63, US-80
Since the newly introduced operator noexcept
makes it
easy (easier than previously) to detect whether or not a
function has been declared with the empty exception
specification (including noexcept
) library functions that
cannot throw should be decorated with the empty
exception specification. Failing to do so and leaving it as a
matter of QoI would be detrimental to portability and
efficiency.
[ Resolution proposed by ballot comment ]
Review the whole library, and apply the noexcept
specification where it is appropriate.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of the combination of n3155, n3156, n3157, n3167 would satisfy this request. The paper n3150 is related to this as well.
[ 2010 Batavia: ]
While the LWG expects to see further papers in this area, sufficient action was taken in Batavia to close the issue as Resolved by the listed papers.
Proposed resolution:
See n3155, n3156, n3157, n3167 and remotely n3150
swap
should not throwSection: 16 [library] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-65
Nothrowing swap
operations are key to many C++ idioms,
notably the common copy/swap idiom to provide the
strong exception safety guarantee.
[ Resolution proposed by ballot comment ]
Where possible, all library types should provide a
swap
operation with an exception specification
guaranteeing no exception shall propagate.
Where noexcept(true)
cannot be guaranteed to
not terminate the program, and the swap
in
questions is a template, an exception specification
with the appropriate conditional expression could
be specified.
[2011-03-13: Daniel comments and drafts wording]
During a survey of the library some main categories for
potential noexcept
swap
function could be isolated:
swap
functions that are specified in terms of already
noexcept
swap
member functions, like that of valarray
.swap
of std::function
, and member and free swap
functions of stream buffers and streams where considered but rejected as good candidates,
because of the danger to potentially impose requirements of existing implementations.
These functions could be reconsidered as candidates in the future.Negative list:
iter_swap
, have not been touched,
because there are no fundamental exceptions constraints on iterator operations in general
(only for specific types, like library container iterators)While evaluating the current state of swap
functions
of library components it was observed that several conditional noexcept
functions have turned into unconditional ones, e.g. in the
header <utility>
synopsis:
template<class T> void swap(T& a, T& b) noexcept;
The suggested resolution shown below also attempts to fix these cases.
[2011-03-22 Daniel redrafts to satisfy new criteria for applying noexcept
.
Parts resolved by N3263-v2 and D3267 are not added here.]
Proposed resolution:
Edit 22.2 [utility] p. 2, header <utility>
synopsis and
22.2.2 [utility.swap] before p. 1, as indicated (The intent is to fix an editorial
omission):
template<class T> void swap(T& a, T& b) noexcept(see below);
Edit the prototype declaration in 22.3.2 [pairs.pair] before p. 34 as indicated (The intent is to fix an editorial omission):
void swap(pair& p) noexcept(see below);
Edit 22.4.1 [tuple.general] p. 2 header <tuple>
synopsis and
22.4.12 [tuple.special] before p. 1 as indicated (The intent is to fix an editorial omission):
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);
Edit 22.4.4 [tuple.tuple], class template tuple
synopsis and
22.4.4.4 [tuple.swap] before p. 1 as indicated (The intent is to fix an editorial omission):
void swap(tuple&) noexcept(see below);
Edit 20.2.2 [memory.syn] p. 1, header <memory>
synopsis as indicated (The
intent is to fix an editorial omission of the proposing paper N3195).
template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;
Edit header <valarray>
synopsis, 29.6.1 [valarray.syn] and
29.6.3.4 [valarray.special] before p. 1 as indicated
[Drafting comment: The corresponding member swap is already noexcept]:
template<class T> void swap(valarray<T>&, valarray<T>&) noexcept;
Section: 16 [library] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-18
The general approach on moving is that a library object after moving out is in a "valid but unspecified state". But this is stated at the single object specifications, which is error prone (especially if the move operations are implicit) and unnecessary duplication.
[ Resolution proposed by ballot comment ]
Consider putting a general statement to the same effect into clause 17.
[2010-11-05 Beman provides exact wording. The wording was inspired by Dave Abrahams' message c++std-lib-28958, and refined with help from Alisdair, Daniel, and Howard. ]
[2011-02-25 P/R wording superseded by N3241. ]
Proposed resolution:
Resolved by N3241
Section: 3.16 [defns.deadlock] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-52
The definition of deadlock in 17.3.7 excludes cases involving a single thread making it incorrect.
[ Resolution proposed by ballot comment ]
The definition should be corrected.
[ 2010 Batavia Concurrency group provides a Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 3.16 [defns.deadlock] as indicated:
deadlock
twoone or more threads are unable to continue execution because each is blocked waiting for one or more of the others to satisfy some condition.
Section: 99 [defns.move.assign.op] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-50
This definition of move-assignment operator is redundant and confusing now that the term move-assignment operator is defined by the core language in subclause 12.8p21.
[ 2010-10-24 Daniel adds: ]
Accepting n3142 provides a superior resolution.
[ Resolution proposed by ballot comment ]
Strike subclause 99 [defns.move.assign.op]. Add a cross-reference to (12.8) to 17.3.12.
Proposed resolution:
Resolved by paper n3142.
Section: 99 [defns.move.ctor] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-51
This definition of move-constructor is redundant and confusing now that the term constructor is defined by the core language in subclause 12.8p3.
[ 2010-10-24 Daniel adds: ]
Accepting n3142 provides a superior resolution.
[ 2010 Batavia: resolved as NAD Editorial by adopting paper n3142. ]
Original proposed resolution preserved for reference:
Strike subclause 17.3.14, [defns.move.ctor]
17.3.14 [defns.move.ctor]
move constructor a constructor which accepts only an rvalue argument of the type being constructed and might modify the argument as a side effect during construction.
Proposed resolution:
Resolved by paper n3142.
Section: 16.3.3.3.3 [bitmask.types] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-53
The bitmask types defined in 27.5.2 and 28.5 contradict the bitmask type requirements in 17.5.2.1.3, and have missing or incorrectly defined operators.
[ Resolution proposed by ballot comment ]
See Appendix 1 - Additional Details
[ 2010 - Rapperswil ]
The paper n3110 was made available during the meeting to resolve this comment, but withdrawn from formal motions to give others time to review the document. There was no technical objection, and it is expected that this paper will go forward at the next meeting.
Proposed resolution:
See n3110.
<atomic>
to free-standing implementationsSection: 16.4.2.5 [compliance] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-57
The atomic operations facility is closely tied to clause 1 and the memory model. It is not easily supplied as an after-market extension, and should be trivial to implement of a single-threaded serial machine. The consequence of not having this facility will be poor interoperability with future C++ libraries that memory model concerns seriously, and attempt to treat them in a portable way.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add <atomic>
to table 15, headers required for a
free-standing implementation.
Section: 16.4.5.9 [res.on.arguments] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-21
Priority: Not Prioritized
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Discussion:
Addresses US-82
16.4.5.9 [res.on.arguments] p.1. b.3: The second Note can benefit by adopting recent nomenclature.
[ Resolution proposed by the ballot comment: ]
Rephrase the Note in terms of xvalue.
[ Pre-Batavia: ]
Walter Brown provides wording.
[Batavia: Immediate]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Amend the note in 17.6.3.9 [res.on.arguments] p1 bullet 3.
[ Note: If a program casts an lvalue to an
rvaluexvalue while passing that lvalue to a library function (e.g. by calling the function with the argumentmove(x)
), the program is effectively asking that function to treat that lvalue as a temporary. The implementation is free to optimize away aliasing checks which might be needed if the argument was anlvalue. — end note]
offsetof
should be marked noexcept
Section: 17.2 [support.types] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-68
There is no reason for the offsetof macro to invoke potentially throwing operations, so the result of noexcept(offsetof(type,member-designator)) should be true.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add to the end of 18.2p4:
No operation invoked by the offsetof macro shall throw an exception, and
noexcept(offsetof(type,member-designator))
shall be true.
exception_ptr
is synchronizedSection: 17.9.7 [propagation] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-19
It is not clear how exception_ptr
is synchronized.
[ Resolution proposed by ballot comment ]
Make clear that accessing in different threads
multiple exception_ptr
objects that all refer to the
same exception introduce a race.
[2011-03-08: Lawrence comments and drafts wording]
I think fundamentally, this issue is NAD, but clarification would not hurt.
Proposed resolution
Add a new paragraph to 17.9.7 [propagation] after paragraph 6 as follows:
[Note: Exception objects have no synchronization requirements, and expressions using them may conflict (6.9.2 [intro.multithread]). — end note]
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
Section: 17.6.4 [alloc.errors] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-71
The thread safety of std::set_new_handler()
,
std::set_unexpected()
, std::set_terminate()
, is
unspecified making the the functions impossible to use in a thread
safe manner.
[ Resolution proposed by ballot comment: ]
The thread safety guarantees for the functions must be specified and new interfaces should be provided to make it possible to query and install handlers in a thread safe way.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of n3122 partially addresses this request. This issue is related to 1366(i).
[ 2010-Batavia: ]
Resolved by adopting n3189.
Proposed resolution:
Resolved in Batavia by accepting n3189.
Section: 17.6.3.5 [new.delete.dataraces] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-14
It is unclear how a user replacement function can
simultaneously satisfy the race-free conditions imposed in
this clause and query the new-handler in case of a failed
allocation with the only available, mutating interface
std::set_new_handler
.
[ Resolution proposed by ballot comment: ]
Offer a non-mutating interface to query the current new-handler.
[ 2010-10-24 Daniel adds: ]
Accepting n3122 would solve this issue. This issue is related to 1365(i).
[ 2010-Batavia: ]
Resolved by adopting n3189.
Proposed resolution:
Resolved in Batavia by accepting n3189.
Section: 99 [exception.unexpected] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-72
Dynamic exception specifications are deprecated, so
clause 18.8.2 that describes library support for this facility
should move to Annex D, with the exception of the
bad_exception
class which is retained to indicate other
failures in the exception dispatch mechanism (e.g. calling
current_exception()
).
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
With the exception of 18.8.2.1 [bad.exception], move clause 18.8.2 directly to Annex D. [bad.exception] should simply become the new 18.8.2.
std::uncaught_exception()
Section: 99 [depr.uncaught] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
Addresses GB-73
The thread safety std::uncaught_exception()
and the
result of the function when multiple threads throw
exceptions at the same time are unspecified. To make the
function safe to use in the presence of exceptions in
multiple threads the specification needs to be updated.
[ Resolution proposed by ballot comment ]
Update this clause to support safe calls from multiple threads without placing synchronization requirements on the user.
[ 2010 Batavia Concurrency group provides a Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 99 [uncaught] p. 1 as follows:
Returns: true
after the current thread has initialized initializing
an exception object (15.1) until a handler for the exception (including unexpected()
or terminate()
)
is activated (15.3). [ Note: This includes stack unwinding (15.2). — end note ]
throw_with_nested
should not use perfect forwardingSection: 17.9.8 [except.nested] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-84
The throw_with_nested
specification passes in its argument as
T&&
(perfect forwarding pattern), but then discusses
requirements on T
without taking into account that T
may be an lvalue-reference type. It is also not clear in the spec that
t
is intended to be perfectly forwarded.
[ Resolution proposed by ballot comment ]
Patch [except.nested] p6-7 to match the intent with regards to
requirements on T
and the use of
std::forward<T>(t)
.
[ 2010-10-24 Daniel adds: ]
Accepting n3144 would solve this issue.
[2010-11-10 Batavia: LWG accepts Howard's updated wording with corrected boo boos reported by Sebastian Gesemann and Pete Becker, which is approved for Immediate adoption this meeting.]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 18.8.7 nested_exception [except.nested] as indicated:
[[noreturn]] template <class T> void throw_with_nested(T&& t);Let
U
beremove_reference<T>::type
6 Requires:
shall be
TUCopyConstructible
.7 Throws: If
is a non-union class type not derived from
TUnested_exception
, an exception of unspecified type that is publicly derived from bothand
TUnested_exception
and constructed fromstd::forward<T>(t)
, otherwise throwsstd::forward<T>(t)
.
Section: 19.5.3.5 [syserr.errcat.objects] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-76
The C++0x FCD recommends, in a note (see 19.5.1.1/1), that users
create a single error_category
object for each user defined error
category and specifies error_category
equality comparsions based on
equality of addresses (19.5.1.3). The Draft apparently ignores this
when specifying standard error category objects in section 19.5.1.5,
by allowing the generic_category()
and system_category()
functions to return distinct objects for each invocation.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Append a new sentence to 19.5.3.5 [syserr.errcat.objects]/1, and append the same sentence to 19.5.1.5/3.
All calls of this function return references to the same object.
forward
is not compatible with access-controlSection: 22.2 [utility] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-90
In n3090, at variance with previous iterations of the idea
discussed in papers and incorporated in WDs,
std::forward
is constrained via std::is_convertible
,
thus is not robust wrt access control. This causes problems in
normal uses as implementation detail of member
functions. For example, the following snippet leads to a
compile time failure, whereas that was not the case for an
implementation along the lines of n2835 (using enable_if
s
instead of concepts for the constraining, of course)
#include <utility> struct Base { Base(Base&&); }; struct Derived : private Base { Derived(Derived&& d) : Base(std::forward<Base>(d)) { } };
In other terms, LWG 1054 can be resolved in a better way, the present status is not acceptable.
[ 2010-10-24 Daniel adds: ]
Accepting n3143 would solve this issue.
Proposed resolution:
Resolved as NAD Editorial by paper n3143.
pair
and tuple
have too many conversionsSection: 22.3 [pairs] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-15
Several function templates of pair
and tuple
allow for too
many implicit conversions, for example:
#include <tuple> std::tuple<char*> p(0); // Error? struct A { explicit A(int){} }; A a = 1; // Error std::tuple<A> ta = std::make_tuple(1); // OK?
[ Resolution proposed by ballot comment ]
Consider to add wording to constrain these function templates.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
Proposed resolution:
See n3140.
pair
copy-assignment not consistent for referencesSection: 22.3.2 [pairs.pair] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-95
Copy-assignment for pair
is defaulted and does not work
for pairs with reference members. This is inconsistent with
conversion-assignment, which deliberately succeeds even
if one or both elements are reference types, just as for
tuple
. The copy-assignment operator should be
consistent with the conversion-assignment operator and
with tuple
's assignment operators.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would provide a superior resolution, because
pair
does not depend on the semantic requirements ofCopyAssignable
.
[ 2010-11 Batavia ]
Resolved by adopting n3140.
Proposed resolution:
Add to pair
synopsis:
pair& operator=(const pair& p);
Add before paragraph 9:
pair& operator=(const pair& p);
Requires:
T1
andT2
shall satisfy the requirements ofCopyAssignable
.Effects: Assigns
p.first
tofirst
andp.second
tosecond
. Returns:*this
.
pair
and tuple
of references need to better specify move-semanticsSection: 22.3 [pairs] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-16
Several pair
and tuple
functions in regard to move
operations are incorrectly specified if the member types
are references, because the result of a std::move
cannot
be assigned to lvalue-references. In this context the usage
of the requirement sets MoveConstructible
and
CopyConstructible
also doesn't make sense, because
non-const lvalue-references cannot satisfy these requirements.
[ Resolution proposed by ballot comment ]
Replace the usage of std::move
by that of
std::forward
and replace MoveConstructible
and
CopyConstructible
requirements by other requirements.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
[ 2010-11 Batavia: ]
Resolved by adopting n3140.
Proposed resolution:
See n3140.
pair
's range support by proper range facilitySection: 99 [pair.range] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-85
While std::pair
may happen to hold a pair of iterators
forming a valid range, this is more likely a coincidence
than a feature guaranteed by the semantics of the pair
template. A distinct range-type should be supplied to
enable the new for-loop syntax rather than overloading an
existing type with a different semantic.
If a replacement facility is required for C++0x, consider n2995.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Strike 20.3.5.4 and the matching declarations in 20.3 header synopsis.
pair
and tuple
constructors should forward
argumentsSection: 22.3 [pairs] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-96
pair
and tuple
constructors and assignment operators use
std::move
when they should use std::forward
. This
causes lvalue references to be erroneously converted to
rvalue references. Related requirements clauses are also
wrong.
[ Resolution proposed by ballot comment ]
See Appendix 1 - Additional Details
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
[ 2010-11 Batavia ]
Resolved by adopting n3140.
Proposed resolution:
See n3140.
pair
and tuple
Section: 22.3 [pairs], 22.4 [tuple] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-97
pair
's class definition in N3092 22.3.2 [pairs.pair]
contains "pair(const pair&) = default;
" and
"pair& operator=(pair&& p);
". The latter is described by
22.3.2 [pairs.pair] p.12-13.
pair(const pair&) = default;
" is a user-declared explicitly defaulted
copy constructor. According to [class.copy]/10, this inhibits
the implicitly-declared move constructor. pair
should be move constructible.
( [class.copy]/7 explains that "pair(pair<U, V>&& p)
"
will never be instantiated to move pair<T1, T2>
to pair<T1, T2>
.)pair& operator=(pair&& p);
" is a user-provided move
assignment operator (according to 9.6.2 [dcl.fct.def.default]/4: "A
special member function is user-provided if it is user-declared and not explicitly defaulted
on its first declaration."). According to [class.copy]/20, this inhibits
the implicitly-declared copy assignment operator. pair
should be copy assignable, and was in C++98/03. (Again,
[class.copy]/7 explains that "operator=(const pair<U, V>& p)
"
will never be instantiated to copy pair<T1, T2>
to pair<T1, T2>
.)pair& operator=(pair&& p);
" is
unconditionally defined, whereas according to [class.copy]/25,
defaulted copy/move assignment operators are defined as
deleted in several situations, such as when non-static data
members of reference type are present.
If "pair(const pair&) = default;
" and "pair& operator=(pair&& p);
"
were removed from pair
's class definition in 22.3.2 [pairs.pair] and from
22.3.2 [pairs.pair]/12-13, pair
would
receive implicitly-declared copy/move constructors and
copy/move assignment operators, and [class.copy]/25 would
apply. The implicitly-declared copy/move constructors
would be trivial when T1
and T2
have trivial copy/move
constructors, according to [class.copy]/13, and similarly for the
assignment operators, according to [class.copy]/27. Notes could
be added as a reminder that these functions would be
implicitly-declared, but such notes would not be necessary
(the Standard Library specification already assumes a
high level of familiarity with the Core Language, and
casual readers will simply assume that pair
is copyable
and movable).
Alternatively, pair
could be given explicitly-defaulted
copy/move constructors and copy/move assignment
operators. This is a matter of style.
tuple
is also affected. tuple
's class definition in 22.4 [tuple] contains:
tuple(const tuple&) = default; tuple(tuple&&); tuple& operator=(const tuple&); tuple& operator=(tuple&&);
They should all be removed or all be explicitly-defaulted,
to be consistent with pair
. Additionally, 22.4.4.2 [tuple.cnstr]/8-9 specifies the
behavior of an explicitly defaulted function, which is currently inconsistent with
pair
.
[ Resolution proposed by ballot comment: ]
Either remove "
pair(const pair&) = default;
" and "pair& operator=(pair&& p);
" frompair
's class definition in 22.3.2 [pairs.pair] and from 22.3.2 [pairs.pair] p.12-13, or give pair explicitly-defaulted copy/move constructors and copy/move assignment operators.
Changetuple
to match.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue: The move/copy constructor will be defaulted, but the corresponding assignment operators need a non-default implementation because they are supposed to work for references as well.
Proposed resolution:
See n3140.
pack_arguments
is poorly namedSection: 22.4.5 [tuple.creation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-98
pack_arguments
is poorly named. It does not reflect the
fact that it is a tuple creation function and that it forwards
arguments.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Rename pack_arguments
to forward_as_tuple
throughout the standard.
tuple_cat
should be a single variadic signatureSection: 22.4.5 [tuple.creation] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple.creation].
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Discussion:
Addresses GB-88
The tuple_cat
template consists of four overloads and that
can concatenate only two tuple
s. A single variadic
signature that can concatenate an arbitrary number of
tuple
s would be preferred.
[ Resolution proposed by ballot comment: ]
Adopt a simplified form of the proposal in n2975, restricted to
tuple
s and neither requiring nor outlawing support for othertuple
-like types.
[ 2010 Rapperswil: Alisdair to provide wording. ]
[ 2010-11-06: Daniel comments and proposes some alternative wording: ]
There are some problems in the wording: First, even though the result type tuple<see below>
implies it, the specification of the contained tuple element types is missing. Second, the term "tuple
protocol" is not defined anywhere and I see no reason why this normative wording should not be a non-normative
note. We could at least give a better approximation, maybe "tuple-like protocol" as indicated from header
<utility>
synopsis. Further, it seems to me that the effects need to contain a combination of std::forward
with the call of get
. Finally I suggest to replace the requirements Move/CopyConstructible
by proper usage of is_constructible
, as indicated by n3140.
[ 2010 Batavia ]
Moved to Ready with Daniel's improved wording.
Proposed resolution:
Note: This alternate proposed resolution works only if 1191(i) has been accepted.
<tuple>
synopsis, as indicated:
namespace std { ... // 20.4.2.4, tuple creation functions: const unspecified ignore; template <class... Types> tuple<VTypes...> make_tuple(Types&&...); template <class... Types> tuple<ATypes...> forward_as_tuple(Types&&...); template<class... Types> tuple<Types&...> tie(Types&...);template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, const tuple<UTypes...>&); template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, const tuple<UTypes...>&); template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, tuple<UTypes...>&&); template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, tuple<UTypes...>&&);template <class... Tuples> tuple<CTypes...> tuple_cat(Tuples&&...); ...
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
8 Requires: All the types inTTypes
shall beCopyConstructible
(Table 35). All the types inUTypes
shall beCopyConstructible
(Table 35).
9 Returns: Atuple
object constructed by copy constructing its firstsizeof...(TTypes)
elements from the corresponding elements oft
and copy constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);
10 Requires: All the types inTTypes
shall beMoveConstructible
(Table 34). All the types inUTypes
shall beCopyConstructible
(Table 35).
11 Returns: Atuple
object constructed by move constructing its firstsizeof...(TTypes)
elements from the corresponding elements oft
and copy constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);
12 Requires: All the types inTTypes
shall beCopyConstructible
(Table 35). All the types inUTypes
shall beMoveConstructible
(Table 34).
13 Returns: Atuple
object constructed by copy constructing its firstsizeof...(TTypes)
elements from the corresponding elements oft
and move constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);
14 Requires: All the types inTTypes
shall beMoveConstructible
(Table 34). All the types inUTypes
shall beMoveConstructible
(Table 34).
15 Returns: Atuple
object constructed by move constructing its firstsizeof...(TTypes)
elements from the corresponding elements oft
and move constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... Tuples> tuple<CTypes...> tuple_cat(Tuples&&... tpls);8 Let
Ti
be thei
th type inTuples
,Ui
beremove_reference<Ti>::type
, andtpi
be thei
th parameter in the function parameter packtpls
, where all indexing is zero-based in the following paragraphs of this sub-clause [tuple.creation].9 Requires: For all
i
,Ui
shall be the type cvi
tuple<Argsi...>
, where cvi
is the (possibly empty)i
th cv-qualifier-seq, andArgsi
is the parameter pack representing the element types inUi
. LetAik
be theki
th type inArgsi
, then for allAik
the following requirements shall be satisfied: IfTi
is deduced as an lvalue reference type, thenis_constructible<Aik, cvi Aik&>::value == true
, otherwiseis_constructible<Aik, cvi Aik&&>::value == true
.10 Remarks: The types in
CTypes
shall be equal to the ordered sequence of the expanded typesArgs0..., Args1..., Argsn-1...
, wheren
equalssizeof...(Tuples)
. Letei...
be thei
th ordered sequence of tuple elements of the resulttuple
object corresponding to the type sequenceArgsi
.11 Returns: A
tuple
object constructed by initializing theki
th type elementeik
inei...
withget<ki>(std::forward<Ti>(tpi))
for each validki
and each element groupei
in order.12 [Note: An implementation may support additional types in the parameter pack
Tuples
, such aspair
andarray
that support thetuple
-like protocol. — end note]
pack_arguments
overly complexSection: 22.4.5 [tuple.creation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-99
pack_arguments
is overly complex.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
This issue resulted from a lack of understanding of
how references are forwarded. The definition of
pack_arguments
should be simply:
template <class... Types>
tuple<
ATypes&&>
pack_arguments(Types&&...t);
Types: Let Ti
be each type in Types
....
Effects: ...
Returns:
tuple<
ATypes&&...>(std::forward<Types>(t)...)
The synopsis should also change to reflect this simpler signature.
tuple
should be removedSection: 99 [tuple.range] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-87
There is no compelling reason to assume a
heterogeneous tuple of two elements holds a pair of
iterators forming a valid range. Unlike std::pair
, there are
no functions in the standard library using this as a return
type with a valid range, so there is even less reason to try
to adapt this type for the new for-loop syntax.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Strike 20.4.2.10 and the matching declarations in the header synopsis in 20.4.
Section: 21.4.3 [ratio.ratio] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-100
LWG 1281 was discussed in Pittsburgh, and the decision
there was to accept the typedef as proposed and move to
Review. Unfortunately the issue was accidentally applied
to the FCD, and incorrectly. The FCD version of the
typedef refers to ratio<N, D>
, but the typedef is intended
to refer to ratio<num, den>
which in general is not the
same type.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Accept the current proposed wording of LWG 1281(i) which adds:
typedef ratio<num, den> type;
Section: 21.4.4 [ratio.arithmetic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-89
The alias representations of the ratio
arithmetic templates
do not allow implementations to avoid overflow, since they
explicitly specify the form of the aliased template
instantiation. For example
ratio_multiply
, ratio<2, LLONG_MAX>
is required to
alias ratio<2*LLONG_MAX, LLONG_MAX*2>
, which
overflows, so is ill-formed. However, this is trivially equal
to ratio<1, 1>
. It also contradicts the opening statement of
21.4.4 [ratio.arithmetic] p. 1 "implementations may use other algorithms to
compute these values".
[ 2010-10-25 Daniel adds: ]
Accepting n3131 would solve this issue.
[Batavia: Resolved by accepting n3210.]
Proposed resolution:
Change the wording in 21.4.4 [ratio.arithmetic] p. 2-5 as follows:
template <class R1, class R2> using ratio_add = see below;2 The type
ratio_add<R1, R2>
shall be a synonym forratio<T1, T2>
ratio<U, V>
such thatratio<U, V>::num
andratio<U, V>::den
are the same as the corresponding members ofratio<T1, T2>
would be in the absence of arithmetic overflow whereT1
has the valueR1::num * R2::den + R2::num * R1::den
andT2
has the valueR1::den * R2::den
. If the required values ofratio<U, V>::num
andratio<U, V>::den
cannot be represented inintmax_t
then the program is ill-formed.
template <class R1, class R2> using ratio_subtract = see below;3 The type
ratio_subtract<R1, R2>
shall be a synonym forratio<T1, T2>
ratio<U, V>
such thatratio<U, V>::num
andratio<U, V>::den
are the same as the corresponding members ofratio<T1, T2>
would be in the absence of arithmetic overflow whereT1
has the valueR1::num * R2::den - R2::num * R1::den
andT2
has the valueR1::den * R2::den
. If the required values ofratio<U, V>::num
andratio<U, V>::den
cannot be represented inintmax_t
then the program is ill-formed.
template <class R1, class R2> using ratio_multiply = see below;4 The type
ratio_multiply<R1, R2>
shall be a synonym forratio<T1, T2>
ratio<U, V>
such thatratio<U, V>::num
andratio<U, V>::den
are the same as the corresponding members ofratio<T1, T2>
would be in the absence of arithmetic overflow whereT1
has the valueR1::num * R2::num
andT2
has the valueR1::den * R2::den
. If the required values ofratio<U, V>::num
andratio<U, V>::den
cannot be represented inintmax_t
then the program is ill-formed.
template <class R1, class R2> using ratio_divide = see below;5 The type
ratio_divide<R1, R2>
shall be a synonym forratio<T1, T2>
ratio<U, V>
such thatratio<U, V>::num
andratio<U, V>::den
are the same as the corresponding members ofratio<T1, T2>
would be in the absence of arithmetic overflow whereT1
has the valueR1::num * R2::den
andT2
has the valueR1::den * R2::num
. If the required values ofratio<U, V>::num
andratio<U, V>::den
cannot be represented inintmax_t
then the program is ill-formed.
Section: 21 [meta] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-17
Speculative compilation for std::is_constructible
and
std::is_convertible
should be limited, similar to the core
language (see 14.8.2 paragraph 8).
[ 2010-10-24 Daniel adds: ]
Accepting n3142 would solve this issue.
Proposed resolution:
Resolved by paper n3142.
Section: 21 [meta] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-18
Several type traits require compiler support, e.g.
std::is_constructible
or std::is_convertible
.
Their current specification seems to imply, that the corresponding
test expressions should be well-formed, even in absense of access:
class X { X(int){} }; constexpr bool test = std::is_constructible<X, int>::value;
The specification does not clarify the context of this test and because it already goes beyond normal language rules, it's hard to argue by means of normal language rules what the context and outcome of the test should be.
[ Resolution proposed by ballot comment ]
Specify that std::is_constructible
and
std::is_convertible
will return true
only for
public constructors/conversion functions.
[ 2010-10-24 Daniel adds: ]
Accepting n3142 would solve this issue.
Proposed resolution:
Resolved by paper n3142.
result_of
should support pointer-to-data-memberSection: 21.3.5 [meta.unary] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-102
Despite Library Issue 520's ("result_of
and pointers to
data members") resolution of CD1, the FCD's result_of
supports neither pointers to member functions nor
pointers to data members. It should.
[ Resolution proposed by ballot comment ]
Ensure result_of
supports pointers to member
functions and pointers to data members.
[ 2010-10-24 Daniel adds: ]
Accepting n3123 would solve this issue.
Proposed resolution:
Resolved by n3123.
noexcept
Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-92
Trivial functions implicitly declare a noexcept
exception
specification, so the references to has_trivial_*
traits in the
has_nothrow_*
traits are redundant, and should be struck for clarity.
[ Resolution proposed by ballot comment ]
For each of the has_nothrow_something
traits,
remove all references to the matching has_trivial_something
traits.
[ 2010-10-24 Daniel adds: ]
Accepting n3142 would solve this issue.
Proposed resolution:
Resolved by n3142.
is_constructible
reports false positivesSection: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-19
The fundamental trait is_constructible
reports false
positives, e.g.
is_constructible<char*, void*>::value
evaluates to true, even though a corresponding variable initialization would be ill-formed.
[ Resolved in Rapperswil by paper N3047. ]
Proposed resolution:
Remove all false positives from the domain of is_constructible
.
Section: 22.10 [function.objects] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-95
The adaptable function protocol supported by
unary_function/binary_function
has been superceded by
lambda expressions and std::bind
. Despite the name, the
protocol is not very adaptable as it requires intrusive
support in the adaptable types, rather than offering an
external traits-like adaption mechanism. This protocol and
related support functions should be deprecated, and we
should not make onerous requirements for the
specification to support this protocol for callable types
introduced in this standard revision, including those
adopted from TR1. It is expected that high-quality
implementations will provide such support, but we should
not have to write robust standard specifications mixing this
restricted support with more general components such as
function
, bind
and reference_wrapper
.
[ Resolution proposed by ballot comment ]
Move clauses 20.8.3, 20.8.9, 20.8.11 and 20.8.12 to Annex D.
Remove the requirements to conditionally derive fromunary/binary_function
from function
,
reference_wrapper
, and the results of calling mem_fn
and bind
.
[ 2010-10-24 Daniel adds: ]
Accepting n3145 would solve this issue.
Proposed resolution:
Resolved by paper N3198.
function
does not need an explicit
default constructorSection: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP-3
explicit
default contructor is defined in std::function
.
Although it is allowed according to 12.3.1, it seems
unnecessary to qualify the constructor as explicit
.
If it is explicit
, there will be a limitation in initializer_list
.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Remove explicit
.
namespace std { template<class> class function; // undefined template<class R, class... ArgTypes> class function<R(ArgTypes...)> : public unary_function<T1, R> // iff sizeof...(ArgTypes) == 1 and ArgTypes contains T1 : public binary_function<T1, T2, R> // iff sizeof...(ArgTypes) == 2 and ArgTypes contains T1 andT2 { public:typedef R result_type; // 20.8.14.2.1, construct/copy/destroy:explicitfunction();
function
does not need an explicit
default constructorSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP-4
Really does the function
require that default constructor is explicit
?
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Remove explicit
.
function(); template <class A> function(allocator_arg_t, const A& a);
unique_ptr<T> == nullptr
Section: 20.2 [memory] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-99
One reason that the unique_ptr
constructor taking a
nullptr_t
argument is not explicit
is to allow conversion
of nullptr
to unique_ptr
in contexts like equality
comparison. Unfortunately operator==
for unique_ptr
is a
little more clever than that, deducing template parameters for both
arguments. This means that nullptr
does not get deduced
as unique_ptr
type, and there are no other comparison
functions to match.
[ Resolution proposed by ballot comment: ]
Add the following signatures to 20.2 [memory] p.1,
<memory>
header synopsis:template<typename T, typename D> bool operator==(const unique_ptr<T, D> & lhs, nullptr_t); template<typename T, typename D> bool operator==(nullptr_t, const unique_ptr<T, D> & rhs); template<typename T, typename D> bool operator!=(const unique_ptr<T, D> & lhs, nullptr_t); template<typename T, typename D> bool operator!=(nullptr_t, const unique_ptr<T, D> & rhs);
[ 2010-11-02 Daniel comments and provides a proposed resolution: ]
The same problem applies to
The number of overloads triple the current number, but I think it is much clearer to provide them explicitly instead of adding wording that attempts to say that "sufficient overloads" are provided. The following proposal makes the declarations explicit. Additionally, the proposal adds the missing declarations for someshared_ptr
as well: In both cases there are no conversions considered because the comparison functions are templates. I agree with the direction of the proposed resolution, but I believe it would be very surprising and inconsistent, if given a smart pointer objectp
, the expressionp == nullptr
would be provided, but notp < nullptr
and the other relational operators. According to 7.6.9 [expr.rel] they are defined if null pointer values meet other pointer values, even though the result is unspecified for all except some trivial ones. But null pointer values are nothing special here: The Library already defines the relational operators for bothunique_ptr
andshared_ptr
and the outcome of comparing non-null pointer values will be equally unspecified. If the idea of supportingnullptr_t
arguments for relational operators is not what the committee prefers, I suggest at least to consider to remove the existing relational operators for bothunique_ptr
andshared_ptr
for consistency. But that would not be my preferred resolution of this issue.shared_ptr
comparison functions for consistency.
[ 2010-11-03 Daniel adds: ]
Issue 1297(i) is remotely related. The following proposed resolution splits this bullet into sub-bullets A and B. Sub-bullet A would also solve 1297(i), but sub-bullet B would not.
A further remark in regard to the proposed semantics of the ordering ofnullptr
against other pointer(-like) values: One might think that the following definition might
be superior because of simplicity:
template<class T> bool operator<(const shared_ptr<T>& a, nullptr_t); template<class T> bool operator>(nullptr_t, const shared_ptr<T>& a);Returns:
false
.
The underlying idea behind this approach is the assumption that nullptr corresponds to the least ordinal pointer value. But this assertion does not hold for all supported architectures, therefore this approach was not followed because it would lead to the inconsistency, that the following assertion could fire:
shared_ptr<int> p(new int); shared_ptr<int> null; bool v1 = p < nullptr; bool v2 = p < null; assert(v1 == v2);
[2011-03-06: Daniel comments]
The current issue state is not acceptable, because the Batavia meeting did not give advice whether choice A or B of bullet 3 should be applied. Option B will now be removed and if this resolution is accepted, issue 1297(i) should be declared as resolved by 1401(i). This update also resyncs the wording with N3242.
Proposed resolution:
Wording changes are against N3242.
<memory>
synopsis as indicated.
noexcept
specifications are only added, where the guarantee exists, that the function
shall no throw an exception (as replacement of "Throws: Nothing". Note that
the noexcept
additions to the shared_ptr
comparisons are editorial, because
they are already part of the accepted paper n3195:
namespace std { […] // [unique.ptr] Class unique_ptr: template <class T> class default_delete; template <class T> class default_delete<T[]>; template <class T, class D = default_delete<T>> class unique_ptr; template <class T, class D> class unique_ptr<T[], D>; template <class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T, class D> bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept; template <class T, class D> bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept; template <class T, class D> bool operator<(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator<=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<=(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& x); // [util.smartptr.weakptr], Class bad_weak_ptr: class bad_weak_ptr; // [util.smartptr.shared], Class template shared_ptr: template<class T> class shared_ptr; // [util.smartptr.shared.cmp], shared_ptr comparisons: template<class T, class U> bool operator==(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator!=(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator<(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator>(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator<=(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator>=(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T> bool operator==(shared_ptr<T> const& a, nullptr_t) noexcept; template<class T> bool operator==(nullptr_t, shared_ptr<T> const& a) noexcept; template<class T> bool operator!=(shared_ptr<T> const& a, nullptr_t) noexcept; template<class T> bool operator!=(nullptr_t, shared_ptr<T> const& a) noexcept; template<class T> bool operator<(shared_ptr<T> const& a, nullptr_t) noexcept; template<class T> bool operator<(nullptr_t, shared_ptr<T> const& a) noexcept; template>class T> bool operator>(shared_ptr<T> const& a, nullptr_t) noexcept; template>class T> bool operator>(nullptr_t, shared_ptr<T> const& a) noexcept; template<class T> bool operator<=(shared_ptr<T> const& a, nullptr_t) noexcept; template<class T> bool operator<=(nullptr_t, shared_ptr<T> const& a) noexcept; template>class T> bool operator>=(shared_ptr<T> const& a, nullptr_t) noexcept; template>class T> bool operator>=(nullptr_t, shared_ptr<T> const& a) noexcept; […] }
namespace std { […] template <class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T, class D> bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept; template <class T, class D> bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept; template <class T, class D> bool operator<(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator<=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<=(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& x); }
Change 20.3.1.6 [unique.ptr.special] p. 4-7 as indicated and add a series of prototype descriptions:
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);? Requires: Let
CT
becommon_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type
. Then the specializationless<CT>
shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.4 Returns:
less<CT>()(x.get(), y.get())
.x.get() < y.get()? Remarks: If
unique_ptr<T1, D1>::pointer
is not implicitly convertible toCT
orunique_ptr<T2, D2>::pointer
is not implicitly convertible toCT
, the program is ill-formed.template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);5 Returns:
!(y < x)
.x.get() <= y.get()template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);6 Returns:
(y < x)
.x.get() > y.get()template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);7 Returns:
!(x < y)
.x.get() >= y.get()
template <class T, class D> bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;? Returns:
!x
.
template <class T, class D> bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;? Returns:
(bool) x
.
template <class T, class D> bool operator<(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>(nullptr_t, const unique_ptr<T, D>& x);? Requires: The specialization
less<unique_ptr<T, D>::pointer>
shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.? Returns:
less<unique_ptr<T, D>::pointer>()(x.get(), nullptr)
.
template <class T, class D> bool operator<(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>(const unique_ptr<T, D>& x, nullptr_t);? Requires: The specialization
less<unique_ptr<T, D>::pointer>
shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.? Returns:
less<unique_ptr<T, D>::pointer>()(nullptr, x.get())
.
template <class T, class D> bool operator<=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& x);? Returns:
!(nullptr < x)
.
template <class T, class D> bool operator<=(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>=(const unique_ptr<T, D>& x, nullptr_t);? Returns:
!(x < nullptr)
.
Change 20.3.2.2 [util.smartptr.shared] p. 1, class template shared_ptr
synopsis as indicated. For consistency reasons the remaining normal relation
operators are added as well:
namespace std { […] // [util.smartptr.shared.cmp], shared_ptr comparisons: template<class T, class U> bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator<=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator>=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T> bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator==(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator!=(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator<(nullptr_t, const shared_ptr<T>& a) noexcept; template>class T> bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept; template>class T> bool operator>(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator<=(nullptr_t, const shared_ptr<T>& a) noexcept; template>class T> bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept; template>class T> bool operator>=(nullptr_t, const shared_ptr<T>& a) noexcept; […] }
template<class T> bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator==(nullptr_t, const shared_ptr<T>& a) noexcept;? Returns:
!a
.
template<class T> bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator!=(nullptr_t, const shared_ptr<T>& a) noexcept;? Returns:
(bool) a
.
template<class T> bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator>(nullptr_t, const shared_ptr<T>& a) noexcept;? Returns:
less<T*>()(a.get(), nullptr)
.
template<class T> bool operator<(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept;? Returns:
less<T*>()(nullptr, a.get())
.
template<class T> bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator>=(nullptr_t, const shared_ptr<T>& a) noexcept;? Returns:
!(nullptr < a)
.
template<class T> bool operator<=(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept;? Returns:
!(a < nullptr)
.
nullptr
constructors for smart pointers should be constexpr
Section: 20.2 [memory] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [memory].
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Discussion:
Addresses GB-100
The unique_ptr
and shared_ptr
constructors taking
nullptr_t
delegate to a constexpr
constructor, and could be
constexpr
themselves.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
In the 20.3.1.3 [unique.ptr.single] synopsis add
"constexpr" to unique_ptr(nullptr_t)
.
In the 20.3.1.4 [unique.ptr.runtime] synopsis add
"constexpr" to unique_ptr(nullptr_t)
.
In the 20.3.2.2 [util.smartptr.shared] synopsis
add "constexpr" to shared_ptr(nullptr_t)
.
allocator_arg
Section: 20.2.7 [allocator.tag] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-85
There are inconsistent definitions for allocator_arg
.
In 20.2 [memory] paragraph 1,
constexpr allocator_arg_t allocator_arg = allocator_arg_t();
and in 20.9.1,
const allocator_arg_t allocator_arg = allocator_arg_t();
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Change "const" to "constexpr" in 20.9.1 as follows.
constexpr allocator_arg_t allocator_arg = allocator_arg_t();
pointer_traits
should have a size_type
memberSection: 20.2.3 [pointer.traits] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [pointer.traits].
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Discussion:
Addresses US-106
pointer_traits
should have a size_type
member for completeness.
Add typedef see below size_type;
to the generic
pointer_traits
template and typedef size_t
size_type;
to pointer_traits<T*>
. Use
pointer_traits::size_type
and
pointer_traits::difference_type
as the defaults for
allocator_traits::size_type
and
allocator_traits::difference_type
.
See Appendix 1 - Additional Details
[ Post-Rapperswil, Pablo provided wording: ]
The original ballot comment reads simply: "pointer_traits
should have a
size_type
for completeness." The additional details reveal, however,
that the desire for a size_type
is actually driven by the needs
of allocator_traits
. The allocator_traits
template should get its
default difference_type
from pointer_traits
but if it did,
it should get its size_type
from the same source. Unfortunately,
there is no obvious meaning for size_type
in pointer_traits
.
Alisdair suggested, however, that the natural relationship between
difference_type
and size_type
can be expressed simply by the
std::make_unsigned<T>
metafunction. Using this metafunction,
we can easily define size_type
for allocator_traits
without
artificially adding size_type
to pointer_traits
.
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
In [allocator.requirements], Table 42, change two rows as follows:
X::size_type
unsigned integral type a type that can represent the size of the largest object in the allocation model
size_tmake_unsigned<X::difference_type>::typeX::difference_type
signed integral type a type that can represent the difference between any two pointers in the allocation model
ptrdiff_tpointer_traits<X::pointer>::difference_type
In [allocator.traits.types], Change the definition of difference_type
and
size_type
as follows:
typedef
see belowdifference_type;
Type:
Alloc::difference_type
if such a type exists, else.
ptrdiff_tpointer_traits<pointer>::difference_type
typedef
see belowsize_type;
Type:
Alloc::size_type
if such a type exists, else.
size_tmake_unsigned<difference_type>::type
scoped_allocator_adaptor
into separate headerSection: 20.6 [allocator.adaptor] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.adaptor].
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Discussion:
Addresses US-107
scoped_allocator_adaptor
should have its own header.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
See Appendix 1 - Additional Details
shared_ptr
constructors taking movable typesSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with Resolved status.
Discussion:
Addresses US-108
shared_ptr
should have the same policy for constructing
from auto_ptr
as unique_ptr
. Currently it does not.
[ Resolved in Rapperswil by paper N3109. ]
Proposed resolution:
Add
template <class Y> explicit shared_ptr(auto_ptr<Y>&);
to [util.smartptr.shared.const] (and to the synopsis).
undeclare_no_pointers
Section: 99 [util.dynamic.safety] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.dynamic.safety].
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Discussion:
Addresses GB-103
The precondition to calling declare_no_pointers
is that no
bytes in the range "have been previously registered" with
this call. As written, this precondition includes bytes in
ranges, even after they have been explicitly unregistered
with a later call to undeclare_no_pointers
.
Proposed resolution:
Update 99 [util.dynamic.safety] p.9:
void declare_no_pointers(char *p, size_t n);
9
Requires: No bytes in the specified rangehave been previously registeredare currently registered withdeclare_no_pointers()
. If the specified range is in an allocated object, then it must be entirely within a single allocated object. The object must be live until the correspondingundeclare_no_pointers()
call. [..]
monotonic_clock
is a distinct type or a typedefSection: 99 [time.clock.monotonic] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.monotonic].
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Discussion:
Addresses US-111
What it means for monotonic_clock
to be a synonym is
undefined. If it may or may not be a typedef, then certain
classes of programs become unportable.
[ Resolution proposed in ballot comment: ]
Require that it be a distinct class type.
[ 2010-11-01 Daniel comments: ]
Paper n3128 addresses this issue by replacing
monotonic_clock
withsteady_clock
, which is not a typedef.
Proposed resolution:
This is resolved by n3191.
monotonic_clock
Section: 99 [time.clock.monotonic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.monotonic].
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Duplicate of: 1411
Discussion:
Addresses GB-107, DE-20
4.1 [intro.compliance] p.9 states that which conditionally
supported constructs are available should be provided in the
documentation for the implementation. This doesn't help programmers trying
to write portable code, as they must then rely on
implementation-specific means to determine the
availability of such constructs. In particular, the presence
or absence of std::chrono::monotonic_clock
may require
different code paths to be selected. This is the only
conditionally-supported library facility, and differs from the
conditionally-supported language facilities in that it has
standard-defined semantics rather than implementation-defined
semantics.
[ Resolution proposed in ballot comment: ]
Provide feature test macro for determining the
presence of std::chrono::monotonic_clock
. Add
_STDCPP_HAS_MONOTONIC_CLOCK
to the
<chrono>
header, which is defined if
monotonic_clock
is present, and not defined if it is
not present.
[ 2010-11-01 Daniel comments: ]
Paper n3128 addresses this issue by replacing
monotonic_clock
withsteady_clock
, which is not conditionally supported, so there is no need to detect it.
Proposed resolution:
This is resolved by n3191.
Section: 99 [time.clock.monotonic] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.monotonic].
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Discussion:
Addresses CH-21
Monotonic clocks are generally easy to provide on all systems and are implicitely required by some of the library facilities anyway.
[ 2010-11-01 Daniel comments: ]
Paper n3128 addresses this issue by replacing
monotonic_clock
withsteady_clock
, which is mandatory.
[ 2010-11-13 Batavia meeting: ]
This is resolved by adopting n3191. The original resolution is preserved for reference:
Make monotonic clocks mandatory.
Strike 99 [time.clock.monotonic] p.2
2
The classmonotonic_clock
is conditionally supported.Change 32.2.4 [thread.req.timing] p.2 accordingly
The member functions whose names end in
_for
take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions.[ Note: Implementations are not required to use a monotonic clock because such a clock may not be available. — end note ]
Proposed resolution:
This is resolved by n3191.
POS_T
and OFF_T
Section: 27.2.4.4 [char.traits.specializations.char16.t] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [char.traits.specializations.char16.t].
View all issues with C++11 status.
Duplicate of: 1444
Discussion:
Addresses GB-109, GB-123
It is not clear what the specification means for
u16streampos
, u32streampos
or wstreampos
when they
refer to the requirements for POS_T
in 21.2.2, as there
are no longer any such requirements. Similarly the annex
D.7 refers to the requirements of type POS_T
in 27.3 that
no longer exist either.
Clarify the meaning of all cross-reference to the
removed type POS_T
.
[ Post-Rapperswil, Daniel provides the wording. ]
When preparing the wording for this issue I first thought about adding both u16streampos
and u32streampos
to the [iostream.forward] header <iosfwd>
synopsis similar to streampos
and wstreampos
,
but decided not to do so, because the IO library does not yet actively support the char16_t
and char32_t
character types. Adding those would misleadingly imply that they would be part of the iostreams. Also, the addition
would make them also similarly equal to a typedef to fpos<mbstate_t>
, as for streampos
and
wstreampos
, so there is no loss for users that would like to use the proper fpos
instantiation for
these character types.
Additionally the way of referencing was chosen to follow the style suggested by NB comment GB 108.
Moved to Tentatively Ready with proposed wording after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The following wording changes are against N3126.
Change [char.traits.specializations.char16_t]p.1 as indicated:
1 - The type
u16streampos
shall be an implementation-defined type that satisfies the requirements forPOS_T
in 21.2.2pos_type
in [iostreams.limits.pos].
Change [char.traits.specializations.char32_t]p.1 as indicated:
1 - The type
u32streampos
shall be an implementation-defined type that satisfies the requirements forPOS_T
in 21.2.2pos_type
in [iostreams.limits.pos].
Change [char.traits.specializations.wchar.t]p.2 as indicated:
2 - The type
wstreampos
shall be an implementation-defined type that satisfies the requirements forPOS_T
in 21.2.2pos_type
in [iostreams.limits.pos].
Change [fpos.operations], Table 124 — Position type requirements as indicated:
Table 124 — Position type requirements Expression Return type ...
...
...
...
O(p)
OFF_T
streamoff
... ...
...
...
o = p - q
OFF_T
streamoff
...
streamsize(o)
O(sz)
streamsize
OFF_T
streamoff
...
Change [depr.ios.members]p.1 as indicated:
namespace std { class ios_base { public: typedef T1 io_state; typedef T2 open_mode; typedef T3 seek_dir; typedefOFF_Timplementation-defined streamoff; typedefPOS_Timplementation-defined streampos; // remainder unchanged }; }
Change [depr.ios.members]p.5+6 as indicated:
5 - The type
streamoff
is an implementation-defined type that satisfies the requirements oftypeOFF_T
(27.5.1)off_type
in [iostreams.limits.pos].6 - The type
streampos
is an implementation-defined type that satisfies the requirements oftypePOS_T
(27.3)pos_type
in [iostreams.limits.pos].
forward_list::erase_after
should not be allowed to throwSection: 23.2 [container.requirements] Status: C++11 Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
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Discussion:
Addresses DE-21
23.2.1/11 provides a general no-throw guarantee for erase() container functions, exceptions from this are explicitly mentioned for individual containers. Because of its different name, forward_list's erase_after() function is not ruled by this but should so.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add a "Throws: Nothing" clause to both
erase_after
overloads in 23.3.3.4, [forwardlist.modifiers].
front/back
on a zero-sized array
should be undefinedSection: 23.3.3.5 [array.zero] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [array.zero].
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Discussion:
Addresses GB-112
Should the effect of calling front/back
on a zero-sized
array
really be implementation defined i.e. require the
implementor to define behaviour?
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Change "implementation defined" to "undefined"
resize(size())
on a deque
Section: 23.3.5.3 [deque.capacity] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2017-03-21
Priority: Not Prioritized
View all other issues in [deque.capacity].
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Discussion:
Addresses GB-113
There is no mention of what happens if sz==size()
. While
it obviously does nothing I feel a standard needs to say
this explicitely.
[ 2010 Batavia ]
Accepted with a simplified resolution turning one of the <
comparisons into <=.
Proposed resolution:
Ammend [deque.capacity]
void resize(size_type sz);
Effects: If sz <= size()
, equivalent to erase(begin() +
sz, end());
. If size() < sz
, appends sz - size()
default
constructedvalue initialized elements to the sequence.
resize(size())
on a list
Section: 23.3.11.3 [list.capacity] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-115
There is no mention of what happens if sz==size()
. While
it obviously does nothing I feel a standard needs to say
this explicitely.
[ Resolution proposed in ballot comment ]
Express the semantics as pseudo-code similarly
to the way it is done for the copying overload that
follows (in p3). Include an else clause that does
nothing and covers the sz==size()
case.
[ 2010 Batavia ]
Accepted with a simplified resolution turning one of the <
comparisons into <=.
Proposed resolution:
Ammend [list.capacity] p1:
void resize(size_type sz);
Effects: If
sz <= size()
, equivalent tolist<T>::iterator it = begin(); advance(it, sz); erase(it, end());
. Ifsize() < sz
, appendssz - size()
default constructedvalue initialized elements to the sequence.
Section: 23.6 [container.adaptors] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.adaptors].
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Duplicate of: 1350
Discussion:
Addresses DE-22, CH-15
With the final acceptance of move operations as special
members and introduction of corresponding suppression
rules of implicitly generated copy operations the some
library types that were copyable in C++03 are no longer
copyable (only movable) in C++03, among them queue
,
priority_queue
, and stack
.
[ 2010-10-26: Daniel comments: ]
Accepting n3112 should fix this.
[2011-02-17: Lawrence comments:]
The only open issue in CH 15 with respect to the concurrency group
was the treatment of atomic_future
. Since we removed atomic_future
in Batavia, I think all that remains is to remove the open issue from
N3112 and adopt it.
[2011-03-23 Madrid meeting]
Resolved by N3264
Proposed resolution:
See n3112
map
constructor accepting an allocator as single parameter should be explicitSection: 23.4.3 [map] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP-6
Constructor accepting an allocator as a single parameter should be qualified as explicit.
namespace std { template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > class map { public: ... map(const Allocator&);
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > class map { public: ... explicit map(const Allocator&);
multimap
constructor accepting an allocator as a single parameter should be explicitSection: 23.4.4 [multimap] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-7
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > class multimap { public: ... explicit multimap(const Allocator&);
set
constructor accepting an allocator as a single parameter should be explicitSection: 23.4.6 [set] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [set].
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Discussion:
Addresses JP-8
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class Compare = less<Key>, class Allocator = allocator<Key> > class set { public: ... explicit set(const Allocator&);
multiset
constructor accepting an allocator as a single parameter should be explicitSection: 23.4.7 [multiset] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP-9
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class Compare = less<Key>, class Allocator = allocator<Key> > class multiset { public: ... explicit multiset(const Allocator&);
unordered_map
constructor accepting an allocator as a single parameter should be explicitSection: 23.5.3 [unord.map] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord.map].
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Discussion:
Addresses JP-10
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T> > > class unordered_map { public: ... explicit unordered_map(const Allocator&);
unordered_multimap
constructor accepting an allocator as a single parameter should be explicitSection: 23.5.4 [unord.multimap] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-11
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T> > > class unordered_multimap { public: ... explicit unordered_multimap(const Allocator&);
unordered_set
constructor accepting an allocator as a single parameter should be explicitSection: 23.5.6 [unord.set] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-12
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > class unordered_set { public: ... explicit unordered_set(const Allocator&);
unordered_multiset
constructor accepting an allocator as a single parameter should be explicitSection: 23.5.7 [unord.multiset] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-13
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > class unordered_multiset { public: ... explicit unordered_multiset(const Allocator&);
is_permutation
must be more restrictiveSection: 26.6.14 [alg.is.permutation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
Addresses US-120
is_permutation
is underspecified for anything but the
simple case where both ranges have the same value type
and the comparison function is an equivalence relation.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Restrict is_permutation
to the case where it is well
specified. See Appendix 1 - Additional Details
random_shuffle
signatures are inconsistentSection: 26.7.13 [alg.random.shuffle] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.random.shuffle].
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Duplicate of: 1433
Discussion:
Addresses US-121, GB-119
random_shuffle
and shuffle
should be consistent in how
they accept their source of randomness: either both by rvalue reference or
both by lvalue reference.
[ Post-Rapperswil, Daniel provided wording ]
The signatures of the shuffle
and random_shuffle
algorithms are different
in regard to the support of rvalues and lvalues of the provided generator:
template<class RandomAccessIterator, class RandomNumberGenerator> void random_shuffle(RandomAccessIterator first, RandomAccessIterator last, RandomNumberGenerator&& rand);
template<class RandomAccessIterator, class UniformRandomNumberGenerator> void shuffle(RandomAccessIterator first, RandomAccessIterator last, UniformRandomNumberGenerator& g);
The first form uses the perfect forwarding signature and that change compared to
C++03
was done intentionally as shown in the first rvalue proposal
papers.
While it is true, that random generators are excellent examples of stateful functors, there still exist good reasons to support rvalues as arguments:
std::generate
and std::generate_n
)
accept both rvalues and lvalues as well.
C++03
. In the specific
cases, where rvalues are provided, the argument will be accepted instead of being rejected.
Arguments have been raised that accepting rvalues is error-prone or even fundamentally wrong. The author of this proposal disagrees with that position for two additional reasons:
instead of writingmy_generator get_generator(int size);
they will just writestd::vector<int> v = ...; std::shuffle(v.begin(), v.end(), get_generator(v.size()));
and feel annoyed about the need for it.std::vector<int> v = ...; auto gen = get_generator(v.size()); std::shuffle(v.begin(), v.end(), gen);
CopyConstructible
and this is obviously a generally useful property for such objects. It is also useful and sometimes necessary to start a
generator with exactly a specific seed again and again and thus to provide a new generator (or a copy) for each call. The
CopyConstructible
requirements allow providing rvalues of generators and thus this idiom must be useful as well.
Therefore preventing [random_]shuffle
to accept rvalues is an unnecessary restriction which doesn't prevent any
user-error, if there would exist one.
Thus this proposal recommends to make both shuffle
functions consistent and perfectly forward-able.
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
<algorithm>
synopsis as indicated:
template<class RandomAccessIterator, class UniformRandomNumberGenerator> void shuffle(RandomAccessIterator first, RandomAccessIterator last, UniformRandomNumberGenerator&& rand);
template<class RandomAccessIterator, class UniformRandomNumberGenerator> void shuffle(RandomAccessIterator first, RandomAccessIterator last, UniformRandomNumberGenerator&& rand);
Section: 29.4.7 [complex.value.ops] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [complex.value.ops].
View all issues with C++11 status.
Discussion:
Addresses GB-120
The complex number functions added for compatibility with the C99 standard library are defined purely as a cross-reference, with no hint of what they should return. This is distinct from the style of documentation for the functions in the earlier standard. In the case of the inverse-trigonometric and hyperbolic functions, a reasonable guess of the functionality may be made from the name, this is not true of the cproj function, which apparently returns the projection on the Reimann Sphere. A single line description of each function, associated with the cross-reference, will greatly improve clarity.
[2010-11-06 Beman provides proposed resolution wording.]
[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 26.4.7 complex value operations [complex.value.ops] as indicated:
template<class T> complex<T> proj(const complex<T>& x);
Returns: the projection of
x
onto the Riemann sphere.
Effects:Remarks: Behaves the same as the C functioncproj
, defined in 7.3.9.4.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> acos(const complex<T>& x);
Returns: the complex arc cosine of
x
.
Effects:Remarks: Behaves the same as the C functioncacos
, defined in 7.3.5.1.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> asin(const complex<T>& x);
Returns: the complex arc sine of
x
.
Effects:Remarks: Behaves the same as the C functioncasin
, defined in 7.3.5.2.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> atan(const complex<T>& x);
Returns: the complex arc tangent of
x
.
Effects:Remarks: Behaves the same as the C functioncatan
, defined in 7.3.5.3.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> acosh(const complex<T>& x);
Returns: the complex arc hyperbolic cosine of
x
.
Effects:Remarks: Behaves the same as the C functioncacosh
, defined in 7.3.6.1.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> asinh(const complex<T>& x);
Returns: the complex arc hyperbolic sine of
x
.
Effects:Remarks: Behaves the same as the C functioncasinh
, defined in 7.3.6.2.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> atanh(const complex<T>& x);
Returns: the complex arc hyperbolic tangent of
x
.
Effects:Remarks: Behaves the same as the C functioncatanh
, defined in 7.3.6.2.
Section: 29.5.4 [rand.eng] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.eng].
View all issues with C++11 status.
Discussion:
Addresses GB-121
All the random number engine types in this clause have a constructor taking an unsigned integer type, and a constructor template for seed sequences. This means that an attempt to create a random number engine seeded by an integer literal must remember to add the appropriate unsigned suffix to the literal, as a signed integer will attempt to use the seed sequence template, yielding undefined behaviour, as per 26.5.1.1p1a. It would be helpful if at least these anticipated cases produced a defined behaviour, either an erroneous program with diagnostic, or a conversion to unsigned int forwarding to the appropriate constructor.
[ 2010-11-03 Daniel comments and provides a proposed resolution: ]
I suggest to apply a similar solution as recently suggested for 1234(i). It is basically a requirement for an implementation to constrain the template.
[
2010-11-04 Howard suggests to use !is_convertible<Sseq, result_type>::value
as minimum requirement instead of the originally proposed !is_scalar<Sseq>::value
.
This would allow for a user-defined type BigNum
, that is convertible to result_type
,
to be used as argument for a seed instead of a seed sequence. The wording has been updated to
reflect this suggestion.
]
[ 2010 Batavia: There were some initial concerns regarding the portability and reproducibility of results when seeded with a negative signed value, but these concerns were allayed after discussion. Thus, after reviewing the issue, the working group concurred with the issue's Proposed Resolution. ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Add the following paragraph at the end of 29.5.4 [rand.eng]:
5 Each template specified in this section [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.
? For every random number engine and for every random number engine adaptor
X
defined in this sub-clause [rand.eng] and in sub-clause [rand.adapt]:
- If the constructor
template<class Sseq> explicit X(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution.- If the member function
template<class Sseq> void seed(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this function shall not participate in overload resolution.The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as seed sequence, if it is implicitly convertible to
X::result_type
.
Section: 29.5.4.3 [rand.eng.mers] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.eng.mers].
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Discussion:
Addresses US-124
The Mersenne twister algorithm is meaningless for word sizes less than two, as there are then insufficient bits available to be “twisted”.
[ Resolution proposed by ballot comment: ]
Insert the following among the relations that are required to hold:
2u < w
.
[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 29.5.4.3 [rand.eng.mers] p. 4 as indicated:
4 The following relations shall hold:
0 < m
,m <= n
,2u < w
,r <= w
,u <= w
,s <= w
,t <= w
,l <= w
,w <= numeric_limits<UIntType>::digits
,a <= (1u<<w) - 1u
,b <= (1u<<w) - 1u
,c <= (1u<<w) - 1u
,d <= (1u<<w) - 1u
, andf <= (1u<<w) - 1u
.
base()
Section: 29.5.5.2 [rand.adapt.disc] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.adapt.disc].
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Discussion:
Addresses US-126
Each adaptor has a member function called base()
which has no definition.
[ Resolution proposed by ballot comment: ]
Give it the obvious definition.
[ 2010-11-03 Daniel comments and provides a proposed resolution: ]
The following proposal adds noexcept
specifiers to the declarations of
the base()
functions as replacement for a "Throws: Nothing" element.
[ 2010 Batavia: The working group reviewed this issue, and recommended to add the following to the Proposed Resolution. ]
a.base()
shall be valid and shall return a const reference to a
's base engine.
After further review, the working group concurred with the Proposed Resolution.
[Batavia: waiting for WEB to review wording]
Proposed resolution:
A random number engine adaptor (commonly shortened to adaptor)
a
of typeA
is a random number engine that takes values produced by some other random number engine, and applies an algorithm to those values in order to deliver a sequence of values with different randomness properties. An engineb
of typeB
adapted in this way is termed a base engine in this context. The expressiona.base()
shall be valid and shall return a const reference toa
's base engine.
discard_block_engine
synopsis, the following declaration:
// property functions const Engine& base() const noexcept;
const Engine& base() const noexcept;? Returns:
e
.
independent_bits_engine
synopsis, the following declaration:
// property functions const Engine& base() const noexcept;
const Engine& base() const noexcept;? Returns:
e
.
shuffle_order_engine
synopsis, the following declaration:
// property functions const Engine& base() const noexcept;
const Engine& base() const noexcept;? Returns:
e
.
densities()
functions?Section: 29.5.9.6.2 [rand.dist.samp.pconst] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.pconst].
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Discussion:
Addresses US-134
These two distributions have a member function called
densities()
which returns a vector<double>
. The
distribution is templated on RealType
. The distribution
also has another member called intervals()
which returns
a vector<RealType>
. Why doesn't densities return
vector<RealType>
as well? If RealType
is long double
,
the computed densities property isn't being computed to
the precision the client desires. If RealType
is float
, the
densities vector is taking up twice as much space as the client desires.
[ Resolution proposed by ballot comment: ]
Change the piecewise constant and linear distributions to hold / return the densities in a
If this is not done, at least correct 29.5.9.6.2 [rand.dist.samp.pconst] p. 13 which describes the return of densities as avector<result_type>
.vector<result_type>
.
[ Batavia 2010: After reviewing this issue, the working group concurred with the first of the suggestions proposed by the NB comment: "Change the piecewise constant and linear distributions to hold/return the densities in a vector. " ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
piecewise_constant_distribution
synopsis
and the prototype description 29.5.9.6.2 [rand.dist.samp.pconst] before p. 13 as indicated:
vector<doubleresult_type> densities() const;
piecewise_linear_distribution
synopsis
and the prototype description 29.5.9.6.3 [rand.dist.samp.plinear] before p. 13 as indicated:
vector<doubleresult_type> densities() const;
piecewise_linear_distribution
Section: 29.5.9.6.3 [rand.dist.samp.plinear] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses US-135
This paragraph says: Let bk = xmin+k·δ for k = 0,...,n, and wk = fw(bk +δ) for k = 0,...,n. However I believe that fw(bk) would be far more desirable. I strongly suspect that this is nothing but a type-o.
[ Resolution proposed by ballot comment: ]
Change p. 10 to read:
Let bk = xmin+k·δ for k = 0,...,n, and wk = fw(bk) for k = 0,...,n.
[ 2010-11-02 Daniel translates into a proposed resolution ]
[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 29.5.9.6.3 [rand.dist.samp.plinear] p. 10 as indicated:
10 Effects: Constructs a
piecewise_linear_distribution
object with parameters taken or calculated from the following values: Letbk = xmin+k·δ
fork = 0, . . . , n
, andwk = fw(bk
for+δ)k = 0, . . . , n
.
Section: 29.7 [c.math] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [c.math].
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Discussion:
Addresses US-136
Floating-point test functions are incorrectly specified.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
See Appendix 1 - Additional Details
Section: 31.7 [iostream.format] Status: Resolved Submitter: INCITS/PJ Plauger Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iostream.format].
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Discussion:
Addresses US-137
Several iostreams member functions are incorrectly specified.
[ Resolution proposed by ballot comment: ]
See Appendix 1 - Additional Details
[ 2010-10-24 Daniel adds: ]
Accepting n3168 would solve this issue.
Proposed resolution:
Addressed by paper n3168.
Section: 31.7 [iostream.format] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iostream.format].
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Discussion:
Addresses US-139
Resolve issue LWG 1328 one way or the other, but
preferably in the direction outlined in the proposed
resolution, which, however, is not complete as-is: in any
case, the sentry must not set ok_ = false
if is.good() == false
,
otherwise istream::seekg
, being an unformatted
input function, does not take any action because the
sentry object returns false when converted to type bool
.
Thus, it remains impossible to seek away from end of file.
Proposed resolution:
Addressed by paper n3168.
basic_stringbuf::str(basic_string)
postconditionsSection: 31.8.2.4 [stringbuf.members] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [stringbuf.members].
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Discussion:
Addresses GB-124
N3092 31.8.2.4 [stringbuf.members] contains this text specifying the postconditions of
basic_stringbuf::str(basic_string)
:
Postconditions: If
mode & ios_base::out
istrue
,pbase()
points to the first underlying character andepptr() >= pbase() + s.size()
holds; in addition, ifmode & ios_base::in
istrue
,pptr() == pbase() + s.data()
holds, otherwisepptr() == pbase()
istrue
. [...]
Firstly, there's a simple mistake: It should be pbase() + s.length()
,
not pbase() + s.data()
.
Secondly, it doesn't match existing implementations. As far as I can tell,
GCC 4.5 does not test for mode & ios_base::in
in the second part
of that sentence, but for mode & (ios_base::app | ios_base_ate)
,
and Visual C++ 9 for mode & ios_base::app
. Besides, the wording of
the C++0x draft doesn't make any sense to me. I suggest changing the second part
of the sentence to one of the following:
Replace ios_base::in
with (ios_base::ate | ios_base::app)
,
but this would require Visual C++ to change (replacing only with
ios_base::ate
would require GCC to change, and would make
ios_base::app
completely useless with stringstreams
):
in addition, if mode & (ios_base::ate | ios_base::app)
is true
,
pptr() == pbase() + s.length()
holds, otherwise pptr() == pbase()
is true
.
Leave pptr()
unspecified if mode & ios_base::app
, but not
mode & ios_base::ate
(implementations already differ in this case, and it
is always possible to use ios_base::ate
to get the effect of appending, so it
is not necessary to require any implementation to change):
in addition, if mode & ios_base::ate
is true
,
pptr() == pbase() + s.length()
holds, if neither mode & ios_base::ate
nor mode & ios_base::app
is true
, pptr() == pbase()
holds,
otherwise pptr() >= pbase() && pptr() <= pbase() + s.length()
(which of the values in this range is unspecified).
Slightly stricter:
in addition, if mode & ios_base::ate
is true
,
pptr() == pbase() + s.length()
holds, if neither
mode & ios_base::ate
nor mode & ios_base::app
is true
,
pptr() == pbase()
holds, otherwise pptr() == pbase() || pptr() == pbase() + s.length()
(which of these two values is unspecified). A small table might help to better explain the three cases.
BTW, at the end of the postconditions is this text: "egptr() == eback() + s.size()
hold".
Is there a perference for basic_string::length
or basic_string::size
? It doesn't really
matter, but it looks a bit inconsistent.
[2011-03-09: Nicolai Josuttis comments and drafts wording]
First, it seems the current wording is just an editorial mistake. When we added issue 432(i) to the draft standard (in n1733), the wording in the issue:
If
mode & ios_base::out
is true, initializes the output sequence such thatpbase()
points to the first underlying character,epptr()
points one past the last underlying character, and if(mode & ios_base::ate)
is true,pptr()
is set equal toepptr()
elsepptr()
is set equal topbase()
.
became:
If
mode & ios_base::out
is true, initializes the output sequence such thatpbase()
points to the first underlying character,epptr()
points one past the last underlying character, andpptr()
is equal toepptr()
ifmode & ios_base::in
is true, otherwisepptr()
is equal topbase()
.
which beside some changes of the order of words changed
ios_base::ate
into
ios_base::in
So, from this point of view, clearly mode & ios_base::ate
was meant.
Nevertheless, with this proposed resolution we'd have no wording regarding ios_base::app
.
Currently the only statements about app
in the Standard are just in two tables:
openmode
effects" says that the effect of
app
is "seek to end before each write"
app
is "a"
Indeed we seem to have different behavior currently in respect to app
: For
stringstream s2(ios_base::out|ios_base::in|ios_base::app); s2.str("s2 hello"); s1 << "more";
"moreello"
)"s2 hellomore"
)BTW, for fstreams, both implementations append when app
is set:
If f2.txt
has contents "xy"
,
fstream f2("f2.txt",ios_base::out|ios_base::in|ios_base::app); f1 << "more";
appends "more"
so that the contents is "xymore"
.
So IMO app
should set the write pointer to the end so that each writing
appends.
str()
of stringbuffer.
Nevertheless, it doesn't hurt IMO if we clarify the behavior of str()
here in respect to app
.
[2011-03-10: P.J.Plauger comments:]
I think we should say nothing special about app
at construction
time (thus leaving the write pointer at the beginning of the buffer).
Leave implementers wiggle room to ensure subsequent append writes as they see
fit, but don't change existing rules for initial seek position.
[Madrid meeting: It was observed that a different issue should be opened that
clarifies the meaning of app
for stringstream
]
Proposed resolution:
Change 31.8.2.4 [stringbuf.members] p. 3 as indicated:
void str(const basic_string<charT,traits,Allocator>& s);2 Effects: Copies the content of
3 Postconditions: Ifs
into thebasic_stringbuf
underlying character sequence and initializes the input and output sequences according tomode
.mode & ios_base::out
is true,pbase()
points to the first underlying character andepptr() >= pbase() + s.size()
holds; in addition, ifmode &
is true,ios_base::inios_base::atepptr() == pbase() +
holds, otherwises.data()s.size()pptr() == pbase()
is true. Ifmode & ios_base::in
is true,eback()
points to the first underlying character, and bothgptr() == eback() and egptr() == eback() + s.size()
hold.
<cinttypes>
Section: 31.8.3 [istringstream] Status: C++11 Submitter: Canada Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses CA-4
Subclause 27.9.2 [c.files] specifies that <cinttypes> has declarations for abs() and div(); however, the signatures are not present in this subclause. The signatures proposed under TR1 ([tr.c99.inttypes]) are not present in FCD (unless if intmax_t happened to be long long). It is unclear as to which, if any of the abs() and div() functions in [c.math] are meant to be declared by <cinttypes>. This subclause mentions imaxabs() and imaxdiv(). These functions, among other things, are not specified in FCD to be the functions from Subclause 7.8 of the C Standard. Finally, <cinttypes> is not specified in FCD to include <cstdint> (whereas <inttypes.h> includes <stdint.h> in C).
[ Post-Rapperswil, Daniel provides wording ]
Subclause [c.files] specifies that <cinttypes>
has declarations for abs()
and div()
;
however, the signatures are not present in this subclause. The signatures proposed under TR1 ([tr.c99.inttypes]) are not
present in FCD (unless if intmax_t
happened to be long long
). It is unclear as to which, if any of the
abs()
and div()
functions in [c.math] are meant to be declared by <cinttypes>
. This
subclause mentions imaxabs()
and imaxdiv()
. These functions, among other things, are not specified in
FCD to be the functions from subclause 7.8 of the C
Standard. Finally, <cinttypes>
is not specified
in FCD to include <cstdint>
(whereas <inttypes.h>
includes <stdint.h>
in C
).
Moved to Tentatively Ready with proposed wording after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
Table 132 describes header
2 - The contents of header<cinttypes>
. [Note: The macros defined by<cinttypes>
are provided unconditionally. In particular, the symbol__STDC_FORMAT_MACROS
, mentioned in footnote 182 of theC
standard, plays no role inC++
. — end note ]<cinttypes>
are the same as the StandardC
library header<inttypes.h>
, with the following changes: 3 - The header<cinttypes>
includes the header<cstdint>
instead of<stdint.h>
. 4 - If and only if the typeintmax_t
designates an extended integer type ([basic.fundamental]), the following function signatures are added:intmax_t abs(intmax_t); imaxdiv_t div(intmax_t, intmax_t);which shall have the same semantics as the function signatures
intmax_t imaxabs(intmax_t)
andimaxdiv_t imaxdiv(intmax_t, intmax_t)
, respectively.
regex_constants
Section: 28.6.4.3 [re.matchflag] Status: C++14 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: 3
View all other issues in [re.matchflag].
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Discussion:
Addresses GB-127
The Bitmask Type requirements in 16.3.3.3.3 [bitmask.types] p.3 say that
all elements on a bitmask type have distinct values, but
28.6.4.3 [re.matchflag] defines regex_constants::match_default
and
regex_constants::format_default
as elements of the
bitmask type regex_constants::match_flag_type
, both with
value 0. This is a contradiction.
[ Resolution proposed by ballot comment: ]
One of the bitmask elements should be removed from the declaration and should be defined separately, in the same manner as
ios_base::adjustfield
,ios_base::basefield
andios_base::floatfield
are defined by [ios::fmtflags] p.2 and Table 120. These are constants of a bitmask type, but are not distinct elements, they have more than one value set in the bitmask.regex_constants::format_default
should be specified as a constant with the same value asregex_constants::match_default
.
[ 2010-10-31 Daniel comments: ]
Strictly speaking, a bitmask type cannot have any element of value 0 at all, because any such value would contradict the requirement expressed in 16.3.3.3.3 [bitmask.types] p. 3:
for any pair Ci and Cj, Ci & Ci is nonzero
So, actually both regex_constants::match_default
and
regex_constants::format_default
are only constants of the type
regex_constants::match_flag_type
, and no bitmask elements.
[ 2010-11-03 Daniel comments and provides a proposed resolution: ]
The proposed resolution is written against N3126 and considered as a further improvement of the fixes suggested by n3110.
Add the following sentence to 28.6.4.3 [re.matchflag] paragraph 1:
1 The type
regex_constants::match_flag_type
is an implementation-defined bitmask type (17.5.2.1.3). Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set. Typeregex_constants::match_flag_type
also defines the constantsregex_constants::match_default
andregex_constants::format_default
.
[ 2011 Bloomington ]
It appears the key problem is the phrasing of the bitmask requirements. Jeremiah supplies updated wording.
Pete Becker has also provided an alternative resolution.
Ammend 16.3.3.3.3 [bitmask.types]:
Change the list of values for "enum bit mask" in p2 from
V0 = 1 << 0, V1 = 1 << 1, V2 = 1 << 2, V3 = 1 << 3, ...
.
to
V0 = 0, V1 = 1 << 0, V2 = 1 << 1, V3 = 1 << 2, ...
.
Here, the names C0, C1, etc. represent bitmask elements for this particular
bitmask type. All such non-zero elements have distinct values such that, for any pair
Ci and Cj where i != j, Ci & Ci is nonzero
and Ci & Cj is zero.
Change bullet 3 of paragraph 4:
TheA non-zero value Y is set in the object X if the expression X & Y is nonzero.
[2014-02-13 Issaquah:]
Proposed resolution:
Ammend 16.3.3.3.3 [bitmask.types] p3:
Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such elements have distinct, nonzero values such that, for any pair Ci and Cj where i != j, Ci & Ci is nonzero and Ci & Cj is zero. Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.
Add the following sentence to 28.6.4.3 [re.matchflag] paragraph 1:
1 The type
regex_constants::match_flag_type
is an implementation-defined bitmask type (17.5.2.1.3). The constants of that type, except formatch_default
andformat_default
, are bitmask elements. Thematch_default
andformat_default
constants are empty bitmasks. Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set.
match_results
behavior for certain operations Section: 28.6.9.5 [re.results.acc] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-126
It's unclear how match_results should behave if it has been default-constructed. The sub_match objects returned by operator[], prefix and suffix cannot point to the end of the sequence that was searched if no search was done. The iterators held by unmatched sub_match objects might be singular.
[ Resolution proposed by ballot comment: ]
Add to match_results::operator[], match_results::prefix and match_results::suffix:
Requires: !empty()
[ 2010-10-24 Daniel adds: ]
Accepting n3158 would solve this issue.
Proposed resolution:
Addressed by paper n3158.
Section: 32.5 [atomics] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1454
Discussion:
Addresses CH-22, GB-128
WG14 currently plans to introduce atomic facilities that are intended to be compatible with the facilities of clause 29. They should be compatible.
[ Resolution proposed by ballot comment ]
Make sure the headers in clause 29 are defined in a way that is compatible with the planned C standard.
[ 2010 Batavia ]
Resolved by adoption of n3193.
Proposed resolution:
Solved by n3193.
Section: 32.5.2 [atomics.syn] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-130
The synopsis for the <atomic>
header lists the macros
ATOMIC_INTEGRAL_LOCK_FREE
and ATOMIC_ADDRESS_LOCK_FREE
.
