Doc. no. D????
Date: 2024-05-26
Project: Programming Language C++
Reply to: Jonathan Wakely <lwgchair@gmail.com>

C++ Standard Library Defect Reports and Accepted Issues (Revision D125)

Revised 2024-05-26 at 09:22:57 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.

Revision History

Accepted Issues


1(i). C library linkage editing oversight

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.


3(i). Atexit registration during atexit() call is not described

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.


5(i). String::compare specification questionable

Section: 23.4.3.7.8 [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 23.4.3 [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 23.4.3.7.8 [string.swap] paragraphs 5 and 6 which read:

int compare(size_type pos, size_type n1,
            charT * s, size_type n2 = npos) const;
Returns:
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,
            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;
Returns:
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.


7(i). String clause minor problems

Section: 23 [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 23.4.3.7.4 [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 23.4.3.7.8 [string.swap]

(6) In table 37, in section 23.2.2 [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."


8(i). Locale::global lacks guarantee

Section: 30.3.1.6 [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 30.3.1.6 [locale.statics], paragraph 2: 

No library function other than locale::global() shall affect the value returned by locale().


9(i). Operator new(0) calls should not yield the same pointer

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.


11(i). Bitset minor problems

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 type bitset<N>::reference such that (*this)[pos] == this->test(pos), and such that (*this)[pos] = val is equivalent to this->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].


13(i). Eos refuses to die

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()"


14(i). Locale::combine should be const

Section: 30.3.1.4 [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 30.3.1 [locale] and also in 30.3.1.4 [locale.members], add "const" to the declaration of member combine:

template <class Facet> locale combine(const locale& other) const; 

15(i). Locale::name requirement inconsistent

Section: 30.3.1.4 [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 30.3.1.4 [locale.members], paragraph 5, replace "locale(name())" with "locale(name().c_str())".


16(i). Bad ctype_byname<char> decl

Section: 30.4.2.5 [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 30.4.2.4 [facet.ctype.special].


17(i). Bad bool parsing

Section: 30.4.3.2.3 [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 30.4.3.2.3 [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() and truename() of the facet obtained by use_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 iterator in is compared to end 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. If val is set, then err is set to str.goodbit; or to str.eofbit if, when seeking another character to match, it is found that (in==end). If val is not set, then err is set to str.failbit; or to (str.failbit|str.eofbit)if the reason for the failure was that (in==end). [Example: for targets true:"a" and false:"abb", the input sequence "a" yields val==true and err==str.eofbit; the input sequence "abc" yields err=str.failbit, with in ending at the 'c' element. For targets true:"1" and false:"0", the input sequence "1" yields val==true and err=str.goodbit. For empty targets (""), any input sequence yields err==str.failbit. --end example]


18(i). Get(...bool&) omitted

Section: 30.4.3.2.2 [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 30.4.3.2.2 [facet.num.get.members] another get member for bool&, copied from the entry in 30.4.3.2 [locale.num.get].


19(i). "Noconv" definition too vague

Section: 30.4.2.5 [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 30.4.2.5.3 [locale.codecvt.virtuals] to read:

noconv: internT and externT 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 and externT are the same type and the converted sequence is identical to the input sequence [from,from_next). to_next is set equal to to, the value of state is unchanged, and there are no changes to the values in [to, to_limit).


20(i). Thousands_sep returns wrong type

Section: 30.4.4.1.3 [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 30.4.4.1.3 [facet.numpunct.virtuals], above paragraph 2, change "string_type" to "char_type".


21(i). Codecvt_byname<> instantiations

Section: 30.3.1.2.1 [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 30.3.1.2.1 [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> 

22(i). Member open vs. flags

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.


23(i). Num_get overflow result

Section: 30.4.3.2.3 [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:

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:

  1. Set failbit upon conversion error (including overflow), and don't store any value.
  2. Set failbit upon conversion error, and also set errno to indicated the precise nature of the error.
  3. Set failbit upon conversion error. If the error was due to overflow, store +-numeric_limits<T>::max() as an overflow indication.

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 30.4.3.2.3 [facet.num.get.virtuals], end of p3:

Stage 3: The result of stage 2 processing can be one of The sequence of chars 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>:

The numeric value to be stored can be one of:

The resultant numeric value is stored in val.

Change 30.4.3.2.3 [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 a long 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 then false is stored. If the value is 1 then true is stored. Otherwise err|=ios_base::failbit is performed and no value true is stored. and ios_base::failbit is assigned to err.

