Doc. no. J16/00-0036
WG21 N1259
Date: 07 Sep 2000
Project: Programming Language C++
Reply to: Matt Austern <austern@research.att.com>

C++ Standard Library Active Issues List (Revision 15)

Reference ISO/IEC IS 14882:1998(E)

Also see:

The purpose of this document is to record the status of issues which have come before the Library Working Group (LWG) of the ANSI (J16) and ISO (WG21) C++ Standards Committee. Issues represent potential defects in the ISO/IEC IS 14882:1998(E) document. Issues are not to be used to request new features or other extensions.

This document contains only library issues which are actively being considered by the Library Working Group.  That is, issues which have a status of New, Open, Review, and Ready.  See "C++ Standard Library Defect Report List" for issues considered defects and "C++ Standard Library Closed Issues List" for issues considered closed.

The issues in these lists are not necessarily formal ISO Defect Reports (DR's).  While some issues will eventually be elevated to official Defect Report status, other issues will be disposed of in other ways.  See Issue Status.

This document is in an experimental format designed for both viewing via a world-wide web browser and hard-copy printing.  It is available as an HTML file for browsing or PDF file for printing.

Prior to Revision 14, library issues lists existed in two slightly different versions; a Committee Version and a Public Version. Beginning with Revision 14 the two versions were combined into a single version.

This document includes [bracketed italicized notes] as a reminder to the LWG of current progress on issues.  Such notes are strictly unofficial and should be read with caution as they may be incomplete or incorrect. Be aware that LWG support for a particular resolution can quickly change if new viewpoints or killer examples are presented in subsequent discussions.

For the most current version of this document see http://www.dkuug.dk/jtc1/sc22/wg21. Requests for further information about this document should include the document number above, reference ISO/IEC 14882:1998(E), and be submitted to Information Technology Industry Council (ITI), 1250 Eye Street NW, Washington, DC 20005.

Public information as to how to obtain a copy of the C++ Standard, join the standards committee, submit an issue, or comment on an issue can be found in the C++ FAQ at http://www.research.att.com/~austern/csc/faq.html. Public discussion of C++ Standard related issues occurs on news:comp.std.c++.

For committee members, files available on the committee's private web site include the HTML version of the Standard itself.  HTML hyperlinks from this issues list to those files will only work for committee members who have downloaded them into the same disk directory as the issues list files.   

Revision history

Issue Status

New - The issue has not yet been reviewed by the LWG.  Any Proposed Resolution is purely a suggestion from the issue submitter, and should not be construed as the view of LWG.

Open - The LWG has discussed the issue but is not yet ready to move the issue forward.  There are several possible reasons for open status:

A Proposed Resolution for an open issue is still not be construed as the view of LWG.  Comments on the current state of discussions are often given at the end of open issues in an italic font. Such comments are for information only and should not be given undue importance. They do not appear in the public version.

Dup - The LWG has reached consensus that the issue is a duplicate of another issue, and will not be further dealt with.  A Rationale identities the duplicated issue's issue number.   

NAD - The LWG has reached consensus that the issue is not a defect in the Standard, and the issue is ready to forward to the full committee as a proposed record of response.  A Rationale discusses the LWG's reasoning.

Review - Exact wording of a Proposed Resolution is now available for review on an issue for which the LWG previously reached informal consensus.

Ready - The LWG has reached consensus that the issue is a defect in the Standard, the Proposed Resolution is correct, and the issue is ready to forward to the full committee for further action as a Defect Report (DR).

DR - (Defect Report) - The full J16 committee has voted to forward the issue to the Project Editor to be processed as a Potential Defect Report. The Project Editor reviews the issue, and then forwards it to the WG21 Convenor, who returns it to the full committee for final disposition.  This issues list accords the status of DR to all these Defect Reports regardless of where they are in that process.

TC - (Technical Corrigenda) - The full WG21 committee has voted to accept the Defect Report's Proposed Resolution as a Technical Corrigenda.   Action on this issue is thus complete and no further action is possible under ISO rules.

RR - (Record of Response) - The full WG21 committee has determined that this issue is not a defect in the Standard. Action on this issue is thus complete and no further action is possible under ISO rules.

Future - In addition to the regular status, the LWG believes that this issue should be revisited at the next revision of the standard.   It is usually paired with NAD.

Issues are always given the status of New when they first appear on the issues list.  They may progress to Open or Review while the LWG is actively working on them. When the LWG has reached consensus on the disposition of an issue, the status will then change to Dup, NAD, or Ready as appropriate.  Once the full J16 committee votes to forward Ready issues to the Project Editor, they are given the status of Defect Report ( DR).  These in turn may become the basis for Technical Corrigenda (TC), or are closed without action other than a Record of Response (RR).  The intent of this LWG process is that only issues which are truly defects in the Standard move to the formal ISO DR status.

Active Issues


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

Section: 18.3 lib.support.start.term  Status: Ready  Submitter: Steve Clamage  Date: 12 Dec 97 Msg: lib-6500

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.

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.

Paper:

See 99-0039/N1215, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.

[Tokyo: Reviewed by the LWG.]


8.  Locale::global lacks guarantee

Section: 22.1.1.5 lib.locale.statics Status: Ready  Submitter: Matt Austern   Date: 24 Dec 97

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 22.1.1.5 [lib.locale.statics], paragraph 2: 

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

[Tokyo: Reviewed by the LWG.]


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

Section: 18.4.1 lib.new.delete  Status: Ready  Submitter: Steve Clamage  Date: 4 Jan 98

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.

18.4.1 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 18.4.1.3, add the following text:

The provisions of 3.7.3 do not apply to these reserved placement forms of operator new and operator delete.

Paper:

See 99-0040/N1216, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.

[Tokyo: Reviewed by the LWG.]


19.  "Noconv" definition too vague

Section: 22.2.1.5.2 lib.locale.codecvt.virtuals Status: Ready  Submitter: Nathan Myers  Date: 6 Aug 98

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 22.2.1.5.2 [lib.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).

[Tokyo: Reviewed by the LWG.]


26.  Bad sentry example

Section: 27.6.1.1.2 lib.istream::sentry  Status: Ready  Submitter: Nathan Myers  Date: 6 Aug 98

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 27.6.1.1.2 lib.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.

[Tokyo: Reviewed by the LWG.]


31.  Immutable locale values

Section: 22.1.1 [lib.localeStatus: Ready  Submitter: Nathan Myers  Date: 6 Aug 98

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 22.1.1 [lib.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.

[Tokyo: Reviewed by the LWG.]


44.  Iostreams use operator== on int_type values

Section: 27 [lib.input.outputStatus: Open  Submitter: Nathan Myers  Date: 6 Aug 98

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:

[Kona: Nathan to supply proposed wording.

Tokyo: the LWG reaffirmed that this is a defect, and requires careful review of clause 27 as the changes are context sensitive.]


49.  Underspecification of ios_base::sync_with_stdio

Section: 27.4.2.4 lib.ios.members.static Status: Open  Submitter: Matt Austern  Date: 21 Jun 98

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 sentenance in 27.4.2.4 lib.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.

[The LWG agrees  (2)  that a definition of synchronized is required.   Jerry Schwarz will work by email with Matt Austern to provide such a definition.

 Tokyo: PJP knows approximate wording, and will help Matt formulate final wording.]


61.  Ambiguity in iostreams exception policy

Section: 27.6.1.3  lib.istream.unformatted Status: Ready  Submitter: Matt Austern   Date:6 Aug 98

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.)"

[Tokyo: The LWG looked to two alternative wordings submitted by Matt, and choose the proposed resolution as better standardese.]


63.  Exception-handling policy for unformatted output

Section: 27.6.2.6  lib.ostream.unformatted Status: Ready  Submitter: Matt Austern   Date:11 Aug 98

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.

[pre-Toronto: this may be affected by the resolution to issue 160.]

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 (exception() & 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.

[Kona: Matt Austern provided the proposed resolution wording.]

[Tokyo: Reviewed by the LWG.]


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

Section: 22.2.1.5 lib.locale.codecvt  Status: Open  Submitter: Matt Austern  Date: 25 Sep 98

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 (22.2.1.5.2, paragraph 11) and basic_filebuf::underflow (27.8.1.4, 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:

[Kona: Matt Austern will attempt wording;  it is very complex.]


86.  String constructors don't describe exceptions

Section: 21.3.1 lib.string.cons Status: Ready  Submitter: Nico Josuttis  Date:  29 Sep 1998

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 21.3.1 lib.string.cons, 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.

[Tokyo: Reviewed by the LWG.] 


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

Section: 21.3.7.9 lib.string.io  Status: Open  Submitter: Nico Josuttis  Date:  29 Sep 1998

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 21.3.7.9 [lib.string.io], paragraph 1, last sentence "Characters are extracted and appended until any of the following occurs:...", replace:

    - end-of-file occurs on the input sequence;

with: 

    - an attempt to extract a character fails; 

In 21.3.7.9 [lib.string.io], paragraph 5, last sentence, replace :

    - end-of-file occurs on the input sequence (in which case, the getline function calls is.setstate(ios_base::eofbit)).
 
with:
 
    - an attempt to extract a character fails 

In 23.3.5.3 [lib.bitset.operators], paragraph 5, last sentence, replace:
 
    - end-of-file occurs on the input sequence; 

with:
 
    - an attempt to extract a character fails; 

[Tokyo: Sentiment was expressed for a single blanket statement which applies to all extractors (string, formatted, or unformatted).  The problem with the proposed resolution is that there is no concept of read failure in iostreams.  If not exception is thrown, sooner or later eof will be reached.]


92.  Incomplete Algorithm Requirements

Section: 25 lib.algorithms  Status: Open  Submitter: Nico Josuttis  Date:  29 Sep 98

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 suprising/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:

In [lib.function.objects] 20.3 Function objects add as new paragraph 6 (or insert after paragraph 1):

Option 1:

Predicates are functions or function objects that fulfill the following requirements:
  - They return a Boolean value (bool or a value convertible to bool)
  - It doesn't matter for the behavior of a predicate how often it is copied or assigned and how often it is called.

Option 2:

- if it's a function:
  - All calls with the same argument values yield the same result.
- if it's a function object:
  - In any sequence of calls to operator () without calling any non-constant member function, all calls with the same argument values yield the same result. 
- After an assignment or copy both objects return the same result for the same values.

[Santa Cruz: The LWG believes that there may be more to this than meets the eye.  It applies to all function objects, particularly predicates.  Two questions: (1) must a function object be copyable? (2) how many times is a function object called?  These are in effect questions about state.  Function objects appear to require special copy semantics to make state work, and may fail if calling alters state and calling occurs an unexpected number of times.

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 25 lib.algorithms after paragraph 9

Pre-Tokyo: Angelika Langer comments:  if the resolution is that algorithms are free to copy and pass around any function objects, then it is a valid question whether they are also allowed to change the type information from reference type to value type.

Tokyo: Nico will discuss this further with Matt as there are multiple problems beyond the underlying problem of no definition of "Predicate".

Post-Tokyo: Nico provided the above proposed resolutions.]