The ATOMIC_INTEGRAL_LOCK_FREE
macro has been replaced with a set of macros
for each integral type, as listed in 32.5.5 [atomics.lockfree].
[Proposed resolution as of comment]
Against FCD, N3092:
In [atomics.syn], header
<atomic>
synopsis replace as indicated:// 29.4, lock-free property#define ATOMIC_INTEGRAL_LOCK_FREE unspecified#define ATOMIC_CHAR_LOCK_FREE implementation-defined #define ATOMIC_CHAR16_T_LOCK_FREE implementation-defined #define ATOMIC_CHAR32_T_LOCK_FREE implementation-defined #define ATOMIC_WCHAR_T_LOCK_FREE implementation-defined #define ATOMIC_SHORT_LOCK_FREE implementation-defined #define ATOMIC_INT_LOCK_FREE implementation-defined #define ATOMIC_LONG_LOCK_FREE implementation-defined #define ATOMIC_LLONG_LOCK_FREE implementation-defined #define ATOMIC_ADDRESS_LOCK_FREE unspecified
[ 2010-10-26: Daniel adds: ]
The proposed resolution below is against the FCD working draft. After application of the editorial issues US-144 and US-146 the remaining difference against the working draft is the usage of implementation-defined instead of unspecified, effectively resulting in this delta:
// 29.4, lock-free property #define ATOMIC_CHAR_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_CHAR16_T_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_CHAR32_T_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_WCHAR_T_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_SHORT_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_INT_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_LONG_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_LLONG_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_ADDRESS_LOCK_FREE unspecified
It is my understanding that the intended wording should be unspecified as for ATOMIC_ADDRESS_LOCK_FREE
but if this is right, we need to use the same wording in 32.5.5 [atomics.lockfree], which consequently uses
the term implementation-defined. I recommend to keep 32.5.2 [atomics.syn] as it currently is and to
fix 32.5.5 [atomics.lockfree] instead as indicated:
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
[2011-02-20: Daniel adapts the proposed wording to N3225 and fixes an editorial omission of applying N3193]
[2011-03-06: Daniel adapts the wording to N3242]
[Proposed Resolution]
Change 32.5.5 [atomics.lockfree] as indicated:
#define ATOMIC_CHAR_LOCK_FREEimplementation-definedunspecified #define ATOMIC_CHAR16_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_CHAR32_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_WCHAR_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_SHORT_LOCK_FREEimplementation-definedunspecified #define ATOMIC_INT_LOCK_FREEimplementation-definedunspecified #define ATOMIC_LONG_LOCK_FREEimplementation-definedunspecified #define ATOMIC_LLONG_LOCK_FREEimplementation-definedunspecified
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
bool
should be addedSection: 32.5.5 [atomics.lockfree] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-154
There is no ATOMIC_BOOL_LOCK_FREE
macro.
Proposed resolution suggested by the NB comment:
Add ATOMIC_BOOL_LOCK_FREE
to 32.5.5 [atomics.lockfree] and to 32.5.2 [atomics.syn]:
[..] #define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified [..]
[2011-03-12: Lawrence comments and drafts wording]
Point: We were missing a macro test for bool
.
Comment: The atomic<bool>
type is the easiest to make lock-free.
There is no harm in providing a macro.
Action: Add an ATOMIC_BOOL_LOCK_FREE
.
Point: We were missing a macro test for pointers.
Comment: The national body comment noting the missing macro
for bool
did not note the lack of a macro for pointers because
ATOMIC_ADDRESS_LOCK_FREE
was present at the time of the comment.
Its removal appears to be an overzealous consequence of removing
atomic_address
.
Action: Add an ATOMIC_POINTER_LOCK_FREE
.
Point: Presumably atomic_is_lock_free()
will be an inline function
producing a constant in those cases in which the macros are useful.
Comment: The point is technically correct, but could use some exposition. Systems providing forward binary compatibility, e.g. mainstream desktop and server systems, would likely have these functions as inline constants only when the answer is true. Otherwise, the function should defer to a platform-specific dynamic library to take advantage of future systems that do provide lock-free support.
Comment: Such functions are not useful in the preprocessor, and not
portably useful in static_assert
.
Action: Preserve the macros.
Point: The required explicit instantiations are atomic<X>
for each
of the types X
in Table 145. Table 145 does not list bool
, so
atomic<bool>
is not a required instantiation.
Comment: I think specialization was intended in the point instead of
instantiation. In any event, 32.5.8 [atomics.types.generic] paragraph 5 does
indirectly require a specialization for atomic<bool>
. Confusion
arises because the specialization for other integral types have a
wider interface than the generic atomic<T>
, but
atomic<bool>
does not.
Action: Add clarifying text.
Point: The name of Table 145, "atomic integral typedefs", is perhaps misleading, since the types listed do not contain all of the "integral" types.
Comment: Granted, though the table describe those with extra operations.
Action: Leave the text as is.
Point: The macros correspond to the types in Table 145, "with the
signed and unsigned variants grouped together". That's a rather
inartful way of saying that ATOMIC_SHORT_LOCK_FREE
applies to
signed short
and unsigned short
. Presumably this also means that
ATOMIC_CHAR_LOCK_FREE
applies to all three char types.
Comment: Yes, it is inartful.
Comment: Adding additional macros to distinguish signed and unsigned would provide no real additional information given the other constraints in the language.
Comment: Yes, it applies to all three char
types.
Action: Leave the text as is.
Point: The standard says that "There are full specializations over the
integral types (char
, signed char
, ...)" bool
is not in the list. But this text is not normative. It simply tells you
that "there are" specializations, not "there shall be" specializations, which would
impose a requirement. The requirement, to the extent that there is
one, is in the header synopsis, which, in N3242, sort of pulls in the
list of types in Table 145.
Comment: The intent was for the specializations to be normative. Otherwise the extra member functions could not be present.
Action: Clarify the text.
[Proposed Resolution]
Edit header
<atomic>
synopsis 32.5.2 [atomics.syn]:namespace std { // 29.3, order and consistency enum memory_order; template <class T> T kill_dependency(T y); // 29.4, lock-free property #define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_POINTER_LOCK_FREE unspecified // 29.5, generic types template<class T> struct atomic; template<> struct atomic<integral>; template<class T> struct atomic<T*>; // 29.6.1, general operations on atomic types // In the following declarations, atomic_type is either // atomic<T> or a named base class for T from // Table 145 or inferred from // Table 146 or from bool. […] }Edit the synopsis of 32.5.5 [atomics.lockfree] and paragraph 1 as follows:
#define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREEimplementation-definedunspecified #define ATOMIC_CHAR16_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_CHAR32_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_WCHAR_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_SHORT_LOCK_FREEimplementation-definedunspecified #define ATOMIC_INT_LOCK_FREEimplementation-definedunspecified #define ATOMIC_LONG_LOCK_FREEimplementation-definedunspecified #define ATOMIC_LLONG_LOCK_FREEimplementation-definedunspecified #define ATOMIC_POINTER_LOCK_FREE unspecified1 The
ATOMIC_…_LOCK_FREE
macros indicate the lock-free property of the corresponding atomic types, with the signed and unsigned variants grouped together. The properties also apply to the corresponding (partial) specializations of theatomic
template. A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.Edit 32.5.8 [atomics.types.generic] paragraph 3, 4, and 6-8 as follows:
2 The semantics of the operations on specializations of
3 Specializations and instantiations of theatomic
are defined in 32.5.8.2 [atomics.types.operations].atomic
template shall have a deleted copy constructor, a deleted copy assignment operator, and aconstexpr
value constructor. 4 Thereareshall be full specializationsoverfor the integral types(char
,signed char
,unsigned char
,short
,unsigned short
,int
,unsigned int
,long
,unsigned long
,long long
,unsigned long long
,char16_t
,char32_t
, andwchar_t
, and any other types needed by the typedefs in the header<cstdint>
)on theatomic
class template. For each integral type integral, the specializationatomic<integral>
provides additional atomic operations appropriate to integral types.[Editor's note: I'm guessing that this is the correct rendering of the text in the paper; if this sentence was intended to impose a requirement, rather than a description, it will have to be changed.]There shall be a specializationatomic<bool>
which provides the general atomic operations as specified in [atomics.types.operations.general]. 5 The atomic integral specializations and the specializationatomic<bool>
shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax. 6 Thereareshall be pointer partial specializationsonof theatomic
class template. These specializations shall have trivial default constructors and trivial destructors. 7 Thereareshall be named types corresponding to the integral specializations ofatomic
, as specified in Table 145. In addition, there shall be named typeatomic_bool
corresponding to the specializationatomic<bool>
. Each named type is either a typedef to the corresponding specialization or a base class of the corresponding specialization. If it is a base class, it shall support the same member functions as the corresponding specialization. 8 Thereareshall be atomic typedefs corresponding to the typedefs in the header<inttypes.h>
as specified in Table 146.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
atomic_bool
Section: 99 [atomics.types.integral] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1463
Discussion:
Addresses GB-132, US-157
The atomic_itype
types and atomic_address
have two
overloads of operator=
; one is volatile
qualified, and the
other is not. atomic_bool
only has the volatile
qualified
version:
bool operator=(bool) volatile;
On a non-volatile
-qualified object this is ambiguous with
the deleted copy-assignment operator
atomic_bool& operator=(atomic_bool const&) = delete;
due to the need for a single standard conversion in each
case when assigning a bool to an atomic_bool
as in:
atomic_bool b; b = true;
The conversions are:
atomic_bool& → atomic_bool volatile&
vs
bool → atomic_bool
[ Proposed resolution as of NB comment: ]
Change 99 [atomics.types.integral] as indicated:
namespace std { typedef struct atomic_bool { [..] bool operator=(bool) volatile; bool operator=(bool); } atomic_bool; [..] }
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue by replacing
atomic_bool
byatomic<bool>
.
[ 2010 Batavia ]
Resolved by adoption of n3193.
Proposed resolution:
Solved by n3193.
Section: 99 [atomics.types.integral] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-160
The last sentence of 99 [atomics.types.integral] p.1 says:
Table 143 shows typedefs to atomic integral classes and the corresponding
<cstdint>
typedefs.
That's nice, but nothing says these are supposed to be part of the implementation, and they are not listed in the synopsis.
[ Proposed resolution as of NB comment ]
1 The name
atomic_itype
and the functions operating on it in the preceding synopsis are placeholders for a set of classes and functions. Throughout the preceding synopsis,atomic_itype
should be replaced by each of the class names in Table 142 and integral should be replaced by the integral type corresponding to the class name.Table 143 shows typedefs to atomic integral classes and the corresponding<cstdint>
typedefs.
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue.
[ 2010-11 Batavia ]
Resolved by adopting n3193.
Proposed resolution:
Solved by n3193.
atomic_address
Section: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-161
atomic_address
has operator+=
and operator-=
, but no
operator++
or operator--
. The template specialization
atomic<Ty*>
has all of them.
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue by replacing
atomic_address
byatomic<void*>
.
[ Resolved in Batavia by accepting n3193. ]
Proposed resolution:
Change 99 [atomics.types.address], class atomic_address
synopsis, as indicated:
namespace std { typedef struct atomic_address { […] void* operator=(const void*) volatile; void* operator=(const void*); void* operator++(int) volatile; void* operator++(int); void* operator--(int) volatile; void* operator--(int); void* operator++() volatile; void* operator++(); void* operator--() volatile; void* operator--(); void* operator+=(ptrdiff_t) volatile; void* operator+=(ptrdiff_t); void* operator-=(ptrdiff_t) volatile; void* operator-=(ptrdiff_t); } atomic_address; […] }
const
breakage by compare_exchange_*
member functionsSection: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-162
The compare_exchange_weak
and compare_exchange_strong
member functions that take
const void*
arguments lead to a silent removal of const
, because the load
member function and other acessors return the stored value as a void*
.
[ Proposed resolution as of NB comment: ]
Change 99 [atomics.types.address], class atomic_address
synopsis, as indicated:
namespace std { typedef struct atomic_address { [..]bool compare_exchange_weak(const void*&, const void*, memory_order, memory_order) volatile;bool compare_exchange_weak(const void*&, const void*, memory_order, memory_order);bool compare_exchange_strong(const void*&, const void*, memory_order, memory_order) volatile;bool compare_exchange_strong(const void*&, const void*, memory_order, memory_order);bool compare_exchange_weak(const void*&, const void*, memory_order = memory_order_seq_cst) volatile;bool compare_exchange_weak(const void*&, const void*, memory_order = memory_order_seq_cst);bool compare_exchange_strong(const void*&, const void*, memory_order = memory_order_seq_cst) volatile;bool compare_exchange_strong(const void*&, const void*, memory_order = memory_order_seq_cst);[..] } atomic_address; [..] }
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue by replacing
atomic_address
byatomic<void*>
.
[ Resolved in Batavia by accepting n3193. ]
Proposed resolution:
Solved by n3193.
atomic<T*>
from atomic_address
breaks type safetySection: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-163
Requiring atomic<T*>
to be derived from atomic_address
breaks type safety:
atomic<double*> ip; char ch; atomic_store(&ip, &ch); *ip.load() = 3.14159;
The last line overwrites ch
with a value of type double
.
[ 2010-10-27 Daniel adds: ]
Resolving this issue will also solve 1469(i)
Accepting n3164 would solve this issue by removingatomic_address
.
[ Resolved in Batavia by accepting n3193. ]
Proposed resolution:
atomic<T*>
synopsis, as indicated:
namespace std { template <class T> struct atomic<T*>: atomic_address{ [..] }; [..] }
4 There are pointer partial specializations on the
atomic
class template.These specializations shall be publicly derived fromThe unit of addition/subtraction for these specializations shall be the size of the referenced type. These specializations shall have trivial default constructors and trivial destructors.atomic_address
.
atomic_address::compare_exchange_*
member functions should match atomic_compare_exchange_*
free functionsSection: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-164
atomic_address
has member functions compare_exchange_weak
and
compare_exchange_strong
that take arguments of type const void*
,
in addition to the void*
versions. If these member functions survive,
there should be corresponding free functions.
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue differently by removing the overloads with
const void*
arguments, because they break type-safety.
[ Resolved in Batavia by accepting n3193. ]
Proposed resolution:
Extend the synopsis around atomic_address
in 99 [atomics.types.address]
as indicated:
namespace std { [..] bool atomic_compare_exchange_weak(volatile atomic_address*, void**, void*); bool atomic_compare_exchange_weak(atomic_address*, void**, void*); bool atomic_compare_exchange_strong(volatile atomic_address*, void**, void*); bool atomic_compare_exchange_strong(atomic_address*, void**, void*); bool atomic_compare_exchange_weak_explicit(volatile atomic_address*, void**, void*, memory_order, memory_order); bool atomic_compare_exchange_weak_explicit(atomic_address*, void**, void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, void**, void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(atomic_address*, void**, void*, memory_order, memory_order); bool atomic_compare_exchange_weak(volatile atomic_address*, const void**, const void*); bool atomic_compare_exchange_weak(atomic_address*, const void**, const void*); bool atomic_compare_exchange_strong(volatile atomic_address*, const void**, const void*); bool atomic_compare_exchange_strong(atomic_address*, const void**, const void*); bool atomic_compare_exchange_weak_explicit(volatile atomic_address*, const void**, const void*, memory_order, memory_order); bool atomic_compare_exchange_weak_explicit(atomic_address*, const void**, const void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, const void**, const void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, const void**, const void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(atomic_address*, const void**, const void*, memory_order, memory_order); [..] }
atomic<T*>
inheritance from atomic_address
breaks type safetySection: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-133
The free functions that operate on atomic_address
can be
used to store a pointer to an unrelated type in an atomic<T*>
without a cast. e.g.
int i; atomic<int*> ai(&i); string s; atomic_store(&ai,&s);
Overload the atomic_store
, atomic_exchange
and
atomic_compare_exchange_[weak/strong]
operations for
atomic<T*>
to allow storing only pointers to T
.
[ 2010-10-27 Daniel adds: ]
Resolving this issue will also solve 1467(i)
Accepting n3164 would solve this issue by removingatomic_address
.
[Resolved in Batavia by accepting n3193. ]
Proposed resolution:
Add the following overloads to 32.5.8 [atomics.types.generic], the synopsis around the specialization
atomic<T*>
, as indicated:
namespace std { [..] template <class T> struct atomic<T*> : atomic_address { [..] }; template<typename T> void atomic_store(atomic<T*>&,T*); template<typename T> void atomic_store(atomic<T*>&,void*) = delete; template<typename T> void atomic_store_explicit(atomic<T*>&,T*,memory_order); template<typename T> void atomic_store_explicit(atomic<T*>&,void*,memory_order) = delete; template<typename T> T* atomic_exchange(atomic<T*>&,T*); template<typename T> T* atomic_exchange(atomic<T*>&,void*) = delete; template<typename T> T* atomic_exchange_explicit(atomic<T*>&,T*,memory_order); template<typename T> T* atomic_exchange_explicit(atomic<T*>&,void*,memory_order) = delete; template<typename T> T* atomic_compare_exchange_weak(atomic<T*>&,T**,T*); template<typename T> T* atomic_compare_exchange_weak(atomic<T*>&,void**,void*) = delete; template<typename T> T* atomic_compare_exchange_weak_explicit(atomic<T*>&,T**,T*,memory_order); template<typename T> T* atomic_compare_exchange_weak_explicit(atomic<T*>&,void**,void*,memory_order) = delete; template<typename T> T* atomic_compare_exchange_strong(atomic<T*>&,T**,T*); template<typename T> T* atomic_compare_exchange_strong(atomic<T*>&,void**,void*) = delete; template<typename T> T* atomic_compare_exchange_strong_explicit(atomic<T*>&,T**,T*,memory_order); template<typename T> T* atomic_compare_exchange_strong_explicit(atomic<T*>&,void**,void*,memory_order) = delete; }
Section: 32.5.8.2 [atomics.types.operations] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-06-15
Priority: Not Prioritized
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Duplicate of: 1470, 1475, 1476, 1477
Discussion:
Addresses US-175, US-165, CH-23, GB-135
99 [atomics.types.operations.req] p. 25: The first sentence is grammatically incorrect.
[ 2010-10-28 Daniel adds: ]
Duplicate issue 1475(i) also has a proposed resolution, but both issues are resolved with below proposed resolution.
[ 2011-02-15 Howard fixes numbering, Hans improves the wording ]
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 6 votes.
Proposed resolution:
Change 99 [atomics.types.operations.req] p. 23 as indicated:
[ Note: For example, t
The effect ofthe compare-and-exchange operationsatomic_compare_exchange_strong
isif (memcmp(object, expected, sizeof(*object)) == 0) memcpy(object, &desired, sizeof(*object)); else memcpy(expected, object, sizeof(*object));— end note ] [..]
Change 99 [atomics.types.operations.req] p. 25 as indicated:
25 Remark:
When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable. — end note ]The weak compare-and-exchange operations may fail spuriously, that is, return false while leaving the contents of memory pointed to byA weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to byexpected
before the operation is the same that same as that of theobject
and the same as that ofexpected
after the operationexpected
andobject
are equal, it may return false and store back toexpected
the same memory contents that were originally there.. [ Note: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., loadlocked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.
Section: 32.5.8.2 [atomics.types.operations] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-136
GB requests normative clarification in 32.5.8.2 [atomics.types.operations] p.4 that concurrent access constitutes a race, as already done on p.6 and p.7.
[ Resolution proposed in ballot comment: ]
Initialisation of atomics:
We believe the intent is that for any atomics there is a distinguished initialisation write, but that this need not happens-before all the other operations on that atomic - specifically so that the initialisation write might be non-atomic and hence give rise to a data race, and hence undefined behaviour, in examples such as this (from Hans):atomic<atomic<int> *> p f() | { atomic<int>x; | W_na x p.store(&x,mo_rlx); | W_rlx p=&x } |(where na is nonatomic and rlx is relaxed). We suspect also that no other mixed atomic/nonatomic access to the same location is intended to be permitted. Either way, a note would probably help.
[2011-02-26: Hans comments and drafts wording]
I think the important point here is to clarify that races on atomics
are possible, and can be introduced as a result of non-atomic
initialization operations. There are other parts of this that remain unclear
to me, such as whether there are other ways to introduce data races on atomics,
or whether the races with initialization also introduce undefined behavior
by the 3.8 lifetime rules. But I don't think that it is necessary to resolve
those issues before releasing the standard. That's particularly true
since we've introduced atomic_init
, which allows easier ways to
construct initialization races.
[2011-03 Madrid]
Accepted to be applied immediately to the WP
Proposed resolution:
Update 99 [atomics.types.operations.req] p. 5 as follows:
constexpr A::A(C desired);5 Effects: Initializes the object with the value
desired
.[ Note: Construction is not atomic. — end note ]Initialization is not an atomic operation (1.10) [intro.multithread]. [Note: It is possible to have an access to an atomic objectA
race with its construction, for example by communicating the address of the just-constructed objectA
to another thread viamemory_order_relaxed
atomic operations on a suitable atomic pointer variable, and then immediately accessingA
in the receiving thread. This results in undefined behavior. — end note]
In response to the editor comment to 99 [atomics.types.operations.req] p. 8: The first Effects element is the correct and intended one:
void atomic_init(volatile A *object, C desired); void atomic_init(A *object, C desired);8 Effects: Non-atomically initializes
*object
with valuedesired
. This function shall only be applied to objects that have been default constructed, and then only once. [ Note: these semantics ensure compatibility with C. — end note ] [ Note: Concurrent access from another thread, even via an atomic operation, constitutes a data race. — end note ][Editor's note: The preceding text is from the WD as amended by N3196. N3193 makes different changes, marked up in the paper as follows:] Effects: Dynamically initializes an atomic variable. Non-atomically That is, non-atomically assigns the value desired to*object
. [ Note: this operation may need to initialize locks. — end note ] Concurrent access from another thread, even via an atomic operation, constitutes a data race.
extern "C"
Section: 32.5.11 [atomics.fences] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-179
The fence functions (32.5.11 [atomics.fences] p.5 + p.6) should be extern "C"
, for C
compatibility.
[2011-02-16 Reflector discussion]
Moved to Tentatively Ready after 6 votes.
Proposed resolution:
<atomic>
synopsis as indicated:
namespace std { [..] // 29.8, fences extern "C" void atomic_thread_fence(memory_order); extern "C" void atomic_signal_fence(memory_order); }
Change 32.5.11 [atomics.fences], p. 5 and p. 6 as indicated:
extern "C" void atomic_thread_fence(memory_order);5 Effects: depending on the value of
order
, this operation: [..]
extern "C" void atomic_signal_fence(memory_order);6 Effects: equivalent to
atomic_thread_fence(order)
, except that synchronizes with relationships are established only between a thread and a signal handler executed in the same thread.
Section: 32.5.11 [atomics.fences] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-137
Thread fence not only establish synchronizes with relationships,
there are semantics of fences that are expressed not in
terms of synchronizes with relationships (for example see 32.5.4 [atomics.order] p.5).
These semantics also need to apply to the use of
atomic_signal_fence
in a restricted way.
[Batavia: Concurrency group discussed issue, and is OK with the proposed resolution.]
[2011-02-26 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Change 32.5.11 [atomics.fences] p. 6 as indicated:
void atomic_signal_fence(memory_order);6 Effects: equivalent to
atomic_thread_fence(order)
, except thatsynchronizes with relationshipsthe resulting ordering constraints are established only between a thread and a signal handler executed in the same thread.
Section: 32.2 [thread.req] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-138
The FCD combines the requirements for lockable objects with those for the standard mutex objects. These should be separate. This is LWG issue 1268(i).
[ Resolution proposed by ballot comment: ]
See attached Appendix 1 - Additional Details
[ 2010-11-01 Daniel comments: ]
Paper n3130 addresses this issue.
Proposed resolution:
Resolved by n3197.
Section: 32.2.4 [thread.req.timing] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-181
The timeout operations are under-specified.
[ Resolution proposed by ballot comment: ]
Define precise semantics for
timeout_until
andtimeout_for
. See n3141 page 193 - Appendix 1 - Additional Details
[ 2010-11-01 Daniel comments: ]
Accepting n3128 would solve this issue.
Proposed resolution:
Resolved by n3191.
Section: 32.4.5 [thread.thread.this] Status: C++11 Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-25
Clock related operations are currently not required not to throw. So "Throws: Nothing." is not always true.
[ Resolution proposed by ballot comment: ]
Either require clock related operations not to throw (in 20.10) or change the Throws clauses in 30.3.2. Also possibly add a note that
abs_time
in the past or negativerel_time
is allowed.
[2011-02-10: Howard Hinnant provides a resolution proposal]
[Previous proposed resolution:]
Change the Operational semantics of C1::now()
in 30.3 [time.clock.req],
Table 59 — Clock
requirements as follows:
Table 59 — Clock
requirementsExpression Return type Operational semantics C1::now()
C1::time_point
Returns a time_point
object
representing the current point in time.
Shall not throw an exception.
[2011-02-19: Daniel comments and suggests an alternative wording]
Imposing the no-throw requirement on C1::now()
of any clock time
is an overly radical step: It has the indirect consequences that representation
types for C1::rep
can never by types with dynamic memory managment,
e.g. my big_int
, which are currently fully supported by the time
utilities. Further-on this strong constraint does not even solve the problem
described in the issue, because we are still left with the fact that any
of the arithmetic operations of C1::rep
, C1::duration
,
and C1::time_point
may throw exceptions.
The alternative proposal uses the following strategy: The general Clock
requirements remain untouched, but we require that any functions of the library-provided
clocks from sub-clause 30.7 [time.clock] and their associated types shall not
throw exceptions. Second, we replace existing noexcept
specifications of
functions from Clause 30 that depend on durations, clocks, or time points by wording that
clarifies that these functions can only throw, if the operations of user-provided durations,
clocks, or time points used as arguments to these functions throw exceptions.
[2011-03-23 Daniel and Peter check and simplify the proposed resolution resulting in this paper]
There is an inherent problem with std::time_point
that it doesn't seem to have an equivalent value
for ((time_t)-1)
that gets returned by C's time()
function to signal a problem, e.g., because
the underlying hardware is unavailable. After a lot of thinking and checks we came to the resolution that
timepoint::max()
should be the value to serve as a value signaling errors in cases where we
prefer to stick with no-throw conditions. Of-course, user-provided representation types don't need to
follow this approach if they prefer exceptions to signal such failures.
now()
and from_time_t()
can remain noexcept
with the solution to
return timepoint::max()
in case the current time cannot be determined or (time_t)-1
is passed
in, respectively.
Based on the previous proposed solution to LWG 1487 we decided that the new TrivialClock
requirements
should define that now()
mustn't throw and return timepoint::max()
to signal a problem. That
is in line with the C standard where (time_t)-1
signals a problem. Together with a fix to a - we assumed -
buggy specifcation in 20.11.3 p2 which uses "happens-before" relationship with something that isn't any action:
2 In Table 59
C1
andC2
denote clock types.t1
andt2
are values returned byC1::now()
where the call returningt1
happens before (1.10) the call returningt2
and both of these calls happen beforeC1::time_point::max()
.
[2011-03-23 Review with Concurrency group suggested further simplifications and Howard pointed out, that we do not need time_point::max() as a special value.]
also the second "happens before" will be changed to "occurs before" in the english meaning. this is to allow a steady clock to wrap.
Peter updates issue accordingly to discussion.[Note to the editor: we recommend removal of the following redundant paragraphs in 32.7.5 [thread.condition.condvarany] p. 18 to p. 21, p. 27, p. 28, p. 30, and p. 31 that are defining details for the wait functions that are given by the Effects element. ]
[Note to the editor: we recommend removal of the following redundant paragraphs in
32.7.4 [thread.condition.condvar]: p24-p26, p33-p34, and p36-p37 that are defining details for the
wait_for
functions. We believe these paragraphs are redundant with respect to the Effects clauses that
define semantics based on wait_until
. An example of such a specification is the wait()
with a predicate.
]
Proposed resolution:
Change p2 in 20.11.3 [time.clock.req] as follows
2 In Table 59
C1
andC2
denote clock types.t1
andt2
are values returned byC1::now()
where the call returningt1
happens before (1.10) the call returningt2
and both of these callshappenoccur beforeC1::time_point::max()
. [ Note: This meansC1
didn't wrap around betweent1
andt2
— end note ]
Add the following new requirement set at the end of sub-clause 30.3 [time.clock.req]: [Comment:
This requirement set is intentionally incomplete. The reason for
this incompleteness is the based on the fact, that if we would make it right for C++0x, we would end up defining
something like a complete ArithmeticLike
concept for TC::rep
, TC::duration
, and TC::time_point
.
But this looks out-of scope for C++0x to me. The effect is that we essentially do not exactly say, which arithmetic
or comparison operations can be used in the time-dependent functions from Clause 30, even though I expect that
all declared functions of duration
and time_point
are well-formed and well-defined. — end comment]
3 [ Note: the relative difference in durations between those reported by a given clock and the SI definition is a measure of the quality of implementation. — end note ]
? A type
TC
meets theTrivialClock
requirements if:
TC
satisfies theClock
requirements (30.3 [time.clock.req]),the types
TC::rep
,TC::duration
, andTC::time_point
satisfy the requirements ofEqualityComparable
( [equalitycomparable]),LessThanComparable
( [lessthancomparable]),DefaultConstructible
( [defaultconstructible]),CopyConstructible
( [copyconstructible]),CopyAssignable
( [copyassignable]),Destructible
( [destructible]), and of numeric types ([numeric.requirements]) [Note: This means in particular, that operations of these types will not throw exceptions — end note ],lvalues of the types
TC::rep
,TC::duration
, andTC::time_point
are swappable (16.4.4.3 [swappable.requirements]),the function
TC::now()
does not throw exceptions, andthe type
TC::time_point::clock
meets theTrivialClock
requirements, recursively.
Modify 30.7 [time.clock] p. 1 as follows:
1 - The types defined in this subclause shall satisfy the
TrivialClock
requirements (20.11.1).
Modify 30.7.2 [time.clock.system] p. 1, class system_clock
synopsis, as follows:
class system_clock { public: typedef see below rep; typedef ratio<unspecified , unspecified > period; typedef chrono::duration<rep, period> duration; typedef chrono::time_point<system_clock> time_point; static const bool is_monotonic is_steady = unspecified; static time_point now() noexcept; // Map to C API static time_t to_time_t (const time_point& t) noexcept; static time_point from_time_t(time_t t) noexcept; };
Modify the prototype declarations in 30.7.2 [time.clock.system] p. 3 + p. 4 as indicated (This
edit also fixes the miss of the static
specifier in these prototype declarations):
static time_t to_time_t(const time_point& t) noexcept;static time_point from_time_t(time_t t) noexcept;
Modify 30.7.7 [time.clock.steady] p. 1, class steady_clock
synopsis, as follows:
class steady_clock { public: typedef unspecified rep; typedef ratio<unspecified , unspecified > period; typedef chrono::duration<rep, period> duration; typedef chrono::time_point<unspecified, duration> time_point; static const bool is_monotonic is_steady = true; static time_point now() noexcept; };
Modify 30.7.8 [time.clock.hires] p. 1, class high_resolution_clock
synopsis, as follows:
class high_resolution_clock { public: typedef unspecified rep; typedef ratio<unspecified , unspecified > period; typedef chrono::duration<rep, period> duration; typedef chrono::time_point<unspecified, duration> time_point; static const bool is_monotonic is_steady = unspecified; static time_point now() noexcept; };
Add a new paragraph at the end of 32.2.4 [thread.req.timing]:
6 The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.
? Implementation-provided clocks that are used for these functions shall meet the
TrivialClock
requirements (30.3 [time.clock.req]).
Edit the synopsis of 32.4.5 [thread.thread.this] before p. 1. [Note: this duplicates edits also in D/N3267]:
template <class Clock, class Duration> void sleep_until(const chrono::time_point<Clock, Duration>& abs_time)noexcept; template <class Rep, class Period> void sleep_for(const chrono::duration<Rep, Period>& rel_time)noexcept;
Modify the prototype specifications in 32.4.5 [thread.thread.this] before p. 4 and p. 6 and re-add a Throws element following the Synchronization elements at p. 5 and p. 7:
template <class Clock, class Duration> void sleep_until(const chrono::time_point<Clock, Duration>& abs_time)noexcept;4 - [...]
5 - Synchronization: None. ? - Throws: Nothing ifClock
satisfies theTrivialClock
requirements (30.3 [time.clock.req]) and operations ofDuration
do not throw exceptions. [Note: Instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
template <class Rep, class Period> void sleep_for(const chrono::duration<Rep, Period>& rel_time)noexcept;6 [...]
7 Synchronization: None. ? Throws: Nothing if operations ofchrono::duration<Rep, Period>
do not throw exceptions. [Note: Instantiations of duration types supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
Fix a minor incorrectness in p. 5: Duration types need to compare against duration<>::zero()
,
not 0
:
3 The expression
[...] 5 Effects: The function attempts to obtain ownership of the mutex within the relative timeout (30.2.4) specified bym.try_lock_for(rel_time)
shall be well-formed and have the following semantics:rel_time
. If the time specified byrel_time
is less than or equal to0
rel_time.zero()
, the function attempts to obtain ownership without blocking (as if by callingtry_lock()
). The function shall return within the timeout specified byrel_time
only if it has obtained ownership of the mutex object. [ Note: As withtry_lock()
, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note ]
Modify the class timed_mutex
synopsis in 32.6.4.3.2 [thread.timedmutex.class] as indicated:
[Note: this duplicates edits also in D/N3267]:
class timed_mutex { public: [...] template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time)noexcept; template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time)noexcept; [...] };
Modify the class recursive_timed_mutex
synopsis in 32.6.4.3.3 [thread.timedmutex.recursive] as indicated:
[Note: this duplicates edits also in D/N3267]:
class recursive_timed_mutex { public: [...] template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time)noexcept; template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time)noexcept; [...] };
Modify the class template unique_lock
synopsis in 32.6.5.4 [thread.lock.unique] as indicated.
[Note: this duplicates edits also in D/N3267]:
template <class Mutex> class unique_lock { public: [...] template <class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time)noexcept; template <class Rep, class Period> unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time)noexcept; [...] };
Modify the constructor prototypes in 32.6.5.4.2 [thread.lock.unique.cons] before p. 14 and p. 17 [Note: this duplicates edits also in D/N3267]:
template <class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time)noexcept;
template <class Rep, class Period> unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time)noexcept;
Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-27
The mutex requirements force try_lock
to be
noexcept(true)
. However, where they are used by the
generic algorithms, those relax this requirement and say
that try_lock
may throw. This means the requirement is
too stringent, also a non-throwing try_lock
does not allow
for a diagnostic such as system_error
that lock()
will give us.
[ Resolution proposed by ballot comment: ]
delete p18, adjust 30.4.4 p1 and p4 accordingly
[ 2010-11-01 Daniel comments: ]
Accepting n3130 would solve this issue.
Proposed resolution:
Resolved by n3197.
try_lock
does not guarantee forward progressSection: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-186
try_lock
does not provide a guarantee of forward progress
because it is allowed to spuriously fail.
[ Resolution proposed by ballot comment: ]
The standard mutex types must not fail spuriously in
try_lock
. See n3141 page 205 - Appendix 1 - Additional Details
[ 2010-11-01 Daniel comments: ]
Paper n3152 addresses this issue.
Proposed resolution:
Resolved by n3209.
Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-188
Mutex requirements should not be bound to threads.
[ Resolution proposed by ballot comment: ]
See Appendix 1 of n3141 - Additional Details, p. 208.
[ 2010-10-24 Daniel adds: ]
Accepting n3130 would solve this issue.
Proposed resolution:
Resolved by n3197.
Section: 32.6.7.2 [thread.once.callonce] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-190
The term "are serialized" is never defined (32.6.7.2 [thread.once.callonce] p. 2).
[ Resolution proposed by ballot comment: ]
Remove the sentence with "are serialized" from
paragraph 2. Add "Calls to call_once
on the same
once_flag
object shall not introduce data races
(17.6.4.8)." to paragraph 3.
[ 2010-11-01 Daniel translates NB comment into wording ]
[ 2011-02-17: Hans proposes an alternative resolution ]
[ 2011-02-25: Hans, Clark, and Lawrence update the suggested wording ]
[2011-02-26 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Change 32.6.7.2 [thread.once.callonce] p.2+3 as indicated:
template<class Callable, class ...Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);[..]
2 Effects:Calls toAn execution ofcall_once
on the sameonce_flag
object are serialized. If there has been a prior effective call tocall_once
on the sameonce_flag object
, the call tocall_once
returns without invokingfunc
. If there has been no prior effective call tocall_once
on the sameonce_flag
object,INVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
is executed. The call tocall_once
is effective if and only ifINVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
returns without throwing an exception. If an exception is thrown it is propagated to the caller.call_once
that does not call itsfunc
is a passive execution. An execution ofcall_once
that calls itsfunc
is an active execution. An active execution shall callINVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
. If such a call tofunc
throws an exception, the execution is exceptional, otherwise it is returning. An exceptional execution shall propagate the exception to the caller ofcall_once
. Among all executions ofcall_once
for any givenonce_flag
: at most one shall be a returning execution; if there is a returning execution, it shall be the last active execution; and there are passive executions only if there is a returning execution. [Note: Passive executions allow other threads to reliably observe the results produced by the earlier returning execution. — end note] 3 Synchronization:The completion of an effective call toFor any givencall_once
on aonce_flag
object synchronizes with (6.9.2 [intro.multithread]) all subsequent calls tocall_once
on the sameonce_flag
object.once_flag
: all active executions occur in a total order; completion of an active execution synchronizes with (6.9.2 [intro.multithread]) the start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.
lock()
postcondition can not be generally achievedSection: 32.7 [thread.condition] Status: C++11 Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-30
If lock.lock()
throws an exception, the postcondition can not be generally achieved.
[ Resolution proposed by ballot comment: ]
Either state that the postcondition might not be achieved, depending on the error condition, or state that
terminate()
is called in this case.
[ 2010-08-13 Peter Sommerlad comments and provides wording ]
32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany]
p. 13, last bullet, and corresponding paragraphs in all wait functions Problem:
Condition variable wait might fail, because the lock cannot be acquired when notified. CH-30 says: "If lock.lock() throws an exception, the postcondition can not be generally achieved." CH-30 proposes: "Either state that the postcondition might not be achieved, depending on the error condition, or state that terminate() is called in this case." The discussion in Rapperswil concluded that callingterminate()
might be too drastic in this case and a corresponding exception should be thrown/passed on and one should use a lock type that allows querying its status, whichunique_lock
allows forstd::condition_variable
We also had some additional observations while discussing in Rapperswil:
- in 32.7.4 [thread.condition.condvar]
wait
with predicate andwait_until
with predicate lack the precondition, postcondition and Error conditions sections. the lack of the precondition would allow to callpred()
without holding the lock.- in 32.7.4 [thread.condition.condvar]
wait_until
andwait_for
and 32.7.5 [thread.condition.condvarany]wait_for
still specify an error condition for a violated precondition. This should be removed.and add the following proposed solution:
[2011-02-27: Daniel adapts numbering to n3225]
Proposed resolution:
void wait(unique_lock<mutex>& lock);
[..]9 Requires:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
ortimed_wait
) threads.
[..]11 Postcondition:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
template <class Predicate> void wait(unique_lock<mutex>& lock, Predicate pred);
?? Requires:
lock.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
ortimed_wait
) threads.
14 Effects:
while (!pred()) wait(lock);
?? Postcondition:
lock.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
?? Throws:
std::system_error
when an exception is required (30.2.2).
?? Error conditions:
- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Clock, class Duration> cv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time);
15 Requires:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.
[..]
[..]17 Postcondition:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
20 Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);
21 Requires:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.
[..]
[..]24 Postcondition:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
26 Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);
?? Requires:
lock.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
ortimed_wait
) threads.
27 Effects:
while (!pred()) if (wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;
28 Returns:
pred()
?? Postcondition:
lock.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
29 [ Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note ]
?? Throws:
std::system_error
when an exception is required (30.2.2).
?? Error conditions:
- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);
30 Requires:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.
[..]
33 Postcondition:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
[..]
37 Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Lock, class Predicate> void wait(Lock& lock, Predicate pred);
[Note: if any of the wait functions exits with an exception it is indeterminate if the
Lock
is held. One can use aLock
type that allows to query that, such as theunique_lock
wrapper. — end note]
11 Effects:
while (!pred()) wait(lock);
[..]
31 Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
Section: 32.7 [thread.condition] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-29
It is unclear if a spurious wake-up during the loop and reentering
of the blocked state due to a repeated execution
of the loop will adjust the timer of the blocking with the
respect to the previously specified rel_time
value.
[ Resolution proposed by ballot comment: ]
Make it clear (e.g. by a note) that when reexecuting the loop the waiting time when blocked will be adjusted with respect to the elapsed time of the previous loop executions.
[ 2010-08-13 Peter Sommerlad comments and provides wording: ]
Problem: It is unclear if a spurious wake-up during the loop and re-entering of the blocked state due to a repeated execution of the loop will adjust the timer of the blocking with the respect to the previously specified
Proposed Resolution from CH29: Make it clear (e.g. by a note) that when re-executing the loop the waiting time when blocked will be adjusted with respect to the elapsed time of the previous loop executions. Discussion in Rapperswil: Assuming the introduction of a mandatoryrel_time
value.steady_clock
proposed by US-181 to the FCD the specification ofcondition_variable::wait_for
can be defined in terms ofwait_until
using thesteady_clock
. This is also interleaving with US-181, because that touches the same paragraph (30.5.1 p 25, p34 and 30.5.2 p 20, p 28 in n3092.pdf) (The "as if" in the proposed solutions should be confirmed by the standardization terminology experts)
[ 2010-11 Batavia: Resolved by applying n3191 ]
- Change 32.7.4 [thread.condition.condvar] paragraph 25,
wait_for
Effects as indicated:template <class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);[..]
25 Effects: as ifreturn wait_until(lock, chrono::steady_clock::now() + rel_time);
Atomically callslock.unlock()
and blocks on*this
.When unblocked, callslock.lock()
(possibly blocking on the lock), then returns.The function will unblock when signaled by a call tonotify_one()
or a call tonotify_all()
, by the elapsed timerel_time
passing (30.2.4), or spuriously.If the function exits via an exception,lock.lock()
shall be called prior to exiting the function scope.- Change 32.7.4 [thread.condition.condvar] paragraph 34,
wait_for
with predicate Effects as indicated:template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);[..]
34 Effects: as ifreturn wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));
Executes a loop: Within the loop the function first evaluatespred()
and exits the loop if the result istrue
.Atomically callslock.unlock()
and blocks on*this
.When unblocked, callslock.lock()
(possibly blocking on the lock).The function will unblock when signaled by a call tonotify_one()
or a call tonotify_all()
, by the elapsed timerel_time
passing (30.2.4), or spuriously.If the function exits via an exception,lock.lock()
shall be called prior to exiting the function scope.The loop terminates whenpred()
returnstrue
or when the time duration specified byrel_time
has elapsed.- Change 32.7.5 [thread.condition.condvarany] paragraph 20,
wait_for
Effects as indicated:template <class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);20 Effects: as if
return wait_until(lock, chrono::steady_clock::now() + rel_time);
Atomically callslock.unlock()
and blocks on*this
.When unblocked, callslock.lock()
(possibly blocking on the lock), then returns.The function will unblock when signaled by a call tonotify_one()
or a call tonotify_all()
, by the elapsed timerel_time
passing (30.2.4), or spuriously.If the function exits via an exception,lock.unlock()
shall be called prior to exiting the function scope.- Change 32.7.5 [thread.condition.condvarany] paragraph 28,
wait_for
with predicate Effects as indicated:template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);28 Effects: as if
return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));
Executes a loop: Within the loop the function first evaluatespred()
and exits the loop if the result istrue
.Atomically callslock.unlock()
and blocks on*this
.When unblocked, callslock.lock()
(possibly blocking on the lock).The function will unblock when signaled by a call tonotify_one()
or a call tonotify_all()
, by the elapsed timerel_time
passing (30.2.4), or spuriously.If the function exits via an exception,lock.unlock()
shall be called prior to exiting the function scope.The loop terminates whenpred()
returnstrue
or when the time duration specified byrel_time
has elapsed.
Proposed resolution:
Resolved by n3191.
Section: 32.10 [futures] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-194
The specification for managing associated asynchronous state is confusing, sometimes omitted, and redundantly specified.
[ Resolution proposed by ballot comment: ]
Define terms-of-art for releasing, making ready, and abandoning an associated asynchronous state. Use those terms where appropriate. See Appendix 1 - Additional Details
Proposed resolution:
Resolved in Batavia by accepting n3192.
Section: 32.10.5 [futures.state] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-195
The intent and meaning of 32.10.5 [futures.state] p10 is not apparent.
10 Accesses to the same shared state conflict (1.10).
[ 2011-03-07 Jonathan Wakely adds: ]
It's not clear which paragraph this refers to, I had to go to the ballot comments where US-195 reveals it's para 8, which in the FCD (N3092) says:
Accesses to the same associated asynchronous state conflict (1.10).
This is now para 10 in N3242:
Accesses to the same shared state conflict (1.10).
[2011-03-07: Lawrence comments and drafts wording]
The intent of this paragraph is to deal with operations,
such as shared_future::get()
, that return a reference
to a value held in the shared state. User code could potentially
conflict when accessing that value.
Lawrence proposed resolution:
Modify 32.10.5 [futures.state] p10 as follows:
10
Accesses to the same shared state conflict (6.9.2 [intro.multithread]).Some operations, e.g.shared_future::get()
( [futures.shared_future]), may return a reference to a value held in their shared state. Accesses and modifications through those references by concurrent threads to the same shared state may potentially conflict (6.9.2 [intro.multithread]). [Note: As a consequence, accesses must either use read-only operations or provide additional synchronization. — end note]
[2011-03-19: Detlef suggests an alternative resolution, shown below.]
Proposed Resolution
Modify 32.10.5 [futures.state] p10 as follows:
10 Accesses to the same shared state conflict (6.9.2 [intro.multithread]). [Note: This explicitely specifies that the shared state is visible in the objects that reference this state in the sense of data race avoidance 16.4.6.10 [res.on.data.races]. — end note]
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-196
The term "are serialized" is not defined (32.10.6 [futures.promise] p. 21, 25).
[ Resolution proposed by ballot comment: ]
Replace "are serialized" with "shall not introduce a data race (17.6.4.8)".
[ 2010-11-02 Daniel translates proposal into proper wording changes ]
[2011-03-19: Detlef comments]
The proposed resolution for 1507(i) would cover this issue as well.
[Proposed Resolution]
- Change 32.10.6 [futures.promise] p. 21 as indicated:
21 Synchronization: calls to
set_value
andset_exception
on a singlepromise
objectare serializedshall not introduce a data race ([res.on.data.races]). [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]- Change 32.10.6 [futures.promise] p. 25 as indicated:
25 Synchronization: calls to
set_value
andset_exception
on a singlepromise
objectare serializedshall not introduce a data race ([res.on.data.races]). [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]
Proposed resolution:
Resolved 2001-03 Madrid by issue 1507.
promise::set_value
and future::get
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-197
There is no defined synchronization between promise::set_value
and future::get
(32.10.6 [futures.promise] p. 21, 25).
[ Resolution proposed by ballot comment: ]
Replace "[Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note]" with the normative "They synchronize with (1.10) any operation on a future object with the same associated asynchronous state marked ready."
[ 2010-11-02 Daniel translates proposal into proper wording changes ]
[2011-03-19: Detlef comments]
The proposed resolution for 1507(i) would cover this issue as well. Effectively it will reject the request but a clarification is added that the normative wording is already in 32.10.5 [futures.state].
- Change 32.10.6 [futures.promise] p. 21 as indicated:
21 Synchronization: calls to
set_value
andset_exception
on a singlepromise
object are serialized.[ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]They synchronize with ([intro.multithread]) any operation on a future object with the same associated asynchronous state marked ready.- Change 32.10.6 [futures.promise] p. 25 as indicated:
25 Synchronization: calls to
set_value
andset_exception
on a singlepromise
object are serialized.[ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]They synchronize with ([intro.multithread]) any operation on a future object with the same associated asynchronous state marked ready.
Proposed resolution:
Resolved 2001-03 Madrid by issue 1507.
promise::XXX_at_thread_exit
functions have no
synchronization requirementsSection: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-199
promise::XXX_at_thread_exit
functions have no
synchronization requirements. Specifying synchronization
for these member functions requires coordinating with the
words in 32.10.6 [futures.promise]/21 and 25, which give synchronization
requirements for promise::set_value
and
promise::set_exception
(32.10.6 [futures.promise] p. 26 ff., p. 29 ff.).
[ Resolution proposed by ballot comment: ]
Change 32.10.6 [futures.promise]/21 to mention
set_value_at_thread_exit
andset_exception_at_thread_exit;
with this text, replace 32.10.6 [futures.promise]/25 and add two new paragraphs, after 32.10.6 [futures.promise]/28 and 32.10.6 [futures.promise]/31.
[2011-03-8: Lawrence comments and drafts wording]
This comment applies as well to other *_at_thread_exit
functions. The following resolution adds synchronization paragraphs
to all of them and edits a couple of related synchronization
paragraphs.
[2011-03-09: Hans and Anthony add some improvements]
[2011-03-19: Detlef comments]
In regard to the suggested part:
These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.
I would like this to change to:
These operations do not provide any ordering guarantees with respect to other operations on the same promise object. [Note: They synchronize with calls to operations on objects that refer to the same shared state according to 32.10.5 [futures.state]. — end note]
The current proposed resolution has exactly the same paragraph at for places. I propose to have it only once as new paragraph 2.
This also covers 1504(i) (US-196) and 1505(i) (US-197). US-197 is essentially rejected with this resolution, but a clarification is added that the normative wording is already in 32.10.5 [futures.state].
Proposed Resolution
Edit 32.6.4.2 [thread.mutex.requirements.mutex] paragraph 5 as follows:
5 The implementation shall provide lock and unlock operations, as described below.
The implementation shall serialize those operations.For purposes of determining the existence of a data race, these behave as atomic operations (6.9.2 [intro.multithread]). The lock and unlock operations on a single mutex shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the mutex. — end note] [ Note: Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization should be used to ensure that mutex objects are initialized and visible to other threads. — end note ]Edit 32.7 [thread.condition] paragraphs 6-9 as follows:
void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);-6- Requires:
lk
is locked by the calling thread and either
- no other thread is waiting on
cond
, orlk.mutex()
returns the same value for each of the lock arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.-7- Effects: transfers ownership of the lock associated with
lk
into internal storage and schedulescond
to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification shall be as iflk.unlock(); cond.notify_all();-?- Synchronization: The call to
-8- Note: The supplied lock will be held until the thread exits, and care must be taken to ensure that this does not cause deadlock due to lock ordering issues. After callingnotify_all_at_thread_exit
and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.9.2 [intro.multithread]) calls to functions waiting oncond
.notify_all_at_thread_exit
it is recommended that the thread should be exited as soon as possible, and that no blocking or time-consuming tasks are run on that thread. -9- Note: It is the user's responsibility to ensure that waiting threads do not erroneously assume that the thread has finished if they experience spurious wakeups. This typically requires that the condition being waited for is satisfied while holding the lock onlk
, and that this lock is not released and reacquired prior to callingnotify_all_at_thread_exit
.Edit 32.10.6 [futures.promise], paragraphs 14-27 as follows:
void promise::set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();-14- Effects: atomically stores the value
-15- Throws:r
in the shared state and makes that state ready (32.10.5 [futures.state]).
future_error
if its shared state already has a stored value or exception, or- for the first version, any exception thrown by the copy constructor of
R
, or- for the second version, any exception thrown by the move constructor of
R
.-16- Error conditions:
promise_already_satisfied
if its shared state already has a stored value or exception.no_state
if*this
has no shared state.-17- Synchronization:
calls toFor purposes of determining the existence of a data race,set_value
andset_exception
on a singlepromise
object are serialized. [ Note: And they synchronize and serialize with other functions through the referred shared state. — end note ]set_value
,set_exception
,set_value_at_thread_exit
, andset_exception_at_thread_exit
behave as atomic operations (6.9.2 [intro.multithread]) on the memory location associated with thepromise
. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.void set_exception(exception_ptr p);-18- Effects: atomically stores the exception pointer
p
in the shared state and makes that state ready (32.10.5 [futures.state]).-19- Throws:
future_error
if its shared state already has a stored value or exception.-20- Error conditions:
promise_already_satisfied
if its shared state already has a stored value or exception.no_state
if*this
has no shared state.-21- Synchronization:
calls toFor purposes of determining the existence of a data race,set_value
andset_exception
on a singlepromise
object are serialized. [ Note: And they synchronize and serialize with other functions through the referred shared state. — end note ]set_value
,set_exception
,set_value_at_thread_exit
, andset_exception_at_thread_exit
behave as atomic operations (6.9.2 [intro.multithread]) on the memory location associated with thepromise
. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.void promise::set_value_at_thread_exit(const R& r); void promise::set_value_at_thread_exit(R&& r); void promise<R&>::set_value_at_thread_exit(R& r); void promise<void>::set_value_at_thread_exit();-22- Effects: Stores the value
-23- Throws:r
in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.future_error
if an error condition occurs. -24- Error conditions:
promise_already_satisfied
if its shared state already has a stored value or exception.no_state
if*this
has no shared state.-??- Synchronization: For purposes of determining the existence of a data race,
set_value
,set_exception
,set_value_at_thread_exit
, andset_exception_at_thread_exit
behave as atomic operations (6.9.2 [intro.multithread]) on the memory location associated with thepromise
. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.void promise::set_exception_at_thread_exit(exception_ptr p);-25- Effects: Stores the exception pointer
-26- Throws:p
in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.future_error
if an error condition occurs. -27- Error conditions:
promise_already_satisfied
if its shared state already has a stored value or exception.no_state
if*this
has no shared state.-??- Synchronization: For purposes of determining the existence of a data race,
set_value
,set_exception
,set_value_at_thread_exit
, andset_exception_at_thread_exit
behave as atomic operations (6.9.2 [intro.multithread]) on the memory location associated with thepromise
. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.Edit 32.10.10.2 [futures.task.members], paragraph 15-21 as follows:
void operator()(ArgTypes... args);-15- Effects:
-16- Throws: aINVOKE(f, t1, t2, ..., tN, R)
, wheref
is the stored task of*this
andt1
,t2
,...
,tN
are the values inargs...
. If the task returns normally, the return value is stored as the asynchronous result in the shared state of*this
, otherwise the exception thrown by the task is stored. The shared state of*this
is made ready, and any threads blocked in a function waiting for the shared state of*this
to become ready are unblocked.future_error
exception object if there is no shared state or the stored task has already been invoked. -17- Error conditions:
promise_already_satisfied
if the shared state is already ready.no_state
if*this
has no shared state.-18- Synchronization: a successful call to
operator()
synchronizes with (6.9.2 [intro.multithread]) a call to any member function of afuture
orshared_future
object that shares the shared state of*this
. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with (6.9.2 [intro.multithread]) the successful return from any member function that detects that the state is set to ready. [ Note:operator()
synchronizes and serializes with other functions through the shared state. — end note ]void make_ready_at_thread_exit(ArgTypes... args);-19- Effects:
-20- Throws:INVOKE(f, t1, t2, ..., tN, R)
, wheref
is the stored task andt1
,t2
,...
,tN
are the values inargs...
. If the task returns normally, the return value is stored as the asynchronous result in the shared state of*this
, otherwise the exception thrown by the task is stored. In either case, this shall be done without making that state ready (32.10.5 [futures.state]) immediately. Schedules the shared state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.future_error
if an error condition occurs. -21- Error conditions:
promise_already_satisfied
if the shared state already has a stored value or exception.no_state
if*this
has no shared state.-??- Synchronization: a successful call to
make_ready_at_thread_exit
synchronizes with (6.9.2 [intro.multithread]) a call to any member function of afuture
orshared_future
object that shares the shared state of*this
. The completion of
the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state
the destructors for all the current thread's variables of thread storage duration
synchronize with (6.9.2 [intro.multithread]) the successful return from any member function that detects that the state is set to ready. [Note:
make_ready_at_thread_exit
synchronizes and serializes with other functions through the shared state. — end note]
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
packaged_task::operator bool()
Section: 32.10.10 [futures.task] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-201
packaged_task
provides operator bool()
to check whether
an object has an associated asynchronous state. The various future
types provide a member function valid()
that does the same thing.
The names of these members should be the same.
[ Resolution proposed by ballot comment: ]
Replaced the name
packaged_task::operator bool()
withpackaged_task::valid()
in the synopsis (32.10.10 [futures.task]/2) and the member function specification (before 32.10.10.2 [futures.task.members]/15).
[
2010-11-02 Daniel translates proposed wording changes into a proper proposed resolution
and verified that no other places implicitly take advantage of packaged_task
conversion to bool.
]
[Resolved in Batavia by accepting n3194. ]
Proposed resolution:
packaged_task
synopsis as indicated:
template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public: typedef R result_type; [..]explicit operatorbool valid() const; [..] };
explicit operatorbool valid() const;15 Returns: true only if
16 Throws: nothing.*this
has an associated asynchronous state.
Section: 32.10 [futures] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-36
Providing only three different possible values for the enum
launch
and saying that launch::any
means either
launch::sync
or launch::async
is very restricting. This
hinders future implementors to provide clever
infrastructures that can simply by used by a call to
async(launch::any,...)
. Also there is no hook for an
implementation to provide additional alternatives to launch
enumeration and no useful means to combine those (i.e.
interpret them like flags). We believe something like
async(launch::sync | launch::async, ...)
should be allowed
and can become especially useful if one could say also
something like async(launch::any & ~launch::sync, ....)
respectively. This flexibility might limit the features usable
in the function called through async()
, but it will allow a
path to effortless profit from improved hardware/software
without complicating the programming model when just
using async(launch::any,...)
[ Resolution proposed by ballot comment: ]
Change in 32.10.1 [futures.overview] 'enum class launch' to allow
further implementation defined values and provide
the following bit-operators on the launch values
(operator|
, operator&
, operator~
delivering a
launch
value).
launch
enums,
but we shouldn't limit the standard to just 32 or 64
available bits in that case and also should keep
the launch enums in their own enum namespace.
Change [future.async] p3 according to the
changes to enum launch
. change --launch::any
to
"the implementation may choose any of the
policies it provides." Note: this can mean that an
implementation may restrict the called function to
take all required information by copy in case it will
be called in a different address space, or even, on
a different processor type. To ensure that a call is
either performed like launch::async
or
launch::sync
describe one should call
async(launch::sync|launch::async,...)
[ 2010-11-02 Daniel comments: ]
The new paper n3113 provides concrete wording.
Proposed resolution:
Resolved by n3188.
packaged_task
constructors need reviewSection: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-207
The constructor that takes R(*)(ArgTypes...)
is not
needed; the constructor that takes a callable type works
for this argument type. More generally, the constructors
for packaged_task should parallel those for function.
[ US-207 Suggested Resolution: ]
Review the constructors for packaged_task and provide the same ones as function, except where inappropriate.
[ 2010-10-22 Howard provides wording, as requested by the LWG in Rapperswil. ]
[2011-02-10 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Alter the list of constructors in both 32.10.10 [futures.task] and in 32.10.10.2 [futures.task.members] as indicated:
template <class F> explicit packaged_task(F f); template <class F, class Allocator> explicit packaged_task(allocator_arg_t, const Allocator& a, F f); explicit packaged_task(R(*f)(ArgTypes...));template <class F> explicit packaged_task(F&& f); template <class F, class Allocator> explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
packaged_task::make_ready_at_thread_exit
has no synchronization requirementsSection: 32.10.10.2 [futures.task.members] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-208
packaged_task::make_ready_at_thread_exit
has no synchronization requirements.
[ Resolution proposed by ballot comment: ]
Figure out what the synchronization requirements should be and write them.
[2011-02-09 Anthony provides a proposed resolution]
[2011-02-19 Additional edits by Hans, shown in the proposed resolution section]
[2011-02-22 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed Resolution
Add a new paragraph following 32.10.10.2 [futures.task.members] p. 19:
void make_ready_at_thread_exit(ArgTypes... args);19 - ...
?? - Synchronization: Following a successful call to
make_ready_at_thread_exit
, the destruction of all objects with thread storage duration associated with the current thread happens before the associated asynchronous state is made ready. The marking of the associated asynchronous state as ready synchronizes with (6.9.2 [intro.multithread]) the successful return from any function that detects that the state is set to ready.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
auto_ptr
Section: 99 [depr.auto.ptr] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-142
auto_ptr
does not appear in the <memory>
synopsis and
[depr.auto.ptr] doesn't say which header declares it.
Conversely, the deprecated binders bind1st
etc. are in the
<functional>
synopsis, this is inconsistent
Either auto_ptr
should be declared in the
<memory>
synopsis, or the deprecated binders
should be removed from the <functional>
synopsis
and appendix D should say which header declares
the binders and auto_ptr
.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Add the following lines to the synopsis of header <memory>
in [memory]/1:
// [depr.auto.ptr], Class auto_ptr (deprecated): template <class X> class auto_ptr;
Section: 20.3.1.2.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Daniel Krügler Opened: 2010-09-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.dltr.dflt].
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Discussion:
The current working draft does specify the default c'tor of default_delete
in a manner
to guarantee static initialization for default-constructed objects of static storage duration
as a consequence of the acceptance of the proposal n2976
but this paper overlooked the fact that the suggested declaration does not ensure that the type
will be a trivial type. The type default_delete
was always considered as a simple wrapper for
calling delete
or delete[]
, respectivly and should be a trivial type.
In agreement with the new settled core language rules this easy to realize by just changing the declaration to
constexpr default_delete() = default;
This proposal also automatically solves the problem, that the semantics of the default constructor of the
partial specialization default_delete<T[]>
is not specified at all. By defaulting its default constructor
as well, the semantics are well-defined.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The following wording changes are against N3126.
default_delete
in [unique.ptr.dltr.dflt] as indicated:
namespace std { template <class T> struct default_delete { constexpr default_delete() = default; template <class U> default_delete(const default_delete<U>&); void operator()(T*) const; }; }
default_delete
default constructor in [unique.ptr.dltr.dflt]/1. This
brings it in harmony with the style used in the partial specialization default_delete<T[]>
. Since there are
neither implied nor explicit members, there is no possibility to misinterpret what the constructor does:
constexpr default_delete();
1 Effects: Default constructs adefault_delete
object.
default_delete
in [unique.ptr.dltr.dflt1] as indicated:
namespace std { template <class T> struct default_delete<T[]> { constexpr default_delete() = default; void operator()(T*) const; template <class U> void operator()(U*) const = delete; }; }
Section: 32.10 [futures] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2010-09-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current WP N3126 contains ambiguous statements about the
behaviour of functions wait_for
/wait_until
in
case the future refers to a deferred function. Moreover, I believe
it describes a disputable intent, different from the one contained
in the original async proposals, that may have been introduced
inadvertently during the "async cleanup" that occurred recently.
Consider the following case:
int f(); future<int> x = async(launch::deferred, f); future_status s = x.wait_for(chrono::milliseconds(100));
This example raises two questions:
f
invoked?s
?According to the current WP, the answer to question 1 is yes,
because 30.6.9/3 says "The first call to a function waiting for the
associated asynchronous state created by this async call to become
ready shall invoke the deferred function in the thread that called
the waiting function". The answer to question 2, however, is not as
clear. According to 30.6.6/23, s should be
future_status::deferred
because x
refers to a
deferred function that is not running, but it should also be
future_status::ready
because after executing f
(and we saw that f
is always executed) the state becomes
ready. By the way, the expression "deferred function that is not
running" is very unfortunate in itself, because it may apply to
both the case where the function hasn't yet started, as well as the
case where it was executed and completed.
While we clearly have a defect in the WP answering to question
2, it is my opinion that the answer to question 1 is wrong, which
is even worse. Consider that the execution of the function
f
can take an arbitrarily long time. Having
wait_for()
invoke f
is a potential violation of
the reasonable expectation that the execution of
x.wait_for(chrono::milliseconds(100))
shall take at most
100 milliseconds plus a delay dependent on the quality of implementation
and the quality of management (as described in paper N3128).
In fact, previous versions of the WP
clearly specified that only function wait()
is required to
execute the deferred function, while wait_for()
and
wait_until()
shouldn't.
The proposed resolution captures the intent that
wait_for()
and wait_until()
should never attempt
to invoke the deferred function. In other words, the P/R provides
the following answers to the two questions above:
future_status::deferred
In order to simplify the wording, the definition of deferred function has been tweaked so that the function is no longer considered deferred once its evaluation has started, as suggested by Howard.
Discussions in the reflector questioned whether
wait_for()
and wait_until()
should return
immediately or actually wait hoping for a second thread to execute
the deferred function. I believe that waiting could be useful only
in a very specific scenario but detrimental in the general case and
would introduce another source of ambiguity: should
wait_for()
return future_status::deferred
or
future_status::timeout
after the wait? Therefore the P/R
specifies that wait_for
/wait_until
shall return
immediately, which is simpler, easier to explain and more useful in
the general case.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The proposed wording changes are relative to the Final Committee Draft, N3126.
Note to the editor: the proposed wording is meant not be in conflict with any change proposed by paper N3128 "C++ Timeout Specification". Ellipsis are deliberately used to avoid any unintended overlapping.
In [futures.unique_future] 30.6.6/22:
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.6/23 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.unique_future] 30.6.6/25:
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.6/26 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.shared_future] 30.6.7/27
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.7/28 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.shared_future] 30.6.6/30:
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.7/31 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.atomic_future] 30.6.8/23
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.8/24 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.atomic_future] 30.6.8/27:
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.8/28 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.async] 30.6.9/3 second bullet:
[...] The first call to a function
waitingrequiring a non-timed wait for the
associated asynchronous state created by this async call to become
ready shall invoke the deferred function in the thread that called
the waiting function; once evaluation of INVOKE(g,
xyz)
begins, the function is no longer considered
deferred all other calls waiting for the same associated
asynchronous state to become ready shall block until the deferred
function has completed.
Section: 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.7 [unord.multiset] Status: C++11 Submitter: Nicolai Josuttis Opened: 2010-10-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
While bucket_size()
is const for unordered_set
, for all other unordered containers it is not defined as
constant member function.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T> > > class unordered_map { public: [..] // bucket interface size_type bucket_count() const; size_type max_bucket_count() const; size_type bucket_size(size_type n) const; [..]
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T> > > class unordered_multimap { public: [..] // bucket interface size_type bucket_count() const; size_type max_bucket_count() const; size_type bucket_size(size_type n) const; [..]
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > class unordered_multiset { public: [..] // bucket interface size_type bucket_count() const; size_type max_bucket_count() const; size_type bucket_size(size_type n) const; [..]
INVOKE
on member data pointer with too many argumentsSection: 22.10.4 [func.require] Status: C++11 Submitter: Howard Hinnant Opened: 2010-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.8.2 [func.require] p1 says:
1 Define
INVOKE(f, t1, t2, ..., tN)
as follows:
(t1.*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
;((*t1).*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is not one of the types described in the previous item;t1.*f
whenf
is a pointer to member data of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
;(*t1).*f
whenf
is a pointer to member data of a classT
andt1
is not one of the types described in the previous item;f(t1, t2, ..., tN)
in all other cases.
The question is: What happens in the 3rd and
4th bullets when N > 1
?
Does the presence of t2, ..., tN
get ignored, or does it make the
INVOKE
ill formed?
Here is sample code which presents the problem in a concrete example:
#include <functional> #include <cassert> struct S { char data; }; typedef char S::*PMD; int main() { S s; PMD pmd = &S::data; std::reference_wrapper<PMD> r(pmd); r(s, 3.0) = 'a'; // well formed? assert(s.data == 'a'); }
Without the "3.0
" the example is well formed.
[Note: Daniel provided wording to make it explicit that the above example is ill-formed. — end note ]
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
Change 20.8.2 [func.require]/1 as indicated:
1 Define
INVOKE(f, t1, t2, ..., tN)
as follows:
- ...
- ...
t1.*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
;(*t1).*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is not one of the types described in the previous item;- ...
conj
specification is now nonsenseSection: 29.4.10 [cmplx.over] Status: C++11 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Pittsburgh, we accepted the resolution of library issue 1137(i), to add a sentence 3 to [cmplx.over]:
All the specified overloads shall have a return type which is the nested
value_type
of the effectively cast arguments.
This was already true for four of the six functions except conj
and
proj
. It is not completely unreasonable to make proj
return
the real value only, but the IEC specification does call for an imaginary part
of -0 in some circumstances. The people who care about these distinctions really
care, and it is required by an international standard.
Making conj
return just the real part breaks it horribly, however. It is
well understood in mathematics that conj(re + i*im)
is (re - i*im)
,
and it is widely used. The accepted new definition makes conj
useful only
for pure real operations. This botch absolutely must be fixed.
[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Remove the recently added paragraph 3 from [cmplx.over]:
3 All the specified overloads shall have a return type which is the nestedvalue_type
of the effectively cast arguments.
noexcept
for Clause 29Section: 32.5 [atomics] Status: Resolved Submitter: Hans Boehm Opened: 2010-11-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-63 for Clause 29
Clause 29 does not specify noexcept for any of the atomic operations. It probably should, though that's not completely clear. In particular, atomics may want to throw in implementations that support transactional memory.
Proposed resolution:
Apply paper N3251, noexcept for the Atomics Library.
Section: 17.6.3.5 [new.delete.dataraces] Status: C++11 Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-34
Technical details:
When the same unit of storage is allocated and deallocated repeatedly, operations on it can't be allowed to race between the allocator and the user program. But I don't see any mention of happens-before in the descriptions of allocation and deallocation functions. Proposed resolution (not wording yet):The call to an allocation function returning a pointer P
must happen-before the matching
deallocation call with P
as a parameter. Otherwise the behavior is undefined. I don't know whether
receiving P
with memory_order_consume
fits this requirement. memory_order_relaxed
does not.
If some memory is passed to a deallocation function, the implementation must ensure that the deallocation call happens-before any allocation call that returns the same memory address.
[2011-02-26: Hans comments and drafts wording]
The second requirement already exists, almost verbatim, as 17.6.3.5 [new.delete.dataraces] p. 1. I think this is where the statement belongs. However, this paragraph requires work to correctly address the first part of the issue.
[Adopted at Madrid, 2011-03]
Proposed resolution:
Change 17.6.3.5 [new.delete.dataraces] p. 1 as follows:
1
The library versions ofFor purposes of determining the existence of data races, the library versions ofoperator new
andoperator delete
, user replacement versions of globaloperator new
andoperator delete
, and the C standard library functionscalloc
,malloc
,realloc
, andfree
shall not introduce data races (6.9.2 [intro.multithread]) as a result of concurrent calls from different threads.operator new
, user replacement versions of globaloperator new
, and the C standard library functionscalloc
andmalloc
shall behave as though they accessed and modified only the storage referenced by the return value. The library versions ofoperator delete
, user replacement versions ofoperator delete
, and the C standard library functionfree
shall behave as though they accessed and modified only the storage referenced by their first argument. The C standard libraryrealloc
function shall behave as though it accessed and modified only the storage referenced by its first argument and by its return value. Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.
resize(size())
on a vector
Section: 23.3.13.3 [vector.capacity] Status: C++11 Submitter: BSI Opened: 2011-03-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-117
23.3.13.3 [vector.capacity] p. 9 (Same as for 23.3.5.3 [deque.capacity] p. 1 i.e.
deque::resize
). There is no mention of what happens if sz==size()
. While
it obviously does nothing I feel a standard needs to say this explicitely.
Suggested resolution:
Append "If sz == size()
, does nothing" to the effects.
[2011-03-24 Daniel comments]
During the edit of this issue some non-conflicting overlap with 2033(i) became obvious.
CopyInsertable
should be MoveInsertable
and there is missing the DefaultConstructible
requirements, but this should be fixed by 2033(i).
Proposed resolution:
Change 23.3.13.3 [vector.capacity] p. 9 as follows:
void resize(size_type sz);9 Effects: If
10 Requires:sz <= size()
, equivalent toerase(begin() + sz, end());
. Ifsize() < sz
, appendssz - size()
value-initialized elements to the sequence.T
shall beCopyInsertable
into*this
.
Section: 16.4.6.10 [res.on.data.races] Status: Resolved Submitter: BSI Opened: 2011-03-24 Last modified: 2016-01-28
Priority: 3
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Discussion:
Addresses GB-111
Section 16.4.6.10 [res.on.data.races], Data Race Avoidance, requires the C++ Standard Library to avoid data races that might otherwise result from two threads making calls to C++ Standard Library functions on distinct objects. The C standard library is part of the C++ Standard Library and some C++ Standary library functions (parts of the Localization library, as well as Numeric Conversions in 21.5), are specified to make use of the C standard library. Therefore, the C++ standard indirectly imposes a requirement on the thread safety of the C standard library. However, since the C standard does not address the concept of thread safety conforming C implementations exist that do no provide such guarantees. This conflict needs to be reconciled.
Suggested resolution by national body comment:
remove the requirement to make use of
strtol()
andsprintf()
since these functions depend on the global C locale and thus cannot be made thread safe.
[2011-03-24 Madrid meeting]
Deferred
[ 2011 Bloomington ]
Alisdair: PJ, does this cause a problem in C?
PJ: Every implementation know of is thread safe.
Pete: There a couple of effects that are specified on strtol() and sprintf() which is a problem.
PJ: When C++ talks about C calls it should be "as if" calling the function.
Pete: Culprit is to string stuff. My fault.
PJ: Not your fault. You did what you were told. Distinct resolution to change wording.
Dietmar: What would we break if we change it back?
Pete: Nothing. If implemented on top of thread safe C library you are just fine.
Alisdair: Anyone want to clean up wording and put it back to what Pete gave us?
Alisdair: No volunteers. Do we want to mark as NAD? We could leave it as deferred.
Stefanus: Did original submitter care about this?
Lawrence: There is some work to make local calls thread safe. The resolution would be to call those thread safe version.
Pete: "As if called under single threaded C program"
Action Item (Alisdair): Write wording for this issue.
[2012, Kona]
Re-opened at the request of the concurrency subgroup, who feel there is an issue that needs clarifying for the (planned) 2017 standard.
Rationale:
No consensus to make a change at this time
[2012, Portland]
The concurrency subgroup decided to encourage the LWG to consider a change to 16.2 [library.c] or thereabouts
to clarify that we are requiring C++-like thread-safety for setlocale
, so that races are not introduced
by C locale accesses, even when the C library allows it. This would require e.g. adding "and data race avoidance"
at the end of 16.2 [library.c] p1:
"The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety and data race avoidance.",
with some further clarifications in the sections mentioned in 1526(i).
This seems to be consistent with existing implementations. This would technically not be constraining C implementation, but it would be further constraining C libraries used for both C and C++.
[Lenexa 2015-05-05: Move to Resolved]
JW: it's a bit odd that the issue title says sould not impose requirements on C libs, then the P/R does exactly that. Does make sense though, previously we imposed an implicit requirement which would not have been met. Now we say it explicitly and require it is met.
STL: I think this is Resolved, it has been fixed in the working paper [support.runtime]/6 is an example where we call out where things can race. That implies that for everything else they don't create races.
JW: I'm not sure, I think we still need the "and data race avoidance" to clarify that the features from C avoid races, even though C99 says no such thing.
STL: [library.c] says that something like sqrt is part of the C++ Standard LIbrary. [res.on.data.races] then applies to them. Would be OK with a note there, but am uncomfortable with "and data race avoidance" which sounds like it's making a very strong guarantee.
ACTION ITEM JW to editorially add note to [library.c] p1: "Unless otherwise specified, the C Standard Library functions shall meet the requirements for data race avoidance (xref [res.on.data.races])"
Move to Resolved?
10 in favor, 0 opposed, 3 abstentions
Proposed resolution:
This wording is relative to N3376.
Change 16.2 [library.c] p1 as indicated:
-1- The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety and data race avoidance.
packaged_task
specialization of uses_allocator
Section: 32.10.10.3 [futures.task.nonmembers] Status: C++11 Submitter: Howard Hinnant Opened: 2010-08-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
[futures.task.nonmembers]/3 says:
template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc>;
This is a declaration, but should be a definition.
Proposed resolution:
Change [futures.task.nonmembers]/3:
template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc>;: true_type {};
basic_regex
uses non existent string_type
Section: 28.6.7.3 [re.regex.assign] Status: C++11 Submitter: Volker Lukas Opened: 2010-10-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In working draft N3126, subclause 28.6.7.3 [re.regex.assign], paragraphs 12, 13 and 19,
the name string_type
is used. This is presumably a typedef for basic_string<value_type>
, where
value_type
is the character type used by basic_regex
. The basic_regex
template however defines no such typedef, and neither does the <regex>
header or the <initializer_list>
header included by <regex>
.
[ 2010-11-03 Daniel comments and suggests alternative wording: ]
The proposed resolution needs to use
basic_string<charT>
instead ofbasic_string<char>
Previous Proposed Resolution:
Make the following changes to [re.regex.assign]:basic_regex& assign(const charT* ptr, flag_type f = regex_constants::ECMAScript);12 Returns:
assign(
.string_typebasic_string<charT>(ptr), f)basic_regex& assign(const charT* ptr, size_t len, flag_type f = regex_constants::ECMAScript);13 Returns:
assign(
.string_typebasic_string<charT>(ptr, len), f)[..] template <class InputIterator> basic_regex& assign(InputIterator first, InputIterator last, flag_type f = regex_constants::ECMAScript);18 Requires: The type
InputIterator
shall satisfy the requirements for an Input Iterator (24.2.3).19 Returns:
assign(
.string_typebasic_string<charT>(first, last), f)
[ 2010 Batavia ]
Unsure if we should just give basic_regex
a string_type
typedef. Looking for when string_type
was
introduced into regex
. Howard to draft wording for typedef typename traits::string_type string_type
, then move to Review.
[ 2011-02-16: Daniel comments and provides an alternative resolution. ]
I'm strongly in favour with the Batavia idea to provide a separate string_type
within
basic_regex
, but it seems to me that the issue resultion should add one more
important typedef, namely that of the traits type! Currently, basic_regex
is the
only template that does not publish the type of the associated traits type. Instead
of opening a new issue, I added this suggestion as part of the proposed wording.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 6 votes.
Proposed resolution:
Change the class template basic_regex
synopsis, 28.6.7 [re.regex] p. 3, as indicated:
namespace std { template <class charT, class traits = regex_traits<charT> > class basic_regex { public: // types: typedef charT value_type; typedef traits traits_type; typedef typename traits::string_type string_type; typedef regex_constants::syntax_option_type flag_type; typedef typename traits::locale_type locale_type; [..] }; }
match_results
does not specify the semantics of operator==
Section: 28.6.9.9 [re.results.nonmember] Status: Resolved Submitter: Daniel Krügler Opened: 2010-10-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
The Returns element of operator==
says:
true
only if the two objects refer to the same match
It is not really clear what this means: The current specification would allow for an
implementation to return true
, only if the address values of m1
and
m2
are the same. While this approach is unproblematic in terms of used operations
this is also a bit unsatisfactory. With identity equality alone there seems to be no convincing
reason to provide this operator at all. It could for example also refer to an comparison based
on iterator values. In this case a user should better know that this will be done, because
there is no guarantee at all that inter-container comparison of iterators
is a feasible operation. This was a clear outcome of the resolution provided in
N3066
for LWG issue 446(i).
It could also mean that a character-based comparison of the individual sub_match
elements should be done - this would be equivalent to applying operator==
to
the subexpressions, prefix and suffix.