-7- Otherwise target sequences are determined "as if" by calling the members falsename() and truename() of the facet obtained by use_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. If and only if a target sequence is uniquely matched, val is set to the corresponding value. Otherwise false is stored and ios_base::failbit is assigned to err.


24(i). "do_convert" doesn't exist

Section: 30.4.2.5 [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 30.4.2.5 [locale.codecvt], paragraph 3, change "do_convert" to "do_in or do_out". Also, in 30.4.2.5.3 [locale.codecvt.virtuals], change "do_convert()" to "do_in or do_out".


25(i). String operator<< uses width() value wrong

Section: 23.4.4.4 [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 23.4.4.4 [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(); ..."


26(i). Bad sentry example

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.


27(i). String::erase(range) yields wrong iterator

Section: 23.4.3.7.5 [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 23.4.3.7.5 [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.


28(i). Ctype<char>is ambiguous

Section: 30.4.2.4.3 [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 30.4.2.4.3 [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].


29(i). Ios_base::init doesn't exist

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


30(i). Wrong header for LC_*

Section: 30.3.1.2.1 [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 30.3.1.2.1 [locale.category], paragraph 2, change "<cctype>" to read "<clocale>".


31(i). Immutable locale values

Section: 30.3.1 [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 30.3.1 [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.


32(i). Pbackfail description inconsistent

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.


33(i). Codecvt<> mentions from_type

Section: 30.4.2.5 [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 30.4.2.5.3 [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.


34(i). True/falsename() not in ctype<>

Section: 30.4.3.3.3 [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 30.4.3.3.3 [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(); 

35(i). No manipulator unitbuf in synopsis

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); 

36(i). Iword & pword storage lifetime omitted

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.


37(i). Leftover "global" reference

Section: 30.3.1 [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 30.3.1 [locale], paragraph 4, delete the parenthesized expression

(or, failing that, in the global locale)


38(i). Facet definition incomplete

Section: 30.3.2 [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 member id as defined in 30.3.1.2.2 [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..." ]


39(i). istreambuf_iterator<>::operator++(int) definition garbled

Section: 25.6.4.4 [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.


40(i). Meaningless normative paragraph in examples

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.


41(i). Ios_base needs clear(), exceptions()

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 a basic_ios<> object or sub-object, the effect is equivalent to calling basic_ios<>::setstate(badbit) on the derived object (which may throw failure).

[Kona: LWG reviewed wording; setstate(failbit) changed to setstate(badbit).]


42(i). String ctors specify wrong default allocator

Section: 23.4.3 [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 23.4.3 [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 23.4.3.2 [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 23.4.3 [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 23.4.3.2 [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 value str.get_allocator().

B. In 23.4.3 [basic.string], and also in 23.4.3.2 [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. ]


44(i). Iostreams use operator== on int_type values

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:

  1. In lib.basic.ios.members paragraph 13 (postcondition clause for 'fill(cT)') change
         fillch == fill()
    
    to
         traits::eq(fillch, fill())
    
  2. In lib.istream.unformatted paragraph 7 (effects clause for 'get(cT,streamsize,cT)'), third bullet, change
         c == delim for the next available input character c
    
    to
         traits::eq(c, delim) for the next available input character c
    
  3. In lib.istream.unformatted paragraph 12 (effects clause for 'get(basic_streambuf<cT,Tr>&,cT)'), third bullet, change
         c == delim for the next available input character c
    
    to
         traits::eq(c, delim) for the next available input character c
    
  4. In lib.istream.unformatted paragraph 17 (effects clause for 'getline(cT,streamsize,cT)'), second bullet, change
         c == delim for the next available input character c
    
    to
         traits::eq(c, delim) for the next available input character c
      
  5. In lib.istream.unformatted paragraph 24 (effects clause for 'ignore(int,int_type)'), second bullet, change
         c == delim for the next available input character c
    
    to
         traits::eq_int_type(c, delim) for the next available input
         character c
    
  6. In lib.istream.unformatted paragraph 25 (notes clause for 'ignore(int,int_type)'), second bullet, change
         The last condition will never occur if delim == traits::eof()
    
    to
         The last condition will never occur if
         traits::eq_int_type(delim, traits::eof()).
    
  7. In lib.istream.sentry paragraph 6 (example implementation for the sentry constructor) change
         while ((c = is.rdbuf()->snextc()) != traits::eof()) {
    
    to
         while (!traits::eq_int_type(c = is.rdbuf()->snextc(), traits::eof())) {
    

List of changes to Chapter 21:

  1. In lib.string::find paragraph 1 (effects clause for find()), second bullet, change
         at(xpos+I) == str.at(I) for all elements ...
    
    to
         traits::eq(at(xpos+I), str.at(I)) for all elements ...
    