94.  May library implementors add template parameters to Standard Library classes?

Section: 17.4.4 lib.conforming  Status: Open  Submitter: Matt Austern  Date: 22 Jan 98

Is it a permitted extension for library implementors to add template parameters to standard library classes, provided that those extra parameters have defaults? For example, instead of defining template <class T, class Alloc = allocator<T> > class vector; defining it as template <class T, class Alloc = allocator<T>, int N = 1> class vector;

The standard may well already allow this (I can't think of any way that this extension could break a conforming program, considering that users are not permitted to forward-declare standard library components), but it ought to be explicitly permitted or forbidden.

Proposed Resolution:

Add a new subclause [presumably 17.4.4.9] following 17.4.4.8 [lib.res.on.exception.handling]:

17.4.4.9 Template Parameters

A specialization of a template class described in the C++ Standard Library behaves the same as if the implementation declares no additional template parameters.

Footnote/ Additional template parameters with default values are thus permitted.

Add "template parameters" to the list of subclauses at the end of 17.4.4 paragraph 1 [lib.conforming].

[Kona:  The LWG agreed the standard needs clarification.  After discussion with John Spicer, it seems added template parameters can be detected by a program using template-template parameters.  A straw vote - "should implementors be allowed to add template parameters?" found no consensus ; 5 - yes, 7 - no.]

[Post-Kona comment from Steve Cleary via comp.std.c++: 

I disagree [with the proposed resolution] for the following reason: consider user library code with template template parameters. For example, a user library object may be templated on the type of underlying sequence storage to use (deque/list/vector), since these classes all take the same number and type of template parameters; this would allow the user to determine the performance tradeoffs of the user library object. A similar example is a user library object templated on the type of underlying set storage (set/multiset) or map storage (map/multimap), which would allow users to change (within reason) the semantic meanings of operations on that object.

I think that additional template parameters should be forbidden in the Standard classes. Library writers don't lose any expressive power, and can still offer extensions because additional template parameters may be provided by a non-Standard implementation class: 

 template <class T, class Allocator = allocator<T>, int N = 1>
 class __vector
 { ... };
 template <class T, class Allocator = allocator<T> >
 class vector: public __vector<T, Allocator>
 { ... };

]

[Tokyo:  Discussed but no action taken.  Still no consensus as to which answer serves the greatest need.]


96.  Vector<bool> is not a container

Section: 23.2.5 lib.vector.bool  Status: Open Submitter: AFNOR  Date: 7 Oct 98

vector<bool> is not a container as its reference and pointer types are not references and pointers.

Also it forces everyone to have a space optimization instead of a speed one.

See also: 99-0008 == N1185 Vector<bool> is Nonconforming, Forces Optimization Choice.

Proposed Resolution:

[In Santa Cruz the LWG felt that this was Not A Defect.]

[In  Dublin many present felt that failure to meet Container requirements was a defect.  There was disagreement as to whether or not the optimization requirements constituted a defect.

The LWG looked at the following resolutions in some detail:

     * Not A Defect.
     * Add a note explaining that vector<bool> does not meet Container requirements.
     * Remove vector<bool>.
     * Add a new category of container requirements which vector<bool> would meet.
     * Rename vector<bool>.

No alternative had  strong, wide-spread, support and every alternative had at least one "over my dead body" response.

There was also mention of a transition scheme something like (1) add vector_bool and deprecate vector<bool> in the next standard. (2) Remove vector<bool> in the following standard.

Modifying container requirements to permit returning proxies (thus allowing container requirements conforming vector<bool>) was also discussed.

It was also noted that there is a partial but ugly workaround in that vector<bool> maybe further specialized with a customer allocator. 

Kona:  Herb Sutter presented his paper J16/99-0035==WG21/N1211, vector<bool>: More Problems, Better Solutions.  Much discussion of a two step approach: a) deprecate, b) provide replacement under a new name.  LWG straw vote on that: 1-favor, 11-could live with, 2-over my dead body.  This resolution was mentioned in the LWG report to the full committee, where several additional committee members indicated over-my-dead-body positions.

Tokyo:  Not discussed by the full LWG;  no one claimed new insights and so time was more productively spent on other issues.  In private discussions it was asserted that requirements for any solution include 1) Increasing the full committee's understanding of the problem, and 2) providing compiler vendors, authors, teachers, and of course users with specific suggestions as to how to apply the eventual solution.] 


98.  Input iterator requirements are badly written

Section: 24.1.1 lib.input.iterators  Status: Open  Submitter: AFNOR  Date: 7 Oct 98

Table 72 in 24.1.1 (lib.input.iterators) specifies semantics for *r++ of:

   { T tmp = *r; ++r; return tmp; }

This does not work for pointers and over constrains implementors.

Proposed Resolution:

Add for *r++: “To call the copy constructor for the type T is allowed but not required.”

[Dublin: Pete Becker will attempt improved wording.]

[Tokyo: The essence of the issue seems to have escaped.  Pete will email Valentin to try to recapture it.]


102.  Bug in insert range in associative containers

Section: 23.1.2 lib.associative.reqmts  Status: Open  Submitter: AFNOR  Date: 7 Oct 98

Table 69 of Containers say that a.insert(i,j) is linear if [i, j) is ordered. It seems impossible to implement, as it means that if [i, j) = [x], insert in an associative container is O(1)!

Proposed Resolution:

N+log (size()) if [i,j) is sorted according to value_comp()

[This may need better specification.  Matt Austern will ask Dave Musser.]


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

Section: 23.1.2 lib.associative.reqmts   Status: Review  Submitter: AFNOR  Date: 7 Oct 98

Set::iterator is described as implementation-defined with a reference to the container requirement; the container requirement says that const_iterator is an iterator pointing to const T and iterator an iterator pointing to T.

23.1.2 paragraph 2 implies that the keys should not be modified to break the ordering of elements. But that is not clearly specified. Especially considering that the current standard requires that iterator for associative containers be different from const_iterator. Set, for example, has the following:

typedef implementation defined iterator;
       // See _lib.container.requirements_

23.1 lib.container.requirements actually requires that iterator type pointing to T (table 65). Disallowing user modification of keys by  changing the standard to require an iterator for associative container to be the same as const_iterator would be overkill since that will unnecessarily significantly restrict the usage of associative container. A class to be used as elements of set, for example, can no longer be modified easily without either redesigning the class (using mutable on fields that have nothing to do with ordering), or using const_cast, which defeats requiring iterator to be const_iterator. The proposed solution goes in line with trusting user knows what he is doing.

Other Options Evaluated:

Option A.   In 23.1.2 lib.associative.reqmts, paragraph 2, after first sentence,  and before "In addition,...", add one line:

Modification of keys shall not change their strict weak ordering.

Option B. Add three new sentences to 23.1.2 lib.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 23.1.2 lib.associative.reqmts, paragraph 3, which currently reads:

The phrase ``equivalence of keys'' means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 in the same container are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false.

  add the following:

For any two keys k1 and k2 in the same container, comp(k1, k2) shall return the same value whenever it is evaluated. [Note: If k2 is removed from the container and later reinserted, comp(k1, k2) must still return a consistent value but this value may be different than it was the first time k1 and k2 were in the same container. This is intended to allow usage like a string key that contains a filename, where comp compares file contents; if k2 is removed, the file is changed, and the same k2 (filename) is reinserted, comp(k1, k2) must again return a consistent value but this value may be different than it was the previous time k2 was in the container.]

Proposed Resolution:

Add the following to 23.1.2 lib.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.

Simply requiring that keys be immutable is not sufficient, because the comparison object may indirectly (via pointers) operate on values outside of the keys.

[Tokyo:  The LWG crafted the proposed resolution and rationale.]


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

Section: 18.6.1 lib.exception para 8, 9 Status: Ready  Submitter: AFNOR  Date: 7 Oct 98

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 18.6.1 lib.exception  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.

[Tokyo: Reviewed by the LWG.] 


109.  Missing binders for non-const sequence elements

Section: 20.3.6 lib.binders  Status: Open  Submitter: Bjarne Stroustrup  Date: 7 Oct 98

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.

[Dublin: Nico volunteered to organize a discussion of this and related issues.  Here it is:]

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:

In 20.3.6.1 [lib.binder.1st] 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 20.3.6.3 [lib.binder.2nd] 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:

5  NAD
3  As Proposed
6  Leave open

Tokyo:  The issue was not discussed.]


111.  istreambuf_iterator::equal overspecified, inefficient

Section: 24.5.3.5 [lib.istreambuf.iterator::equal]   Status: Open  Submitter: Nathan Myers  Date: 15 Oct 98

The member istreambuf_iterator<>::equal is specified to be unnecessarily inefficient. While this does not affect the efficiency of conforming implementations of iostreams, because they can "reach into" the iterators and bypass this function, it does affect users who use istreambuf_iterators.

The inefficiency results from a too-scrupulous definition, which requires a "true" result if neither iterator is at eof. In practice these iterators can only usefully be compared with the "eof" value, so the extra test implied provides no benefit, but slows down users' code.

The solution is to weaken the requirement on the function to return true only if both iterators are at eof.

Proposed Resolution:

Replace 24.5.3.5 [lib.istreambuf.iterator::equal], paragraph 1,

-1- Returns: true if and only if both iterators are at end-of-stream, or neither is at end-of-stream, regardless of what streambuf object they use.

with

-1- Returns: true if and only if both iterators are at end-of-stream, regardless of what streambuf object they use.

[Dublin:  People present saw no compelling reason to make change.  There is also concern over not-equal.  The issue is being held open for input from Nathan.

Kona:  Did not discuss due to lack of time.

Tokyo:  Still no discussion.]


112.  Minor typo in ostreambuf_iterator constructor

Section: 24.5.4.1 lib.ostreambuf.iter.cons  Status: Ready  Submitter: Matt Austern  Date: 20 Oct 98

The requires clause for ostreambuf_iterator's constructor from an ostream_type (24.5.4.1, paragraph 1) reads "s is not null". However, s is a reference, and references can't be null.

Proposed Resolution:

In 24.5.4.1 lib.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.

[Tokyo: Reviewed by the LWG.] 


114.  Placement forms example in error twice

Section: 18.4.1.3 [lib.new.delete.placement] Status: Ready  Submitter: Steve Clamage  Date: 28 Oct 1998

Section 18.4.1.3 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 likey to fail.

Proposed Resolution:

Replace the first line of code in the example in 18.4.1.3 [lib.new.delete.placement] with:

void* place = operator new(sizeof(Something));

[Kona: See issue 196 (forwarded from Core), which is the same issue but with a different resolution. Need to resolve the difference.

Tokyo: Reviewed by the LWG, which resolved the difference.]


115.  Typo in strstream constructors

Section: D.7.4.1 [depr.strstream.consStatusReady  Submitter: Steve Clamage  Date: 2 Nov 1998

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 sbwith 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 D.7.3.1 [depr.ostrstream.cons] paragraph 2 and D.7.4.1 [depr.strstream.cons]  paragraph 2, change the first condition to (mode&app)==0 and the second condition to (mode&app)!=0.