Proposed resolution:
Addressed by paper n3158.
Section: 27.4.3.2 [string.require] Status: C++14 Submitter: José Daniel García Sánchez Opened: 2010-10-21 Last modified: 2016-11-12
Priority: 0
View all other issues in [string.require].
View all issues with C++14 status.
Discussion:
Clause 21.4.1 [string.require]p3 states:
No
erase()
orpop_back()
member function shall throw any exceptions.
However in 21.4.6.5 [string.erase] p2 the first version of erase
has
Throws:
out_of_range
ifpos > size()
.
[2011-03-24 Madrid meeting]
Beman: Don't want to just change this, can we just say "unless otherwise specified"?
Alisdair: Leave open, but update proposed resolution to say something like "unless otherwise specified". General agreement that it should be corrected but not a stop-ship. Action: Update proposed wording for issue 2003 as above, but leave Open.[2014-02-12 Issaquah meeting]
Jeffrey: Madrid meeting's proposed wording wasn't applied, and it's better than the original proposed wording. However, this sentence is only doing 3 functions' worth of work, unlike the similar paragraphs in 23.2.2 [container.requirements.general]. Suggest just putting "Throws: Nothing" on the 3 functions.
[2014-02-13 Issaquah meeting]
Move as Immmediate
Proposed resolution:
Remove [string.require]p/3:
3 Noerase()
orpop_back()
member function shall throw any exceptions.
Add to the specifications of iterator erase(const_iterator p);
, iterator erase(const_iterator first, const_iterator last);
,
and void pop_back();
in 27.4.3.7.5 [string.erase]:
Throws: Nothing
duration::operator*
has template parameters in funny orderSection: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
View all issues with C++11 status.
Discussion:
In [time] and [time.duration.nonmember] we have:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);
Everywhere else, we always have <rep, period>
in that order for a given
type. But here, we have Period
and Rep2
in reverse order for
<Rep2, Period>
. This is probably of little importance, since the
template parameters are seldom spelled out for a function like this. But changing it
now will eliminate a potential source of future errors and confusion.
Proposed resolution:
Change the signature in [time] and [time.duration.nonmember] to:
template <class Rep1, classPeriodRep2, classRep2Period> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);
unordered_map::insert(T&&)
protection should apply to map
tooSection: 23.4.3.4 [map.modifiers], 23.4.4.3 [multimap.modifiers], 23.5.3.4 [unord.map.modifiers], 23.5.4.3 [unord.multimap.modifiers] Status: C++14 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [map.modifiers].
View all issues with C++14 status.
Discussion:
In [unord.map.modifiers], the signature:
template <class P> pair<iterator, bool> insert(P&& obj);
now has an added Remarks paragraph:
Remarks: This signature shall not participate in overload resolution unless
P
is implicitly convertible tovalue_type
.
The same is true for unordered_multimap
.
map
nor multimap
have this constraint, even though it is a
Good Thing(TM) in those cases as well.
[ The submitter suggests: Add the same Remarks clause to [map.modifiers] and [multimap.modifiers]. ]
[ 2010-10-29 Daniel comments: ]
I believe both paragraphs need more cleanup: First, the current Requires element conflict with the Remark;
second, it seems to me that the whole single Requires element is intended to be split into a Requires
and an Effects element; third, the reference to tuple
is incorrect (noticed by Paolo Carlini);
fourth, it refers to some non-existing InputIterator
parameter relevant for a completely different
overload; sixth, the return type of the overload with hint is wrong.
The following proposed resolution tries to solve these issues as well and uses similar wording as for
the corresponding unordered containers. Unfortunately it has some redundancy over Table 99, but I did
not remove the specification because of the more general template parameter P
- the Table 99
requirements apply only for an argument identical to value_type
.
- Change 23.4.3.4 [map.modifiers] around p. 1 as indicated:
template <class P> pair<iterator, bool> insert(P&& x); template <class P>pair<iterator, bool>insert(const_iterator position, P&& x);1 Requires:
P
shall be convertible tovalue_type
is constructible fromstd::forward<P>(x)
..IfP
is instantiated as a reference type, then the argumentx
is copied from. Otherwisex
is considered to be an rvalue as it is converted tovalue_type
and inserted into the map. Specifically, in such casesCopyConstructible
is not required ofkey_type
ormapped_type
unless the conversion fromP
specifically requires it (e.g., ifP
is atuple<const key_type, mapped_type>
, thenkey_type
must beCopyConstructible
). The signature takingInputIterator
parameters does not requireCopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type,mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
? Effects: Insertsx
converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(x)
. For the second form, the iteratorposition
is a hint pointing to where the search should start. ? Returns: For the first form, thebool
component of the returnedpair
object indicates whether the insertion took place and the iterator component - or for the second form the returned iterator - points to the element with key equivalent to the key ofvalue_type(x)
. ? Complexity: Logarithmic in general, but amortized constant ifx
is inserted right beforeposition
. ? Remarks: These signatures shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
.- Change 23.4.4.3 [multimap.modifiers] around p. 1 as indicated:
template <class P> iterator insert(P&& x); template <class P> iterator insert(const_iterator position, P&& x);1 Requires:
P
shall be convertible tovalue_type
is constructible fromstd::forward<P>(x)
.IfP
is instantiated as a reference type, then the argumentx
is copied from. Otherwisex
is considered to be an rvalue as it is converted tovalue_type
and inserted into the map. Specifically, in such casesCopyConstructible
is not required ofkey_type
ormapped_type
unless the conversion fromP
specifically requires it (e.g., ifP
is atuple<const key_type, mapped_type>
, thenkey_type
must beCopyConstructible
). The signature takingInputIterator
parameters does not requireCopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type, mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
? Effects: Insertsx
converted tovalue_type
. For the second form, the iteratorposition
is a hint pointing to where the search should start. ? Returns: An iterator that points to the element with key equivalent to the key ofvalue_type(x)
. ? Complexity: Logarithmic in general, but amortized constant ifx
is inserted right beforeposition
. ? Remarks: These signatures shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
.
[ 2010 Batavia: ]
We need is_convertible
, not is_constructible
, both in ordered and unordered containers.
[ 2011 Bloomington ]
The effects of these inserts can be concisely stated in terms of emplace(). Also, the correct term is "EmplaceConstructible", not "constructible".
New wording by Pablo, eliminating duplicate requirements already implied by the effects clause. Move to Review.
[ 2011-10-02 Daniel comments and refines the proposed wording ]
Unfortunately the template constraints expressed as "
P
is implicitly convertible tovalue_type
" reject the intended effect to support move-only key types, which was the original intention when the library became move-enabled through the rvalue-reference proposals by Howard (This can clearly be deduced from existing carefully selected wording that emphasizes thatCopyConstructible
is only required for special situations involving lvalues or const rvalues as arguments). The root of the problem is based on current core rules, where an "implicitly converted" value has copy-initialization semantics. Consider a move-only key typeKM
, some mapped typeT
, and a source valuep
of typeP
equal tostd::pair<KM, T>
, this is equivalent to:std::pair<const KM, T> dest = std::move(p);Now 9.5 [dcl.init] p16 b6 sb2 says that the effects of this heterogeneous copy-initialization (
But the actual code that is required (with the default allocator) is simply a direct-initialization fromp
has a different type thandest
) are as-if a temporary of the target typestd::pair<const KM, T>
is produced from the rvaluep
of typeP
(which is fine), and this temporary is used to initializedest
. This second step cannot succeed, because we cannot move fromconst KM
toconst KM
. This means thatstd::is_convertible<P, std::pair<const KM, T>>::value
is false.P
tovalue_type
, so imposing an implicit conversion is more than necessary. Therefore I strongly recommend to reduce the "overload participation" constraint tostd::is_constructible<std::pair<const KM, T>, P>::value
instead. This change is the only change that has been performed to the previous proposed wording from Pablo shown below.
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup, after much discussion on Daniel's analysis of Copy Initialization and move semantics, which we ultimately agreed with.
[2012, Portland: applied to WP]
Proposed resolution:
template <class P> pair<iterator, bool> insert(P&& x); template <class P>pair<iterator, bool>insert(const_iterator position, P&& x);1 Requires:P
shall be convertible tovalue_type
.IfP
is instantiated as a reference type, then the argumentx
is copied from. Otherwisex
is considered to be an rvalue as it is converted tovalue_type
and inserted into the map. Specifically, in such casesCopyConstructible
is not required ofkey_type
ormapped_type
unless the conversion fromP
specifically requires it (e.g., ifP
is atuple<const key_type, mapped_type>
, thenkey_type
must beCopyConstructible
). The signature takingInputIterator
parameters does not requireCopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type,mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
? Effects: The first form is equivalent toreturn emplace(std::forward<P>(x))
. The second form is equivalent toreturn emplace_hint(position, std::forward<P>(x))
. ? Remarks: These signatures shall not participate in overload resolution unlessstd::is_constructible<value_type, P&&>::value
is true.
template <class P> iterator insert(P&& x); template <class P> iterator insert(const_iterator position, P&& x);1 Requires:P
shall be convertible tovalue_type
.IfP
is instantiated as a reference type, then the argumentx
is copied from. Otherwisex
is considered to be an rvalue as it is converted tovalue_type
and inserted into the map. Specifically, in such casesCopyConstructible
is not required ofkey_type
ormapped_type
unless the conversion fromP
specifically requires it (e.g., ifP
is atuple<const key_type, mapped_type>
, thenkey_type
must beCopyConstructible
). The signature takingInputIterator
parameters does not requireCopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type, mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
? Effects: The first form is equivalent toreturn emplace(std::forward<P>(x))
. The second form is equivalent toreturn emplace_hint(position, std::forward<P>(x))
. ? Remarks: These signatures shall not participate in overload resolution unlessstd::is_constructible<value_type, P&&>::value
is true.
template <class P> pair<iterator, bool> insert(P&& obj);1 Requires:2 Effects: equivalent tovalue_type
is constructible fromstd::forward<P>(obj)
.return emplace(std::forward<P>(obj))
.Inserts obj converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(obj)
.3 Returns: The bool component of the returned pair object indicates whether the insertion took place and the iterator component points to the element with key equivalent to the key ofvalue_type(obj)
.4 Complexity: Average case O(1), worst case O(size()).53 Remarks: This signature shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
std::is_constructible<value_type, P&&>::value
is true.template <class P> iterator insert(const_iterator hint, P&& obj);6 Requires:value_type
is constructible fromstd::forward<P>(obj)
.7? Effects: equivalent toreturn emplace_hint(hint, std::forward<P>(obj))
.Inserts obj converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(obj)
. The iterator hint is a hint pointing to where the search should start.8 Returns: An iterator that points to the element with key equivalent to the key ofvalue_type(obj)
.9 Complexity: Average case O(1), worst case O(size()).10? Remarks: This signature shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
std::is_constructible<value_type, P&&>::value
is true.
template <class P> iterator insert(P&& obj);1 Requires:2 Effects: equivalent tovalue_type
is constructible fromstd::forward<P>(obj)
.return emplace(std::forward<P>(obj))
.Inserts obj converted tovalue_type
.3 Returns: An iterator that points to the element with key equivalent to the key ofvalue_type(obj)
.4 Complexity: Average case O(1), worst case O(size()).53 Remarks: This signature shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
std::is_constructible<value_type, P&&>::value
is true.template <class P> iterator insert(const_iterator hint, P&& obj);6 Requires:value_type
is constructible fromstd::forward<P>(obj)
.7? Effects: equivalent toreturn emplace_hint(hint, std::forward<P>(obj))
.Inserts obj converted tovalue_type
. The iterator hint is a hint pointing to where the search should start.8 Returns: An iterator that points to the element with key equivalent to the key ofvalue_type
(obj).9 Complexity: Average case O(1), worst case O(size()).10? Remarks: This signature shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
std::is_constructible<value_type, P&&>::value
is true.
map<>::at()
Section: 23.4.3.3 [map.access] Status: C++11 Submitter: Matt Austern Opened: 2010-11-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [map.access].
View all issues with C++11 status.
Discussion:
In [map.access]/9, the Returns clause for map<Key, T>::at(x)
says
that it returns "a reference to the element whose key is equivalent to x
." That can't be right.
The signature for at()
says that its return type is T
, but the elements
of map<Key, T>
have type pair<const K, T>
. (I checked [unord.map.elem]
and found that its specification of at()
is correct. This is a problem for map
only.)
Proposed resolution:
Change the wording in [map.access]/9 so it's identical to what we already say for operator[]
,
which is unambiguous and correct.
Returns: A reference to the
element whose key is equivalentmapped_type
corresponding tox
in*this
.
packaged_task::operator()
Section: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: Pete Becker Opened: 2010-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task.members].
View all issues with C++11 status.
Discussion:
The Throws clause for packaged_task::operator()
says that it throws "a
future_error
exception object if there is no associated asynchronous
state or the stored task has already been invoked." However, the Error
Conditions clause does not define an error condition when the stored task has
already been invoked, only when the associated state is already ready (i.e. the
invocation has completed).
[2011-02-17 Anthony provides an alternative resolution]
Previous proposed resolution:
Change the first bullet item in 32.10.10.2 [futures.task.members] /22:
void operator()(ArgTypes... args);20 ...
21 ...
22 Error conditions:
promise_already_satisfied
ifthe associated asynchronous state is already readyoperator()
has already been called.no_state
if*this
has no associated asynchronous state.
[Adopted at Madrid, 2011-03]
Proposed resolution:
Change the first bullet item in 32.10.10.2 [futures.task.members] p. 17:
void operator()(ArgTypes... args);15 ...
16 ...
17 Error conditions:
promise_already_satisfied
if theassociated asynchronous state is already readystored task has already been invoked.no_state
if*this
has no associated asynchronous state.
Change the first bullet item in 32.10.10.2 [futures.task.members] p. 21:
void make_ready_at_thread_exit(ArgTypes... args);19 ...
20 ...
21 Error conditions:
promise_already_satisfied
if theassociated asynchronous state already has a stored value or exceptionstored task has already been invoked.no_state
if*this
has no associated asynchronous state.
Section: 27.4.5 [string.conversions] Status: C++14 Submitter: Alisdair Meredith Opened: 2010-07-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.conversions].
View all issues with C++14 status.
Discussion:
The functions (w
)stoi
and (w
)stof
are specified in terms of calling C library APIs for potentially wider
types. The integer and floating-point versions have subtly different
behaviour when reading values that are too large to convert. The
floating point case will throw out_of_bound
if the read value
is too large to convert to the wider type used in the implementation,
but behaviour is undefined if the converted value cannot narrow to a
float. The integer case will throw out_of_bounds
if the
converted value cannot be represented in the narrower type, but throws
invalid_argument
, rather than out_of_range
, if the
conversion to the wider type fails due to overflow.
Suggest that the Throws clause for both specifications should be consistent, supporting the same set of fail-modes with the matching set of exceptions.
Proposed resolution:
21.5p3 [string.conversions]
int stoi(const string& str, size_t *idx = 0, int base = 10); long stol(const string& str, size_t *idx = 0, int base = 10); unsigned long stoul(const string& str, size_t *idx = 0, int base = 10); long long stoll(const string& str, size_t *idx = 0, int base = 10); unsigned long long stoull(const string& str, size_t *idx = 0, int base = 10);...
3 Throws:
invalid_argument
ifstrtol
,strtoul
,strtoll
, orstrtoull
reports that no conversion could be performed. Throwsout_of_range
ifstrtol
,strtoul
,strtoll
orstrtoull
setserrno
toERANGE
, or if the converted value is outside the range of representable values for the return type.
21.5p6 [string.conversions]
float stof(const string& str, size_t *idx = 0); double stod(const string& str, size_t *idx = 0); long double stold(const string& str, size_t *idx = 0);...
6 Throws:
invalid_argument
ifstrtod
orstrtold
reports that no conversion could be performed. Throwsout_of_range
ifstrtod
orstrtold
setserrno
toERANGE
or if the converted value is outside the range of representable values for the return type.
is_* traits
for binding operations can't be meaningfully specializedSection: 22.10.15.2 [func.bind.isbind] Status: C++14 Submitter: Sean Hunt Opened: 2010-07-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.bind.isbind].
View all issues with C++14 status.
Discussion:
22.10.15.2 [func.bind.isbind] says for is_bind_expression
:
Users may specialize this template to indicate that a type should be treated as a subexpression in a
bind
call.
But it also says:
If
T
is a type returned frombind
,is_bind_expression<T>
shall be publicly derived fromintegral_constant<bool, true>
, otherwise fromintegral_constant<bool, false>
.
This means that while the user is free to specialize, any specialization
would have to be false
to avoid violating the second
requirement. A similar problem exists for is_placeholder
.
[ 2010 Batavia (post meeting session) ]
Alisdair recognises this is clearly a bug introduced by some wording he
wrote, the sole purpose of this metafunction is as a customization point
for users to write their own bind
-expression types that participate
in the standard library bind
protocol. The consensus was that this
should be fixed in Madrid, moved to Open.
[2011-05-13 Jonathan Wakely comments and provides proposed wording]
The requirements are that is_bind_expression<T>::value
is true when T
is a type returned from bind
, false for any other type, except when
there's a specialization involving a user-defined type (N.B. 16.4.5.2.1 [namespace.std]
means we don't need to say e.g. is_bind_expression<string>
is false.)
integral_constant<bool, false>
and for implementations
to provide specializations for the unspecified types returned from
bind
. User-defined specializations can do whatever they like, as long
as is_bind_expression::value
is sane. There's no reason to forbid
users from defining is_bind_expression<user_defined_type>::value=false
if that's what they want to do.
Similar reasoning applies to is_placeholder
, but a further issue is
that 22.10.15.2 [func.bind.isbind] contains wording for is_placeholder
but
contains no definition of it and the sub-clause name only refers to
is_bind_expression
. The wording below proposes splitting paragraphs 3
and 4 of 22.10.15.2 [func.bind.isbind] into a new sub-clause covering
is_placeholder
.
If the template specializations added by the proposed wording are too
vague then they could be preceded by "for exposition only" comments
[2011-05-18 Daniel comments and provides some refinements to the P/R]
Both bind
-related type traits should take advantage of the
UnaryTypeTrait requirements. Additionally, the updated wording does not
imply that the implementation provides several specializations. Wording was
used similar to the specification of the uses_allocator
type trait
(which unfortunately is not expressed in terms of BinaryTypeTrait requirements).
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS.
Change 22.10.15.2 [func.bind.isbind] to:
namespace std { template<class T> struct is_bind_expression; // see below: integral_constant<bool, see below> { };}-1-
-2-is_bind_expression
can be used to detect function objects generated bybind
.bind
usesis_bind_expression
to detect subexpressions.Users may specialize this template to indicate that a type should be treated as a subexpression in abind
call.IfInstantiations of theT
is a type returned frombind
,is_bind_expression<T>
shall be publicly derived fromintegral_constant<bool, true>
, otherwise fromintegral_constant<bool, false>
is_bind_expression
template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic oftrue_type
ifT
is a type returned frombind
, otherwise it shall have a BaseCharacteristic offalse_type
. A program may specialize this template for a user-defined typeT
to have a BaseCharacteristic oftrue_type
to indicate thatT
should be treated as a subexpression in abind
call..-3-is_placeholder
can be used to detect the standard placeholders_1
,_2
, and so on.bind
usesis_placeholder
to detect placeholders. Users may specialize this template to indicate a placeholder type.-4- IfT
is the type ofstd::placeholders::_J
,is_placeholder<T>
shall be publicly derived fromintegral_constant<int, J>
, otherwise fromintegral_constant<int, 0>
.
Insert a new sub-clause immediately following sub-clause 22.10.15.2 [func.bind.isbind], the suggested sub-clause tag is [func.bind.isplace]:
is_placeholder
[func.bind.isplace]namespace std { template<class T> struct is_placeholder; // see below }-?-
-?- Instantiations of theis_placeholder
can be used to detect the standard placeholders_1
,_2
, and so on.bind
usesis_placeholder
to detect placeholders.is_placeholder
template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic ofintegral_constant<int, J>
ifT
is the type ofstd::placeholders::_J
, otherwise it shall have a BaseCharacteristic ofintegral_constant<int, 0>
. A program may specialize this template for a user-defined typeT
to have a BaseCharacteristic ofintegral_constant<int, N>
withN > 0
to indicate thatT
should be treated as a placeholder type.
Section: 27.4.4.4 [string.io] Status: C++14 Submitter: James Kanze Opened: 2010-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.io].
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Discussion:
What should the following code output?
#include <string> #include <iostream> #include <iomanip> int main() { std::string test("0X1Y2Z"); std::cout.fill('*'); std::cout.setf(std::ios::internal, std::ios::adjustfield); std::cout << std::setw(8) << test << std::endl; }
I would expect "**0X1Y2Z
", and this is what the compilers I have access
to (VC++, g++ and Sun CC) do. But according to the standard, it should be
"0X**1Y2Z
":
27.4.4.4 [string.io]/5:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]). After constructing a
sentry
object, if this object returnstrue
when converted to a value of typebool
, determines padding as described in 28.3.4.3.3.3 [facet.num.put.virtuals], then inserts the resulting sequence of characters seq as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size()
; then callsos.width(0)
.
28.3.4.3.3.3 [facet.num.put.virtuals]/5:
[…]
Stage 3: A local variable is initialized as
fmtflags adjustfield= (flags & (ios_base::adjustfield));The location of any padding is determined according to Table 88.
If
str.width()
is nonzero and the number ofcharT
's in the sequence after stage 2 is less thanstr.width()
, then enough fill characters are added to the sequence at the position indicated for padding to bring the length of the sequence tostr.width()
.str.width(0)
is called.
Table 88 — Fill padding State Location adjustfield == ios_base::left
pad after adjustfield == ios_base::right
pad before adjustfield == internal
and a sign occurs in the representationpad after the sign adjustfield == internal
and representation after stage 1 began with 0x or 0Xpad after x or X otherwise pad before
Although it's not 100% clear what "the sequence after stage 2" should mean here,
when there is no stage 2, the only reasonable assumption is that it is the
contents of the string being output. In the above code, the string being output
is "0X1Y2Z
", which starts with "0X
", so the padding should be
inserted "after x or X", and not before the string. I believe that this is a
defect in the standard, and not in the three compilers I tried.
[ 2010 Batavia (post meeting session) ]
Consensus that all known implementations are consistent, and disagree with the standard. Preference is to fix the standard before implementations start trying to conform to the current spec, as the current implementations have the preferred form. Howard volunteered to drught for Madrid, move to Open.
[2011-03-24 Madrid meeting]
Daniel Krügler volunteered to provide wording, interacting with Dietmar and Bill.
[2011-06-24 Daniel comments and provides wording]
The same problem applies to the output provided by const char*
and similar
character sequences as of 31.7.6.3.4 [ostream.inserters.character] p. 5. and even for
single character output (!) as described in 31.7.6.3.4 [ostream.inserters.character] p. 1,
just consider the character value '-' where '-' is the sign character. In this case
Table 91 — "Fill padding" requires to pad after the sign, i.e. the output
for the program
#include <iostream> #include <iomanip> int main() { char c = '-'; std::cout.fill('*'); std::cout.setf(std::ios::internal, std::ios::adjustfield); std::cout << std::setw(2) << c << std::endl; }
According to the current wording this program should output "-*
", but
all tested implementations output "*-
" instead.
money_put
functions.
[ 2011 Bloomington ]
Move to Review, the resolution seems correct but it would be nice if some factoring of the common words were proposed.
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup.
While better factoring of the common words is desirable, it is also editorial and should not hold up the progress of this issue. As the edits impact two distinct clauses, it is not entirely clear what a better factoring should look like.
[2012, Portland: applied to WP]
Proposed resolution:
The new wording refers to the FDIS numbering.
Change 27.4.4.4 [string.io]/5 as indicated:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);-5- Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]). After constructing a sentry object, if this object returns
true
when converted to a value of typebool
, determines padding asdescribed in [facet.num.put.virtuals],follows: AcharT
character sequence is produced, initially consisting of the elements defined by the range[str.begin(), str.end())
. Ifstr.size()
is less thanos.width()
, then enough copies ofos.fill()
are added to this sequence as necessary to pad to a width ofos.width()
characters. If(os.flags() & ios_base::adjustfield) == ios_base::left
istrue
, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. Tthen inserts the resulting sequence of charactersseq
as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size()
; then callsos.width(0)
.
Change 31.7.6.3.4 [ostream.inserters.character]/1 as indicated (An additional editorial fix is suggested for the first prototype declaration):
template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, charT c}); template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, char c); // specialization template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, char c); // signed and unsigned template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, signed char c); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, unsigned char c);-1- Effects: Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of
out
. After a sentry object is constructed it inserts characters. In casec
has typechar
and the character type of the stream is notchar
, then the character to be inserted isout.widen(c)
; otherwise the character isc
. Padding is determined asdescribed in [facet.num.put.virtuals]follows: A character sequence is produced, initially consisting of the insertion character. Ifout.width()
is greater than one, then enough copies ofout.fill()
are added to this sequence as necessary to pad to a width ofout.width()
characters. If(out.flags() & ios_base::adjustfield) == ios_base::left
istrue
, the fill characters are placed after the insertion character; otherwise, they are placed before the insertion character.The insertion character and any required padding are inserted intowidth(0)
is called.out
; then callsos.width(0)
.
Change 31.7.6.3.4 [ostream.inserters.character]/5 as indicated:
template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, const charT* s); template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, const char* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const char* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const signed char* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const unsigned char* s);[…]
-5- Padding is determined asdescribed in [facet.num.put.virtuals]. Thefollows: A character sequence is produced, initially consisting of the elements defined by then
characters starting ats
are widened usingout.widen
([basic.ios.members])n
characters starting ats
widened usingout.widen
([basic.ios.members]). Ifn
is less thanout.width()
, then enough copies ofout.fill()
are added to this sequence as necessary to pad to a width ofout.width()
characters. If(out.flags() & ios_base::adjustfield) == ios_base::left
istrue
, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. The widened characters and any required padding are inserted intoout
. Callswidth(0)
.
pair
, not tuple
Section: 23.4 [associative] Status: Resolved Submitter: Paolo Carlini Opened: 2010-10-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I'm seeing something strange in the paragraphs 23.4.3.4 [map.modifiers] and 23.4.4.3 [multimap.modifiers]:
they both talk about tuple<const key_type, mapped_type>
but I think they
should be talking about pair<const key_type, mapped_type>
because, among
other reasons, a tuple
is not convertible to a pair
. If I replace tuple
with pair
everything makes sense to me.
[ 2010-11-07 Daniel comments ]
This is by far not the only necessary fix within both sub-clauses. For details see the 2010-10-29 comment in 2005(i).
[2011-03-24 Madrid meeting]
Paolo: Don't think we can do it now.
Daniel K: Agrees.[ 2011 Bloomington ]
Consensus that this issue will be resolved by 2005(i), but held open until that issue is resolved.
Proposed resolution:
Apply the resolution proposed by the 2010-10-29 comment in 2005(i).
constexpr
?Section: 16.4.6.7 [constexpr.functions] Status: C++14 Submitter: Matt Austern Opened: 2010-11-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [constexpr.functions].
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Discussion:
Suppose that a particular function is not tagged as constexpr in the standard, but that, in some particular implementation, it is possible to write it within the constexpr constraints. If an implementer tags such a function as constexpr, is that a violation of the standard or is it a conforming extension?
There are two questions to consider. First, is this allowed under the as-if rule? Second, if it does not fall under as-if, is there (and should there be) any special license granted to implementers to do this anyway, sort of the way we allow elision of copy constructors even though it is detectable by users?
I believe that this does not fall under "as-if", so implementers probably don't have that freedom today. I suggest changing the WP to grant it. Even if we decide otherwise, however, I suggest that we make it explicit.
[ 2011 Bloomington ]
General surprise this was not already in 'Ready' status, and so moved.
[ 2012 Kona ]
Some concern expressed when presented to full committee for the vote to WP status that this issue had been resolved without sufficient thought of the consequences for diverging library implementations, as users may use SFINAE to observe different behavior from otherwise identical code. Issue moved back to Review status, and will be discussed again in Portland with a larger group. Note for Portland: John Spicer has agreed to represent Core's concerns during any such discussion within LWG.
[2013-09 Chicago]
Straw poll: LWG strongly favoured to remove from implementations the freedom to add constexpr
.
Matt provides new wording.
[2013-09 Chicago]
Move to Immediate after reviewing Matt's new wording, apply the new wording to the Working Paper.
Proposed resolution:
In 16.4.6.7 [constexpr.functions], change paragraph 1 to:
This standard explicitly requires that certain standard library functions are
constexpr
[dcl.constexpr]. An implementation shall not declare any standard library function signature asconstexpr
except for those where it is explicitly required. Within any header that provides any non-defining declarations ofconstexpr
functions or constructors an implementation shall provide corresponding definitions.
Section: 16.4.5.3.3 [macro.names] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2010-11-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A program is currently forbidden to use keywords as macro names. This restriction should be strengthened to include all identifiers
that could be used by the library as attribute-tokens (for example noreturn
, which is used by header <cstdlib>
)
and the special identifiers introduced recently for override control (these are not currently used in the library public interface,
but could potentially be used by the implementation or in future revisions of the library).
[2011-02-10 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Modify 16.4.5.3.3 [macro.names] paragraph 2 as follows:
A translation unit shall not
#define
or#undef
names lexically identical to keywords, to the identifiers listed in Table X [Identifiers with special meaning], or to the attribute-tokens described in clause 7.6 [dcl.attr].
Section: 21.3.5 [meta.unary] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2010-11-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to N3126 ‑ 3.9/9,
"Scalar types, trivial class types (Clause 9), arrays of such types and cv‑qualified versions of these types (3.9.3) are collectively called trivial types."
Thus, an array (possibly of unknown bound) can be trivial type, non‑trivial type, or an array type whose triviality cannot be determined because its element type is incomplete.
According to N3126 ‑ Table 45, preconditions for std::is_trivial
are
defined as follows:
"T
shall be a complete type, (possibly cv-qualified) void
,
or an array of unknown bound"
It seems that "an array of unknown bound" should be changed to "an
array of unknown bound of a complete element type". Preconditions for
some other templates (e.g., std::is_trivially_copyable
,
std::is_standard_layout
, std::is_pod
, and std::is_literal_type
) should
be changed similarly.
On the other hand, some preconditions look too restrictive. For
example, std::is_empty
and std::is_polymorphic
might accept any
incomplete non‑class type.
[2011-02-18: Daniel provides wording proposal]
While reviewing the individual preconditions I could find three different groups of either too weakening or too strengthening constraints:
is_empty/is_polymorphic/is_abstract/has_virtual_destructor:
These traits can only apply for non‑union class types, otherwise the result must always be false
is_base_of:
Similar to the previous bullet, but the current wording comes already near to that ideal, it only misses to add the non‑union aspect.
is_trivial/is_trivially_copyable/is_standard_layout/is_pod/is_literal_type:
These traits always require that std::remove_all_extents<T>::type
to be cv void
or
a complete type.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
Modify the pre-conditions of the following type traits in 21.3.5.4 [meta.unary.prop], Table 48 — Type property predicates:
Table 48 — Type property predicates Template Condition Preconditions ... template <class T>
struct is_trivial;T
is a trivial type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_trivially_copyable;T
is a trivially copyable
type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_standard_layout;T
is a standard-layout
type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_pod;T
is a POD type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_literal_type;T
is a literal type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_empty;T
is a class type, but not a
union type, with no
non-static data members
other than bit-fields of
length 0, no virtual
member functions, no
virtual base classes, and
no base class B for which
is_empty<B>::value
is
false.IfT
shall be a complete type,
(possibly cv-qualified)void
, or
an array of unknown boundT
is a non‑union class type,T
shall be a complete type.template <class T>
struct is_polymorphic;T
is a polymorphic
class (10.3)IfT
shall be a complete type,
type, (possibly cv-qualified)void
, or
an array of unknown boundT
is a non‑union class type,T
shall be a complete type.template <class T>
struct is_abstract;T
is an abstract
class (10.4)IfT
shall be a complete type,
type, (possibly cv-qualified)void
, or
an array of unknown boundT
is a non‑union class type,T
shall be a complete type.... template <class T>
struct has_virtual_destructor;T
has a virtual
destructor (12.4)IfT
shall be a complete type,
(possibly cv-qualified)void
, or
an array of unknown boundT
is a non‑union class type,T
shall be a complete type.
Modify the pre-conditions of the following type traits in 21.3.7 [meta.rel], Table 50 — Type relationship predicates:
Table 50 — Type relationship predicates Template Condition Comments ... template <class Base, class
Derived>
struct is_base_of;Base
is a base class of
Derived
(10) without
regard to cv-qualifiers
orBase
andDerived
are not unions and
name the same class
type without regard to
cv-qualifiersIf Base
andDerived
are
non‑union class types
and are different types
(ignoring possible cv-qualifiers)
thenDerived
shall be a complete
type. [ Note: Base classes that
are private, protected, or
ambigious are, nonetheless, base
classes. — end note ]...
Allocators
must be no-throw swappableSection: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2010-11-17 Last modified: 2017-07-30
Priority: 2
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Discussion:
During the Batavia meeting it turned out that there is a definition
hole for types satisfying the Allocators
requirements: The problem
became obvious when it was discussed whether all swap
functions
of Containers
with internal data handles can be safely tagged
with noexcept
or not. While it is correct that the implicit
swap
function of an allocator is required to be a no-throw
operation (because move/copy-constructors and assignment operators are
required to be no-throw functions), there are no such requirements
for specialized swap
overloads for a particular allocator.
Containers
are
required to support swappable Allocators
, when the value
allocator_traits<>::propagate_on_container_swap
evaluates
to true
.
[2011-02-10 Alberto, Daniel, and Pablo collaborated on the proposed wording]
The proposed resolution (based on N3225) attempts to solve the following problems:
X::propagate_on_container_copy_assignment
, X::propagate_on_container_move_assignment
, and
X::propagate_on_container_swap
only describe operations, but no requirements. In fact, if and only
if these compile-time predicates evaluate to true
, the additional requirements
CopyAssignable
, no-throw MoveAssignable
, and no-throw lvalue Swappable
,
respectively, are imposed on the allocator types.a.get_allocator() == b.get_allocator()
or allocator_traits<allocator_type>::propagate_on_container_swap::value == true
), which should be cleaned up.[2011-04-08 Pablo comments]
I'm implementing a version of list now and I actually do find it impossible to write an exception-safe assignment operator unless I can assume that allocator assignment does not throw. (The problem is that I use a sentinel node and I need to allocate a new sentinel using the new allocator without destroying the old one -- then swap the allocator and sentinel pointer in atomically, without risk of an exception leaving one inconsistent with the other.
Please update the proposed resolution to add the nothrow requirement to copy-assignment.[2014-02-14 Issaquah: Move to Ready]
Fix a couple of grammar issues related to calling swap
and move to Ready.
Proposed resolution:
Adapt the following three rows from Table 44 — Allocator requirements:
Table 44 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault X::propagate_on_container_copy_assignment
Identical to or derived from true_type
orfalse_type
true_type
only if an allocator of typeX
should be copied
when the client container is copy-assigned. See Note B, below.false_type
X::propagate_on_container_move_assignment
Identical to or derived from true_type
orfalse_type
true_type
only if an allocator of typeX
should be moved
when the client container is move-assigned. See Note B, below.false_type
X::propagate_on_container_swap
Identical to or derived from true_type
orfalse_type
true_type
only if an allocator of typeX
should be swapped
when the client container is swapped. See Note B, below.false_type
Following 16.4.4.6 [allocator.requirements] p. 3 insert a new normative paragraph:
Note B: If
X::propagate_on_container_copy_assignment::value
is true,X
shall satisfy theCopyAssignable
requirements (Table 39 [copyassignable]) and the copy operation shall not throw exceptions. IfX::propagate_on_container_move_assignment::value
is true,X
shall satisfy theMoveAssignable
requirements (Table 38 [moveassignable]) and the move operation shall not throw exceptions. IfX::propagate_on_container_swap::value
is true, lvalues ofX
shall be swappable (16.4.4.3 [swappable.requirements]) and theswap
operation shall not throw exceptions.
Modify 23.2.2 [container.requirements.general] p. 8 and p. 9 as indicated:
8 - [..] The allocator may be replaced only via assignment or
9 - The expressionswap()
. Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only ifallocator_traits<allocator_type>::propagate_on_container_copy_assignment::value
,allocator_traits<allocator_type>::propagate_on_container_move_assignment::value
, orallocator_traits<allocator_type>::propagate_on_container_swap::value
is true within the implementation of the corresponding container operation.The behavior of a call to a container's. In all container types defined in this Clause, the memberswap
function is undefined unless the objects being swapped have allocators that compare equal orallocator_traits<allocator_type>::propagate_on_container_swap::value
is trueget_allocator()
returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement.a.swap(b)
, for containersa
andb
of a standard container type other thanarray
, shall exchange the values ofa
andb
without invoking any move, copy, or swap operations on the individual container elements. Lvalues of aAnyCompare
,Pred
, orHash
objectstypes belonging toa
andb
shall be swappable and shall be exchanged byunqualified calls to non-membercallingswap
as described in 16.4.4.3 [swappable.requirements]. Ifallocator_traits<allocator_type>::propagate_on_container_swap::value
istrue
, then lvalues ofallocator_type
shall be swappable and the allocators ofa
andb
shall also be exchangedusing an unqualified call to non-memberby callingswap
as described in 16.4.4.3 [swappable.requirements]. Otherwise,theythe allocators shall not be swapped, and the behavior is undefined unlessa.get_allocator() == b.get_allocator()
. Every iterator referring to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with valuea.end()
before the swap will have valueb.end()
after the swap.
std::reference_wrapper
makes incorrect usage of std::result_of
Section: 22.10.6 [refwrap] Status: C++11 Submitter: Nikolay Ivchenkov Opened: 2010-11-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
std::reference_wrapper
's function call operator uses wrong
type encoding for rvalue-arguments. An rvalue-argument of type T
must
be encoded as T&&
, not as just T
.
#include <functional> #include <iostream> #include <string> #include <type_traits> #include <utility> template <class F, class... Types> typename std::result_of<F (Types...)>::type f1(F f, Types&&... params) { return f(std::forward<Types...>(params...)); } template <class F, class... Types> typename std::result_of<F (Types&&...)>::type f2(F f, Types&&... params) { return f(std::forward<Types...>(params...)); } struct Functor { template <class T> T&& operator()(T&& t) const { return static_cast<T&&>(t); } }; int main() { typedef std::string const Str; std::cout << f1(Functor(), Str("1")) << std::endl; // (1) std::cout << f2(Functor(), Str("2")) << std::endl; // (2) }
Lets consider the function template f1
(which is similar to
std::reference_wrapper
's function call operator). In the invocation
(1) F
is deduced as 'Functor
' and Types
is deduced as type sequence
which consists of one type 'std::string const
'. After the substitution
we have the following equivalent:
template <> std::result_of<F (std::string const)>::type f1<Functor, std::string const>(Functor f, std::string const && params) { return f(std::forward<const std::string>(params)); }
The top-level cv-qualifier in the parameter type of 'F (std::string const)
' is removed, so we have
template <> std::result_of<F (std::string)>::type f1<Functor, std::string const>(Functor f, std::string const && params) { return f(std::forward<const std::string>(params)); }
Let r
be an rvalue of type 'std::string
' and cr
be an rvalue of type
'std::string const
'. The expression Str("1")
is cr
. The corresponding
return type for the invocation
Functor().operator()(r)
is 'std::string &&
'. The corresponding return type for the invocation
Functor().operator()(cr)
is 'std::string const &&
'.
std::result_of<Functor (std::string)>::type
is the same type as the
corresponding return type for the invocation Functor().operator()(r)
,
i.e. it is 'std::string &&
'. As a consequence, we have wrong reference
binding in the return statement in f1
.
Now lets consider the invocation (2) of the function template f2
. When
the template arguments are substituted we have the following equivalent:
template <> std::result_of<F (std::string const &&)>::type f2<Functor, std::string const>(Functor f, std::string const && params) { return f(std::forward<const std::string>(params)); }
std::result_of<F (std::string const &&)>::type
is the same type as
'std::string const &&
'. This is correct result.
[ 2010-12-07 Jonathan Wakely comments and suggests a proposed resolution ]
I agree with the analysis and I think this is a defect in the standard, it would be a shame if it can't be fixed.
In the following example one would expectf(Str("1"))
and
std::ref(f)(Str("2"))
to be equivalent but the current wording makes
the invocation through reference_wrapper
ill-formed:
#include <functional> #include <string> struct Functor { template <class T> T&& operator()(T&& t) const { return static_cast<T&&>(t); } }; int main() { typedef std::string const Str; Functor f; f( Str("1") ); std::ref(f)( Str("2") ); // error }
[ 2010-12-07 Daniel comments and refines the proposed resolution ]
There is one further defect in the usage of result_of
within
reference_wrapper
's function call operator: According to 22.10.6.5 [refwrap.invoke] p. 1
the invokable entity of type T
is provided as lvalue, but
result_of
is fed as if it were an rvalue. This does not only lead to
potentially incorrect result types, but it will also have the effect that
we could never use the function call operator with a function type,
because the type encoding used in result_of
would form an invalid
function type return a function type. The following program demonstrates
this problem:
#include <functional> void foo(int) {} int main() { std::ref(foo)(0); // error }
The correct solution is to ensure that T
becomes T&
within result_of
, which solves both problems at once.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Change the synopsis in 22.10.6 [refwrap] paragraph 1:
namespace std { template <class T> class reference_wrapper { public : [...] // invocation template <class... ArgTypes> typename result_of<T&(ArgTypes&&...)>::type operator() (ArgTypes&&...) const; }; }
Change the signature in 22.10.6.5 [refwrap.invoke] before paragraph 1
template <class... ArgTypes> typename result_of<T&(ArgTypes&&... )>::type operator()(ArgTypes&&... args) const;1 Returns:
INVOKE(get(), std::forward<ArgTypes>(args)...)
. (20.8.2)
regex_traits::isctype
Returns clause is wrongSection: 28.6.6 [re.traits] Status: C++14 Submitter: Jonathan Wakely Opened: 2010-11-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.traits].
View all issues with C++14 status.
Discussion:
Addresses GB 10
28.6.6 [re.traits] p. 12 says:
returns true if
f
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence "w" is not equal to0
andc == '_'
If the bitmask value corresponding to "w" has a non-zero value (which
it must do) then the bitwise or with any value is also non-zero, and
so isctype('_', f)
returns true for any f
. Obviously this is wrong,
since '_'
is not in every ctype
category.
There's a similar problem with the following phrases discussing the "blank" char class.
[2011-05-06: Jonathan Wakely comments and provides suggested wording]
DR 2019(i) added isblank
support to <locale>
which simplifies the
definition of regex_traits::isctype
by removing the special case for the "blank" class.
regex_traits::lookup_classname
.
I then refer to that table in the Returns clause of regex_traits::isctype
to expand on the "in an unspecified manner" wording which is too vague. The conversion
can now be described using the "is set" term defined by 16.3.3.3.3 [bitmask.types] and
the new table to convey the intented relationship between e.g.
[[:digit:]] and ctype_base::digit
, which is not actually stated in the
FDIS.
The effects of isctype
can then most easily be described in code,
given an "exposition only" function prototype to do the not-quite-so-unspecified conversion
from char_class_type
to ctype_base::mask
.
The core of LWG 2018 is the "bitwise or'ed" wording which gives the
wrong result, always evaluating to true for all values of f
. That is
replaced by the condition (f&x) == x
where x
is the result of calling
lookup_classname
with "w". I believe that's necessary, because the
"w" class could be implemented by an internal "underscore" class i.e.
x = _Alnum|_Underscore
in which case (f&x) != 0
would give the wrong
result when f==_Alnum
.
The proposed resolution also makes use of ctype::widen
which addresses
the problem that the current wording only talks about "w" and '_' which assumes
charT
is char. There's still room for improvement here:
the regex grammar in 28.6.12 [re.grammar] says that the class names in the
table should always be recognized, implying that e.g. U"digit" should
be recognized by regex_traits<char32_t>
, but the specification of
regex_traits::lookup_classname
doesn't cover that, only mentioning
char
and wchar_t
. Maybe the table should not distinguish narrow and
wide strings, but should just have one column and add wording to say
that regex_traits
widens the name as if by using use_facet<ctype<charT>>::widen()
.
Another possible improvement would be to allow additional
implementation-defined extensions in isctype
. An implementation is
allowed to support additional class names in lookup_classname
, e.g.
[[:octdigit:]] for [0-7] or [[:bindigit:]] for [01], but the current
definition of isctype provides no way to use them unless ctype_base::mask
also supports them.
[2011-05-10: Alberto and Daniel perform minor fixes in the P/R]
[ 2011 Bloomington ]
Consensus that this looks to be a correct solution, and the presentation as a table is a big improvement.
Concern that the middle section wording is a little muddled and confusing, Stefanus volunteered to reword.
[ 2013-09 Chicago ]
Stefanus provides improved wording (replaced below)
[ 2013-09 Chicago ]
Move as Immediate after reviewing Stefanus's revised wording, apply the new wording to the Working Paper.
Proposed resolution:
This wording is relative to the FDIS.
Modify 28.6.6 [re.traits] p. 10 as indicated:
template <class ForwardIterator> char_class_type lookup_classname( ForwardIterator first, ForwardIterator last, bool icase = false) const;-9- Returns: an unspecified value that represents the character classification named by the character sequence designated by the iterator range [
-10- Remarks: Forfirst
,last
). If the parametericase
is true then the returned mask identifies the character classification without regard to the case of the characters being matched, otherwise it does honor the case of the characters being matched.(footnote 335) The value returned shall be independent of the case of the characters in the character sequence. If the name is not recognized then returns a value that compares equal to0
.regex_traits<char>
, at least thenames "d", "w", "s", "alnum", "alpha", "blank", "cntrl", "digit", "graph", "lower", "print", "punct", "space", "upper" and "xdigit"narrow character names in Table X shall be recognized. Forregex_traits<wchar_t>
, at least thenames L"d", L"w", L"s", L"alnum", L"alpha", L"blank", L"cntrl", L"digit", L"graph", L"lower", L"print", L"punct", L"space", L"upper" and L"xdigit"wide character names in Table X shall be recognized.
Modify 28.6.6 [re.traits] p. 12 as indicated:
bool isctype(charT c, char_class_type f) const;-11- Effects: Determines if the character
-12- Returns:c
is a member of the character classification represented byf
.ConvertsGiven an exposition-only function prototypef
into a valuem
of typestd::ctype_base::mask
in an unspecified manner, and returns true ifuse_facet<ctype<charT> >(getloc()).is(m, c)
is true. Otherwise returns true iff
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence "w" is not equal to0
andc == '_'
, or iff
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence "blank" is not equal to0
andc
is one of an implementation-defined subset of the characters for whichisspace(c, getloc())
returns true, otherwise returns false.template<class C> ctype_base::mask convert(typename regex_traits<C>::char_class_type f);that returns a value in which each
ctype_base::mask
value corresponding to a value inf
named in Table X is set, then the result is determined as if by:ctype_base::mask m = convert<charT>(f); const ctype<charT>& ct = use_facet<ctype<charT>>(getloc()); if (ct.is(m, c)) { return true; } else if (c == ct.widen('_')) { charT w[1] = { ct.widen('w') }; char_class_type x = lookup_classname(w, w+1); return (f&x) == x; } else { return false; }[Example:
regex_traits<char> t; string d("d"); string u("upper"); regex_traits<char>::char_class_type f; f = t.lookup_classname(d.begin(), d.end()); f |= t.lookup_classname(u.begin(), u.end()); ctype_base::mask m = convert<char>(f); // m == ctype_base::digit|ctype_base::upper— end example]
[Example:
regex_traits<char> t; string w("w"); regex_traits<char>::char_class_type f; f = t.lookup_classname(w.begin(), w.end()); t.isctype('A', f); // returns true t.isctype('_', f); // returns true t.isctype(' ', f); // returns false— end example]
At the end of 28.6.6 [re.traits] add a new "Table X — Character class names and corresponding ctype masks":
Table X — Character class names and corresponding ctype masks Narrow character name Wide character name Corresponding ctype_base::mask
value"alnum"
L"alnum"
ctype_base::alnum
"alpha"
L"alpha"
ctype_base::alpha
"blank"
L"blank"
ctype_base::blank
"cntrl"
L"cntrl"
ctype_base::cntrl
"digit"
L"digit"
ctype_base::digit
"d"
L"d"
ctype_base::digit
"graph"
L"graph"
ctype_base::graph
"lower"
L"lower"
ctype_base::lower
"print"
L"print"
ctype_base::print
"punct"
L"punct"
ctype_base::punct
"space"
L"space"
ctype_base::space
"s"
L"s"
ctype_base::space
"upper"
L"upper"
ctype_base::upper
"w"
L"w"
ctype_base::alnum
"xdigit"
L"xdigit"
ctype_base::xdigit
isblank
not supported by std::locale
Section: 28.3.3.3.1 [classification] Status: C++11 Submitter: Jonathan Wakely Opened: 2010-11-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
C99 added isblank
and iswblank
to <locale.h>
but <locale>
does not
provide any equivalent.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 6 votes.
Proposed resolution:
Add to 28.3.3.3.1 [classification] synopsis:
template <class charT> bool isgraph (charT c, const locale& loc); template <class charT> bool isblank (charT c, const locale& loc);
Add to 28.3.4.2 [category.ctype] synopsis:
static const mask xdigit = 1 << 8; static const mask blank = 1 << 9; static const mask alnum = alpha | digit; static const mask graph = alnum | punct;
constexpr
functions have invalid effectsSection: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: Daniel Krügler Opened: 2010-12-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
View all issues with C++11 status.
Discussion:
As of issue 1171(i) several time-utility functions have been marked constexpr
.
Alas this was done without adapting the corresponding return elements, which has the effect that
none of current arithmetic functions of class template duration
marked as constexpr
can ever be constexpr
functions (which makes them ill-formed, no diagnostics required as
of recent core rules), because they invoke a non-constant expression, e.g. 30.5.6 [time.duration.nonmember]/2:
template <class Rep1, class Period1, class Rep2, class Period2> constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); 2 Returns: CD(lhs) += rhs.
The real problem is, that we cannot defer to as-if rules here: The returns element
specifies an indirect calling contract of a potentially user-defined function. This cannot be
the +=
assignment operator of such a user-defined type, but must be the corresponding
immutable binary operator+
(unless we require that +=
shall be an immutable function
which does not really makes sense).
[2011-02-17 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
The suggested wording changes are against the working draft N3242. Additional to the normative wording changes some editorial fixes are suggested.
In 30.5.6 [time.duration.nonmember], change the following arithmetic function specifications as follows:
template <class Rep1, class Period1, class Rep2, class Period2> constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);2 Returns:
CD(lhs) += rhs
CD(CD(lhs).count() + CD(rhs).count())
.
template <class Rep1, class Period1, class Rep2, class Period2> constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);3 Returns:
CD(lhs) -= rhs
CD(CD(lhs).count() - CD(rhs).count())
.
template <class Rep1, class Period, class Rep2> constexpr duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);4 Remarks: This operator shall not participate in overload resolution unless
5 Returns:Rep2
is implicitly convertible toCR(Rep1, Rep2)
.duration<CR(Rep1, Rep2), Period>(d) *= s
CD(CD(d).count() * s)
.
[...]
template <class Rep1, class Period, class Rep2> constexpr duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);8 Remarks: This operator shall not participate in overload resolution unless
9 Returns:Rep2
is implicitly convertible toCR(Rep1, Rep2)
andRep2
is not an instantiation ofduration
.duration<CR(Rep1, Rep2), Period>(d) /= s
CD(CD(d).count() / s)
.
[...]
template <class Rep1, class Period, class Rep2> constexpr duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);11 Remarks: This operator shall not participate in overload resolution unless
12 Returns:Rep2
is implicitly convertible toCR(Rep1, Rep2)
andRep2
is not an instantiation ofduration
.duration<CR(Rep1, Rep2), Period>(d) %= s
CD(CD(d).count() % s)
template <class Rep1, class Period1, class Rep2, class Period2> constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);13 Returns:
common_type<duration<Rep1, Period1>, duration<Rep2, Period2> >::type(lhs) %= rhs
CD(CD(lhs).count() % CD(rhs).count())
.
result_of
Section: 22.10.15.4 [func.bind.bind], 32.10.1 [futures.overview], 32.10.9 [futures.async] Status: C++14 Submitter: Daniel Krügler Opened: 2010-12-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.bind.bind].
View all issues with C++14 status.
Discussion:
Issue 2017(i) points out some incorrect usages of result_of
in the
declaration of the function call operator overload of reference_wrapper
,
but there are more such specification defects:
[..] The effect of
g(u1, u2, ..., uM)
shall beINVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type)
[..]
but fd
is defined as "an lvalue of type FD
constructed from std::forward<F>(f)
". This means that
the above usage must refer to result_of<FD cv & (V1, V2, ..., VN)>
instead.
Similar in 22.10.15.4 [func.bind.bind] p. 10 bullet 2 we have:
if the value of
is_bind_expression<TiD>::value
is true, the argument istid(std::forward<Uj>(uj)...)
and its typeVi
isresult_of<TiD cv (Uj...)>::type
Again, tid
is defined as "lvalue of type TiD
constructed from std::forward<Ti>(ti)
". This means that
the above usage must refer to result_of<TiD cv & (Uj...)>
instead. We also have similar defect as in
2017(i) in regard to the argument types, this leads us to the further corrected form
result_of<TiD cv & (Uj&&...)>
. This is not the end: Since the Vi
are similar sensitive to the argument problem, the last part must say:
Vi
is result_of<TiD cv & (Uj&&...)>::type &&"
(The bound arguments Vi
can never be void
types, therefore we don't need
to use the more defensive std::add_rvalue_reference
type trait)
The function template async
is declared as follows (the other overload has the same problem):
template <class F, class... Args> future<typename result_of<F(Args...)>::type> async(F&& f, Args&&... args);
This usage has the some same problems as we have found in reference_wrapper
(2017(i)) and more: According to
the specification in 32.10.9 [futures.async] the effective result type is that of the call of
INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)
First, decay_copy
potentially modifies the effective types to decay<F>::type
and decay<Args>::type...
.
Second, the current specification is not really clear, what the value category of callable type or the arguments shall be: According
to the second bullet of 32.10.9 [futures.async] p. 3:
Invocation of the deferred function evaluates
INVOKE(g, xyz)
whereg
is the stored value ofdecay_copy(std::forward<F>(f))
andxyz
is the stored copy ofdecay_copy(std::forward<Args>(args))...
.
This seems to imply that lvalues are provided in contrast to the direct call expression of 32.10.9 [futures.async] p. 2 which implies rvalues instead. The specification needs to be clarified.
[2011-06-13: Daniel comments and refines the proposed wording changes]
The feedback obtained following message c++std-lib-30745 and follow-ups point to the intention, that
the implied provision of lvalues due to named variables in async
should be provided as rvalues to support
move-only types, but the functor type should be forwarded as lvalue in bind
.
bind
were newly invented, the value strategy could be improved, because now we have a preference of
ref &
qualified function call operator overloads. But such a change seems to be too late now.
User-code that needs to bind a callable object with an ref &&
qualified function call
operator (or conversion function to function pointer) needs to use a corresponding wrapper similar to reference_wrapper
that forwards the reference as rvalue-reference instead.
The wording has been adapted to honor these observations and to fit to FDIS numbering as well.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
The suggested wording changes are against the FDIS.
Change 22.10.15.4 [func.bind.bind] p. 3 as indicated:
template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-2- Requires:
-3- Returns: A forwarding call wrapperis_constructible<FD, F>::value
shall be true. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall be true.INVOKE(fd, w1, w2, ..., wN)
(20.8.2) shall be a valid expression for some valuesw1
,w2
, ...,wN
, whereN == sizeof...(bound_args)
.g
with a weak result type (20.8.2). The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of<FD cv & (V1, V2, ..., VN)>::type)
, where cv represents the cv-qualifiers ofg
and the values and types of the bound argumentsv1
,v2
, ...,vN
are determined as specified below. […]
Change 22.10.15.4 [func.bind.bind] p. 7 as indicated:
template<class R, class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-6- Requires:
-7- Returns: A forwarding call wrapperis_constructible<FD, F>::value
shall be true. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall be true.INVOKE(fd, w1, w2, ..., wN)
shall be a valid expression for some valuesw1
,w2
, ...,wN
, whereN == sizeof...(bound_args)
.g
with a nested typeresult_type
defined as a synonym forR
. The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), R)
, where the values and types of the bound argumentsv1
,v2
, ...,vN
are determined as specified below. […]
Change 22.10.15.4 [func.bind.bind] p. 10 as indicated:
-10- The values of the bound arguments
v1
,v2
, ...,vN
and their corresponding typesV1
,V2
, ...,VN
depend on the typesTiD
derived from the call to bind and the cv-qualifiers cv of the call wrapperg
as follows:
- if
TiD
isreference_wrapper<T>
, the argument istid.get()
and its typeVi
isT&
;- if the value of
is_bind_expression<TiD>::value
istrue
, the argument istid(std::forward<Uj>(uj)...)
and its typeVi
isresult_of<TiD cv & (Uj&&...)>::type&&
;- if the value
j
ofis_placeholder<TiD>::value
is not zero, the argument isstd::forward<Uj>(uj)
and its typeVi
isUj&&
;- otherwise, the value is
tid
and its typeVi
isTiD cv &
.
This resolution assumes that the wording of 32.10.9 [futures.async] is intended to provide rvalues
as arguments of INVOKE
.
Change the function signatures in header <future>
synopsis 32.10.1 [futures.overview] p. 1
and in 32.10.9 [futures.async] p. 1 as indicated:
template <class F, class... Args> future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type> async(F&& f, Args&&... args); template <class F, class... Args> future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type> async(launch policy, F&& f, Args&&... args);
Change 32.10.9 [futures.async] as indicated: (Remark: There is also a tiny editorial correction
in p. 4 that completes one ::
scope specifier)
-3- Effects: […]
- […]
- if
policy & launch::deferred
is non-zero — StoresDECAY_COPY(std::forward<F>(f))
andDECAY_COPY(std::forward<Args>(args))...
in the shared state. These copies off
andargs
constitute a deferred function. Invocation of the deferred function evaluatesINVOKE(std::move(g), std::move(xyz))
whereg
is the stored value ofDECAY_COPY(std::forward<F>(f))
andxyz
is the stored copy ofDECAY_COPY(std::forward<Args>(args))...
. The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4) on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called the waiting function. Once evaluation ofINVOKE(std::move(g), std::move(xyz))
begins, the function is no longer considered deferred. [ Note: If this policy is specified together with other policies, such as when using apolicy
value oflaunch::async | launch::deferred
, implementations should defer invocation or the selection of the policy when no more concurrency can be effectively exploited. — end note ]
-4- Returns: an object of type
future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
that refers to the associated asynchronous state created by this call toasync
.
reference_wrapper<T>::result_type
is underspecifiedSection: 22.10.6 [refwrap] Status: C++11 Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [refwrap].
View all issues with C++11 status.
Discussion:
Issue 1295(i) correctly removed function types and references to function types from the bullet 1 of 22.10.4 [func.require] p. 3 because neither function types nor function references satisfy the requirements for a target object which is defined to be an object of a callable type. This has the effect that the reference in 22.10.6 [refwrap] p. 2
reference_wrapper
has a weak result type (20.8.2).
is insufficient as a reference to define the member type result_type
when the template argument
T
is a function type.
Extend the definition of a weak result type in 22.10.4 [func.require] p. 3 to both function types and references thereof. This extension must be specified independend from the concept of a call wrapper, though.
Add one extra sentence to 22.10.6 [refwrap] p. 2 that simply defines the member type
result_type
for reference_wrapper<T>
, when T
is a function type.
I checked the current usages of weak result type to have a base to argue for one or the other
approach. It turns out, that there is no further reference to this definition in regard to
function types or references thereof. The only other reference can be found in
22.10.15.4 [func.bind.bind] p. 3, where g
is required to be a class type.
[2011-02-23 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
The suggested wording changes are against the working draft N3242.
Change 22.10.6 [refwrap] p. 2 as indicated:
2
reference_wrapper<T>
has a weak result type (20.8.2). IfT
is a function type,result_type
shall be a synonym for the return type ofT
.
lock_guard
and unique_lock
Section: 32.6.5.2 [thread.lock.guard], 32.6.5.4 [thread.lock.unique] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.lock.guard].
View all issues with Resolved status.
Discussion:
There are two different *Lockable
requirements imposed on template arguments
of the class template lock_guard
as of 32.6.5.2 [thread.lock.guard] p. 1+2:
1 [..] The supplied
Mutex
type shall meet theBasicLockable
requirements (30.2.5.2).
2 The supplied
Mutex
type shall meet theLockable
requirements (30.2.5.3).
The Lockable
requirements include the availability of a member function try_lock()
,
but there is no operational semantics in the specification of lock_guard
that would rely
on such a function. It seems to me that paragraph 2 should be removed.
Mutex
should
always provide the try_lock()
member function, because several member functions of
unique_lock
(unique_lock(mutex_type& m, try_to_lock_t)
or bool try_lock()
)
take advantage of such a function without adding extra requirements for this.
It seems that the requirement subset BasicLockable
should be removed.
I searched for further possible misusages of the *Lockable
requirements, but could not
find any more.
[2011-02-23]
Howard suggests an alternative approach in regard to unique_lock
: The current
minimum requirements on its template argument should better be reduced to BasicLockable
instead of the current Lockable
, including ammending member-wise constraints where required.
This suggestions was supported by Anthony, Daniel, and Pablo.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
The suggested wording changes are against the working draft N3242.
Remove 32.6.5.2 [thread.lock.guard] p. 2 completely:
2 The suppliedMutex
type shall meet theLockable
requirements (30.2.5.3).Change 32.6.5.4 [thread.lock.unique] p. 1-3 as indicated. The intend is to make
BasicLockable
the fundamental requirement forunique_lock
. We also update the note to reflect these changes and synchronize one remaining reference of 'mutex' by the proper term 'lockable object' in sync to the wording changes oflock_guard
:1 [..] The behavior of a program is undefined if the contained pointer
pm
is not null and themutexlockable object pointed to bypm
does not exist for the entire remaining lifetime (3.8) of theunique_lock
object. The suppliedMutex
type shall meet theBasicLockable
requirements (30.2.5.2).[Editor's note: BasicLockable is redundant, since the following additional paragraph requires Lockable.]
2 The suppliedMutex
type shall meet theLockable
requirements (30.2.5.3).3 [ Note:
unique_lock<Mutex>
meets theBasicLockable
requirements. IfMutex
meets theLockable
requirements ([thread.req.lockable.req]),unique_lock<Mutex>
also meets theLockable
requirements and ifMutex
meets theTimedLockable
requirements (30.2.5.4),unique_lock<Mutex>
also meets theTimedLockable
requirements. — end note ]Modify 32.6.5.4.2 [thread.lock.unique.cons] to add the now necessary member-wise additional constraints for
Lockable
:unique_lock(mutex_type& m, try_to_lock_t) noexcept;8 Requires: The supplied
9 Effects: Constructs an object of typeMutex
type shall meet theLockable
requirements ([thread.req.lockable.req]). Ifmutex_type
is not a recursive mutex the calling thread does not own the mutex.unique_lock
and callsm.try_lock()
.Modify 32.6.5.4.3 [thread.lock.unique.locking] to add the now necessary member-wise additional constraints for
Lockable
:bool try_lock();? Requires: The supplied
4 Effects:Mutex
type shall meet theLockable
requirements ([thread.req.lockable.req]).pm->try_lock()
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
atomic<integral>
and atomic<T*>
Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 5 and 6 of 32.5.8 [atomics.types.generic] impose different requirements on implementations for
specializations of the atomic
class template for integral types and for pointer types:
5 The atomic integral specializations and the specialization
atomic<bool>
shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax.
6 There are pointer partial specializations on the
atomic
class template. These specializations shall have trivial default constructors and trivial destructors.
It looks like an oversight to me, that for pointer specializations the requirements for standard layout and support for aggregate initialization syntax are omitted. In fact, this been confirmed by the N3193 proposal author. I suggest to impose the same implementation requirements for pointer types as for integral types, this should not impose unrealistic requirements on implementations.
[2011-02-10 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed Resolution
The suggested wording changes are against the working draft N3242.
Change 32.5.8 [atomics.types.generic] p. 6 as indicated:
6 There are pointer partial specializations on the
atomic
class template. These specializations shall have standard layout, trivial default constructors, and trivial destructors. They shall each support aggregate initialization syntax.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
packaged_task
Section: 32.10.10.2 [futures.task.members] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to 32.10.10.2 [futures.task.members] p. 7 bullet 2:
packaged_task& operator=(packaged_task&& other);7 Effects:
[...]
packaged_task<R, ArgTypes...>(other).swap(*this)
.
The argument other
given to the move constructor is an lvalue and must be converted into
an rvalue via appropriate usage of std::move
.
Proposed Resolution
The suggested wording changes are against the working draft N3242.
Change 32.10.10.2 [futures.task.members] p. 7 bullet 2 as indicated:
packaged_task& operator=(packaged_task&& other);7 Effects:
[...]
packaged_task(std::move(other)).swap(*this)
.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
packaged_task
Section: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2011-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Related with LWG issue 1514(i).
The move constructor of packaged_task
does not specify how the stored task is constructed.
The obvious way is to move-construct it using the task stored in the argument. Moreover, the
constructor should be provided with a throws clause similar to one used for the other constructors,
as the move constructor of the stored task is not required to be nothrow.
As for the other constructors, the terms "stores a copy of f
" do not reflect the intent, which is
to allow f
to be moved when possible.
[2011-02-25: Alberto updates wording]
[2011-02-26 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
(wording written assuming LWG 1514(i) is also accepted)
Change 32.10.10.2 [futures.task.members] paragraph 3:
3 Effects: constructs a new
packaged_task
object with an associated asynchronous state andstores a copy ofinitializes the object's stored task withf
as the object's stored taskstd::forward<F>(f)
. The constructors that take anAllocator
argument use it to allocate memory needed to store the internal data structures.
Change 32.10.10.2 [futures.task.members] paragraph 5:
5 Effects: constructs a new
packaged_task
object and transfers ownership ofother
's associated asynchronous state to*this
, leavingother
with no associated asynchronous state. Moves the stored task fromother
to*this
.
messages_base::catalog
overspecifiedSection: 28.3.4.8.2 [locale.messages] Status: C++14 Submitter: Howard Hinnant Opened: 2011-02-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 28.3.4.8.2 [locale.messages], messages_base::catalog
is specified to be a typedef to int
.
This type is subsequently used to open, access and close catalogs.
However, an OS may have catalog/messaging services that are indexed and managed by types other than int
.
For example POSIX
, publishes the following messaging API:
typedef unspecified nl_catd; nl_catd catopen(const char* name , int oflag); char* catgets(nl_catd catd, int set_id, int msg_id, const char* s); int catclose(nl_catd catd);
I.e., the catalog is managed with an unspecified type, not necessarily an int
.
Mac OS uses a void*
for nl_catd
(which is conforming to the POSIX
standard).
The current messages_base
spec effectively outlaws using the built-in OS messaging service
supplied for this very purpose!
[2011-02-24: Chris Jefferson updates the proposed wording, changing unspecified to unspecified signed integral type]
[2011-03-02: Daniel updates the proposed wording, changing unspecified signed integral type to
unspecified signed integer type (We don't want to allow for bool
or char
)]
[2011-03-24 Madrid meeting]
Consensus that this resolution is the direction we would like to see.
Proposed resolution:
Modify 28.3.4.8.2 [locale.messages]:
namespace std { class messages_base { public: typedefintunspecified signed integer type catalog; }; ... }
noexcept
' on basic_regex
move-assignment operatorSection: 28.6.7 [re.regex] Status: C++11 Submitter: Jonathan Wakely Opened: 2011-02-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
N3149 replaced
the "Throws: nothing
" clause on basic_regex::assign(basic_regex&&)
with
the noexcept
keyword. The effects of the move-assignment operator are defined in terms of
the assign()
function, so the "Throws: nothing
" applied there too, and a
noexcept
-specification should be added there too.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 7 votes.
Proposed resolution:
Modify the basic_regex
synopsis in 28.6.7 [re.regex] p. 3:
namespace std { template <class charT, class traits = regex_traits<charT> > class basic_regex { public: ... basic_regex& operator=(const basic_regex&); basic_regex& operator=(basic_regex&&) noexcept; basic_regex& operator=(const charT* ptr); ... }; }
Modify 28.6.7.3 [re.regex.assign] p. 2:
basic_regex& operator=(basic_regex&& e) noexcept;2 Effects: returns
assign(std::move(e))
.
packaged_task::result_type
should be removedSection: 32.10.10 [futures.task] Status: C++11 Submitter: Anthony Williams Opened: 2010-11-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
packaged_task::operator()
always returns void
, regardless of the return
type of the wrapped task. However, packaged_task::result_type
is a
typedef to the return type of the wrapped task. This is inconsistent
with other uses of result_type
in the standard, where it matches the
return type of operator()
(e.g. function
, owner_less
). This is confusing.
It also violates the TR1 result_of
protocol, and thus makes
packaged_task
harder to use with anything that respects that protocol.
Finally, it is of little use anyway.
packaged_task::result_type
should therefore be removed.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Alter the class definition of packaged_task
in 32.10.10 [futures.task] p. 2 as follows:
template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public:typedef R result_type;[...] };
std::future<>::share()
only applies to rvaluesSection: 32.10.7 [futures.unique.future] Status: C++11 Submitter: Anthony Williams Opened: 2011-02-17 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
As specified, future<>::share()
has the signature
shared_future<R> share() &&;
This means that it can only be applied to rvalues. One of the key benefits of share()
is
that it can be used with the new auto
facility:
std::promise<some_long_winded_type_name> some_promise; auto f = some_promise.get_future(); // std::future auto sf = std::move(f).share();
share()
is sufficiently explicit that the move should not be required. We should be able to write:
auto sf = f.share();
[2011-02-22 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Alter the declaration of share()
to remove the "&&" rvalue qualifier in
[futures.unique_future] p. 3, and [futures.unique_future] p. 11:
shared_future<R> share()&&;
async
functionSection: 32.10.9 [futures.async] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2011-02-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Clause 32.10.9 [futures.async] has undergone significant rewording in Batavia. Due to co-presence of at least three different sources of modification there is a part where changes have overlapped (marked by an Editor's note), which should be reconciled. Moreover, I believe that a few non-overlapping sentences are now incorrect and should be fixed, so the problem cannot be handled editorially. (See c++std-lib-29667.)
[Adopted in Madrid, 2011-03]
Proposed resolution:
Edit 32.10.5 [futures.state], paragraph 3 as follows.
An asynchronous return object is an object that reads results from an associated asynchronous state. A waiting function of an asynchronous return object is one that potentially blocks to wait for the associated asynchronous state to be made ready. If a waiting function can return before the state is made ready because of a timeout (30.2.5), then it is a timed waiting function, otherwise it is a non-timed waiting function.
Edit within 32.10.9 [futures.async] paragraph 3 bullet 2 as follows.
Effects: [...]
- if
policy & launch::deferred
is non-zero — [...] The associated asynchronous state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4 [futures.state])requiring a non-timed waiton an asynchronous return object referring tothethis associated asynchronous statecreated by thisshall invoke the deferred function in the thread that called the waiting functionasync
call to become ready;.onceOnce evaluation ofINVOKE(g, xyz)
begins, the function is no longer considered deferred. [...]
Edit 32.10.9 [futures.async] paragraph 5 as follows.
Synchronization: Regardless of the provided
policy
argument,
- the invocation of
async
synchronizes with (1.10) the invocation off
. [Note: this statement applies even when the corresponding future object is moved to another thread. —end note]; and- the completion of the function
f
is sequenced before (1.10) the associated asynchronous state is made ready. [Note:f
might not be called at all, so its completion might never happen. —end note]
IfIf the implementation chooses thepolicy & launch::async
is non-zero,launch::async
policy,
- a call to a waiting function on an asynchronous return object that shares the associated asynchronous state created by this
async
call shall block until the associated thread has completed, as if joined (30.3.1.5);thethe associated thread completion synchronizes with (1.10) the return from the first function that successfully detects the ready status of the associated asynchronous state or with the return from the last function that releases the associated asynchronous statejoin()
on the created thread objectreturns, whichever happens first.[Editor's note: N3196 changes the following sentence as indicated. N3188 removes the sentence. Please pick one.] If the invocation is deferred, the completion of the invocation of the deferred function synchronizes with the successful return from a call to a waiting function on the associated asynchronous state.
reserve
, shrink_to_fit
, and resize
functionsSection: 23.3.13.3 [vector.capacity], 23.3.5.3 [deque.capacity] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2011-02-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I have several questions with regard to the working paper N3225 (C++0x working draft):
Where the working draft specifies preconditions for shrink_to_fit
member function of std::vector
and std::deque
?
Where the working draft specifies preconditions for 'void reserve(size_type n)
'
member function of std::vector
?
Does a call to 'void resize(size_type sz)
' of std::vector
require
the element type to be DefaultConstructible
? If yes, why such
requirement is not listed in the Requires paragraph?
Does a call to 'void resize(size_type sz)
' of std::vector
require
the element type to be MoveAssignable
because the call erase(begin() + sz, end())
mentioned in the Effects paragraph would require the element type to be MoveAssignable
?
Why CopyInsertable
requirement is used for 'void resize(size_type sz)
' of std::vector
instead of MoveInsertable
requirement?
[2011-06-12: Daniel comments and provides wording]
According to my understanding of the mental model of vector (and to some parts for deque) the some requirements are missing in the standard as response to above questions:
shrink_to_fit
for both std::vector
and std::deque
should impose the MoveInsertable
requirements. The reason for this is, that these containers
can host move-only types. For a container type X
the C++03 idiom X(*this).swap(*this)
imposes the CopyInsertable
requirements which would make the function call ill-formed,
which looks like an unacceptable restriction to me. Assuming the committee decides to support the
move-only case, further wording has to be added for the situation where such a move-only type could
throw an exception, because this can leave the object in an unspecified state. This seems consistent
with the requirements of reserve
, which seems like a very similar function to me (for
vector
). And this brings us smoothly to the following bullet:
I agree that we are currently missing to specify the preconditions of the reserve
function.
My interpretation of the mental model of this function is that it should work for move-only types, which
seems to be supported by the wording used in 23.3.13.3 [vector.capacity] p2:
[…] If an exception is thrown other than by the move constructor of a non-CopyInsertable type, there are no effects.
Given this statement, the appropriate requirement is MoveInsertable
into the vector
.
vector::resize(size_type)
misses to list the DefaultConstructible
requirements.
erase
implies the MoveAssignable
requirements. I don't think that this implication is intended. This function requires "append" and
"pop-back" effects, respectively, where the former can be realized in terms of MoveInsertable
requirements. The same fix in regard to using pop_back
instead of erase
is necessary
for the two argument overload of resize
as well (no MoveAssignable
is required).
CopyInsertable
requirement is incorrect and should be MoveInsertable
instead.In addition to above mentioned items, the proposed resolution adds a linear complexity bound for
shrink_to_fit
and attempts to resolve the related issue 2066(i).
[ 2011 Bloomington ]
Move to Ready.
Note for editor: we do not normally refer to 'linear time' for complexity requirements, but there is agreement that any clean-up of such wording is editorial.
Proposed resolution:
This wording is relative to the FDIS.
Edit 23.3.5.3 [deque.capacity] as indicated [Remark: The suggested change of p4 is
not redundant, because CopyInsertable
is not necessarily a refinement of MoveInsertable
in contrast to the fact that CopyConstructible
is a refinement of MoveConstructible
]:
void resize(size_type sz);-1- Effects: If
-2- Requires:sz <= size()
, equivalent tocallingerase(begin() + sz, end());
pop_back()
size() - sz
times. Ifsize() < sz
, appendssz - size()
value-initialized elements to the sequence.T
shall beMoveInsertable
into*this
andDefaultConstructible
.
void resize(size_type sz, const T& c);-3- Effects: If
sz <= size()
, equivalent to callingpop_back()
size() - sz
times. Ifsize() < sz
, appendssz - size()
copies ofc
to the sequence.if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) erase(begin()+sz, end()); else ; // do nothing-4- Requires:
T
shall beMoveInsertable
into*this
andCopyInsertable
into*this
.
void shrink_to_fit();-?- Requires:
-?- Complexity: Takes at most linear time in the size of the sequence. -5- Remarks:T
shall beMoveInsertable
into*this
.shrink_to_fit
is a non-binding request to reduce memory use but does not change the size of the sequence. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ]
Edit 23.3.13.3 [vector.capacity] as indicated including edits that also resolve 2066(i)
[Remark: The combined listing of MoveInsertable
and CopyInsertable
before p12 is not redundant,
because CopyInsertable
is not necessarily a refinement of MoveInsertable
in contrast to the
fact that CopyConstructible
is a refinement of MoveConstructible
]:
[…]
void reserve(size_type n);-?- Requires:
-2- Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage allocation accordingly. AfterT
shall beMoveInsertable
into*this
.reserve()
,capacity()
is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
. If an exception is thrown other than by the move constructor of a non-CopyInsertable
type, there are no effects. -3- Complexity: It does not change the size of the sequence and takes at most linear time in the size of the sequence. -4- Throws:length_error
ifn > max_size()
.[footnote 266] -5- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call toreserve()
until the time when an insertion would make the size of the vector greater than the value ofcapacity()
.
void shrink_to_fit();-?- Requires:
-?- Complexity: Takes at most linear time in the size of the sequence. -6- Remarks:T
shall beMoveInsertable
into*this
.shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.
[…]
void resize(size_type sz);-9- Effects: If
-10- Requires:sz <= size()
, equivalent tocallingerase(begin() + sz, end());
pop_back()
size() - sz
times. Ifsize() < sz
, appendssz - size()
value-initialized elements to the sequence.T
shall beinto
CopyMoveInsertable*this
andDefaultConstructible
. -??- Remarks: If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.
void resize(size_type sz, const T& c);-11- Effects: If
sz <= size()
, equivalent to callingpop_back()
size() - sz
times. Ifsize() < sz
, appendssz - size()
copies ofc
to the sequence.if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) erase(begin()+sz, end()); else ; // do nothing-??- Requires:
-12-T
shall beMoveInsertable
into*this
andCopyInsertable
into*this
.RequiresRemarks: If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.
Section: 32.5.4 [atomics.order] Status: Resolved Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This violates the core intent of the memory model, as stated in the note in 6.9.2 [intro.multithread] p. 21.
This was discovered by Mark Batty, and pointed out in their POPL 2011 paper, "Mathematizing C++ Concurrency", section 4, "Sequential consistency of SC atomics". The problem is quite technical, but well-explained in that paper.
This particular issue was not understood at the time the FCD comments were generated. But it is closely related to a number of FCD comments. It should have arisen from US-171, though that's not the actual history.
This issue has been under discussion for several months in a group that included a half dozen or so of the most interested committee members. The P/R represents a well-considered consensus among us:
[2011-03-16: Jens updates wording]
Modify 32.5.4 [atomics.order] p.3, so that the normative part reads:
3 There shall be a single total order S on all
memory_order_seq_cst
operations, consistent with the "happens before" order and modification orders for all affected locations, such that eachmemory_order_seq_cst
operation that loads a value observes either the last preceding modification according to this order S, A (if any), or the result of an operation X thatis notdoes not happen before A and that is notmemory_order_seq_cst
. [ Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the "happens before" ordering. — end note ]
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
atomic
free functions incorrectly specifiedSection: 32.5.2 [atomics.syn] Status: Resolved Submitter: Pete Becker Opened: 2011-03-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In earlier specifications of atomics the template specialization atomic<integer>
was derived from atomic_integer
(e.g. atomic<int>
was derived from atomic_int
),
and the working draft required free functions such as
int atomic_load(const atomic_int*)
for each of the atomic_integer
types. This worked fine with normal function overloading.