  2. In lib.string::rfind paragraph 1 (effects clause for rfind()), second bullet, change
         at(xpos+I) == str.at(I) for all elements ...
    
    to
         traits::eq(at(xpos+I), str.at(I)) for all elements ...
    
  3. In lib.string::find.first.of paragraph 1 (effects clause for find_first_of()), second bullet, change
         at(xpos+I) == str.at(I) for all elements ...
    
    to
         traits::eq(at(xpos+I), str.at(I)) for all elements ...
    
  4. In lib.string::find.last.of paragraph 1 (effects clause for find_last_of()), second bullet, change
         at(xpos+I) == str.at(I) for all elements ...
    
    to
         traits::eq(at(xpos+I), str.at(I)) for all elements ...
    
  5. In lib.string::find.first.not.of paragraph 1 (effects clause for find_first_not_of()), second bullet, change
         at(xpos+I) == str.at(I) for all elements ...
    
    to
         traits::eq(at(xpos+I), str.at(I)) for all elements ...
    
  6. In lib.string::find.last.not.of paragraph 1 (effects clause for find_last_not_of()), second bullet, change
         at(xpos+I) == str.at(I) for all elements ...
    
    to
         traits::eq(at(xpos+I), str.at(I)) for all elements ...
    
  7. In lib.string.ios paragraph 5 (effects clause for getline()), second bullet, change
         c == delim for the next available input character c 
    
    to
         traits::eq(c, delim) for the next available input character c 
    

Notes:


46(i). Minor Annex D errors

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;

47(i). Imbue() and getloc() Returns clauses swapped

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.


48(i). Use of non-existent exception constructor

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.


49(i). Underspecification of ios_base::sync_with_stdio

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 returns false.

to:

true if the previous state of the standard iostream objects (27.3) was synchronized and otherwise returns false.

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.]


50(i). Copy constructor and assignment operator of ios_base

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.


51(i). Requirement to not invalidate iterators missing

Section: 24.2 [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"


52(i). Small I/O problems

Section: 31.5.3.2 [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 for resolution.]

Second, 31.5.3.2 [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".


53(i). Basic_ios destructor unspecified

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".


54(i). Basic_streambuf's destructor

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.


55(i). Invalid stream position is undefined

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))"


56(i). Showmanyc's return type

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.


57(i). Mistake in char_traits

Section: 23.2.4.6 [char.traits.specializations.wchar.t] Status: TC1 Submitter: Matt Austern Opened: 1998-07-01 Last modified: 2016-01-28

Priority: Not Prioritized

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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 23.2.4.6 [char.traits.specializations.wchar.t] paragraph 3 which begins "The types streampos and wstreampos may be different..." .


59(i). Ambiguity in specification of gbump

Section: 31.6.3.4.2 [streambuf.get.area] Status: TC1 Submitter: Matt Austern Opened: 1998-07-28 Last modified: 2016-01-28

Priority: Not Prioritized

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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.


60(i). What is a formatted input function?

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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Duplicate of: 162, 163, 166

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.


61(i). Ambiguity in iostreams exception policy

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.


62(i). Sync's return value

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 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".


63(i). Exception-handling policy for unformatted output

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 an ios::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.


64(i). Exception handling in 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 throw ios_base::failure (27.4.4.3). If it inserted no characters because it caught an exception thrown while extracting characters from sb and failbit is on in exceptions() (27.4.4.3), then the caught exception is rethrown.


66(i). Strstreambuf::setbuf

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.


68(i). Extractors for char* should store null at end

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.


69(i). Must elements of a vector be contiguous?

Section: 24.3.11 [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 24.3.11 [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 than bool, then it obeys the identity &v[n] == &v[0] + n for all 0 <= 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:


70(i). Uncaught_exception() missing throw() specification

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 14.6.3 [except.uncaught], paragraph 1, 17.9 [support.exception], and 99 [uncaught], add "throw()" to uncaught_exception().


71(i). Do_get_monthname synopsis missing argument

Section: 30.4.6.2 [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 30.4.6.2.3 [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 30.4.6.2 [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;

74(i). Garbled text for codecvt::do_max_length

Section: 30.4.2.5 [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 30.4.2.5.3 [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) and stateT value state. The specialization codecvt<char, char, mbstate_t>::do_max_length() returns 1.