[ Project Editor in lib-6682 indicated that these changes have already been made as editorial. ]

[Tokyo: Reviewed by the LWG.]


117basic_ostream uses nonexistent num_put member functions

Section: 27.6.2.5.2 lib.ostream.inserters.arithmetic   Status: Open  Submitter: Matt Austern  Date: 20 Nov 98

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 locale­dependent 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 or 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<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();

[Dublin: The LWG feels this is probably correct, but would like to review it one more time with additional technical experts.  Issue 118 is related.

Tokyo:  Matt should speak to PJP; the first example is too simplistic for signed int and signed short.]


118basic_istream uses nonexistent num_get member functions

Section: 27.6.1.2.2 lib.istream.formatted.arithmetic   Status: Review  Submitter: Matt Austern  Date: 20 Nov 98

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 22.2.2.1.1 lib.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 27.6.1.2.2 Arithmetic Extractors [lib.istream.formatted.arithmetic], 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 < SHRT_MIN || SHRT_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 < INT_MIN || INT_MAX < lval))
		err = ios_base::failbit;
  setstate(err);

[Dublin: What about do_get? Aren't two functions need there too?  Also, the LWG would like to see full wording for the Proposed Resolution.

Post-Tokyo: PJP provided the above wording.]


120. Can an implementor add specializations?

Section: 17.4.3.1 lib.reserved.names Status: Review  Submitter: Judy Ward Date: 15 Dec 1998

Section 17.4.3.1 says:

It is undefined for a C++ program to add declarations or definitions to namespace std or namespaces within namespace std unless otherwise specified. A program may add template specializations for any standard library template to namespace std. Such a specialization (complete or partial) of a standard library template results in undefined behavior unless the declaration depends on a user-defined name of external linkage and unless the specialization meets the standard library requirements for the original template...

This implies that it is ok for library users to add specializations, but not implementors. A user program can actually detect this, for example, the following manual instantiation will not compile if the implementor has made ctype<wchar_t> a specialization:

#include <locale>
#include <wchar.h>

template class std::ctype<wchar_t>; // can't be specialization

Lib-7047 [Matt Austern] comments: 

The status quo is unclear, and probably contradictory. This issue applies both the explicit instantiations and tospecializations, since it is not permitted to provide both a specialization and an explicit instantiation.

The specialization issue is actually more serious than the instantiation one. One could argue that there is a consistent status quo as far as instantiations go, but one can't argue that in the case of specializations. The standard must either (1) give library implementors license to provide explicit specializations of any library template; or (2) give a complete list of exactly which specializations must be provided, and forbid library implementors from providing any specializations not on that list. At present the standard does neither.

Proposed Resolution:

Append to 17.4.3.1 lib.reserved.names paragraph 1:

A program may manually instantiate any templates in the standard library only if the declaration depends on a user-defined name of external linkage and the instantiation meets the standard library requirements for the original template.

[Kona: Wording should be added to the effect that users will not be allowed to manual instantiate any templates in the standard library.   Judy will work on the proposed wording.  Also see issue 177.

Post-Tokyo: Judy Ward provided the above wording.]


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

Section: 27.5.2 lib.streambuf Status: Ready  Submitter: Judy Ward Date: 15 Dec 1998

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 27.5.2 lib.streambuf paragraphs 2 and 3 (the above two sentences).

Rationale:

The streambuf synopsis already has a declaration for the typedefs and that is sufficient.

[Tokyo: Reviewed by the LWG.]


123. Should valarray helper arrays fill functions be const?

Section: 26.3.5.4 lib.slice.arr.fill, 26.3.7.4 lib.gslice.array.fill, 26.3.8.4 lib.mask.array.fill, 26.3.9.4 lib.indirect.array..fill Status: Open  Submitter: Judy Ward Date: 15 Dec 1998

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 26.3.5.2 lib.slice.arr.assign is const:

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

but this one in Section 26.3.5.4 lib.slice.arr.fill, is not:

    void operator=(const T&);

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

Proposed Resolution:

Make the operator=(const T&) versions of slice_array, gslice_array, indirect_array, and mask_array const member functions.

[Dublin: Pete Becker spoke to Daveed Vandevoorde about this and will work on a proposed resolution.

Tokyo: Discussed together with the AFNOR paper 00-0023/N1246.  The current helper slices now violate language rules due to a core language change (but most compilers don't check, so the violation has previously gone undetected).  Major surgery is being asked for in this and other valarray proposals (see issue 77 Rationale), and a complete design review is needed before making piecemeal changes.  Robert Klarer will work on formulating the issues. ]


127.  auto_ptr<> conversion issues

Section: 20.4.5 lib.auto.ptr  Status: Ready  Submitter: Greg Colvin  Date: 17 Feb 1999

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_refAlso 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 implementers, 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 20.4.5 lib.auto.ptr, paragraph 2, and  20.4.5.3 lib.auto.ptr.conv, paragraph 2, make the conversion to auto_ptr_ref const:

template<class Y> operator auto_ptr_ref<Y>() const throw();

Proposed Resolution:

In 20.4.5 lib.auto.ptr, paragraph 2, move the auto_ptr_ref definition to namespace scope.

In 20.4.5 lib.auto.ptr, 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 20.4.5.3 lib.auto.ptr.conv:

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.

[Tokyo: Reviewed by the LWG.]


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

Section: 27.6.1.3 lib.istream.unformatted and 27.6.2.4 lib.ostream.seeks  Status: Ready  Submitter: Angelika Langer  Date: February 22, 1999  

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  27.6.1.3 lib.istream.unformatted and to the Effects: clause of  seekp() in 27.6.2.4 lib.ostream.seeks:

In case of failure, the function calls setstate(failbit) (which may throw ios_base::failure).

[Tokyo: Reviewed by the LWG.]


134.  vector constructors over specified

Section: 23.2.4.1 lib.vector.cons Status: Ready  Submitter: Howard Hinnant Date: 6 Mar 99

The complexity description says: "It does at most 2N calls to the copy constructor of T and logN reallocations if they are just input iterators ...".

This appears to be overly restrictive, dictating the precise memory/performance tradeoff for the implementor.

Proposed Resolution:

Change 23.2.4.1 lib.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, since it is impossible to determine the distance between first and last and then do copying.

[Dublin:  The issues hinges on whether at "most 2N calls" is correct or not.  There was a feeling that 2N is correct, but the issue will be left open to allow Howard to further analyze the complexity.

Tokyo:  Needs to be integrated with issue 144.  The LWG now agrees Howard is correct for vector, but is concerned about deque.

Post-Tokyo:  Howard Hinnant analyzed proposed resolutions for deque constructors in both 134 and 144, and says that the wording in 144 is better for deque.  Thus 134 has been modified to deal only with vector, and 144 will resolve the issue for deque.]


136.  seekp, seekg setting wrong streams?

Section: 27.6.1.3 lib.istream.unformatted Status: Review  Submitter: Howard Hinnant Date: 6 Mar 99

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 my issue either by changing basic_istream and basic_ostream, or by changing basic_stringbuf. I actually 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 actually a bit weird. There's no similar requirement for basic_streambuf<>::seekpos, or for basic_filebuf<>::seekoff or basic_filebuf<>::seekpos.]


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

Section: 22.1.1 lib.locale   Status: Ready  Submitter: Angelika Langer  Date: 17 Mar 1999

Section  22.1.1 lib.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 22.1.2 lib.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.

[Dublin:  The opinion of other iostream experts is required.

Tokyo:  Reviewed by the LWG.]


142.  lexicographical_compare complexity wrong

Section: 25.3.8 lib.alg.lex.comparison  Status: Ready  Submitter: Howard Hinnant  Date: 20 Jun 99

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 25.3.8 [lib.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]

[Kona: Matt Austern provided the proposed resolution wording at the request of the LWG.

Tokyo:  Reviewed by the LWG.]


144.  Deque constructor complexity wrong

Section: 23.2.1.1 lib.deque.cons  Status: Ready  Submitter: Herb Sutter  Date: 9 May 99

In 23.2.1.1 paragraph 6, the deque ctor that takes an iterator range appears to have complexity requirements which are incorrect, and which contradict the complexity requirements for insert(). I suspect that the text in question, below, was taken from vector:

Complexity: If the iterators first and last are forward iterators, bidirectional iterators, or random access iterators the constructor makes only N calls to the copy constructor, and performs no reallocations, where N is last - first.

The word "reallocations" does not really apply to deque. Further, all of the following appears to be spurious:

It makes at most 2N calls to the copy constructor of T and log N reallocations if they are input iterators.1)

1) The complexity is greater in the case of input iterators because each element must be added individually: it is impossible to determine the distance between first abd last before doing the copying.

This makes perfect sense for vector, but not for deque. Why should deque gain an efficiency advantage from knowing in advance the number of elements to insert?

Proposed Resolution:

In 23.2.1.1 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.

[Kona: Reviewed by the LWG.

Tokyo:  Needs to be integrated with issue 134.

Post-Tokyo:  Howard Hinnant analyzed proposed resolutions for deque constructors in both 134 and 144, and says that the wording in 144 is better for deque.  Thus 134 has been modified to deal only with vector, and 144 will resolve the issue for deque.]


146.  complex<T> Inserter and Extractor need sentries

Section: 26.2.6 lib.complex.ops Status: Ready  Submitter:  Angelika Langer Date:12 May 99

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 26.2.6 lib.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.

[Tokyo: Reviewed by the LWG.]


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

Section: 17.4.4.3 lib.global.functions  Status: Ready  Submitter: Lois Goldthwaite Date: 4 Jun 99

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

[Kona:  Because operator new and delete are global, the proposed resolution was changed from "non-member" to "global or non-member. ]

[Tokyo: Reviewed by the LWG.]


153.  Typo in narrow() semantics

Section:: 22.2.1.3.2 lib.facet.ctype.char.members  Status: Review  Submitter: Dietmar Kühl Date: 20 Jul 99

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.

Proposed resolution:

Change 22.2.1.3.2 lib.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?]


159.  Strange use of underflow()

Section:: 27.5.2.4.3 lib.streambuf.virt.get  Status: Ready  Submitter: Dietmar Kühl Date: 20 Jul 99

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 27.5.2.4.3 lib.streambuf.virt.get paragraph 1,  showmanyc()returns clause,  by replacing the word "supplied" with the words "extracted from the stream".

[Tokyo: Reviewed by the LWG.]


164.  do_put() has apparently unused fill argument

Section:: 22.2.5.3.2 lib.locale.time.put.virtuals  Status: Ready  Submitter: Angelika Langer Date: 23 Jul 99

In [lib.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 22.2.5.3.2 lib.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.

[Tokyo: Reviewed by the LWG.]


165xsputn(), pubsync() never called by basic_ostream members?