For the post-Batavia working draft, N3193 removed the requirement that atomic<integer>
be derived from atomic_integer
and replaced the free functions taking pointers to
atomic_integer
with template functions taking atomic_type*
, such as
template <class T> T atomic_load(const atomic_type*);
and a code comment explaining that atomic_type
can be either atomic<T>
or a
named base class of atomic<T>
. The latter possibility is supposed to allow existing
implementations based on the previous specification to continue to conform.
From history, this allowance seems to imply that functions like atomic_load
can be non-template
free functions, as they were before. The explicit requirements do not allow this, and, by requiring that
they be templates, make them far more complicated. As the specification is currently written, code that
uses an implementation that uses a base class would have to provide an explicit template type:
atomic<int> my_atomic_int; atomic_load<int>(&my_atomic_int);
That type argument isn't needed when atomic_type
is atomic<T>
, but cautious
users would always provide it to make their code portable across different implementations of the
standard library.
One possibility for the implementor would be to do some template meta-programming to infer the type
T
when there are no function parameters of type T
, but without running afoul of the
prohibition on adding parameters with default values (16.4.6.4 [global.functions]/3).
So the promise that implementations of the previous specification continue to conform has not been met. The specification of these free functions should be rewritten to support library code written to the previous specification or the vacuous promise should be removed.
[2011-03-08: Lawrence comments and drafts wording:]
One of the goals is to permit atomics code to compile under both C and C++. Adding explicit template arguments would defeat that goal.
The intent was to permit the normal function overloads foratomic_int
when atomic_int
is distinct from atomic<int>
. That intent was not reflected in the wording.
Proposed Resolution
Explicitly permit free functions.
Edit within the header
<atomic>
synopsis 32.5.2 [atomics.syn] as follows:// 29.6.1, general operations on atomic types// In the following declarations, atomic_type is either //// In the following declarations, atomic-type is either //atomic<T>
or a named base class forT
from // Table 145 or inferred from // Table 146.atomic<T>
or a named base class forT
from // Table 145 or inferred from // Table 146. // If it isatomic<T>
, then the declaration is a template // declaration prefixed withtemplate <class T>
template <class T>bool atomic_is_lock_free(const volatileatomic_typeatomic-type*);template <class T>bool atomic_is_lock_free(constatomic_typeatomic-type*);template <class T>void atomic_init(volatileatomic_typeatomic-type*, T);template <class T>void atomic_init(atomic_typeatomic-type*, T);template <class T>void atomic_store(volatileatomic_typeatomic-type*, T);template <class T>void atomic_store(atomic_typeatomic-type*, T);template <class T>void atomic_store_explicit(volatileatomic_typeatomic-type*, T, memory_order);template <class T>void atomic_store_explicit(atomic_typeatomic-type*, T, memory_order);template <class T>T atomic_load(const volatileatomic_typeatomic-type*);template <class T>T atomic_load(constatomic_typeatomic-type*);template <class T>T atomic_load_explicit(const volatileatomic_typeatomic-type*, memory_order);template <class T>T atomic_load_explicit(constatomic_typeatomic-type*, memory_order);template <class T>T atomic_exchange(volatileatomic_typeatomic-type*, T);template <class T>T atomic_exchange(atomic_typeatomic-type*, T);template <class T>T atomic_exchange_explicit(volatileatomic_typeatomic-type*, T, memory_order);template <class T>T atomic_exchange_explicit(atomic_typeatomic-type*, T, memory_order);template <class T>bool atomic_compare_exchange_weak(volatileatomic_typeatomic-type*, T*, T);template <class T>bool atomic_compare_exchange_weak(atomic_typeatomic-type*, T*, T);template <class T>bool atomic_compare_exchange_strong(volatileatomic_typeatomic-type*, T*, T);template <class T>bool atomic_compare_exchange_strong(atomic_typeatomic-type*, T*, T);template <class T>bool atomic_compare_exchange_weak_explicit(volatileatomic_typeatomic-type*, T*, T, memory_order, memory_order);template <class T>bool atomic_compare_exchange_weak_explicit(atomic_typeatomic-type*, T*, T. memory_order, memory_order);template <class T>bool atomic_compare)exchange_strong_explicit(volatileatomic_typeatomic-type*, T*, T, memory_order, memory_order);template <class T>bool atomic_compare_exchange_strong_explicit(atomic_typeatomic-type*, T*, T, memory_order, memory_order); // 29.6.2, templated operations on atomic types// In the following declarations, atomic_type is either //template <class T> T atomic_fetch_add(volatileatomic<T>
or a named base class forT
from // Table 145 or inferred from // Table 146.atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_add(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_add_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_add_explicit(atomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_sub(volatileatomic-typeatomic<T>*, T); template <class T> T atomic_fetch_sub(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_sub_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_sub_explicit(atomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_and(volatileatomic-typeatomic<T>*, T); template <class T> T atomic_fetch_and(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_and_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_and_explicit(atomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_or(volatileatomic-typeatomic<T>*, T); template <class T> T atomic_fetch_or(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_or_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_or_explicit(atomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_xor(volatileatomic-typeatomic<T>*, T); template <class T> T atomic_fetch_xor(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_xor_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_xor_explicit(atomic-typeatomic<T>*, T, memory_order); // 29.6.3, arithmetic operations on atomic types // In the following declarations, atomic-integral is either //atomic<T>
or a named base class forT
from // Table 145 or inferred from // Table 146. // If it isatomic<T>
, // then the declaration is a template specialization declaration prefixed with //template <>
template <>integral atomic_fetch_add(volatile atomic-integral*, integral);template <>integral atomic_fetch_add(atomic-integral*, integral);template <>integral atomic_fetch_add_explicit(volatile atomic-integral*, integral, memory_order);template <>integral atomic_fetch_add_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_sub(volatile atomic-integral*, integral);template <>integral atomic_fetch_sub(atomic-integral*, integral);template <>integral atomic_fetch_sub_explicit(volatile atomic-integral*, integral, memory_order);template <>integral atomic_fetch_sub_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_and(volatile atomic-integral*, integral);template <>integral atomic_fetch_and(atomic-integral*, integral);template <>integral atomic_fetch_and_explicit(volatile atomic-integral*, integral, memory_order);template <>integral atomic_fetch_and_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_or(volatile atomic-integral*, integral);template <>integral atomic_fetch_or(atomic-integral*, integral);template <>integral atomic_fetch_or_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_or_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_xor(volatile atomic-integral*, integral);template <>integral atomic_fetch_xor(atomic-integral*, integral);template <>integral atomic_fetch_xor_explicit(volatile atomic-integral*, integral, memory_order);template <>integral atomic_fetch_xor_explicit(atomic-integral*, integral, memory_order);Edit [atomics.types.operations.general] paragraph 1+2 as follows:
-1- The implementation shall provide the functions and function templates identified as "general operations on atomic types" in 32.5.2 [atomics.syn].
-2- In the declarations of these functions and function templates, the name atomic-type refers to eitheratomic<T>
or to a named base class forT
from Table 145 or inferred from Table 146.In [atomics.types.operations.templ] delete paragraph 2:
-1- The implementation shall declare but not define the function templates identified as "templated operations on atomic types" in 32.5.2 [atomics.syn].
-2- In the declarations of these templates, the name atomic-type refers to eitheratomic<T>
or to a named base class forT
from Table 145 or inferred from Table 146.Edit [atomics.types.operations.arith] paragraph 1+2 as follows:
-1- The implementation shall provide the functions and function template specializations identified as "arithmetic operations on atomic types" in 32.5.2 [atomics.syn].
-2- In the declarations of these functions and function template specializations, the name integral refers to an integral type and the name atomic-integral refers to eitheratomic<
integral>
or to a named base class for integral from Table 145 or inferred from Table 146.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
std::reverse
and std::copy_if
Section: 26.7.1 [alg.copy], 26.7.10 [alg.reverse] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2011-03-02 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [alg.copy].
View all other issues in [alg.copy].
View all issues with C++14 status.
Discussion:
In the description of std::reverse
Effects: For each non-negative integer
i <= (last - first)/2
, appliesiter_swap
to all pairs of iteratorsfirst + i
,(last - i) - 1
.
should be changed to
Effects: For each non-negative integer
i < (last - first)/2
, appliesiter_swap
to all pairs of iteratorsfirst + i
,(last - i) - 1
.
Here i
shall be strictly less than (last - first)/2
.
In the description of std::copy_if
Returns paragraph is missing.
[2011-03-02: Daniel drafts wording]
Proposed resolution:
Modify 26.7.10 [alg.reverse] p. 1 as indicated:
1 Effects: For each non-negative integer
i <
, applies=(last - first)/2iter_swap
to all pairs of iteratorsfirst + i
,(last - i) - 1
.
Add the following Returns element after 26.7.1 [alg.copy] p. 9:
template<class InputIterator, class OutputIterator, class Predicate> OutputIterator copy_if(InputIterator first, InputIterator last, OutputIterator result, Predicate pred);8 Requires: The ranges
9 Effects: Copies all of the elements referred to by the iterator[first,last)
and[result,result + (last - first))
shall not overlap.i
in the range[first,last)
for whichpred(*i)
is true. ?? Returns: The end of the resulting range. 10 Complexity: Exactlylast - first
applications of the corresponding predicate. 11 Remarks: Stable.
is_convertible
Section: 21 [meta] Status: Resolved Submitter: Daniel Krügler Opened: 2011-03-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [meta].
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Discussion:
When n3142 was suggested, it concentrated on constructions, assignments, and destructions, but overlooked to complement the single remaining compiler-support trait
template <class From, class To> struct is_convertible;
with the no-throw and triviality related aspects as it had been done with the other expression-based traits. Specifically, the current specification misses to add the following traits:
template <class From, class To> struct is_nothrow_convertible; template <class From, class To> struct is_trivially_convertible;
In particular the lack of is_nothrow_convertible
is severely restricting. This
was recently recognized when the proposal for decay_copy
was prepared by
n3255.
There does not exist a portable means to define the correct conditional noexcept
specification for the decay_copy
function template, which is declared as:
template <class T> typename decay<T>::type decay_copy(T&& v) noexcept(???);
The semantics of decay_copy
bases on an implicit conversion which again
influences the overload set of functions that are viable here. In most circumstances
this will have the same effect as comparing against the trait
std::is_nothrow_move_constructible
, but there is no guarantee for that being
the right answer. It is possible to construct examples, where this would lead
to the false result, e.g.
struct S { S(const S&) noexcept(false); template<class T> explicit S(T&&) noexcept(true); };
std::is_nothrow_move_constructible
will properly honor the explicit template
constructor because of the direct-initialization context which is part of the
std::is_constructible
definition and will in this case select it, such that
std::is_nothrow_move_constructible<S>::value == true
, but if we had
the traits is_nothrow_convertible
, is_nothrow_convertible<S, S>::value
would evaluate to false
, because it would use the copy-initialization context
that is part of the is_convertible
definition, excluding any explicit
constructors and giving the opposite result.
The decay_copy
example is surely not one of the most convincing examples, but
is_nothrow_convertible
has several use-cases, and can e.g. be used to express
whether calling the following implicit conversion function could throw an exception or not:
template<class T, class U> T implicit_cast(U&& u) noexcept(is_nothrow_convertible<U, T>::value) { return std::forward<U>(u); }
Therefore I suggest to add the missing trait is_nothrow_convertible
and for
completeness also the missing trait is_trivially_convertible
to 21 [meta].
[2011-03-24 Madrid meeting]
Daniel K: This is a new feature so out of scope.
Pablo: Any objections to moving 2040 to Open? No objections.[Bloomington, 2011]
Move to NAD Future, this would be an extension to existing functionality.
[LEWG, Kona 2017]
Fallen through the cracks since 2011, but we should discuss it. Alisdair points out that triviality is
about replacing operations with memcpy
, so be sure this is false for int
->float
.
[Rapperswil, 2018]
Resolved by the adoption of p0758r1.
Proposed resolution:
Ammend the following declarations to the header <type_traits>
synopsis
in 21.3.3 [meta.type.synop]:
namespace std { … // 20.9.6, type relations: template <class T, class U> struct is_same; template <class Base, class Derived> struct is_base_of; template <class From, class To> struct is_convertible; template <class From, class To> struct is_trivially_convertible; template <class From, class To> struct is_nothrow_convertible; … }
Modify Table 51 — "Type relationship predicates" as indicated. The removal of the
remaining traces of the trait is_explicitly_convertible
is an editorial
step, it was removed by n3047:
Table 51 — Type relationship predicates Template Condition Comments … template <class From, class To>
struct is_convertible;see below From
andTo
shall be complete
types, arrays of unknown bound, or
(possibly cv-qualified)void
types.template <class From, class To>
struct is_explicitly_convertible;is_constructible<To, From>::value
a synonym for a two-argument
version ofis_constructible
.
An implementation may define it
as an alias template.template <class From, class To>
struct is_trivially_convertible;is_convertible<From,
is
To>::valuetrue
and the
conversion, as defined by
is_convertible
, is known
to call no operation that is
not trivial ([basic.types], [special]).From
andTo
shall be complete
types, arrays of unknown bound,
or (possibly cv-qualified)void
types.template <class From, class To>
struct is_nothrow_convertible;is_convertible<From,
is
To>::valuetrue
and the
conversion, as defined by
is_convertible
, is known
not to throw any
exceptions ([expr.unary.noexcept]).From
andTo
shall be complete
types, arrays of unknown bound,
or (possibly cv-qualified)void
types.…
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++11 Submitter: Howard Hinnant Opened: 2011-03-09 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [facet.num.get.virtuals].
View all other issues in [facet.num.get.virtuals].
View all issues with C++11 status.
Discussion:
num_get
Stage 2 accumulation changed between C++03 and the current C++0x working draft. The sentences:
If it is not discarded, then a check is made to determine if
c
is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.
have been dropped from 28.3.4.3.2.3 [facet.num.get.virtuals], Stage 2, paragraph 3 that begins:
If
discard
is true, […]
Consider this code:
#include <sstream> #include <iostream> int main(void) { std::istringstream s("8cz"); long i = 0; char c; s >> i; if (!s.fail()) std::cout << "i = " << i << '\n'; else { std::cout << "s >> i failed\n"; s.clear(); } s >> c; if (!s.fail()) std::cout << "c = " << c << '\n'; else std::cout << "s >> c failed\n"; }
C++0x currently prints out:
s >> i failed c = z
However C++03 conforming implementations will output:
i = 8 c = c
I believe we need to restore C++03 compatibility.
Proposed resolution:
Add to 28.3.4.3.2.3 [facet.num.get.virtuals], Stage 2:
If
If the character is either discarded or accumulated then in is advanced bydiscard
is true, then if'.'
has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if'.'
has already been accumulated, the character is discarded and Stage 2 terminates. If it is not discarded, then a check is made to determine ifc
is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.++in
and processing returns to the beginning of stage 2.
forward_list::before_begin()
to forward_list::end()
Section: 23.3.7.3 [forward.list.iter] Status: C++11 Submitter: Joe Gottman Opened: 2011-03-13 Last modified: 2023-02-07
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
For an object c
of type forward_list<X, Alloc>
, the iterators
c.before_begin()
and c.end()
are part of the same underlying sequence,
so the expression c.before_begin() == c.end()
must be well-defined.
But the standard says nothing about what the result of this expression
should be. The forward iterator requirements says no dereferenceable
iterator is equal to a non-dereferenceable iterator and that two
dereferenceable iterators are equal if and only if they point to the
same element. But since before_begin()
and end()
are both
non-dereferenceable, neither of these rules applies.
Many forward_list
methods, such as insert_after()
, have a
precondition that the iterator passed to them must not be equal to
end()
. Thus, user code might look like the following:
void foo(forward_list<int>& c, forward_list<int>::iterator it) { assert(it != c.end()); c.insert_after(it, 42); }
Conversely, before_begin()
was specifically designed to be used with
methods like insert_after()
, so if c.before_begin()
is passed to
this function the assertion must not fail.
[2011-03-14: Daniel comments and updates the suggested wording]
The suggested wording changes are necessary but not sufficient. Since there
does not exist an equivalent semantic definition of cbefore_begin()
as
we have for cbegin()
, this still leaves the question open whether
the normative remark applies to cbefore_begin()
as well. A simple fix
is to define the operational semantics of cbefore_begin()
in terms of
before_begin()
.
[2011-03-24 Madrid meeting]
General agreement that this is a serious bug.
Pablo: Any objections to moving 2042 to Immediate? No objections.Proposed resolution:
Add to the definition of forward_list::before_begin()
[forwardlist.iter]
the following:
iterator before_begin(); const_iterator before_begin() const; const_iterator cbefore_begin() const;-1- Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by
-?- Effects:begin()
.cbefore_begin()
is equivalent toconst_cast<forward_list const&>(*this).before_begin()
. -?- Remarks:before_begin() == end()
shall equal false.
Section: 16.4.6.8 [algorithm.stable] Status: C++14 Submitter: Pablo Halpern Opened: 2011-03-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
16.4.6.8 [algorithm.stable] specified the meaning of "stable" when applied to the different types of algorithms. The second bullet says:
— For the remove algorithms the relative order of the elements that are not removed is preserved.
There is no description of what "stable" means for copy algorithms, even though the term is
applied to copy_if
(and perhaps others now or in the future). Thus, copy_if
is using the term without a precise definition.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS.
In 16.4.6.8 [algorithm.stable] p. 1 change as indicated:
When the requirements for an algorithm state that it is “stable” without further elaboration, it means:
- For the sort algorithms the relative order of equivalent elements is preserved.
- For the remove and copy algorithms the relative order of the elements that are not removed is preserved.
- For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range precede the elements from the second range.
forward_list::merge
and forward_list::splice_after
with unequal allocatorsSection: 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Pablo Halpern Opened: 2011-03-24 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
View all issues with C++14 status.
Discussion:
list::merge
and list::splice
have the requirement that the two lists being merged or
spliced must use the same allocator. Otherwise, moving list nodes from one container to the other would
corrupt the data structure. The same requirement is needed for forward_list::merge
and
forward_list::splice_after
.
[ 2011 Bloomington ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
In [forwardlist.ops] p. 1 change as indicated:
void splice_after(const_iterator position, forward_list<T,Allocator>& x); void splice_after(const_iterator position, forward_list<T,Allocator>&& x);1 - Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
).get_allocator() == x.get_allocator()
.&x != this
.
In [forwardlist.ops] p. 5 change as indicated:
void splice_after(const_iterator position, forward_list<T,Allocator>& x, const_iterator i); void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator i);5 - Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
). The iterator followingi
is a dereferenceable iterator inx
.get_allocator() == x.get_allocator()
.
In [forwardlist.ops] p. 9 change as indicated:
void splice_after(const_iterator position, forward_list<T,Allocator>& x, const_iterator first, const_iterator last); void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator first, const_iterator last);9 - Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
). (first
,last
) is a valid range inx
, and all iterators in the range (first
,last
) are dereferenceable.position
is not an iterator in the range (first
,last
).get_allocator() == x.get_allocator()
.
In [forwardlist.ops] p. 18 change as indicated:
void merge(forward_list<T,Allocator>& x); void merge(forward_list<T,Allocator>&& x); template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp); template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);18 - Requires:
comp
defines a strict weak ordering ([alg.sorting]), and*this
andx
are both sorted according to this ordering.get_allocator() == x.get_allocator()
.
unique_ptr
Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++14 Submitter: Daniel Krügler Opened: 2011-04-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.asgn].
View all issues with C++14 status.
Discussion:
The semantics described in 20.3.1.3.4 [unique.ptr.single.asgn] p. 6
Effects: Transfers ownership from
u
to*this
as if […] followed by an assignment fromstd::forward<D>(u.get_deleter())
.
contradicts to the pre-conditions described in p. 4:
Requires: If E is not a reference type, assignment of the deleter from an rvalue of type
E
shall be well-formed and shall not throw an exception. Otherwise,E
is a reference type and assignment of the deleter from an lvalue of typeE
shall be well-formed and shall not throw an exception.
Either the pre-conditions are incorrect or the semantics should be an assignment from
std::forward<E>(u.get_deleter())
, instead.
It turns out that this contradiction is due to an incorrect transcription from the proposed
resolution of 983(i) to the finally accepted proposal
n3073 (see
bullet 12) as confirmed by Howard Hinnant, thus the type argument provided to std::forward
must be fixed as indicated.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS.
Edit 20.3.1.3.4 [unique.ptr.single.asgn] p. 6 as indicated:
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;4 - Requires: If
5 - Remarks: This operator shall not participate in overload resolution unless:E
is not a reference type, assignment of the deleter from an rvalue of typeE
shall be well-formed and shall not throw an exception. Otherwise,E
is a reference type and assignment of the deleter from an lvalue of typeE
shall be well-formed and shall not throw an exception.
unique_ptr<U, E>::pointer
is implicitly convertible topointer
andU
is not an array type.6 - Effects: Transfers ownership from
7 - Returns:u
to*this
as if by callingreset(u.release())
followed by an assignment fromstd::forward<
.DE>(u.get_deleter())*this
.
mem_fn
overloadsSection: 22.10 [function.objects], 22.10.16 [func.memfn] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [function.objects].
View all issues with C++14 status.
Discussion:
The mem_fn
overloads for member functions are redundant and misleading
and should be removed from the post-C++11 WP.
mem_fn
was specified by
a single signature:
template<class R, class T> unspecified mem_fn(R T::* pm);
and was accompanied by the remark "Implementations may implement mem_fn
as a set of overloaded function templates." This remark predates variadic templates
and was presumably to allow implementations to provide overloads for a limited
number of function parameters, to meet the implementation-defined limit on numbers of
template parameters.
CopyConstructible
concept
(those overloads first appeared in N2322.) The overloads failed to
account for varargs member functions (i.e. those declared with an
ellipsis in the parameter-declaration-clause) e.g.
struct S { int f(int, ...); };
Syntactically such a function would be handled by the original
mem_fn(R T::* pm)
signature, the only minor drawback being that there
would be no CopyConstructible
requirement on the parameter list. (Core
DR 547 clarifies that partial specializations can be written to match
cv-qualified and ref-qualified functions to support the case where R T::*
matches a pointer to member function type.)
mem_fn(R T::* pm)
signature.
Concepts were removed from the draft and N3000 restored the original
single signature and accompanying remark.
LWG 1230(i) was opened to strike the remark again and to add an overload
for member functions (this overload was unnecessary for syntactic reasons and
insufficient as it didn't handle member functions with cv-qualifiers and/or
ref-qualifiers.)
920(i) (and 1230(i)) were resolved by restoring a full set of
(non-concept-enabled) overloads for member functions with cv-qualifiers and ref-qualifiers,
but as in the concept-enabled draft there were no overloads for member functions with
an ellipsis in the parameter-declaration-clause. This is what is present in the FDIS.
Following the thread beginning with message c++std-lib-30675, it is my
understanding that all the mem_fn
overloads for member functions are
unnecessary and were only ever added to allow concept requirements.
I'm not aware of any reason implementations cannot implement mem_fn
as
a single function template. Without concepts the overloads are
redundant, and the absence of overloads for varargs functions can be
interpreted to imply that varargs functions are not intended to work
with mem_fn
. Clarifying the intent by adding overloads for varargs
functions would expand the list of 12 redundant overloads to 24, it
would be much simpler to remove the 12 redundant overloads entirely.
[Bloomington, 2011]
Move to Review.
The issue and resolution appear to be correct, but there is some concern that the wording of INVOKE may be different depending on whether you pass a pointer-to-member-data or pointer-to-member-function. That might make the current wording necessary after all, and then we might need to add the missing elipsis overloads.
There was some concern that the Remark confirming implementors had freedom to implement this as a set of overloaded functions may need to be restored if we delete the specification for these overloads.
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change the <functional>
synopsis 22.10 [function.objects] p. 2 as follows:
namespace std { […] // [func.memfn], member function adaptors: template<class R, class T> unspecified mem_fn(R T::*);template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...)); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);[…] }
Change 22.10.16 [func.memfn] as follows:
template<class R, class T> unspecified mem_fn(R T::*);template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...)); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);
is_destructible
is underspecifiedSection: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2011-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++14 status.
Discussion:
The conditions for the type trait is_destructible
to be true
are described in Table 49 — Type property predicates:
For a complete type
T
and given
template <class U> struct test { U u; };
,
test<T>::~test()
is not deleted.
This specification does not say what the result would be for function types or for abstract types:
X
the instantiation test<X>
is already ill-formed, so we cannot say anything about whether the destructor
would be deleted or not.which has the same consequence as for abstract types, namely that the corresponding instantiation ofIf a declaration acquires a function type through a type dependent on a template-parameter and this causes a declaration that does not use the syntactic form of a function declarator to have function type, the program is ill-formed.
[ Example:template<class T> struct A { static T t; }; typedef int function(); A<function> a; // ill-formed: would declare A<function>::t // as a static member function— end example ]
test
is already ill-formed and we cannot say anything
about the destructor.
To solve this problem, I suggest to specify function types as trivially and nothrowing destructible, because above mentioned rule is very special for templates. For non-templates, a typedef can be used to introduce a member as member function as clarified in 9.3.4.6 [dcl.fct] p. 10.
For abstract types, two different suggestions have been brought to my attention: Either declare them as unconditionally non-destructible or check whether the expression
std::declval<T&>().~T()
is well-formed in an unevaluated context. The first solution is very easy to specify, but the second version has the advantage for providing more information to user-code. This information could be quite useful, if generic code is supposed to invoke the destructor of a reference to a base class indirectly via a delete expression, as suggested by Howard Hinnant:
template <class T> my_pointer<T>::~my_pointer() noexcept(is_nothrow_destructible<T>::value) { delete ptr_; }
Additional to the is_destructible
traits, its derived forms is_trivially_destructible
and is_nothrow_destructible
are similarly affected, because their wording refers to "the indicated
destructor" and probably need to be adapted as well.
[ 2011 Bloomington ]
After discussion about to to handle the exceptional cases of reference types, function types (available by defererencing a function pointer)
and void
types, Howard supplied proposed wording.
[ 2011-08-20 Daniel comments and provides alternatives wording ]
The currently proposed wording would have the consequence that every array type is not destructible, because the pseudo-destructor requires a scalar type with the effect that the expression
std::declval<T&>().~T()
is not well-formed for e.g. T
equal to int[3]
. The intuitive
solution to fix this problem would be to adapt the object type case to refer to
the expression
std::declval<U&>().~U()
with U
equal to remove_all_extents<T>::type
, but that
would have the effect that arrays of unknown bounds would be destructible, if
the element type is destructible, which was not the case before (This was
intentionally covered by the special "For a complete type T" rule in
the FDIS).
Let
U
beremove_all_extents<T>::type
.
For incomplete types and function types,is_destructible<T>::value
isfalse
.
For object types, if the expressionstd::declval<U&>().~U()
is well-formed
when treated as an unevaluated operand (Clause 5), thenis_destructible<T>::value
istrue
, otherwise it isfalse
.
For reference types,is_destructible<T>::value
istrue
.
This wording also harmonizes with the "unevaluated operand" phrase used in other places, there does not exist the definition of an "unevaluated context"
Note: In the actually proposed wording this wording has been slightly reordered with the same effects.Howard's (old) proposed resolution:
Update 21.3.5.4 [meta.unary.prop], table 49:
template <class T> struct is_destructible;
For a complete typeT
and giventemplate <class U> struct test { U u; };
,test<T>::~test()
is not deleted.
For object types, if the expression:std::declval<T&>().~T()
is well-formed in an unevaluated context thenis_destructible<T>::value
istrue
, otherwise it isfalse
.
Forvoid
types,is_destructible<T>::value
isfalse
.
For reference types,is_destructible<T>::value
istrue
.
For function types,is_destructible<T>::value
isfalse
.T
shall be a complete type, (possibly cv-qualified)void
, or an array of unknown bound.
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup.
[2012, Portland: applied to WP]
Proposed resolution:
Update 21.3.5.4 [meta.unary.prop], table 49:
template <class T>
struct is_destructible; |
T and given template <class U> struct test { U u; }; , test<T>::~test() is not deleted.
For reference types, is_destructible<T>::value is true .For incomplete types and function types, is_destructible<T>::value is false .For object types and given U equal to remove_all_extents<T>::type ,if the expression std::declval<U&>().~U() is well-formed when treated as anunevaluated operand (Clause 7 [expr]), then is_destructible<T>::value is true ,otherwise it is false . |
T shall be a complete type, (possibly cv-qualified) void , or an array of unknown bound. |
allocator_traits
to define member typesSection: 23.5 [unord] Status: C++14 Submitter: Tom Zieberman Opened: 2011-04-29 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord].
View all issues with C++14 status.
Discussion:
The unordered associative containers define their member types reference
,
const_reference
, pointer
, const_pointer
in terms of
their template parameter Allocator
(via allocator_type
typedef). As
a consequence, only the allocator types, that provide sufficient typedefs, are usable
as allocators for unordered associative containers, while other containers do not have
this deficiency. In addition to that, the definitions of said typedefs are different
from ones used in the other containers. This is counterintuitive and introduces a certain
level of confusion. These issues can be fixed by defining pointer
and
const_pointer
typedefs in terms of allocator_traits<Allocator>
and by defining reference
and const_reference
in terms of
value_type
as is done in the other containers.
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Change 23.5.3.1 [unord.map.overview] paragraph 3 as indicated:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > class unordered_map { public: // types typedef Key key_type; typedef std::pair<const Key, T> value_type; typedef T mapped_type; typedef Hash hasher; typedef Pred key_equal; typedef Allocator allocator_type; typedef typenameallocator_typeallocator_traits<Allocator>::pointer pointer; typedef typenameallocator_typeallocator_traits<Allocator>::const_pointer const_pointer; typedeftypename allocator_type::referencevalue_type& reference; typedeftypename allocator_type::const_referenceconst value_type& const_reference; typedef implementation-defined size_type; typedef implementation-defined difference_type; […] }; }
Change 23.5.4.1 [unord.multimap.overview] paragraph 3 as indicated:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > class unordered_multimap { public: // types typedef Key key_type; typedef std::pair<const Key, T> value_type; typedef T mapped_type; typedef Hash hasher; typedef Pred key_equal; typedef Allocator allocator_type; typedef typenameallocator_typeallocator_traits<Allocator>::pointer pointer; typedef typenameallocator_typeallocator_traits<Allocator>::const_pointer const_pointer; typedeftypename allocator_type::referencevalue_type& reference; typedeftypename allocator_type::const_referenceconst value_type& const_reference; typedef implementation-defined size_type; typedef implementation-defined difference_type; […] }; }
Change 23.5.6.1 [unord.set.overview] paragraph 3 as indicated:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > class unordered_set { public: // types typedef Key key_type; typedef Key value_type; typedef Hash hasher; typedef Pred key_equal; typedef Allocator allocator_type; typedef typenameallocator_typeallocator_traits<Allocator>::pointer pointer; typedef typenameallocator_typeallocator_traits<Allocator>::const_pointer const_pointer; typedeftypename allocator_type::referencevalue_type& reference; typedeftypename allocator_type::const_referenceconst value_type& const_reference; typedef implementation-defined size_type; typedef implementation-defined difference_type; […] }; }
Change 23.5.7.1 [unord.multiset.overview] paragraph 3 as indicated:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > class unordered_multiset { public: // types typedef Key key_type; typedef Key value_type; typedef Hash hasher; typedef Pred key_equal; typedef Allocator allocator_type; typedef typenameallocator_typeallocator_traits<Allocator>::pointer pointer; typedef typenameallocator_typeallocator_traits<Allocator>::const_pointer const_pointer; typedeftypename allocator_type::referencevalue_type& reference; typedeftypename allocator_type::const_referenceconst value_type& const_reference; typedef implementation-defined size_type; typedef implementation-defined difference_type; […] }; }
tuple
constructors for more than one parameterSection: 22.4.4 [tuple.tuple], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Ville Voutilainen Opened: 2011-05-01 Last modified: 2016-01-28
Priority: 2
View all other issues in [tuple.tuple].
View all issues with Resolved status.
Discussion:
One of my constituents wrote the following:
-------snip------------ So far the only use I've found forstd::tuple
is as an ad-hoc type to emulate
multiple return values. If the tuple ctor was made non-explicit one could
almost think C++ supported multiple return values especially when combined
with std::tie()
.
// assume types line_segment and point // assume function double distance(point const&, point const&) std::tuple<point, point> closest_points(line_segment const& a, line_segment const& b) { point ax; point bx; /* some math */ return {ax, bx}; } double distance(line_segment const& a, line_segment const& b) { point ax; point bx; std::tie(ax, bx) = closest_points(a, b); return distance(ax, bx); }
-------snap----------
See also the messages starting from lib-29330. Some notes:pair
allows such a returndecltype
refuses {1, 2}
I would recommend making non-unary tuple
constructors non-explicit.
[Bloomington, 2011]
Move to NAD Future, this would be an extension to existing functionality.
[Portland, 2012]
Move to Open at the request of the Evolution Working Group.
[Lenexa 2015-05-05]
VV: While in the area of tuples, LWG 2051 should have status of WP, it is resolved by Daniel's "improving pair and tuple" paper.
MC: status Resolved, by N4387
Proposed resolution:
Resolved by the adoption of N4387.mapped_type
and value_type
for associative containersSection: 23.2.7 [associative.reqmts] Status: Resolved Submitter: Marc Glisse Opened: 2011-05-04 Last modified: 2016-01-28
Priority: 2
View other active issues in [associative.reqmts].
View all other issues in [associative.reqmts].
View all issues with Resolved status.
Discussion:
(this is basically reopening the first part of issue 2006(i), as discussed in the thread starting at c++std-lib-30698 )
Section 23.2.7 [associative.reqmts] In Table 102, several uses ofT
(which means mapped_type
here) should
be value_type
instead. This is almost editorial. For instance:
a_uniq.emplace(args)Requires:
Effects: Inserts aT
shall beEmplaceConstructible
intoX
from args.T
objectt
constructed withstd::forward<Args>(args)...
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key oft
.
[ 2011 Bloomington ]
Not even an exhaustive list of problem locations. No reason to doubt issue.
Pablo agrees to provide wording.
[ 2011-09-04 Pablo Halpern provides improved wording ]
[2014-02-15 post-Issaquah session : move to Resolved]
AJM to replace this note with a 'Resolved By...' after tracking down the exact sequence of events, but clearly resolved in C++14 DIS.
Proposed resolution:
In both section 23.2.7 [associative.reqmts] Table 102 and 23.2.8 [unord.req], Table 103, make the following text replacements:
Original text, in FDIS | Replacement text |
T is CopyInsertable into X and CopyAssignable . |
value_type is CopyInsertable into X , key_type is CopyAssignable , and
mapped_type is CopyAssignable (for containers having a mapped_type ) |
T is CopyInsertable |
value_type is CopyInsertable |
T shall be CopyInsertable |
value_type shall be CopyInsertable |
T shall be MoveInsertable |
value_type shall be MoveInsertable |
T shall be EmplaceConstructible |
value_type shall be EmplaceConstructible |
T object |
value_type object |
[
Notes to the editor: The above are carefully selected
phrases that can be used for global search-and-replace within
the specified sections without accidentally making changes to
correct uses T
.
]
regex
bitmask typesSection: 28.6.4 [re.const] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-05-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.const].
View all issues with C++14 status.
Discussion:
When N3110 was applied to the WP some redundant "static" keywords were added and one form of initializer which isn't valid for enumeration types was replaced with another form of invalid initializer.
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Change 28.6.4.2 [re.synopt] as indicated:
namespace std { namespace regex_constants { typedef T1 syntax_option_type;staticconstexpr syntax_option_type icase = unspecified ;staticconstexpr syntax_option_type nosubs = unspecified ;staticconstexpr syntax_option_type optimize = unspecified ;staticconstexpr syntax_option_type collate = unspecified ;staticconstexpr syntax_option_type ECMAScript = unspecified ;staticconstexpr syntax_option_type basic = unspecified ;staticconstexpr syntax_option_type extended = unspecified ;staticconstexpr syntax_option_type awk = unspecified ;staticconstexpr syntax_option_type grep = unspecified ;staticconstexpr syntax_option_type egrep = unspecified ; } }
Change 28.6.4.3 [re.matchflag] as indicated:
namespace std { namespace regex_constants { typedef T2 match_flag_type;staticconstexpr match_flag_type match_default= 0{};staticconstexpr match_flag_type match_not_bol = unspecified ;staticconstexpr match_flag_type match_not_eol = unspecified ;staticconstexpr match_flag_type match_not_bow = unspecified ;staticconstexpr match_flag_type match_not_eow = unspecified ;staticconstexpr match_flag_type match_any = unspecified ;staticconstexpr match_flag_type match_not_null = unspecified ;staticconstexpr match_flag_type match_continuous = unspecified ;staticconstexpr match_flag_type match_prev_avail = unspecified ;staticconstexpr match_flag_type format_default= 0{};staticconstexpr match_flag_type format_sed = unspecified ;staticconstexpr match_flag_type format_no_copy = unspecified ;staticconstexpr match_flag_type format_first_only = unspecified ; } }
Change 28.6.4.4 [re.err] as indicated:
namespace std { namespace regex_constants { typedef T3 error_type;staticconstexpr error_type error_collate = unspecified ;staticconstexpr error_type error_ctype = unspecified ;staticconstexpr error_type error_escape = unspecified ;staticconstexpr error_type error_backref = unspecified ;staticconstexpr error_type error_brack = unspecified ;staticconstexpr error_type error_paren = unspecified ;staticconstexpr error_type error_brace = unspecified ;staticconstexpr error_type error_badbrace = unspecified ;staticconstexpr error_type error_range = unspecified ;staticconstexpr error_type error_space = unspecified ;staticconstexpr error_type error_badrepeat = unspecified ;staticconstexpr error_type error_complexity = unspecified ;staticconstexpr error_type error_stack = unspecified ; } }
time_point
constructors need to be constexpr
Section: 30.6 [time.point] Status: Resolved Submitter: Anthony Williams Opened: 2011-05-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
In 30.6 [time.point], time_point::min()
and time_point::max()
are listed as constexpr
. However, time_point
has no constexpr
constructors,
so is not a literal type, and so these functions cannot be constexpr
without adding a
constexpr
constructor for implementation purposes.
constexpr
to the constructors of time_point
. The effects of
the constructor template basically imply that the member function time_since_epoch()
is
intended to be constexpr
as well.
[2012, Portland]
Resolved by adopting paper n3469.
Proposed resolution:
This wording is relative to the FDIS.
Alter the class template definition in 30.6 [time.point] as follows:
template <class Clock, class Duration = typename Clock::duration> class time_point { […] public: // 20.11.6.1, construct: constexpr time_point(); // has value epoch constexpr explicit time_point(const duration& d); // same as time_point() + d template <class Duration2> constexpr time_point(const time_point<clock, Duration2>& t); // 20.11.6.2, observer: constexpr duration time_since_epoch() const; […] };
Alter the declarations in 30.6.2 [time.point.cons]:
constexpr time_point();-1- Effects: Constructs an object of type
time_point
, initializingd_
withduration::zero()
. Such atime_point
object represents the epoch.
constexpr explicit time_point(const duration& d);-2- Effects: Constructs an object of type
time_point
, initializingd_
withd
. Such atime_point
object represents the epoch+ d
.
template <class Duration2> constexpr time_point(const time_point<clock, Duration2>& t);-3- Remarks: This constructor shall not participate in overload resolution unless
-4- Effects: Constructs an object of typeDuration2
is implicitly convertible toduration
.time_point
, initializingd_
witht.time_since_epoch()
.
Alter the declaration in 30.6.3 [time.point.observer]:
constexpr duration time_since_epoch() const;-1- Returns: d_.
std::move
in std::accumulate
and other algorithmsSection: 26.10 [numeric.ops] Status: Resolved Submitter: Chris Jefferson Opened: 2011-01-01 Last modified: 2020-09-06
Priority: 3
View all other issues in [numeric.ops].
View all issues with Resolved status.
Discussion:
The C++0x draft says std::accumulate
uses: acc = binary_op(acc, *i)
.
acc = binary_op(std::move(acc), *i)
can lead to massive improvements (particularly,
it means accumulating strings is linear rather than quadratic).
Consider the simple case, accumulating a bunch of strings of length 1 (the same argument holds for other length buffers).
For strings s
and t
, s+t
takes time length(s)+length(t)
, as you have to copy
both s
and t
into a new buffer.
So in accumulating n
strings, step i
adds a string of length i-1
to a string of length
1, so takes time i
.
Therefore the total time taken is: 1+2+3+...+n
= O(n2
)
std::move(s)+t
, for a "good" implementation, is amortized time length(t)
, like vector
,
just copy t
onto the end of the buffer. So the total time taken is:
1+1+1+...+1
(n
times) = O(n
). This is the same as push_back
on a vector
.
I'm trying to decide if this implementation might already be allowed. I suspect it might not
be (although I can't imagine any sensible code it would break). There are other algorithms
which could benefit similarly (inner_product
, partial_sum
and
adjacent_difference
are the most obvious).
Is there any general wording for "you can use rvalues of temporaries"?
The reflector discussion starting with message c++std-lib-29763 came to the conclusion
that above example is not covered by the "as-if" rules and that enabling this behaviour
would seem quite useful.
[ 2011 Bloomington ]
Moved to NAD Future. This would be a larger change than we would consider for a simple TC.
[2017-02 in Kona, LEWG responds]
Like the goal.
What is broken by adding std::move() on the non-binary-op version?
A different overload might be selected, and that would be a breakage. Is it breakage that we should care about?
We need to encourage value semantics.
Need a paper. What guidance do we give?
Use std::reduce() (uses generalized sum) instead of accumulate which doesn’t suffer it.
Inner_product and adjacent_difference also. adjacent_difference solves it half-way for “val” object, but misses the opportunity for passing acc as std::move(acc)
.
[2017-06-02 Issues Telecon]
Ville to encourage Eelis to write a paper on the algorithms in <numeric>, not just for accumulate.
Howard pointed out that this has already been done for the algorithms in <algorithm>
Status to Open; Priority 3
[2017-11-11, Albuquerque]
This was resolved by the adoption of P0616r0.
Proposed resolution:
future_errc
enums start with value 0 (invalid value for broken_promise
)Section: 32.10.1 [futures.overview] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-05-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.overview].
View all issues with C++14 status.
Discussion:
In 32.10.1 [futures.overview] enum class future_errc
is defined as follows:
enum class future_errc { broken_promise, future_already_retrieved, promise_already_satisfied, no_state };
With this declaration broken_promise
has value 0, which means that
for a future_error f
with this code
f.code().operator bool()
yields false, which makes no sense. 0 has to be reserved for "no error". So, the enums defined here have to start with 1.
Howard, Anthony, and Jonathan have no objections.[Discussion in Bloomington 2011-08-16]
Previous resolution:
This wording is relative to the FDIS.
In 32.10.1 [futures.overview], header
<future>
synopsis, fix the declaration offuture_errc
as follows:namespace std { enum class future_errc {broken_promise,future_already_retrieved = 1, promise_already_satisfied, no_state, broken_promise }; […] }
Is this resolution overspecified? These seem to be all implementation-defined. How do users add new values and not conflict with established error codes?
PJP proxy says: over-specified. boo.
Other error codes: look for is_error_code_enum
specializations. Only one exists io_errc
Peter: I don't see any other parts of the standard that specify error codes where we have to do something similar.
Suggest that for every place where we add an error code, the following:
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
In 32.10.1 [futures.overview], header <future>
synopsis, fix
the declaration of future_errc
as follows:
namespace std { enum class future_errc { broken_promise = implementation defined, future_already_retrieved = implementation defined, promise_already_satisfied = implementation defined, no_state = implementation defined }; […] }
In 32.10.1 [futures.overview], header <future>
synopsis, add a paragraph after paragraph 2 as follows:
future_errc
are distinct and not zero.
time_point + duration
semantics should be made constexpr
conformingSection: 30.6.6 [time.point.nonmember] Status: Resolved Submitter: Daniel Krügler Opened: 2011-05-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.point.nonmember].
View all issues with Resolved status.
Discussion:
It has been observed by LWG 2054(i) that the specification of some time_point
member functions
already imply that time_point
needs to be a literal type and suggests to specify the constructors
and the member function time_since_epoch()
as constexpr
functions at the
minimum necessary. Adding further constexpr
specifier to other operations should
clearly be allowed and should probably be done as well. But to allow for further constexpr
functions in the future requires that their semantics is compatible to operations allowed in constexpr
functions. This is already fine for all operations, except this binary plus operator:
template <class Clock, class Duration1, class Rep2, class Period2> time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type> operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);-1- Returns:
CT(lhs) += rhs
, whereCT
is the type of the return value.
for similar reasons as those mentioned in 2020(i). The semantics should be fixed to allow
for making them constexpr
. This issue should also be considered as a placeholder for a request
to make the remaining time_point
operations similarly constexpr
as had been done for
duration
.
[2012, Portland]
Resolved by adopting paper n3469.
Proposed resolution:
This wording is relative to the FDIS.
In 30.6.6 [time.point.nonmember], p.1 change the Returns element semantics as indicated:
template <class Clock, class Duration1, class Rep2, class Period2> time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type> operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);-1- Returns:
, where
CT(lhs) += rhsCT(lhs.time_since_epoch() + rhs)CT
is the type of the return value.
valarray
and begin/end
Section: 29.6 [numarray] Status: C++14 Submitter: Gabriel Dos Reis Opened: 2011-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [numarray].
View all issues with C++14 status.
Discussion:
It was just brought to my attention that the pair of functions
begin/end
were added to valarray
component.
Those additions strike me as counter to the long standing agreement
that valarray<T>
is not yet another container. Valarray values
are in general supposed to be treated as a whole, and as such
has a loose specification allowing expression template techniques.
begin/end
- or at least for the
const valarray<T>&
version. I strongly believe those
are defects.
[This issue was discussed on the library reflector starting from c++std-lib-30761. Some of the key conclusions of this discussion were:]
begin/end
members were added to allow valarray
to participate
in the new range-based for-loop by n2930
and not to make them container-like.begin/end
become invalidated. To fix this, these invalidation rules need at
least to reflect the invalidation rules of the references returned by the
operator[]
overloads of valarray
(29.6.2.4 [valarray.access]).
begin/end
, if the
replacement type technique is used (which was clearly part of the design of valarray
).
Providing such additional overloads would also lead to life-time problems in examples like
begin(x + y)
where x
and y
are expressions involving valarray
objects. To fix this, the begin/end
overloads could be explicitly excluded from the
general statements of 29.6.1 [valarray.syn] p.3-5. This would make it unspecified
whether the expression begin(x + y)
would be well-formed, portable code would
need to write this as begin(std::valarray<T>(x + y))
.[ 2011 Bloomington ]
The intent of these overloads is entirely to support the new for syntax, and not to create new containers.
Stefanus provides suggested wording.
[2012, Kona]
Moved to Tenatively Ready by post-meeting issues processing group, after confirmation from Gaby.
[2012, Portland: applied to WP]
Proposed resolution:
In 29.6.1 [valarray.syn]/4, make the following insertion:
4 Implementations introducing such replacement types shall provide additional functions and operators as follows:
const valarray<T>&
other than begin
and end
(29.6.10 [valarray.range]), identical functions taking the replacement types shall be added;
const valarray<T>&
arguments, identical functions taking every combination
of const valarray<T>&
and replacement types shall be added.
In 29.6.10 [valarray.range], make the following insertion:
1 In the begin
and end
function templates that follow, unspecified1 is a type that meets
the requirements of a mutable random access iterator (24.2.7) whose value_type
is the template parameter
T
and whose reference
type is T&
. unspecified2 is a type that meets the
requirements of a constant random access iterator (24.2.7) whose value_type
is the template parameter
T
and whose reference
type is const T&
.
2 The iterators returned by begin
and end
for an array are guaranteed to be valid until the
member function resize(size_t, T)
(29.6.2.8 [valarray.members]) is called for that array or until
the lifetime of that array ends, whichever happens first.
map::erase
Section: 23.4.3 [map] Status: C++17 Submitter: Christopher Jefferson Opened: 2011-05-18 Last modified: 2017-07-30
Priority: 3
View all other issues in [map].
View all issues with C++17 status.
Discussion:
map::erase
(and several related methods) took an iterator in C++03, but take a const_iterator
in C++0x. This breaks code where the map's key_type
has a constructor which accepts an iterator
(for example a template constructor), as the compiler cannot choose between erase(const key_type&)
and erase(const_iterator)
.
#include <map> struct X { template<typename T> X(T&) {} }; bool operator<(const X&, const X&) { return false; } void erasor(std::map<X,int>& s, X x) { std::map<X,int>::iterator it = s.find(x); if (it != s.end()) s.erase(it); }
[ 2011 Bloomington ]
This issue affects only associative container erase
calls, and is not more general, as these are the
only functions that are also overloaded on another single arguement that might cause confusion - the erase
by key method. The complete resolution should simply restore the iterator
overload in addition to the
const_iterator
overload for all eight associative containers.
Proposed wording supplied by Alan Talbot, and moved to Review.
[2012, Kona]
Moved back to Open by post-meeting issues processing group.
Pablo very unhappy about case of breaking code with ambiguous conversion between both iterator types.
Alisdair strongly in favor of proposed resolution, this change from C++11 bit Chris in real code, and it took a while to track down the cause.
Move to open, bring in front of a larger group
Proposed wording from Jeremiah:
erase(key)
shall not participate in overload resolution if iterator
is
convertible to key
.
Note that this means making erase(key)
a template-method
Poll Chris to find out if he already fixed his code, or fixed his library
Jeremiah - allow both overloads, but enable_if
the const_iterator
form as
a template, requiring is_same
to match only const_iterator
.
Poll PJ to see if he has already applied this fix?
[2015-02 Cologne]
AM: To summarize, we changed a signature and code broke. At what point do we stop and accept breakage in increasingly obscure code? VV: libc++ is still broken, but libstdc++ works, so they've fixed this — perhaps using this PR? [Checks] Yes, libstdc++ uses this solution, and has a comment pointing to LWG 2059. AM: This issue hasn't been looked at since Kona. In any case, we already have implementation experience now.
AM: I'd say let's ship it. We already have implementation experience (libstdc++ and MSVS). MC: And "tentatively ready" lets me try to implement this and see how it works.Proposed resolution:
Editorial note: The following things are different between 23.2.7 [associative.reqmts] p.8 and 23.2.8 [unord.req] p.10. These should probably be reconciled.
- First uses the convention "denotes"; second uses the convention "is".
- First redundantly says: "If no such element exists, returns a.end()." in erase table entry, second does not.
23.2.7 [associative.reqmts] Associative containers
8 In Table 102, X
denotes an associative container class, a
denotes a value of X
, a_uniq
denotes a value of X
when X
supports unique keys, a_eq
denotes a value of X
when
X
supports multiple keys, u
denotes an identifier, i
and j
satisfy input iterator
requirements and refer to elements implicitly convertible to value_type
, [i,j)
denotes a valid range,
p
denotes a valid const iterator to a
, q
denotes a valid dereferenceable const iterator to a
,
r
denotes a valid dereferenceable iterator to a, [q1, q2)
denotes a valid range of const iterators
in a
, il
designates an object of type initializer_list<value_type>
, t
denotes a value of
X::value_type
, k
denotes a value of X::key_type
and c
denotes a value of type
X::key_compare
. A
denotes the storage allocator used by X
, if any, or
std::allocator<X::value_type>
otherwise, and m
denotes an allocator of a type convertible to A
.
23.2.7 [associative.reqmts] Associative containers Table 102
Add row:
a.erase(r) |
iterator |
erases the element pointed to by r . Returns an iterator pointing to the element immediately following r
prior to the element being erased. If no such element exists, returns a.end() .
|
amortized constant |
23.2.8 [unord.req] Unordered associative containers
10 In table 103: X
is an unordered associative container class, a
is an object of type X
,
b
is a possibly const object of type X
, a_uniq
is an object of type X
when
X
supports unique keys, a_eq
is an object of type X
when X
supports equivalent keys,
i
and j
are input iterators that refer to value_type
, [i, j)
is a valid range,
p
and q2
are valid const iterators to a
, q
and q1
are valid dereferenceable
const iterators to a
, r
is a valid dereferenceable iterator to a, [q1,q2)
is a
valid range in a
, il
designates an object of type initializer_list<value_type>
,
t
is a value of type X::value_type
, k
is a value of type key_type
, hf
is a
possibly const value of type hasher
, eq
is a possibly const value of type key_equal
,
n
is a value of type size_type
, and z
is a value of type float
.
23.2.8 [unord.req] Unordered associative containers Table 103
Add row:
a.erase(r) |
iterator |
Erases the element pointed to by r . Returns the iterator immediately following r prior to the erasure.
|
Average case O(1), worst case O(a.size() ). |
23.4.3.1 [map.overview] Class template map overview p. 2
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.4.4.1 [multimap.overview] Class template multimap overview p. 2
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.4.6.1 [set.overview] Class template set overview p. 2
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.4.7.1 [multiset.overview] Class template multiset overview
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.5.3.1 [unord.map.overview] Class template unordered_map overview p. 3
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.5.4.1 [unord.multimap.overview] Class template unordered_multimap overview p. 3
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.5.6.1 [unord.set.overview] Class template unordered_set overview p. 3
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.5.7.1 [unord.multiset.overview] Class template unordered_multiset overview p. 3
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
C.6.12 [diff.cpp03.containers] C.2.12 Clause 23: containers library
23.2.3, 23.2.4
Change: Signature changes: from iterator to const_iterator parameters
Rationale: Overspecification. Effects: The signatures of the following member functions changed from taking an iterator to taking a const_iterator:
Valid C++ 2003 code that uses these functions may fail to compile with this International Standard.
make_move_iterator
and arraysSection: 24.2 [iterator.synopsis], 24.5.4 [move.iterators] Status: C++14 Submitter: Marc Glisse Opened: 2011-05-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.synopsis].
View all issues with C++14 status.
Discussion:
The standard library always passes template iterators by value and never by reference,
which has the nice effect that an array decays to a pointer. There is one exception:
make_move_iterator
.
#include <iterator> int main(){ int a[]={1,2,3,4}; std::make_move_iterator(a+4); std::make_move_iterator(a); // fails here }
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Modify the header <iterator>
synopsis in 24.2 [iterator.synopsis]:
namespace std { […] template <class Iterator> move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i); […] }
Modify the class template move_iterator
synopsis in 24.5.4.2 [move.iterator]:
namespace std { […] template <class Iterator> move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i); }
Modify 24.5.4.9 [move.iter.nonmember]:
template <class Iterator> move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i);-3- Returns:
move_iterator<Iterator>(i)
.
std::function
swapsSection: 22.10.17.3 [func.wrap.func], 22.10.17.3.3 [func.wrap.func.mod] Status: C++17 Submitter: Daniel Krügler Opened: 2011-05-28 Last modified: 2020-09-06
Priority: 2
View all other issues in [func.wrap.func].
View all issues with C++17 status.
Discussion:
Howard Hinnant observed in reflector message c++std-lib-30841 that 22.10.17.3 [func.wrap.func]
makes the member swap noexcept
, even though the non-member swap is not noexcept
.
noexcept
specifier at the member swap is incorrect and should be removed.
But if we allow for a potentially throwing member swap of std::function
, this causes
another conflict with the exception specification for the following member function:
template<class F> function& operator=(reference_wrapper<F> f) noexcept;
Effects:
function(f).swap(*this);
Note that in this example the sub-expression function(f)
does not cause any problems,
because of the nothrow-guarantee given in 22.10.17.3.2 [func.wrap.func.con] p. 10. The problem
is located in the usage of the swap which could potentially throw given the general latitude.
std::function
should be noexcept), or this function needs to be adapted as well,
e.g. by taking the exception-specification away or by changing the semantics.
One argument for "swap-may-throw" would be to allow for small-object optimization techniques
where the copy of the target may throw. But given the fact that the swap function has been guaranteed
to be "Throws: Nothing" from TR1 on, it seems to me that that there would still be opportunities to
perform small-object optimizations just restricted to the set of target copies that cannot throw.
In my opinion member swap of std::function
has always been intended to be no-throw, because
otherwise there would be no good technical reason to specify the effects of several member
functions in terms of the "construct-swap" idiom (There are three functions that are defined
this way), which provides the strong exception safety in this case. I suggest to enforce that both
member swap and non-member swap of std::function
are nothrow functions as it had been guaranteed
since TR1 on.
[ 2011 Bloomington ]
Dietmar: May not be swappable in the first place.
Alisdair: This is wide contact. Then we should be taking noexcept off instead of putting it on. This is preferred resolution.
Pablo: This is bigger issue. Specification of assignment in terms of swap is suspect to begin with. It is over specification. How this was applied to string is a better example to work from.
Pablo: Two problems: inconsistency that should be fixed (neither should have noexcept), the other issues is that assignment should not be specified in terms of swap. There are cases where assignment should succeed where swap would fail. This is easier with string as it should follow container rules.
Action Item (Alisdair): There are a few more issues found to file.
Dave: This is because of allocators? The allocator makes this not work.
Howard: There is a type erased allocator in shared_ptr. There is a noexcept allocator in shared_ptr.
Pablo: shared_ptr is a different case. There are shared semantics and the allocator does move around. A function does not have shared semantics.
Alisdair: Function objects think they have unique ownership.
Howard: In function we specify semantics with copy construction and swap.
Action Item (Pablo): Write this up better (why assignment should not be defined in terms of swap)
Howard: Not having trouble making function constructor no throw.
Dietmar: Function must allocate memory.
Howard: Does not put stuff that will throw on copy or swap in small object optimization. Put those on heap. Storing allocator, but has to be no throw copy constructable.
Pablo: Are you allowed to or required to swap or move allocators in case or swap or move.
Dave: An allocator that is type erased should be different...
Pablo: it is
Dave: Do you need to know something about allocator types? But only at construction time.
Pablo: You could have allocators that are different types.
Dave: Swap is two ended operation.
Pablo: Opinion is that both have to say propagate on swap for them to swap.
John: It is not arbitrary. If one person says no. No is no.
Howard: Find noexcept swap to be very useful. Would like to move in that direction and bring container design along.
Dave: If you have something were allocator must not propagate you can detect that at construction time.
...
Pablo: Need to leave this open and discuss in smaller group.
Alisdair: Tried to add boost::any as TR2 proposal and ran into this issue. Only the first place where we run into issues with type erased allocators. Suggest we move it to open.
Action Item: Move to open.
Action Item (Pablo works with Howard and Daniel): Address the more fundamental issue (which may be multiple issues) and write up findings.
Previous resolution [SUPERSEDED]:
This wording is relative to the FDIS.
Modify the header
<functional>
synopsis in 22.10 [function.objects] as indicated:namespace std { […] template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; […] }Modify the class template
function
synopsis in 22.10.17.3 [func.wrap.func] as indicated:namespace std { […] // [func.wrap.func.alg], specialized algorithms: template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; […] }Modify 22.10.17.3.8 [func.wrap.func.alg] as indicated:
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;-1- Effects:
f1.swap(f2);
[2014-02-28 (Post Issaquah), Pablo provides more information]
For cross-referencing purposes: The resolution of this issue should be
harmonized with any resolution to LWG 2370(i), which addresses
inappropriate noexcept
s in some function constructors.
We have the following choices:
swap()
does not throw
Discussion: This definition is desirable, and allows assignment to be implemented with the strong exception guarantee, but it does have consequences: The implementation cannot use the small-object optimization for a function-object
F
unlessF
isNothrowMovable
(nothrow-swappable is unimportant becauseF
is not swapped with anotherF
). Note that many functors written before C++11 will not have move constructors decorated withnoexcept
, so this limitation could affect a lot of code.It is not clear what other implementation restrictions might be needed. Allocators are required not to throw on move or copy. Is that sufficient?
swap()
can throw
Discussion: This definition gives maximum latitude to implementation to use small-object optimization. However, the strong guarantee on assignment is difficult to achieve. Should we consider giving up on the strong guarantee? How much are we willing to pessimize code for exceptions?
swap()
will not throw if both functions have NoThrowMoveable
functors
Discussion: This definition is similar to option 2, but gives slightly stronger guarantees. Here,
swap()
can throw, but the programmer can theoretically prevent that from happening. This should be straight-forward to implement and gives the implementation a lot of latitude for optimization. However, because this is a dynamic decision, the program is not as easy to reason about. Also, the strong guarantee for assignment is compromized as in option 2.
[2016-08-02, Ville, Billy, and Billy comment and reinstantiate the original P/R]
We (Ville, Billy, and Billy) propose to require that function
's swap
is noexcept
in all cases.
libstdc++ does not throw in their swap
. It is not noexcept
today, but the small functor
optimization only engages for trivially copyable types.
msvc++ checks is_nothrow_move_constructible
before engaging the small functor optimization and marks
its swap
noexcept
libc++ marks swap
noexcept
(though I have not looked at its implementation)
Moreover, many of the concerns that were raised by providing this guarantee are no longer applicable now that
P0302 has been accepted, which removes allocator support from std::function
.
[2016-08 Chicago]
Tues PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Modify the header <functional>
synopsis in 22.10 [function.objects] as indicated:
namespace std { […] template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; […] }
Modify the class template function
synopsis in 22.10.17.3 [func.wrap.func] as indicated:
namespace std { […] // [func.wrap.func.alg], specialized algorithms: template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; […] }
Modify 22.10.17.3.8 [func.wrap.func.alg] as indicated:
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;-1- Effects: As if by:
f1.swap(f2);
Section: 27.4.3 [basic.string] Status: C++17 Submitter: Howard Hinnant Opened: 2011-05-29 Last modified: 2017-07-30
Priority: 3
View other active issues in [basic.string].
View all other issues in [basic.string].
View all issues with C++17 status.
Discussion:
27.4.3.2 [string.require]/p4 says that basic_string
is an "allocator-aware"
container and behaves as described in 23.2.2 [container.requirements.general].
allocator_traits<allocator_type>::propagate_on_container_move_assignment::value
is false, and if the allocators stored in the lhs and rhs sides are not equal, then move
assigning a string has the same semantics as copy assigning a string as far as resources are
concerned (resources can not be transferred). And in this event, the lhs may have to acquire
resources to gain sufficient capacity to store a copy of the rhs.
However 27.4.3.3 [string.cons]/p22 says:
basic_string<charT,traits,Allocator>& operator=(basic_string<charT,traits,Allocator>&& str) noexcept;Effects: If
*this
andstr
are not the same object, modifies*this
as shown in Table 71. [Note: A valid implementation isswap(str)
. — end note ]
These two specifications for basic_string::operator=(basic_string&&)
are in conflict with
each other. It is not possible to implement a basic_string
which satisfies both requirements.
basic_string
is defined as:
basic_string& assign(basic_string&& str) noexcept;Effects: The function replaces the string controlled by
*this
with a string of lengthstr.size()
whose elements are a copy of the string controlled bystr
. [ Note: A valid implementation isswap(str)
. — end note ]
It seems contradictory that this member can be sensitive to propagate_on_container_swap
instead
of propagate_on_container_move_assignment
. Indeed, there is a very subtle chance for undefined
behavior here: If the implementation implements this in terms of swap
, and if
propagate_on_container_swap
is false, and if the two allocators are unequal, the behavior
is undefined, and will likely lead to memory corruption. That's a lot to go wrong under a member
named "assign".
[ 2011 Bloomington ]
Alisdair: Can this be conditional noexcept
?
Pablo: We said we were not going to put in many conditional noexcept
s. Problem is not allocator, but non-normative definition. It says swap is a valid operation which it is not.
Dave: Move assignment is not a critical method.
Alisdair: Was confusing assignment and construction.
Dave: Move construction is critical for efficiency.
Kyle: Is it possible to test for noexcept
.
Alisdair: Yes, query the noexcept
operator.
Alisdair: Agreed there is a problem that we cannot unconditionally mark these operations as noexcept
.
Pablo: How come swap is not defined in alloc
Alisdair: It is in utility.
Pablo: Swap has a conditional noexcept
. Is no throw move constructable, is no throw move assignable.
Pablo: Not critical for strings or containers.
Kyle: Why?
Pablo: They do not use the default swap.
Dave: Important for deduction in other types.
Alisdair: Would change the policy we adopted during FDIS mode.
Pablo: Keep it simple and get some vendor experience.
Alisdair: Is this wording correct? Concerned with bullet 2.
Pablo: Where does it reference containers section.
Alisdair: String is a container.
Alisdair: We should not remove redundancy piecemeal.
Pablo: I agree. This is a deviation from rest of string. Missing forward reference to containers section.
Pablo: To fix section 2. Only the note needs to be removed. The rest needs to be a forward reference to containers.
Alisdair: That is a new issue.
Pablo: Not really. Talking about adding one sentence, saying that basic string is a container.
Dave: That is not just a forward reference, it is a semantic change.
PJ: We intended to make it look like a container, but it did not satisfy all the requirements.
Pablo: Clause 1 is correct. Clause 2 is removing note and noexcept
(do not remove the rest). Clause 3 is correct.
Alisdair: Not sure data() is correct (in clause 2).
Conclusion: Move to open, Alisdair and Pablo volunteered to provide wording
[ originally proposed wording: ]
This wording is relative to the FDIS.
Modify the class template basic_string
synopsis in 27.4.3 [basic.string]:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: […] basic_string& operator=(basic_string&& str)noexcept; […] basic_string& assign(basic_string&& str)noexcept; […] }; }
Remove the definition of the basic_string
move assignment operator from 27.4.3.3 [string.cons]
entirely, including Table 71 — operator=(const basic_string<charT, traits, Allocator>&&)
.
This is consistent with how we define move assignment for the containers in Clause 23:
basic_string<charT,traits,Allocator>& operator=(basic_string<charT,traits,Allocator>&& str) noexcept;
-22- Effects: If*this
andstr
are not the same object, modifies*this
as shown in Table 71. [ Note: A valid implementation isswap(str)
. — end note ]-23- If*this
andstr
are the same object, the member has no effect.-24- Returns:*this
Table 71 —operator=(const basic_string<charT, traits, Allocator>&&)
ElementValuedata()
points at the array whose first element was pointed at bystr.data()
size()
previous value ofstr.size()
capacity()
a value at least as large assize()
Modify the paragraphs prior to 27.4.3.7.3 [string.assign] p.3 as indicated (The first insertion recommends a separate paragraph number for the indicated paragraph):
basic_string& assign(basic_string&& str)noexcept;-?- Effects: Equivalent to
-3- Returns:*this = std::move(str)
.The function replaces the string controlled by*this
with a string of lengthstr.size()
whose elements are a copy of the string controlled bystr
. [ Note: A valid implementation isswap(str)
. — end note ]*this
[ 2012-08-11 Joe Gottman observes: ]
One of the effects of
basic_string
's move-assignment operator (27.4.3.3 [string.cons], Table 71) is
Element Value data()
points at the array whose first element was pointed at by str.data()
If a string implementation uses the small-string optimization and the input string
str
is small enough to make use of it, this effect is impossible to achieve. To use the small string optimization, a string has to be implemented using something likeunion { char buffer[SMALL_STRING_SIZE]; char *pdata; };When the string is small enough to fit inside
Resolution proposal: Change Table 71 to read:buffer
, thedata()
member function returnsstatic_cast<const char *>(buffer)
, and sincebuffer
is an array variable, there is no way to implement move so that the moved-to string'sbuffer
member variable is equal tothis->buffer
.
Element Value data()
points at the array whose first element was pointed at bythat contains the same characters in the same order asstr.data()
str.data()
contained beforeoperator=()
was called
[2015-05-07, Lenexa]
Howard suggests improved wording
Move to ImmediateProposed resolution:
This wording is relative to N4431.
Modify the class template basic_string
synopsis in 27.4.3 [basic.string]:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: […] basic_string& assign(basic_string&& str) noexcept( allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value); […] }; }
Change 27.4.3.3 [string.cons]/p21-23:
basic_string& operator=(basic_string&& str) noexcept( allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value);-21- Effects:
IfMove assigns as a sequence container ([container.requirements]), except that iterators, pointers and references may be invalidated.*this
andstr
are not the same object, modifies*this
as shown in Table 71. [ Note: A valid implementation isswap(str)
. — end note ]-22- If-23- Returns:*this
andstr
are the same object, the member has no effect.*this
Table 71 —operator=(basic_string&&)
effectsElementValuedata()
points at the array whose first element was pointed at bystr.data()
size()
previous value ofstr.size()
capacity()
a value at least as large assize()
Modify the paragraphs prior to 27.4.3.7.3 [string.assign] p.3 as indicated
basic_string& assign(basic_string&& str) noexcept( allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value);-3- Effects: Equivalent to
-4- Returns:*this = std::move(str)
.The function replaces the string controlled by*this
with a string of lengthstr.size()
whose elements are a copy of the string controlled bystr
. [ Note: A valid implementation isswap(str)
. — end note ]*this
noexcept
issues in basic_string
Section: 27.4.3 [basic.string] Status: C++14 Submitter: Howard Hinnant Opened: 2011-05-29 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
The following inconsistencies regarding noexcept
for basic_string
are noted.
noexcept
:
void swap(basic_string& str);
But the global swap is marked noexcept
:
template<class charT, class traits, class Allocator>
void swap(basic_string<charT,traits,Allocator>& lhs,
basic_string<charT,traits,Allocator>& rhs) noexcept;
But only in the definition, not in the synopsis.
All comparison operators are markednoexcept
in their definitions, but not in the synopsis.
The compare function that takes a pointer:
int compare(const charT *s) const;
is not marked noexcept
. But some of the comparison functions which are marked noexcept
(only in their definition) are specified to call the throwing compare operator:
template<class charT, class traits, class Allocator> bool operator==(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs) noexcept;
Returns:
lhs.compare(rhs) == 0
.
All functions with a narrow contract should not be declared as noexcept
according to
the guidelines presented in n3279.
Among these narrow contract functions are the swap
functions (23.2.2 [container.requirements.general] p. 8)
and functions with non-NULL
const charT*
parameters.
[2011-06-08 Daniel provides wording]
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS. Both move-assignment operator and the moving assign
function are not touched by this issue, because they are handled separately by issue 2063(i).
Modify the header <string>
synopsis in 27.4 [string.classes] as
indicated (Rationale: Adding noexcept
to these specific overloads is in sync with
applying the same rule to specific overloads of the member functions find
, compare
, etc.
This approach deviates from that taken in n3279,
but seems more consistent given similar application for comparable member functions):
#include <initializer_list> namespace std { […] template<class charT, class traits, class Allocator> bool operator==(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator!=(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator<(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator>(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator<=(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator>=(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] }
Modify the class template basic_string
synopsis in 27.4.3 [basic.string] as
indicated (Remark 1: The noexcept
at the move-constructor is fine, because even for a
small-object optimization there is no problem here, because basic_string::value_type
is required to be a non-array POD as of 27.1 [strings.general] p1, Remark 2: This
proposal removes the noexcept
at single character overloads of find
, rfind
,
etc. because they are defined in terms of potentially allocating functions. It seems like
an additional issue to me to change the semantics in terms of non-allocating functions and
adding noexcept
instead):
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: […] // [string.ops], string operations: […] size_type find (charT c, size_type pos = 0) constnoexcept; […] size_type rfind(charT c, size_type pos = npos) constnoexcept; […] size_type find_first_of(charT c, size_type pos = 0) constnoexcept; […] size_type find_last_of (charT c, size_type pos = npos) constnoexcept; […] size_type find_first_not_of(charT c, size_type pos = 0) constnoexcept; […] size_type find_last_not_of (charT c, size_type pos = npos) constnoexcept; […] }; }
Modify 27.4.3.8.2 [string.find] before p5 and before p7 as indicated:
size_type find(const charT* s, size_type pos = 0) constnoexcept; […] size_type find(charT c, size_type pos = 0) constnoexcept;-7- Returns:
find(basic_string<charT,traits,Allocator>(1,c), pos)
.
Modify [string.rfind] before p7 as indicated:
size_type rfind(charT c, size_type pos = npos) constnoexcept;-7- Returns:
rfind(basic_string<charT,traits,Allocator>(1,c),pos)
.
Modify [string.find.first.of] before p7 as indicated:
size_type find_first_of(charT c, size_type pos = 0) constnoexcept;-7- Returns:
find_first_of(basic_string<charT,traits,Allocator>(1,c), pos)
.
Modify [string.find.last.of] before p7 as indicated:
size_type find_last_of(charT c, size_type pos = npos) constnoexcept;-7- Returns:
find_last_of(basic_string<charT,traits,Allocator>(1,c),pos)
.
Modify [string.find.first.not.of] before p7 as indicated:
size_type find_first_not_of(charT c, size_type pos = 0) constnoexcept;-7- Returns:
find_first_not_of(basic_string(1, c), pos)
.
Modify [string.find.last.not.of] before p7 as indicated:
size_type find_last_not_of(charT c, size_type pos = npos) constnoexcept;-7- Returns:
find_last_not_of(basic_string(1, c), pos)
.
Modify [string.operator==] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator==(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator==(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op!=] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator!=(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator!=(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op<] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator<(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator<(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op>] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator>(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator>(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op<=] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator<=(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator<=(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op>=] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator>=(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator>=(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify 27.4.4.3 [string.special] as indicated (Remark: The change of the semantics guarantees as of 16.3.2.4 [structure.specifications] p4 that the "Throws: Nothing" element of member swap is implied):
template<class charT, class traits, class Allocator> void swap(basic_string<charT,traits,Allocator>& lhs, basic_string<charT,traits,Allocator>& rhs)noexcept;-1- Effects: Equivalent to
lhs.swap(rhs);
Section: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-06-06 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
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Discussion:
The example in 16.4.4.6 [allocator.requirements] says SimpleAllocator
satisfies
the requirements of Table 28 — Allocator requirements, but it doesn't support comparison
for equality/inequality.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS.
Modify the example in 16.4.4.6 [allocator.requirements] p5 as indicated:
-5- […]
[ Example: the following is an allocator class template supporting the minimal interface that satisfies the requirements of Table 28:template <class Tp> struct SimpleAllocator { typedef Tp value_type; SimpleAllocator(ctor args); template <class T> SimpleAllocator(const SimpleAllocator<T>& other); Tp *allocate(std::size_t n); void deallocate(Tp *p, std::size_t n); }; template <class T, class U> bool operator==(const SimpleAllocator<T>&, const SimpleAllocator<U>&); template <class T, class U> bool operator!=(const SimpleAllocator<T>&, const SimpleAllocator<U>&);— end example ]
vector::resize(size_type)
Section: 23.3.13.3 [vector.capacity] Status: Resolved Submitter: Rani Sharoni Opened: 2011-03-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In C++1x (N3090) there are two version of vector::resize
— 23.3.13.3 [vector.capacity]:
void resize(size_type sz); void resize(size_type sz, const T& c);
The text in 23.3.13.3 [vector.capacity]/12 only mentions "no effects on throw" for the two args version of resize:
Requires: If an exception is thrown other than by the move constructor of a non-
CopyConstructible
T
there are no effects.
This seems like unintentional oversight since resize(size)
is
semantically the same as resize(size, T())
.
Additionally, the C++03 standard only specify single version of resize
with default for the second argument - 23.2.4:
void resize(size_type sz, T c = T());
Therefore not requiring same guarantees for both version of resize is in fact a regression.
[2011-06-12: Daniel comments]
The proposed resolution for issue 2033(i) should solve this issue as well.
[ 2011 Bloomington ]
This issue will be resolved by issue 2033(i), and closed when this issue is applied.
[2012, Kona]
Resolved by adopting the resolution in issue 2033(i) at this meeting.
Proposed resolution:
Apply the proposed resolution of issue 2033(i)
packaged_task
should have deleted copy c'tor with const parameterSection: 32.10.10 [futures.task] Status: C++14 Submitter: Daniel Krügler Opened: 2011-06-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task].
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Discussion:
Class template packaged_task
is a move-only type with the following form of the
deleted copy operations:
packaged_task(packaged_task&) = delete; packaged_task& operator=(packaged_task&) = delete;
Note that the argument types are non-const. This does not look like a typo to me, this form seems to exist from the very first proposing paper on N2276. Using either of form of the copy-constructor did not make much difference before the introduction of defaulted special member functions, but it makes now an observable difference. This was brought to my attention by a question on a German C++ newsgroup where the question was raised why the following code does not compile on a recent gcc:
#include <utility>
#include <future>
#include <iostream>
#include <thread>
int main(){
std::packaged_task<void()> someTask([]{ std::cout << std::this_thread::get_id() << std::endl; });
std::thread someThread(std::move(someTask)); // Error here
// Remainder omitted
}
It turned out that the error was produced by the instantiation of some return type
of std::bind
which used a defaulted copy-constructor, which leads to a
const declaration conflict with [class.copy] p8.
packaged_task
to prevent such problems.
A similar problem exists for class template basic_ostream
in 31.7.6.2 [ostream]:
namespace std { template <class charT, class traits = char_traits<charT> > class basic_ostream : virtual public basic_ios<charT,traits> { […] // 27.7.3.3 Assign/swap basic_ostream& operator=(basic_ostream& rhs) = delete; basic_ostream& operator=(const basic_ostream&& rhs); void swap(basic_ostream& rhs); };
albeit this could be considered as an editorial swap of copy and move assignment operator, I suggest to fix this as part of this issue as well.
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Modify the class template basic_ostream
synopsis in 31.7.6.2 [ostream]
as indicated (Note: The prototype signature of the move assignment operator in 31.7.6.2.3 [ostream.assign]
is fine):
namespace std { template <class charT, class traits = char_traits<charT> > class basic_ostream : virtual public basic_ios<charT,traits> { […] // 27.7.3.3 Assign/swap basic_ostream& operator=(const basic_ostream& rhs) = delete; basic_ostream& operator=(constbasic_ostream&& rhs); void swap(basic_ostream& rhs); };
Modify the class template packaged_task
synopsis in 32.10.10 [futures.task] p2
as indicated:
namespace std { template<class> class packaged_task; // undefined template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public: […] // no copy packaged_task(const packaged_task&) = delete; packaged_task& operator=(const packaged_task&) = delete; […] }; […] }
basic_string
move constructorSection: 27.4.3.3 [string.cons] Status: C++14 Submitter: Bo Persson Opened: 2011-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.cons].
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Discussion:
Sub-clause 27.4.3.3 [string.cons] contains these constructors in paragraphs 2 and 3:
basic_string(const basic_string<charT,traits,Allocator>& str); basic_string(basic_string<charT,traits,Allocator>&& str) noexcept;[…]
-3- Throws: The second form throws nothing if the allocator's move constructor throws nothing.
How can it ever throw anything if it is marked noexcept
?
[2011-07-11: Daniel comments and suggests wording changes]
Further, according to paragraph 18 of the same sub-clause:
basic_string(const basic_string& str, const Allocator& alloc); basic_string(basic_string&& str, const Allocator& alloc);[…]
-18- Throws: The second form throws nothing ifalloc == str.get_allocator()
unless the copy constructor forAllocator
throws.
The constraint "unless the copy constructor for Allocator
throws"
is redundant, because according to Table 28 — Allocator requirements, the expressions
X a1(a); X a(b);
impose the requirement: "Shall not exit via an exception".