75(i). Contradiction in codecvt::length's argument types

Section: 30.4.2.5 [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 30.4.2.5 [locale.codecvt], and also in 30.4.2.6 [locale.codecvt.byname], change the stateT argument type on both member length() and member do_length() from

const stateT&

to

stateT&

In 30.4.2.5.3 [locale.codecvt.virtuals], add to the definition for member do_length a paragraph:

Effects: The effect on the state argument is ``as if'' it called do_in(state, from, from_end, from, to, to+max, to) for to pointing to a buffer of at least max elements.


76(i). Can a codecvt facet always convert one internal character at a time?

Section: 30.4.2.5 [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 (30.4.2.5.3 [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 30.4.2.5.3 [locale.codecvt.virtuals] paragraph 2:

A codecvt facet that is used by basic_filebuf (31.10 [file.streams]) must have the property that if

    do_out(state, from, from_end, from_next, to, to_lim, to_next)

would return ok, where from != from_end, then

    do_out(state, from, from + 1, from_next, to, to_end, to_next)

must also return ok, and that if

    do_in(state, from, from_end, from_next, to, to_lim, to_next)

would return ok, where to != to_lim, then

    do_in(state, from, from_end, from_next, to, to + 1, to_next)

must also return ok. [Footnote: Informally, this means that basic_filebuf assumes that the mapping from internal to external characters is 1 to N: a codecvt that is used by basic_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.


78(i). Typo: event_call_back

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".


79(i). Inconsistent declaration of polar()

Section: 28.4.2 [complex.syn], 28.4.7 [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 28.4.2 [complex.syn] polar is declared as follows:

   template<class T> complex<T> polar(const T&, const T&); 

In 28.4.7 [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 28.4.2 [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); 

80(i). Global Operators of complex declared twice

Section: 28.4.2 [complex.syn], 28.4.3 [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.


83(i). String::npos vs. string::max_size()

Section: 23.4.3 [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 23.4.3 [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 exceed max_size() then the operation throws length_error.

Rationale:

The LWG believes length_error is the correct exception to throw.


86(i). String constructors don't describe exceptions

Section: 23.4.3.2 [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 23.4.3.2 [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.


90(i). Incorrect description of operator >> for strings

Section: 23.4.4.4 [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 23.4.4.4 [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.


91(i). Description of operator>> and getline() for string<> might cause endless loop

Section: 23.4.4.4 [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 23.4.4.4 [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 23.4.4.4 [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.


92(i). Incomplete Algorithm Requirements

Section: 27 [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 27 [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 27 [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.]


98(i). Input iterator requirements are badly written

Section: 25.3.5.3 [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 25.3.5.3 [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 for a similar problem.

Proposed resolution:

In Table 72 in 25.3.5.3 [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.


103(i). set::iterator is required to be modifiable, but this allows modification of keys

Section: 24.2.7 [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_

24.2 [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 24.2.7 [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 24.2.7 [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 and const_iterator are constant iterators. It is unspecified whether or not iterator and const_iterator are the same type."

Option C. To 24.2.7 [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 24.2.7 [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 and const_iterator are constant iterators. It is unspecified whether or not iterator and const_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.]


106(i). Numeric library private members are implementation defined

Section: 28.6.5 [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:


108(i). Lifetime of exception::what() return unspecified

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().


109(i). Missing binders for non-const sequence elements

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.]


110(i). istreambuf_iterator::equal not const

Section: 25.6.4 [istreambuf.iterator], 25.6.4.4 [istreambuf.iterator.ops] Status: TC1 Submitter: Nathan Myers Opened: 1998-10-15 Last modified: 2023-02-07

Priority: Not Prioritized

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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 25.6.4 [istreambuf.iterator] and also in [istreambuf.iterator::equal], replace:

bool equal(istreambuf_iterator& b);

with:

bool equal(const istreambuf_iterator& b) const;

112(i). Minor typo in ostreambuf_iterator constructor

Section: 25.6.5.2 [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 25.6.5.2 [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.


114(i). Placement forms example in error twice

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));

115(i). Typo in strstream constructors

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.


117(i). basic_ostream uses nonexistent num_put member functions

Section: 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.)


118(i). basic_istream uses nonexistent num_get member functions

Section: 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

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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 30.4.3.2.2 [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.]


119(i). Should virtual functions be allowed to strengthen the exception specification?

Section: 16.4.6.13 [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 16.4.6.13 [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 16.4.6.13 [res.on.exception.handling] from:

     "may strengthen the exception-specification for a function"

to:

     "may strengthen the exception-specification for a non-virtual function".


120(i). Can an implementor add specializations?

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.


122(i). streambuf/wstreambuf description should not say they are specializations

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.


123(i). Should valarray helper arrays fill functions be const?