Section:: 27.6.2.1 lib.ostream  Status: Review  Submitter: Dietmar Kühl Date: 20 Jul 99

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.  Improper use of traits_type::length()

Section:: 27.6.2.5.4 lib.ostream.inserters.character  Status: Review  Submitter: Dietmar Kühl Date: 20 Jul 99

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 27.6.2.5.4 (lib.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 an formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts characters. The number len of characters starting at s to be inserted is

- traits::length(s) if the second argument is of type const charT* 
- char_traits<char>::length(s) if the second argument is of type char and charT is not char 
- char_traits<signed char>::length(s) if the second argument is of type signed char and charT is not signed char 
- char_traits<unsigned char>::length(s) if the second argument is of type unsigned char and charT is not unsigned char

Padding is determined as described in lib.facet.num.put.virtuals. The len 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).

[Kona: It is clear to the LWG there is a defect here.  Dietmar will supply specific wording.

Post-Tokyo: Dietmar supplied the above wording.]


170.  Inconsistent definition of traits_type

Section:: 27.7.4 lib.stringstream  Status: Ready  Submitter: Dietmar Kühl Date: 20 Jul 99

The classes basic_stringstream (27.7.4, lib.stringstream), basic_istringstream (27.7.2, lib.istringstream), and basic_ostringstream (27.7.3, lib.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:

To the declarations of basic_ostringstream (27.7.3, lib.ostringstream) and basic_stringstream (27.7.4, lib.stringstream) add:

typedef traits traits_type;

[Tokyo: Reviewed by the LWG.]


171.  Strange seekpos() semantics due to joint position

Section:: 27.8.1.4 lib.filebuf.virtuals  Status: Review  Submitter: Dietmar Kühl Date: 20 Jul 99

Overridden virtual functions, seekpos()

In 27.8.1.1 (lib.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() return false.

[Kona: Dietmar is working on a proposed resolution.]

[Post-Tokyo: Dietmar supplied the above wording.]


179.  Comparison of const_iterators to iterators doesn't work

Section: 23.1 lib.container.requirements Status: Review Submitter: Judy Ward Date: 2 Jul 1998

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;
}

Proposed Resolution:

In section 23.1 (lib.container.requirements) after paragraph 7 add:

It is possible to mix iterators and const_iterators in iterator comparison operations.

[Kona: The LWG does wish the example to work.  Judy will provide wording.]

[Post-Tokyo: Judy supplied the above wording at the request of the LWG.]


181.  make_pair() unintended behavior

Section: 20.2.2 lib.pairs  Status: Review  Submitter: Andrew Koenig Date: 3 Aug 99

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 20.2 [lib.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 20.2.2 [lib.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.

[Kona:  The LWG agreed that this is a probable defect, but would like to see fixes spelled out to verify the fix isn't worse that the problem.

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.

Tokyo:  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.

Post-Tokyo: Nico Josuttis provided the above proposed resolution at the request of the LWG.]


182.  Ambiguous references to size_t

Section: 17 lib.library  Status: Review  Submitter: Al Stevens  Date: 15 Aug 99

Many references to size_t throughout the document omit the std:: namespace qualification.

For example, 17.4.3.4 [lib.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 17.4.3.4 [lib.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.  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.]


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

Section: 27.6.3  lib.std.manip  Status: Review Submitter: Andy Sawyer Date: 7 Jul 99

27.6.3 [lib.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 27.6.3 [lib.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 foonote]

     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, 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.  numeric_limits<bool> wording problems

Section: 18.2.1.5 lib.numeric.special  Status: Review  Submitter: Gabriel Dos Reis Date: 21 Jul 99

bools are defined by the standard to be of integer types, as per 3.9.1/7 [basic.fundamental]. 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 18.2.1.5 [lib.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 T min() throw() { return false; }
         static T 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 T epsilon() throw() { return bool(0); }
         static T round_error() throw() { return bool(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 T infinity() throw() { return bool(0); }
         static T quiet_NaN() throw() { return bool(0); }
         static T signaling_NaN() throw() { return bool(0); }
         static T denorm_min() throw() { return bool(0); }

         static const bool is_iec559 = false;
         static const bool is_bounded = false;
         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.  Questionable use of term "inline"

Section: 20.3 lib.function.objects  Status: Review  Submitter: UK Panel  Date: 26 Jul 99

Paragraph 4 of 20.3 [lib.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 20.3 [lib.function.objects] paragraph 1, remove the sentence:

They are important for the effective use of the library.

Remove 20.3 [lib.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 20.3 [lib.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.  bitset::set() second parameter should be bool

Section: 23.3.5.2 lib.bitset.members  Status: Review  Submitter: Darin Adler  Date:  13 Aug 99

In section 23.3.5.2 [lib.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 23.3.5[lib.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 23.3.5.2[lib.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 Sawyers will work on better P/R wording.

Post-Tokyo: Andy provided the above wording.]


197.  max_size() underspecified

Section: 20.1.5 lib.allocator.requirements, 23.1 lib.container.requirements  Status: Review  Submitter: Andy Sawyer  Date: 21 Oct 99

Must the value returned by max_size() be unchanged from call to call?

Must the value returned from max_size() be meaningful?

Possible meanings identified in lib-6827:

1) The largest container the implementation can support given "best case" conditions - i.e. assume the run-time platform is "configured to the max", and no overhead from the program itself. This may possibly be determined at the point the library is written, but certainly no later than compile time.

2) The largest container the program could create, given "best case" conditions - i.e. same platform assumptions as (1), but take into account any overhead for executing the program itself. (or, roughly "storage=storage-sizeof(program)"). This does NOT include any resource allocated by the program. This may (or may not) be determinable at compile time.

3) The largest container the current execution of the program could create, given knowledge of the actual run-time platform, but again, not taking into account any currently allocated resource. This is probably best determined at program start-up.

4) The largest container the current execution program could create at the point max_size() is called (or more correctly at the point max_size() returns :-), given it's current environment (i.e. taking into account the actual currently available resources). This, obviously, has to be determined dynamically each time max_size() is called.

Proposed Resolution:

Change 20.1.5 lib.allocator.requirements table 32 max_size() wording from:

      the largest value that can meaningfully be passed to X::allocate
to:
      the value of the largest constant expression (5.19 expr.const) that could ever meaningfully be passed to X::allocate

Change 23.1 lib.container.requirements table 65 max_size() wording from:

      size() of the largest possible container.
to:
      the value of the largest constant expression (5.19 expr.const) that could ever meaningfully be returned by X::size().

[Kona: The LWG informally discussed this and asked Andy Sawyer to submit an issue.

Tokyo: The LWG believes (1) above is the intended meaning.

Post-Tokyo:  Beman Dawes supplied the above resolution at the request of the LWG.   21.3.3 lib.string.capacity was not changed because it references max_size() in 23.1.  The term "compile-time" was avoided because it is not defined anywhere in the standard (even though it is used several places in the library clauses).]


198.  Validity of pointers and references unspecified after iterator destruction

Section: 24.1 lib.iterator.requirements  Status: Open  Submitter: Beman Dawes  Date: 3 Nov 99

Is a pointer or reference obtained from an iterator still valid after destruction of the iterator?

    // assume iter is a dereferenceable iterator with value_type T

    T& r = *iter;
    T* p = &*iter;

    // are r and p still valid at this point even though the iterators
    // they were obtained from have been destroyed?

If pointers and references must remain valid after iterator destruction, it is not possible to implement standard conforming containers which return iterators to cached elements. This is a particular problem for large disk-based containers like B-trees as they cannot be portably implemented without caching elements.

Three well-known implementations of <algorithm> seem to be written as if pointers and references do not remain valid after iterator destruction. Thus these implementations appear to already conform to the proposed resolution. Whether this is by design or happenstance isn't known.

The standard doesn't appear to address this question. It needs to be made clear to both users and implementors.

Proposed Resolution:

Add a new paragraph to 24.1 lib.iterator.requirements:

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

[Tokyo: The LWG reformulated the question purely in terms of iterators.  The answer to the question is "no, pointers and references don't remain valid after iterator destruction."  PJP explained that implementors use considerable care to avoid such ephemeral pointers and references.  Several LWG members said that they thought that the standard did not actually specify the lifetime of pointers and references obtained from iterators, except possibly input iterators.

Post-Tokyo:   The issue has been reformulated purely in terms of iterators. ]


199.  What does allocate(0) return?

Section: 20.1.5 lib.allocator.requirements Status: Ready  Submitter: Matt Austern   Date: 19 Nov 99

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.

Original proposed resolutions:

Alternative A: Add a note to the allocate row of Table 32: "[Note: If n == 0, the return value is a null pointer. --end note]"

Alternative B: Add a note to the allocate row of Table 32: "[Note: The return value is not a null pointer even when n == 0. --end note]"

Proposed Resolution:

Add a note to the allocate row of Table 32: "[Note: If n == 0, the return value is unspecified. --end note]"

[Tokyo:  The LWG says 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. Thus the LWG formulated the above proposed resolution.]


200.  Forward iterator requirements don't allow constant iterators

Section: 24.1.3 lib.forward.iterators Status: Open  Submitter: Matt Austern   Date: 19 Nov 99

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, change the return type column for *a from "T&" to "T& if X is mutable, otherwise const T&".

[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.]


201.  Numeric limits terminology wrong

Section: 18.2.1 lib.limits Status: Ready  Submitter: Stephen Cleary   Date: 21 Dec 1999

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.

Proposed Resolution:

Change 18.2 [lib.support.limits] para 1 from:

The headers <limits>, <climits>, and <cfloat> supply characteristics of implementation-dependent fundamental types (3.9.1).

to:

The headers <limits>, <climits>, and <cfloat> supply characteristics of implementation-dependent arithmetic types (3.9.1).

Change 18.2.1 [lib.limits] para 1 from:

The numeric_limits component provides a C++ program with information about various properties of the implementation's representation of the fundamental types.

to:

The numeric_limits component provides a C++ program with information about various properties of the implementation's representation of the arithmetic types.

Change 18.2.1 [lib.limits] para 2 from:

Specializations shall be provided for each fundamental type. . .

to:

Specializations shall be provided for each arithmetic type. . .

Change 18.2.1 [lib.limits] para 4 from:

Non-fundamental standard types. . .

to:

Non-arithmetic standard types. . .

Change 18.2.1.1 [lib.numeric.limits] para 1 from:

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

to:

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

[Tokyo:  Reviewed by the LWG.]


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

Section: 25.2.8 lib.alg.unique Status: Open  Submitter: Andrew Koenig   Date: 13 Jan 00

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:

Options:

1. Impose an explicit requirement that the predicate be an equivalence relation. 

2. Drop the word "equal" from the description to make it clear that the intent is to compare pairs of adjacent elements.

3. Change the effects to:

Effects: Eliminates all but the first element e from every consecutive group of elements referred to by the iterator i in the range [first, last) for which the following corresponding conditions hold: e == *i or pred(e,*i) != false.

If we adopt (2), we also need to decide whether pred(*i, *(i - 1)) is really what we meant, or whether pred(*(i - 1), i) is more appropriate.

A LWG member [Nico Josuttis] comments:

First, I agree that the current wording is simply wrong. However, to follow all [known] current implementations I propose [option 3 above].

[Tokyo:  The issue was discussed at length without reaching consensus.