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Change 27.4.3.3 [string.cons] p3 as indicated (This move constructor has a wide
contract and is therefore safely marked as noexcept
):
basic_string(const basic_string<charT,traits,Allocator>& str); basic_string(basic_string<charT,traits,Allocator>&& str) noexcept;-2- Effects: Constructs an object of class
basic_string
as indicated in Table 64. In the second form,str
is left in a valid state with an unspecified value.-3- Throws: The second form throws nothing if the allocator's move constructor throws nothing.
Change 27.4.3.3 [string.cons] p18 as indicated (This move-like constructor may throw, if the allocators don't compare equal, but not because of a potentially throwing allocator copy constructor, only because the allocation attempt may fail and throw an exception):
basic_string(const basic_string& str, const Allocator& alloc); basic_string(basic_string&& str, const Allocator& alloc);[…]
-18- Throws: The second form throws nothing ifalloc == str.get_allocator()
unless the copy constructor for.Allocator
throws
allocate_shared
should use allocator_traits<A>::construct
Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: Resolved Submitter: Jonathan Wakely Opened: 2011-07-11 Last modified: 2017-07-16
Priority: 2
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Discussion:
20.3.2.2.7 [util.smartptr.shared.create] says:
-2- Effects: Allocates memory suitable for an object of type
T
and constructs an object in that memory via the placement new expression::new (pv) T(std::forward<Args>(args)...)
. The templateallocate_shared
uses a copy of a to allocate memory. If an exception is thrown, the functions have no effect.
This explicitly requires placement new rather than using
allocator_traits<A>::construct(a, (T*)pv, std::forward<Args>(args)...)
In most cases that would result in the same placement new expression,
but would allow more control over how the object is constructed e.g.
using scoped_allocator_adaptor
to do uses-allocator construction, or
using an allocator declared as a friend to construct objects with no
public constructors.
[2011-08-16 Bloomington:]
Agreed to fix in principle, but believe that make_shared
and
allocate_shared
have now diverged enough that their descriptions
should be separated. Pablo and Stefanus to provide revised wording.
Daniel's (old) proposed resolution:
This wording is relative to the FDIS.
Change the following paragraphs of 20.3.2.2.7 [util.smartptr.shared.create] as indicated (The suggested removal of the last sentence of p1 is not strictly required to resolve this issue, but is still recommended, because it does not say anything new but may give the impression that it says something new):
template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args); template<class T, class A, class... Args> shared_ptr<T> allocate_shared(const A& a, Args&&... args);-1- Requires: For the template
-2- Effects: Allocates memory suitable for an object of typemake_shared
, tThe expression::new (pv) T(std::forward<Args>(args)...)
, wherepv
has typevoid*
and points to storage suitable to hold an object of typeT
, shall be well formed. For the templateallocate_shared
, the expressionallocator_traits<A>::construct(a, pt, std::forward<Args>(args)...)
, wherept
has typeT*
and points to storage suitable to hold an object of typeT
, shall be well formed.A
shall be an allocator ([allocator.requirements]).The copy constructor and destructor ofA
shall not throw exceptions.T
and constructs an object in that memory. The templatemake_shared
constructs the object via the placement new expression::new (pv) T(std::forward<Args>(args)...)
. The templateallocate_shared
uses a copy ofa
to allocate memory and constructs the object by callingallocator_traits<A>::construct(a, pt, std::forward<Args>(args)...)
. If an exception is thrown, the functions have no effect. -3- Returns: Ashared_ptr
instance that stores and owns the address of the newly constructed object of typeT
. -4- Postconditions:get() != 0 && use_count() == 1
-5- Throws:bad_alloc
, or, for the templatemake_shared
, an exception thrown from the constructor ofT
, or, for the templateallocate_shared
, an exception thrown fromA::allocate
or fromallocator_traits<A>::construct
from the constructor of. -6- Remarks: Implementations are encouraged, but not required, to perform no more than one memory allocation. [ Note: This provides efficiency equivalent to an intrusive smart pointer. — end note ] -7- [ Note: These functions will typically allocate more memory thanT
sizeof(T)
to allow for internal bookkeeping structures such as the reference counts. — end note ]
[2011-12-04: Jonathan and Daniel improve wording]
See also c++std-lib-31796
[2013-10-13, Ville]
This issue is related to 2089(i).
[2014-02-15 post-Issaquah session : move to Tentatively NAD]
STL: This takes an allocator, but then ignores its construct. That's squirrely.
Alisdair: The convention is when you take an allocator, you use its construct.
STL: 23.2.2 [container.requirements.general]/3, argh! This fills me with despair, but I understand it now.
STL: Ok, this is some cleanup.
STL: You're requiring b
to be of type A
and not being rebound, is that an overspecification?
Pablo: Good point. Hmm, that's only a requirement on what must be well-formed.
STL: If it's just a well-formed requirement, then why not just use a directly?
Pablo: Yeah, the well-formed requirement is overly complex. It's not a real call, we could just use a directly. It makes it harder to read.
Alisdair: b
should be an allocator in the same family as a
.
Pablo: This is a well-formed requirement, I wonder if it's the capital A that's the problem here. It doesn't matter here, this is way too much wording.
Alisdair: It's trying to tie the constructor arguments into the allocator requirements.
Pablo: b
could be struck, that's a runtime quality. The construct will work with anything that's in the family of A
.
Alisdair: The important part is the forward
of Args
.
Pablo: A
must be an allocator, and forward
Args
must work with that.
Alisdair: First let's nail down A
.
Pablo: Then replace b
with a
, and strike the rest.
STL: You need pt
's type, at least.
Pablo: There's nothing to be said about runtime constraints here, this function doesn't even take a pt
.
STL: Looking at the Effects, I believe b
is similarly messed up, we can use a2
to construct an object.
Alisdair: Or any allocator in the family of a
.
STL: We say this stuff for the deallocate too, it should be lifted up.
STL: "owns the address" is weird.
Alisdair: shared_ptr owns pointers, although it does sound funky.
Walter: "to destruct" is ungrammatical.
STL: "When ownership is given up" is not what we usually say.
Alisdair: I think the Returns clause is the right place to say this.
STL: The right place to say this is shared_ptr
's dtor, we don't want to use Core's "come from" convention.
Alisdair: I'm on the hook to draft cleaner wording.
[2015-10, Kona Saturday afternoon]
AM: I was going to clean up the wording, but haven't done it yet.
Defer until we have new wording.
[2016-03, Jacksonville]
Alisdair: we need to figure out whether we should call construct or not; major implementation divergence
STL: this does not grant friendship, does it?
Jonathan: some people want it.
Thomas: scoped allocator adapter should be supported, so placement new doesn't work
Alisdair: this makes the make_ functions impossible
Thomas: you don't want to use those though.
Alisdair: but people use that today, at Bloomberg
Alisdair: and what do we do about fancy pointers?
Jonathan: we constrain it to only non-fancy pointers.
STL: shared_ptr has never attempted to support fancy pointers; seems like a paper is needed.
Poll: call construct:6 operator new: 0 don't care: 4
Poll: should we support fancy pointers? Yes: 1 No: 4 don't care: 4
STL: 20.8.2.2.6p2: 'and pv->~T()' is bogus for void
STL: 20.8.2.2.6p4: is this true even if we're going to allocate a bit more?
Alisdair: yes
Alisdair: coming up with new wording
[2016-08, Chicago Monday PM]
Alisdair to provide new wording this week
[2017-07 Toronto]
Resolved by the adoption of P0674R1 in Toronto
Proposed resolution:
This wording is relative to the FDIS.
Change the following paragraphs of 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);template<class T, class A, class... Args> shared_ptr<T> allocate_shared(const A& a, Args&&... args);
-1- Requires: The expression
::new (pv) T(std::forward<Args>(args)...)
, where pv
has type void*
and points to storage suitable to hold an object of type T
, shall be well
formed. A
shall be an allocator (16.4.4.6 [allocator.requirements]). The copy constructor
and destructor of A
shall not throw exceptions.
return allocate_shared<T>(allocator<T>(), std::forward<Args>(args)...);
Allocates memory suitable for an object of type
T
and constructs an object in that memory via the placement new expression
::new (pv) T(std::forward<Args>(args)...)
. The template allocate_shared
uses a copy
of a
to allocate memory. If an exception is thrown, the functions have no effect.
std::allocator
may not be instantiated, the expressions ::new (pv) T(std::forward<Args>(args)...)
and
pv->~T()
may be evaluated directly — end note].
shared_ptr
instance that stores and owns the address of the newly constructed
object of type T
.get() != 0 && use_count() == 1
bad_alloc
, or an exception thrown from A::allocate
or from the
constructor of T
.sizeof(T)
to allow
for internal bookkeeping structures such as the reference counts. — end note]Add the following set of new paragraphs immediately following the previous paragraph 7 of 20.3.2.2.7 [util.smartptr.shared.create]:
template<class T, class A, class... Args> shared_ptr<T> allocate_shared(const A& a, Args&&... args);
-?- Requires: The expressions
allocator_traits<A>::construct(b, pt, std::forward<Args>(args)...)
and
allocator_traits<A>::destroy(b, pt)
shall be well-formed and well-defined,
where b
has type A
and is a copy of a
and where pt
has type T*
and points to storage suitable to hold an object of type T
.
A
shall meet the allocator requirements (16.4.4.6 [allocator.requirements]).
a2
of type allocator_traits<A>::rebind_alloc<unspecified>
that compares equal to
a
to allocate memory suitable for an object of type T
.
Uses a copy b
of type A
from a
to construct an object of type T
in
that memory by calling allocator_traits<A>::construct(b, pt, std::forward<Args>(args)...)
.
If an exception is thrown, the function has no effect.
-?- Returns: A shared_ptr
instance that stores and owns the address of the newly constructed
object of type T
. When ownership is given up, the effects are as follows: Uses a copy b2
of type A
from a
to destruct an object of type T
by calling
allocator_traits<A>::destroy(b2, pt2)
where pt2
has type T*
and refers to the newly constructed object. Then uses an object of type
allocator_traits<A>::rebind_alloc<unspecified>
that compares equal to
a
to deallocate the allocated memory.
-?- Postconditions: get() != 0 && use_count() == 1
-?- Throws: Nothing unless memory allocation or allocator_traits<A>::construct
throws an exception.
-?- Remarks: Implementations are encouraged, but not required, to perform no more than one memory
allocation. [Note: Such an implementation provides efficiency equivalent to an intrusive smart
pointer. — end note]
-?- [Note: This function will typically allocate more memory than sizeof(T)
to allow for internal
bookkeeping structures such as the reference counts. — end note]
std::valarray
move-assignmentSection: 29.6.2.3 [valarray.assign] Status: C++14 Submitter: Paolo Carlini Opened: 2011-05-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [valarray.assign].
View all issues with C++14 status.
Discussion:
Yesterday I noticed that the language we have in the FDIS about std::valarray
move assignment
is inconsistent with the resolution of LWG 675. Indeed, we guarantee constant complexity (vs linear
complexity). We also want it to be noexcept, that is more subtle, but again it's at variance with all
the containers.
std::valarray
? Shall we maybe just strike or fix the as-if, consider it
some sort of pasto from the copy-assignment text, thus keep the noexcept and constant complexity requirements
(essentially the whole operation would boild down to a swap of POD data members). Or LWG 675(i) should be
explicitly extended to std::valarray
too? In that case both noexcept and constant complexity
would go, I think, and the operation would boil down to the moral equivalent of clear()
(which
doesn't really exist in this case) + swap
?
Howard: I agree the current wording is incorrect. The complexity should be linear in size()
(not
v.size()
) because the first thing this operator needs to do is resize(0)
(or clear()
as you put it).
noexcept
.
As for proper wording, here's a first suggestion:
Effects:
Complexity: linear.*this
obtains the value ofv
. The value ofv
after the assignment is not specified.
See also reflector discussion starting with c++std-lib-30690.
[2012, Kona]
Move to Ready.
Some discussion on the types supported by valarray
concludes that the wording is
trying to say something similar to the core wording for trivial types, but significantly
predates it, and does allow for types with non-trivial destructors. Howard notes that
the only reason for linear complexity, rather than constant, is to support types with
non-trivial destructors.
AJM suggests replacing the word 'value' with 'state', but straw poll prefers moving forward with the current wording, 5 to 2.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
In 29.6.2.3 [valarray.assign] update as follows:
valarray<T>& operator=(valarray<T>&& v) noexcept;3 Effects:
4 Complexity:*this
obtains the value ofv
.If the length ofThe value ofv
is not equal to the length of*this
, resizes*this
to make the two arrays the same length, as if by callingresize(v.size())
, before performing the assignment.v
after the assignment is not specified.ConstantLinear.
Section: 99 [depr.temporary.buffer] Status: C++17 Submitter: Kazutoshi Satoda Opened: 2011-08-10 Last modified: 2025-03-13
Priority: 3
View all other issues in [depr.temporary.buffer].
View all issues with C++17 status.
Discussion:
According to [temporary.buffer] p1+2:
template <class T> pair<T*, ptrdiff_t> get_temporary_buffer(ptrdiff_t n) noexcept;-1- Effects: Obtains a pointer to storage sufficient to store up to
-2- Returns: A pair containing the buffer's address and capacity (in the units ofn
adjacentT
objects. It is implementation-defined whether over-aligned types are supported (3.11).sizeof(T)
), or a pair of 0 values if no storage can be obtained or ifn <= 0
.
I read this as prohibiting to return a buffer of which capacity is less than n
, because
such a buffer is not sufficient to store n
objects.
(for the return value, a pair
P
) [...] the buffer pointed to byP.first
is large enough to holdP.second
objects of typeT
.P.second
is greater than or equal to 0, and less than or equal tolen
.
There seems to be two different targets of the "up to n" modification: The capacity of obtained buffer, and the actual number that the caller will store into the buffer.
First I read as the latter, and got surprised seeing that libstdc++ implementation can return a smaller buffer. I started searching aboutget_temporary_buffer()
. After reading a quote from TC++PL at
stackoverflow,
I realized that the former is intended.
Such misinterpretation seems common:
JIS standard (Japanese translation of ISO/IEC standard) says nothing
like "up to". I think the editor misinterpreted the original wording,
and omitted words for "up to" as it is redundant. (If a buffer is
sufficient to store n
objects, it is also sufficient to store
up to n
objects.)
Rogue Wave implementation doesn't return smaller buffer, instead, it can return larger buffer on some circumstances. Apache STDCXX is a derived version of that implementation, and publicly accessible:
Specializations of the
get_temporary_buffer()
function template attempt to allocate a region of storage sufficiently large to store at leastn
adjacent objects of typeT
.
I know one commercial compiler package based on Rogue Wave implementation, and its implementation is essentially same as the above.
[2014-05-18, Daniel comments and suggests concrete wording]
The provided wording attempts to clarify the discussed capacity freedom, but it also makes it clearer that the returned
memory is just "raw memory", which is currently not really clear. In addition the wording clarifies that the deallocating
return_temporary_buffer
function does not throw exceptions, which I believe is the intention when the preconditions
of the functions are satisfied. Then, my understanding is that we can provide to return_temporary_buffer
a
null pointer value if that was the value, get_temporary_buffer()
had returned. Furthermore, as STL noticed, the current
wording seemingly allows multiple invocations of return_temporary_buffer
with the same value returned by
get_temporary_buffer
; this should be constrained similar to the wording we have for operator delete
(unfortunately
we miss such wording for allocators).
[2015-05, Lenexa]
MC: move to ready? in favor: 14, opposed: 0, abstain: 0
Proposed resolution:
This wording is relative to N3936.
Change [temporary.buffer] as indicated:
template <class T> pair<T*, ptrdiff_t> get_temporary_buffer(ptrdiff_t n) noexcept;-1- Effects: Obtains a pointer to uninitialized, contiguous storage for
-?- Remarks: CallingN
adjacent objects of typeT
, for some non-negative numberN
.Obtains a pointer to storage sufficient to store up toIt is implementation-defined whether over-aligned types are supported (3.11).n
adjacentT
objects.get_temporary_buffer
with a positive numbern
is a non-binding request to return storage forn
objects of typeT
. In this case, an implementation is permitted to return instead storage for a non-negative numberN
of such objects, whereN != n
(includingN == 0
). [Note: The request is non-binding to allow latitude for implementation-specific optimizations of its memory management. — end note]. -2- Returns: Ifn <= 0
or if no storage could be obtained, returns a pairP
such thatP.first
is a null pointer value andP.second == 0
; otherwise returns a pairP
such thatP.first
refers to the address of the uninitialized storage andP.second
refers to its capacityN
(in the units ofsizeof(T)
).Apair
containing the buffer's address and capacity (in the units ofsizeof(T)
), or a pair of 0 values if no storage can be obtained or ifn <= 0
.template <class T> void return_temporary_buffer(T* p);-3- Effects: Deallocates the
-4- Requires:buffer to whichstorage referenced byp
pointsp
.The buffer shall have been previously allocated byp
shall be a pointer value returned by an earlier call toget_temporary_buffer
which has not been invalidated by an intervening call toreturn_temporary_buffer(T*)
. -?- Throws: Nothing.
std::reverse_copy
Section: 26.7.10 [alg.reverse] Status: C++14 Submitter: Peter Miller Opened: 2011-08-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.reverse].
View all issues with C++14 status.
Discussion:
The output of the program below should be:
"three two one null \n"
But when std::reverse_copy
is implemented as described in N3291 26.7.10 [alg.reverse]
it's:
"null three two one \n"
because there's an off by one error in 26.7.10 [alg.reverse]/4; the definition should read:
*(result + (last - first) - 1 - i) = *(first + i)
Test program:
#include <algorithm> #include <iostream> template <typename BiIterator, typename OutIterator> auto reverse_copy_as_described_in_N3291( BiIterator first, BiIterator last, OutIterator result ) -> OutIterator { // 25.3.10/4 [alg.reverse]: // "...such that for any non-negative integer i < (last - first)..." for ( unsigned i = 0; i < ( last - first ); ++i ) // "...the following assignment takes place:" *(result + (last - first) - i) = *(first + i); // 25.3.10/6 return result + (last - first); } int main() { using std::begin; using std::end; using std::cout; static const char*const in[3] { "one", "two", "three" }; const char* out[4] { "null", "null", "null", "null" }; reverse_copy_as_described_in_N3291( begin( in ), end( in ), out ); for ( auto s : out ) cout << s << ' '; cout << std::endl; return 0; }
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 26.7.10 [alg.reverse] p4 as follows:
template<class BidirectionalIterator, class OutputIterator> OutputIterator reverse_copy(BidirectionalIterator first, BidirectionalIterator last, OutputIterator result);-4- Effects: Copies the range [
-5- Requires: The ranges [first,last
) to the range [result,result+(last-first)
) such that for any non-negative integeri < (last - first)
the following assignment takes place:*(result + (last - first) - 1 - i) = *(first + i)
.first,last
) and [result,result+(last-first)
) shall not overlap. -6- Returns:result + (last - first)
. -7- Complexity: Exactlylast - first
assignments.
Section: 6.9.2 [intro.multithread], 32.5.5 [atomics.lockfree], 99 [atomics.types.operations.req] Status: Resolved Submitter: Torvald Riegel Opened: 2011-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [intro.multithread].
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Discussion:
According to 6.9.2 [intro.multithread] p2:
"Implementations should ensure that all unblocked threads eventually make progress."
Which assumptions can an implementation make about the thread scheduling? This is relevant for how implementations implement compare-exchange with load-linked / store conditional (LL-SC), and atomic read-modifiy-write operations with load...compare-exchange-weak loops.
32.5.5 [atomics.lockfree] p2 declares the lock-free property for a particular object. However, "lock-free" is never defined, and in discussions that I had with committee members it seemed as if the standard's lock-free would be different from what lock-free means in other communities (eg, research, text books on concurrent programming, etc.).
Following 99 [atomics.types.operations.req] p7 is_lock_free()
returns "true if the object is lock-free". What is returned if the object is only
sometimes lock-free?
Basically, I would like to see clarifications for the progress guarantees so that users know what they can expect from implementations (and what they cannot expect!), and to give implementors a clearer understanding of which user expectations they have to implement.
Elaborate on the intentions of the progress guarantee in 6.9.2 [intro.multithread] p2. As I don't know about your intentions, it's hard to suggest a resolution.
Define the lock-free property. The definition should probably include the following points:
[2011-12-01: Hans comments]
6.9.2 [intro.multithread] p2 was an intentional compromise, and it was understood at the time that it was not a precise statement. The wording was introduced by N3209, which discusses some of the issues. There were additional reflector discussions.
This is somewhat separable from the question of what lock-free means, which is probably a more promising question to focus on.[2012, Kona]
General direction: lock-free means obstruction-free. Leave the current "should" recommendation for progress. It would take a lot of effort to try to do better.
[2012, Portland: move to Open]
The current wording of 6.9.2 [intro.multithread] p2 doesn't really say very much. As far as we can tell the term lock-free is nowhere defined in the standard.
James: we would prefer a different way to phrase it.
Hans: the research literature includes the term abstraction-free which might be a better fit.
Detlef: does Posix define a meaning for blocking (or locking) that we could use?
Hans: things like compare-exchange-strong can wait indefinitely.
Niklas: what about spin-locks -- still making no progress.
Hans: suspect we can only give guidance, at best. The lock-free meaning from the theoretical commmunity (forard progress will be made) is probably too strong here.
Atrur: what about livelocks?
Hans: each atomic modification completes, even if the whole thing is blocked.
Moved to open.
[2013-11-06: Jason Hearne-McGuiness comments]
Related to this issue, the wording in 32.5.5 [atomics.lockfree] p2,
In any given program execution, the result of the lock-free query shall be consistent for all pointers of the same type.
should be made clearer, because the object-specific (non-static) nature of the is_lock_free()
functions
from 32.5.8 [atomics.types.generic] and 32.5.8.2 [atomics.types.operations] imply that for different
instances of pointers of the same type the returned value could be different.
Proposed resolution:
[2014-02-16 Issaquah: Resolved by paper n3927]
CopyConstructible
requirement in set constructorsSection: 23.4.6.2 [set.cons] Status: C++17 Submitter: Jens Maurer Opened: 2011-08-20 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
23.4.6.2 [set.cons] paragraph 4 says:
Requires: If the iterator's dereference operator returns an lvalue or a non-const rvalue, then
Key
shall beCopyConstructible
.
I'm confused why a "non-const rvalue" for the return value of the iterator
would require CopyConstructible
; isn't that exactly the situation
when you'd want to apply the move constructor?
multimap
seems better in that regard
([multimap.cons] paragraph 3):
Requires: If the iterator's dereference operator returns an lvalue or a const rvalue
pair<key_type, mapped_type>
, then bothkey_type
and mapped_type shall beCopyConstructible
.
Obviously, if I have a const rvalue, I can't apply the move constructor (which will likely attempt modify its argument).
Dave Abrahams: I think you are right. Proposed resolution: drop "non-" from 23.4.6.2 [set.cons] paragraph 3.[2012, Kona]
The wording is in this area will be affected by Pablo's paper being adopted at this meeting. Wait for that paper to be applied before visiting this issue - deliberately leave in New status until the next meeting.
Proposed resolution from Kona 2012:
This wording is relative to the FDIS.
Change 23.4.6.2 [set.cons] p3 as follows:
template <class InputIterator> set(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());-3- Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range [
-4- Requires: If the iterator's dereference operator returns an lvalue or afirst,last
).non-const rvalue, thenKey
shall beCopyConstructible
. -5- Complexity: Linear inN
if the range [first,last
) is already sorted usingcomp
and otherwiseN logN
, whereN
islast - first
.
[2014-05-18, Daniel comments]
According to Pablo, the current P/R correctly incorporates the changes from his paper (which was adopted in Kona)
[2014-06-10, STL comments and suggests better wording]
N1858 was voted into WP N2284 but was "(reworded)", introducing the "non-const" damage.
N1858 wanted to add this for map:Requires: Does not require
CopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type, mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
And this for set:
Requires:
Key
must beCopyConstructible
only if the dereferencedInputIterator
returns an lvalue or const rvalue.
(And similarly for multi.)
This was reworded to N2284 23.3.1.1 [map.cons]/3 and N2284 23.3.3.1 [set.cons]/4, and it slightly changed over the years into N3936 23.4.4.2 [map.cons]/3 and N3936 23.4.6.2 [set.cons]/4. In 2005/2007, this was the best known way to say "hey, we should try to move this stuff", as the fine-grained element requirements were taking shape. Then in 2010, N3173 was voted into WP N3225, adding the definition ofEmplaceConstructible
and modifying the container requirements tables to make the range constructors require
EmplaceConstructible
.
After looking at this history and double-checking our implementation (where map
/set
range construction goes through
emplacement, with absolutely no special-casing for map
's pairs), I am convinced that N3173 superseded N1858 here.
(Range-insert()
and the unordered containers are unaffected.)
Previous resolution [SUPERSEDED]:
This wording is relative to the N3936.
Change 23.4.6.2 [set.cons] p4 as follows:
template <class InputIterator> set(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());-3- Effects: Constructs an empty
-4- Requires: If the iterator's indirection operator returns an lvalue or aset
using the specified comparison object and allocator, and inserts elements from the range [first,last
).non-const rvalue, thenKey
shall beCopyInsertible
into*this
. -5- Complexity: Linear inN
if the range [first,last
) is already sorted usingcomp
and otherwiseN logN
, whereN
islast - first
.
[2015-02 Cologne]
GR: Do requirements supersede rather than compose [container requirements and per-function requirements]? AM: Yes, they supersede.
AM: This looks good. Ready? Agreement.Proposed resolution:
This wording is relative to the N4296.
Remove 23.4.3.2 [map.cons] p3:
template <class InputIterator> map(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());[…]
-3- Requires: If the iterator's indirection operator returns an lvalue or aconst
rvaluepair<key_type, mapped_type>
, then bothkey_type
andmapped_type
shall beCopyInsertible
into*this
.
Remove 23.4.4.2 [multimap.cons] p3:
template <class InputIterator> multimap(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());[…]
-3- Requires: If the iterator's indirection operator returns an lvalue or aconst
rvaluepair<key_type, mapped_type>
, then bothkey_type
andmapped_type
shall beCopyInsertible
into*this
.
Remove 23.4.6.2 [set.cons] p4:
template <class InputIterator> set(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());[…]
-4- Requires: If the iterator's indirection operator returns an lvalue or a non-const
rvalue, thenKey
shall beCopyInsertible
into*this
.
Remove 23.4.7.2 [multiset.cons] p3:
template <class InputIterator> multiset(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());[…]
-3- Requires: If the iterator's indirection operator returns an lvalue or aconst
rvalue, thenKey
shall beCopyInsertible
into*this
.
async()
incompleteSection: 32.10.9 [futures.async] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-08-29 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.async].
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Discussion:
The current throw specification of async()
does state:
-6- Throws:
system_error
if policy islaunch::async
and the implementation is unable to start a new thread.
First it seems not clear whether this only applies if policy equals
launch::async
of if the async
launch mode flag is set
(if policy|launch::async!=0
)
More generally, I think what we want to say is that if the implementation cannot successfully execute on one of the policies allowed, then it must choose another. The principle would apply to implementation-defined policies as well.
Peter Sommerlad:
Should not throw. That was the intent. "is async" meat exactly.
[2012, Portland: move to Tentatively NAD Editorial]
If no launch policy, it is undefined behavior.
Agree with Lawrence, should try all the allowed policies. We will rephrase so that
the policy argument should be lauch::async
. Current wording seems good enough.
We believe this choice of policy statement is really an editorial issue.
[2013-09 Chicago]
If all the implementors read it and can't get it right - it is not editorial. Nico to provide wording
No objections to revised wording, so moved to Immediate.
Accept for Working Paper
Proposed resolution:
This wording is relative to N3691.
Change 32.10.9 [futures.async] p6, p7 as indicated:
-6- Throws:
system_error
ifpolicy
is==
launch::async
and the implementation is unable to start a new thread.-7- Error conditions:
resource_unavailable_try_again
— ifpolicy
is==
launch::async
and the system is unable to start a new thread.
once_flag
becomes invalidSection: 32.6.7 [thread.once] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
In function call_once
32.6.7.2 [thread.once.callonce]
paragraph 4 and 5 specify for call_once()
:
Throws:
Error conditions:system_error
when an exception is required (32.2.2 [thread.req.exception]), or any exception thrown byfunc
.
invalid_argument
— if theonce_flag
object is no longer valid.
However, nowhere in 32.6.7 [thread.once] is specified, when a once-flag becomes invalid.
As far as I know this happens if the flag is used for different functions. So we either have to have to insert a sentence/paragraph in30.4.4.2 Function call_once [thread.once.callonce]
or
30.4.4 Call once [thread.once]
explaining when a once_flag
becomes invalidated or we should state as error condition something like:
invalid_argument
— if the func
used in combination with the once_flag
is different
from a previously passed func
for the same once_flag
Anthony Williams:
A
If the library can detect that this is the case then it will throw this exception. If it cannot detect such a case then it will never be thrown.once_flag
is invalidated if you destroy it (e.g. it is an automatic object, or heap allocated and deleted, etc.)
Jonathan Wakely:
I have also wondered how that error can happen in C++, where the type system will reject a non-callable type being passed to
If acall_once()
and should prevent aonce_flag
being used after its destructor runs.once_flag
is used after its destructor runs then it is indeed undefined behaviour, so implementations are already free to throw any exception (or set fire to a printer) without the standard saying so. My assumption was that it's an artefact of basing the API on pthreads, which says:The
pthread_once()
function may fail if:[EINVAL]
If eitheronce_control
orinit_routine
is invalid.
Pete Becker:
Yes, probably. We had to clean up several UNIXisms that were in the original design.
[2012, Kona]
Remove error conditions, move to Review.
[2012, Portland: move to Tentatively Ready]
Concurrency move to Ready, pending LWG review.
LWG did not have time to perform the final review in Portland, so moving to tentatively ready to reflect the Concurrency belief that the issue is ready, but could use a final inspection from library wordsmiths.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3337.
Change 32.6.7.2 [thread.once.callonce] as indicated:
template<class Callable, class ...Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);[…]
-4- Throws:system_error
when an exception is required (30.2.2), or any exception thrown byfunc
.-5- Error conditions:
invalid_argument
— if theonce_flag
object is no longer valid.
Allocator
requirements should include CopyConstructible
Section: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
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Discussion:
As discussed in c++std-lib-31054 and c++std-lib-31059, the Allocator
requirements implicitly require CopyConstructible
because
a.select_on_container_copy_construction()
and
container.get_allocator()
both return a copy by value, but the
requirement is not stated explicitly anywhere.
CopyConstructible
.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change Table 28 — Allocator requirements in 16.4.4.6 [allocator.requirements]:
Expression | Return type | Assertion/note pre-/post-condition | Default |
---|---|---|---|
X a1(a);
|
Shall not exit via an exception. post: a1 == a
|
||
…
|
|||
X a1(move(a));
|
Shall not exit via an exception. post: a1 equals the prior valueof a .
|
Change 16.4.4.6 [allocator.requirements] paragraph 4:
An allocator type
X
shall satisfy the requirements ofCopyConstructible
(16.4.4.2 [utility.arg.requirements]). TheX::pointer
,X::const_pointer
,X::void_pointer
, andX::const_void_pointer
types shall satisfy the requirements ofNullablePointer
(16.4.4.4 [nullablepointer.requirements]). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception.X::pointer
andX::const_pointer
shall also satisfy the requirements for a random access iterator (24.3 [iterator.requirements]).
weak_ptr::owner_before
Section: 20.3.2.3 [util.smartptr.weak], 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++14 Submitter: Ai Azuma Opened: 2011-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.weak].
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Discussion:
Is there any reason why weak_ptr::owner_before
member function templates are not const-qualified?
Daniel Krügler:
I don't think so. To the contrary, without these to be const member function templates, the semantics of the specializations
It is amusing to note that this miss has remain undetected from the accepted paper n2637 on. For the suggested wording changes see below.owner_less<weak_ptr<T>>
andowner_less<shared_ptr<T>>
described in 20.3.2.4 [util.smartptr.ownerless] is unclear.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change the class template weak_ptr
synopsis in 20.3.2.3 [util.smartptr.weak]
as indicated:
namespace std { template<class T> class weak_ptr { public: typedef T element_type; […] template<class U> bool owner_before(shared_ptr<U> const& b) const; template<class U> bool owner_before(weak_ptr<U> const& b) const; }; […] }
Change the prototypes in 20.3.2.3.6 [util.smartptr.weak.obs] before p6 as indicated:
template<class U> bool owner_before(shared_ptr<U> const& b) const; template<class U> bool owner_before(weak_ptr<U> const& b) const;
basic_istream::ignore
Section: 31.7.5.4 [istream.unformatted] Status: C++14 Submitter: Krzysztof Żelechowski Opened: 2011-09-11 Last modified: 2016-08-09
Priority: Not Prioritized
View all other issues in [istream.unformatted].
View all issues with C++14 status.
Discussion:
31.7.5.4 [istream.unformatted] in N3242 currently has the following to say about the
semantics of basic_istream::ignore
:
[..]. Characters are extracted until any of the following occurs:
- if
n != numeric_limits<streamsize>::max()
(18.3.2),n
characters are extracted
This statement, apart from being slightly ungrammatical, indicates that if
(n == numeric_limits<streamsize>::max()
), the method returns without
extracting any characters.
[..]. Characters are extracted until any of the following occurs:
(n != numeric_limits<streamsize>::max())
(18.3.2) and (n
) characters have been extracted so far.
[2013-04-20, Bristol]
Resolution: Ready.
[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to the FDIS.
Change 31.7.5.4 [istream.unformatted] p25 as indicated:
basic_istream<charT,traits>& ignore(streamsize n = 1, int_type delim = traits::eof());-25- Effects: Behaves as an unformatted input function (as described in 31.7.5.4 [istream.unformatted], paragraph 1). After constructing a
sentry
object, extracts characters and discards them. Characters are extracted until any of the following occurs:
ifn != numeric_limits<streamsize>::max()
( [limits.numeric]),andn
charactersarehave been extracted so far- end-of-file occurs on the input sequence (in which case the function calls
setstate(eofbit)
, which may throwios_base::failure
(31.5.4.4 [iostate.flags]));traits::eq_int_type(traits::to_int_type(c), delim)
for the next available input characterc
(in which casec
is extracted).
Section: 29.7 [c.math] Status: C++14 Submitter: Daniel Krügler Opened: 2011-09-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [c.math].
View all issues with C++14 status.
Discussion:
29.7 [c.math] ends with a description of a rule set for "sufficient overloads" in p11:
Moreover, there shall be additional overloads sufficient to ensure:
- If any argument corresponding to a
double
parameter has typelong double
, then all arguments corresponding todouble
parameters are effectively cast tolong double
.- Otherwise, if any argument corresponding to a
double
parameter has typedouble
or an integer type, then all arguments corresponding todouble
parameters are effectively cast todouble
.- Otherwise, all arguments corresponding to
double
parameters are effectively cast tofloat
.
My impression is that this rule set is probably more generic as intended, my assumption is that it is written to mimic the C99/C1x rule set in 7.25 p2+3 in the "C++" way:
-2- Of the
-3- Use of the macro invokes a function whose generic parameters have the corresponding real type determined as follows:<math.h>
and<complex.h>
functions without anf
(float
) orl
(long double
) suffix, several have one or more parameters whose corresponding real type isdouble
. For each such function, exceptmodf
, there is a corresponding type-generic macro. (footnote 313) The parameters whose corresponding real type isdouble
in the function synopsis are generic parameters. Use of the macro invokes a function whose corresponding real type and type domain are determined by the arguments for the generic parameters. (footnote 314)
- First, if any argument for generic parameters has type
long double
, the type determined islong double
.- Otherwise, if any argument for generic parameters has type
double
or is of integer type, the type determined isdouble
.- Otherwise, the type determined is
float
.
where footnote 314 clarifies the intent:
If the type of the argument is not compatible with the type of the parameter for the selected function, the behavior is undefined.
The combination of the usage of the unspecific term "cast" with otherwise no further constraints (note that C constraints the valid set to types that C++ describes as arithmetic types, but see below for one important difference) has the effect that it requires the following examples to be well-formed and well-defined:
#include <cmath> enum class Ec { }; struct S { explicit operator long double(); }; void test(Ec e, S s) { std::sqrt(e); // OK, behaves like std::sqrt((float) e); std::sqrt(s); // OK, behaves like std::sqrt((float) s); }
GCC 4.7 does not accept any of these examples.
I found another example where the C++ rule differs from the C set, but in this case I'm not so sure, which direction C++ should follow. The difference is located in the fact, that in C enumerated types are integer types as described in 6.2.5 p17 (see e.g. n1569 or n1256): "The type char, the signed and unsigned integer types, and the enumerated types are collectively called integer types. The integer and real floating types are collectively called real types." This indicates that in C the following code#include <math.h> enum E { e }; void test(void) { sqrt(e); // OK, behaves like sqrt((double) e); }
seems to be well-defined and e
is cast to double
, but in C++
referring to
#include <cmath> enum E { e }; void test() { std::sqrt(e); // OK, behaves like sqrt((float) e); }
is also well-defined (because of our lack of constraints) but we
must skip bullet 2 (because E is not an integer type) and effectively
cast e
to float
. Accepting this, we would introduce
a silent, but observable runtime difference for C and C++.
Howard provided wording to solve the issue.
[2012, Kona]
Moved to Ready. The proposed wording reflects both original intent from TR1, and current implementations.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 29.7 [c.math] p11 as indicated:
Moreover, there shall be additional overloads sufficient to ensure:
- If any arithmetic argument corresponding to a
double
parameter has typelong double
, then all arithmetic arguments corresponding todouble
parameters are effectively cast tolong double
.- Otherwise, if any arithmetic argument corresponding to a
double
parameter has typedouble
or an integer type, then all arithmetic arguments corresponding todouble
parameters are effectively cast todouble
.- Otherwise, all arithmetic arguments corresponding to
double
parametersare effectively cast tohave typefloat
.
iostream_category()
and noexcept
Section: 31.5 [iostreams.base] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-09-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iostreams.base].
View all issues with C++14 status.
Discussion:
In <system_error>
we have:
const error_category& generic_category() noexcept; const error_category& system_category() noexcept;
In <future>
we have:
const error_category& future_category() noexcept;
But in <ios>
we have:
const error_category& iostream_category();
Is there any reason that iostream_category()
is not declared with
noexcept
or is this an oversight?
Daniel:
This looks like an oversight to me. We made the above mentioned changes as part of noexcept-ifying the thread library butiostream_category()
was skipped, so it seems
to be forgotten. There should be no reason, why it cannot
be noexcept
. When doing so, we should also make these functions
noexcept
(similar to corresponding overloads):
error_code make_error_code(io_errc e); error_condition make_error_condition(io_errc e);
Suggested wording provided by Daniel.
[2013-04-20, Bristol]
Unanimous.
Resolution: move to tentatively ready.[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to the FDIS.
Change [iostreams.base.overview], header <ios>
synopsis
as indicated:
#include <iosfwd> namespace std { […] error_code make_error_code(io_errc e) noexcept; error_condition make_error_condition(io_errc e) noexcept; const error_category& iostream_category() noexcept; }
Change the prototype declarations in 31.5.6 [error.reporting] as indicated:
error_code make_error_code(io_errc e) noexcept;
-1- Returns:
error_code(static_cast<int>(e), iostream_category())
.
error_condition make_error_condition(io_errc e) noexcept;
-2- Returns:
error_condition(static_cast<int>(e), iostream_category())
.
const error_category& iostream_category() noexcept;
-3- Returns: A reference to an object of a type derived from class
-4- The object’serror_category
.default_error_condition
andequivalent
virtual functions shall behave as specified for the classerror_category
. The object’sname
virtual function shall return a pointer to the string"iostream"
.
std::terminate
problemSection: 17.9.5 [exception.terminate] Status: Resolved Submitter: Daniel Krügler Opened: 2011-09-25 Last modified: 2024-11-07
Priority: 3
View all issues with Resolved status.
Discussion:
Andrzej Krzemienski reported the following on comp.std.c++:
In N3290, which is to become the official standard, in 17.9.5.4 [terminate], paragraph 1 reads
Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1), in effect immediately after evaluating the throw-expression (18.8.3.1). May also be called directly by the program.
It is not clear what is "in effect". It was clear in previous drafts where paragraphs 1 and 2 read:
Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1). May also be called directly by the program.
Effects: Calls theterminate_handler
function in effect immediately after evaluating the throw-expression (18.8.3.1), if called by the implementation, or calls the currentterminate_handler
function, if called by the program.It was changed by N3189. The same applies to function unexpected (D. 11.4, paragraph 1).
Assuming the previous wording is still intended, the wording can be read "unlessstd::terminate
is called by the program, we will use the handler that was in effect immediately after evaluating the throw-expression". This assumes that there is some throw-expression connected to every situation that triggers the call tostd::terminate
. But this is not the case:
- In case
std::thread
is assigned to or destroyed while being joinable there is no throw-expression involved.- In case
std::unexpected
is called by the program,std::terminate
is triggered by the implementation - no throw-expression involved.- In case a destructor throws during stack unwinding we have two throw-expressions involved.
Which one is referred to?
In casestd::nested_exception::rethrow_nested
is called for an object that has captured no exception, there is no throw-expression involved directly (and may no throw be involved even indirectly). Next, 17.9.5.1 [terminate.handler], paragraph 2 saysRequired behavior: A
terminate_handler
shall terminate execution of the program without returning to the caller.This seems to allow that the function may exit by throwing an exception (because word "return" implies a normal return).
One could argue that words "terminate execution of the program" are sufficient, but then why "without returning to the caller" would be mentioned. In case such handler throws, noexcept specification in functionstd::terminate
is violated, andstd::terminate
would be called recursively - shouldstd::abort
not be called in case of recursivestd::terminate
call? On the other hand some controlled recursion could be useful, like in the following technique.
The here mentioned wording changes by N3189 in regard to 17.9.5.4 [terminate] p1
were done for a better separation of effects (Effects element) and additional normative
wording explanations (Remarks element), there was no meaning change intended. Further,
there was already a defect existing in the previous wording, which was not updated when
further situations where defined, when std::terminate
where supposed to be
called by the implementation.
terminate_handler
function, so should be moved just after
"Effects: Calls the current terminate_handler
function."
It seems ok to allow a termination handler to exit via an exception, but the
suggested idiom should better be replaced by a more simpler one based on
evaluating the current exception pointer in the terminate handler, e.g.
void our_terminate (void) { std::exception_ptr p = std::current_exception(); if (p) { ... // OK to rethrow and to determine it's nature } else { ... // Do something else } }
[2011-12-09: Daniel comments]
[2012, Kona]
Move to Open.
There is an interaction with Core issues in this area that Jens is already supplying wording for. Review this issue again once Jens wording is available.
Alisdair to review clause 15.5 (per Jens suggestion) and recommend any changes, then integrate Jens wording into this issue.
[2024-11-07 Status changed: Open → Resolved.]
Resolved by the resolution of LWG 2111(i).
Proposed resolution:
std::allocator::construct
should use uniform initializationSection: 20.2.10.2 [allocator.members] Status: Resolved Submitter: David Krauss Opened: 2011-10-07 Last modified: 2020-09-06
Priority: 2
View all other issues in [allocator.members].
View all issues with Resolved status.
Discussion:
When the EmplaceConstructible
(23.2.2 [container.requirements.general]/13) requirement is used
to initialize an object, direct-initialization occurs. Initializing an aggregate or using a std::initializer_list
constructor with emplace requires naming the initialized type and moving a temporary. This is a result of
std::allocator::construct
using direct-initialization, not list-initialization (sometimes called "uniform
initialization") syntax.
std::allocator<T>::construct
to use list-initialization would, among other things, give
preference to std::initializer_list
constructor overloads, breaking valid code in an unintuitive and
unfixable way — there would be no way for emplace_back
to access a constructor preempted by
std::initializer_list
without essentially reimplementing push_back
.
std::vector<std::vector<int>> v; v.emplace_back(3, 4); // v[0] == {4, 4, 4}, not {3, 4} as in list-initialization
The proposed compromise is to use SFINAE with std::is_constructible
, which tests whether direct-initialization
is well formed. If is_constructible
is false, then an alternative std::allocator::construct
overload
is chosen which uses list-initialization. Since list-initialization always falls back on direct-initialization, the
user will see diagnostic messages as if list-initialization (uniform-initialization) were always being used, because
the direct-initialization overload cannot fail.
std::initializer_list
satisfy a constructor, such as trying to emplace-insert a value of {3, 4}
in
the above example. The workaround is to explicitly specify the std::initializer_list
type, as in
v.emplace_back(std::initializer_list<int>(3, 4))
. Since this matches the semantics as if
std::initializer_list
were deduced, there seems to be no real problem here.
The other case is when arguments intended for aggregate initialization satisfy a constructor. Since aggregates cannot
have user-defined constructors, this requires that the first nonstatic data member of the aggregate be implicitly
convertible from the aggregate type, and that the initializer list have one element. The workaround is to supply an
initializer for the second member. It remains impossible to in-place construct an aggregate with only one nonstatic
data member by conversion from a type convertible to the aggregate's own type. This seems like an acceptably small
hole.
The change is quite small because EmplaceConstructible
is defined in terms of whatever allocator is specified,
and there is no need to explicitly mention SFINAE in the normative text.
[2012, Kona]
Move to Open.
There appears to be a real concern with initializing aggregates, that can be performed only using brace-initialization. There is little interest in the rest of the issue, given the existence of 'emplace' methods in C++11.
Move to Open, to find an acceptable solution for intializing aggregates. There is the potential that EWG may have an interest in this area of language consistency as well.
[2013-10-13, Ville]
This issue is related to 2070(i).
[2015-02 Cologne]
Move to EWG, Ville to write a paper.
[2015-09, Telecon]
Ville: N4462 reviewed in Lenexa. EWG discussion to continue in Kona.
[2016-08 Chicago]
See N4462
The notes in Lenexa say that Marshall & Jonathan volunteered to write a paper on this
[2018-08-23 Batavia Issues processing]
P0960 (currently in flight) should resolve this.
[2020-01 Resolved by the adoption of P0960 in Kona.]
Proposed resolution:
This wording is relative to the FDIS.
Change 20.2.10.2 [allocator.members] p12 as indicated:
template <class U, class... Args> void construct(U* p, Args&&... args);12 Effects:
::new((void *)p) U(std::forward<Args>(args)...)
ifis_constructible<U, Args...>::value
istrue
, else::new((void *)p) U{std::forward<Args>(args)...}
m.try_lock_for()
Section: 32.6.4.3 [thread.timedmutex.requirements] Status: C++14 Submitter: Pete Becker Opened: 2011-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.timedmutex.requirements].
View all issues with C++14 status.
Discussion:
32.6.4.3 [thread.timedmutex.requirements]/4 says, in part,
"Requires: If the tick period of [the argument] is not exactly convertible … [it] shall be rounded up …"
This doesn't belong in the requires clause. It's an effect. It belongs in paragraph 5. Nitpickingly, this would be a technical change: as written it imposes an obligation on the caller, while moving it imposes an obligation on the callee. Although that's certainly not what was intended.
Peter Dimov comments: Not to mention that it should round down, not up. :-) Incidentally, I see that the wrongtry_lock
requirement that the caller shall not own
the mutex has entered the standard, after all. Oh well. Let's hope that the real world
continues to ignore it.
[2012, Kona]
Remove the offending sentence from the requirements clause. Do not add it back anywhere else. The implementation already must have license to wake up late, so the rounding is invisible.
Move to Review.[2012, Portland]
Concurrency move to Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3337.
Change 32.6.4.3 [thread.timedmutex.requirements]/4 as indicated:
-3- The expression
-4- Requires:m.try_lock_for(rel_time)
shall be well-formed and have the following semantics:If the tick period ofIfrel_time
is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period.m
is of typestd::timed_mutex
, the calling thread does not own the mutex.
condition_variable_any
Section: 32.7.5 [thread.condition.condvarany] Status: C++14 Submitter: Pete Becker Opened: 2011-10-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvarany].
View all issues with C++14 status.
Discussion:
32.7.5 [thread.condition.condvarany]/4 says, in part, that
condition_variable_any()
throws an exception
"if any native handle type manipulated is not available".
condition_variable()
[32.7.4 [thread.condition.condvar]/4],
"if some non-memory resource limitation prevents initialization"? If not,
it should be worded the same way.
[2012, Kona]
Copy the corresponding wording from the condition_variable
constructor in 32.7.4 [thread.condition.condvar] p4.
[2012, Portland]
Concurrency move to Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3337.
Change 32.6.4.3 [thread.timedmutex.requirements]/4 as indicated:
condition_variable_any();[…]
-4- Error conditions:
resource_unavailable_try_again
—if any native handle type manipulated is not availableif some non-memory resource limitation prevents initialization.operation_not_permitted
— if the thread does not have the privilege to perform the operation.
condition_variable::wait
with predicateSection: 32.7.4 [thread.condition.condvar] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2011-10-27 Last modified: 2015-10-20
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
View all issues with C++14 status.
Discussion:
the Throws: clause of condition_variable::wait/wait_xxx
functions that
take a predicate argument is:
Throws:
system_error
when an exception is required (32.2.2 [thread.req.exception]).
If executing the predicate throws an exception, I would expect such exception to propagate unchanged
to the caller, but the throws clause seems to indicate that it gets mutated into a system_error
.
That's because of 16.3.2.4 [structure.specifications]/4:
F
's semantics contains a Throws:, Postconditions:, or Complexity: element, then that supersedes
any occurrences of that element in the code sequence."
Is my interpretation correct? Does it match the intent?
Daniel comments:
I don't think that this interpretation is entirely correct, the wording does not say that
std::system_error
or a derived class must be thrown, it simply is underspecified
in this regard (The extreme interpretation is that the behaviour would be undefined, but
that would be too far reaching I think). We have better wording for this in
32.6.7.2 [thread.once.callonce] p4, where it says:
"Throws: system_error
when an exception is required (32.2.2 [thread.req.exception]),
or any exception thrown by func
."
or in 32.4.5 [thread.thread.this] p6/p9:
"Throws: Nothing if Clock
satisfies the TrivialClock
requirements
(30.3 [time.clock.req]) and operations of Duration
do not throw exceptions.
[ Note: instantiations of time point types and clocks supplied by the implementation
as specified in 30.7 [time.clock] do not throw exceptions. — end note ]"
So, the here discussed Throws elements should add lines along the lines of
"Any exception thrown by operations of pred
."
and similar wording for time-related operations:
"Any exception thrown by operations of Duration
",
"Any exception thrown by operations of chrono::duration<Rep, Period>
"
[2011-11-28: Ganesh comments and suggests wording]
As for the discussion about the exception thrown by the manipulation of time-related objects, I believe the argument applies to all functions declared in 32 [thread]. Therefore, instead of adding wording to each member, I would simply move those requirements from 32.4.5 [thread.thread.this] p6/p9 to a new paragraph in 32.2.4 [thread.req.timing].
As for 32.7.5 [thread.condition.condvarany], the member functionswait()
and
wait_until()
are described only in terms of the Effects: clause (so strictly speaking,
they need no changes), however, wait_for()
is described with a full set of clauses
including Throws: and Error conditions:. Either we should add those clauses to wait/wait_until
with changes similar to the one above, or remove paragraphs 29 to 34 entirely. By the way,
even paragraph 26 could be removed IMHO.
[2012, Kona]
We like the idea behind the proposed resolution.
Modify the first addition to read instead: "Functions that specify a timeout, will throw if an operation on a clock, time point, or time duration throws an exception." In the note near the bottom change "even if the timeout has already expired" to "or if the timeout has already expired". (This is independent, but the original doesn't seem to make sense.) Move to Review.[2012, Portland]
Concurrency move to Ready with slightly ammended wording.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3337.
Add a new paragraph at the end of 32.2.4 [thread.req.timing]:
[…]
-6- The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution. -7- Implementation-provided clocks that are used for these functions shall meet theTrivialClock
requirements (30.3 [time.clock.req]). -?- Functions that specify a timeout, will throw if, during the execution of this function, a clock, time point, or time duration throws an exception. [ Note: instantiations of clock, time point and duration types supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
Change 32.4.5 [thread.thread.this] as indicated:
template <class Clock, class Duration> void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);;-4- Effects: Blocks the calling thread for the absolute timeout (32.2.4 [thread.req.timing]) specified by
-5- Synchronization: None. -6- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).abs_time
.Nothing ifClock
satisfies theTrivialClock
requirements (30.3 [time.clock.req]) and operations ofDuration
do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
template <class Rep, class Period> void sleep_for(const chrono::duration<Rep, Period>& rel_time);;-7- Effects: Blocks the calling thread for the relative timeout (32.2.4 [thread.req.timing]) specified by
-8- Synchronization: None. -9- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).rel_time
.Nothing if operations ofchrono::duration<Rep, Period>
do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
Change 32.6.4.3 [thread.timedmutex.requirements] as indicated:
-3- The expression m.try_lock_for(rel_time)
shall be well-formed and have the following semantics:
rel_time
. If the time specified by rel_time
is less than or equal to rel_time.zero()
, the
function attempts to obtain ownership without blocking (as if by calling try_lock()
). The function
shall return within the timeout specified by rel_time
only if it has obtained ownership of the mutex
object. [Note: As with try_lock()
, there is no guarantee that ownership will be obtained if the lock
is available, but implementations are expected to make a strong effort to do so. — end note]
[…]
-8- Synchronization: If try_lock_for()
returns true
, prior unlock()
operations on the same object
synchronize with (6.9.2 [intro.multithread]) this operation.
-9- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).m.try_lock_until(abs_time)
shall be well-formed and have the following semantics:
[…]
-12- Effects: The function attempts to obtain ownership of the mutex. If abs_time
has already passed, the
function attempts to obtain ownership without blocking (as if by calling try_lock()
). The function
shall return before the absolute timeout (32.2.4 [thread.req.timing]) specified by abs_time
only
if it has obtained ownership of the mutex object. [Note: As with try_lock()
, there is no guarantee
that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort
to do so. — end note]
[…]
-15- Synchronization: If try_lock_until()
returns true, prior unlock()
operations on the same object
synchronize with (6.9.2 [intro.multithread]) this operation.
-16- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).Change 32.7.4 [thread.condition.condvar] as indicated:
template <class Predicate> void wait(unique_lock<mutex>& lock, Predicate pred);[…]
-15- Effects: Equivalent to:while (!pred()) wait(lock);[…]
-17- Throws:std::system_error
when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown bypred
. […]
template <class Clock, class Duration> cv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time);[…]
-23- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]) or timeout-related exceptions (32.2.4 [thread.req.timing]). […]
template <class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);[…]
-26- Effects:as ifEquivalent to:return wait_until(lock, chrono::steady_clock::now() + rel_time);[…]
-29- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]) or timeout-related exceptions (32.2.4 [thread.req.timing]). […]
template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);[…]
-32- Effects: Equivalent to:while (!pred()) if (wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;[…] -36- Throws:
-33- Returns:pred()
std::system_error
when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown bypred
. […]
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);[…]
-39- Effects:as ifEquivalent to:return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));[…]
-42- Returns:[…] -44- Throws:pred()
system_error
when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown bypred
. […]
Change 32.7.5 [thread.condition.condvarany] as indicated:
template <class Lock, class Predicate> void wait(Lock& lock, Predicate pred);-14- Effects: Equivalent to:
while (!pred()) wait(lock);
template <class Lock, class Clock, class Duration> cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);[…]
-18- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]) or any timeout-related exceptions (32.2.4 [thread.req.timing]). […]
template <class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);[…]
-20- Effects:as ifEquivalent to:return wait_until(lock, chrono::steady_clock::now() + rel_time);[…]
-23- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]) or any timeout-related exceptions (32.2.4 [thread.req.timing]). […]
template <class Lock, class Clock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);-25- Effects: Equivalent to:
while (!pred()) if (wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;-26-
-27- [Note: The returned value indicates whether the predicate evaluates toReturns:[Note: There is no blocking ifpred()
pred()
is initiallytrue
, or if the timeout has already expired. — end note]true
regardless of whether the timeout was triggered. end note]
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);-28- Effects:
as ifEquivalent to:return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));
-29- [Note: There is no blocking ifpred()
is initiallytrue
, even if the timeout has already expired. — end note]-30- Postcondition:lock
is locked by the calling thread.-31- Returns:pred()
-32- [Note: The returned value indicates whether the predicate evaluates totrue
regardless of whether the timeout was triggered. — end note]-33- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]).-34- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.
duration
conversion overflow shouldn't participate in overload resolutionSection: 30.5.2 [time.duration.cons] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2011-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.cons].
View all issues with C++14 status.
Discussion:
30.5.2 [time.duration.cons] says:
template <class Rep2, class Period2> constexpr duration(const duration<Rep2, Period2>& d);Remarks: This constructor shall not participate in overload resolution unless
treat_as_floating_point<rep>::value
istrue
or bothratio_divide<Period2, period>::den
is1
andtreat_as_floating_point<Rep2>::value
isfalse
.
The evaluation of ratio_divide<Period2, period>::den
could make
ratio_divide<Period2, period>::num
overflow.
period
=ratio<1,1000>
)
from an exa-second (Period2
=ratio<1018>
).
ratio_divide<ratio<1018>, ratio<1,1000>>::num
is
1021
which overflows which makes the compiler error.
If the function f
is overloaded with milliseconds and seconds
void f(milliseconds); void f(seconds);
The following fails to compile.
duration<int,exa> r(1); f(r);
While the conversion to seconds work, the conversion to milliseconds make the program fail at compile time.
In my opinion, this program should be well formed and the constructor from duration<int,exa>
to milliseconds shouldn't participate in overload resolution as the result can not be represented.
[2012, Kona]
Move to Review.
Pete: The wording is not right.
Howard: Will implement this to be sure it works.
Jeffrey: If ratio needs a new hook, should it be exposed to users for their own uses?
Pete: No.
Move to Review, Howard to implement in a way that mere mortals can understand.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to the FDIS.
Change the following paragraphs of 30.5.2 [time.duration.cons] p4 indicated:
template <class Rep2, class Period2> constexpr duration(const duration<Rep2, Period2>& d);Remarks: This constructor shall not participate in overload resolution unless no overflow is induced in the conversion and
treat_as_floating_point<rep>::value
istrue
or bothratio_divide<Period2, period>::den
is1
andtreat_as_floating_point<Rep2>::value
isfalse
. [ Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note ]
future::get
in regard to MoveAssignable
Section: 32.10.7 [futures.unique.future] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-02 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with C++14 status.
Discussion:
[futures.unique_future] paragraph 15 says the following:
R future::get();
…
-15- Returns:future::get()
returns the value stored in the object’s shared state. If the type of the value is
MoveAssignable
the returned value is moved, otherwise it is copied.
There are some problems with the description:
"If the type of the value isMoveAssignable
the returned value is moved, otherwise it is copied."
MoveAssignable
? This should be
based solely on a pure expression-based requirement, if this is an requirement for implementations.
MoveAssignable
to the plain expression part std::is_move_assignable
would solvs (1), but raises another question, namely why a move-assignment should be relevant
for a function return based on the value stored in the future state? We would better fall back to
std::is_move_constructible
instead.
The last criticism I have is about the part
"the returned value is moved, otherwise it is copied" because an implementation won't be able to recognize what the user-defined type will do during an expression that is prepared by the implementation. I think the wording is intended to allow a move by seeding with an rvalue expression viastd::move
(or equivalent), else the result will be an effective
copy construction.
[2011-11-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib.]
Proposed resolution:
This wording is relative to the FDIS.
Change [futures.unique_future] paragraph 15 as indicated:
R future::get();
…
-15- Returns:future::get()
returns the value v
stored in the object’s shared
state as std::move(v)
. If the type of the value is
MoveAssignable
the returned value is moved, otherwise it is copied.
packaged_task
constructors should be constrainedSection: 32.10.10.2 [futures.task.members] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task.members].
View all issues with C++14 status.
Discussion:
With the proposed resolution of 2067(i), this no longer selects the copy constructor:
std::packaged_task<void()> p1; std::packaged_task<void()> p2(p1);
Instead this constructor is a better match:
template <class F> explicit packaged_task(F&& f);
This attempts to package a packaged_task
, which internally tries to
copy p2
, which fails because the copy constructor is deleted. For at
least one implementation the resulting error message is much less
helpful than the expected "cannot call deleted function" because it
happens after instantiating several more templates rather than in the
context where the constructor is called.
F
cannot be deduced as (possibly cv)
packaged_task&
or packaged_task
. It could be argued
this constraint is already implied because packaged_task
is not
copyable and the template constructors require that "invoking a copy of f
shall behave the same as invoking f
".
Daniel points out that the variadic constructor of std::thread
described in 32.4.3.3 [thread.thread.constr] has a similar problem and
suggests a similar wording change, which has been integrated below.
An alternative is to declare thread(thread&)
and
packaged_task(packaged_task&)
as deleted.
[2012, Portland]
This issue appears to be more about library specification than technical concurrency issues, so should be handled in LWG.
[2013, Chicago]
Move to Immediate resolution.
Howard volunteered existing implementation experience with the first change, and saw no issue that the second would introduce any new issue.
Proposed resolution:
This wording is relative to the FDIS.
Insert a new Remarks element to 32.4.3.3 [thread.thread.constr] around p3 as indicated:
template <class F, class ...Args> explicit thread(F&& f, Args&&... args);
-3- Requires: F
and each Ti
in Args
shall satisfy the MoveConstructible
requirements. INVOKE(DECAY_COPY ( std::forward<F>(f)), DECAY_COPY (std::forward<Args>(args))...)
(20.8.2) shall be a valid expression.
decay<F>::type
is the same type as std::thread
.
Insert a new Remarks element to 32.10.10.2 [futures.task.members] around p2 as indicated:
template <class F> packaged_task(F&& f); template <class F, class Allocator> explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
-2- Requires: INVOKE(f, t1, t2, ..., tN, R)
, where t1, t2, ..., tN
are values of the corresponding
types in ArgTypes...
, shall be a valid expression. Invoking a copy of f
shall behave the same as invoking f
.
decay<F>::type
is the same type as std::packaged_task<R(ArgTypes...)>
.
promise::set_value
and promise::set_value_at_thread_exit
Section: 32.10.6 [futures.promise] Status: C++14 Submitter: Pete Becker Opened: 2011-11-14 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
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Discussion:
32.10.6 [futures.promise]/16 says that promise::set_value(const R&)
throws any exceptions
thrown by R
's copy constructor, and that promise_set_value(R&&)
throws any exceptions
thrown by R
's move constructor.
promise::set_value_at_thread_exit
. It
has no corresponding requirements, only that these functions throw "future_error
if an error condition
occurs."
Daniel suggests wording to fix this: The approach is a bit more ambitious and also attempts to fix wording glitches
of 32.10.6 [futures.promise]/16, because it would be beyond acceptable efforts of implementations to
determine whether a constructor call of a user-defined type will indeed call a copy constructor or move constructor
(in the first case it might be a template constructor, in the second case it might also be a copy-constructor,
if the type has no move constructor).
[2012, Portland: move to Review]
Moved to Review by the concurrency working group, with no further comments.
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
This wording is relative to the FDIS.
Change 32.10.6 [futures.promise]/16 as indicated:
void promise::set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();[…]
-16- Throws:
future_error
if its shared state already has a stored value or exception, or- for the first version, any exception thrown by the
copy constructor ofconstructor selected to copy an object ofR
, or- for the second version, any exception thrown by the
move constructor ofconstructor selected to move an object ofR
.
Change 32.10.6 [futures.promise]/22 as indicated:
void promise::set_value_at_thread_exit(const R& r); void promise::set_value_at_thread_exit(R&& r); void promise<R&>::set_value_at_thread_exit(R& r); void promise<void>::set_value_at_thread_exit();[…]
-16- Throws:future_error
if an error condition occurs.
future_error
if its shared state already has a stored value or exception, or- for the first version, any exception thrown by the constructor selected to copy an object of
R
, or- for the second version, any exception thrown by the constructor selected to move an object of
R
.
va_start()
usageSection: 17.14 [support.runtime] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [support.runtime].
View all issues with C++14 status.
Discussion:
In 17.14 [support.runtime] p3 we find (emphasis mine):
The restrictions that ISO C places on the second parameter to the
va_start()
macro in header<stdarg.h>
are different in this International Standard. The parameterparmN
is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).227 If the parameterparmN
is declared with a function, array, or reference type, or with a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.
It seems astonishing that the constraints on function types and array types imposes these
on the declared parameter parmN
, not to the adjusted one (which would
not require this extra wording, because that is implicit). This seems to say that a function
definition of the form (Thanks to Johannes Schaub for this example)
#include <stdarg.h> void f(char const paramN[], ...) { va_list ap; va_start(ap, paramN); va_end(ap); }
would produce undefined behaviour when used.
Similar wording exists in C99 and in the most recent C11 draft in 7.16.1.4 p4 In my opinion the constraints in regard to array types and function types are unnecessary and should be relaxed. Are there really implementations out in the wild that would (according to my understanding incorrectly) provide the declared and not the adjusted type ofparamN
as deduced type to va_start()
?
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 17.14 [support.runtime] p3 as indicated:
The restrictions that ISO C places on the second parameter to the
va_start()
macro in header<stdarg.h>
are different in this International Standard. The parameterparmN
is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).227 If the parameterparmN
isdeclared withof afunction, array, orreference type, orwithof a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.
launch::async
policy usedSection: 32.10.9 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-14 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.async].
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Discussion:
32.10.9 [futures.async] p5 says
If the implementation chooses the
launch::async
policy,
- a call to a waiting function on an asynchronous return object that shares the shared state created by this
async
call shall block until the associated thread has completed, as if joined (32.4.3.6 [thread.thread.member]);
That should say a non-timed waiting function, otherwise, calling a timed waiting function can block indefinitely waiting for the associated thread to complete, rather than timing out after the specified time.
Sincestd::thread
does not provide a timed_join()
function (nor does
Pthreads, making it impossible on many platforms) there is no way for a timed waiting
function to try to join but return early due to timeout, therefore timed waiting
functions either cannot guarantee to timeout or cannot be used to meet the requirement
to block until the thread is joined. In order to allow timed waiting functions to
timeout the requirement should only apply to non-timed waiting functions.
[2012, Portland: move to Review]
Detlef: Do we actually need this fix — is it detectable?
Yes — you will never get a timeout. Should we strike the whole paragraph?
Hans: issue with thread local destruction.
Niklas: I have a strong expectation that a timed wait will respect the timeout
agreed
Detlef: we want a timed wait that does not time out to return like a non-timed wait; but is this implementable?
Pablo: Could we simply append ", or else time out"
Detlef: the time out on the shared state needs implementing anyway, even if the underlying O/S does not support a timed join.
Hans: the net effect is the timeout does not cover the thread local destruction... ah, I see what you're doing
Detlef: happy with Pablo's proposal
Wording proposed is to append after the word "joined" add ", or else time out"
Moved to review with this wording.
[2013, Bristol]
"Non-timed" made the new wording redundant and the result overly weak. Remove it.
Attempted to move to add this to the working paper (Concurrency motion 2) without the addition of "non-timed". Motion was withdrawn after Jonathan Wakely expressed implementability concerns.[2013-09, Chicago]
Discussion of interaction with the absence of a Posix timed join.
Jonathan Wakely withdrew his objection, so moved to Immediate. Accept for Working PaperProposed resolution:
[This wording is relative to the FDIS.]
Change 32.10.9 [futures.async] p5 as indicated:
If the implementation chooses the
launch::async
policy,
- a call to a waiting function on an asynchronous return object that shares the shared state created by this
async
call shall block until the associated thread has completed, as if joined, or else time out (32.4.3.6 [thread.thread.member]);
Section: 21.3.8 [meta.trans] Status: C++17 Submitter: Daniel Krügler Opened: 2011-11-18 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
Table 53 — "Reference modifications" says in regard to the type trait
add_lvalue_reference
(emphasize mine)
If
T
names an object or function type then the member typedef type shall nameT&
;
The problem with this specification is that function types with cv-qualifier or ref-qualifier, like
void() const void() &
are also affected by the first part of the rule, but this would essentially mean, that
instantiating add_lvalue_reference
with such a type would attempt to form
a type that is not defined in the C++ type system, namely
void(&)() const void(&)() &
The general policy for TransformationTraits is to define always some meaningful
mapping type, but this does not hold for add_lvalue_reference
, add_rvalue_reference
,
and in addition to these two for add_pointer
as well. The latter one would
attempt to form the invalid types
void(*)() const void(*)() &
A possible reason why those traits were specified in this way is that in C++03 (and that means for TR1), cv-qualifier were underspecified in the core language and several compilers just ignored them during template instantiations. This situation became fixed by adopting CWG issues 295 and 547.
While there is possibly some core language clarification needed (see reflector messages starting from c++std-core-20740), it seems also clear that the library should fix the specification. The suggested resolution follows the style of the specification of the support conceptsPointeeType
and ReferentType
defined in
N2914.
[2012-02-10, Kona]
Move to NAD.
These cv- and ref-qualified function types are aberrations in the type system, and do
not represent any actual entity defined by the language. The notion of cv- and ref-
qualification applies only to the implicit *this
reference in a member function.
However, these types can be produced by quirks of template metaprogramming, the question
remains what the library should do about it. For example, add_reference
returns
the original type if passed a reference type, or a void
type. Conversely,
add_pointer
will return a pointer to the referenced type when passed a reference.
It is most likely that the 'right' answer in any case will depend on the context that the question is being asked, in terms of forming these obscure types. The best the LWG can do is allow an error to propagate back to the user, so they can provide their own meaningful answer in their context - with additional metaprogramming on their part. The consensus is that if anyone is dangerous enough with templates to get themselves into this problem, they will also have the skills to resolve the problem themselves. This is not going to trip up the non-expert developer.
Lastly, it was noted that this problem arises only because the language is inconsistent in providing us these nonsense types that do no really represent anything in the language. There may be some way Core or Evolution could give us a more consistent type system so that the LWG does not need to invent an answer at all, should this question need resolving. This is another reason to not specify anything at the LWG trait level at this time, leaving the other working groups free to produce the 'right' answer that we can then follow without changing the meaning of existing, well-defined programs.
[2012-02-10, post-Kona]
Move back to Open. Daniel is concerned that this is not an issue we can simply ignore, further details to follow.
[2012-10-06, Daniel comments]
This issue really should be resolved as a defect: First, the argument that "forming these obscure types"
should "allow an error to propagate" is inconsistent with the exact same "obscure type" that would be formed
when std::add_lvalue_reference<void>
wouldn't have an extra rules for void
types, which
also cannot form references. The originally proposed resolution attempts to apply the same solution for the same
common property of void
types and function types with cv-qualifiers or ref-qualifier.
These functions had the property of ReferentType
during concept time (see
CWG 749 bullet three for the final
wording).
void
types shows, that this extra rule is not so unexpected. Further, any usage
of the result types of these traits as argument types or return types of functions would make these ill-formed
(and in a template context would be sfinaed away), so the expected effects are rarely unnoticed. Checking
all existing explicit usages of the traits add_rvalue_reference
, add_lvalue_reference
, and
add_pointer
didn't show any example where the error would be silent: add_rvalue_reference
is used to specify the return value of declval()
and the instantiation of declval<void() const>()
would be invalid, because of the attempt to return a function type. Similarly, add_lvalue_reference
is used to specify the return type of unique_ptr<T>::operator*()
. Again, any instantiation with
void() const
wouldn't remain unnoticed. The trait add_pointer
is used to specify the trait
std::decay
and this is an interesting example, because it is well-formed when instantiated with void
types, too, and is heavily used throughout the library specification. All use-cases would not be negatively affected
by the suggested acceptance of function types with cv-qualifiers or ref-qualifier, because they involve
types that are either function arguments, function parameters or types were references are formed from.
The alternative would be to add an additional extra rule that doesn't define a type member 'type' when
we have a function type with cv-qualifiers or ref-qualifier. This is better than the
current state but it is not superior than the proposal to specify the result as the original type, because
both variants are sfinae-friendly. A further disadvantage of the "non-type" approach here would be that any
usage of std::decay
would require special protection against these function types, because
instantiating std::decay<void() const>
again would lead to a deep, sfinae-unfriendly error.
The following example demonstrates the problem: Even though the second f
template is the best final
match here, the first one will be instantiated. During that process std::decay<T>::type
becomes instantiated as well and will raise a deep error, because as part of the implementation the trait
std::add_pointer<void() const>
becomes instantiated:
#include <type_traits> template<class T> typename std::decay<T>::type f(T&& t); template<class T, class U> U f(U u); int main() { f<void() const>(0); }
When the here proposed resolution would be applied this program would be well-formed and selects the expected function.
Previous resolution from Daniel [SUPERSEDED]:
Change Table 53 — "Reference modifications" in 21.3.8.3 [meta.trans.ref] as indicated:
Table 53 — Reference modifications Template Comments …
template <class T>
struct
add_lvalue_reference;If T
names an objecttype
or ifT
names a function type that does not have
cv-qualifiers or a ref-qualifier then the member typedeftype
shall nameT&
; otherwise, ifT
names a type "rvalue reference toT1
" then
the member typedeftype
shall nameT1&
; otherwise,type
shall nameT
.template <class T>
struct
add_rvalue_reference;If T
names an objecttype
or ifT
names a function type that does not have
cv-qualifiers or a ref-qualifier then the member typedeftype
shall nameT&&
; otherwise,type
shall nameT
. [Note: This rule reflects
the semantics of reference collapsing (9.3.4.3 [dcl.ref]). For example, when a typeT
names a typeT1&
, the typeadd_rvalue_reference<T>::type
is not an
rvalue reference. — end note]Change Table 56 — "Pointer modifications" in 21.3.8.6 [meta.trans.ptr] as indicated:
Table 56 — Pointer modifications Template Comments …
template <class T>
struct add_pointer;The member typedeftype
shall name the same type as
IfT
names a function type that has cv-qualifiers or a ref-qualifier
then the member typedeftype
shall nameT
; otherwise, it
shall name the same type asremove_reference<T>::type*
.
The following revised proposed resolution defines - in the absence of a proper core language definition - a new
term referenceable type as also suggested by the resolution for LWG 2196(i) as an
umbrella of the negation of void
types and function types with cv-qualifiers or ref-qualifier.
This simplifies and minimizes the requires wording changes.
[ 2013-09-26, Daniel synchronizes wording with recent draft ]
[ 2014-05-18, Daniel synchronizes wording with recent draft and comments ]
My impression is that this urgency of action this issue attempts to point out is partly caused by the fact that even for the most recent C++14 compilers the implementations have just recently changed to adopt the core wording. Examples for these are bug reports to gcc or clang.
Occasionally the argument has been presented to me that the suggested changes to the traits affected by this issue would lead to irregularities compared to other traits, especially the lack of guarantee thatadd_pointer
might not return
a pointer or that add_(l/r)value_reference
might not return a reference type. I would like to point out that this
kind of divergence is actually already present in most add/remove
traits: For example, we have no guarantee that
add_const
returns a const type (Reference types or function types get special treatments), or that add_rvalue_reference
returns an rvalue-reference (e.g. when applied to an lvalue-reference type).
Zhihao Yuan brought to my attention, that the originally proposing paper
N1345 carefully discussed these design choices.
[2015-05, Lenexa]
MC: move to Ready: in favor: 16, opposed: 0, abstain: 1
STL: have libstdc++, libc++ implemented this? we would need to change your implementation
Proposed resolution:
This wording is relative to N3936.
Change Table 53 — "Reference modifications" in 21.3.8.3 [meta.trans.ref] as indicated:
Template | Comments |
---|---|
…
|
|
template <class T>
|
If T names then the member typedef type shall name T& ; otherwise, T names a type "rvalue reference to T1 " thenthe member typedef type shall name T1& ; otherwise,type shall name T .[Note: This rule reflects the semantics of reference collapsing (9.3.4.3 [dcl.ref]). — end note] |
template <class T>
|
If T names then the member typedef type shall name T&& ; otherwise, type shall name T . [Note: This rule reflectsthe semantics of reference collapsing (9.3.4.3 [dcl.ref]). For example, when a type T names a type T1& , the type add_rvalue_reference_t<T> is not anrvalue reference. — end note] |
Change Table 56 — "Pointer modifications" in 21.3.8.6 [meta.trans.ptr] as indicated:
Template | Comments |
---|---|
…
|
|
template <class T>
|
If T names a referenceable type or a (possibly cv-qualified) void type thentype shall name the same type asremove_reference_t<T>* ; otherwise, type shall name T .
|
std::launch
an implementation-defined type?Section: 32.10.1 [futures.overview] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.10.1 [futures.overview] says std::launch
is an
implementation-defined bitmask type, which would usually mean the
implementation can choose whether to define an enumeration type, or a
bitset
, or an integer type. But in the case of std::launch
it's
required to be a scoped enumeration type,
enum class launch : unspecified { async = unspecified, deferred = unspecified, implementation-defined };
so what is implementation-defined about it, and what is an implementation supposed to document about its choice?
[2011-12-02 Moved to Tentatively Ready after 6 positive votes on c++std-lib.]
Proposed resolution:
This wording is relative to the FDIS.
Change 32.10.1 [futures.overview] paragraph 2 as indicated:
The enum type
launch
isan implementation-defineda bitmask type (16.3.3.3.3 [bitmask.types]) withlaunch::async
andlaunch::deferred
denoting individual bits. [ Note: Implementations can provide bitmasks to specify restrictions on task interaction by functions launched byasync()
applicable to a corresponding subset of available launch policies. Implementations can extend the behavior of the first overload ofasync()
by adding their extensions to the launch policy under the “as if” rule. — end note ]
std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment
Section: 20.2.10 [default.allocator] Status: C++14 Submitter: Ai Azuma Opened: 2011-11-08 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [default.allocator].
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Discussion:
"std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment::value
"
is specified as "false", according to (20.2.10 [default.allocator]) and (20.2.9.2 [allocator.traits.types]).
However, according to (23.2.2 [container.requirements.general]), this specification leads to the unneeded requirements
(MoveInsertable
and MoveAssignable
of the value type) on the move assignment operator of containers
with the default allocator.
typedef std::true_type propagate_on_container_move_assignment;
"
in the definition of std::allocator
class template,
std::allocator_traits
" class template for "std::allocator
"
class template, in which "propagate_on_container_move_assignment
"
nested typedef is specified as "std::true_type
".
Pablo prefers the first resolution.
[2011-12-02: Pablo comments]
This issue has potentially some overlap with 2108(i). Should the trait always_compare_equal
been added, this issue's resolution should be improved based on that.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 20.2.10 [default.allocator], the class template allocator
synopsis as indicated:
namespace std { template <class T> class allocator; // specialize forvoid
: template <> class allocator<void> { public: typedef void* pointer; typedef const void* const_pointer; // reference-to-void
members are impossible. typedef void value_type; template <class U> struct rebind { typedef allocator<U> other; }; }; template <class T> class allocator { public: typedef size_t size_type; typedef ptrdiff_t difference_type; typedef T* pointer; typedef const T* const_pointer; typedef T& reference; typedef const T& const_reference; typedef T value_type; template <class U> struct rebind { typedef allocator<U> other; }; typedef true_type propagate_on_container_move_assignment; […] }; }
unique_lock
move-assignment should not be noexcept
Section: 32.6.5.4 [thread.lock.unique] Status: C++14 Submitter: Anthony Williams Opened: 2011-11-27 Last modified: 2017-07-05
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
I just noticed that the unique_lock
move-assignment operator is declared noexcept
. This
function may call unlock()
on the wrapped mutex, which may throw.
noexcept
specification from unique_lock::operator=(unique_lock&&)
in 32.6.5.4 [thread.lock.unique] and 32.6.5.4.2 [thread.lock.unique.cons].