Section: 28.6.5.4 [slice.arr.fill], 28.6.7.4 [gslice.array.fill], 28.6.8.4 [mask.array.fill], 28.6.9.4 [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 28.6.5.2 [slice.arr.assign] is const:

    void operator=(const valarray<T>&) const;

but this one in Section 28.6.5.4 [slice.arr.fill] is not:

    void operator=(const T&);

The description of the semantics for these two functions is similar.

Proposed resolution:

28.6.5 [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;
    

28.6.5.4 [slice.arr.fill] slice_array fill function

Change the function declaration

      void operator=(const T&);
    

to

      void operator=(const T&) const;
    

28.6.7 [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;
    

28.6.7.4 [gslice.array.fill] gslice_array fill function

Change the function declaration

      void operator=(const T&);
    

to

      void operator=(const T&) const;
    

28.6.8 [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;
    

28.6.8.4 [mask.array.fill] mask_array fill function

Change the function declaration

      void operator=(const T&);
    

to

      void operator=(const T&) const;
    

28.6.9 [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;
    

28.6.9.4 [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, this now matters: these functions are now callable in more circumstances. In many existing implementations, both versions are already const.


124(i). ctype_byname<charT>::do_scan_is & do_scan_not return type should be const charT*

Section: 30.4.2.3 [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 30.4.2.3 [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 30.4.2.3 [locale.ctype.byname] do_scan_is() and do_scan_not() to return a const charT*.


125(i). valarray<T>::operator!() return type is inconsistent

Section: 28.6.2 [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 28.6.2 [template.valarray] valarray<T>::operator!() is declared to return a valarray<T>, but in Section 28.6.2.6 [valarray.unary] it is declared to return a valarray<bool>. The latter appears to be correct.

Proposed resolution:

Change in Section 28.6.2 [template.valarray] the declaration of operator!() so that the return type is valarray<bool>.


126(i). typos in Effects clause of ctype::do_narrow()

Section: 30.4.2.2.3 [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 30.4.2.2.3 [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)) )

127(i). auto_ptr<> conversion issues

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 the auto_ptr p that r holds a reference to.
Returns: *this.


129(i). Need error indication from seekp() and seekg()

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 throw ios_base::failure).

Rationale:

Setting failbit is the usual error reporting mechanism for streams


130(i). Return type of container::erase(iterator) differs for associative containers

Section: 24.2.7 [associative.reqmts], 24.2.4 [sequence.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 1999-03-02 Last modified: 2016-01-28

Priority: Not Prioritized

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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 24.2.7 [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 24.2.7 [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 24.4.4 [map], in the map class synopsis; and in 24.4.5 [multimap], in the multimap class synopsis; and in 24.4.6 [set], in the set class synopsis; and in 24.4.7 [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. ]


132(i). list::resize description uses random access iterators

Section: 24.3.10.3 [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 24.3.10.3 [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.]


133(i). map missing get_allocator()

Section: 24.4.4 [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 24.4.4 [map], paragraph 2, after operator= in the map declaration:

    allocator_type get_allocator() const;

134(i). vector constructors over specified

Section: 24.3.11.2 [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 24.3.11.2 [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.


136(i). seekp, seekg setting wrong streams?

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.]


137(i). Do use_facet and has_facet look in the global locale?

Section: 30.3.1 [locale] Status: TC1 Submitter: Angelika Langer Opened: 1999-03-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 30.3.1 [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 30.3.2 [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.


139(i). Optional sequence operation table description unclear

Section: 24.2.4 [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.


141(i). basic_string::find_last_of, find_last_not_of say pos instead of xpos

Section: 23.4.3.7.4 [string.insert], 23.4.3.7.6 [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();


142(i). lexicographical_compare complexity wrong

Section: 27.8.11 [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 27.8.11 [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]


144(i). Deque constructor complexity wrong

Section: 24.3.8.2 [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 24.3.8.2 [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 24.3.8.2 [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.


146(i). complex<T> Inserter and Extractor need sentries

Section: 28.4.6 [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 28.4.6 [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.


147(i). Library Intro refers to global functions that aren't global

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.


148(i). Functions in the example facet BoolNames should be const

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!"; }


149(i). Insert should return iterator to first element inserted

Section: 24.2.4 [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:

24.2.4 [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) void iterator Inserts n copies of t before p.
a.insert(p,i,j) void iterator Each iterator in the range [i,j) shall be dereferenced exactly once. pre: i and j are not iterators into a. Inserts copies of elements in [i, j) before p
a.insert(p,il) void iterator a.insert(p, il.begin(), il.end()).

Add after p6 24.2.4 [sequence.reqmts]:

-6- ...