Straw vote: 

Option 1 - preferred by 2 people.
Option 2 - preferred by 0 people.
Option 3 - preferred by 3 people.
Many abstentions.]


207.   ctype<char> members return clause incomplete

Section: 22.2.1.3.2 lib.facet.ctype.char.members  Status: Open  Submitter: Robert Klarer Date: 2 Nov 99

Proposed Resolution:

Change the returns clause in 22.2.1.3.2 lib.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 the returns clause in 22.2.1.3.2 lib.facet.ctype.char.members paragraph 11 from:

    Returns: do_narrow(low, high, to).

to:

    Returns: do_narrow(c) or do_narrow(low, high, to), respectively.

[Post-Tokyo:  This appears to be a duplicate of issue 153.]


208. Unnecessary restriction on past-the-end iterators

Section: 24.1 lib.iterators  Status: Ready  Submitter: Stephen Cleary Date: 02 Feb 00

In 24.1 paragraph 5, it is stated ". . . Dereferenceable and past-the-end values are always non-singular."

This places an unnecessary restriction on past-the-end iterators for containers with forward iterators (for example, a singly-linked list). If the past-the-end value on such a container was a well-known singular value, it would still satisfy all forward iterator requirements.

Removing this restriction would allow, for example, a singly-linked list without a "footer" node.

This would have an impact on existing code that expects past-the-end iterators obtained from different (generic) containers being not equal.

Proposed Resolution:

Change 24.1 [lib.iterators] paragraph 5, the last sentence, from:

Dereferenceable and past-the-end values are always non-singular.

to:

Dereferenceable values are always non-singular. 

[Tokyo:  After discussion of the meaning of "non-singular", and working out several examples, the LWG changed the proposed resolution to simply strike the words "and past-the-end".]


209.  basic_string declarations inconsistent

Section: 21.3 lib.basic.string  Status: Ready  Submitter: Igor Stauder  Date: 11 Feb 00

In Section 21.3 [lib.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 21.3 [lib.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);

[Tokyo: 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.  distance first and last confused

Section: 25 lib.algorithms  Status: Ready  Submitter: Lisa Lippincott  Date: 15 Feb 00

In paragraph 9 of section 25 [lib.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 25 [lib.algorithms] change "a-b" to "b-a".

[Tokyo:  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.  operator>>(istream&, string&) doesn't set failbit

Section: 21.3.7.9 lib.string.io  Status: Ready  Submitter: Scott Snyder  Date: 4 Feb 00

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 27.6.1.2 [lib.istream.formatted] and 27.6.1.3 [lib.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 21.3.7.9 [lib.string.io] paragraph 8.)

Proposed Resolution:

Insert new paragraph after paragraph 2 in section 21.3.7.9 [lib.string.io]:

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

[Tokyo: Reviewed by the LWG.]


212.  Empty range behavior unclear for several algorithms

Section: 25.3.7 lib.alg.min.max  Status: Ready  Submitter: Nico Josuttis  Date: 26 Feb 00

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 25.3.7 - Minimum and maximum [lib.alg.min.max], paragraphs 7 and 9 append: Returns last if first==last.

[Tokyo: 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.  set::find() missing const overload

Section: 23.3.3 23.3.4 lib.set  Status: Review  Submitter: Judy Ward  Date: 28 Feb 00

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_ranger() in section 23.3.3 lib.set and section 23.3.4 lib.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.]


216.  setbase manipulator description flawed

Section: 27.6.3 lib.std.manip  Status: Review  Submitter: Hyman Rosen  Date: 29 Feb 00

27.6.3 lib.std.manip paragraph 5 says:

smanip setbase(int base);

Returns: An object s of unspecified type such that if out is an (instance of) basic_ostream then the expression out<<s behaves as if f(s) were called, 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, int base)
{
  // set basefield
  str.setf(n == 8 ? ios_base::oct :
                n == 10 ? ios_base::dec :
                n == 16 ? ios_base::hex :
                  ios_base::fmtflags(0), ios_base::basefield);
  return str;
}

There are two problems here. First, f takes two parameters, so the description needs to say that out<<s and in>>s behave as if f(s,base) had been called. Second, f is has a parameter named base, but is written as if the parameter was named n.

Actually, there's a third problem. The paragraph has grammatical errors. There needs to be an "and" after the first comma, and the "Where f" sentence fragment needs to be merged into its preceding sentence. You may also want to format the function a little better. The formatting above is more-or-less what the Standard contains.

Proposed Resolution:

The resolution of this defect is subsumed by the proposed resolution for issue 183.

[Tokyo:  The LWG agrees that this is a defect and notes that it occurs additional places in the section, all requiring fixes.

Post-Tokyo:  The resolution was combined with issue 183 as they affect the same text.]


217.  Facets example (Classifying Japanese characters) contains errors

Section: 22.2.8 lib.facets.examples Status: Ready  Submitter: Martin Sebor  Date: 29 Feb 00

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;
}

[Tokyo:  Reviewed by the LWG.]


220. ~ios_base() usage valid?

Section: 27.4.2.7 lib.ios.base.cons  Status: Ready  Submitter: Jonathan Schilling, Howard Hinnant  Date 13 Mar 00

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.

[Tokyo: Reviewed by the LWG and accepted after changing "calling ios_base member functions." to "calling basic_ios::init".]


221.  num_get<>::do_get stage 2 processing broken

Section: 22.2.2.1.2 lib.facet.num.get.virtuals  Status: Review  Submitter: Matt Austern Date: 14 Mar 00

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+-";

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

Section: 17.3.1.3 lib.structure.specifications  Status: Ready  Submitter: Judy Ward  Date: 17 Mar 00

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 21.3.1 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 17.3.1.3 [lib.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.

[Tokyo:  First it was observed that the standard is somewhat inconsistent, but that a failure to note a throw condition in a throws clause does not grant permission not to throw.  Then it was noted that the inconsistent wording is in a footnote, and thus non-normative.  The proposed resolution from the LWG clarifies the footnote.]


223.  reverse algorithm should use iter_swap rather than swap

Section: 25.2.9 lib.alg.reverse  Status: Ready  Submitter: Dave Abrahams  Date: 21 Mar 00

Shouldn't the effects say "applies iter_swap to all pairs..."?

Proposed Resolution:

In 25.2.9 lib.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.

[Tokyo: Reviewed by the LWG.]


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

Section: 23.1.2 lib.associative.reqmts  Status: Ready  Submitter: Ed Brey  Date: 23 Mar 00

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 "N" to "size()".

[Tokyo: Reviewed by the LWG.  Proposed resolution changed after discussion of how complexity is described in the standard.  It was noted that the standard does not always use "big-O notation" in the strict sense.]


225. std:: algorithms use of other unqualified algorithms

Section: 17.4.4.3 lib.global.functions, 25 lib.algorithms  Status: Open  Submitter: Dave Abrahams  Date: 01 Apr 00

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 17.4.4.3 lib.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 (basic.lookup.koenig).

[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.]


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

Section: 17.4.3.1 lib.reserved.names  Status: Open  Submitter: Dave Abrahams  Date: 01 Apr 00

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.]

Proposed Resolution:

[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.]

Dave Abrahams and Peter Dimov <pdimov@mmltd.net> have proposed an alternative resolution that involves core changes:

7.3.3/9:

14/2:

14/4:

14.5.4:

14.5.4/1:

14.5.4/4:

14.5.4/5:

14.5.4/6:

14.5.4/7:

14.5.4/9:

14.5.4/11 (new paragraph):

A function template partial specialization specializes a primary template if and only if, after substituting the template arguments provided in the specialization template argument list into the primary template declaration, the resulting function signature matches that of the specialization.

[Note: each function template partial specialization specializes at most one primary template.]

14.5.4/12 (new paragraph):

[Example:

template<class T> void f(T  x);      // primary template #1
template<class U> void f(U* y);      // primary template #2

template<class V> void f<V*>(V* z);  // specialization of #1, T = V*
template<class W> void f<W*>(W** w); // specialization of #2, U = W*

-- end example.]

14.5.4.1/1:

14.5.4.2:

14.5.4.2/1: (change to):

For two template partial specializations (that specialize the same primary template,) the first is at least as specialized as the second if, given the following rewrite to two function templates, the first function template is at least as specialized as the second according to the ordering rules for function templates (14.5.5.2):

14.5.4.2/2 (change example to):

template<int I, int J, class T> class X { };
template<int I, int J>          class X<I, J, int> { };           // #1
template<int I>                 class X<I, I, int> { };           // #2

template<int I, int J, class T> class __unique;

template<int I, int J>          void __f(__unique<I, J, int>);    // #A
template<int I>                 void __f(__unique<I, I, int>);    // #B

14.5.4.2/3 (new paragraph):

[Example:

template<class T, class U, class V> U  f            (V);
template<class U, class V>8         U  f<int, U, V>   (V);    // #1
template<class T>                   T  f<int, T, T>   (T);    // #2

template<class T, class U, class V> class __unique;

template<class U, class V>  void __f(__unique<int, U, V>);    // #A
template<class T>           void __f(__unique<int, T, T>);    // #B

-- end example.]


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

Section: 25.2.2 lib.alg.swap  Status: Ready  Submitter: Dave Abrahams  Date: 09 Apr 00

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)

[Tokyo: Reviewed by the LWG.  Also see issue 230, identifying other places in the standard where Assignable is specified without also specifying CopyConstructible.]


228.  Incorrect specification of "..._byname" facets

Section: 22.2 lib.locale.categories  Status: New  Submitter: Dietmar Kühl  Date: 20 Apr 00

The sections 22.2.1.2 (lib.locale.ctype.byname), 22.2.1.4 (lib.locale.ctype.byname.special), 22.2.1.6 (lib.locale.codecvt.byname), 22.2.3.2 (lib.locale.numpunct.byname), 22.2.4.2 (lib.locale.collate.byname), 22.2.5.4 (lib.locale.time.put.byname), 22.2.6.4 (lib.locale.moneypunct.byname), and 22.2.7.2 (lib.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 'ctpye_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_bymame<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 22.2.1.4 (lib.locale.ctype.byname.special) to become:

     namespace std {
       template <> class ctype_byname<char> : public ctype<char> {
       public:
         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
         virtual catalog do_open(const basic_string<char>&, const locale&) const;
         virtual string_type  do_get(catalog, int set, int msgid,
                                const string_type& dfault) const;
         virtual void    do_close(catalog) const;
       };
     }

Remove section 22.2.1.4  (lib.locale.ctype.byname.special) 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.]


229.  Unqualified references of other library entities

Section: 17.4.1.1 lib.contents  Status: New  Submitter: Steve Clamage  Date: 19 Apr 00

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.

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 issue 229.]


230.  Assignable specified without also specifying CopyConstructible

Section: 17 lib.library  Status: New  Submitter: Beman Dawes  Date: 26 Apr 00

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, 26.4.1 [lib.accumulate].