Daniel:
I think the situation is actually a bit more complex as it initially looks.
First, the effects of the move-assignment operator are (emphasize mine):
Effects: If
owns
callspm->unlock()
.
Now according to the BasicLockable
requirements:
3 Requires: The current execution agent shall hold a lock on
m.unlock()
m
. 4 Effects: Releases a lock onm
held by the current execution agent. Throws: Nothing.
This shows that unlock itself is a function with narrow contract and for this reasons no unlock function of a mutex or lock itself does have a noexcept specifier according to our mental model.
Now the move-assignment operator attempts to satisfy these requirement of the function and calls it only when it assumes that the conditions are ok, so from the view-point of the caller of the move-assignment operator it looks as if the move-assignment operator would in total a function with a wide contract. The problem with this analysis so far is, that it depends on the assumed correctness of the state "owns". Looking at the construction or state-changing functions, there do exist several ones that depend on caller-code satisfying the requirements and there is one guy, who looks most suspicious:11 Requires: The calling thread own the mutex.
unique_lock(mutex_type& m, adopt_lock_t);
[…]
13 Postconditions:pm == &m
andowns == true
.
because this function does not even call lock()
(which may, but is not
required to throw an exception if the calling thread does already own the mutex).
So we have in fact still a move-assignment operator that might throw an exception,
if the mutex was either constructed or used (call of lock) incorrectly.
[Issaquah 2014-02-11: Move to Immediate after SG1 review]
Proposed resolution:
This wording is relative to the FDIS.
Change 32.6.5.4 [thread.lock.unique], class template unique_lock
synopsis as indicated:
namespace std { template <class Mutex> class unique_lock { public: typedef Mutex mutex_type; […] unique_lock(unique_lock&& u) noexcept; unique_lock& operator=(unique_lock&& u)noexcept; […] }; }
Change 32.6.5.4.2 [thread.lock.unique.cons] around p22 as indicated:
unique_lock& operator=(unique_lock&& u)noexcept;-22- Effects: If
-23- Postconditions:owns
callspm->unlock()
.pm == u_p.pm
andowns == u_p.owns
(whereu_p
is the state ofu
just prior to this construction),u.pm == 0
andu.owns == false
. -24- [Note: With a recursive mutex it is possible for both*this
andu
to own the same mutex before the assignment. In this case,*this
will own the mutex after the assignment andu
will not. — end note] -??- Throws: Nothing.
const_iterator
's value_type
Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Jeffrey Yasskin Opened: 2011-11-28 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [container.requirements.general].
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Discussion:
In the FDIS, Table 96 specifies X::const_iterator
as a "constant iterator type
whose value type is T
". However, Table 97 specifies X::const_reverse_iterator
as an "iterator type whose value type is const T
" and which is defined as
reverse_iterator<const_iterator>
. But reverse_iterator::value_type
is
just "typename iterator_traits<Iterator>::value_type
" 24.5.1.2 [reverse.iterator],
so const_iterator
and const_reverse_iterator
must have the same value_type
.
The resolution to issue 322(i) implies that
const_reverse_iterator
should change.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change Table 97 — "Reversible container requirements" as indicated
Expression | Return type | Assertion/note pre-/post-condition | Complexity |
---|---|---|---|
X::reverse_-
|
iterator type whose value type is T
|
reverse_iterator<iterator>
|
compile time |
X::const_-
|
constant iterator type whose value type is
|
reverse_iterator<const_iterator>
|
compile time |
move_iterator
wrapping iterators returning prvaluesSection: 24.5.4 [move.iterators] Status: C++17 Submitter: Dave Abrahams Opened: 2011-11-30 Last modified: 2017-07-30
Priority: 3
View all other issues in [move.iterators].
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Discussion:
Shouldn't move_iterator
be specialized so that if the iterator it wraps
returns a prvalue when dereferenced, the move_iterator
also returns by
value? Otherwise, it creates a dangling reference.
move_iterator<I>::reference
would do.
A direction might be testing on is_reference<iterator_traits<I>::reference>
,
or is_reference<decltype(*declval<I>())>
.
Daniel: I would prefer to use a consistent style among the iterator adaptors, so I
suggest to keep with the iterator_traits
typedefs if possible.
using reference = typename conditional< is_reference<typename iterator_traits<Iterator>::reference>::value, value_type&&, value_type >::type;
We might also want to ensure that if Iterator
's reference
type is
a reference, the referent is equal to value_type
(after removal of cv-qualifiers).
In general we have no such guarantee.
value_type&&
, should we use
value_type
or should we keep the reference
type of the wrapped iterator?
Daniel: This suggestion looks appealing at first, but the problem here is that using this typedef
can make it impossible for move_iterator
to satisfy its contract, which means returning
an rvalue of the value type (Currently it says rvalue-reference, but this must be fixed as of
this issue anyway). I think that user-code can reasonably expect that when it has constructed
an object m
of move_iterator<It>
, where It
is a valid
mutable iterator type, the expression
It::value_type&& rv = *m;
is well-formed.
Let's setR
equal to iterator_traits<Iterator>::reference
in the following. We can discuss the following situations:
R
is a reference type: We can only return the corresponding xvalue of R
,
if value_type
is reference-related to the referent type, else this is presumably no
forward iterator and we cannot say much about it, except that it must be convertible to
value_type
, so it better should return a prvalue.R
is not a reference type: In this case we can rely on a conversion to
value_type
again, but not much more. Assume we would return R
directly,
this might turn out to have a conversion to an lvalue-reference type of the value type (for
example). If that is the case, this would indirectly violate the contract of
move_iterator
.
In regard to the first scenario I suggest that implementations are simply required to
check that V2 = remove_cv<remove_reference<R>::type>::type
is equal
to the value type V1
as a criterion to return this reference as an xvalue, in all other
cases it should return the value type directly as prvalue.
reference
has
the correct cv-qualification, if R
is a real reference.
It is possible to improve this a bit by indeed supporting reference-related types,
this would require to test is_same<V1, V2>::value || is_base_of<V1, V2>::value
instead. I'm unsure whether (a) this additional effort is worth it and (b) a strict reading of
the forward iterator requirements seems not to allow to return a reference-related type (Whether
this is a defect or not is another question).
[2011-12-05: Marc Glisse comments and splits into two resolution alternatives]
I guess I am looking at the speed of:
value_type x; x = *m;
(copy construction would likely trigger copy elision and thus be neutral) instead of the validity of:
value_type&& x = *m;
In this sense, Daniels earlier proposition that ignored value_type
and just did
switch_lvalue_ref_to_rvalue_ref<reference>
was easier to understand (and it didn't
require thinking about reference related types).
[2012, Kona]
Move to Review.
Alisdair: This only applies to input iterators, so keep that in mind when thinking about this.
STL: I see what B is doing, but not A.
Howard: I agree.
Alisdair: Should we use add_rvalue_reference
?
STL: No, we do not want reference collapsing.
STL: Re A, messing with the CV qualification scares me.
Alisdair: Agree. That would break my intent.
STL: Actually I don't think it's actually wrong, but I still don't see what it's doing.
Alisdair: A is picking the value type, B is picking the proxy type.
Howard: I like returning the proxy type.
STL: Returning a reference (B) seems right, because the requirements say "reference". I suspect that B works correctly if you have a move iterator wrapping a move iterator wrapping a thing. I think that A would mess up the type in the middle.
Considerable discussion about which version is correct, checking various examples.
STL: Still think B is right. Still don't understand A. In A we are losing the proxyness.
Howard: Agree 100%. We don't want to lose the proxy. If it's const, so be it.
STL: B is also understandable by mortals.
Howard: Remove to review, keep A but move it out of the proposed resolution area (but keep it for rational).
Alisdair: Adding an explanatory note might be a good idea, if someone wants to write one.
Walter: Concerned about losing the word "reference" in p.1.
Howard: move_iterator
will return an xvalue or a prvalue, both of which are rvalues.
[Proposed resolution A, rejected in preference to the currently proposed resolution (B)
Change 24.5.4 [move.iterators] p1 as indicated:
Class template Change 24.5.4.2 [move.iterator], class template Immediately following the class template
-?- Let
]move_iterator
is an iterator adaptor with the same behavior as the underlying iterator
except that its dereference operator implicitly converts the value returned by the underlying iterator's
dereference operator to an rvalue referenceof the value type. Some generic algorithms
can be called with move iterators to replace copying with moving.
move_iterator
synopsis, as indicated:
namespace std {
template <class Iterator>
class move_iterator {
public:
typedef Iterator iterator_type;
typedef typename iterator_traits<Iterator>::difference_type difference_type;
typedef Iterator pointer;
typedef typename iterator_traits<Iterator>::value_type value_type;
typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
typedef
value_type&&see below reference;
[…]
};
}
move_iterator
synopsis in
24.5.4.2 [move.iterator] insert a new paragraph as indicated:R
be iterator_traits<Iterator>::reference
and
let V
be iterator_traits<Iterator>::value_type
. If
is_reference<R>::value
is true
and if
remove_cv<remove_reference<R>::type>::type
is the same type as V
,
the template instantiation move_iterator<Iterator>
shall define the nested type
named reference
as a synonym for remove_reference<R>::type&&
,
otherwise as a synonym for V
.
[2012, Portland: Move to Tentatively Ready]
AJM wonders if the implied trait might be useful elsewhere, and worth adding to type traits as a transformation type trait.
Suspicion that the Range SG might find such a trait useful, but wait until there is clear additional use of such a trait before standardizing.
Minor wording tweak to use add_rvalue_reference
rather than manually adding the &&
,
then move to Tentatively Ready.
[2013-01-09 Howard Hinnant comments]
I believe the P/R for LWG 2106 is incorrect (item 3). The way it currently reads, move_iterator<I>::reference
is always an lvalue reference. I.e. if R
is an lvalue reference type, then reference becomes
add_rvalue_reference<R>::type
which is just R
. And if R
is not a reference type,
then reference becomes R
(which is also just R
;-)).
I believe the correct wording is what was there previously:
-?- Let R
be iterator_traits<Iterator>::reference
. If is_reference<R>::value
is true, the template instantiation move_iterator<Iterator>
shall define the nested type named
reference
as a synonym for remove_reference<R>::type&&
, otherwise as a synonym for
R
.
Additionally Marc Glisse points out that move_iterator<I>::operator*()
should return
static_cast<reference>(*current)
, not std::move(*current)
.
Previous resolution:
This wording is relative to the FDIS.
Change 24.5.4 [move.iterators] p1 as indicated:
Class template
move_iterator
is an iterator adaptor with the same behavior as the underlying iterator except that its dereference operator implicitly converts the value returned by the underlying iterator's dereference operator to an rvaluereference. Some generic algorithms can be called with move iterators to replace copying with moving.Change 24.5.4.2 [move.iterator], class template
move_iterator
synopsis, as indicated:namespace std { template <class Iterator> class move_iterator { public: typedef Iterator iterator_type; typedef typename iterator_traits<Iterator>::difference_type difference_type; typedef Iterator pointer; typedef typename iterator_traits<Iterator>::value_type value_type; typedef typename iterator_traits<Iterator>::iterator_category iterator_category; typedefvalue_type&&see below reference; […] }; }Immediately following the class template
move_iterator
synopsis in 24.5.4.2 [move.iterator] insert a new paragraph as indicated:-?- Let
R
beiterator_traits<Iterator>::reference
. Ifis_reference<R>::value
istrue
, the template instantiationmove_iterator<Iterator>
shall define the nested type namedreference
as a synonym foradd_rvalue_reference<R>::type
, otherwise as a synonym forR
.
[2014-05-19, Daniel comments]
The term instantiation has been changed to specialization in the newly added paragraph as suggested by STL and much preferred by myself.
[2014-05-19 Library reflector vote]
The issue has been identified as Tentatively Ready based on five votes in favour.
Proposed resolution:
This wording is relative to N3936.
Change 24.5.4 [move.iterators] p1 as indicated:
Class template
move_iterator
is an iterator adaptor with the same behavior as the underlying iterator except that its indirection operator implicitly converts the value returned by the underlying iterator's indirection operator to an rvaluereference. Some generic algorithms can be called with move iterators to replace copying with moving.
Change 24.5.4.2 [move.iterator], class template move_iterator
synopsis, as indicated:
namespace std { template <class Iterator> class move_iterator { public: typedef Iterator iterator_type; typedef typename iterator_traits<Iterator>::difference_type difference_type; typedef Iterator pointer; typedef typename iterator_traits<Iterator>::value_type value_type; typedef typename iterator_traits<Iterator>::iterator_category iterator_category; typedefvalue_type&&see below reference; […] }; }
Immediately following the class template move_iterator
synopsis in
24.5.4.2 [move.iterator] insert a new paragraph as indicated:
-?- Let
R
beiterator_traits<Iterator>::reference
. Ifis_reference<R>::value
istrue
, the template specializationmove_iterator<Iterator>
shall define the nested type namedreference
as a synonym forremove_reference<R>::type&&
, otherwise as a synonym forR
.
Edit [move.iter.op.star] p1 as indicated:
reference operator*() const;-1- Returns:
.
std::movestatic_cast<reference>(*current)
Section: 16.4.4.6 [allocator.requirements] Status: Resolved Submitter: Jonathan Wakely Opened: 2011-12-01 Last modified: 2018-12-03
Priority: 3
View other active issues in [allocator.requirements].
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Discussion:
Whether two allocator objects compare equal affects the complexity of
container copy and move assignments and also the possibility of an
exception being thrown by container move assignments. The latter point
means container move assignment cannot be noexcept
when
propagate_on_container_move_assignment
(POCMA) is false for the
allocator because there is no way to detect at compile-time if two
allocators will compare equal. LWG 2013(i) means this affects all
containers using std::allocator
, but even if that is resolved, this
affects all stateless allocators which do not explicitly define POCMA
to true_type
.
allocator_traits
, but that would be duplicating information that is
already defined by the type's equality operator if that operator
always returns true. Requiring users to write operator==
that simply
returns true and also explicitly override a trait to repeat the same
information would be unfortunate and risk user errors that allow the
trait and actual operator==
to disagree.
Dave Abrahams suggested a better solution in message c++std-lib-31532,
namely to allow operator==
to return true_type
, which is convertible
to bool
but also detectable at compile-time. Adopting this as the
recommended way to identify allocator types that always compare equal
only requires a slight relaxation of the allocator requirements so
that operator==
is not required to return bool
exactly.
The allocator requirements do not make it clear that it is well-defined
to compare non-const values, that should be corrected too.
In message c++std-lib-31615 Pablo Halpern suggested an always_compare_equal
trait that could still be defined, but with a sensible default value rather
than requiring users to override it, and using that to set sensible values for
other allocator traits:
Do we still need
[…] One point that I want to ensure doesn't get lost is that if we adopt some sort ofalways_compare_equal
if we can have anoperator==
that returnstrue_type
? What would its default value be?is_empty<A> || is_convertible<decltype(a == a), true_type>::value
, perhaps? One benefit I see to such a definition is that stateless C++03 allocators that don't use thetrue_type
idiom will still benefit from the new trait.always_compare_equal
-like trait, thenpropagate_on_container_swap
andpropagate_on_container_move_assignment
should default toalways_compare_equal
. Doing this will eliminate unnecessary requirements on the container element type, as per [LWG 2103(i)].
Optionally, operator==
for std::allocator
could be made to return
true_type
, however if LWG 2103(i) is adopted that is less important.
always_compare_equal
,
all_objects_(are_)equivalent
, or all_objects_compare_equal
.
[2014-11-07 Urbana]
Resolved by N4258
Proposed resolution:
This wording is relative to the FDIS.
Change Table 27 — "Descriptive variable definitions" in 16.4.4.6 [allocator.requirements]:
Variable | Definition |
---|---|
a3, a4
|
const ) type X
|
b
|
a value of (possibly const ) type Y
|
Change Table 28 — "Allocator requirements" in 16.4.4.6 [allocator.requirements]:
Expression | Return type | Assertion/note pre-/post-condition | Default |
---|---|---|---|
|
convertible to bool
|
returns true only if storage allocated from each can be deallocated via the other. operator== shall be reflexive,symmetric, and transitive, and shall not exit via an exception. |
|
|
convertible to bool
|
same as
|
|
a3 == b
|
convertible to bool
|
same as a3 ==
|
|
a3 != b
|
convertible to bool
|
same as !(a3 == b)
|
|
[…]
|
|||
a.select_on_-
|
X
|
Typically returns either a orX()
|
return a;
|
X::always_compares_equal
|
Identical to or derived from true_type orfalse_type
|
true_type if the expression x1 == x2 isguaranteed to be true for any two (possiblyconst ) values x1, x2 of type X , whenimplicitly converted to bool . See Note B, below.
|
true_type , if is_empty<X>::value is true or ifdecltype(declval<const X&>() == declval<const X&>()) is convertible to true_type , otherwise false_type .
|
[…]
|
Note A: […]
Note B: IfX::always_compares_equal::value
or XX::always_compares_equal::value
evaluate
to true
and an expression equivalent to x1 == x2
or x1 != x2
for any two values
x1, x2
of type X
evaluates to false
or true
, respectively, the behaviour
is undefined.
Change class template allocator_traits
synopsis, 20.2.9 [allocator.traits] as indicated:
namespace std { template <class Alloc> struct allocator_traits { typedef Alloc allocator_type; […] typedef see below always_compares_equal; typedef see below propagate_on_container_copy_assignment; […] }; }
Insert the following between 20.2.9.2 [allocator.traits.types] p6 and p7 as indicated:
typedef see below always_compares_equal;-?- Type:
Alloc::always_compares_equal
if such a type exists; otherwise,true_type
ifis_empty<Alloc>::value
istrue
or ifdecltype(declval<const Alloc&>() == declval<const Alloc&>())
is convertible totrue_type
; otherwise,false_type
.
typedef see below propagate_on_container_copy_assignment;-7- Type:
Alloc::propagate_on_container_copy_assignment
if such a type exits, otherwisefalse_type
.
Change class template allocator
synopsis, 20.2.10 [default.allocator] as indicated:
namespace std { template <class T> class allocator; // specialize forvoid
: template <> class allocator<void> { public: typedef void* pointer; typedef const void* const_pointer; // reference-to-void
members are impossible. typedef void value_type; template <class U> struct rebind { typedef allocator<U> other; }; }; template <class T> class allocator { public: typedef size_t size_type; typedef ptrdiff_t difference_type; typedef T* pointer; typedef const T* const_pointer; typedef T& reference; typedef const T& const_reference; typedef T value_type; template <class U> struct rebind { typedef allocator<U> other; }; typedef true_type always_compares_equal; […] }; }
hash
specializationsSection: 19.5.7 [syserr.hash], 20.3.3 [util.smartptr.hash], 22.10.19 [unord.hash], 17.7.6 [type.index.synopsis], 27.4.6 [basic.string.hash], 23.3.14 [vector.bool], 32.4.3.2 [thread.thread.id] Status: C++14 Submitter: Daniel Krügler Opened: 2011-12-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
20.3.3 [util.smartptr.hash] p2 is specified as follows:
Requires: the template specializations shall meet the requirements of class template
hash
(20.8.12).
The problem here is the usage of a Requires element, which is actually a pre-condition that a user of a component has to satisfy. But the intent of this wording is actually to be a requirement on implementations. The Requires element should be removed here and the wording should be improved to say what it was intended for.
We have similar situations in basically all other places where the specification of library-providedhash
specializations is defined. Usually, the Requires element is incorrect. In the
special case of hash<unique_ptr<T, D>>
the implementation depends on
the behaviour of hash
specializations, that could be user-provided. In this case
the specification needs to separate the requirements on these specializations and those
that are imposed on the implementation.
[2012, Kona]
Update wording and move to Review.
Believe a simpler formulation is to simply string the term Requires: and leave the current wording intact, rather than strike the whole clause and replace it.
[Originally proposed wording for reference
Change 19.5.7 [syserr.hash] as indicated:
-1- Change 22.9.3 [bitset.hash] as indicated:
-1- Change 20.3.3 [util.smartptr.hash] as indicated:
-1-
template <> struct hash<error_code>;
Requires: the template specialization shall meet the requirements
of class template The header
hash
(22.10.19 [unord.hash])<system_error>
provides a definition for a specialization of the
template hash<error_code>
. The requirements for the members of
this specialization are given in sub-clause 22.10.19 [unord.hash].
template <size_t N> struct hash<bitset<N> >;
Requires: the template specialization shall meet the requirements
of class template The header
hash
(22.10.19 [unord.hash])<bitset>
provides a definition for a partial specialization of the
hash
class template for specializations of class template bitset<N>
.
The requirements for the members of instantiations of this specialization are given
in sub-clause 22.10.19 [unord.hash].
template <class T, class D> struct hash<unique_ptr<T, D> >;
Requires: the template specialization shall meet the requirements
of class template The header
hash
(22.10.19 [unord.hash])<memory>
provides a definition for a partial specialization of the
hash
class template for specializations of class template unique_ptr<T, D>
.
The requirements for the members of instantiations of this specialization are given
in sub-clause 22.10.19 [unord.hash]. For an object p
of type
UP
, where UP
is unique_ptr<T, D>
,
hash<UP>()(p)
shall evaluate to the same value as
hash<typename UP::pointer>()(p.get())
. The specialization
hash<typename UP::pointer>
shall be well-formed.hash<typename UP::pointer>
shall be well-formed and well-defined [Note: the general requirements
of class template hash
(22.10.19 [unord.hash]) are implied —
end note].
template <class T> struct hash<shared_ptr<T> >;-2-
Requires: the template specialization shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<memory>
provides a definition for a partial specialization of thehash
class template for specializations of class templateshared_ptr<T>
. The requirements for the members of instantiations of this specialization are given in sub-clause 22.10.19 [unord.hash]. For an objectp
of typeshared_ptr<T>
,hash<shared_ptr<T> >()(p)
shall evaluate to the same value ashash<T*>()(p.get())
.
Change 22.10.19 [unord.hash] p2 as indicated: [Comment: For unknown reasons the extended integer types are not mentioned here, which looks like an oversight to me and makes also the wording more complicated. See 2119(i) for this part of the problem. — end comment]
template <> struct hash<bool>; template <> struct hash<char>; […] template <> struct hash<long double>; template <class T> struct hash<T*>;-2-
Requires: the template specializations shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<functional>
provides definitions for specializations of thehash
class template for each cv-unqualified arithmetic type except for the extended integer types. This header also provides a definition for a partial specialization of thehash
class template for any pointer type. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].
Change [type.index.hash] p1 as indicated:
template <> struct hash<type_index>;-1-
Requires: the template specialization shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<typeindex>
provides a definition for a specialization of the templatehash<type_index>
. The requirements for the members of this specialization are given in sub-clause 22.10.19 [unord.hash]. For an objectindex
of typetype_index
,hash<type_index>()(index)
shall evaluate to the same result asindex.hash_code()
.
Change 27.4.6 [basic.string.hash] p1 as indicated:
template <> struct hash<string>; template <> struct hash<u16string>; template <> struct hash<u32string>; template <> struct hash<wstring>;-1-
Requires: the template specializations shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<string>
provides definitions for specializations of thehash
class template for the typesstring
,u16string
,u32string
, andwstring
. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].
Change 23.3.14 [vector.bool] p7 as indicated:
template <class Allocator> struct hash<vector<bool, Allocator> >;-7-
Requires: the template specialization shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<vector>
provides a definition for a partial specialization of thehash
class template for specializations of class templatevector<bool, Allocator>
. The requirements for the members of instantiations of this specialization are given in sub-clause 22.10.19 [unord.hash].
Change 32.4.3.2 [thread.thread.id] p14 as indicated:
template <> struct hash<thread::id>;-14-
Requires: the template specialization shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<thread>
provides a definition for a specialization of the templatehash<thread::id>
. The requirements for the members of this specialization are given in sub-clause 22.10.19 [unord.hash].
[2012, Portland: Move to Tentatively Ready]
No further wording issues, so move to Tentatively Ready (post meeting issues processing).
[2013-04-20 Bristol]
Proposed resolution:
Change 19.5.7 [syserr.hash] as indicated:
template <> struct hash<error_code>;-1-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash].
Change 22.9.3 [bitset.hash] as indicated:
template <size_t N> struct hash<bitset<N> >;-1-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
Change 20.3.3 [util.smartptr.hash] as indicated:
template <class T, class D> struct hash<unique_ptr<T, D> >;-1-
-?- Requires: The specializationRequires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]). For an objectp
of typeUP
, whereUP
isunique_ptr<T, D>
,hash<UP>()(p)
shall evaluate to the same value ashash<typename UP::pointer>()(p.get())
.The specializationhash<typename UP::pointer>
shall be well-formed.hash<typename UP::pointer>
shall be well-formed and well-defined, and shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
template <class T> struct hash<shared_ptr<T> >;-2-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]). For an objectp
of typeshared_ptr<T>
,hash<shared_ptr<T> >()(p)
shall evaluate to the same value ashash<T*>()(p.get())
.
Change 22.10.19 [unord.hash] p2 as indicated: [Comment: For unknown reasons the extended integer types are not mentioned here, which looks like an oversight to me and makes also the wording more complicated. See 2119(i) for this part of the problem. — end comment]
template <> struct hash<bool>; template <> struct hash<char>; […] template <> struct hash<long double>; template <class T> struct hash<T*>;-2-
Requires: tThe template specializations shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
Change [type.index.hash] p1 as indicated:
template <> struct hash<type_index>;-1-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]). For an objectindex
of typetype_index
,hash<type_index>()(index)
shall evaluate to the same result asindex.hash_code()
.
Change 27.4.6 [basic.string.hash] p1 as indicated:
template <> struct hash<string>; template <> struct hash<u16string>; template <> struct hash<u32string>; template <> struct hash<wstring>;-1-
Requires: tThe template specializations shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
Change 23.3.14 [vector.bool] p7 as indicated:
template <class Allocator> struct hash<vector<bool, Allocator> >;-7-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
Change 32.4.3.2 [thread.thread.id] p14 as indicated:
template <> struct hash<thread::id>;-14-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
remove
can't swap but note says it mightSection: 26.7.8 [alg.remove] Status: C++14 Submitter: Howard Hinnant Opened: 2011-12-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.remove].
View all issues with C++14 status.
Discussion:
26.7.8 [alg.remove]/p1 says:
1 Requires: The type of
*first
shall satisfy theMoveAssignable
requirements (Table 22).
This means that remove
/remove_if
can only use move assignment to permute the sequence. But then
26.7.8 [alg.remove]/p6 (non-normatively) contradicts p1:
6 Note: each element in the range
[ret,last)
, whereret
is the returned value, has a valid but unspecified state, because the algorithms can eliminate elements by swapping with or moving from elements that were originally in that range.
[2012, Kona]
Move to Ready.
Alisdair notes we could extend permission to use swap
if it is available, but there
is no interest. Accept the proposed resolution as written.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 26.7.8 [alg.remove] as indicated:
template<class ForwardIterator, class T> ForwardIterator remove(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class Predicate> ForwardIterator remove_if(ForwardIterator first, ForwardIterator last, Predicate pred);[…]
-6-Note: each element in the range[ret,last)
, whereret
is the returned value, has a valid but unspecified state, because the algorithms can eliminate elements byswapping with ormoving from elements that were originally in that range.
unexpected
/terminate
handler is called from the exception handling runtime?Section: 17.9.5.4 [terminate], 99 [unexpected] Status: C++17 Submitter: Howard Hinnant Opened: 2011-12-06 Last modified: 2017-07-30
Priority: 3
View all other issues in [terminate].
View all issues with C++17 status.
Discussion:
Prior to N3242, modified by N3189, we said this about unexpected()
:
Effects: Calls the
unexpected_handler
function in effect immediately after evaluating the throw-expression (D.13.1), if called by the implementation, or calls the currentunexpected_handler
, if called by the program.
and this about terminate()
:
Effects: Calls the
terminate_handler
function in effect immediately after evaluating the throw-expression (18.8.3.1), if called by the implementation, or calls the currentterminate_handler
function, if called by the program.
But now in both places we say:
Calls the current
unexpected_handler
function.
and:
Calls the current
terminate
function.
The difference is that in C++98/03 if a destructor reset a handler during stack unwinding, that new handler was
not called if the unwinding later led to unexpected()
or terminate()
being called. But these new
words say that this new handler is called. This is an ABI-breaking change in the way exceptions are handled.
Was this change intentional?
[2011-12-09: Daniel comments]
There was no such semantic change intended. It was an unfortunate side effect when trying to better separate different responsibilities in the previous wording.
A related issue is 2088(i).[2012-01-30: Howard comments]
The C++98/03 wording is somewhat ambiguous:
Calls the terminate_handler function in effect immediately after evaluating the throw-expression...
There are potentially two throw-expressions being referred to here, and it is not clear if this sentence is referring to just the first or both:
throw assignment-expression;
throw;
There is ample evidence in current implementations that it is understood that only 1. was meant. But clearly both 1 and 2 could have been meant. We need a clarification. Does an execution of a rethrow (throw;) update which handlers can potentially be called?
throw;
// update handlers to get_xxx()?My opinion: Go with existing practice, and clarify what that practice is, if surveys find that everyone does the same thing. Gcc 4.2 and Apple do 1. only, and do not reset the handlers to the current handlers on throw;.
If current practice is not unanimously one way or the other, I have no strong opinion. I have not found a motivating use case for the use of any particular handler. Most applications set the handlers once at the beginning of the program and then do not change them, and so will not be impacted by whatever decision is made here.[2014-02-15 Issaquah: Move to Review]
STL: Original change in N3242 came from trying to make set/get exception handler thread safe. The issue requests we revert to 98/03, which Howard notes was already ambiguous.
Alisdair: Issue author thinks we made this change in C++11 without taking into account Itanium ABI, which cannot implement the new semantic (without breaking compatibility).
Alisdair: original change in N3242 was trying to solve the problem of which handler is called when the handler is changing in another thread, but this turns out to be an issue in even the single-threaded case.
Pablo: despite wanting to make it thread safe, you are still changing a global
STL and Marshall confirm that there is real implementation divergance on the question, so we cannot pick just one behavior if we want to avoid breaking exisitng practice.
Alisdair: not sure who to talk to across all library vendors to fix, need more information for progress (IBM and Sun)
STL: Howard did identify a problem with the wording as well: throw;
is a throw expression,
but we typically want to re-activate the in-flight exception, not throw a new copy.
Pablo: wondering why all of this wording is here (N3189)? It looks like we were trying to handle another thread
changing handler between a throw
and terminate
in current thread.
Alisdair: Anything working with exception handling should have used only thread-local resources, but that ship has sailed. We must account for the same exception object being re-thrown in multiple threads simultaneously, with no happens-before relationships.
Room: Why on earth would we care about exactly which way the program dies when the terminate calls are racing?!
Pablo: Reasonable to set the handler once (in main
) and never change it.
Pablo: If willing to put lots of work into this, you could say at point of a throw
these handlers become
thread local but that is overkill. We want destructors to be able to change these handlers (if only for backwards
compatibility).
Alisdair: the "do it right" is to do something per thread, but that is more work than vendors will want to do. Want to say setting handler while running multiple threads is unspecified.
Pablo: possible all we need to do is say it is always the current handler
STL: That prevents an implementation or single threaded program from calling a new handler after a throw
,
probably should say if terminate
is called by the implementation (during EH), any handler that was
current can be called. Leaves it up in the air as to when the handler is captured, supporting the diverging
existing practices.
Jeffrey: use happens before terminology to avoid introducing races
STL: Give this to concurrency?
Jeffrey: It is in clause 18, generally LWG and not SG1 territory.
Alisdair: Concerned about introducing happens before into fundamental exception handling since it would affect single threaded performance as well. Want to give to concurrency or LEWG/EWG, we are into language design here.
Jeffrey: suspect LEWG won't have a strong opinion. I don't want it to be ours!!!
Pablo: Might be a case for core>
Alisdair: Would be happier if at least one core person were in the discussion.
STL: No sympathy for code that tries to guard the terminate handler.
Alisdair: We are back to set it once, globally. Want to be clear that if set_terminate
is called just once,
when EH is not active, and never changed again, then the user should get the handler from that specific call.
AlisdairM: "unspecified which handler is called if an exception is active when set_terminate
is called."
This supports existing behaviors, and guarantees which handler is called in non-conentious situations. Implicit
assumption that a funtion becomes a handler only after a successful call to set_handler
, so we are not
leaving a door open to the implementation inventing entirely new handlers of its own.
Consensus.
Poll to confirm status as P1: new consensus is P3
Action: Alisdair provides new wording. Drop from P1 to P3, and move to Review.
[2015-05, Lenexa]
HH: we accidentally changed semantics of which handler gets called during exception unwinding. This was attempt to put it back.
Discovered implementations don't really do anything. […] Fine with unspecified behavior to move this week.
STL/MC: observed different behavior
STL: legitimizes all implementations and tells users to not do this
Move to ready? 9/0/1
Proposed resolution:
Amend 17.9.5.4 [terminate] as indicated:
[[noreturn]] void terminate() noexcept;
Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1)
, in effect immediately after throwing the exception. May also be called directly by the program.
Effects: Calls a
terminate_handler
function. It is unspecified whichterminate_handler
function will be called if an exception is active during a call toset_terminate
. Otherwise cCalls the currentterminate_handler
function. [Note: A defaultterminate_handler
is always considered a callable handler in this context. — end note]
Amend 99 [unexpected] as indicated:
[[noreturn]] void unexpected();
Remarks: Called by the implementation when a function exits via an exception not allowed by its exception-specification (15.5.2)
, in effect after evaluating the throw-expression (D.11.1). May also be called directly by the program.
Effects: Calls an
unexpected_handler
function. It is unspecified whichunexpected_handler
function will be called if an exception is active during a call toset_unexpected
. Otherwise cCalls the currentunexpected_handler
function. [Note: A defaultunexpected_handler
is always considered a callable handler in this context. — end note]
Section: 16.4.6 [conforming], 20.2.9 [allocator.traits], 20.6.1 [allocator.adaptor.syn] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-30 Last modified: 2016-01-28
Priority: 1
View all other issues in [conforming].
View all issues with C++14 status.
Discussion:
It is a very established technique for implementations to derive internally from user-defined class types that are used to customize some library component, e.g. deleters and allocators are typical candidates. The advantage of this approach is to possibly take advantage of the empty-base-class optimization (EBCO).
Whether or whether not libraries did take advantage of such a detail didn't much matter in C++03. Even though there did exist a portable idiom to prevent that a class type could be derived from, this idiom has never reached great popularity: The technique required to introduce a virtual base class and it did not really prevent the derivation, but only any construction of such a type. Further, such types are not empty as defined by thestd::is_empty
trait, so
could easily be detected by implementations from TR1 on.
With the new C++11 feature of final classes and final member functions it is now very easy to define an empty,
but not derivable from class type. From the point of the user it is quite natural to use this feature for
types that he or she did not foresee to be derivable from.
On the other hand, most library implementations (including third-party libraries) often take advantage of EBCO
applied to user-defined types used to instantiate library templates internally. As the time of submitting this
issue the following program failed to compile on all tested library implementations:
#include <memory>
struct Noop final {
template<class Ptr>
void operator()(Ptr) const {}
};
std::unique_ptr<int, Noop> up;
In addition, many std::tuple
implementations with empty, final classes as element types failed as well,
due to a popular inheritance-based implementation technique. EBCO has also a long tradition to be
used in library containers to efficiently store potentially stateless, empty allocators.
__is_final
or __is_derivable
to make EBCO possible in the current form but excluding non-derivable class types. As of this writing this
seems to happen already. Problem is that without a std::is_derivable
trait, third-party libraries
have no portable means to do the same thing as standard library implementations. This should be a good
reason to make such a trait public available soon, but seems not essential to have now. Further, this issue
should also be considered as a chance to recognice that EBCO has always been a very special corner case
(There exist parallels to the previously existing odd core language rule that did make the interplay
between std::auto_ptr
and std::auto_ptr_ref
possible) and that it would be better to
provide explicit means for space-efficient storage, not necessarily restricted to inheritance relations,
e.g. by marking data members with a special attribute.
At least two descriptions in the current standard should be fixed now for better clarification:
As mentioned by Ganesh, 20.2.9 [allocator.traits] p1 currently contains a (non-normative) note "Thus, it is always possible to create a derived class from an allocator." which should be removed.
As pointed out by Howard, the specification of scoped_allocator_adaptor
as of
20.6.1 [allocator.adaptor.syn] already requires derivation from OuterAlloc
, but
only implies indirectly the same for the inner allocators due to the exposition-only
description of member inner
. This indirect implication should be normatively required for
all participating allocators.
[2012, Kona]
What we really need is a type trait to indicate if a type can be derived from. Howard reports Clang and libc++ have had success with this approach.
Howard to provide wording, and AJM to alert Core that we may be wanting to add a new trait that requires compiler support.
[2014-02, Issaquah: Howard and Daniel comment and provide wording]
Several existing C++11 compilers do already provide an internal __is_final
intrinsic (e.g. clang and gcc) and therefore
we believe that this is evidence enough that this feature is implementable today.
is_final
query should result in a true outcome
if and only if the current existing language definition holds that a complete class type (either union or non-union)
has been marked with the class-virt-specifier final
— nothing more.
The following guidelines lead to the design decision and the wording choice given below:
It has been expressed several times that a high-level trait such as "is_derivable
" would be preferred and
would be more useful for non-experts. One problem with that request is that it is astonishingly hard to find a common denominator
for what the precise definition of this trait should be, especially regarding corner-cases. Another example of getting very
differing points of view is to ask a bunch of C++ experts what the best definition of the is_empty
trait
should be (which can be considered as a kind of higher-level trait).
Once we have a fundamental trait like is_final
available, we can easily define higher-level traits in the future
on top of this by a proper logical combination of the low-level traits.
A critical question is whether providing such a low-level compile-time introspection might be considered as disadvantageous,
because it could constrain the freedom of existing implementations even further and whether a high-level trait would solve
this dilemma. We assert that since C++11 the static introspection capabilities are already very large and we believe that
making the presence or absence of the final
keyword testable does not make the current situation worse.
Below code example demonstrates the intention and the implementability of this feature:
#include <type_traits> namespace std { template <class T> struct is_final : public integral_constant<bool, __is_final(T)> {}; } // std // test it union FinalUnion final { }; union NonFinalUnion { }; class FinalClass final { }; struct NonFinalClass { }; class Incomplete; int main() { using std::is_final; static_assert( is_final<const volatile FinalUnion>{}, ""); static_assert(!is_final<FinalUnion[]>{}, ""); static_assert(!is_final<FinalUnion[1]>{}, ""); static_assert(!is_final<NonFinalUnion>{}, ""); static_assert( is_final<FinalClass>{}, ""); static_assert(!is_final<FinalClass&>{}, ""); static_assert(!is_final<FinalClass*>{}, ""); static_assert(!is_final<NonFinalClass>{}, ""); static_assert(!is_final<void>{}, ""); static_assert(!is_final<int>{}, ""); static_assert(!is_final<Incomplete>{}, ""); // error incomplete type 'Incomplete' used in type trait expression }
[2014-02-14, Issaquah: Move to Immediate]
This is an important issue, that we really want to solve for C++14.
Move to Immediate after polling LEWG, and then the NB heads of delegation.
Proposed resolution:
This wording is relative to N3797.
Change 21.3.3 [meta.type.synop], header <type_traits>
synopsis, as indicated
namespace std { […] // 20.10.4.3, type properties: […] template <class T> struct is_empty; template <class T> struct is_polymorphic; template <class T> struct is_abstract; template <class T> struct is_final; […] }
Change 21.3.5.4 [meta.unary.prop], Table 49 — Type property predicates, as indicated
Template | Condition | Preconditions |
---|---|---|
template <class T> struct is_abstract;
|
[…] | […] |
template <class T> struct is_final;
|
T is a class type marked with the class-virt-specifier final (11 [class]).[Note: A union is a class type that can be marked with final . — end note]
|
If T is a class type, T shall be a complete type
|
After 21.3.5.4 [meta.unary.prop] p5 add one further example as indicated:
[Example:
// Given: struct P final { }; union U1 { }; union U2 final { }; // the following assertions hold: static_assert(!is_final<int>::value, "Error!"); static_assert( is_final<P>::value, "Error!"); static_assert(!is_final<U1>::value, "Error!"); static_assert( is_final<U2>::value, "Error!");— end example]
bool
" requirementsSection: 16.4.4.4 [nullablepointer.requirements], 24.3.5.3 [input.iterators], 24.3.5.7 [random.access.iterators], 26.1 [algorithms.general], 26.8 [alg.sorting], 32.2.1 [thread.req.paramname] Status: Resolved Submitter: Daniel Krügler Opened: 2011-12-09 Last modified: 2025-03-13
Priority: 3
View all other issues in [nullablepointer.requirements].
View all issues with Resolved status.
Discussion:
As of 16.4.4.2 [utility.arg.requirements] Table 17/18, the return types of the expressions
a == b
or
a < b
for types satisfying the EqualityComparable
or LessThanComparable
types, respectively, are required to be "convertible to bool
" which corresponds to
a copy-initialization context. But several newer parts of the library that refer to
such contexts have lowered the requirements taking advantage of the new terminology of
"contextually convertible to bool
" instead, which corresponds to a
direct-initialization context (In addition to "normal" direct-initialization constructions,
operands of logical operations as well as if
or switch
conditions also
belong to this special context).
EqualityComparable
but also specify that the expression
a != b
shall be just "contextually convertible to bool
". The same discrepancy
exists for requirement set NullablePointer
in regard to several equality-related expressions.
a < b
contextually convertible tobool
as well as for all derived comparison functions, so strictly speaking we could have a random access
iterator that does not satisfy the LessThanComparable
requirements, which looks like an
artifact to me.
LessThanComparable
or
EqualityComparable
we still would have the problem that some current specifications
are actually based on the assumption of implicit convertibility instead of "explicit convertibility", e.g.
20.3.1.6 [unique.ptr.special] p3:
template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);-3- Returns:
x.get() != y.get()
.
Similar examples exist in 20.3.1.3.3 [unique.ptr.single.dtor] p2, 20.3.1.3.4 [unique.ptr.single.asgn] p9, 20.3.1.3.5 [unique.ptr.single.observers] p1+3+8, etc.
In all these places the expressions involving comparison functions (but not those of the conversion of aNullablePointer
to bool
!) assume to be "convertible to bool
". I think this
is a very natural assumption and all delegations of the comparison functions of some type X
to some
other API type Y
in third-party code does so assuming that copy-initialization semantics will
just work.
The actual reason for using the newer terminology can be rooted back to LWG 556(i). My hypotheses
is that the resolution of that issue also needs a slight correction. Why so?
The reason for opening that issue were worries based on the previous "convertible to bool
"
wording. An expressions like "!pred(a, b)
" might not be well-formed in those situations, because
operator!
might not be accessible or might have an unusual semantics (and similarly for other logical
operations). This can indeed happen with unusual proxy return types, so the idea was that the evaluation of
Predicate
, BinaryPredicate
(26.1 [algorithms.general] p8+9), and Compare
(26.8 [alg.sorting] p2) should be defined based on contextual conversion to bool
.
Unfortunately this alone is not sufficient: In addition, I think, we also want the predicates
to be (implicitly) convertible to bool
! Without this wording, several conditions are plain wrong,
e.g. 26.6.6 [alg.find] p2, which talks about "pred(*i) != false
" (find_if
) and
"pred(*i) == false
" (find_if_not
). These expressions are not within a boolean context!
While we could simply fix all these places by proper wording to be considered in a "contextual conversion to
bool
", I think that this is not the correct solution: Many third-party libraries already refer to
the previous C++03 Predicate
definition — it actually predates C++98 and is as old as the
SGI specification. It seems to be a high price to
pay to switch to direct initialization here instead of fixing a completely different specification problem.
A final observation is that we have another definition for a Predicate
in 32.2.1 [thread.req.paramname] p2:
If a parameter is
Predicate
,operator()
applied to the actual template argument shall return a value that is convertible tobool
.
The problem here is not that we have two different definitions of Predicate
in the standard — this
is confusing, but this fact alone is not a defect. The first (minor) problem is that this definition does not properly
apply to function objects that are function pointers, because operator()
is not defined in a strict sense.
But the actually worse second problem is that this wording has the very same
problem that has originally lead to
LWG 556(i)! We only need to look at 32.7.4 [thread.condition.condvar] p15 to recognice this:
while (!pred()) wait(lock);
The negation expression here looks very familiar to the example provided in LWG 556(i) and is sensitive
to the same "unusual proxy" problem. Changing the 32.2.1 [thread.req.paramname] wording to a corresponding
"contextual conversion to bool
" wouldn't work either, because existing specifications rely on "convertible
to bool
", e.g. 32.7.4 [thread.condition.condvar] p32+33+42 or 32.7.5 [thread.condition.condvarany]
p25+26+32+33.
bool
" the actual problem of that
issue has not been fixed. What actually needs to be required here is some normative wording that basically
expresses something along the lines of:
The semantics of any contextual conversion to
bool
shall be equivalent to the semantics of any implicit conversion tobool
.
This is still not complete without having concepts, but it seems to be a better approximation. Another way of solving this issue would be to define a minimum requirements table with equivalent semantics. The proposed wording is a bit simpler but attempts to express the same thing.
[2012, Kona]
Agree with Daniel that we potentially broke some C++03 user code, accept the changes striking "contextually" from tables. Stefan to provide revised wording for section 25, and figure out changes to section 30.
Move to open, and then to Review when updated wording from Stefan is available.
[2012-10-12, STL comments]
The current proposed resolution still isn't completely satisfying. It would certainly be possible for the Standard to
require these various expressions to be implicitly and contextually convertible to bool
, but that would have
a subtle consequence (which, I will argue, is undesirable - regardless of the fact that it dates all the way back to
C++98/03). It would allow users to provide really wacky types to the Standard Library, with one of two effects:
Standard Library implementations would have to go to great lengths to respect such wacky types, essentially using
static_cast<bool>
when invoking any predicates or comparators.
Otherwise, such wacky types would be de facto nonportable, because they would make Standard Library implementations explode.
Effect B is the status quo we're living with today. What Standard Library implementations want to do with pred(args)
goes beyond "if (pred(args))
" (C++03), contextually converting pred(args)
to bool
(C++11), or
implicitly and contextually converting pred(args)
to bool
(the current proposed resolution).
Implementations want to say things like:
if (pred(args)) if (!pred(args)) if (cond && pred(args)) if (cond && !pred(args))
These are real examples taken from Dinkumware's implementation. There are others that would be realistic
("pred(args) && cond
", "cond || pred(args)
", etc.)
pred(args)
to be implicitly and contextually convertible to bool
doesn't prevent operator!()
from being overloaded and returning std::string
(as a wacky example). More
ominously, it doesn't prevent operator&&()
and operator||()
from being overloaded and destroying
short-circuiting.
I would like LWG input before working on Standardese for a new proposed resolution. Here's an outline of what I'd like to do:
Introduce a new "concept" in 16.4.4 [utility.requirements], which I would call BooleanTestable
in the
absence of better ideas.
Centralize things and reduce verbosity by having everything simply refer to BooleanTestable
when necessary.
I believe that the tables could say "Return type: BooleanTestable
", while Predicate/BinaryPredicate/Compare
would need the incantation "shall satisfy the requirements of BooleanTestable".
Resolve the tug-of-war between users (who occasionally want to do weird things) and implementers (who don't want to have to contort their code) by requiring that:
Given a BooleanTestable x
, x
is both implicitly and contextually convertible to bool
.
Given a BooleanTestable x
, !x
is BooleanTestable
. (This is intentionally "recursive".)
Given a BooleanTestable x
, bool t = x, t2(x), f = !x;
has the postcondition t == t2 && t != f
.
Given a BooleanTestable x
and a BooleanTestable y
of possibly different types, "x && y
"
and "x || y
" invoke the built-in operator&&()
and operator||()
, triggering short-circuiting.
bool
is BooleanTestable
.
I believe that this simultaneously gives users great latitude to use types other than bool
, while allowing
implementers to write reasonable code in order to get their jobs done. (If I'm forgetting anything that implementers
would want to say, please let me know.)
About requirement (I): As Daniel patiently explained to me, we need to talk about both implicit conversions and
contextual conversions, because it's possible for a devious type to have both "explicit operator bool()
"
and "operator int()
", which might behave differently (or be deleted, etc.).
About requirement (IV): This is kind of tricky. What we'd like to say is, "BooleanTestable
can't ever trigger
an overloaded logical operator". However, given a perfectly reasonable type Nice
- perhaps even bool
itself! -
other code (perhaps a third-party library) could overload operator&&(Nice, Evil)
. Therefore, I believe
that the requirement should be "no first use" - the Standard Library will ask for various BooleanTestable
types
from users (for example, the result of "first != last
" and the result of "pred(args)
"), and as long
as they don't trigger overloaded logical operators with each other, everything is awesome.
About requirement (V): This is possibly redundant, but it's trivial to specify, makes it easier for users to understand
what they need to do ("oh, I can always achieve this with bool
"), and provides a "base case" for requirement
(IV) that may or may not be necessary. Since bool
is BooleanTestable
, overloading
operator&&(bool, Other)
(etc.) clearly makes the Other
type non-BooleanTestable
.
This wording is relative to the FDIS.
Change Table 25 — "
NullablePointer
requirements" in 16.4.4.4 [nullablepointer.requirements] as indicated:
Table 25 — NullablePointer
requirementsExpression Return type Operational semantics […]
a != b
contextuallyconvertible tobool
!(a == b)
a == np
np == acontextuallyconvertible tobool
a == P()
a != np
np != acontextuallyconvertible tobool
!(a == np)
Change Table 107 — "Input iterator requirements" in 24.3.5.3 [input.iterators] as indicated:
Table 107 — Input iterator requirements (in addition to Iterator) Expression Return type Operational semantics Assertion/note
pre-/post-conditiona != b
contextuallyconvertible tobool
!(a == b)
pre: (a, b)
is in the domain of==
.[…]
Change Table 111 — "Random access iterator requirements" in 24.3.5.7 [random.access.iterators] as indicated:
Table 111 — Random access iterator requirements (in addition to bidirectional iterator) Expression Return type Operational semantics Assertion/note
pre-/post-condition[…]
a < b
contextuallyconvertible tobool
b - a > 0
<
is a total ordering relationa > b
contextuallyconvertible tobool
b < a
>
is a total ordering relation opposite to<
.a >= b
contextuallyconvertible tobool
!(a < b)
a <= b
contextuallyconvertible tobool
!(a > b)
Change 26.1 [algorithms.general] p8+9 as indicated:
-8- The
-9- ThePredicate
parameter is used whenever an algorithm expects a function object (22.10 [function.objects]) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable astrue
. In other words, if an algorithm takesPredicate pred
as its argument and first as its iterator argument, it should work correctly in the constructpred(*first)
implicitly or contextually converted tobool
(Clause 7.3 [conv]). The function objectpred
shall not apply any non-constant function through the dereferenced iterator.BinaryPredicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and typeT
whenT
is part of the signature returns a value testable astrue
. In other words, if an algorithm takesBinaryPredicate binary_pred
as its argument andfirst1
andfirst2
as its iterator arguments, it should work correctly in the constructbinary_pred(*first1, *first2)
implicitly or contextually converted tobool
(Clause 7.3 [conv]).BinaryPredicate
always takes the first iterator'svalue_type
as its first argument, that is, in those cases whenT
value is part of the signature, it should work correctly in the constructbinary_pred(*first1, value)
implicitly or contextually converted tobool
(Clause 7.3 [conv]).binary_pred
shall not apply any non-constant function through the dereferenced iterators.Change 26.8 [alg.sorting] p2 as indicated:
-2-
Compare
is a function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of typeCompare
, when implicitly or contextually converted tobool
(7.3 [conv]), yieldstrue
if the first argument of the call is less than the second, andfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.Change 32.2.1 [thread.req.paramname] p2 as indicated:
-2-
If a parameter isPredicate
, operator() applied to the actual template argument shall return a value that is convertible tobool
Predicate
is a function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of typePredicate
, when implicitly or contextually converted tobool
(7.3 [conv]), yieldstrue
if the corresponding test condition is satisfied, andfalse
otherwise.
[2014-05-20, Daniel suggests concrete wording based on STL's proposal]
The presented wording follows relatively closely STL's outline with the following notable exceptions:
A reference to BooleanTestable
in table "Return Type" specifications seemed very unusual to me and
I found no "prior art" for this in the Standard. Instead I decided to follow the usual style to add a symbol
with a specific meaning to a specific paragraph that specifies symbols and their meanings.
STL's requirement IV suggested to directly refer to built-in operators &&
and ||
. In my
opinion this concrete requirement isn't needed if we simply require that two BooleanTestable
operands behave
equivalently to two those operands after conversion to bool
(each of them).
I couldn't find a good reason to require normatively that type bool
meets the requirements of BooleanTestable
: My
assertion is that after having defined them, the result simply falls out of this. But to make this a bit clearer, I added
also a non-normative note to these effects.
[2014-06-10, STL comments]
In the current wording I would like to see changed the suggested changes described by bullet #6:
In 23.2.2 [container.requirements.general] p4 undo the suggested change
Then change the 7 occurrences of "convertible to bool
" in the denoted tables to "bool
".
[2015-05-05 Lenexa]
STL: Alisdair wanted to do something here, but Daniel gave us updated wording.
[2015-07 Telecon]
Alisdair: Should specify we don't break short circuiting.
Ville: Looks already specified because that's the way it works for bool.
Geoffrey: Maybe add a note about the short circuiting.
B2/P2 is somewhat ambiguous. It implies that B has to be both implicitly convertible to bool and contextually convertible to bool.
We like this, just have nits.
Status stays Open.
Marshall to ping Daniel with feedback.
[2016-02-27, Daniel updates wording]
The revised wording has been updated from N3936 to N4567.
To satisfy the Kona 2015 committee comments, the wording in
[booleantestable.requirements] has been improved to better separate the two different requirements of "can be
contextually converted to bool
" and "can be implicitly converted to bool
. Both are necessary because
it is possible to define a type that has the latter property but not the former, such as the following one:
2016-08-07, Daniel: The below example has been corrected to reduce confusion about the performed conversions as indicated by the delta markers:
using Bool = int; struct OddBoolean { explicit operator bool() const = delete; operator Bool() const;OddBoolean(bool) = delete; OddBoolean(Bool){}} ob; bool b2 = ob; // OK bool b1(ob); // ErrorOddBoolean b2 = true; // OK OddBoolean b1(true); // Error
In [booleantestable.requirements] a note has been added to ensure that an implementation is not allowed to break any short-circuiting semantics.
I decided to separate LWG 2587(i)/2588(i) from this issue. Both these issues aren't exactly the same but depending on the committee's position, their resolution might benefit from the new vocabulary introduced here.
[2021-06-25; Daniel comments]
The paper P2167R1 is provided to resolve this issue.
[2022-05-01; Daniel comments]
The paper P2167R2 is provided to resolve this issue.
Previous resolution [SUPERSEDED]:
This wording is relative to N4567.
Change 16.4.4.2 [utility.arg.requirements] p1, Table 17 — "EqualityComparable requirements", and Table 18 — "LessThanComparable requirements" as indicated:
-1- […] In these tables,
[…]T
is an object or reference type to be supplied by a C++ program instantiating a template;a
,b
, andc
are values of type (possiblyconst
)T
;s
andt
are modifiable lvalues of typeT
;u
denotes an identifier;rv
is an rvalue of typeT
;andv
is an lvalue of type (possiblyconst
)T
or an rvalue of typeconst T
; andBT
denotes a type that meets theBooleanTestable
requirements ([booleantestable.requirements]).
Table 17 — EqualityComparable
requirements [equalitycomparable]Expression Return type Requirement a == b
convertible to
bool
BT
==
is an equivalence relation, that is, it has the following properties: […][…]
Table 18 — LessThanComparable
requirements [lessthancomparable]Expression Return type Requirement a < b
convertible to
bool
BT
<
is a strict weak ordering relation (26.8 [alg.sorting])Between 16.4.4.3 [swappable.requirements] and 16.4.4.4 [nullablepointer.requirements] insert a new sub-clause as indicated:
?.?.?.?BooleanTestable
requirements [booleantestable.requirements]-?- A
An objectBooleanTestable
type is a boolean-like type that also supports conversions tobool
. A typeB
meets theBooleanTestable
requirements if the expressions described in Table ?? are valid and have the indicated semantics, and ifB
also satisfies all the other requirements of this sub-clause [booleantestable.requirements].b
of typeB
can be implicitly converted tobool
and in addition can be contextually converted tobool
(Clause 4). The result values of both kinds of conversions shall be equivalent. [Example: The typesbool
,std::true_type
, andstd::bitset<>::reference
areBooleanTestable
types. — end example] For the purpose of Table ??, letB2
andBn
denote types (possibly both equal toB
or to each other) that meet theBooleanTestable
requirements, letb1
denote a (possiblyconst
) value ofB
, letb2
denote a (possiblyconst
) value ofB2
, and lett1
denote a value of typebool
. [Note: These rules ensure what an implementation can rely on but doesn't grant it license to break short-circuiting behavior of aBooleanTestable
type. — end note]Somewhere within the new sub-clause [booleantestable.requirements] insert the following new Table (?? denotes the assigned table number):
Table ?? — BooleanTestable
requirements [booleantestable]Expression Return type Operational semantics bool(b1)
bool
Remarks: bool(b1) == t1
for every value
b1
implicitly converted tot1
.!b1
Bn
Remarks: bool(b1) == !bool(!b1)
for
every valueb1
.b1 && b2
bool
bool(b1) && bool(b2)
b1 || b2
bool
bool(b1) || bool(b2)
Change 16.4.4.4 [nullablepointer.requirements] p5 and Table 25 — "NullablePointer requirements" as indicated:
[…]
-5- In Table 25,u
denotes an identifier,t
denotes a non-const
lvalue of typeP
,a
andb
denote values of type (possiblyconst
)P
,andnp
denotes a value of type (possiblyconst
)std::nullptr_t
, andBT
denotes a type that meets theBooleanTestable
requirements ([booleantestable.requirements]). […]
Table 25 — NullablePointer
requirements [nullablepointer]Expression Return type Operational semantics …
a != b
contextually convertible tobool
BT
[…] a == np
np == a
contextually convertible tobool
BT
[…] a != np
np != a
contextually convertible tobool
BT
[…] Change 22.4.9 [tuple.rel] as indicated;
template<class... TTypes, class... UTypes> constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);-1- Requires: For all
[…]i
, where0 <= i
andi < sizeof...(TTypes)
,get<i>(t) == get<i>(u)
is a valid expression returning a type thatis convertible tomeets thebool
BooleanTestable
requirements ([booleantestable.requirements]).sizeof...(TTypes) == sizeof...(UTypes)
.template<class... TTypes, class... UTypes> constexpr bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);-4- Requires: For all
[…]i
, where0 <= i
andi < sizeof...(TTypes)
,get<i>(t) < get<i>(u)
andget<i>(u) < get<i>(t)
are valid expressions returning types thatare convertible tomeet thebool
BooleanTestable
requirements ([booleantestable.requirements]).sizeof...(TTypes) == sizeof...(UTypes)
.Change 23.2.2 [container.requirements.general], Table 95 — "Container requirements", and Table 97 — "Optional container operations" as indicated:
-4- In Tables 95, 96, and 97
X
denotes a container class containing objects of typeT
,a
andb
denote values of typeX
,u
denotes an identifier,r
denotes a non-const
value of typeX
,andrv
denotes a non-const
rvalue of typeX
, andBT
denotes a type that meets theBooleanTestable
requirements ([booleantestable.requirements]).
Table 95 — Container requirements Expression Return type […] …
a == b
convertible to
bool
BT
[…] a != b
convertible to
bool
BT
[…] …
a.empty()
convertible to
bool
BT
[…] […]
Table 97 — Optional container requirements Expression Return type […] …
a < b
convertible to
bool
BT
[…] a > b
convertible to
bool
BT
[…] a <= b
convertible to
bool
BT
[…] a >= b
convertible to
bool
BT
[…] Change 24.3.1 [iterator.requirements.general], Table 106 — "Input iterator requirements", and Table 110 — "Random access iterator requirements" as indicated:
-12- In the following sections,
a
andb
denote values of typeX
orconst X
,difference_type
andreference
refer to the typesiterator_traits<X>::difference_type
anditerator_traits<X>::reference
, respectively,n
denotes a value ofdifference_type
,u
,tmp
, andm
denote identifiers,r
denotes a value ofX&
,t
denotes a value of value typeT
,o
denotes a value of some type that is writable to the output iterator, andBT
denotes a type that meets theBooleanTestable
requirements ([booleantestable.requirements]).
Table 106 — Input iterator requirements Expression Return type […] a != b
contextually convertible to
bool
BT
[…] […]
Table 110 — Random access iterator requirements Expression Return type […] …
a < b
contextually convertible to
bool
BT
[…] a > b
contextually convertible to
bool
BT
[…] a >= b
contextually convertible to
bool
BT
[…] a <= b
contextually convertible to
bool
BT
[…] Change 26.1 [algorithms.general] p8+p9 as indicated:
[Drafting note: The wording changes below also fix (a) unusual wording forms used ("should work") which are unclear in which sense they are imposing normative requirements and (b) the problem, that the current wording seems to allow that the predicate may mutate a call argument, if that is not a dereferenced iterator. Upon applying the new wording it became obvious that the both the previous and the new wording has the effect that currently algorithms such as
adjacent_find
,search_n
,unique
, andunique_copy
are not correctly described (because they have no iterator argument namedfirst1
), which could give raise to a new library issue. — end drafting note]-8- The
-9- ThePredicate
parameter is used whenever an algorithm expects a function object (20.9) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable astrue
.In other words, iIf an algorithm takesPredicate pred
as its argument andfirst
as its iterator argument,it should work correctly in the constructthe expressionpred(*first)
contextually converted tobool
(Clause 4)pred(*first)
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]). The function objectpred
shall not apply any non-constant function throughthe dereferenced iteratorits argument.BinaryPredicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and typeT
whenT
is part of the signature returns a value testable astrue
.In other words, iIf an algorithm takesBinaryPredicate binary_pred
as its argument andfirst1
andfirst2
as its iterator arguments,it should work correctly in the constructthe expressionbinary_pred(*first1, *first2)
contextually converted tobool
(Clause 4)binary_pred(*first1, *first2)
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]).BinaryPredicate
always takes the first iterator'svalue_type
as its first argument, that is, in those cases whenT
value is part of the signature,it should work correctly in the constructthe expressionbinary_pred(*first1, value)
contextually converted tobool
(Clause 4)binary_pred(*first1, value)
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]).binary_pred
shall not apply any non-constant function throughthe dereferenced iteratorsany of its arguments.Change 26.8 [alg.sorting] p2 as indicated:
[…]
-2-Compare
is a function object type (20.9).The return value of the function call operation applied to an object of typeCompare
, when contextually converted tobool
(Clause 4), yieldstrue
if the first argument of the call is less than the second, andfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. Leta
andb
denote two argument values whose types depend on the corresponding algorithm. Then the expressioncomp(a, b)
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]). The return value ofcomp(a, b)
, converted tobool
, yieldstrue
if the first argumenta
is less than the second argumentb
, andfalse
otherwise. It is assumed thatcomp
will not apply any non-constant function throughthe dereferenced iteratorany of its arguments. […]Change 31.5.3.3 [fpos.operations] and Table 126 — "Position type requirements" as indicated:
-1- Operations specified in Table 126 are permitted. In that table,
P
refers to an instance offpos
,[…]
o
refers to a value of typestreamoff
,
BT
refers to a type that meets theBooleanTestable
requirements ([booleantestable.requirements]),[…]
Table 126 — Position type requirements Expression Return type […] …
p == q
convertible tobool
BT
[…] p != q
convertible tobool
BT
[…] Change 32.2.1 [thread.req.paramname] p1 as indicated:
-1- Throughout this Clause, the names of template parameters are used to express type requirements.
If a template parameter is namedPredicate
,operator()
applied to the template argument shall return a value that is convertible tobool
Predicate
is a function object type (22.10 [function.objects]). Letpred
denote an lvalue of typePredicate
. Then the expressionpred()
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]). The return value ofpred()
, converted tobool
, yieldstrue
if the corresponding test condition is satisfied, andfalse
otherwise.
[2022-11-05; Daniel comments]
The paper P2167R3 has been reviewed and accepted by LWG and would solve this issue.
[2022-11-22 Resolved by P2167R3 accepted in Kona. Status changed: Open → Resolved.]
Proposed resolution:
unique_ptr
for array does not support cv qualification conversion of actual argumentSection: 20.3.1.4 [unique.ptr.runtime] Status: Resolved Submitter: Alf P. Steinbach Opened: 2011-12-16 Last modified: 2017-09-07
Priority: 1
View all other issues in [unique.ptr.runtime].
View all issues with Resolved status.
Discussion:
Addresses US 16
N3290 20.3.1.4.2 [unique.ptr.runtime.ctor] "unique_ptr
constructors":
These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to
pointer
. [Note: One implementation technique is to create private templated overloads of these members. — end note]
This language excludes even pointer
itself as type for the actual argument.
#include <memory>
using namespace std;
struct T {};
T* foo() { return new T; }
T const* bar() { return foo(); }
int main()
{
unique_ptr< T const > p1( bar() ); // OK
unique_ptr< T const [] > a1( bar() ); // OK
unique_ptr< T const > p2( foo() ); // OK
unique_ptr< T const [] > a2( foo() ); // ? this is line #15
}
The intent seems to be clearly specified in 20.3.1.4 [unique.ptr.runtime]/1 second bullet:
— Pointers to types derived from
T
are rejected by the constructors, and byreset
.
But the following language in 20.3.1.4.2 [unique.ptr.runtime.ctor] then rejects far too much...
Proposed new wording of N3290 20.3.1.4.2 [unique.ptr.runtime.ctor] "unique_ptr
constructors":
These constructors behave the same as in the primary template except that actual argument pointers
p
to types derived fromT
are rejected by the constructors. [Note: One implementation technique is to create private templated overloads of these members. — end note]
This will possibly capture the intent better, and avoid the inconsistency between the non-array and array
versions of unique_ptr
, by using nearly the exact same phrasing as for the paragraph explaining
the intent.
[2012-08-25 Geoffrey Romer comments in c++std-lib-32978]
The current P/R seems to intend to support at least two different implementation techniques — additional unusable templates that catch forbidden arguments or replacing existing constructors by templates that ensure ill-formed code inside the template body, when the requirements are not met. It seems unclear whether the current wording allows the second approach, though. It should be considered to allow both strategies or if that is not possible the note should be clearer.
The very same problem exists for the reset
member function, but even worse, because the current
specification is more than clear that the deleted reset
function will catch all cases not equal to
pointer
. It seems confusing at best to have different policies for the constructor and for the reset
function. In this case, the question in regard to implementation freedom mentioned above is even more important.
It's awkward to refer to "the constructors" twice in the same sentence; I suggest revising the sentence as
"...except that they do not accept argument pointers p
to types derived from T
"
[2012-12-20: Geoffrey Romer comments and provides a revised resolution]
The array specialization of unique_ptr
differs from the primary template in several ways,
including the following:
unique_ptr<T[], D>
cannot be constructed from a plain pointer whose type is not
exactly unique_ptr<T[], D>::pointer
or nullptr_t
.
unique_ptr<T[], D>
cannot be constructed from a unique_ptr<U[], E>&&
unless U
is exactly T
and E
is exactly D
.
unique_ptr<T[], D>
cannot be moveassigned from a unique_ptr<U[], E>&&
unless U
is exactly T
and E
is exactly D
.
unique_ptr<T[], D>::reset
cannot take an argument whose type is not exactly
unique_ptr<T[], D>::pointer
or nullptr_t
.
default_delete<T[]>
cannot be constructed from a default_delete<U[]>
unless
U
is exactly T
.
default_delete<T[]>::operator()
cannot be called on a pointer whose type is not
exactly T*
.
The common intent of all these restrictions appears to be to disallow implicit conversions from pointer-to-derived-class to pointer-to-base-class in contexts where the pointer is known to point to an array, because such conversions are inherently unsafe; deleting or subscripting the result of such a conversion leads to undefined behavior (see also CWG 1504). However, these restrictions have the effect of disallowing all implicit conversions in those contexts, including most notably cv-qualification, but also user-defined conversions, and possibly others. This PR narrows all those restrictions, to disallow only unsafe pointer-to-derived to pointer-to-base conversions, while allowing all others.
I removed the nebulous language stating that certain functions "will not accept" certain arguments. Instead I use explicitly deleted template functions, which participate in overload resolution only for pointer-to-derived to pointer-to-base conversions. This is more consistent with the existing text and easier to express correctly than an approach based on declaring certain types of calls to be ill-formed, but may produce inferior compiler diagnostics. Wherever possible, this PR defines the semantics of template specializations in terms of their differences from the primary template. This improves clarity and minimizes the risk of unintended differences (e.g. LWG 2169(i), which this PR also fixes). This PR also makes it explicit that the specialization inherits the description of all members, not just member functions, from the primary template and, in passing, clarifies the default definition of pointer in the specialization. This resolution only disallows pointer-to-derived to pointer-to-base conversions between ordinary pointer types; if user-defined pointer types provide comparable conversions, it is their responsibility to ensure they are safe. This is consistent with C++'s general preference for expressive power over safety, and for assuming the user knows what they're doing; furthermore, enforcing such a restriction on user-defined types appears to be impractical without cooperation from the user. The "base class without regard to cv-qualifiers" language is intended to parallel the specification ofstd::is_base_of
.
Jonathan Wakely has a working implementation of this PR patched into libstdc++.
Previous resolution:
This wording is relative to the FDIS.
Change 20.3.1.4.2 [unique.ptr.runtime.ctor] as indicated:
explicit unique_ptr(pointer p) noexcept; unique_ptr(pointer p, see below d) noexcept; unique_ptr(pointer p, see below d) noexcept;These constructors behave the same as in the primary template except that
they do not accept pointer types which are convertible toargument pointerspointer
p
to types derived fromT
are rejected by the constructors. [Note: One implementation technique is to create private templated overloads of these members. — end note]
[2014-02, Issaquah]
GR: want to prevent unsafe conversions. Standard is inconsistent how it does this. for reset()
has deleted function
template capturing everything except the known-safe cases. Other functions use SFINAE. Main reason this is hard is that
unique_ptr
supports fancy pointers. Have to figure out how to handle them and what requirements to put on them.
Requirements are minimal, not even required to work with pointer_traits
.
pointer_traits
doesn't work
GR: ways to get fancy pointers to work is to delegate responsibility for preventing unsafe conversions to the fancy pointers themselves.
Howard doesn't like that, he wants even fancy pointers to be prevented from doing unsafe conversions in unique_ptr
contexts.
AM: Howard says unique_ptr
was meant to be very very safe under all conditions, this open a hole in that. Howard wants to
eke forward and support more, but not if we open any holes in type safety.
GR: do we need to be typesafe even for fancy types with incorrect pointer_traits
?
AM: that would mean it's only unsafe for people who lie by providing a broken specialization of pointer_traits
GR: probably can't continue with ambiguity between using SFINAE and ill-formedness. Would appreciate guidance on direction used for that.
STL: difference is observable in convertibility using type traits.
STL: for reset()
which doesn't affect convertibility ill-formed allows static_assert
, better diagnostic.
For assignment it's detectable and has traits, constraining them is better.
EN: I strongly prefer constraints than static_asserts
STL: if we could rely on pointer_traits
that might be good. Alternatively could we add more machinery to deleter?
make deleter say conversions are allowed, otherwise we lock down all conversions. basically want to know if converting U
to
T
is safe.
Previous resolution [SUPERSEDED]:
This wording is relative to N3485.
Revise 20.3.1.2.3 [unique.ptr.dltr.dflt1] as follows
namespace std { template <class T> struct default_delete<T[]> { constexpr default_delete() noexcept = default; template <class U> default_delete(const default_delete<U>&) noexcept; void operator()(T*) const; template <class U> void operator()(U*) const = delete; }; }-?- Descriptions are provided below only for member functions that have behavior different from the primary template.
template <class U> default_delete(const default_delete<U>&) noexcept;-?- This constructor behaves the same as in the primary template except that it shall not participate in overload resolution unless:
U
is an array type, and
V*
is implicitly convertible toT*
, and
T
is not a base class ofV
(without regard to cv-qualifiers),where
V
is the array element type ofU
.void operator()(T* ptr) const;-1- Effects: calls
-2- Remarks: Ifdelete[]
onptr
.T
is an incomplete type, the program is ill-formed.template <class U> void operator()(U*) const = delete;-?- Remarks: This function shall not participate in overload resolution unless
T
is a base class ofU
(without regard to cv-qualifiers).Revise 20.3.1.3 [unique.ptr.single]/3 as follows:
If the type
remove_reference<D>::type::pointer
exists, thenunique_ptr<T, D>::pointer
shall be a synonym forremove_reference<D>::type::pointer
. Otherwiseunique_ptr<T, D>::pointer
shall be a synonym for. The type
Telement_type*unique_ptr<T, D>::pointer
shall satisfy the requirements ofNullablePointer
(16.4.4.4 [nullablepointer.requirements]).Revise 20.3.1.4 [unique.ptr.runtime] as follows:
namespace std { template <class T, class D> class unique_ptr<T[], D> { public: typedef see below pointer; typedef T element_type; typedef D deleter_type; // 20.7.1.3.1, constructors constexpr unique_ptr() noexcept; explicit unique_ptr(pointer p) noexcept; template <class U> explicit unique_ptr(U* p) = delete; unique_ptr(pointer p, see below d) noexcept; template <class U> unique_ptr(U* p, see below d) = delete; unique_ptr(pointer p, see below d) noexcept; template <class U> unique_ptr(U* p, see below d) = delete; unique_ptr(unique_ptr&& u) noexcept; constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { } template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept; // destructor ~unique_ptr(); // assignment unique_ptr& operator=(unique_ptr&& u) noexcept; template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept; unique_ptr& operator=(nullptr_t) noexcept; // 20.7.1.3.2, observers T& operator[](size_t i) const; pointer get() const noexcept; deleter_type& get_deleter() noexcept; const deleter_type& get_deleter() const noexcept; explicit operator bool() const noexcept; // 20.7.1.3.3 modifiers pointer release() noexcept; void reset(pointer p = pointer()) noexcept;void reset(nullptr_t) noexcept;template <class U> void reset(U*) = delete; void swap(unique_ptr& u) noexcept; // disable copy from lvalue unique_ptr(const unique_ptr&) = delete; unique_ptr& operator=(const unique_ptr&) = delete; }; }-1- A specialization for array types is provided with a slightly altered interface.
Conversions
between different types offromunique_ptr<T[], D>
unique_ptr<Derived[]>
tounique_ptr<Base[]>
, whereBase
is a base class ofDerived
, fromauto_ptr
, or to or from the non-array forms ofunique_ptr
produce an ill-formed program.Pointers to types derived from
T
are rejected by the constructors, and byreset
.The observers
operator*
andoperator->
are not provided.The indexing observer
operator[]
is provided.The default deleter will call
delete[]
.-2- Descriptions are provided below only for
-3- The template argumentmember functions that have behavior differentmembers that differ from the primary template.T
shall be a complete type.Revise 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:
explicit unique_ptr(pointer p) noexcept; unique_ptr(pointer p, see below d) noexcept; unique_ptr(pointer p, see below d) noexcept;template <class U> explicit unique_ptr(U* p) = delete; template <class U> unique_ptr(U* p, see below d) = delete; template <class U> unique_ptr(U* p, see below d) = delete;
These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to pointer. [Note: One implementation technique is to create private templated overloads of these members. — end note]These constructors shall not participate in overload resolution unless:
pointer
is a pointer type, and
U*
is implicitly convertible topointer
, and
T
is a base class ofU
(without regard to cv-qualifiers).The type of
d
is determined as in the corresponding non-deleted constructors.template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;-?- This constructor behaves the same as in the primary template, except that it shall not participate in overload resolution unless:
unique_ptr<U, E>::pointer
is implicitly convertible topointer
, and
U
is an array type, andeither
D
is a reference type andE
is the same type asD
, orD
is not a reference type andE
is implicitly convertible toD
, andeither at least one of
pointer
andunique_ptr<U, E>::pointer
is not a pointer type, orT
is not a base class of the array element type ofU
(without regard to cv-qualifiers).Insert a new sub-clause following 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:
??
unique_ptr
assignment [unique.ptr.runtime.asgn]template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;-?- This operator behaves the same as in the primary template, except that it shall not participate in overload resolution unless:
unique_ptr<U, E>::pointer
is implicitly convertible topointer
, and
U
is an array type, andeither
D
is a reference type andE
is the same type asD
, orD
is not a reference type andE
is implicitly convertible toD
, andeither at least one of
pointer
andunique_ptr<U, E>::pointer
is not a pointer type, orT
is not a base class of the array element type ofU
(without regard to cv-qualifiers).Revise 20.3.1.4.5 [unique.ptr.runtime.modifiers] as follows:
void reset(pointer p = pointer()) noexcept; void reset(nullptr_t p) noexcept;template <class U> void reset(U*) = delete;
-1- Effects: Ifget() == nullptr
there are no effects. Otherwiseget_deleter()(get())
.-2- Postcondition:-?- This function shall not participate in overload resolution unless:get() == p
.
pointer
is a pointer type, and
U*
is implicitly convertible topointer
, and
T
is a base class ofU
(without regard to cv-qualifiers).
[2014-06 Rapperswil]
Discussion of N4042 and general agreement that this paper resolves the substance of this issue and should be adopted with minor edits. Geoffrey Romer will provide an updated paper.
[2014-06 post-Rapperswil]
As described in N4089.
[2014-11-07 Urbana]
Resolved by N4089
Proposed resolution:
See proposed wording in N4089.
hash
specializations for extended integer typesSection: 22.10.19 [unord.hash] Status: C++17 Submitter: Daniel Krügler Opened: 2011-12-16 Last modified: 2017-07-30
Priority: 3
View all other issues in [unord.hash].
View all issues with C++17 status.
Discussion:
According to the header <functional>
synopsis 22.10 [function.objects]
and to the explicit description in 22.10.19 [unord.hash] class template
hash
specializations shall be provided for all arithmetic types that are
not extended integer types. This is not explicitly mentioned, but neither the list
nor any normative wording does include them, so it follows by implication.
numeric_limits
corresponding specializations are required. I would
expect that an unordered_map
with key type std::uintmax_t
would
just work, but that depends now on whether this type is an extended integer type
or not.
This issue is not asking for also providing specializations for the
cv-qualified arithmetic types. While this is surely a nice-to-have feature,
I consider that restriction as a more secondary problem in practice.
The proposed resolution also fixes a problem mentioned in 2109(i) in regard
to confusing requirements on user-defined types and those on implementations.
[2012, Kona]
Move to Open.
Agreed that it's a real issue and that the proposed wording fixes it. However, the wording change is not minimal and isn't consistent with the way we fixed hash wording elsewhere.
Alisdair will provide updated wording.
[2014-05-06 Geoffrey Romer suggests alternative wording]
Previous resolution from Daniel [SUPERSEDED]:
This wording is relative to the FDIS.
Change 22.10.19 [unord.hash] p2 as indicated:
template <> struct hash<bool>; template <> struct hash<char>; […] template <> struct hash<long double>; template <class T> struct hash<T*>;-2-
Requires: the template specializations shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<functional>
provides definitions for specializations of thehash
class template for each cv-unqualified arithmetic type. This header also provides a definition for a partial specialization of thehash
class template for any pointer type. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].