The iterator returned from a.insert(p,n,t) points to the copy of the first element inserted into a, or p if n == 0.

The iterator returned from a.insert(p,i,j) points to the copy of the first element inserted into a, or p if i == j.

The iterator returned from a.insert(p,il) points to the copy of the first element inserted into a, or p if il is empty.

p2 24.3.8 [deque] Update class definition, change return type from void to iterator:

void iterator insert(const_iterator position, size_type n, const T& x);
template <class InputIterator>
  void iterator insert(const_iterator position, InputIterator first, InputIterator last);
  void iterator insert(const_iterator position, initializer_list<T>);

24.3.8.4 [deque.modifiers] change return type from void to iterator on following declarations:

  void iterator insert(const_iterator position, size_type n, const T& x);
template <class InputIterator>
  void iterator 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:

void iterator insert_after(const_iterator position, initializer_list<T> il);
void iterator insert_after(const_iterator position, size_type n, const T& x);
template <class InputIterator>
  void iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

p8 [forwardlist.modifiers] change return type from void to iterator:

void iterator 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>
  void iterator 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:

void iterator insert_after(const_iterator position, initializer_list<T> il);

change return type from void to iterator on following declarations:

p2 24.3.10 [list] Update class definition, change return type from void to iterator:

void iterator insert(const_iterator position, size_type n, const T& x);

template <class InputIterator>
void iterator insert(const_iterator position, InputIterator first, InputIterator last);

void iterator insert(const_iterator position, initializer_list<T>);

24.3.10.4 [list.modifiers] change return type from void to iterator on following declarations:

void iterator insert(const_iterator position, size_type n, const T& x);

template <class InputIterator>
  void iterator insert(const_iterator position, InputIterator first, InputIterator last);

Add the following (missing) declaration

iterator insert(const_iterator position, initializer_list<T>);

p2 24.3.11 [vector]

Update class definition, change return type from void to iterator:

void iterator insert(const_iterator position, T&& x);

void iterator insert(const_iterator position, size_type n, const T& x);

template <class InputIterator>
  void iterator insert(const_iterator position, InputIterator first, InputIterator last);

void iterator insert(const_iterator position, initializer_list<T>);

24.3.11.5 [vector.modifiers] change return type from void to iterator on following declarations:

void iterator insert(const_iterator position, size_type n, const T& x);

template <class InputIterator>
  void iterator insert(const_iterator position, InputIterator first, InputIterator last);

Add the following (missing) declaration

iterator insert(const_iterator position, initializer_list<T>);

p1 24.3.12 [vector.bool] Update class definition, change return type from void to iterator:

void iterator insert (const_iterator position, size_type n, const bool& x);

template <class InputIterator>
  void iterator insert(const_iterator position, InputIterator first, InputIterator last);

  void iterator insert(const_iterator position, initializer_list<bool> il);

p5 23.4.3 [basic.string] Update class definition, change return type from void to iterator:

void iterator insert(const_iterator p, size_type n, charT c);

template<class InputIterator>
  void iterator insert(const_iterator p, InputIterator first, InputIterator last);

void iterator insert(const_iterator p, initializer_list<charT>);

p13 23.4.3.7.4 [string.insert] change return type from void to iterator:

void iterator 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 if n == 0.

p15 23.4.3.7.4 [string.insert] change return type from void to iterator:

template<class InputIterator>
  void iterator 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 if first == last.

p17 23.4.3.7.4 [string.insert] change return type from void to iterator:

void iterator 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 if il 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.


150(i). Find_first_of says integer instead of iterator

Section: 27.6.9 [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 27.6.9 [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) ...


151(i). Can't currently clear() empty container

Section: 24.2.4 [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


152(i). Typo in scan_is() semantics

Section: 30.4.2.2.3 [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 30.4.2.2.3 [locale.ctype.virtuals] paragraphs 4 and 6, change the returns clause from:

"... such that is(*p) would..."

to:  "... such that is(m, *p) would...."


153(i). Typo in narrow() semantics

Section: 30.4.2.4.3 [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 30.4.2.4.3 [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 30.4.2.4.3 [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, which addresses the same paragraphs.]


154(i). Missing double specifier for do_get()

Section: 30.4.3.2.3 [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 doubles is actually a problem I found quite often in production code, too).

Proposed resolution:

In 30.4.3.2.3 [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.


155(i). Typo in naming the class defining the class 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.


156(i). Typo in imbue() description

Section: 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&".


158(i). Underspecified semantics for 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."


159(i). Strange use of 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".


160(i). Typo: Use of non-existing function 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.]