Proposed Resolution:

In [lib.container.requirements] 23.1 table 65 for value_type: change "T is Assignable" to "T is CopyConstructible and Assignable"

In [lib.associative.reqmts] 23.1.2  table 69 X::key_type; change "Key is Assignable" to "Key is CopyConstructible and Assignable"

In [lib.input.iterators] 24.1.1 paragraph 3, which reads:

[Note: For input iterators, a == b does not imply ++a == ++b. (Equality does not guarantee the substitution property or referential transparency.) Algorithms on input iterators should never attempt to pass through the same iterator twice. They should be single pass algorithms. Value type T is not required to be an Assignable type (23.1). These algorithms can be used with istreams as the source of the input data through the istream_iterator class. ]

Change "... not required to be an Assignable type (23.1)" to "... not required to be a CopyConstructible (20.1.3) or Assignable type (23.1)".

In [lib.output.iterators] 24.1.2 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:

In [lib.alg.replace] 25.2.4 paragraph 1 and 4 respectively, change:

1 Requires: Type T is Assignable (23.1) (and, for replace(), EqualityComparable (20.1.1)).

4 Requires: Type T is Assignable (23.1) (and, for replace_copy(), EqualityComparable

to:

1 Requires: Type T is CopyConstructible ( 20.1.3) and Assignable (23.1) (and, for replace(), EqualityComparable (20.1.1)).

4 Requires: Type T is CopyConstructible ( 20.1.3) and Assignable (23.1) (and, for replace_copy(), EqualityComparable


In [lib.alg.fill] 25.2.5 paragraph 1, change:

1 Requires: Type T is Assignable (23.1), Size is convertible to an integral type (4.7, 12.3).

to:

1 Requires: Type T is CopyConstructible ( 20.1.3) and Assignable (23.1), Size is convertible to an integral type (4.7, 12.3).

[Post-Tokyo: Beman Dawes submitted this issue at the request of the LWG .

He asks that the [lib.alg.replace] 25.2.4 and [lib.alg.fill] 25.2.5 changes be studied carefully, as it is not clear that CopyConstructible is really a requirement and may be overspecification.]


231.  Precision in iostream?

Section: 22.2.2.2.2 lib.facet.num.put.virtuals Status: New  Submitter: James Kanze, Stephen Clamage  Date: 25 Apr 00

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 << 1.23 << '\n' ;
        return 0 ;
    }

From my C experience, I would expect "1e+00"; this is what printf( "%.0e" , 1.23 ) ; 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:


232.  "depends" poorly defined in 17.4.3.1

Section: 17.4.3.1 lib.reserved.names  Status: New  Submitter: Peter Dimov  Date: 18 Apr 00

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


233.  Insertion hints in associative containers

Section: 23.1.2 lib.associative.reqmts Status: New  Submitter: Andrew Koenig   Date: 30 Apr 2000

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.

Proposed Resolution:

My personal opinion is that in the best of all possible worlds, the standard would say that x is inserted into mm at the point closest to (the point immediately ahead of) p. That would give the user a way of controlling the order in which elements appear that have equal keys. Doing so would be particularly easy in two cases that I suspect are common:
mm.insert(mm.begin(), x);
mm.insert(mm.end(), x);

These examples would allow x to be inserted at the beginning and end, respectively, of the set of elements with the same key as x.


234.  Typos in allocator definition

Section: 20.4.1.1 lib.allocator.members  Status: New  Submitter: Dietmar Kühl  Date: 24 Apr 2000

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.  No specification of default ctor for reverse_iterator

Section: 24.4.1.1 lib.reverse.iterator  Status: New  Submitter: Dietmar Kühl  Date: 24 Apr 2000

The declaration of reverse_iterator lists a default constructor. However, no specification is given what this constructor should do.

Proposed Resolution:


236.  ctype<char>::is() member modifies facet

Section: 24.2.1.3.2 lib.facet.ctype.char.members  Status: New  Submitter: Dietmar Kühl  Date: 24 Apr 2000

The description of the is() member in paragraph 4 of lib.facet.ctype.char.members is broken: According to this description, the second form of the is() method modifies the masks in the ctype object. The correct semantics if, of course, to obtain an array of masks. The corresponding method in the general case, ie. the do_is() method as described in lib-locales.html#lib.locale.ctype.virtuals paragraph 1 does the right thing.

Proposed resolution:

Change paragraph 4 from

The second form, for all *p in the range [low, high), assigns vec[p-low] to table()[(unsigned char)*p].
to become

The second form, for all *p in the range [low, high), assigns table()[(unsigned char)*p] to vec[p-low].

237.  Undefined expression in complexity specification

Section: 23.2.2.1 lib.list.cons  Status: New  Submitter: Dietmar Kühl  Date: 24 Apr 2000

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 std::distance(first, last).

238.  Contradictory results of stringbuf initialization.

Section: 27.7.1.1 lib.stringbuf.cons  Status: New  Submitter: Dietmar Kühl  Date: 11 May 2000

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:


239.  Complexitiy of unique() and/or unique_copy incorrect

Section: 25.2.8 lib.alg.unique Status: New  Submitter: Angelika Langer   Date: May 15 2000

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 25.2.8 lib.alg.unique  to:

Complexity: Exactly last - first - 1 applications of the corresponding predicate.

240.  Complexity of adjacent_find() is meaningless

Section: 25.1.5 lib.alg.adjacent.find Status: New  Submitter: Angelika Langer  Date: May 15 2000

The complexity section of adjacent_find is defective.

template<class ForwardIterator, class BinaryPredicate>
       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 25.1.5 lib.alg.adjacent.find to something like:

Complexity: Exactly find_if(first, last, unary_pred) - first applications of the binary predicate pred, where i is the iterator specified in the Returns section as if there were a unary predicate unary_pred that yields true if pred(v,*i) yields true and false if pred(v,*i) yields false for any value v in the range [first,i].

241.  Does unique_copy() require CopyConstructible and Assignable?

Section: 25.2.8 lib.alg.unique Status: New  Submitter: Angelika Langer  Date: May 15 2000

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() [lib.accumulate].

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:

none, since the intent is not clear


242.  Side effects of function objects

Section: 25.2.3 lib.alg.transform and 26.4 lib.numeric.ops Status: New  Submitter: Angelika Langer  Date: May 15 2000

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 [intro.execution]as:

-7- Accessing an object designated by a volatile lvalue (basic.lval), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.
As a consequence, the function call operator of a function object supplied to any of the algorithms listed above cannot modify data members, cannot invoke any function that has a side effect, and cannot even create and modify temporary objects.  It is difficult to imagine a function object that is still useful under these severe limitations. For instance, any non-trivial transformator supplied to transform() might involve creation and modification of temporaries, which is prohibited according to the current wording of the standard.

On the other hand, popular implementations of these algorithms exhibit uniform and predictable behavior when invoked with a side-effect-producing function objects. It looks like the strong requirement is not needed for efficient implementation of these algorithms.

The requirement of  side-effect-free function objects could be replaced by a more relaxed basic requirement (which would hold for all function objects supplied to any algorithm in the standard library):

A function objects supplied to an algorithm shall not invalidate any iterator or sequence that is used by the algorithm. Invalidation of the sequence includes destruction of the sorting order if the algorithm relies on the sorting order (see section 25.3 - Sorting and related operations [lib.alg.sorting]).
I can't judge whether it is intended that the function objects supplied to transform(), accumulate(), inner_product(), partial_sum(), or adjacent_difference() shall not modify sequence elements through dereferenced iterators.

It is debatable whether this issue is a defect or a change request. Since the consequences for user-supplied function objects are drastic and limit the usefulness of the algorithms significantly I would consider it a defect.

Proposed Resolution:

Add the basic requirement to section 25 [lib.algorithms], maybe in conjunction with a definition of a function object:

Several algorithms in this section can be supplied with a function object.  A function object may be a pointer to function, or an object of a type with an appropriate function call operator. A function objects shall not invalidate any iterator or sequence that is used by the algorithm. For the algorithms that rely on the sorting order of the input sequences (see section 25.3 - Sorting and related operations [lib.alg.sorting]) the function object shall not alter the sorting order.


Remove the requirement regarding side effects of function objects supplied to transform(), accumulate(), inner_product(), partial_sum(), and adjacent_difference() from the Requires sections of 25.2.3 [lib.alg.transform] and 26.4 [lib.numeric.ops].

or

Replace the requirement regarding side effects of function objects supplied to transform(), accumulate(), inner_product(), partial_sum(), and adjacent_difference() from the Requires sections of 25.2.3 [lib.alg.transform] and 26.4 [lib.numeric.ops] by:

The function object shall not modify its argument(s).

243get and getline when sentry reports failure

Section: 27.6.1.3 lib.istream.unformatted Status: New  Submitter: Martin Sebor   Date: May 15 2000

basic_istream<>::get(), and basic_istream<>::getline(), are unclear with respect to the behavior and side-effects of the named functions in case of an error.

27.6.1.3, p1 states that "... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input..." It is not clear from this (or the rest of the paragraph) what precisely the behavior should be when the sentry ctor exits by throwing an exception or when the sentry object returns false. In particular, what is the number of characters extracted that gcount() returns supposed to be?

27.6.1.3 p8 and p19 say about the effects of get() and getline(): "... In any case, it then stores a null character (using charT()) into the next successive location of the array." Is not clear whether this sentence applies if either of the conditions above holds (i.e., when sentry fails).

Proposed Resolution:

Proposed resolution 1:
Add to 27.6.1.3, p1 after the sentence

"... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input."
the following
"Otherwise, if the sentry constructor exits by throwing an exception, the exception is not caught. If the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0 and the function's argument is left unchanged."

Proposed resolution 2 (supported by Jerry Schwarz in Message c++std-lib-7618):
Add to 27.6.1.3, p1 after the sentence

"... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input."
the following
"Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array."
Personally I am inclined to support resolution 1 since it would seem to be consistent with the general philosophy of the input functions to not modify the argument on failure.

244.  Must find's third argument be CopyConstructible?

Section: 25.1.2 lib.alg.find Status: New  Submitter: Andrew Koenig   Date: 02 May 2000

Is the following implementation of find acceptable?

	template<class Iter, class X>
	Iter find(Iter begin, Iter end, const X& x)
	{
	    X x1 = x;		// this is the crucial statement
	    while (begin != end && *begin != x1)
	        ++begin;
	    return begin;
	}
If the answer is yes, then it is implementation-dependent as to whether the following fragment is well formed:
	vector<string> v;

	find(v.begin(), v.end(), "foo");
At issue is whether there is a requirement that the third argument of find be CopyConstructible. There may be no problem here, but analysis is necessary.

Proposed Resolution:


245.  Which operations on istream_iterator triger input operations?

Section: 24.5.1 lib.istream.iterator Status: New  Submitter: Andrew Koenig   Date: 02 May 2000

I do not think the standard specifies what operation(s) on istream iterators trigger input operations. So, for example:

	istream_iterator<int> i(cin);

	int n = *i++;
I do not think it is specified how many integers have been read from cin. The number must be at least 1, of course, but can it be 2? More?