[2015-05, Lenexa]
STL: the new PR is very simple and could resolve that nicely
MC: the older PR is rather longish
anybody have any objections to this approach?
what people want to have as a status?
STL: I want to have Ready
MC: move to ready: in favor: 13, opposed: 0, abstain: 4
Proposed resolution:
This wording is relative to N3936.
Change 22.10.19 [unord.hash] p1 as indicated:
The unordered associative containers defined in 23.5 use specializations of the class template
hash
as the default hash function. For all object typesKey
for which there exists a specializationhash<Key>
, and for all integral and enumeration types (7.2)Key
, the instantiationhash<Key>
shall: […]
async
do if neither 'async
' nor 'deferred
' is set in policy?Section: 32.10.9 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-01-01 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.async].
View all other issues in [futures.async].
View all issues with C++14 status.
Discussion:
Implementations already disagree, one returns an invalid future with
no shared state, one chooses policy == async
and one chooses policy ==
deferred
, see c++std-lib-30839, c++std-lib-30840 and c++std-lib-30844.
It's not clear if returning an invalid future is allowed by the current wording.
If the intention is to allow an empty future to be returned, then 32.10.9 [futures.async] p3 and p4 should be adjusted to clarify that a shared state might not be created and an invalid future might be returned.
If the intention is that a valid future is always returned, p3 should say something about the case where none of the conditions applies.
[2012, Portland: move to Review]
We could make it undefined if no launch policy is defined.
Hans: If no launch policy is specified the behaviour is undefined
Artur: or implementation defined?
Hans: no: we don't want people to do this
[Proposed wording]
This wording is relative to N3376
Add a third bullet to the end of the list in 30.6.8p3
"if no valid launch policy is provided the behaviour is undefined"
Moved to review
[2013-04-19, Bristol]
Detlef provides new wording
Previous wording:
[This wording is relative to N3376]
Add a third bullet to the end of the list in 32.10.9 [futures.async]p3
– if no valid launch policy is provided the behaviour is undefined
[2013-09 Chicago]
If no policy is given, it should be undefined, so moved to Immediate.
Accept for Working Paper
Proposed resolution:
[This wording is relative to N3485]
Add a third bullet to the end of the list in 32.10.9 [futures.async]p3
– If no value is set in the launch policy, or a value is set that is neither specified in this International Standard or by the implementation, the behaviour is undefined.
merge()
stability for lists versus forward listsSection: 23.3.11.5 [list.ops], 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Nicolai Josuttis Opened: 2012-01-15 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [list.ops].
View all issues with C++14 status.
Discussion:
forward_list::merge()
is specified in [forwardlist.ops], p19 as follows:
This operation shall be stable: for equivalent elements in the two lists, the elements from
*this
shall always precede the elements fromx
.
But list::merge()
is only specified in 23.3.11.5 [list.ops], p24 as follows:
Remarks: Stable.
Note that in general we define "stable" only for algorithms (see 3.60 [defns.stable] and 16.4.6.8 [algorithm.stable]) so for member function we should explain it everywhere we use it.
Thus for lists we have to add:Stable: for equivalent elements in the two lists, the elements from the list always precede the elements from the argument list.
This, BTW, was the specification we had with C++03.
In addition, I wonder whether we also have some guarantees regarding stability saying that the order of equivalent elements of each list merged remains stable (which would be my interpretation of just saying "stable", BTW). Thus, I'd expect that for equivalent elements we guarantee that*this
(in the same order as on entry)[2012, Kona]
Move to Open.
STL says we need to fix up 17.6.5.7 to be stronger, and then the remarks for merge should just say "Remarks: Stable (see 17.6.5.7)"
Assigned to STL for word-smithing.
[ 2013-04-14 STL provides rationale and improved wording ]
Step 1: Centralize all specifications of stability to 16.4.6.8 [algorithm.stable].
Step 2: 3.60 [defns.stable] and 16.4.6.8 [algorithm.stable] talk about "algorithms", without mentioning "container member functions". There's almost no potential for confusion here, but there's a simple way to increase clarity without increasing verbosity: make the container member functions explicitly cite 16.4.6.8 [algorithm.stable]. For consistency, we can also update the non-member functions.
Step 3: Fix the "so obvious, we forgot to say it" bug in 16.4.6.8 [algorithm.stable]: a "stable" merge of equivalent elements A B C D and W X Y Z produces A B C D W X Y Z, never D C B A X W Z Y.
Step 3.1: Say "(preserving their original order)" to be consistent with "the relative order [...] is preserved" in 16.4.6.8 [algorithm.stable]'s other bullet points.
Step 4: Copy part of list::merge()
's wording to forward_list::merge()
, in order to properly connect
with 16.4.6.8 [algorithm.stable]'s "first range" and "second range".
[2013-04-18, Bristol]
Original wording saved here:
This wording is relative to the FDIS.
Change 23.3.11.5 [list.ops] as indicated:
void merge(list<T,Allocator>& x); void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>& x, Compare comp); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);[…]
-24- Remarks:StableThis operation shall be stable: for equivalent elements in the two lists, the elements from*this
shall always precede the elements fromx
and the order of equivalent elements of*this
andx
remains stable. If(&x != this)
the range[x.begin(), x.end())
is empty after the merge. No elements are copied by this operation. The behavior is undefined ifthis->get_allocator() != x.get_allocator()
.Change [forwardlist.ops] as indicated:
void merge(forward_list<T,Allocator>& x); void merge(forward_list<T,Allocator>&& x); template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp); template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);[…]
-19- Effects: Mergesx
into*this
. This operation shall be stable: for equivalent elements in the two lists, the elements from*this
shall always precede the elements fromx
and the order of equivalent elements of*this
andx
remains stable.x
is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.
Proposed resolution:
This wording is relative to the N3485.
Change 16.4.6.8 [algorithm.stable]/1 as indicated:
When the requirements for an algorithm state that it is “stable” without further elaboration, it means:
[…]
- For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).
Change [forwardlist.ops] as indicated:
void remove(const T& value); template <class Predicate> void remove_if(Predicate pred);-12- Effects: Erases all the elements in the list referred by a list iterator
-13- Throws: Nothing unless an exception is thrown by the equality comparison or the predicate. -??- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]i
for which the following conditions hold:*i == value
(forremove()
),pred(*i)
is true (forremove_if()
).This operation shall be stable: the relative order of the elements that are not removed is the same as their relative order in the original list.Invalidates only the iterators and references to the erased elements.void merge(forward_list& x); void merge(forward_list&& x); template <class Compare> void merge(forward_list& x, Compare comp) template <class Compare> void merge(forward_list&& x, Compare comp)[…]
-19- Effects: Mergesthe two sorted rangesx
into*this
[begin(), end())
and[x.begin(), x.end())
.This operation shall be stable: for equivalent elements in the two lists, the elements from*this
shall always precede the elements fromx
.x
is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
. -20- Remarks: Stable (16.4.6.8 [algorithm.stable]). The behavior is undefined ifthis->get_allocator() != x.get_allocator()
. […]void sort(); template <class Compare> void sort(Compare comp);[…]
-23- Effects: Sorts the list according to theoperator<
or thecomp
function object.This operation shall be stable: the relative order of the equivalent elements is preserved.If an exception is thrown the order of the elements in*this
is unspecified. Does not affect the validity of iterators and references. -??- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]
Change 23.3.11.5 [list.ops] as indicated:
void remove(const T& value); template <class Predicate> void remove_if(Predicate pred);[…]
-17- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]void merge(list& x); void merge(list&& x); template <class Compare> void merge(list& x, Compare comp) template <class Compare> void merge(list&& x, Compare comp)[…]
-24- Remarks: Stable (16.4.6.8 [algorithm.stable]). […] […]void sort(); template <class Compare> void sort(Compare comp);[…]
-30- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]
Change 26.7.1 [alg.copy]/12 as indicated:
template<class InputIterator, class OutputIterator, class Predicate> OutputIterator copy_if(InputIterator first, InputIterator last, OutputIterator result, Predicate pred);[…]
-12- Remarks: Stable (16.4.6.8 [algorithm.stable]).
Change 26.7.8 [alg.remove] as indicated:
template<class ForwardIterator, class T> ForwardIterator remove(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class Predicate> ForwardIterator remove_if(ForwardIterator first, ForwardIterator last, Predicate pred);[…]
-4- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]template<class InputIterator, class OutputIterator, class T> OutputIterator remove_copy(InputIterator first, InputIterator last, OutputIterator result, const T& value); template<class InputIterator, class OutputIterator, class Predicate> OutputIterator remove_copy_if(InputIterator first, InputIterator last, OutputIterator result, Predicate pred);[…]
-11- Remarks: Stable (16.4.6.8 [algorithm.stable]).
Change 26.8.2.2 [stable.sort]/4 as indicated:
template<class RandomAccessIterator> void stable_sort(RandomAccessIterator first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void stable_sort(RandomAccessIterator first, RandomAccessIterator last, Compare comp);[…]
-4- Remarks: Stable (16.4.6.8 [algorithm.stable]).
Change 26.8.6 [alg.merge] as indicated:
template<class InputIterator1, class InputIterator2, class OutputIterator> OutputIterator merge(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, OutputIterator result); template<class InputIterator1, class InputIterator2, class OutputIterator, class Compare> OutputIterator merge(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, OutputIterator result, Compare comp);[…]
-5- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]template<class BidirectionalIterator> void inplace_merge(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> void inplace_merge(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last, Compare comp);[…]
-9- Remarks: Stable (16.4.6.8 [algorithm.stable]).
merge()
allocator requirements for lists versus forward listsSection: 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Nicolai Josuttis Opened: 2012-01-15 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
View all issues with C++14 status.
Discussion:
Sub-clause 23.3.11.5 [list.ops], p24 states for lists:
The behavior is undefined if
this->get_allocator() != x.get_allocator()
.
But there is nothing like that for forward lists in [forwardlist.ops], although I would expect the same undefined behavior there.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Add a new paragraph after [forwardlist.ops] p19 as indicated:
void merge(forward_list<T,Allocator>& x); void merge(forward_list<T,Allocator>&& x); template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp); template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);[…]
-19- Effects: […] -?- Remarks: The behavior is undefined ifthis->get_allocator() != x.get_allocator()
.
raw_storage_iterator
Section: 99 [depr.storage.iterator] Status: C++17 Submitter: Jonathan Wakely Opened: 2012-01-23 Last modified: 2017-07-30
Priority: 3
View all other issues in [depr.storage.iterator].
View all issues with C++17 status.
Discussion:
Aliaksandr Valialkin pointed out that raw_storage_iterator
only supports constructing
a new object from lvalues so cannot be used to construct move-only types:
template <typename InputIterator, typename T> void move_to_raw_buffer(InputIterator first, InputIterator last, T *raw_buffer) { std::move(first, last, std::raw_storage_iterator<T *, T>(raw_buffer)); }
This could easily be solved by overloading operator=
for rvalues.
raw_storage_iterator
causes exception-safety problems when used with any
generic algorithm. I suggest leaving it alone and not encouraging its use.
[2014-11-11, Jonathan provides improved wording]
In Urbana LWG decided to explicitly say the value is constructed from an rvalue.
Previous resolution from Jonathan [SUPERSEDED]:This wording is relative to N3337.
Add a new signature to the synopsis in [storage.iterator] p1:
namespace std { template <class OutputIterator, class T> class raw_storage_iterator : public iterator<output_iterator_tag,void,void,void,void> { public: explicit raw_storage_iterator(OutputIterator x); raw_storage_iterator<OutputIterator,T>& operator*(); raw_storage_iterator<OutputIterator,T>& operator=(const T& element); raw_storage_iterator<OutputIterator,T>& operator=(T&& element); raw_storage_iterator<OutputIterator,T>& operator++(); raw_storage_iterator<OutputIterator,T> operator++(int); }; }Insert the new signature and a new paragraph before p4:
raw_storage_iterator<OutputIterator,T>& operator=(const T& element); raw_storage_iterator<OutputIterator,T>& operator=(T&& element);-?- Requires: For the first signature
-4- Effects: Constructs a value fromT
shall beCopyConstructible
. For the second signatureT
shall beMoveConstructible
.element
at the location to which the iterator points. -5- Returns: A reference to the iterator.
[2015-05, Lenexa]
MC: Suggestion to move it to Ready for incorporation on Friday
MC: move to ready: in favor: 12, opposed: 0, abstain: 3
Proposed resolution:
This wording is relative to N4140.
Add a new signature to the synopsis in [storage.iterator] p1:
namespace std { template <class OutputIterator, class T> class raw_storage_iterator : public iterator<output_iterator_tag,void,void,void,void> { public: explicit raw_storage_iterator(OutputIterator x); raw_storage_iterator<OutputIterator,T>& operator*(); raw_storage_iterator<OutputIterator,T>& operator=(const T& element); raw_storage_iterator<OutputIterator,T>& operator=(T&& element); raw_storage_iterator<OutputIterator,T>& operator++(); raw_storage_iterator<OutputIterator,T> operator++(int); }; }
Insert a new paragraph before p4:
raw_storage_iterator<OutputIterator,T>& operator=(const T& element);-?- Requires:
-4- Effects: Constructs a value fromT
shall beCopyConstructible
.element
at the location to which the iterator points. -5- Returns: A reference to the iterator.
Insert the new signature and a new paragraph after p5:
raw_storage_iterator<OutputIterator,T>& operator=(T&& element);-?- Requires:
-?- Effects: Constructs a value fromT
shall beMoveConstructible
.std::move(element)
at the location to which the iterator points. -?- Returns: A reference to the iterator.
cbegin/cend
Section: 24.2 [iterator.synopsis], 24.7 [iterator.range] Status: C++14 Submitter: Dmitry Polukhin Opened: 2012-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.synopsis].
View all issues with C++14 status.
Discussion:
All standard containers support cbegin/cend
member functions but corresponding global functions are
missing. Proposed resolution it to add global cbegin/cend
functions by analogy with global begin/end
functions. This addition will unify things for users.
[2012, Kona]
STL: Range-based for loops do not use global begin
/end
(anymore).
Alisdair: We will have to make sure these will be available through many headers.
STL: Do this, including r
and cr
. This won't add any additional work.
Matt: Users will find it strange if these are not all available.
Alisdair: Should we have these available everywhere begin/end are available?
Marshall: Yes. Not any extra work.
Howard: Adding all of these means we need all of <iterator>
.
STL: We already need it all.
Matt: We have to be careful what we are requiring if we include the r
versions.
Jeffrey: If we include r
, should they adapt if the container does not define reverse iteration?
STL: No. No special behavior. Should fail to compile. Up to user to add the reverse code--it's easy.
Howard: Anyway it will SFINAE out.
Alisdair: Error messages due to SFINAE are harder to understand than simple failure to compile.
STL: Agrees that SFINAE makes error messages much worse.
Action: STL to provide additional wording for the r
variants.
Move to Review once that wording is availalbe.
[ 2013-04-14 STL provides rationale and improved wording ]
Step 1: Implement std::cbegin/cend()
by calling std::begin/end()
. This has numerous advantages:
cbegin/cend()
members were invented.initializer_list
, which is extremely minimal and lacks cbegin/cend()
members.Step 2: Like std::begin/end()
, implement std::rbegin/rend()
by calling c.rbegin/rend()
.
Note that C++98/03 had the Reversible Container Requirements
.
Step 3: Also like std::begin/end()
, provide overloads of std::rbegin/rend()
for arrays.
Step 4: Provide overloads of std::rbegin/rend()
for initializer_list
, because it lacks
rbegin/rend()
members. These overloads follow 17.11.5 [support.initlist.range]'s signatures. Note that
because these overloads return reverse_iterator
, they aren't being specified in <initializer_list>
.
Step 5: Like Step 1, implement std::crbegin/crend()
by calling std::rbegin/rend()
.
Original wording saved here:
This wording is relative to N3337.
In 24.2 [iterator.synopsis], header iterator synopsis, add the following declarations:
namespace std { […] // 24.6.5, range access: template <class C> auto begin(C& c) -> decltype(c.begin()); template <class C> auto begin(const C& c) -> decltype(c.begin()); template <class C> auto end(C& c) -> decltype(c.end()); template <class C> auto end(const C& c) -> decltype(c.end()); template <class C> auto cbegin(const C& c) -> decltype(c.cbegin()); template <class C> auto cend(const C& c) -> decltype(c.cend()); template <class T, size_t N> T* begin(T (&array)[N]); template <class T, size_t N> T* end(T (&array)[N]); template <class T, size_t N> const T* cbegin(T (&array)[N]); template <class T, size_t N> const T* cend(T (&array)[N]); }In 24.7 [iterator.range] after p5 add the following series of paragraphs:
template <class C> auto cbegin(const C& c) -> decltype(c.cbegin());-?- Returns:
c.cbegin()
.template <class C> auto cend(const C& c) -> decltype(c.cend());-?- Returns:
c.cend()
.template <class T, size_t N> const T* cbegin(T (&array)[N]);-?- Returns:
array
.template <class T, size_t N> const T* cend(T (&array)[N]);-?- Returns:
array + N
.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3485.
In 24.2 [iterator.synopsis], header iterator synopsis, add the following declarations:
namespace std { […] // 24.6.5, range access: template <class C> auto begin(C& c) -> decltype(c.begin()); template <class C> auto begin(const C& c) -> decltype(c.begin()); template <class C> auto end(C& c) -> decltype(c.end()); template <class C> auto end(const C& c) -> decltype(c.end()); template <class T, size_t N> T* begin(T (&array)[N]); template <class T, size_t N> T* end(T (&array)[N]); template <class C> auto cbegin(const C& c) -> decltype(std::begin(c)); template <class C> auto cend(const C& c) -> decltype(std::end(c)); template <class C> auto rbegin(C& c) -> decltype(c.rbegin()); template <class C> auto rbegin(const C& c) -> decltype(c.rbegin()); template <class C> auto rend(C& c) -> decltype(c.rend()); template <class C> auto rend(const C& c) -> decltype(c.rend()); template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]); template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]); template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il); template <class E> reverse_iterator<const E*> rend(initializer_list<E> il); template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c)); template <class C> auto crend(const C& c) -> decltype(std::rend(c)); }
At the end of 24.7 [iterator.range], add:
template <class C> auto cbegin(const C& c) -> decltype(std::begin(c));-?- Returns:
std::begin(c)
.template <class C> auto cend(const C& c) -> decltype(std::end(c));-?- Returns:
std::end(c)
.template <class C> auto rbegin(C& c) -> decltype(c.rbegin()); template <class C> auto rbegin(const C& c) -> decltype(c.rbegin());-?- Returns:
c.rbegin()
.template <class C> auto rend(C& c) -> decltype(c.rend()); template <class C> auto rend(const C& c) -> decltype(c.rend());-?- Returns:
c.rend()
.template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]);-?- Returns:
reverse_iterator<T*>(array + N)
.template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]);-?- Returns:
reverse_iterator<T*>(array)
.template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il);-?- Returns:
reverse_iterator<const E*>(il.end())
.template <class E> reverse_iterator<const E*> rend(initializer_list<E> il);-?- Returns:
reverse_iterator<const E*>(il.begin())
.template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c));-?- Returns:
std::rbegin(c)
.template <class C> auto crend(const C& c) -> decltype(std::rend(c));-?- Returns:
std::rend(c)
.
std::initializer_list
Section: 16.4.5.2.1 [namespace.std], 17.11 [support.initlist] Status: C++17 Submitter: Richard Smith Opened: 2012-01-18 Last modified: 2017-07-30
Priority: 3
View other active issues in [namespace.std].
View all other issues in [namespace.std].
View all issues with C++17 status.
Discussion:
Since the implementation is intended to magically synthesize instances of std::initializer_list
(rather than by a constructor call, for instance), user specializations of this type can't generally be
made to work. I can't find any wording which makes such specializations ill-formed, though, which leads
me to suspect that they're technically legal under the provisions of 16.4.5.2.1 [namespace.std] p1.
[2012, Kona]
This sounds correct, but we need wording for a resolution.
Marshall Clow volunteers to produce wording.
[2014-02-19, Jonathan Wakely provides proposed wording]
[2014-03-27, Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N3936.
Add new new paragraph below 17.11 [support.initlist] p2:
-2- An object of type
-?- If an explicit specialization or partial specialization ofinitializer_list<E>
provides access to an array of objects of typeconst E
. […]initializer_list
is declared, the program is ill-formed.
Section: 32.5.4 [atomics.order] Status: C++14 Submitter: Mark Batty Opened: 2012-02-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [atomics.order].
View all other issues in [atomics.order].
View all issues with C++14 status.
Discussion:
C11 issue 407
It seems that both C11 and C++11 are missing the following two derivatives of this rule:
For atomic modifications
A
andB
of an atomic objectM
, if there is amemory_order_seq_cst
fenceX
such thatA
is sequenced beforeX
, andX
precedesB
inS
, thenB
occurs later thanA
in the modification order ofM
.
For atomic modifications
A
andB
of an atomic objectM
, if there is amemory_order_seq_cst
fenceY
such thatY
is sequenced beforeB
, andA
precedesY
inS
, thenB
occurs later thanA
in the modification order ofM
.
Above wording has been suggested for the Technical Corrigendum of C11 via issue 407, details can be found here.
[2012-03-19: Daniel proposes a slightly condensed form to reduce wording duplications]
[2012-03-20: Hans comments]
The usage of the term atomic operations in 32.5.4 [atomics.order] p7 is actually incorrect and should better be replaced by atomic modifications as used in the C11 407 wording.
There seems to be a similar wording incorrectness used in 6.9.2 [intro.multithread] p17 which should be corrected as well.[2012, Portland: move to Review]
Olivier: does the fence really participate in the modifications?
Hans: S is the total set of all sequentially consistent operations, and sequentially consistent fences are in S.
Olivier: this sort of combination of a pair of half-open rules seems to imply the write must make it to main memory
But not all implementations treat a fence as a memory operation; cannot observe the half-open rule.
Hans: not sure this is actually prevented here. You could defer until the next load. What the wording doesn't quite show is that the third bullet in the new wording is already in the standard.
Hans: it is the interaction between fences on one side and other memory modifications on the other that is being defined here.
Pablo: S is not directly observable; it is a hypothetic ordering.
Moved to review
Hans: to alert C liaison
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
This wording is relative to N3376.
[Drafting note: The project editor is kindly asked to consider to replace in 6.9.2 [intro.multithread] p17 the phrase "before an operation B on M" by "before a modification B of M".]
Change 32.5.4 [atomics.order] paragraph 7 as indicated: [Drafting note: Note that the wording change intentionally does also replace the term atomic operation by atomic modification]
-7- For atomic operations A and B on an atomic object M, if there are
For atomic modifications A and B of an atomic object M, B occurs
later than A in the modification order of M if:
memory_order_seq_cst
fences X and Y such that A is sequenced before X,
Y is sequenced before B, and X precedes Y in S, then B
occurs later than A in the modification order of M.
memory_order_seq_cst
fence X such that A is sequenced before X,
and X precedes B in S, or
memory_order_seq_cst
fence Y such that Y is sequenced before B,
and A precedes Y in S, or
memory_order_seq_cst
fences X and Y such that A is sequenced
before X, Y is sequenced before B, and X precedes Y in S.
memory_order_seq_cst
ensures sequential consistency only for a program that is free of data races
and uses exclusively memory_order_seq_cst
operations. Any use of weaker ordering will invalidate this
guarantee unless extreme care is used. In particular, memory_order_seq_cst
fences ensure a total order
only for the fences themselves. Fences cannot, in general, be used to restore sequential consistency for atomic
operations with weaker ordering specifications. — end note ]
std::function
ambiguitySection: 22.10.17.3.2 [func.wrap.func.con] Status: C++14 Submitter: Ville Voutilainen Opened: 2012-02-28 Last modified: 2016-01-28
Priority: 2
View all other issues in [func.wrap.func.con].
View all issues with C++14 status.
Discussion:
Consider the following:
#include <functional> void f(std::function<void()>) {} void f(std::function<void(int)>) {} int main() { f([]{}); f([](int){}); }
The calls to f
in main
are ambiguous. Apparently because the
conversion sequences to std::function
from the lambdas are identical.
The standard specifies that the function object given to std::function
"shall be Callable (20.8.11.2) for argument types ArgTypes
and
return type R
." It doesn't say that if this is not the case, the
constructor isn't part of the overload set.
invoke
is possible but was deferred for a later point in time. Defining a type trait for
the Callable requirement would also be possible, so there seem to be no technical
reasons why the template constructor of std::function
should not be
constrained. The below suggested wording does this without introducing a special
trait for this. This corresponds to the way that has been used to specify the
result_of
trait. Note that the definition of the Callable
requirement is perfectly suitable for this, because it is a pure syntactically
based requirement and can be directly transformed into a constrained template.
The suggested resolution also applies such wording to the "perfectly forwarding"
assignment operator
template<class F> function& operator=(F&&);
The positive side-effect of this is that it automatically implements a solution to a problem similar to that mentioned in issue 1234(i).
It would be possible to apply similar constraints to the member signaturestemplate<class F> function& operator=(reference_wrapper<F>); template<class F, class A> void assign(F&&, const A&);
as well. At this point there does not seem to be a pestering reason to do so.
[2012-10 Portland: Move to Review]
STL: This is a real issue, but does not like a resolution relying on a SFINAEable metafunction that is not specified and available to the users.
packaged_task
has the same issue.
STL strongly wants to see an is_callable
type trait to clarify the proposed wording.
Jeremiah concerned about holding up what appears to be a correct resolution for a hypothetical better one later - the issue is real.
Why must f
by CopyConstructible? Surely MoveConstructible would be sufficient?
Answer: because function
is CopyConstructible, and the bound functor is type-erased
so must support all the properties of function
itself.
Replace various applications of declval
in the proposed resolution with simply using
the passed functor object, f
.
Alisdair to apply similar changes to packaged_task
.
[2012-11-09, Vicente J. Botet Escriba provides another example]
Consider the following:
class AThreadWrapper { public: explicit operator std::thread(); ... }; std::thread th = std::thread(AThreadWrapper); // call to conversion operator intended
The call to the conversion operator is overloaded with the thread constructor. But thread constructor requirement
makes it fail as AThreadWrapper
is not a Callable and the compiler tries to instantiate the thread
constructor and fails.
[2014-02-14 Issaquah meeting: Move to Immediate]
Proposed resolution:
This wording is relative to N3376.
Change the following paragraphs in 22.10.17.3.2 [func.wrap.func.con]:
[Editorial comment: The removal of the seemingly additional no-throw
requirements of copy constructor and destructor of A
is recommended,
because they are already part of the Allocator requirements. Similar clean-up
has been suggested by 2070(i) — end comment]
template<class F> function(F f); template<class F, class A> function(allocator_arg_t, const A& a, F f);-7- Requires:
-?- Remarks: These constructors shall not participate in overload resolution unlessF
shall beCopyConstructible
.f
shall be Callable (22.10.17.3 [func.wrap.func]) for argument typesArgTypes
and return typeR
. The copy constructor and destructor ofA
shall not throw exceptions.f
is Callable (22.10.17.3 [func.wrap.func]) for argument typesArgTypes...
and return typeR
.[…]
template<class F> function& operator=(F&& f);-18- Effects:
-19- Returns:function(std::forward<F>(f)).swap(*this);
*this
-?- Remarks: This assignment operator shall not participate in overload resolution unlessdeclval<typename decay<F>::type&>()
is Callable (22.10.17.3 [func.wrap.func]) for argument typesArgTypes...
and return typeR
.
Section: 16.4.6.4 [global.functions] Status: C++17 Submitter: Yakov Galka Opened: 2012-01-25 Last modified: 2017-07-30
Priority: 3
View all other issues in [global.functions].
View all issues with C++17 status.
Discussion:
16.4.6.4 [global.functions] says
Unless otherwise specified, global and non-member functions in the standard library shall not use functions from another namespace which are found through argument-dependent name lookup (3.4.2).
This sounds clear enough. There are just two problems:
Both implementations I tested (VS2005 and GCC 3.4.3) do unqualified calls to the comma operator in some parts of the library with operands of user-defined types.
The standard itself does this in the description of some algorithms. E.g. uninitialized_copy
is defined as:
Effects:
for (; first != last; ++result, ++first) ::new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);
If understood literally, it is required to call operator,(ForwardIterator, InputIterator)
.
[2013-03-15 Issues Teleconference]
Moved to Open.
There are real questions here, that may require a paper to explore and answer properly.
[2014-05-18, Daniel comments and suggests concrete wording]
Other issues, such as 2114(i) already follow a similar spirit as the one suggested by bullet 2 of the issue submitter. I assert that consideration of possible user-provided overloads of the comma-operator were not intended by the original wording and doing so afterwards would unnecessarily complicate a future conceptualization of the library and would needlessly restrict implementations.
I don't think that a paper is needed to solve this issue, there is a simply way to ensure that the code-semantics excludes consideration of user-provided comma operators. The provided wording below clarifies this by explicitly casting the first argument of the operator tovoid
.
[2015-05, Lenexa]
DK: is putting it in the middle the right place for it?
STL: either works, but visually putting it in the middle is clearer, and for "++it1, ++i2, ++it3" it needs to be
done after the second comma, so "++it1, (void) ++i2, (void) ++it3" is better than "(void) ++it1, ++i2, (void) ++it3"
ZY: for INVOKE
yesterday we used static_cast<void>
but here we're using C-style cast, why?
STL: for INVOKE
I want to draw attention that there's an intentional coercion to void
because that's
the desired type. Here we only do it because that's the best way to prevent the problem, not because we specifically want a
void
type.
Move to Ready: 9 in favor, none opposed, 1 abstention
Proposed resolution:
This wording is relative to N3936.
Change 26.11.5 [uninitialized.copy] as indicated:
template <class InputIterator, class ForwardIterator> ForwardIterator uninitialized_copy(InputIterator first, InputIterator last, ForwardIterator result);-1- Effects:
for (; first != last; ++result, (void) ++first) ::new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);[…]
template <class InputIterator, class Size,class ForwardIterator> ForwardIterator uninitialized_copy_n(InputIterator first, Size n, ForwardIterator result);-3- Effects:
for (; n > 0; ++result, (void) ++first, --n) ::new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);
Change 26.8.11 [alg.lex.comparison] p3 as indicated:
template<class InputIterator1, class InputIterator2> bool lexicographical_compare(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2); template<class InputIterator1, class InputIterator2, class Compare> bool lexicographical_compare(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, Compare comp);-3- Remarks: […]
for ( ; first1 != last1 && first2 != last2 ; ++first1, (void) ++first2) { if (*first1 < *first2) return true; if (*first2 < *first1) return false; } return first1 == last1 && first2 != last2;
condition_variable::wait()
Section: 32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany] Status: C++14 Submitter: Pete Becker Opened: 2012-03-06 Last modified: 2015-10-03
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
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Discussion:
condition_varible::wait()
(and, presumably, condition_variable_any::wait()
, although
I haven't looked at it) says that it calls lock.unlock()
, and if condition_variable::wait()
exits by an exception it calls lock.lock()
on the way out. But if the initial call to
lock.unlock()
threw an exception, does it make sense to call lock.lock()
? We simply
don't know the state of that lock object, and it's probably better not to touch it.
wait()
call has been unblocked, it calls lock.lock()
. If lock.lock()
throws an exception, what happens? The requirement is:
If the function exits via an exception,
lock.lock()
shall be called prior to exiting the function scope.
That can be read in two different ways. One way is as if it said "lock.lock()
shall have been called …",
i.e. the original, failed, call to lock.lock()
is all that's required. But a more natural reading is
that wait has to call lock.lock()
again, even though it already failed.
lock.unlock()
and the final call to lock.lock()
. Each one should have its own requirement.
Lumping them together muddles things.
[2012, Portland: move to Open]
Pablo: unlock
failing is easy -- the call leaves it locked.
The second case, trying to lock
fails -- what can you do?
This is an odd state as we had it locked before was called wait.
Maybe we should call terminate
as we cannot meet the post-conditions.
We could throw a different exception.
Hans: calling terminate
makes sense as we're likely to call it soon anyway
and at least we have some context.
Detlef: what kind of locks might be being used?
Pablo: condition variables are 'our' locks so this is less of a problem.
condition_variable_any
might be more problematic.
The general direction is to call terminate
if the lock cannot be reacquired.
Pablo: Can we change the wording to 'leaves the mutex locked' ?
Hans: so if the unlock
throws we simply propagate the exception.
Move the issue to open and add some formal wording at a later time.
[2013-09 Chicago: Resolved]
Detlef improves wording. Daniel suggests to introduce a Remarks element for the special "If the function fails to meet the postcondition..." wording and applies this to the proposed wording.
Proposed resolution:
This wording is relative to N3691.
Edit 32.7.4 [thread.condition.condvar] as indicated:
void wait(unique_lock<mutex>& lock);[…]
-10- Effects:
Atomically calls
lock.unlock()
and blocks on*this
.When unblocked, calls
lock.lock()
(possibly blocking on the lock), then returns.The function will unblock when signaled by a call to
notify_one()
or a call tonotify_all()
, or spuriously.
If the function exits via an exception,lock.lock()
shall be called prior to exiting the function scope.-?- Remarks: If the function fails to meet the postcondition,
-11- Postcondition:std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. -12- Throws: Nothing.system_error
when an exception is required (30.2.2)-13- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Predicate> void wait(unique_lock<mutex>& lock, Predicate pred);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -16- Postcondition:lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. -17- Throws:timeout-related exceptions (30.2.4)system_error
when an exception is required (30.2.2),,or any exception thrown bypred
.-18- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Clock, class Duration> cv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time);[…]
-20- Effects:
[…]
If the function exits via an exception,
lock.lock()
shall be called prior to exiting the functionscope.-?- Remarks: If the function fails to meet the postcondition,
-21- Postcondition:std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. […] -23- Throws:timeout-related exceptions (30.2.4).system_error
when an exception is required (30.2.2) or-24- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -28- Postcondition:lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. […] -29- Throws:timeout-related exceptions (30.2.4).system_error
when an exception is required (30.2.2) or-30- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -33- Postcondition:lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. […] -35- Throws:timeout-related exceptions (30.2.4)system_error
when an exception is required (30.2.2),,or any exception thrown bypred
.-36- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -40- Postcondition:lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. […] -42- Throws:timeout-related exceptions (30.2.4)system_error
when an exception is required (30.2.2),,or any exception thrown bypred
.-43- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.
Edit 32.7.5 [thread.condition.condvarany] as indicated:
template<class Lock> void wait(Lock& lock);[…]
-10- Effects:
Atomically calls
lock.unlock()
and blocks on*this
.When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, or spuriously.
If the function exits via an exception,lock.lock()
shall be called prior to exiting the function scope.-?- Remarks: If the function fails to meet the postcondition,
-11- Postcondition:std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]lock
is locked by the calling thread. -12- Throws: Nothing.system_error
when an exception is required (30.2.2)-13- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Lock, class Clock, class Duration> cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);[…]
-15- Effects:
[…]
If the function exits via an exception,
lock.lock()
shall be called prior to exiting the functionscope.-?- Remarks: If the function fails to meet the postcondition,
-16- Postcondition:std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]lock
is locked by the calling thread. […] -18- Throws:timeout-related exceptions (30.2.4).system_error
when an exception is required (30.2.2) or-19- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -22- Postcondition:lock
is locked by the calling thread. […] -23- Throws:timeout-related exceptions (30.2.4).system_error
when an exception is required (30.2.2) or-24- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.
atomic_flag::clear
should not accept memory_order_consume
Section: 32.5.10 [atomics.flag] Status: C++14 Submitter: Ben Viglietta Opened: 2012-03-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.flag].
View all issues with C++14 status.
Discussion:
N3376 32.5.10 [atomics.flag]/7 says
this about atomic_flag::clear
:
Requires: The
order
argument shall not bememory_order_acquire
ormemory_order_acq_rel
.
In addition, memory_order_consume
should be disallowed, since it doesn't meaningfully apply to store operations.
It's already disallowed on the analogous atomic<T>::store
. The proposed updated text would be:
Requires: The
order
argument shall not bememory_order_consume
,memory_order_acquire
, ormemory_order_acq_rel
.
[2012, Portland: move to Review]
Hans: this is a clear oversight.
Moved to review
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
[This wording is relative to N3376.]
void atomic_flag_clear(volatile atomic_flag *object) noexcept; void atomic_flag_clear(atomic_flag *object) noexcept; void atomic_flag_clear_explicit(volatile atomic_flag *object, memory_order order) noexcept; void atomic_flag_clear_explicit(atomic_flag *object, memory_order order) noexcept; void atomic_flag::clear(memory_order order = memory_order_seq_cst) volatile noexcept; void atomic_flag::clear(memory_order order = memory_order_seq_cst) noexcept;-7- Requires: The
-8- Effects: Atomically sets the value pointed to byorder
argument shall not bememory_order_consume
,memory_order_acquire
, ormemory_order_acq_rel
.object
or bythis
to false. Memory is affected according to the value oforder
.
Section: 16.4.5.2.1 [namespace.std], 19.5 [syserr], 20.2.8.1 [allocator.uses.trait], 22.10.15.2 [func.bind.isbind], 22.10.15.3 [func.bind.isplace], 22.10.19 [unord.hash], 21.3.8.7 [meta.trans.other], 28.3.3.1 [locale], 28.3.4.2.5 [locale.codecvt], 28.6.11.1.5 [re.regiter.incr] Status: C++20 Submitter: Loïc Joly Opened: 2012-03-08 Last modified: 2021-02-25
Priority: 4
View other active issues in [namespace.std].
View all other issues in [namespace.std].
View all issues with C++20 status.
Discussion:
The expression "user-defined type" is used in several places in the standard, but I'm not sure what it means. More specifically, is a type defined in the standard library a user-defined type?
From my understanding of English, it is not. From most of the uses of this term in the standard, it seem to be considered as user defined. In some places, I'm hesitant, e.g. 16.4.5.2.1 [namespace.std] p1:A program may add a template specialization for any standard library template to namespace
std
only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.
Does it mean we are allowed to add in the namespace std
a specialization for
std::vector<std::pair<T, U>>
, for instance?
[ 2012-10 Portland: Move to Deferred ]
The issue is real, in that we never define this term and rely on a "know it when I see it" intuition. However, there is a fear that any attempt to pin down a definition is more likely to introduce bugs than solve them - getting the wording for this precisely correct is likely far more work than we are able to give it.
There is unease at simple closing as NAD, but not real enthusiasm to provide wording either. Move to Deferred as we are not opposed to some motivated individual coming back with full wording to review, but do not want to go out of our way to encourage someone to work on this in preference to other issues.
[2014-02-20 Re-open Deferred issues as Priority 4]
[2015-03-05 Jonathan suggests wording]
I dislike the suggestion to change to "user-provided" type because I already find the difference between user-declared / user-provided confusing for special member functions, so I think it would be better to use a completely different term. The core language uses "user-defined conversion sequence" and "user-defined literal" and similar terms for things which the library provides, so I think we should not refer to "user" at all to distinguish entities defined outside the implementation from things provided by the implementation.
I propose "program-defined type" (and "program-defined specialization"), defined below. The P/R below demonstrates the scope of the changes required, even if this name isn't adopted. I haven't proposed a change for "User-defined facets" in [locale].[Lenexa 2015-05-06]
RS, HT: The core language uses "user-defined" in a specific way, including library things but excluding core language things, let's use a different term.
MC: Agree.
RS: "which" should be "that", x2
RS: Is std::vector<MyType> a "program-defined type"?
MC: I think it should be.
TK: std::vector<int> seems to take the same path.
JW: std::vector<MyType> isn't program-defined, we don't need it to be, anything that depends on that also depends on =MyType.
TK: The type defined by an "explicit template specialization" should be a program-defined type.
RS: An implicit instantiation of a "program-defined partial specialization" should also be a program-defined type.
JY: This definition formatting is horrible and ugly, can we do better?
RS: Checking ISO directives.
RS: Define "program-defined type" and "program-defined specialization" instead, to get rid of the angle brackets.
JW redrafting.
[2017-09-12]
Jonathan revises wording as per Lenexa discussion
Previous resolution [SUPERSEDED]:This wording is relative to N4296.
Add a new sub-clause to [definitions]:
17.3.? [defns.program.defined]
program-defined
<type> a class type or enumeration type which is not part of the C++ standard library and not defined by the implementation. [Note: Types defined by the implementation include extensions (4.1 [intro.compliance]) and internal types used by the library. — end note]program-defined
<specialization> an explicit template specialization or partial specialization which is not part of the C++ standard library and not defined by the implementation.Change 16.4.5.2.1 [namespace.std] paragraph 1+2:
-1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace
-2- The behavior of a C++ program is undefined if it declares […] A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of astd
or to a namespace within namespacestd
unless otherwise specified. A program may add a template specialization for any standard library template to namespacestd
only if the declaration depends on auserprogram-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.userprogram-defined type and the instantiation meets the standard library requirements for the original template.Change 19.5 [syserr] paragraph 4:
-4- The
is_error_code_enum
andis_error_condition_enum
may be specialized foruserprogram-defined types to indicate that such types are eligible for classerror_code
and classerror_condition
automatic conversions, respectively.Change 20.2.8.1 [allocator.uses.trait] paragraph 1:
-1- Remarks: automatically detects […]. A program may specialize this template to derive from
true_type
for auserprogram-defined typeT
that does not have a nestedallocator_type
but nonetheless can be constructed with an allocator where either: […]Change 22.10.15.2 [func.bind.isbind] paragraph 2:
-2- Instantiations of the
is_bind_expression
template […]. A program may specialize this template for auserprogram-defined typeT
to have aBaseCharacteristic
oftrue_type
to indicate thatT
should be treated as a subexpression in abind
call.Change 22.10.15.3 [func.bind.isplace] paragraph 2:
-2- Instantiations of the
is_placeholder
template […]. A program may specialize this template for auserprogram-defined typeT
to have aBaseCharacteristic
ofintegral_constant<int, N>
withN > 0
to indicate thatT
should be treated as a placeholder type.Change 22.10.19 [unord.hash] paragraph 1:
The unordered associative containers defined in 23.5 use specializations of the class template
hash
[…], the instantiationhash<Key>
shall:
[…]
[…]
[…]
[…]
satisfy the requirement that the expression
h(k)
, whereh
is an object of typehash<Key>
andk
is an object of typeKey
, shall not throw an exception unlesshash<Key>
is auserprogram-defined specialization that depends on at least oneuserprogram-defined type.Change 21.3.8.6 [meta.trans.ptr] Table 57 (
common_type
row):
Table 57 — Other transformations Template Condition Comments …
template <class... T>
struct common_type;The member typedef type
shall be
defined or omitted as specified below.
[…]. A program may
specialize this trait if at least one
template parameter in the
specialization is auserprogram-defined type.
[…]…
Change 28.3.4.2.5 [locale.codecvt] paragraph 3:
-3- The specializations required in Table 81 (22.3.1.1.1) […]. Other encodings can be converted by specializing on a
userprogram-definedstateT
type.[…]Change 28.6.11.1.5 [re.regiter.incr] paragraph 8:
-8- [Note: This means that a compiler may call an implementation-specific search function, in which case a
userprogram-defined specialization ofregex_search
will not be called. — end note]
[2018-3-14 Wednesday evening issues processing; move to Ready]
After this lands, we need to audit Annex C to find "user-defined type" Example: [diff.cpp03.containers]/3
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4687.
Add a new sub-clause to [definitions]:
20.3.? [defns.program.defined.spec]
program-defined specialization
explicit template specialization or partial specialization that is not part of the C++ standard library and not defined by the implementation20.3.? [defns.program.defined.type]
program-defined type
class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or an instantiation of a program-defined specialization[Drafting note: ISO directives say the following Note should be labelled as a "Note to entry" but the C++ WP doesn't follow that rule (yet). — end drafting note]
[Note: Types defined by the implementation include extensions (4.1 [intro.compliance]) and internal types used by the library. — end note]
Change 16.4.5.2.1 [namespace.std] paragraph 1+2:
-1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std
or to a
namespace within namespace std
unless otherwise specified. A program may add a template specialization
for any standard library template to namespace std
only if the declaration depends on a
userprogram-defined type and the specialization meets the standard library requirements for the
original template and is not explicitly prohibited.
Change 19.5 [syserr] paragraph 4:
-4- The is_error_code_enum
and is_error_condition_enum
may be specialized for
userprogram-defined types to indicate that such types are eligible for class error_code
and class error_condition
automatic conversions, respectively.
Change 20.2.8.1 [allocator.uses.trait] paragraph 1:
-1- Remarks: automatically detects […]. A program may specialize this template to derive from
true_type
for a userprogram-defined type T
that does not have a nested
allocator_type
but nonetheless can be constructed with an allocator where either: […]
Change 22.10.15.2 [func.bind.isbind] paragraph 2:
-2- Instantiations of the is_bind_expression
template […]. A program may specialize
this template for a userprogram-defined type T
to have a BaseCharacteristic
of true_type
to indicate that T
should be treated as a subexpression in a bind
call.
Change 22.10.15.3 [func.bind.isplace] paragraph 2:
-2- Instantiations of the is_placeholder
template […]. A program may specialize this template for a
userprogram-defined type T
to have a BaseCharacteristic
of
integral_constant<int, N>
with N > 0
to indicate that T
should be
treated as a placeholder type.
Change 22.10.19 [unord.hash] paragraph 1:
The unordered associative containers defined in 23.5 use specializations of the class template hash
[…],
the instantiation hash<Key>
shall:
[…]
[…]
[…]
[…]
satisfy the requirement that the expression h(k)
, where h
is an object of type
hash<Key>
and k
is an object of type Key
, shall not throw an exception unless
hash<Key>
is a userprogram-defined specialization that depends on at least one
userprogram-defined type.
Change 21.3.8.6 [meta.trans.ptr] Table 57 (common_type
row):
Table 57 — Other transformations Template Condition Comments …
template <class... T>
struct common_type;The member typedef type
shall be
defined or omitted as specified below.
[…]. A program may
specialize this trait if at least one
template parameter in the
specialization is auserprogram-defined type.
[…]…
Change 28.3.4.2.5 [locale.codecvt] paragraph 3:
-3- The specializations required in Table 81 (22.3.1.1.1) […]. Other encodings can be converted
by specializing on a userprogram-defined stateT
type.[…]
Change 28.6.11.1.5 [re.regiter.incr] paragraph 8:
-8- [Note: This means that a compiler may call an implementation-specific search function, in which case
a userprogram-defined specialization of regex_search
will not be called. —
end note]
notify_all_at_thread_exit
synchronization requirement?Section: 32.7 [thread.condition] Status: C++14 Submitter: Pete Becker Opened: 2012-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition].
View all issues with C++14 status.
Discussion:
notify_all_at_thread_exit
has the following synchronization requirement:
Synchronization: The call to
notify_all_at_thread_exit
and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.9.2 [intro.multithread]) calls to functions waiting oncond
.
The functions waiting on cond
have already been called, otherwise they wouldn't be waiting. So how can a subsequent
call to notify_all_at_thread_exit
synchronize with them?
[2012-03-09 Jeffrey Yasskin comments:]
I think the text should say that "notify_all_at_thread_exit
and destructor calls are sequenced before
the lk.unlock()
", and leave it at that, unless there's a funny implementation I haven't thought of.
[2012-03-19 Hans Boehm comments:]
I think the synchronization clause should just be replaced with (modulo wording tweaks):
"The impliedlk.unlock()
call is sequenced after the destruction of all objects with thread storage duration
associated with the current thread."
as Jeffrey suggested.
To use this correctly, the notifying thread has to essentially acquire the lock, set a variable indicating it's done,
call notify_all_at_thread_exit()
, while the waiting thread acquires the lock, and repeatedly waits on the
cv until the variable is set, and then releases the lock. That ensures that we have the proper synchronizes with
relationship as a result of the lock.
[2012, Portland: move to Review]
The lk.unlock()
refers back to the wording the previous paragraph.
Moved to review
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
This wording is relative to N3376.
Modify 32.7 [thread.condition] p8 as indicated:
void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);[…]
-8- Synchronization:The call toThe impliednotify_all_at_thread_exit
and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.9.2 [intro.multithread]) calls to functions waiting oncond
lk.unlock()
call is sequenced after the destruction of all objects with thread storage duration associated with the current thread.
common_type
trait produces reference typesSection: 21.3.8.7 [meta.trans.other] Status: C++14 Submitter: Doug Gregor Opened: 2012-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
View all issues with C++14 status.
Discussion:
The type computation of the common_type
type trait is defined as
template <class T, class U> struct common_type<T, U> { typedef decltype(true ? declval<T>() : declval<U>()) type; };
This means that common_type<int, int>::type
is int&&
, because
declval<int>()
returns int&&
decltype
returns T&&
when its expression is an xvalue (9.2.9.3 [dcl.type.simple] p4)
Users of common_type
do not expect to get a reference type as the result; the expectation is that
common_type
will return a non-reference type to which all of the types can be converted.
std::unique_ptr
's
operator<
in 20.3.1.6 [unique.ptr.special] (around p4) is also broken: In the most typical case
(with default deleter), the determination of the common pointer type CT will instantiate
std::less<CT>
which can now be std::less<T*&&>
, which will
not be the specialization of pointer types that guarantess a total order.
Given the historic constext of common_type
original specification, the proper resolution to me
seems to be using std::decay
instead of std::remove_reference
:
template <class T, class U> struct common_type<T, U> { typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type; };
At that time rvalues had no identity in this construct and rvalues of non-class types have no cv-qualification. With this change we would ensure that
common_type<int, int>::type == common_type<const int, const int>::type == int
Note that this harmonizes with the corresponding heterogenous case, which has already the exact same effect:
common_type<int, long>::type == common_type<const int, const long>::type == long
[2012-10-11 Daniel comments]
While testing the effects of applying the proposed resolution I noticed that this will have the effect that the unary
form of common_type
, like
common_type<int>
is not symmetric to the n-ary form (n > 1). This is unfortunate, because this difference comes especially to effect when
common_type
is used with variadic templates. As an example consider the following make_array
template:
#include <array>
#include <type_traits>
#include <utility>
template<class... Args>
std::array<typename std::common_type<Args...>::type, sizeof...(Args)>
make_array(Args&&... args)
{
typedef typename std::common_type<Args...>::type CT;
return std::array<CT, sizeof...(Args)>{static_cast<CT>(std::forward<Args>(args))...};
}
int main()
{
auto a1 = make_array(0); // OK: std::array<int, 1>
auto a2 = make_array(0, 1.2); // OK: std::array<double, 2>
auto a3 = make_array(5, true, 3.1415f, 'c'); // OK: std::array<float, 4>
int i = 0;
auto a1b = make_array(i); // Error, attempt to form std::array<int&, 1>
auto a2b = make_array(i, 1.2); // OK: std::array<double, 2>
auto a2c = make_array(i, 0); // OK: std::array<int, 2>
}
The error for a1b
only happens in the unary case and it is easy that it remains unnoticed
during tests. You cannot explain that reasonably to the user here.
std::decay
to the result of the
std::common_type
deduction. But if this is necessary here, I wonder why it should also be applied to
the binary case, where it gives the wrong illusion of a complete type decay? The other way around: Why is
std::decay
not also applied to the unary case as well?
This problem is not completely new and was already observed for the original std::common_type
specification.
At this time the decltype
rules had a similar asymmetric effect when comparing
std::common_type<const int, const int>::type
(equal to 'int
' at this time)
with:
std::common_type<const int>::type
(equal to 'const int
')
and I wondered whether the unary form shouldn't also perform the same "decay" as the n-ary form.
This problem makes me think that the current resolution proposal might not be ideal and I expect differences in implementations (for those who consider to apply this proposed resolution already). I see at least three reasonable options:Accept the current wording suggestion for LWG 2141 as it is and explain that to users.
Keep std::common_type
as currently specified in the Standard and tell users to use
std::decay
where needed. Also fix other places in the library, e.g. the comparison
functions of std::unique_ptr
or a most of the time library functions.
Apply std::decay
also in the unary specialization of std::common_type
with
the effect that std::common_type<const int&>::type
returns int
.
[2012-10-11 Marc Glisse comments]
If we are going with decay everywhere, I wonder whether we should also decay in the 2-argument version before
and not only after. So if I specialize common_type<mytype, double>
,
common_type<const mytype, volatile double&>
would automatically work.
[2012-10-11 Daniel provides wording for bullet 3 of his list:]
Change 21.3.8.7 [meta.trans.other] p3 as indicated:
template <class T> struct common_type<T> { typedef typename decay<T>::type type; }; template <class T, class U> struct common_type<T, U> { typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type; };
[2013-03-15 Issues Teleconference]
Moved to Review.
Want to carefully consider the effect of decay
vs. remove_reference
with respect
to constness before adopting, although this proposed resolution stands for review in Bristol.
[2013-04-18, Bristol meeting]
Previous wording:
This wording is relative to N3376.
In 21.3.8.7 [meta.trans.other] p3, change the
common_type
definition totemplate <class T, class U> struct common_type<T, U> { typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type; };
[2013-04-18, Bristol]
Move to Ready
[2013-09-29, Chicago]
Accepted for the working paper
Proposed resolution:
This wording is relative to N3485.
Change 21.3.8.7 [meta.trans.other] p3 as indicated:
template <class T> struct common_type<T> { typedef typename decay<T>::type type; }; template <class T, class U> struct common_type<T, U> { typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type; };
packaged_task::operator()
synchronization too broad?Section: 32.10.10.2 [futures.task.members] Status: C++14 Submitter: Pete Becker Opened: 2012-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task.members].
View all issues with C++14 status.
Discussion:
According to 32.10.10.2 [futures.task.members] p.18:
[A] successful call to [
packaged_task::
]operator()
synchronizes with a call to any member function of afuture
orshared_future
object that shares the shared state of*this
.
This requires that the call to operator()
synchronizes with calls to future::wait_for
,
future::wait_until
, shared_future::wait_for
, and shared_future::wait_until
,
even when these functions return because of a timeout.
[2012, Portland: move to Open]
If it said "a successful return from" (or "a return from" to cover exceptions) the problem would be more obvious.
Detlef: will ask Anthony Williams to draft some wording.
Moved to open (Anthony drafted to draft)
[2013-09, Chicago: move to Ready]
Anthony's conclusion is that the offending paragraph is not needed. Already included in the statement that the state is made ready.
Recommendation: Remove 32.10.10.2 [futures.task.members] p18 (the synchronization clause). Redundant because of 32.10.5 [futures.state] p9.Moved to Ready
Proposed resolution:
This wording is relative to N3691.
Remove 32.10.10.2 [futures.task.members] p18 as indicated:
void operator()(ArgTypes... args);[…]
-18- Synchronization: a successful call tooperator()
synchronizes with (1.10) a call to any member function of afuture
orshared_future
object that shares the shared state of*this
. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with (1.10) the successful return from any member function that detects that the state is set to ready. [Note:operator()
synchronizes and serializes with other functions through the shared state. — end note]
ios_base::xalloc
should be thread-safeSection: 31.5.2 [ios.base] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2012-03-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ios.base].
View all issues with C++14 status.
Discussion:
The static function ios_base::xalloc()
could be called from multiple threads and is not covered by
16.4.5.10 [res.on.objects] and 16.4.6.10 [res.on.data.races]. Adding a thread-safety requirement
should not impose a significant burden on implementations, as the function can be easily implemented with
hopefully lock-free atomics.
[2013-04-20, Bristol]
Unanimous.
Resolution: move tentatively ready. (Inform Bill about this issue.)[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to N3376.
In 31.5.2.6 [ios.base.storage] add a new paragraph after paragraph 1:
static int xalloc();-1- Returns:
-?- Remarks: Concurrent access to this function by multiple threads shall not result in a data race (6.9.2 [intro.multithread]).index ++
.
noexcept
specification in type_index
Section: 17.7.7 [type.index] Status: C++14 Submitter: Daniel Krügler Opened: 2012-03-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [type.index].
View all issues with C++14 status.
Discussion:
The class type type_index
is a thin wrapper of type_info
to
adapt it as a valid associative container element. Similar to type_info
,
all member functions have an effective noexcept(true)
specification, with the
exception of hash_code()
and name()
. The actual effects of these
functions is a direct call to type_info
's hash_code()
and name
function, but according to 17.7 [support.rtti] these are both noexcept
functions, so there is no reason for not declaring them as noexcept
, too. In fact,
one of the suggested changes of the original proposing paper
N2530
specifically was to ensure that type_info
would get a hash_code()
function that guarantees not to throw exceptions (during that time the hash
requirements did not allow to exit with an exception). From this we can conclude that
type_index::hash_code()
was intended to be nothrow.
noexcept
.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Modify the class type_index
synopsis, [type.index.overview] as indicated:
namespace std { class type_index { public: type_index(const type_info& rhs) noexcept; bool operator==(const type_index& rhs) const noexcept; bool operator!=(const type_index& rhs) const noexcept; bool operator< (const type_index& rhs) const noexcept; bool operator<= (const type_index& rhs) const noexcept; bool operator> (const type_index& rhs) const noexcept; bool operator>= (const type_index& rhs) const noexcept; size_t hash_code() const noexcept; const char* name() const noexcept; private: const type_info* target; // exposition only // Note that the use of a pointer here, rather than a reference, // means that the default copy/move constructor and assignment // operators will be provided and work as expected. }; }
Modify the prototype definitions in [type.index.members] as indicated:
size_t hash_code() const noexcept;-8- Returns:
target->hash_code()
const char* name() const noexcept;-9- Returns:
target->name()
error_category
default constructorSection: 19.5.3 [syserr.errcat] Status: C++14 Submitter: Howard Hinnant Opened: 2012-03-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [syserr.errcat].
View all issues with C++14 status.
Discussion:
Should error_category
have a default constructor?
Classes may be derived from
error_category
to support categories of errors in addition to those defined in this International Standard.
How shall classes derived from error_category
construct their base?
error_category
was default-constructible. That is still the case in
N2241, because no other
constructor is declared. Then later N2422
(issue 6) declares
the copy constructor as deleted, but doesn't add a default constructor, causing it to be no longer
default-constructible. That looks like an oversight to me, and I think there should be a public default
constructor.
Daniel: A default-constructor indeed should be provided to allow user-derived classes as described by the
standard. I suggest this one to be both noexcept
and constexpr
. The latter allows
user-derived non-abstract classes to take advantage of the special constant initialization rule
of [basic.start.init] p2 b2 for objects with static (or thread) storage duration in namespace
scope. Note that a constexpr
constructor is feasible here, even though there exists a non-trivial
destructor and even though error_category
is not a literal type (see std::mutex
for a similar
design choice).
In addition to that the proposed resolution fixes another minor glitch: According to 16.3.3.5 [functions.within.classes]
virtual destructors require a semantics description.
Alberto Ganesh Barbati: I would suggest to remove =default
from the constructor instead.
Please consider that defaulting a constructor or destructor may actually define them as deleted under certain
conditions (see 11.4.5 [class.ctor]/5 and 11.4.7 [class.dtor]/5). Removing =default
is easier than providing wording to ensures that such conditions do not occur.
[2012-10 Portland: move to Ready]
The issue is real and the resolution looks good.
Are there similar issues elsewhere in this clause?
Potential to add constexpr
to more constructors, but clearly a separable issue.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Modify the class error_category
synopsis, 19.5.3.1 [syserr.errcat.overview] as indicated:
[Drafting note: According to the general
noexcept
library guidelines
destructors should not have any explicit exception specification. This destructor was overlooked during the paper
analysis — end note]
namespace std { class error_category { public: constexpr error_category() noexcept; virtual ~error_category()noexcept; error_category(const error_category&) = delete; error_category& operator=(const error_category&) = delete; virtual const char* name() const noexcept = 0; virtual error_condition default_error_condition(int ev) const noexcept; virtual bool equivalent(int code, const error_condition& condition) const noexcept; virtual bool equivalent(const error_code& code, int condition) const noexcept; virtual string message(int ev) const = 0; bool operator==(const error_category& rhs) const noexcept; bool operator!=(const error_category& rhs) const noexcept; bool operator<(const error_category& rhs) const noexcept; }; }
Before 19.5.3.2 [syserr.errcat.virtuals] p1 insert a new prototype description as indicated:
virtual ~error_category();-?- Effects: Destroys an object of class
error_category
.
Before 19.5.3.3 [syserr.errcat.nonvirtuals] p1 insert a new prototype description as indicated:
constexpr error_category() noexcept;-?- Effects: Constructs an object of class
error_category
.
Allocator
's allocate
functionSection: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Daniel Krügler Opened: 2012-03-05 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++14 status.
Discussion:
According to Table 28 — "Allocator requirements", the expression
a.allocate(n, u)
expects as second argument a value u
that is described in Table 27 as:
a value of type
YY::const_pointer
obtained by callingYY::allocate
, or elsenullptr
.
This description leaves it open, whether or whether not a value of type YY::const_void_pointer
is
valid or not. The corresponding wording in C++03 is nearly the same, but in C++03 there did not exist the concept of
a general void_pointer
for allocators. There is some evidence for support of void pointers because
the general allocator_traits
template declares
static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);
and the corresponding function for std::allocator<T>
is declared as:
pointer allocate(size_type, allocator<void>::const_pointer hint = 0);
As an additional minor wording glitch (especially when comparing with the NullablePointer
requirements imposed on
const_pointer
and const_void_pointer
), the wording seems to exclude lvalues of type
std::nullptr_t
, which looks like an unwanted artifact to me.
[ 2012-10 Portland: Move to Ready ]
No strong feeling that this is a big issue, but consensus that the proposed resolution is strictly better than the current wording, so move to Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change Table 27 — "Descriptive variable definitions" in 16.4.4.6 [allocator.requirements]:
Variable | Definition |
---|---|
u
|
a value of type YY::const_pointer obtained by calling YY::allocate , or else
nullptr XX::const_void_pointer obtained by conversion from a result
value of YY::allocate , or else a value of type (possibly const) std::nullptr_t .
|
std::hash
Section: 22.10.19 [unord.hash] Status: C++14 Submitter: Ville Voutilainen Opened: 2012-04-10 Last modified: 2016-08-03
Priority: Not Prioritized
View all other issues in [unord.hash].
View all issues with C++14 status.
Discussion:
The paper proposes various hashing improvements. What it doesn't mention is hashing of enums; enums are integral types, and users expect them to have built-in hashing support, rather than having to convert enums to ints for uses with unordered containers and other uses of hashes. Daniel Krügler explains in c++std-lib-32412 that this is not achievable with a SFINAEd hash specialization because it would require a partial specialization with a type parameter and a non-type parameter with a default argument, which is currently not allowed, and hence the fixes in N3333 should be adopted instead.
[2012-10 Portland: Move to Open]
We agree this is a real issue that should be resolved, by specifying such a hash.
It is not clear that we should specify this as calling hash on the underlying_type
,
or whether that is overspecification and we merely require that the hash be supplied.
STL already has shipped an implementation, and is keen to provide wording.
[ 2013-04-14 STL provides rationale and improved wording ]
Rationale:
This can be achieved by inserting a very small tweak to the Standardese. We merely have to require that hash<Key>
be valid when Key
is an "enumeration type" (which includes both scoped and unscoped enums). This permits, but does
not require, hash<Enum>
to behave identically to hash<underlying_type<Enum>::type>
, following
existing precedent — note that when unsigned int
and unsigned long
are the same size,
hash<unsigned int>
is permitted-but-not-required to behave identically to hash<unsigned long>
.
static_assert
nicely, explode horribly at compiletime or runtime, etc.
While we're in the neighborhood, this proposed resolution contains an editorial fix. The 22.10 [function.objects]
synopsis says "base template", which doesn't appear anywhere else in the Standard, and could confuse users into
thinking that they need to derive from it. The proper phrase is "primary template".
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3485.
In 22.10 [function.objects], header functional synopsis, edit as indicated:
namespace std { […] // 20.8.12, hash functionbaseprimary template: template <class T> struct hash; […] }
In 22.10.19 [unord.hash]/1 edit as indicated:
-1- The unordered associative containers defined in 23.5 [unord] use specializations of the class template
hash
as the defaulthash
function. For all object typesKey
for which there exists a specializationhash<Key>
, and for all enumeration types (9.8.1 [dcl.enum]) Key, the instantiationhash<Key>
shall: […]
Section: 22.10 [function.objects] Status: C++14 Submitter: Scott Meyers Opened: 2012-02-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [function.objects].
View all issues with C++14 status.
Discussion:
22.10 [function.objects] p5 says:
To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects correspondingly provide typedefs
argument_type
andresult_type
for function objects that take one argument andfirst_argument_type
,second_argument_type
, andresult_type
for function objects that take two arguments.
I have two concerns about this paragraph. First, the wording appears to prescribe a requirement for all
function objects in valid C++ programs, but it seems unlikely that that is the intent. As such, the scope
of the requirement is unclear. For example, there is no mention of these typedefs in the specification for
closures (5.1.2), and Daniel Krügler has explained in the thread at
http://tinyurl.com/856plkn that conforming implementations can
detect the difference between closures with and without these typedefs. (Neither gcc 4.6 nor VC10 appear
to define typedefs such as result_type
for closure types. I have not tested other compilers.)
std::bind
, as Howard Hinnant explains in the thread at http://tinyurl.com/6q5bos4.
From what I can tell, the standard already defines which adaptability typedefs must be provided by various
kinds of function objects in the specifications for those objects. Examples include the function objects
specified in 22.10.6 [refwrap]- [negators]. I therefore suggest that
22.10 [function.objects]/5 simply be removed from the standard. I don't think it adds anything
except opportunities for confusion.
[2012-10 Portland: Move to Open]
This wording caused confusion earlier in the week when reviewing Stefan's paper on greater<>
.
This phrasing sounds normative, but is actually descriptive but uses unfortunate wording.
The main reason this wording exists is to document the protocol required to support the legacy binders in Annex D.
Stefan points out that unary_negate
and binary_negate
have not been deprecated and rely
on this. He plans a paper to remove this dependency.
Consensus that this wording is inadequate, confusing, and probably should be removed. However, that leaves a big hole in the specification for the legacy binders, that needs filling.
While not opposed to striking this paragraph, we will need the additional wording to fix the openning hole before this issue can move forward.
[ 2013-04-14 STL provides rationale ]
Rationale:
I've concluded that Scott's original proposed resolution was correct and complete. There are two sides to this story: the producers and the consumers of these typedefs.
Producers: As Scott noted, the Standard clearly documents which function objects must provide these
typedefs. Some function objects must provide them unconditionally (e.g. plus<T>
(for T != void
),
22.10.7 [arithmetic.operations]/1), some conditionally (e.g. reference_wrapper<T>
,
22.10.6 [refwrap]/2-4), and some don't have to provide them at all (e.g. lambdas, 7.5.6 [expr.prim.lambda]).
These requirements are clear, so we shouldn't change them or even add informative notes. Furthermore, because these
typedefs aren't needed in the C++11 world with decltype
/perfect forwarding/etc., we shouldn't add more
requirements to provide them.
Consumers: This is what we were concerned about at Portland. However, the consumers also clearly document
their requirements in the existing text. For example, reference_wrapper<T>
is also a conditional consumer,
and 22.10.6 [refwrap] explains what typedefs it's looking for. We were especially concerned about the old negators
and the deprecated binders, but they're okay too. [negators] clearly says that
unary_negate<Predicate>
requires Predicate::argument_type
to be a type, and
binary_negate<Predicate>
requires Predicate::first_argument_type
and Predicate::second_argument_type
to be types. (unary_negate
/binary_negate
provide result_type
but they don't consume it.)
99 [depr.lib.binders] behaves the same way with Fn::first_argument_type
, Fn::second_argument_type
,
and Fn::result_type
. No additional wording is necessary.
A careful reading of 22.10 [function.objects]/5 reveals that it wasn't talking about anything beyond the mere
existence of the mentioned typedefs — for example, it didn't mention that the function object's return type should be
result_type
, or even convertible to result_type
. As the producers and consumers are certainly talking about
the existence of the typedefs (in addition to clearly implying semantic requirements), we lose nothing by deleting the
unnecessary paragraph.
[2013-04-18, Bristol]
Previous wording:
Remove 22.10 [function.objects] p5:
To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects correspondingly provide typedefsargument_type
andresult_type
for function objects that take one argument andfirst_argument_type
,second_argument_type
, andresult_type
for function objects that take two arguments.
Proposed resolution:
This wording is relative to N3485.
Edit 22.10 [function.objects] p5:
[Note:To enable adaptors and other components to manipulate function objects that take one or two arguments
it is required that the function objectsmany of the function objects in this clause correspondingly provide typedefsargument_type
andresult_type
for function objects that take one argument andfirst_argument_type
,second_argument_type
, andresult_type
for function objects that take two arguments.— end note]
find_end
Section: 26.6.8 [alg.find.end] Status: C++14 Submitter: Andrew Koenig Opened: 2012-03-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
26.6.8 [alg.find.end] describes the behavior of find_end as returning:
The last iterator
i
in the range[first1,last1 - (last2 - first2))
such that for any nonnegative integern < (last2 - first2)
, the following corresponding conditions hold:*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false
.
Does "for any" here mean "for every" or "there exists a"? I think it means the former, but it could be interpreted either way.
Daniel: The same problem exists for the following specifications from Clause 26 [algorithms]:[2013-04-20, Bristol]
Unanimous agreement on the wording.
Resolution: move to tentatively ready[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to N3376.
Change 26.6.8 [alg.find.end] p2 as indicated:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 find_end(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 find_end(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);[…]
-2- Returns: The last iteratori
in the range[first1,last1 - (last2 - first2))
such that foranyevery nonnegative integern < (last2 - first2)
, the following corresponding conditions hold:*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false
. Returnslast1
if[first2,last2)
is empty or if no such iterator is found.
Change 26.6.15 [alg.search] p2 and p6 as indicated:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);[…]
-2- Returns: The first iteratori
in the range[first1,last1 - (last2-first2))
such that foranyevery nonnegative integern
less thanlast2 - first2
the following corresponding conditions hold:*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false
. Returnsfirst1
if[first2,last2)
is empty, otherwise returnslast1
if no such iterator is found.
[…]
template<class ForwardIterator, class Size, class T> ForwardIterator search_n(ForwardIterator first, ForwardIterator last, Size count, const T& value); template<class ForwardIterator, class Size, class T, class BinaryPredicate> ForwardIterator search_n(ForwardIterator first, ForwardIterator last, Size count, const T& value, BinaryPredicate pred);[…]
-6- Returns: The first iteratori
in the range[first,last-count)
such that foranyevery non-negative integern
less thancount
the following corresponding conditions hold:*(i + n) == value, pred(*(i + n),value) != false
. Returnslast
if no such iterator is found.
Change 26.7.10 [alg.reverse] p4 as indicated:
template<class BidirectionalIterator, class OutputIterator> OutputIterator reverse_copy(BidirectionalIterator first, BidirectionalIterator last, OutputIterator result);[…]
-4- Effects: Copies the range[first,last)
to the range[result,result+(last-first))
such that foranyevery non-negative integeri < (last - first)
the following assignment takes place:*(result + (last - first) - i) = *(first + i)
.
Change 26.8.5 [alg.partitions] p5 and p9 as indicated:
template<class ForwardIterator, class Predicate> ForwardIterator partition(ForwardIterator first, ForwardIterator last, Predicate pred);[…]
-5- Returns: An iteratori
such that foranyevery iteratorj
in the range[first,i) pred(*j) != false
, and foranyevery iteratork
in the range[i,last), pred(*k) == false
.
[…]
template<class BidirectionalIterator, class Predicate> BidirectionalIterator stable_partition(BidirectionalIterator first, BidirectionalIterator last, Predicate pred);[…]
-9- Returns: An iteratori
such that foranyevery iteratorj
in the range[first,i), pred(*j) != false
, and foranyevery iteratork
in the range[i,last), pred(*k) == false
. The relative order of the elements in both groups is preserved.
Change 26.8 [alg.sorting] p5 as indicated:
-5- A sequence is sorted with respect to a comparator
comp
if foranyevery iteratori
pointing to the sequence andanyevery non-negative integern
such thati + n
is a valid iterator pointing to an element of the sequence,comp(*(i + n), *i) == false
.
Change 26.8.3 [alg.nth.element] p1 as indicated:
template<class RandomAccessIterator> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare comp);-1- After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also foranyevery iteratori
in the range[first,nth)
andanyevery iteratorj
in the range[nth,last)
it holds that:!(*i > *j)
orcomp(*j, *i) == false
.
Change 26.8.4.2 [lower.bound] p2 as indicated:
template<lass ForwardIterator, class T> ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class T, class Compare> ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last, const T& value, Compare comp);[…]
-2- Returns: The furthermost iteratori
in the range[first,last]
such that foranyevery iteratorj
in the range[first,i)
the following corresponding conditions hold:*j < value
orcomp(*j, value) != false
.
Change 26.8.4.3 [upper.bound] p2 as indicated:
template<lass ForwardIterator, class T> ForwardIterator upper_bound(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class T, class Compare> ForwardIterator upper_bound(ForwardIterator first, ForwardIterator last, const T& value, Compare comp);[…]
-2- Returns: The furthermost iteratori
in the range[first,last]
such that foranyevery iteratorj
in the range[first,i)
the following corresponding conditions hold:!(value < *j)
orcomp(value, *j) == false
.
Change 26.8.9 [alg.min.max] p21 and p23 as indicated:
template<class ForwardIterator> ForwardIterator min_element(ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Compare> ForwardIterator min_element(ForwardIterator first, ForwardIterator last, Compare comp);-21- Returns: The first iterator
i
in the range[first,last)
such that foranyevery iteratorj
in the range[first,last)
the following corresponding conditions hold:!(*j < *i)
orcomp(*j, *i) == false
. Returnslast
iffirst == last
.
[…]
template<class ForwardIterator> ForwardIterator max_element(ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Compare> ForwardIterator max_element(ForwardIterator first, ForwardIterator last, Compare comp);-23- Returns: The first iterator
i
in the range[first,last)
such that foranyevery iteratorj
in the range[first,last)
the following corresponding conditions hold:!(*i < *j)
orcomp(*i, *j) == false
. Returnslast
iffirst == last
.
basic_string<>::swap
semantics ignore allocatorsSection: 27.4.3.2 [string.require] Status: Resolved Submitter: Robert Shearer Opened: 2012-04-13 Last modified: 2018-11-25
Priority: 3
View all other issues in [string.require].
View all issues with Resolved status.
Discussion:
In C++11, basic_string
is not described as a "container", and is not governed by the allocator-aware
container semantics described in sub-clause 23.2 [container.requirements]; as a result, and
requirements or contracts for the basic_string
interface must be documented in Clause
27 [strings].
swap
member function with no requirements, and
with guarantees to execute in constant time without throwing. Fulfilling such a contract is not reasonable
in the presence of unequal non-propagating allocators.
In contrast, 23.2.2 [container.requirements.general] p7 declares the behavior of member swap
for containers with unequal non-propagating allocators to be undefined.
Resolution proposal:
Additional language from Clause 23 [containers] should probably be copied to Clause
27 [strings]. I will refrain from an exactly recommendation, however, as I am raising further
issues related to the language in Clause 23 [containers].
[2013-03-15 Issues Teleconference]
Moved to Open.
Alisdair has offered to provide wording.
Telecon notes that 23.2.2 [container.requirements.general]p13 says that string
is an
allocator-aware container.
Resolved by the adoption of P1148 in San Diego.
Proposed resolution:
Section: 29.5.3.3 [rand.req.urng] Status: Resolved Submitter: John Salmon Opened: 2012-04-26 Last modified: 2020-10-06
Priority: 4
View all other issues in [rand.req.urng].
View all issues with Resolved status.
Discussion:
The expressions G::min()
and G::max()
in Table 116 in 29.5.3.3 [rand.req.urng] are specified
as having "compile-time" complexity.
static int min();
then is the method required to have a constexpr
qualifier? I believe the standard would benefit from
clarification of this point.
[2018-12-08; Tim Song comments]
This issue was resolved by P0898R3 and the subsequent editorial rewrite of this subclause.
[2020-10-06; Reflector discussions]
Resolved by P0898R3 voted in Rapperswil.
Rationale:
Resolved by P0898R3.
Proposed resolution:
__bool_true_false_are_defined
should be removedSection: 17.14 [support.runtime] Status: Resolved Submitter: Thomas Plum Opened: 2012-04-30 Last modified: 2020-11-09
Priority: 4
View all other issues in [support.runtime].
View all issues with Resolved status.
Discussion:
Since C99, the C standard describes a macro named __bool_true_false_are_defined
.
#if !defined(__bool_true_false_are_defined) #define bool int /* or whatever */ #define true 1 #define false 0 #endif
Then when the project was compiled by a "new" compiler that implemented bool
as defined by the
evolving C++98 or C99 standards, those lines would be skipped; but when compiled by an "old" compiler that
didn't yet provide bool
, true
, and false
, then the #define
's would provide a
simulation that worked for most purposes.
Headers
<csetjmp>
(nonlocal jumps),<csignal>
(signal handling),<cstdalign>
(alignment),<cstdarg>
(variable arguments),<cstdbool>
(__bool_true_false_are_defined
).<cstdlib>
(runtime environmentgetenv()
,system()
), and<ctime>
(system clockclock()
,time()
) provide further compatibility with C code.
However, para 2 says
"The contents of these headers are the same as the Standard C library headers
<setjmp.h>
,<signal.h>
,<stdalign.h>
,<stdarg.h>
,<stdbool.h>
,<stdlib.h>
, and<time.h>
, respectively, with the following changes:",
and para 8 says
"The header
<cstdbool>
and the header<stdbool.h>
shall not define macros namedbool
,true
, orfalse
."
Thus para 8 doesn't exempt the C++ implementation from the arguably clear requirement of the C standard, to
provide a macro named __bool_true_false_are_defined
defined to be 1.
__bool_true_false_are_defined
. In that case, the name belongs to implementers to provide, or not, as
they choose.
[2013-03-15 Issues Teleconference]
Moved to Open.
While not strictly necessary, the clean-up look good.
We would like to hear from our C liaison before moving on this issue though.
[2015-05 Lenexa]
LWG agrees. Jonathan provides wording.
[2017-03-04, Kona]
The reference to <cstdbool> in p2 needs to be struck as well. Continue the discussion on the reflector once the DIS is available.
Previous resolution [SUPERSEDED]:
This wording is relative to N4296.
Edit the footnote on 16.4.2.3 [headers] p7:
176) In particular, including any of the standard headers
<stdbool.h>
,<cstdbool>
,<iso646.h>
or<ciso646>
has no effect.Edit 17.14 [support.runtime] p1 as indicated (and remove the index entry for
__bool_true_false_are_defined
):-1- Headers
<csetjmp>
(nonlocal jumps),<csignal>
(signal handling),<cstdalign>
(alignment),<cstdarg>
(variable arguments),<cstdbool>
,(__bool_true_false_are_defined
).<cstdlib>
(runtime environmentgetenv()
,system()
), and<ctime>
(system clockclock()
,time()
) provide further compatibility with C code.Remove Table 38 — Header
<cstdbool>
synopsis [tab:support.hdr.cstdbool] from 17.14 [support.runtime]
Table 38 — Header <cstdbool>
synopsisType Name(s) Macro: __bool_true_false_are_defined
[2019-03-18; Daniel comments and eliminates previously proposed wording]
With the approval of P0619R4 in Rapperswil, the offending wording has now been eliminated from the working draft. I suggest to close this issue as: Resolved by P0619R4.