161(i). Typo: 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...


164(i). do_put() has apparently unused fill argument

Section: 30.4.6.4.2 [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 30.4.6.4.2 [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 30.4.6.4.2 [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.


165(i). 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:

  1. state in a footnote that this means that xsputn() will never be called by any ostream member and that this is intended.
  2. relax the restriction and allow calling 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.]


167(i). Improper use of 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:

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>


168(i). Typo: formatted vs. unformatted

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."


169(i). Bad efficiency of overflow() mandated

Section: 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.


170(i). Inconsistent definition of 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;

171(i). Strange seekpos() semantics due to joint position

Section: 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:

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.]


172(i). Inconsistent types for 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.

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.


173(i). Inconsistent types for 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.

Proposed resolution:

In 31.10.3.5 [filebuf.virtuals] paragraph 9, change all uses of int in the description of setbuf() to streamsize.


174(i). Typo: 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;"


175(i). Ambiguity for 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().


176(i). 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.


179(i). Comparison of const_iterators to iterators doesn't work

Section: 24.2 [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 24.2 [container.requirements] paragraph 7:

In the expressions

    i == j
    i != j
    i < j
    i <= j
    i >= j
    i > j
    i - j
  

Where 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.)


180(i). Container member iterator arguments constness has unintended consequences

Section: 23.4.3 [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 23.4.3 [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 23.4.3.7.4 [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 23.4.3.7.5 [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.


181(i). make_pair() unintended behavior

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.


182(i). Ambiguous references to size_t

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, 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.]


183(i). I/O stream manipulators don't work for wide character streams

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 involves the same paragraphs.]

[Post-Tokyo: The issues list maintainer combined the proposed resolution of this issue with the proposed resolution for issue 216 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.]


184(i). numeric_limits<bool> wording problems

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.]


185(i). Questionable use of term "inline"

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.]


186(i). bitset::set() second parameter should be bool

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.


187(i). iter_swap underspecified

Section: 27.7.3 [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.


189(i). setprecision() not specified correctly

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)".


193(i). Heap operations description incorrect

Section: 27.8.8 [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 27.8.8 [alg.heap.operations] property (1) from:

(1) *a is the largest element

to:

(1) There is no element greater than *a


195(i). Should 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() or is.rdbuf()->sgetc() returns traits::eof(), the function calls setstate(failbit | eofbit) (which may throw ios_base::failure).


198(i). Validity of pointers and references unspecified after iterator destruction

Section: 25.3.4 [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*(), 25.5.1.5 [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 25.3.4 [iterator.concepts]:

Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.

Replace paragraph 1 of 25.5.1.5 [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 25.3.4 [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. 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.


199(i). What does 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.


200(i). Forward iterator requirements don't allow constant iterators

Section: 25.3.5.5 [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 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.]


201(i). Numeric limits terminology wrong

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-dependent fundamental arithmetic 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 the fundamental arithmetic types.

-2- Specializations shall be provided for each fundamental arithmetic type, both floating point and integer, including bool. The member is_specialized shall be true for all such specializations of numeric_limits.

-4- Non-fundamentalarithmetic standard types, such as complex<T> (26.3.2), shall not have specializations.

Change 17.3.5 [numeric.limits] to:

-1- The member is_specialized makes it possible to distinguish between fundamental types, which have specializations, and non-scalar types, which do not.


202(i). unique() effects unclear when predicate not an equivalence relation

Section: 27.7.9 [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 27.7.9 [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 or pred(*(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 27.7.9 [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 or pred(*(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.


206(i). operator new(size_t, nothrow) may become unlinked to ordinary operator new if ordinary version replaced

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:

-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 the default (possibly replaced) operator new(std::size_t) or operator new(std::size_t, const std::nothrow_t&).

-13- Default behavior:

-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 of calloc, malloc, or realloc, 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) or operator 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 operator new(std::size_t, const std::nothrow_t&). This nothrow version of operator new[] returns a pointer obtained as if acquired from the ordinary version. 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) operator new[](std::size_t size). This requirement is binding on a replacement version of this function.

-8- Default behavior: Returns operator new(size, nothrow).

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 to operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&).

-12- Default behavior: Calls operator delete(ptr) or operator delete[](ptr, std::nothrow) respectively.

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. ]


208(i). Unnecessary restriction on past-the-end iterators

Section: 25.3.4 [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 25.3.4 [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.


209(i). basic_string declarations inconsistent

Section: 23.4.3 [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 23.4.3 [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 23.4.3 [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.


210(i). distance first and last confused

Section: 27 [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 27 [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 27 [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.