Proposed Resolution:


246a.insert(p,t) is incorrectly specified

Section: 23.1.2 lib.associative.reqmts Status: New  Submitter: Mark Rodgers   Date: 19 May 2000

Closed issue 192 raised several problems with the specification of this function, but was rejected as Not A Defect because it was too big a change with unacceptable impacts on existing implementations. However, issues remain that could be addressed with a smaller change and with little or no consequent impact.

  1. The specification is inconsistent with the original proposal and with several implementations.

    The initial implementation by Hewlett Packard only ever looked immediately before p, and I do not believe there was any intention to standardise anything other than this behaviour. Consequently, current implementations by several leading implementers also look immediately before p, and will only insert after p in logarithmic time. I am only aware of one implementation that does actually look after p, and it looks before p as well. It is therefore doubtful that existing code would be relying on the behaviour defined in the standard, and it would seem that fixing this defect as proposed below would standardise existing practice.

  2. The specification is inconsistent with insertion for sequence containers.

    This is difficult and confusing to teach to newcomers. All insert operations that specify an iterator as an insertion location should have a consistent meaning for the location represented by that iterator.

  3. As specified, there is no way to hint that the insertion should occur at the beginning of the container, and the way to hint that it should occur at the end is long winded and unnatural.

    For a container containing n elements, there are n+1 possible insertion locations and n+1 valid iterators. For there to be a one-to-one mapping between iterators and insertion locations, the iterator must represent an insertion location immediately before the iterator.

  4. When appending sorted ranges using insert_iterators, insertions are guaranteed to be sub-optimal.

    In such a situation, the optimum location for insertion is always immediately after the element previously inserted. The mechanics of the insert iterator guarantee that it will try and insert after the element after that, which will never be correct. However, if the container first tried to insert before the hint, all insertions would be performed in amortised constant time.

Proposed Resolution:

In 23.1.2 [lib.associative.reqmts] paragraph 7, table 69, make the following changes in the row for a.insert(p,t):

assertion/note pre/post condition:
Change the last sentence from

"iterator p is a hint pointing to where the insert should start to search."
to
"iterator p is a hint indicating that immediately before p may be a correct location where the insertion could occur."

complexity:
Change the words "right after" to "immediately before".


247vector, deque::insert complexity

Section: 23.3.4.4 lib.vector.modifiers Status: New  Submitter: Lisa Lippincott   Date: 06 June 2000

Paragraph 2 of 23.3.4.3 [lib.vector.modifiers] describes the complexity of vector::insert:

Complexity: If first and last are forward iterators, bidirectional iterators, or random access iterators, the complexity is linear in the number of elements in the range [first, last) plus the distance to the end of the vector. If they are input iterators, the complexity is proportional to the number of elements in the range [first, last) times the distance to the end of the vector.

First, this fails to address the non-iterator forms of insert.

Second, the complexity for input iterators misses an edge case -- it requires that an arbitrary number of elements can be added at the end of a vector in constant time.

At the risk of strengthening the requirement, I suggest simply

Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.

For input iterators, one may achieve this complexity by first inserting at the end of the vector, and then using rotate.

I looked to see if deque had a similar problem, and was surprised to find that deque places no requirement on the complexity of inserting multiple elements (23.2.1.3 [lib.deque.modifiers], paragraph 3):

Complexity: In the worst case, inserting a single element into a deque takes time linear in the minimum of the distance from the insertion point to the beginning of the deque and the distance from the insertion point to the end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to the copy constructor of T.

I suggest:

Complexity: The complexity is linear in the number of elements inserted plus the shorter of the distances to the beginning and end of the deque. Inserting a single element at either the beginning or the end of a deque causes a single call to the copy constructor of T.

Proposed Resolution:


248.  time_get fails to set eofbit

Section: 22.2.5 lib.category.time Status: New  Submitter: Martin Sebor   Date: 22 June 2000

There is no requirement that any of time_get member functions set ios::eofbit when they reach the end iterator while parsing their input. Since members of both the num_get and money_get facets are required to do so (22.2.2.1.2, and 22.2.6.1.2, respectively), time_get members should follow the same requirement for consistency.

Proposed Resolution:

Proposed Resolution 1:
Add paragraph 2 to section 22.2.5.1 with the following text:

If the end iterator is reached during parsing by any of the get() member functions, the member sets ios_base::eofbit in err.

Proposed Resolution 2:
Change the last sentence of paragraph 2 in section 22.2 from

The get() members take an ios_base::iostate& argument whose value they ignore, but set to ios_base::failbit in case of a parse error."
to
The get() members take an ios_base::iostate& argument whose value they ignore, but in which they set ios_base::failbit in case of a parse error and ios_base::eofbit if they reach the end of input during parsing.

249.  Return Type of auto_ptr::operator=

Section: 20.4.5 lib.auto.ptr Status: New  Submitter: Joseph Gottman <joegottman@worldnet.att.net>   Date: 30 Jun 2000

According to section 20.4.5, the function auto_ptr::operator=() returns a reference to an auto_ptr. The reason that operator=() usually returns a reference is to facilitate code like

    int x,y,z;
    x = y = z = 1;
However, given analogous code for auto_ptrs,
    auto_ptr<int> x, y, z;
    z.reset(new int(1));
    x = y = z;
the result would be that z and y would both be set to NULL, instead of all the auto_ptrs being set to the same value. This makes such cascading assignments useless and counterintuitive for auto_ptrs.

Proposed Resolution:

Change auto_ptr::operator=() to return void instead of an auto_ptr reference.

250.  splicing invalidates iterators

Section: 23.2.2.4 lib.list.ops Status: New  Submitter: Brian Parker <brianp@research.canon.com.au>   Date: 14 Jul 2000

Section 23.2.2.4 [lib.list.ops] states that

  void splice(iterator position, list<T, Allocator>& x);
invalidates all iterators and references to list x. This is unnecessary and defeats an important feature of splice. In fact, the SGI STL guarantees that iterators to x remain valid after splice.

Proposed Resolution:

I think that this clause (and the other splice clauses) should be reworded to- "all iterators and references remain valid, including iterators that point to elements of x."

251.  basic_stringbuf missing allocator_type

Section: 27.7.1 lib.stringbuf Status: New  Submitter: Martin Sebor   Date: 28 Jul 2000

The synopsis for the template class basic_stringbuf doesn't list a typedef for the template parameter Allocator. This makes it impossible to determine the type of the allocator at compile time. It's also inconsistent with all other template classes in the library that do provide a typedef for the Allocator parameter.

Proposed Resolution:

Add to the synopsis of the template class basic_stringbuf the typedef:
  typedef Allocator allocator_type;

252.  missing casts/C-style casts used in iostreams

Section: 27.7 lib.string.streams Status: New  Submitter: Martin Sebor   Date: 28 Jul 2000

27.7.2.2, p1 uses a C-style cast rather than the more appropriate const_cast<> in the Returns clause for basic_istringstream<>::rdbuf(). The same C-style cast is being used in 27.7.3.2, p1, D.7.2.2, p1, and D.7.3.2, p1, and perhaps elsewhere. 27.7.6, p1 and D.7.2, p2 are missing the cast altogether.

C-style casts have not been deprecated, so the first part of this issue is stylistic rather than a matter of correctness.

Proposed Resolution:

In 27.7.2.2, p1 replace

  -1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.
with
  -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In 27.7.3.2, p1 replace

  -1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.
with
  -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In 27.7.6, p1, replace

  -1- Returns: &sb
with
  -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In D.7.2, p2 replace

  -2- Returns: &sb. 
with
  -2- Returns: const_cast<strstreambuf*>(&sb).

253.  valarray helper functions are almost entirely useless

Section: 26.3.2.1 lib.valarray.cons and 26.3.2.2 lib.valarray.assign Status: New  Submitter: Robert Klarer   Date: 31 Jul 2000

This discussion is adapted from message c++std-lib-7056 posted November 11, 1999. I don't think that anyone can reasonably claim that the problem described below is NAD.

These valarray constructors can never be called:

   template <class T>
         valarray<T>::valarray(const slice_array<T> &);
   template <class T>
         valarray<T>::valarray(const gslice_array<T> &);
   template <class T>
         valarray<T>::valarray(const mask_array<T> &);
   template <class T>
         valarray<T>::valarray(const indirect_array<T> &);

Similarly, these valarray assignment operators cannot be called:

     template <class T>
     valarray<T> valarray<T>::operator=(const slice_array<T> &);
     template <class T>
     valarray<T> valarray<T>::operator=(const gslice_array<T> &);
     template <class T>
     valarray<T> valarray<T>::operator=(const mask_array<T> &);
     template <class T>
     valarray<T> valarray<T>::operator=(const indirect_array<T> &);

Please consider the following example:

   #include <valarray>
   using namespace std;

   int main()
   {
       valarray<double> va1(12);
       valarray<double> va2(va1[slice(1,4,3)]); // line 1
   }

Since the valarray va1 is non-const, the result of the sub-expression va1[slice(1,4,3)] at line 1 is an rvalue of type const std::slice_array<double>. This slice_array rvalue is then used to construct va2. The constructor that is used to construct va2 is declared like this:

     template <class T>
     valarray<T>::valarray(const slice_array<T> &);

Notice the constructor's const reference parameter. When the constructor is called, a slice_array must be bound to this reference. The rules for binding an rvalue to a const reference are in 8.5.3, paragraph 5 (see also 13.3.3.1.4). Specifically, paragraph 5 indicates that a second slice_array rvalue is constructed (in this case copy-constructed) from the first one; it is this second rvalue that is bound to the reference parameter. Paragraph 5 also requires that the constructor that is used for this purpose be callable, regardless of whether the second rvalue is elided. The copy-constructor in this case is not callable, however, because it is private. Therefore, the compiler should report an error.

Since slice_arrays are always rvalues, the valarray constructor that has a parameter of type const slice_array<T> & can never be called. The same reasoning applies to the three other constructors and the four assignment operators that are listed at the beginning of this post. Furthermore, since these functions cannot be called, the valarray helper classes are almost entirely useless.

Proposed Resolution:

Adopt section 2 of 00-0023/N1246. Sections 1 and 5 of that paper have already been classified as "Request for Extension". Sections 3 and 4 are reasonable generalizations of section 2, but they do not resolve an obvious inconsistency in the standard.


254.  Exception types in clause 19 are constructed from std::string

Section: 19.1 lib.std.exceptions Status: New  Submitter: Dave Abrahams   Date: 01 Aug 2000

Many of the standard exception types which implementations are required to throw are constructed with a const std::string& parameter. For example:

     19.1.5  Class out_of_range                          [lib.out.of.range]
     namespace std {
       class out_of_range : public logic_error {
       public:
         explicit out_of_range(const string& what_arg);
       };
     }

   1 The class out_of_range defines the type of objects  thrown  as  excep-
     tions to report an argument value not in its expected range.

     out_of_range(const string& what_arg);

     Effects:
       Constructs an object of class out_of_range.
     Postcondition:
       strcmp(what(), what_arg.c_str()) == 0.