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Rationale:
Resolved by P0619R4.Proposed resolution:
reserve(n)
reserves for n-1
elementsSection: 23.2.8 [unord.req] Status: C++17 Submitter: Daniel James Opened: 2012-05-07 Last modified: 2017-07-30
Priority: 3
View other active issues in [unord.req].
View all other issues in [unord.req].
View all issues with C++17 status.
Discussion:
I think that unordered containers' reserve
doesn't quite do what it should. I'd expect after calling
x.reserve(n)
to be able to insert n
elements without invalidating iterators. But as
the standard is written (I'm looking at n3376), I think the guarantee only holds for n-1
elements.
max_load_factor
of 1
, reserve(n)
is equivalent to
rehash(ceil(n/1))
, ie. rehash(n)
. rehash(n)
requires that the bucket
count is >= n
, so it can be n
(Table 103). The rule is that insert
shall not affect the validity of iterators if (N + n) < z * B
(23.2.8 [unord.req] p15).
But for this case the two sides of the equation are equal, so insert
can affect the validity of iterators.
[2013-03-16 Howard comments and provides wording]
Given the following:
LF := load_factor() MLF := max_load_factor() S := size() B := bucket_count() LF == S/B
The container has an invariant:
LF <= MLF
Therefore:
MLF >= S/B S <= MLF * B B >= S/MLF
[2013-03-15 Issues Teleconference]
Moved to Open.
Howard to provide rationale and potentally revised wording.
[2012-02-12 Issaquah : recategorize as P3]
Jonathan Wakely: submitter is Boost.Hash maintainer. Think it's right.
Marshall Clow: even if wrong it's more right than what we have now
Geoffrey Romer: issue is saying rehash should not leave container in such a state that a notional insertion of zero elements should not trigger a rehash
AJM: e.g. if you do a range insert from an empty range
AJM: we don't have enough brainpower to do this now, so not priority zero
Recategorised as P3
[Lenexa 2015-05-06: Move to Ready]
Proposed resolution:
This wording is relative to N3485.
In 23.2.8 [unord.req] Table 103 — Unordered associative container requirements, change the post-condition
in the row for a.rehash(n)
to:
Post:a.bucket_count() >= a.size() / a.max_load_factor()
anda.bucket_count() >= n
.
In 23.2.8 [unord.req]/p15 change
Theinsert
andemplace
members shall not affect the validity of iterators if(N+n) <= z * B
, whereN
is the number of elements in the container prior to the insert operation,n
is the number of elements inserted,B
is the container's bucket count, andz
is the container's maximum load factor.
atomic_flag
initializationSection: 32.5.10 [atomics.flag] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2012-05-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.flag].
View all issues with C++14 status.
Discussion:
32.5.10 [atomics.flag]/4 describes the ATOMIC_FLAG_INIT
, but it's not quite clear about a
couple of points:
it's said that ATOMIC_FLAG_INIT
"can be used to initialize an object of type atomic_flag
"
and the following example:
std::atomic_flag guard = ATOMIC_FLAG_INIT;
is presented. It's not clear whether the macro can also be used in the other initialization contexts:
std::atomic_flag guard ATOMIC_FLAG_INIT; std::atomic_flag guard {ATOMIC_FLAG_INIT}; struct A { std::atomic_flag flag; A(); }; A::A() : flag (ATOMIC_FLAG_INIT); A::A() : flag {ATOMIC_FLAG_INIT};
Please also note that examples are non-normative, according to the ISO directives, meaning that the wording presents no normative way to use the macro.
it's said that "It is unspecified whether an uninitialized atomic_flag
object has an initial state
of set or clear.". I believe the use of "uninitialized" is inappropriate. First of all, if an object is
uninitialized it is obvious that we cannot assert anything about its state. Secondly, it doesn't address the
following cases:
std::atomic_flag a; // object is "initialized" by trivial default constructor std::atomic_flag a {}; // object is value-initialized static std::atomic_flag a; // object is zero-initialized
strictly speaking a trivial constructor "initializes" the object, although it doesn't actually initialize the sub-objects.
it's said that "For a static-duration object, that initialization shall be static.". Considering the following example:
struct A { A(); // user-provided, not constexpr std::atomic_flag flag = ATOMIC_FLAG_INIT; // possibly other non-static data members }; static A a;
The object a.flag
(as a sub-object of the object a
) has static-duration, yet the initialization
has to be dynamic because A::A
is not constexpr
.
[2012, Portland]
We would like to be able to allow more initialisation contexts for example:
However we need further input from experts with implementation specific knowledge to identify which additional contexts (if any) would be universally valid.
Moved to open
[2013, Chicago]
Move to Immediate, following review.
Some discussion over the explicit use of only copy initialization, and not direct initialization. This is necessary to
allow the implementation of atomic_flag
as an aggregate, and may be further reviewed in the future.
Accept for Working Paper
Proposed resolution:
[This wording is relative to N3376.]
Change 32.5.10 [atomics.flag]/4 as follows:
The macro
ATOMIC_FLAG_INIT
shall be defined in such a way that it can be used to initialize an object of typeatomic_flag
to the clear state. The macro can be used in the form:atomic_flag guard = ATOMIC_FLAG_INIT;It is unspecified whether the macro can be used in other initialization contexts. For a complete static-duration object, that initialization shall be static.
It is unspecified whether an uninitializedUnless initialized withATOMIC_FLAG_INIT
, it is unspecified whether anatomic_flag
object has an initial state of set or clear.[ Example:atomic_flag guard = ATOMIC_FLAG_INIT;
— end example ]
resize
Section: 23.3.13.3 [vector.capacity] Status: C++17 Submitter: Daniel Krügler Opened: 2012-06-07 Last modified: 2017-07-30
Priority: 1
View other active issues in [vector.capacity].
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View all issues with C++17 status.
Discussion:
As part of resolving LWG issue 2033(i) a wording change was done for resize()
to
respect the problem mentioned in the question:
Does a call to 'void resize(size_type sz)' of
std::vector
require the element type to beMoveAssignable
because the callerase(begin() + sz, end())
mentioned in the Effects paragraph would require the element type to beMoveAssignable
?
The wording change was to replace in 23.3.5.3 [deque.capacity] and 23.3.13.3 [vector.capacity]:
-1- Effects: If
sz <= size()
, equivalent toerase(begin() + sz, end())
; […]
by:
-1- Effects: If
sz <= size()
, equivalent to callingpop_back() size() - sz
times. […]
The overlooked side-effect of this wording change is that this implies a destruction order
of the removed elements to be in reverse order of construction, but the previous version
did not impose any specific destruction order due to the way how the semantics of erase
is specified in Table 100.
#include <vector>
#include <iostream>
struct Probe {
int value;
Probe() : value(0) {}
Probe(int value) : value(value) {}
~Probe() { std::cout << "~Probe() of " << value << std::endl; }
};
int main() {
std::vector<Probe> v;
v.push_back(Probe(1));
v.push_back(Probe(2));
v.push_back(Probe(3));
std::cout << "---" << std::endl;
v.resize(0);
}
the last three lines of the output for every compiler I tested was:
~Probe() of 1 ~Probe() of 2 ~Probe() of 3
but a conforming implementation would now need to change this order to
~Probe() of 3 ~Probe() of 2 ~Probe() of 1
This possible stringent interpretation makes sense, because one can argue that sequence containers
(or at least std::vector
) should have the same required destruction order of it's elements,
as elements of a C array or controlled by memory deallocated with an array delete
have.
I also learned that libc++ does indeed implement std::vector::resize
in a way that the
second output form is observed.
std::vector
this was not required in C++03 and this request may be too strong. My current
suggestion would be to restore the effects of the previous wording in regard to the destruction order,
because otherwise several currently existing implementations would be broken just because of this
additional requirement.
[2013-03-15 Issues Teleconference]
Moved to Open.
Jonathan says that he believes this is a valid issue.
Walter wonders if this was intended when we made the previous change - if so, this would be NAD.
Jonathan said that Issue 2033(i) doesn't mention ordering.
Walter then asked if anyone is really unhappy that we're destroying items in reverse order of construction.
Jonathan points out that this conflicts with existing practice (libstc++, but not libc++).
Jonathan asked for clarification as to whether this change was intended by 2033(i).
[2014-06 Rapperswil]
Daniel points out that the ordering change was not intended.
General agreement that implementations should not be required to change.[2014-06-28 Daniel provides alternative wording]
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 23.3.5.3 [deque.capacity] as indicated: [Drafting note: The chosen wording form is similar to that for
forward_list
. Note that the existing Requires element already specifies the necessary operational requirements
on the value type. — end drafting note]
void resize(size_type sz);-1- Effects: If
[…]sz <
, erases the last=size()size() - sz
elements from the sequenceequivalent to calling. Otherwisepop_back() size() - sz
timesIf, appendssize() <= sz
sz - size()
default-inserted elements to the sequence.void resize(size_type sz, const T& c);-3- Effects: If
[…]sz <
, erases the last=size()size() - sz
elements from the sequenceequivalent to calling. Otherwisepop_back() size() - sz
timesIf, appendssize() < sz
sz - size()
copies ofc
to the sequence.
Change 23.3.13.3 [vector.capacity] as indicated: [Drafting note: See deque
for the rationale of the
used wording. — end drafting note]
void resize(size_type sz);-12- Effects: If
[…]sz <
, erases the last=size()size() - sz
elements from the sequenceequivalent to calling. Otherwisepop_back() size() - sz
timesIf, appendssize() < sz
sz - size()
default-inserted elements to the sequence.void resize(size_type sz, const T& c);-15- Effects: If
[…]sz <
, erases the last=size()size() - sz
elements from the sequenceequivalent to calling. Otherwisepop_back() size() - sz
timesIf, appendssize() < sz
sz - size()
copies ofc
to the sequence.
allocator_traits::max_size
missing noexcept
Section: 16.4.4.6 [allocator.requirements], 20.2.9.3 [allocator.traits.members], 20.2.9 [allocator.traits] Status: C++14 Submitter: Bo Persson Opened: 2012-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++14 status.
Discussion:
N3376 describes in 20.2.9.3 [allocator.traits.members]/7
static size_type max_size(Alloc& a);Returns:
a.max_size()
if that expression is well-formed; otherwise,numeric_limits<size_type>::max()
.
The max_size
function is supposed to call one of two functions that are both noexcept
.
To make this intermediate function useful for containers, it should preserve the noexcept
attribute.
static size_type max_size(Alloc& a) noexcept;
[2012-08-05 Daniel comments]
On the first sight this does not seem like a defect of the specification, because the Allocator requirements in
16.4.4.6 [allocator.requirements] (Table 28) do not impose a no-throw requirement onto max_size()
;
the table just describes the fall-back implementation for max_size()
if a given allocator does
not provide such a function.
std::allocator
as a special model of this concept and is allowed to increase the exception-guarantees
for max_size()
, but this does not imply a corresponding rules for other allocators.
Furthermore, max_size()
of Containers is not specified in terms of
Allocator::max_size()
, so again this is not a real contradiction.
Nonetheless I think that the following stronger decision should be considered:
Require that for all Allocators (as specified in 16.4.4.6 [allocator.requirements]) max_size()
never throws an exception. This would it make much more useful to call this function in situations where no
exception should leave the context.
Require that for all Allocators (as specified in 16.4.4.6 [allocator.requirements]) max_size()
can be called on const allocator object. Together with the previous item this would allow an implementation
of a container's max_size()
function to delegate to the allocator's max_size()
function.
In regard to the second statement it should be mentioned that there are two current specification deviations from that in the draft:
The synopsis of 20.2.9 [allocator.traits] uses a const allocator argument as part of the signature
of the max_size
function.
Both the synopsis of 20.6.1 [allocator.adaptor.syn] and the member specification in
20.6.4 [allocator.adaptor.members] p8 declare scoped_allocator_adaptor::max_size
as const member function, but this function delegates to
allocator_traits<OuterAlloc>::max_size(outer_allocator())
where outer_allocator()
resolves to the member function overload returning a
const outer_allocator_type&
.
The question arises whether these current defects actually point to a defect in the Allocator requirements and should be fixed there.
[ 2012-10 Portland: Move to Review ]
Consensus that the change seems reasonable, and that for any given type the template is intantiated with the contract should be 'wide' so this meets the guidelines we agreed in Madrid for C++11.
Some mild concern that while we don't imagine many allocator implementations throwing on this method,
it is technically permited by current code that we would not be breaking, by turning throw
expressions into disguised terminate
calls. In this case, an example might be an
instrumented 'logging' allocator that writes every function call to a log file or database, and might
throw if that connection/file were no longer available.
Another option would be to make exception spefication a conditional no-except, much like we do for
some swap
functions and assignment operators. However, this goes against the intent of the
Madrid adoption of noexcept
which is that vendors are free to add such extensions, but we
look for a clear line in the library specification, and do not want to introduce conditional-noexcept
piecemeal. A change in our conventions here would require a paper addressing the library specification
as a whole.
Consensus was to move forward, but move the issue only to Review rather than Ready to allow time for further comments. This issue should be considered 'Ready' next time it is reviewed unless we get such comments in the meantime.
[2013-04-18, Bristol]
Proposed resolution:
In 20.2.9 [allocator.traits] and 20.2.9.3 [allocator.traits.members]/7, change the function signature to
static size_type max_size(Alloc& a) noexcept;
nth_element
requires inconsistent post-conditionsSection: 26.8.3 [alg.nth.element] Status: C++14 Submitter: Peter Sommerlad Opened: 2012-07-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.nth.element].
View all issues with C++14 status.
Discussion:
The specification of nth_element refers to operator<
whereas all sorting without a compare function is based on
operator<
. While it is true that for all regular types both operators should be defined accordingly, all other
sorting algorithms only rely on existence of operator<
. So I guess the paragraph p1
After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also for any iteratori
in the range[first,nth)
and any iteratorj
in the range[nth,last)
it holds that:!(*i > *j)
orcomp(*j, *i) == false
.
should read
After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also for any iteratori
in the range[first,nth)
and any iteratorj
in the range[nth,last)
it holds that:!(*j < *i)
orcomp(*j, *i) == false
.
Note only "!(*i > *j)
" was changed to "!(*j < *i)
" and it would be more symmetric with
comp(*j, *i)
as well.
[ 2012-10 Portland: Move to Ready ]
This is clearly correct by inspection, moved to Ready by unanimous consent.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change 26.8.3 [alg.nth.element] p1 as indicated:
template<class RandomAccessIterator> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare comp);-1- After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also for any iteratori
in the range[first,nth)
and any iteratorj
in the range[nth,last)
it holds that:or
!(*i > *j)!(*j < *i)comp(*j, *i) == false
.
vector.emplace(vector.begin(), vector.back())
?Section: 23.3.13.5 [vector.modifiers], 23.2 [container.requirements] Status: C++20 Submitter: Howard Hinnant Opened: 2012-07-07 Last modified: 2021-02-25
Priority: 2
View all other issues in [vector.modifiers].
View all issues with C++20 status.
Discussion:
Nikolay Ivchenkov recently brought the following example on the std-discussion newsgroup, asking whether the following program well-defined:
#include <iostream> #include <vector> int main() { std::vector<int> v; v.reserve(4); v = { 1, 2, 3 }; v.emplace(v.begin(), v.back()); for (int x : v) std::cout << x << std::endl; }
Nikolay Ivchenkov:
I think that an implementation ofvector
's 'emplace' should initialize an intermediate object with
v.back()
before any shifts take place, then perform all necessary shifts and finally replace the
value pointed to by v.begin()
with the value of the intermediate object. So, I would expect the
following output:
3 1 2 3
GNU C++ 4.7.1 and GNU C++ 4.8.0 produce other results:
2 1 2 3
Howard Hinnant:
I believe Nikolay is correct that vector should initialize an intermediate object withv.back()
before any shifts take place. As Nikolay pointed out in another email, this appears to be the only way to
satisfy the strong exception guarantee when an exception is not thrown by T
's copy constructor,
move constructor, copy assignment operator, or move assignment operator as specified by
23.3.13.5 [vector.modifiers]/p1. I.e. if the emplace construction throws, the vector must remain unaltered.
That leads to an implementation that tolerates objects bound to the function parameter pack of the emplace
member function may be elements or sub-objects of elements of the container.
My position is that the standard is correct as written, but needs a clarification in this area. Self-referencing
emplace
should be legal and give the result Nikolay expects. The proposed resolution of LWG 760(i)
is not correct.
[2015-02 Cologne]
LWG agrees with the analysis including the assessment of LWG 760(i) and would appreciate a concrete wording proposal.
[2015-04-07 dyp comments]
The Standard currently does not require that creation of such intermediate objects is legal. 23.2.4 [sequence.reqmts] Table 100 — "Sequence container requirements" currently specifies:
Table 100 — Sequence container requirements Expression Return type Assertion/note
pre-/post-condition…
a.emplace(p, args);
iterator
Requires: T
isEmplaceConstructible
intoX
fromargs
. Forvector
anddeque
,T
is alsoMoveInsertable
intoX
andMoveAssignable
. […]…
The EmplaceConstructible
concept is defined via
allocator_traits<A>::construct
in 23.2.2 [container.requirements.general] p15.5 That's surprising to me
since the related concepts use the suffix Insertable
if they
refer to the allocator. An additional requirement such as
std::is_constructible<T, Args...>
is necessary to allow
creation of intermediate objects.
The creation of intermediate objects also affects other functions, such
as vector.insert
. Since aliasing the vector is only allowed for
the single-element forms of insert
and emplace
(see
526(i)), the range-forms are not affected. Similarly,
aliasing is not allowed for the rvalue-reference overload. See also LWG
2266(i).
There might be a problem with a requirement of
std::is_constructible<T, Args...>
related to the issues
described in LWG 2461(i). For example, a scoped allocator
adapter passes additional arguments to the constructor of the value
type. This is currently not done in recent implementations of libstdc++
and libc++ when creating the intermediate objects, they simply create
the intermediate object by perfectly forwarding the arguments. If such
an intermediate object is then moved to its final destination in the
vector, a change of the allocator instance might be required —
potentially leading to an expensive copy. One can also imagine worse
problems, such as run-time errors (allocators not comparing equal at
run-time) or compile-time errors (if the value type cannot be created
without the additional arguments). I have not looked in detail into this
issue, but I'd be reluctant adding a requirement such as
std::is_constructible<T, Args...>
without further
investigation.
It should be noted that the creation of intermediate objects currently
is inconsistent in libstdc++ vs libc++. For example, libstdc++ creates
an intermediate object for vector.insert
, but not
vector.emplace
, whereas libc++ does the exact opposite in this
respect.
A live demo of the inconsistent creation of intermediate objects can be found here.
[2015-10, Kona Saturday afternoon]
HH: If it were easy, it'd have wording. Over the decades I have flipped 180 degrees on this. My current position is that it should work even if the element is in the same container.
TK: What's the implentation status? JW: Broken in GCC. STL: Broken in MSVS. Users complain about this every year.
MC: 526 says push_back has to work.
HH: I think you have to make a copy of the element anyway for reasons of exception safety. [Discussion of exception guarantees]
STL: vector has strong exception guarantees. Could we not just provide the Basic guarantee here.
HH: It would terrify me to relax that guarantee. It'd be an ugly, imperceptible runtime error.
HH: I agree if we had a clean slate that strong exception safety is costing us here, and we shouldn't provide it if it costs us.
STL: I have a mail here, "how can vector provide the strong guarantee when inserting in the middle".
HH: The crucial point is that you only get the strong guarantee if the exception is thrown by something other than the copy and move operations that are used to make the hole.
STL: I think we need to clean up the wording. But it does mandate currently that the self-emplacement must work, because nothings says that you can't do it. TK clarifies that a) self-emplacement must work, and b) you get the strong guarantee only if the operations for making the hole don't throw, otherwise basic. HH agrees. STL wants this to be clear in the Standard.
STL: Should it work for deque, too? HH: Yes.
HH: I will attempt wording for this.
TK: Maybe mail this to the reflector, and maybe someone has a good idea?
JW: I will definitely not come up with anything better, but I can critique wording.
Moved to Open; Howard to provide wording, with feedback from Jonathan.
[2017-01-25, Howard suggests wording]
[2018-1-26 issues processing telecon]
Status to 'Tentatively Ready' after adding a period to Howard's wording.
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Modify in 23.2.4 [sequence.reqmts] Table 87 — "Sequence container requirements" as indicated:
Table 87 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-condition[…]
a.emplace(p, args)
iterator
Requires: T
isEmplaceConstructible
intoX
fromargs
. Forvector
anddeque
,T
is also
MoveInsertable
intoX
andMoveAssignable
.
Effects: Inserts an object of typeT
constructed with
std::forward<Args>(args)...
beforep
.
[Note:args
may directly or indirectly refer to a value
ina
. — end note]
std::atomic<X>
requires X
to be nothrow default constructibleSection: 32.5.8 [atomics.types.generic], 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Jonathan Wakely Opened: 2012-07-19 Last modified: 2016-01-28
Priority: 4
View all other issues in [atomics.types.generic].
View all issues with Resolved status.
Discussion:
As raised in c++std-lib-32781, this fails to compile even though the default constructor is not used:
#include <atomic> struct X { X() noexcept(false) {} X(int) { } }; std::atomic<X> x(3);
This is because atomic<T>
's default constructor is declared to be non-throwing and
is explicitly-defaulted on its first declaration:
atomic() noexcept = default;
This is ill-formed if the implicitly-declared default constructor would not be non-throwing.
Possible solutions:atomic<T>
atomic<T>::atomic()
from the overload set if T
is not nothrow default constructible.
noexcept
from atomic<T>::atomic()
, allowing it to be
deduced (but the default constructor is intended to be always noexcept)
atomic<T>::atomic()
on its first declaration (but makes the default constructor
user-provided and so prevents atomic<T>
being trivial)
[2012, Portland: move to Core]
Recommend referring to core to see if the constructor noexcept
mismatch
can be resolved there. The issue is not specific to concurrency.
[2015-04-09 Daniel comments]
CWG issue 1778, which had been created in behalf of this LWG issue, has been resolved as a defect.
[2015-10, Kona]
Mark as resolved
Proposed resolution:
Section: 26.8.8 [alg.heap.operations] Status: C++17 Submitter: Peter Sommerlad Opened: 2012-07-09 Last modified: 2017-07-30
Priority: 3
View all other issues in [alg.heap.operations].
View all issues with C++17 status.
Discussion:
Another similar issue to the operator<
vs greater in nth_element
but not as direct occurs
in 26.8.8 [alg.heap.operations]:
-1- A heap is a particular organization of elements in a range between two random access iterators
[a,b)
. Its two key properties are:
- There is no element greater than
*a
in the range and*a
may be removed bypop_heap()
, or a new element added bypush_heap()
, in O(log(N
)) time.
As noted by Richard Smith, it seems that the first bullet should read:
*a
is not less than any element in the range
Even better the heap condition could be stated here directly, instead of leaving it unspecified, i.e.,
Each element at
(a+2*i+1)
and(a+2*i+2)
is less than the element at(a+i)
, if those elements exist, fori>=0
.
But may be that was may be intentional to allow other heap organizations?
See also follow-up discussion of c++std-lib-32780.[2016-08 Chicago]
Walter provided wording
Tues PM: Alisdair & Billy(MS) to improve the wording.
[2016-08-02 Chicago LWG]
Walter provides initial Proposed Resolution. Alisdair objects to perceived complexity of the mathematical phrasing.
Previous resolution [SUPERSEDED]:
[Note to editor: As a drive-by editorial adjustment, please replace the current enumerated list format by the numbered bullet items shown below.]
Change [alg.heap.operations]:
1 A heap is a particular organization of elements in a range between two random access iterators [a, b)
. Its two key properties aresuch that:(1.1) --
There is no element greater than*a
in the range and
For alli >= 0
,
comp(a[i], a[L])
is false whenever L = 2*i+1 < b-a,
and
comp(a[i], a[R])
is false whenever R = 2*i+2 < b-a.
(1.2) --
*a
may be removed bypop_heap()
, or a new element added bypush_heap()
, in O(log(N)) time.
[2016-08-03 Chicago LWG]
Walter and Billy O'Neal provide revised Proposed Resolution, superseding yesterday's.
Thurs PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 26.8.8 [alg.heap.operations] as indicated:
Note to project editor: As a drive-by editorial adjustment, please replace the current enumerated list format by numbered bullet items.
-1- A heap is a particular organization of elements in a range between two random access iterators
[a, b)
. Its two key properties aresuch that:
(1.1) —
There is no element greater thanWith , for all , ,*a
in the range andcomp(a[
], a[])
isfalse
.[Note to the project editor: In LaTeX the above insertion should be expressed as follows:
With $N =b
-a
$, for all $i$, $0 < i < N$,comp(a[$\left \lfloor{\frac{i-1}{2}}\right \rfloor$], a[$i$])
isfalse
.](1.2) —
*a
may be removed bypop_heap()
, or a new element added bypush_heap()
, in 𝒪(log(N)) time.
uniform_real_distribution
constructorSection: 29.5.9.2.2 [rand.dist.uni.real] Status: C++17 Submitter: Marshall Clow Opened: 2012-07-14 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
uniform_real says in 29.5.9.2.2 [rand.dist.uni.real] p1:
A
uniform_real_distribution
random number distribution produces random numbersx
,a ≤ x < b
,
but also that (29.5.9.2.2 [rand.dist.uni.real] p2):
explicit uniform_real_distribution(RealType a = 0.0, RealType b = 1.0);-2- Requires:
a ≤ b
andb - a ≤ numeric_limits<RealType>::max()
.
If you construct a uniform_real_distribution<RealType>(a, b)
where there are no representable
numbers between 'a' and 'b' (using RealType
's representation) then you cannot satisfy
29.5.9.2.2 [rand.dist.uni.real].
a == b
.
[2014-11-04 Urbana]
Jonathan provides wording.
[2014-11-08 Urbana]
Moved to Ready with the note.
There remains concern that the constructors are permitting values that may (or may not) be strictly outside the domain of the function, but that is a concern that affects the design of the random number facility as a whole, and should be addressed by a paper reviewing and addressing the whole clause, not picked up in the issues list one distribution at a time. It is still not clear that such a paper would be uncontroversial.
Proposed resolution:
This wording is relative to N4140.
Add a note after paragraph 1 before the synopsis in 29.5.9.2.2 [rand.dist.uni.real]:
-1- A
uniform_real_distribution
random number distribution produces random numbers , , distributed according to the constant probability density function.
[Note: This implies that is undefined when
a == b
. — end note]
Drafting note: should be in math font, and
a == b
should be in code font.
reset()
requirements in unique_ptr
specializationSection: 20.3.1.4.5 [unique.ptr.runtime.modifiers] Status: C++14 Submitter: Geoffrey Romer Opened: 2012-07-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.runtime.modifiers].
View all issues with C++14 status.
Discussion:
In 20.3.1.4.5 [unique.ptr.runtime.modifiers]/p1-2 of N3376, the description of reset()
in the
array specialization of unique_ptr
partially duplicates the description of the base template method
(as specified in 20.3.1.3.6 [unique.ptr.single.modifiers]/p3-5), but lacks some significant requirements.
Specifically, the text introduced in LWG 998(i), and item 13 of LWG 762(i), is present
only in the base template, not the specialization.
reset()
operation order addressed by LWG 998(i)
applies just as much to the derived template as to the base template, and the derived template has just as
much need to rely on get_deleter()(get())
being well-defined, well-formed, and not throwing exceptions
(arguably some of those properties follow from the fact that T
is required to be a complete type, but
not all).
Assuming the derived template's reset()
semantics are intended to be identical to the base template's,
there is no need to explicitly specify the semantics of reset(pointer p)
at all (since
20.3.1.4 [unique.ptr.runtime]/3 specifies "Descriptions are provided below only for member functions that
have behavior different from the primary template."), and reset(nullptr_t p)
can be specified by
reference to the 'pointer' overload. This is more concise, and eliminates any ambiguity about intentional vs.
accidental discrepancies.
[2012-10 Portland: Move to Ready]
This resolution looks blatantly wrong, as it seems to do nothing but defer to primary template
where we should describe the contract here.
Ongoing discussion points out that the primary template has a far more carefully worded semantic
for reset(p)
that we would want to copy here.
STL points out that we need the nullptr
overload for this dynamic-array form, as there is
a deleted member function template that exists to steal overloads of pointer-to-derived, avoiding
undifined behavior, so we need the extra overload.
Finally notice that there is blanket wording further up the clause saying we describe only changes from the primary template, so the proposed wording is in fact exactly correct. Move to Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change 20.3.1.4.5 [unique.ptr.runtime.modifiers] as indicated:
void reset(pointer p = pointer()) noexcept;void reset(nullptr_t p) noexcept;-1- Effects:
IfEquivalent toget() == nullptr
there are no effects. Otherwiseget_deleter()(get())
reset(pointer())
.-2- Postcondition:get() == p
.
DefaultConstructible
Section: 16.4.4.2 [utility.arg.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2012-07-19 Last modified: 2017-07-30
Priority: 2
View all other issues in [utility.arg.requirements].
View all issues with C++17 status.
Discussion:
The lack of the definition of the DefaultConstructible
requirements in C++03 was fixed
by LWG 724(i) at a time where the core rules of list-initialization were slightly
different than today, at that time value-initialization (shortly) was the primary rule for
class types, i.e. just before applying CWG 1301,
CWG 1324, and
CWG 1368.
DefaultConstructible
requirements anymore, because we require that
T u{};
value-initializes the object u
.
[ 2012-10 Portland: Move to Core ]
We are not qualified to pick apart the Core rules quickly at this point, but the consensus is that if the core language has changed in this manner, then the fix should similarly be applied in Core - this is not something that we want users of the language to have to say every time they want to Value initialize (or aggregate initialize) an object.
More to Open until we get a clear response from Core, Alisdair to file an issue with Mike.
[2013-04 Bristol: Back to Library]
The Core Working group opened, discussed, and resolved CWG 1578 as NAD for this library-related problem: Empty aggregate initialization and value-initialization are different core language concepts, and this difference can be observed (e.g. for a type with a deleted default-constructor).
[2014-02-15 Issaquah: Move to Ready]
AM: core says still LWG issue, wording has been non-controversial, move to ready?
NJ: what about durations? think they are ok
Ville: pair
and a few other have value initialize
AM: look at core 1578
AM: value initialize would require ()
, remove braces from third row?
STL: no
PH: core has new issue on aggregates and non-aggregates.
AM: right, they said does not affect this issue
NJ: why ok with pair
and tuple
?
STL: will use ()
, tuple
of aggregates with deleted constructor is ill-formed
Ville: aggregate with reference can't have ()
STL: {}
would be an issue too
Ville: aggregate with reference will have ()
deleted implicitly
Move to Ready.
Proposed resolution:
This wording is relative to N3691.
Change Table 19 in 16.4.4.2 [utility.arg.requirements] as indicated:
Expression | Post-condition |
---|---|
T t;
|
object t is default-initialized
|
T u{};
|
object u is value-initialized or aggregate-initialized
|
T() T{}
|
a temporary object of type T is value-initialized or aggregate-initialized
|
atomic_compare_exchange_*
accept v == nullptr
arguments?Section: 99 [depr.util.smartptr.shared.atomic] Status: C++14 Submitter: Howard Hinnant Opened: 2012-07-28 Last modified: 2017-11-29
Priority: Not Prioritized
View all other issues in [depr.util.smartptr.shared.atomic].
View all issues with C++14 status.
Discussion:
Looking at [util.smartptr.shared.atomic]/p31
template<class T> bool atomic_compare_exchange_strong_explicit(shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure);-31- Requires:
p
shall not be null.
What about v
? Can it be null? And if so, what happens?
Requires:
p
shall not be null.
It looks like a simple oversight to me that we did not add for the atomic_compare_exchange_*
:
Requires:
p
shall not be null andv
shall not be null.
[2012-10 Portland: Move to Ready]
This is clearly the right thing to do, and Lawrence concurs.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change [util.smartptr.shared.atomic] as indicated:
template<class T> bool atomic_compare_exchange_weak( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);-27- Requires:
[…]p
shall not be null andv
shall not be null.template<class T> bool atomic_compare_exchange_weak_explicit( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure); template<class T> bool atomic_compare_exchange_strong_explicit( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure);-31- Requires:
[…]p
shall not be null andv
shall not be null.
wstring_convert::converted()
should be noexcept
Section: 99 [depr.conversions.string] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-04-22
Priority: Not Prioritized
View other active issues in [depr.conversions.string].
View all other issues in [depr.conversions.string].
View all issues with C++14 status.
Discussion:
There is no reason wstring_convert::converted()
shouldn't be noexcept
.
wstring_convert::state()
and wbuffer_convert::state()
to be noexcept
too, depending on the requirements on mbstate_t
.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
Defer the separate discsussion of state()
to another issue, if anyone is ever motivated
to file one.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Edit in the class template wstring_convert
synopsis [conversions.string] p2:
size_t converted() const noexcept;
Edit the signature before [conversions.string] p6:
size_t converted() const noexcept;
wstring_convert
and wbuffer_convert
validitySection: 99 [depr.conversions.string], 99 [depr.conversions.buffer] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-09-07
Priority: Not Prioritized
View other active issues in [depr.conversions.string].
View all other issues in [depr.conversions.string].
View all issues with C++14 status.
Discussion:
See discussion following c++std-lib-32710.
It's not specified what happens ifwstring_convert
and wbuffer_convert
objects are constructed
with null Codecvt
pointers.
Should the constructors have preconditions that the pointers are not null? If not, are conversions expected to
fail, or is it undefined to attempt conversions if the pointers are null?
There are no observer functions to check whether objects were constructed with valid Codecvt
pointers.
If the types are made movable such observers would be necessary even if the constructors require non-null
pointers (see also LWG 2176(i)).
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Insert a new paragraph before [conversions.string] paragraph 16:
wstring_convert(Codecvt *pcvt = new Codecvt); wstring_convert(Codecvt *pcvt, state_type state); wstring_convert(const byte_string& byte_err, const wide_string& wide_err = wide_string());-?- Requires: For the first and second constructors
-16- Effects: The first constructor shall storepcvt != nullptr
.pcvt
incvtptr
and default values incvtstate
,byte_err_string
, andwide_err_string
. The second constructor shall storepcvt
incvtptr
,state
incvtstate
, and default values inbyte_err_string
andwide_err_string
; moreover the stored state shall be retained between calls tofrom_bytes
andto_bytes
. The third constructor shall storenew Codecvt
incvtptr
,state_type()
in cvtstate,byte_err
inbyte_err_string
, andwide_err
inwide_err_string
.
Insert a new paragraph before [conversions.buffer] paragraph 10:
wbuffer_convert(std::streambuf *bytebuf = 0, Codecvt *pcvt = new Codecvt, state_type state = state_type());-?- Requires:
-10- Effects: The constructor constructs a stream buffer object, initializespcvt != nullptr
.bufptr
tobytebuf
, initializescvtptr
topcvt
, and initializescvtstate
tostate
.
wstring_convert
and wbuffer_convert
Section: 99 [depr.conversions.string], 99 [depr.conversions.buffer] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-09-07
Priority: Not Prioritized
View other active issues in [depr.conversions.string].
View all other issues in [depr.conversions.string].
View all issues with C++14 status.
Discussion:
See discussion following c++std-lib-32699.
The constructors forwstring_convert
and wbuffer_convert
should be explicit, to avoid
implicit conversions which take ownership of a Codecvt
pointer and delete it unexpectedly.
Secondly, [conversions.buffer] p11 describes a destructor which is not declared in the class
synopsis in p2.
Finally, and most importantly, the definitions in [conversions.string] and
[conversions.buffer] imply implicitly-defined copy constructors and assignment operators, which
would do shallow copies of the owned Codecvt
objects and result in undefined behaviour in the
destructors.
Codecvt
is not required to be CopyConstructible
, so deep copies are not possible.
The wstring_convert
and wstring_buffer
types could be made move-only, but the proposed
resolution below doesn't do so because of the lack of preconditions regarding null Codecvt
pointers
and the absence of observer functions that would allow users to check preconditions (see also LWG 2175(i)).
[2013-03-15 Issues Teleconference]
Moved to Review.
Jonathan pointed out that you can have an implicit constructor that takes ownership of a heap reference, which would result an unexpected deletion.
No-one really likes the 'naked new' in the interface here, either.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3376.
Edit the class template wstring_convert
synopsis in [conversions.string] p2:
explicit wstring_convert(Codecvt *pcvt = new Codecvt); wstring_convert(Codecvt *pcvt, state_type state); explicit wstring_convert(const byte_string& byte_err, const wide_string& wide_err = wide_string()); ~wstring_convert(); wstring_convert(const wstring_convert&) = delete; wstring_convert& operator=(const wstring_convert&) = delete;
Edit the signatures before [conversions.string] p16:
explicit wstring_convert(Codecvt *pcvt = new Codecvt); wstring_convert(Codecvt *pcvt, state_type state); explicit wstring_convert(const byte_string& byte_err, const wide_string& wide_err = wide_string());
Edit the class template wbuffer_convert
synopsis in [conversions.buffer] p2:
explicit wbuffer_convert(std::streambuf *bytebuf = 0, Codecvt *pcvt = new Codecvt, state_type state = state_type()); ~wbuffer_convert(); wbuffer_convert(const wbuffer_convert&) = delete; wbuffer_convert& operator=(const wbuffer_convert&) = delete;
Edit the signature before [conversions.buffer] p10:
explicit wbuffer_convert(std::streambuf *bytebuf = 0, Codecvt *pcvt = new Codecvt, state_type state = state_type());
Copy/MoveInsertable
Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Loïc Joly Opened: 2012-08-10 Last modified: 2017-09-07
Priority: Not Prioritized
View other active issues in [container.requirements.general].
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Discussion:
See also discussion following c++std-lib-32883 and c++std-lib-32897.
The requirements onCopyInsertable
and MoveInsertable
are either incomplete, or complete but hard to
figure out.
From e-mail c++std-lib-32897:
Pablo Halpern:
I agree that we need semantic requirements for all of the *Insertable
concepts analogous to the requirements
we have on similar concepts.
Howard Hinnant:
I've come to believe that the standard is actually correct as written in this area. But it is really hard
to read. I would have no objection whatsoever to clarifications to CopyInsertable
as you suggest (such as the
post-conditions on v
). And I do agree with you that the correct approach to the clarifications is to
confirm that CopyInsertable
implies MoveInsertable
.
[2012, Portland: Move to Tentatively Ready]
Move to Tentatively Ready by unanimous consent.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Edit 23.2.2 [container.requirements.general] p13 as indicated:
-13- […] Given a container type
X
having anallocator_type
identical toA
and avalue_type
identical toT
and given an lvaluem
of typeA
, a pointerp
of typeT*
, an expressionv
of type (possiblyconst
)T
, and an rvaluerv
of typeT
, the following terms are defined. IfX
is not allocator-aware, the terms below are defined as ifA
werestd::allocator<T>
— no allocator object needs to be created and user specializations ofstd::allocator<T>
are not instantiated:
T
isDefaultInsertable
intoX
means that the following expression is well-formed:allocator_traits<A>::construct(m, p);An element of
X
is default-inserted if it is initialized by evaluation of the expressionallocator_traits<A>::construct(m, p);where
p
is the address of the uninitialized storage for the element allocated withinX
.
T
isinto
CopyMoveInsertableX
means that the following expression is well-formed:allocator_traits<A>::construct(m, p, rv);and when evaluated the following postconditions hold: The value of
*p
is equivalent to the value ofrv
before the evaluation. [Note:rv
remains a valid object. Its state is unspecified — end note]
T
isinto
MoveCopyInsertableX
means that, in addition to satisfying theMoveInsertable
requirements, the following expression is well-formed:allocator_traits<A>::construct(m, p,rv);and when evaluated the following postconditions hold: The value of
v
is unchanged and is equivalent to*p
.
T
isEmplaceConstructible
intoX
fromargs
, for zero or more argumentsargs
, means that the following expression is well-formed:allocator_traits<A>::construct(m, p, args);
T
isErasable
fromX
means that the following expression is well-formed:allocator_traits<A>::destroy(m, p);[Note: A container calls
allocator_traits<A>::construct(m, p, args)
to construct an element atp
usingargs
. The default construct instd::allocator
will call::new((void*)p) T(args)
, but specialized allocators may choose a different definition. — end note]
enable_shared_from_this
and construction from raw pointersSection: 20.3.2.7 [util.smartptr.enab], 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: Daniel Krügler Opened: 2012-08-16 Last modified: 2017-09-07
Priority: 3
View all other issues in [util.smartptr.enab].
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Discussion:
On reflector message c++std-lib-32927, Matt Austern asked whether the following example should be well-defined or not
struct X : public enable_shared_from_this<X> { }; auto xraw = new X; shared_ptr<X> xp1(xraw); shared_ptr<X> xp2(xraw);
pointing out that 20.3.2.2.2 [util.smartptr.shared.const] does not seem to allow it, since
xp1
and xp2
aren't allowed to share ownership, because each of them is required to have
use_count() == 1
. Despite this wording it might be reasonable (and technical possible)
to implement that request.
The
shared_ptr
constructors that create unique pointers can detect the presence of anenable_shared_from_this
base and assign the newly createdshared_ptr
to its__weak_this member
.
Now according to the specification in 20.3.2.2.2 [util.smartptr.shared.const] p3-7:
template<class Y> explicit shared_ptr(Y* p);
the notion of creating unique pointers can be read to be included by this note, because the post-condition
of this constructor is unique() == true
. Evidence for this interpretation seems to be weak, though.
auto xraw = new X; shared_ptr<X> xp1(xraw); shared_ptr<X> xp2(xraw);
He also pointed out that the current post-conditions of the affected shared_ptr
constructor
would need to be reworded.
shared_ptr
objects to prevent them from owning the same underlying object without sharing the
ownership. It might be useful to add such a requirement.
[2013-03-15 Issues Teleconference]
Moved to Open.
More discussion is needed to pick a direction to guide a proposed resolution.
[2013-05-09 Jonathan comments]
The note says the newly created shared_ptr
is assigned to the weak_ptr
member. It doesn't
say before doing that the shared_ptr
should check if the weak_ptr
is non-empty and possibly
share ownership with some other pre-existing shared_ptr
.
[2015-08-26 Daniel comments]
LWG issue 2529(i) is independent but related to this issue.
[2016-03-16, Alisdair comments]
This issues should be closed as Resolved
by paper p0033r1 at Jacksonville.
Proposed resolution:
std::seed_seq
operationsSection: 29.5.8.1 [rand.util.seedseq] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
View all issues with C++14 status.
Discussion:
29.5.8.1 [rand.util.seedseq] p1 says upfront:
No function described in this section 29.5.8.1 [rand.util.seedseq] throws an exception.
This constraint seems non-implementable to me when looking especially at the members
template<class T> seed_seq(initializer_list<T> il); template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);
which have the effect of invoking v.push_back()
for the exposition-only member of
type std::vector
(or its equivalent) over all elements of the provided range, so
out-of-memory exceptions are always possible and the seed_seq
object doesn't seem
to be constructible this way.
InputIterator
might also throw exceptions.
Aside to that it should me mentioned, that a default constructor of vector<uint_least32_t>
in theory can also throw exceptions, even though this seems less of a problem to me in this context, because
such an implementation could easily use a different internal container in seed_seq
that can hold
this no-throw exception guarantee.
Secondly, a slightly different problem category related to exceptions occurs for the member templates
template<class RandomAccessIterator> void generate(RandomAccessIterator begin, RandomAccessIterator end); template<class OutputIterator> void param(OutputIterator dest) const;
where the actual operations performed by the implementation would never need to throw, but since they invoke operations of a user-provided customization point, the overall operation, like for example
copy(v.begin(), v.end(), dest);
could also throw exceptions. In this particular example we can just think of a std::back_insert_iterator
applied to a container that needs to allocate its elements used as the type for OutputIterator
.
std::seed_seq
except the template generate
is actually needed within the library implementation, as mentioned in the
discussion of LWG 2124(i).
I suggest to remove the general no-exception constraints for operations of std::seed_seq
except for
member size()
and the default constructor and to provide specific wording for generate()
and
param()
to ensure that the algorithm itself is a nothrow operation, which is especially for
generate()
important, because the templates specified in 29.5.4 [rand.eng] and
29.5.5 [rand.adapt] also depend on this property indirectly, which is further discussed in LWG
2181(i).
Howard:
I suggest to use a different form for the exception specification, something similar to
22.10.15.4 [func.bind.bind] p4:
Throws: Nothing unless an operation on
RandomAccessIterator
throws an exception.
Daniel:
The currently suggested "what and when" form seems a bit more specific and harmonizes with the form used for function templategenerate_canonical
from 29.5.8.2 [rand.util.canonical].
[2013-04-20, Bristol]
Open an editorial issue on the exception wording ("Throws: What and when").
Solution: move to tentatively ready.[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to N3376.
Edit 29.5.8.1 [rand.util.seedseq] p1 as indicated:
-1- No function described in this section 29.5.8.1 [rand.util.seedseq] throws an exception.
Edit 29.5.8.1 [rand.util.seedseq] around p2 as indicated:
seed_seq();-2- Effects: Constructs a
-?- Throws: Nothing.seed_seq
object as if by default-constructing its memberv
.
Edit 29.5.8.1 [rand.util.seedseq] around p7 as indicated:
template<class RandomAccessIterator> void generate(RandomAccessIterator begin, RandomAccessIterator end);-7- Requires:
-8- Effects: Does nothing ifRandomAccessIterator
shall meet the requirements of a mutable random access iterator (Table 111) type. Moreover,iterator_traits<class RandomAccessIterator>::value_type
shall denote an unsigned integer type capable of accommodating 32-bit quantities.begin == end
. Otherwise, withs = v.size()
andn = end - begin
, fills the supplied range[begin, end)
according to the following algorithm […] -?- Throws: What and whenRandomAccessIterator
operations ofbegin
andend
throw.
Edit 29.5.8.1 [rand.util.seedseq] around p9 as indicated:
size_t size() const;-9- Returns: The number of 32-bit units that would be returned by a call to
-??- Throws: Nothing. -10- Complexity: constant time.param()
.
Edit 29.5.8.1 [rand.util.seedseq] around p11 as indicated:
template<class OutputIterator> void param(OutputIterator dest) const;-11- Requires:
-12- Effects: Copies the sequence of prepared 32-bit units to the given destination, as if by executing the following statement:OutputIterator
shall satisfy the requirements of an output iterator (Table 108) type. Moreover, the expression*dest = rt
shall be valid for a valuert
of typeresult_type
.copy(v.begin(), v.end(), dest);-??- Throws: What and when
OutputIterator
operations ofdest
throw.
Section: 29.5.3.2 [rand.req.seedseq], 29.5.4 [rand.eng], 29.5.5 [rand.adapt] Status: C++17 Submitter: Daniel Krügler Opened: 2012-08-18 Last modified: 2017-07-30
Priority: 3
View all other issues in [rand.req.seedseq].
View all issues with C++17 status.
Discussion:
LWG issue 2180(i) points out some deficiences in regard to the specification of the library-provided
type std::seed_seq
regarding exceptions, but there is another specification problem
in regard to general types satisfying the seed sequence constraints (named SSeq
) as described in
29.5.3.2 [rand.req.seedseq].
Except where specified otherwise, no function described in this section 29.5.4 [rand.eng]/29.5.5 [rand.adapt] throws an exception.
This constraint causes problems, because the described templates in these sub-clauses depend on operations of
SSeq::generate()
which is a function template, that depends both on operations provided by the
implementor of SSeq
(e.g. of std::seed_seq
), and those of the random access iterator type
provided by the caller. With class template linear_congruential_engine
we have just one example for a user
of SSeq::generate()
via:
template<class Sseq> linear_congruential_engine<>::linear_congruential_engine(Sseq& q); template<class Sseq> void linear_congruential_engine<>::seed(Sseq& q);
None of these operations has an exclusion rule for exceptions.
As described in 2180(i) the wording forstd::seed_seq
should and can be fixed to ensure that
operations of seed_seq::generate()
won't throw except from operations of the provided iterator range,
but there is no corresponding "safety belt" for user-provided SSeq
types, since 29.5.3.2 [rand.req.seedseq]
does not impose no-throw requirements onto operations of seed sequences.
A quite radical step to fix this problem would be to impose general no-throw requirements on the expression
q.generate(rb,re)
from Table 115, but this is not as simple as it looks initially, because this
function again depends on general types that are mutable random access iterators. Typically, we do not
impose no-throw requirements on iterator operations and this would restrict general seed sequences where
exceptions are not a problem. Furthermore, we do not impose comparable constraints for other expressions,
like that of the expression g()
in Table 116 for good reasons, e.g. random_device::operator()
explicitly states when it throws exceptions.
A less radical variant of the previous suggestion would be to add a normative requirement on the expression
q.generate(rb,re)
from Table 115 that says: "Throws nothing if operations of rb
and re
do not throw exceptions". Nevertheless we typically do not describe conditional Throws elements in proper
requirement sets elsewhere (Container requirements excluded, they just describe the containers from Clause 23)
and this may exclude resonable implementations of seed sequences that could throw exceptions under rare
situations.
The iterator arguments provided to SSeq::generate()
for operations in templates of 29.5.4 [rand.eng] and
29.5.5 [rand.adapt] are under control of implementations, so we could impose stricter exceptions requirements
on SSeq::generate()
for SSeq
types that are used to instantiate member templates in 29.5.4 [rand.eng]
and 29.5.5 [rand.adapt] solely.
We simply add extra wording to the introductive parts of 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]
that specify that operations of the engine (adaptor) templates that depend on a template parameter SSeq
throw no exception unless SSeq::generate()
throws an exception.
Given these options I would suggest to apply the variant described in the fourth bullet.
The proposed resolution attempts to reduce a lot of the redundancies of requirements in the introductory paragraphs of 29.5.4 [rand.eng] and 29.5.5 [rand.adapt] by introducing a new intermediate sub-clause "Engine and engine adaptor class templates" following sub-clause 29.5.2 [rand.synopsis]. This approach also solves the problem that currently 29.5.4 [rand.eng] also describes requirements that apply for 29.5.5 [rand.adapt] (Constrained templates involving theSseq
parameters).
[2013-04-20, Bristol]
Remove the first bullet point:
?- Throughout this sub-clause general requirements and conventions are described that apply to every class template specified in sub-clause 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]. Phrases of the form "in those sub-clauses" shall be interpreted as equivalent to "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".
Replace "in those sub-clauses" with "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".
Find another place for the wording. Daniel: These are requirements on the implementation not on the types. I'm not comfortable in moving it to another place without double checking. Improve the text (there are 4 "for"s): for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses. Move the information of this paragraph to the paragraphs it refers to:"-?- Descriptions are provided in those sub-clauses only for engine operations that are not described in 29.5.3.4 [rand.req.eng], for adaptor operations that are not described in 29.5.3.5 [rand.req.adapt], or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses."
Alisdair: I prefer duplication here than consolidation/reference to these paragraphs.
The room showed weakly favjust or for duplication.Previous resolution from Daniel [SUPERSEDED]:
Add a new sub-clause titled "Engine and engine adaptor class templates" following sub-clause 29.5.2 [rand.synopsis] (but at the same level) and add one further sub-clause "General" as child of the new sub-clause as follows:
Engine and engine adaptor class templates [rand.engadapt] General [rand.engadapt.general]-?- Throughout this sub-clause general requirements and conventions are described that apply to every class template specified in sub-clause 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]. Phrases of the form "in those sub-clauses" shall be interpreted as equivalent to "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".
-?- Except where specified otherwise, the complexity of each function specified in those sub-clauses is constant. -?- Except where specified otherwise, no function described in those sub-clauses throws an exception. -?- Every function described in those sub-clauses that has a function parameterq
of typeSSeq&
for a template type parameter namedSSeq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws. -?- Descriptions are provided in those sub-clauses only for engine operations that are not described in 29.5.3.4 [rand.req.eng], for adaptor operations that are not described in 29.5.3.5 [rand.req.adapt], or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses. -?- Each template specified in those sub-clauses requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold. -?- For every random number engine and for every random number engine adaptorX
defined in those sub-clauses:
if the constructor
template <class Sseq> explicit X(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution;if the member function
template <class Sseq> void seed(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this function shall not participate in overload resolution;The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as a seed sequence if it is implicitly convertible to
X::result_type
.Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:
-1- Each type instantiated from a class template specified in this section 29.5.4 [rand.eng] satisfies the requirements of a random number engine (29.5.3.4 [rand.req.eng]) type and the general implementation requirements specified in sub-clause [rand.engadapt.general].
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant.-3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception.-4- Descriptions are provided in this section 29.5.4 [rand.eng] only for engine operations that are not described in 29.5.3.4 [rand.req.eng] […]-5- Each template specified in this section 29.5.4 [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. […]-6- For every random number engine and for every random number engine adaptorX
defined in this subclause (29.5.4 [rand.eng]) and in sub-clause 29.5.4 [rand.eng]: […]Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:
-1- Each type instantiated from a class template specified in this section
29.5.4 [rand.eng]29.5.5 [rand.adapt] satisfies the requirements of a random number engine adaptor (29.5.3.5 [rand.req.adapt]) type and the general implementation requirements specified in sub-clause [rand.engadapt.general].-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant.-3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception.-4- Descriptions are provided in this section 29.5.5 [rand.adapt] only for engine operations that are not described in 29.5.3.5 [rand.req.adapt] […]-5- Each template specified in this section 29.5.5 [rand.adapt] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. […]
[2014-02-09, Daniel provides alternative resolution]
[Lenexa 2015-05-07: Move to Ready]
LWG 2181 exceptions from seed sequence operations
STL: Daniel explained that I was confused. I said, oh, seed_seq says it can throw if the RanIt throws. Daniel says the RanIts are provided by the engine. Therefore if you give a seed_seq to an engine, it cannot throw, as implied by the current normative text. So what Daniel has in the PR is correct, if slightly unnecessary. It's okay to have explicitly non-overlapping Standardese even if overlapping would be okay.
Marshall: And this is a case where the std:: on seed_seq is a good thing.
STL: Meh.
STL: And that was my only concern with this PR. I like the latest PR much better than the previous.
Marshall: Yes. There's a drive-by fix for referencing the wrong section. Other than that, the two are the same.
STL: Alisdair wanted the repetition instead of centralization, and I agree.
Marshall: Any other opinions?
Jonathan: I'll buy it.
STL: For a dollar?
Hwrd: I'll buy that for a nickel.
Marshall: Any objections to Ready? I don't see a point in Immediate.
Jonathan: Absolutely agree.
Marshall: 7 for ready, 0 opposed, 0 abstain.
[2014-05-22, Daniel syncs with recent WP]
[2015-10-31, Daniel comments and simplifies suggested wording changes]
Upon Walter Brown's suggestion the revised wording does not contain any wording changes that could be considered as editorial.
Previous resolution from Daniel [SUPERSEDED]:
This wording is relative to N3936.
Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:
-1- Each type instantiated from a class template specified in this section 29.5.4 [rand.eng] satisfies the requirements of a random number engine (29.5.3.4 [rand.req.eng]) type.
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant. -3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception. -?- Every function described in this section 29.5.4 [rand.eng] that has a function parameterq
of typeSseq&
for a template type parameter namedSseq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws. -4- Descriptions are provided in this section 29.5.4 [rand.eng] only for engine operations that are not described in 29.5.3.4 [rand.req.eng] or for operations where there is additional semantic information. In particular, declarations for copy constructors,forcopy assignment operators,forstreaming operators,and forequality operators, and inequality operators are not shown in the synopses. -5- Each template specified in this section 29.5.4 [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold. -6- For every random number engine and for every random number engine adaptorX
defined in this subclause (29.5.4 [rand.eng]) and in sub-clause 29.5.5 [rand.adapt]:
if the constructor
template <class Sseq> explicit X(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution;if the member function
template <class Sseq> void seed(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this function shall not participate in overload resolution;The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as a seed sequence if it is implicitly convertible to
X::result_type
.Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:
-1- Each type instantiated from a class template specified in this section
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant. -3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception. -?- Every function described in this section 29.5.5 [rand.adapt] that has a function parameter29.5.4 [rand.eng]29.5.5 [rand.adapt] satisfies the requirements of a random number engine adaptor (29.5.3.5 [rand.req.adapt]) type.q
of typeSseq&
for a template type parameter namedSseq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws. -4- Descriptions are provided in this section 29.5.5 [rand.adapt] only for adaptor operations that are not described in section 29.5.3.5 [rand.req.adapt] or for operations where there is additional semantic information. In particular, declarations for copy constructors,forcopy assignment operators,forstreaming operators,and forequality operators, and inequality operators are not shown in the synopses. -5- Each template specified in this section 29.5.5 [rand.adapt] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.Edit the contents of sub-clause 29.5.9.1 [rand.dist.general] p2 as indicated: [Drafting note: These editorial changes are just for consistency with those applied to 29.5.4 [rand.eng] and 29.5.5.1 [rand.adapt.general] — end drafting note]
-2- Descriptions are provided in this section 29.5.9 [rand.dist] only for distribution operations that are not described in 29.5.3.6 [rand.req.dist] or for operations where there is additional semantic information. In particular, declarations for copy constructors,
forcopy assignment operators,forstreaming operators,and forequality operators, and inequality operators are not shown in the synopses.
Proposed resolution:
This wording is relative to N4527.
Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:
-1- […]
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant. -3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception. -?- Every function described in this section 29.5.4 [rand.eng] that has a function parameterq
of typeSseq&
for a template type parameter namedSseq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws. […]
Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:
-1- […]
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant. -3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception. -?- Every function described in this section 29.5.5 [rand.adapt] that has a function parameterq
of typeSseq&
for a template type parameter namedSseq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws.
Container::[const_]reference
types are misleadingly specifiedSection: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-20 Last modified: 2017-07-05
Priority: 0
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++14 status.
Discussion:
According to Table 96 (Container requirements) the return type of X::reference
and
X::const_reference
is "lvalue of T
" and "const
lvalue of T
",
respectively. This does not make much sense, because an lvalue is an expression category, not a type.
It could also refer to an expression that has a type, but this doesn't make sense either in this
context, because obviously X::[const_]reference
are intended to refer to types.
vector<bool>
has no real reference type for X::[const_]reference
and this definition presumably is intended to cover such situations as well, one might think that the wording is
just a sloppy form of "type that represents a [const] lvalue of T
". But this is also problematic,
because basically all proxy reference expressions are rvalues.
It is unclear what the intention is. A straightward way of fixing this wording could make
X::[const_]reference
identical to [const] T&
. This holds for all Library containers
except for vector<bool>
.
Another way of solving this definition problem would be to impose a requirement that holds for both
references and reference-like proxies. Both X::reference
and X::const_reference
would need to be convertible to const T&
. Additionally X::reference
would need to
support for a mutable container an assignment expression of the form
declval<X::reference>() = declval<T>()
(this presentation intentionally does not require
declval<X::reference&>() = declval<T>()
).
Further, the Table 96 does not impose any relations between X::reference
and X::const_reference
.
It seems that at least X::reference
needs to be convertible to X::const_reference
.
A related question is whether X::reference
is supposed to be a mutable reference-like type,
irrespective of whether the container is an immutable container or not. The way, type match_results
defines reference
identical to const_reference
indicates one specific interpretation (similarly,
the initializer_list
template also defines member type reference
equal to const value_type&
).
Note that this can be a different decision as that for iterator
and const_iterator
,
e.g. for sets the type X::reference
still is a mutable reference, even though iterator
is described as constant iterator.
The proposed resolution is incomplete in regard to the last question.
[2013-03-15 Issues Teleconference]
Moved to Review.
Alisdair notes that this looks like wording in the right direction. Wonders about congruence of these typedefs and the similar ones for iterators.
[2013-09 Chicago]
Moved to Ready.
Consensus that the requirements should require real references, just like iterators, as containers are required to support at least ForwardIterators, which have the same restriction on references.
Matt will file a new issue for some additional concerns with regex match_results
.
[2014-02-10, Daniel comments]
The new issue opened by Matt is LWG 2306(i).
[Issaquah 2014-02-11: Move to Immediate]
Issue should have been Ready in pre-meeting mailing.
Proposed resolution:
This wording is relative to N3376.
Change Table 96 — "Container requirements" as indicated:
Expression | Return type | Operational Semantics |
Assertion/note pre-/post-condition |
Complexity |
---|---|---|---|---|
X::reference
|
T&
|
compile time | ||
X::const_reference
|
const T&
|
compile time |
match_results
constructorsSection: 28.6.9.2 [re.results.const], 28.6.9.7 [re.results.all] Status: C++20 Submitter: Pete Becker Opened: 2012-08-29 Last modified: 2021-02-25
Priority: 3
View all other issues in [re.results.const].
View all issues with C++20 status.
Discussion:
28.6.9.2 [re.results.const] p1 says:
In all
match_results
constructors, a copy of theAllocator
argument shall be used for any memory allocation performed by the constructor or member functions during the lifetime of the object.
There are three constructors:
match_results(const Allocator& = Allocator()); match_results(const match_results& m); match_results(match_results&& m) noexcept;
The second and third constructors do no have an Allocator
argument, so despite the "all match_results
constructors", it is not possible to use "the Allocator
argument" for the second and third constructors.
Allocator
value is move constructed from
m.get_allocator()
, but doesn't require using that allocator to allocate memory.
The same basic problem recurs in 28.6.9.7 [re.results.all], which gives the required return value for
get_allocator()
:
Returns: A copy of the
Allocator
that was passed to the object's constructor or, if that allocator has been replaced, a copy of the most recent replacement.
Again, the second and third constructors do not take an Allocator
, so there is nothing that meets this
requirement when those constructors are used.
[2018-06-02, Daniel comments and provides wording]
The introductory wording of match_results
says in 28.6.9 [re.results] p2:
The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container (26.2.1, 26.2.3) except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
This wording essentially brings us to 23.2.2 [container.requirements.general] which describes in p8 in general the usage of allocators:
[…] Copy constructors for these container types obtain an allocator by calling
allocator_traits<allocator_ type>::select_on_container_copy_construction
on the allocator belonging to the container being copied. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. […]
The constructors referred to in the issue discussion are the copy constructor and move constructor of match_results
,
so we know already what the required effects are supposed to be.
[…] All other constructors for these container types take a
const allocator_type&
argument. [Note: If an invocation of a constructor uses the default value of an optional allocator argument, then the Allocator type must support value-initialization. — end note] A copy of this allocator is used for any memory allocation and element construction performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. […]
Further requirements imposed on two of the three match_results
constructors can be derived from Table 80 —
"Allocator-aware container requirements" via the specified expressions
X() X(m) X(rv)
In other words: The existing wording does already say everything that it said by 28.6.9.2 [re.results.const] p1 (end even more), except for possibly the tiny problem that
match_results(const Allocator& a = Allocator());
uses "const Allocator&
" instead of "const allocator_type&
" in the signature, albeit even
that deviation shouldn't change the intended outcome, which is IMO crystal-clear when looking at sub-clauses
23.2.2 [container.requirements.general] and 23.2.4 [sequence.reqmts] as a whole.
Either strike 28.6.9.2 [re.results.const] p1 completely; or
Replace 28.6.9.2 [re.results.const] p1 by referring to the specification of allocators in 23.2.2 [container.requirements.general] and 23.2.4 [sequence.reqmts].
My suggestion is to favour for the first option, because attempting to provide extra wording that refers to allocators
and the three constructors may lead to the false impression that no further allocator-related
requirements hold for type match_results
which are not explicitly repeated here again.
[2018-06, Rapperswil]
The group agrees with the provided resolution. Move to Ready.
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Edit 28.6.9.2 [re.results.const] as indicated:
-1- In allmatch_results
constructors, a copy of theAllocator
argument shall be used for any memory allocation performed by the constructor or member functions during the lifetime of the object.
match_results
assignmentsSection: 28.6.9.2 [re.results.const], 28.6.9.7 [re.results.all] Status: C++20 Submitter: Pete Becker Opened: 2012-08-29 Last modified: 2021-02-25
Priority: 3
View all other issues in [re.results.const].
View all issues with C++20 status.
Discussion:
The effects of the two assignment operators are specified in Table 141. Table 141 makes no mention of allocators,
so, presumably, they don't touch the target object's allocator. That's okay, but it leaves the question:
match_results::get_allocator()
is supposed to return "A copy of the Allocator that was passed to the
object's constructor or, if that allocator has been replaced, a copy of the most recent replacement"; if assignment
doesn't replace the allocator, how can the allocator be replaced?
[2018-06-04, Daniel comments and provides wording]
Similar to the reasoning provided in the 2018-06-02 comment in LWG 2183(i), it is possible to refer to
the introductory wording of match_results
which says in 28.6.9 [re.results] p2:
The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container (26.2.1, 26.2.3) except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
Again, similar to LWG 2183(i), this allows us to deduce the required effects of the copy/move assignment operators discussed here, because 23.2.2 [container.requirements.general] p8 also says:
[…] The allocator may be replaced only via assignment or
swap()
. Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only ifallocator_traits<allocator_type>::propagate_on_container_copy_assignment::value
,allocator_traits<allocator_type>::propagate_on_container_move_assignment::value
, orallocator_traits<allocator_type>::propagate_on_container_swap::value
istrue
within the implementation of the corresponding container operation. In all container types defined in this Clause, the memberget_allocator()
returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement. […]
So this wording already specifies everything we need, except for the problem that
28.6.9 [re.results] p2 quoted above restricts to operations supported by a const-qualified sequence
container, which of-course would exclude the copy assignment and the move assignment operators.
But given that these mutable definitions are defined for match_results
, it seems that the only fix
needed is to adjust 28.6.9 [re.results] p2 a bit to ensure that both assignment operators are
covered (again) by the general allocator-aware container wording.
[2018-06, Rapperswil]
The group generally likes the suggested direction, but would prefer the changed wording to say effectively "except that only copy assignment, move assignment, and operations defined...". Once applied, move to ready.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Edit 28.6.9 [re.results] as indicated:
-2- The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container (23.2.2 [container.requirements.general], 23.2.4 [sequence.reqmts]) except that besides copy assignment and move assignment only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
[2018-06-06, Daniel updates wording]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Edit 28.6.9 [re.results] as indicated:
-2- The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container (23.2.2 [container.requirements.general], 23.2.4 [sequence.reqmts]) except that only copy assignment, move assignment, and operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
future/shared_future::wait_for/wait_until
Section: 32.10.7 [futures.unique.future], 32.10.8 [futures.shared.future] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2012-09-20 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with C++14 status.
Discussion:
The functions future::wait_for
, future::wait_until
, shared_future::wait_for
, and
shared_future::wait_for
can throw any timeout-related exceptions. It would be better if the wording
could be more explicit. This is in line with the changes proposed in LWG 2093(i)'s Throws element
of condition_variable::wait
with predicate.
[2012, Portland: move to Review]
The phrase timeout-related exception does not exist.
2093(i) was put in review, and there is some dependency here with this issue.
If you provide a user-defined clock that throws, we need to put back an exception to allow that to be done.
We will put this in review and say that this cannot go in before 2093(i).
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
[This resolution should not be adopted before resolving 2093(i)]
[This wording is relative to N3376.]
Change [futures.unique_future] as indicated:
template <class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;-21- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the relative timeout (32.2.4 [thread.req.timing]) specified by
-22- Returns: […] -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).rel_time
has expired.template <class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;-23- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout (32.2.4 [thread.req.timing]) specified by
-24- Returns: […] -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).abs_time
has expired.
Change [futures.shared_future] as indicated:
template <class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;-23- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the relative timeout (32.2.4 [thread.req.timing]) specified by
-24- Returns: […] -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).rel_time
has expired.template <class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;-25- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout (32.2.4 [thread.req.timing]) specified by
-26- Returns: […] -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).abs_time
has expired.
async/launch::deferred
Section: 32.10.9 [futures.async] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2012-09-20 Last modified: 2017-07-05
Priority: Not Prioritized
View other active issues in [futures.async].
View all other issues in [futures.async].
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Discussion:
The description of the effects of async
when the launch policy is launch::deferred
doesn't
state what is done with the result of the deferred function invocation and the possible exceptions as it is done
for the asynchronous function when the policy is launch::async
.
[2012, Portland: move to Open]
Detlef: agree with the problem but not with the resolution. The wording should be applied to all launch policies rather than having to be separately specified for each one.
Hans: we should redraft to factor out the proposed text outside the two bullets. Needs to be carefully worded to be compatible with the resolution of 2120(i) (see above).
Moved to open
[Issaquah 2014-02-11: Move to Immediate after SG1 review]
Proposed resolution:
[This wording is relative to N3376.]
Change 32.10.9 [futures.async] p3 bullet 2 as indicated:
template <class F, class... Args> future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type> async(F&& f, Args&&... args); template <class F, class... Args> future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type> async(launch policy, F&& f, Args&&... args);-2- Requires: […]
-3- Effects:: The first function behaves the same as a call to the second function with apolicy
argument oflaunch::async | launch::deferred
and the same arguments forF
andArgs
. […] The further behavior of the second function depends on thepolicy
argument as follows (if more than one of these conditions applies, the implementation may choose any of the corresponding policies):
if
policy & launch::async
is non-zero […]if
policy & launch::deferred
is non-zero — StoresDECAY_COPY(std::forward<F>(f))
andDECAY_COPY(std::forward<Args>(args))...
in the shared state. These copies off
andargs
constitute a deferred function. Invocation of the deferred function evaluatesINVOKE(std::move(g), std::move(xyz))
whereg
is the stored value ofDECAY_COPY(std::forward<F>(f))
andxyz
is the stored copy ofDECAY_COPY(std::forward<Args>(args))...
. Any return value is stored as the result in the shared state. Any exception propagated from the execution of the deferred function is stored as the exceptional result in the shared state. The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (32.10.5 [futures.state]) on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called the waiting function. Once evaluation ofINVOKE(std::move(g), std::move(xyz))
begins, the function is no longer considered deferred. [Note: If this policy is specified together with other policies, such as when using a policy value oflaunch::async | launch::deferred
, implementations should defer invocation or the selection of thepolicy
when no more concurrency can be effectively exploited. — end note]
vector<bool>
is missing emplace
and emplace_back
member functionsSection: 23.3.14 [vector.bool] Status: C++14 Submitter: Nevin Liber Opened: 2012-09-21 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [vector.bool].
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Discussion:
It should have them so that it more closely matches the vector<T>
interface, as this helps when
writing generic code.
[2012, Portland: Move to Tentatively Ready]
Question on whether the variadic template is really needed, but it turns out to be needed to support
emplace
of no arguments.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change the class template vector<bool>
synopsis, 23.3.14 [vector.bool] p1, as indicated:
namespace std { template <class Allocator> class vector<bool, Allocator> { public: […] // modifiers: template <class... Args> void emplace_back(Args&&... args); void push_back(const bool& x); void pop_back(); template <class... Args> iterator emplace(const_iterator position, Args&&... args); iterator insert(const_iterator position, const bool& x); […] }; }
operator&
Section: 24.5.1.6 [reverse.iter.elem] Status: C++14 Submitter: Alisdair Meredith Opened: 2012-09-23 Last modified: 2021-06-06
Priority: 1
View all other issues in [reverse.iter.elem].
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Discussion:
The specification for reverse_iterator::operator->
returns the address of the object yielded by dereferencing
with operator*
, but does not have the usual
wording about returning the true address of the object. As
reverse_iterator
requires the adapted iterator have
at least the bidirectional iterator category, we know that
the returned reference is a true reference, and not a proxy,
hence we can use std::addressof
on the reference
to get the right answer.
This will most likely show itself as an issue with a list
or vector
of a type with such an overloaded operator,
where algorithms are likely to work with a forward iteration, but
not with reverse iteration.
[2013-04-20, Bristol]
Resolution: Goes to open now and move to review as soon as Daniel proposes a new wording.
[2014-02-12 Issaquah meeting]
Use std::addressof
as the library uses elsewhere, then move as Immediate.
Proposed resolution:
Revise [reverse.iter.opref] p1, as indicated:
Returns: addressof
.
&(operator*())
Section: 32.7 [thread.condition] Status: C++14 Submitter: Hans Boehm Opened: 2012-09-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition].
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Discussion:
The condition variable specification possibly leaves it unclear whether the effect of a notify_one()
call can effectively be delayed, so that a call unblocks a wait()
call that happens after the
notify_one
call. (For notify_all()
this is not detectable, since it only results in spurious
wake-ups.) Although this may at first glance seem like a contrived interpretation, it gains relevance since
glibc in fact allows the analogous behavior (see here)
and it is currently controversial whether this is correct and the Posix specification allows it (see
here).
The following proposed resolution disallows the glibc implementation, remaining consistent with the believed intent of C++11. To make that clear, we require that the "unspecified total order" O from 32.7 [thread.condition] p4 be consistent with happens-before. We also intend that the 3 components of a wait occur in order in O, but stating that explicitly seems too pedantic. Since they are numbered, it appears clear enough that they are sequenced one after the other.
Another uncertainty with the current phrasing is whether there is a single total order that includes all c.v. accesses, or one total order per c.v. We believe it actually doesn't matter, because there is no way to tell the difference, but this requires a bit more thought. We resolved it one way, just to remove the potential ambiguity.
[2012, Portland: Move to Review]
This is linked to a glibc issue, and a POSIX specification issue.
We believe the proposed wording fixes the ambiguity in C++ and is compatible with the proposed resolution for Posix (which confirms the glibc behaviour as illegal).
Moved to review (Detlef hopes to send some improved wording to the reflector).
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
This wording is relative to N3376.
Change 32.7 [thread.condition] p4 as indicated:
-4- The implementation shall behave as if all executions of
notify_one
,notify_all
, and each part of thewait
,wait_for
, andwait_until
executions are executed insomea single unspecified total order consistent with the "happens before" order.
match_results(match_results&&)
Section: 28.6.9.2 [re.results.const] Status: C++23 Submitter: Pete Becker Opened: 2012-10-02 Last modified: 2023-11-22
Priority: 4
View all other issues in [re.results.const].
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Discussion:
28.6.9.2 [re.results.const]/3: "Move-constructs an object of class match_results
satisfying the same
postconditions as Table 141."
Table 141 lists various member functions and says that their results should be the results of the corresponding member
function calls on m
. But m
has been moved from, so the actual requirement ought to be based on the
value that m
had before the move construction, not on m
itself.
In addition to that, the requirements for the copy constructor should refer to Table 141.
Ganesh: Also, the requirements for move-assignment should refer to Table 141. Further it seems as if in Table 141 all phrases of "for all integersn < m.size()
" should be replaced by "for all unsigned integers
n < m.size()
".
[2019-03-26; Daniel comments and provides wording]
The previous Table 141 (Now Table 128 in N4810) has been modified to cover now the effects of move/copy constructors and move/copy assignment operators. Newly added wording now clarifies that for move operations the corresponding values refer to the values of the move source before the operation has started.
Re Ganesh's proposal: Note that no further wording is needed for the move-assignment operator, because in the current working draft the move-assignment operator's Effects: element refers already to Table 128. The suggested clarification of unsigned integers has been implemented by referring to non-negative integers instead. Upon suggestion from Casey, the wording also introduces Ensures: elements that refer to Table 128 and as drive-by fix eliminates a "Throws: Nothing." element from anoexcept
function.
Previous resolution [SUPERSEDED]:
This wording is relative to N4810.
Add a new paragraph at the beginning of 28.6.9.2 [re.results.const] as indicated:
-?- Table 128 lists the postconditions of
match_results
copy/move constructors and copy/move assignment operators. For move operations, the results of the expressions depending on the parameterm
denote the values they had before the respective function calls.Modify 28.6.9.2 [re.results.const] as indicated:
match_results(const match_results& m);-3- Effects: Constructs
-?- Ensures: As indicated in Table 128.an object of classa copy ofmatch_results
, asm
.match_results(match_results&& m) noexcept;-4- Effects: Move constructs
-?- Ensures: As indicated in Table 128.an object of classfrommatch_results
m
satisfying the same postconditions as Table 128. Additionally, the storedAllocator
value is move constructed fromm.get_allocator()
.-5- Throws: Nothing.match_results& operator=(const match_results& m);-6- Effects: Assigns
-?- Ensures: As indicated in Table 128.m
to*this
.The postconditions of this function are indicated in Table 128.match_results& operator=(match_results&& m);-7- Effects: Move
-?- Ensures: As indicated in Table 128.-assignsm
to*this
.The postconditions of this function are indicated in Table 128.Modify 28.6.9.2 [re.results.const], Table 128 — "
match_results
assignment operator effects", as indicated:
Table 128 — match_results
assignment operator effectscopy/move operation postconditionsElement Value ready()
m.ready()
size()
m.size()
str(n)
m.str(n)
for all non-negative integersn < m.size()
prefix()
m.prefix()
suffix()
m.suffix()
(*this)[n]
m[n]
for all non-negative integersn < m.size()
length(n)
m.length(n)
for all non-negative integersn < m.size()
position(n)
m.position(n)
for all non-negative integersn < m.size()
[2021-06-25; Daniel comments and provides new wording]
The revised wording has been rebased to N4892. It replaces the now obsolete Ensures: element by the Postconditions: element and also restores the copy constructor prototype that has been eliminated by P1722R2 as anchor point for the link to Table 140.
[2021-06-30; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Add a new paragraph at the beginning of 28.6.9.2 [re.results.const] as indicated:
-?- Table 140 [tab:re.results.const] lists the postconditions of
match_results
copy/move constructors and copy/move assignment operators. For move operations, the results of the expressions depending on the parameterm
denote the values they had before the respective function calls.
Modify 28.6.9.2 [re.results.const] as indicated:
explicit match_results(const Allocator& a);-1- Postconditions:
ready()
returnsfalse
.size()
returns0
.match_results(const match_results& m);-?- Postconditions: As specified in Table 140.
match_results(match_results&& m) noexcept;-2- Effects: The stored
-3- Postconditions: As specified in Table 140.Allocator
value is move constructed fromm.get_allocator()
.match_results& operator=(const match_results& m);-4- Postconditions: As specified in Table 140.
match_results& operator=(match_results&& m);-5- Postconditions: As specified in Table 140.
Modify 28.6.9.2 [re.results.const], Table 140 — "match_results
assignment operator effects",
[tab:re.results.const], as indicated:
Table 140 — match_results
assignment operator effectscopy/move operation postconditionsElement Value ready()
m.ready()
size()
m.size()
str(n)
m.str(n)
for all non-negative integersn < m.size()
prefix()
m.prefix()
suffix()
m.suffix()
(*this)[n]
m[n]
for all non-negative integersn < m.size()
length(n)
m.length(n)
for all non-negative integersn < m.size()
position(n)
m.position(n)
for all non-negative integersn < m.size()
std::abs(0u)
is unclearSection: 29.7 [c.math] Status: C++17 Submitter: Daniel Krügler Opened: 2012-10-02 Last modified: 2017-07-30
Priority: 2
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Discussion:
In C++03 the following two programs are invalid:
#include <cmath> int main() { std::abs(0u); }
#include <cstdlib> int main() { std::abs(0u); }
because none of the std::abs()
overloads is a best match.
In C++11 the additional "sufficient overload" rule from 29.7 [c.math] p11 (see also LWG
2086(i)) can be read to be applicable to the std::abs()
overloads as well, which
can lead to the following possible conclusions:
The program
#include <type_traits> #include <cmath> static_assert(std::is_same<decltype(std::abs(0u)), double>(), "Oops"); int main() { std::abs(0u); // Calls std::abs(double) }
is required to be well-formed, because of sub-bullet 2 ("[..] or an integer type [..]") of 29.7 [c.math] p11 (Note that the current resolution of LWG 2086