211(i). operator>>(istream&, string&) doesn't set failbit

Section: 23.4.4.4 [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 23.4.4.4 [string.io] paragraph 8.)

Proposed resolution:

Insert new paragraph after paragraph 2 in section 23.4.4.4 [string.io]:

If the function extracts no characters, it calls is.setstate(ios::failbit) which may throw ios_base::failure (27.4.4.3).


212(i). Empty range behavior unclear for several algorithms

Section: 27.8.9 [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 27.8.9 [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.


214(i). set::find() missing const overload

Section: 24.4.6 [set], 24.4.7 [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 24.4.6 [set] and section 24.4.7 [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.]


217(i). Facets example (Classifying Japanese characters) contains errors

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;
}

220(i). ~ios_base() usage valid?

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.


221(i). num_get<>::do_get stage 2 processing broken

Section: 30.4.3.2.3 [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.


222(i). Are throw clauses necessary if a throw is already implied by the effects clause?

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 23.4.3.2 [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.


223(i). reverse algorithm should use iter_swap rather than swap

Section: 27.7.10 [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 27.7.10 [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.


224(i). clear() complexity for associative containers refers to undefined N

Section: 24.2.7 [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.


225(i). std:: algorithms use of other unqualified algorithms

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 accordingly. Any resolution of issue 225 should be coordinated with the resolution of issue 229.]

[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 deals with the question of when it is appropriate for the standard to explicitly specify otherwise.


226(i). User supplied specializations or overloads of namespace std function templates

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).

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 28.6.3.3 [valarray.transcend] paragraph 1, state that the valarray transcendentals use unqualified lookup.


227(i). std::swap() should require CopyConstructible or DefaultConstructible arguments

Section: 27.7.3 [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)


228(i). Incorrect specification of "..._byname" facets

Section: 30.4 [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 30.4.2.3 [locale.ctype.byname], 30.4.2.6 [locale.codecvt.byname], 30.4.4.2 [locale.numpunct.byname], 30.4.5.2 [locale.collate.byname], 30.4.6.5 [locale.time.put.byname], 30.4.7.5 [locale.moneypunct.byname], and 30.4.8.3 [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 30.4.2.5 [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.]

[Copenhagen: proposed resolution was revised slightly, to remove three last virtual functions from messages_byname.]


229(i). Unqualified references of other library entities

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, 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. Any resolution of issue 225 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.]


230(i). Assignable specified without also specifying CopyConstructible

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 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, 28.5.3 [rand.req].

Proposed resolution:

In 24.2 [container.requirements] table 65 for value_type: change "T is Assignable" to "T is CopyConstructible and Assignable"

In 24.2.7 [associative.reqmts] table 69 X::key_type; change "Key is Assignable" to "Key is CopyConstructible and Assignable"

In 25.3.5.4 [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 27.7.5 [alg.replace] and 27.7.6 [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.]

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.


231(i). Precision in iostream?

Section: 30.4.3.3.3 [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 30.4.3.3.3 [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.]


232(i). "depends" poorly defined in 17.4.3.1

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.]

[post-Toronto: Judy provided the above proposed resolution and rationale.]


233(i). Insertion hints in associative containers

Section: 24.2.7 [associative.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 2000-04-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 192, 246

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 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 24.2.7 [associative.reqmts] to:

Associative container requirements
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. iterator 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 after before p.

234(i). Typos in allocator definition

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".


235(i). No specification of default ctor for reverse_iterator

Section: 25.5.1.2 [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 25.5.1.4 [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 type Iterator.

[pre-Copenhagen: Dietmar provide wording for proposed resolution.]


237(i). Undefined expression in complexity specification

Section: 24.3.8.2 [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).


238(i). Contradictory results of stringbuf initialization.

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.


239(i). Complexity of unique() and/or unique_copy incorrect

Section: 27.7.9 [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 27.7.9 [alg.unique] to:

Complexity: For nonempty ranges, exactly last - first - 1 applications of the corresponding predicate.


240(i). Complexity of adjacent_find() is meaningless

Section: 27.6.10 [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 27.6.10 [alg.adjacent.find] to:

For a nonempty range, exactly min((i - first) + 1, (last - first) - 1) applications of the corresponding predicate, where i is adjacent_find's return value.

[Copenhagen: the original resolution specified an upper bound. The LWG preferred an exact count.]


241(i). Does unique_copy() require CopyConstructible and Assignable?

Section: 27.7.9 [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(), 28.5.3 [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. ]


242(i). Side effects of function objects

Section: 27.7.4 [alg.transform], 28.5 [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()