There are at least two problems with this:

  1. A program which is low on memory may end up throwing std::bad_alloc instead of out_of_range because memory runs out while constructing the exception object.
  2. An obvious implementation which stores a std::string data member may end up invoking terminate() during exception unwinding because the exception object allocates memory (or rather fails to) as it is being copied.

There may be no cure for (1) other than changing the interface to out_of_range, though one could reasonably argue that (1) is not a defect. Personally I don't care that much if out-of-memory is reported when I only have 20 bytes left, in the case when out_of_range would have been reported. People who use exception-specifications might care a lot, though.

There is a cure for (2), but it isn't completely obvious. I think a note for implementors should be made in the standard. Avoiding possible termination in this case shouldn't be left up to chance. The cure is to use a reference-counted "string" implementation in the exception object. I am not neccessarily referring to a std::string here; any simple reference-counting scheme for a NTBS would do.

Further discussion, in email:

...I'm not so concerned about (1). After all, a library implementation can add const char* constructors as an extension, and users don't need to avail themselves of the standard exceptions, though this is a lame position to be forced into. FWIW, std::exception and std::bad_alloc don't require a temporary basic_string.

...I don't think the fixed-size buffer is a solution to the problem, strictly speaking, because you can't satisfy the postcondition
  strcmp(what(), what_arg.c_str()) == 0
For all values of what_arg (i.e. very long values). That means that the only truly conforming solution requires a dynamic allocation.

Proposed Resolution:


255.  Why do basic_streambuf<>::pbump() and gbump() take an int?

Section: 27.5.2 lib.streambuf Status: New  Submitter: Martin Sebor   Date: 12 Aug 2000

The basic_streambuf members gbump() and pbump() are specified to take an int argument. This requirement prevents the functions from effectively manipulating buffers larger than std::numeric_limits<int>::max() characters. It also makes the common use case for these functions somewhat difficult as many compilers will issue a warning when an argument of type larger than int (such as ptrdiff_t on LLP64 architectures) is passed to either of the function. Since it's often the result of the subtraction of two pointers that is passed to the functions, a cast is necessary to silence such warnings. Finally, the usage of a native type in the functions signatures is inconsistent with other member functions (such as sgetn() and sputn()) that manipulate the underlying character buffer. Those functions take a streamsize argument.

Proposed Resolution:

Change the signatures of these functions in the synopsis of template class basic_streambuf (27.5.2) and in their descriptions (27.5.2.3.1, p4 and 27.5.2.3.2, p4) to take a streamsize argument.

Although this change has the potential of changing the ABI of the library, the change will affect only platforms where int is different than the definition of streamsize. However, since both functions are typically inline (they are on all known implementations), even on such platforms the change will not affect any user code unless it explicitly relies on the existing type of the functions (e.g., by taking their address). Such a possibility is IMO quite remote.

Alternate Suggestion from Howard Hinnant, c++std-lib-7780:

This is something of a nit, but I'm wondering if streamoff wouldn't be a better choice than streamsize. The argument to pbump and gbump MUST be signed. But the standard has this to say about streamsize (27.4.1/2/Footnote):

[Footnote: streamsize is used in most places where ISO C would use size_t. Most of the uses of streamsize could use size_t, except for the strstreambuf constructors, which require negative values. It should probably be the signed type corresponding to size_t (which is what Posix.2 calls ssize_t). --- end foonote]

This seems a little weak for the argument to pbump and gbump. Should we ever really get rid of strstream, this footnote might go with it, along with the reason to make streamsize signed.


256.  typo in 27.4.4.2, p17: copy_event does not exist

Section: 27.4.4.2 lib.basic.ios.members Status: New  Submitter: Martin Sebor   Date: 21 Aug 2000

27.4.4.2, p17 says

-17- Before copying any parts of rhs, calls each registered callback pair (fn,index) as (*fn)(erase_event,*this,index). After all parts but exceptions() have been replaced, calls each callback pair that was copied from rhs as (*fn)(copy_event,*this,index).

The name copy_event isn't defined anywhere. The intended name was copyfmt_event.

Proposed Resolution:

Replace copy_event with copyfmt_event in the named paragraph.

257.  STL functional object and iterator inheritance.

Section: 20.3.1 lib.base and 24.3.2 lib.iterator.basic Status: New  Submitter: Robert Dick <dickrp@venus.ee.Princeton.EDU>   Date: 17 Aug 2000

According to the November 1997 Draft Standard, the results of deleting an object of a derived class through a pointer to an object of its base class are undefined if the base class has a non-virtual destructor. Therefore, it is potentially dangerous to publicly inherit from such base classes.

Defect:
The STL design encourages users to publicly inherit from a number of classes which do nothing but specify interfaces, and which contain non-virtual destructors.

Attribution:
Wil Evers and William E. Kempf suggested this modification for functional objects.

Proposed Resolution:

Proposed correction:

When a base class in the standard library is useful only as an interface specifier, i.e., when an object of the class will never be directly instantiated, specify that the class contains a protected destructor. This will prevent deletion through a pointer to the base class without performance, or space penalties (on any implementation I'm aware of).

As an example, replace...

    template <class Arg, class Result>
    struct unary_function {
            typedef Arg    argument_type;
            typedef Result result_type;
    };

... with...

    template <class Arg, class Result>
    struct unary_function {
            typedef Arg    argument_type;
            typedef Result result_type;
    protected:
            ~unary_function() {}
    };

Affected definitions:
 20.3.1 [lib.function.objects] -- unary_function, binary_function
 24.3.2 [lib.iterator.basic] -- iterator


258.  Missing allocator requirement

Section: 20.1.5 lib.allocator.requirements Status: New  Submitter: Matt Austern   Date: 22 Aug 2000

From lib-7752:

I've been assuming (and probably everyone else has been assuming) that allocator instances have a particular property, and I don't think that property can be deduced from anything in Table 32.

I think we have to assume that allocator type conversion is a homomorphism. That is, if x1 and x2 are of type X, where X::value_type is T, and if type Y is X::template rebind<U>::other, then Y(x1) == Y(x2) if and only if x1 == x2.

Further discussion: Howard Hinant writes, in lib-7757:

I think I can prove that this is not proveable by Table 32. And I agree it needs to be true except for the "and only if". If x1 != x2, I see no reason why it can't be true that Y(x1) == Y(x2). Admittedly I can't think of a practical instance where this would happen, or be valuable. But I also don't see a need to add that extra restriction. I think we only need:

if (x1 == x2) then Y(x1) == Y(x2)

If we decide that == on allocators is transitive, then I think I can prove the above. But I don't think == is necessarily transitive on allocators. That is:

Given x1 == x2 and x2 == x3, this does not mean x1 == x3.

Example:

x1 can deallocate pointers from: x1, x2, x3
x2 can deallocate pointers from: x1, x2, x4
x3 can deallocate pointers from: x1, x3
x4 can deallocate pointers from: x2, x4

x1 == x2, and x2 == x4, but x1 != x4

Proposed Resolution:


259basic_string::operator[] and const correctness

Section: 21.1.4 lib.string.access Status: New  Submitter: Chris Newton <chrisnewton@btinternet.com>   Date:

Paraphrased from a message that Chris Newton posted to comp.std.c++:

The standard's description of basic_string<>::operator[] seems to violate const correctness.

The standard (21.3.4/1) says that "If pos < size(), returns data()[pos]." The types don't work. The return value of data() is const charT*, but operator[] has a non-const version whose return type is reference.

Proposed Resolution:


260.  Inconsistent return type of istream_iterator::operator++(int)

Section: 24.5.1.2 lib.istream.iterator.ops Status: New  Submitter: Martin Sebor   Date: 27 Aug 2000

The synopsis of istream_iterator::operator++(int) in 24.5.1 shows it as returning the iterator by value. 24.5.1.2, p5 shows the same operator as returning the iterator by reference. That's incorrect given the Effects clause below (since a temporary is returned). The `&' is probably just a typo.

Proposed Resolution:

Change the declaration in 24.5.1.2, p5 from

 istream_iterator<T,charT,traits,Distance>& operator++(int);
 
to
 istream_iterator<T,charT,traits,Distance> operator++(int);
 
(that is, remove the `&').

261.  Missing description of istream_iterator::operator!=

Section: 24.5.1.2 lib.istream.iterator.ops Status: New  Submitter: Martin Sebor   Date: 27 Aug 2000

24.5.1, p3 lists the synopsis for

   template <class T, class charT, class traits, class Distance>
        bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
                        const istream_iterator<T,charT,traits,Distance>& y);

but there is no description of what the operator does (i.e., no Effects or Returns clause) in 24.5.1.2.

Proposed Resolution:

Add paragraph 7 to the end of section 24.5.1.2 with the following text:

   template <class T, class charT, class traits, class Distance>
        bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
                        const istream_iterator<T,charT,traits,Distance>& y);

-7- Returns: !(x == y).


262.  Bitmask operator ~ specified incorrectly

Section: 17.3.2.1.2 lib.bitmask.types Status: New  Submitter: Beman Dawes   Date: 03 Sep 2000

The ~ operation should be applied after the cast to int_type.

Proposed Resolution:

Change 17.3.2.1.2 [lib.bitmask.types] operator~ from:

   bitmask operator~ ( bitmask X )
     { return static_cast< bitmask>(static_cast<int_type>(~ X)); }

to:

   bitmask operator~ ( bitmask X )
     { return static_cast< bitmask>(~static_cast<int_type>(X)); }

263.  Severe restriction on basic_string reference counting

Section: 21.3 lib.basic.string Status: New  Submitter: Kevlin Henney <kevlin@curbralan.com>   Date: 04 Sep 2000

The note in paragraph 6 suggests that the invalidation rules for references, pointers, and iterators in paragraph 5 permit a reference- counted implementation (actually, according to paragraph 6, they permit a "reference counted implemenation", but this is a minor editorial fix).

However, the last sub-bullet is so worded as to make a reference-counted implementation unviable. In the following example none of the conditions for iterator invalidation are satisfied:

    // first example: "*******************" should be printed twice
    string original = "some arbitrary text", copy = original;
    const string & alias = original;

    string::const_iterator i = alias.begin(), e = alias.end();
    for(string::iterator j = original.begin(); j != original.end(); ++j)
        *j = '*';
    while(i != e)
        cout << *i++;
    cout << endl;
    cout << original << endl;

Similarly, in the following example:

    // second example: "some arbitrary text" should be printed out
    string original = "some arbitrary text", copy = original;
    const string & alias = original;

    string::const_iterator i = alias.begin();
    original.begin();
    while(i != alias.end())
        cout << *i++;

I have tested this on three string implementations, two of which were reference counted. The reference-counted implementations gave "surprising behaviour" because they invalidated iterators on the first call to non-const begin since construction. The current wording does not permit such invalidation because it does not take into account the first call since construction, only the first call since various member and non-member function calls.

Proposed Resolution:

Change the following sentence in 21.3 paragraph 5 from

Subsequent to any of the above uses except the forms of insert() and erase() which return iterators, the first call to non-const member functions operator[](), at(), begin(), rbegin(), end(), or rend().

to

Following construction or any of the above uses, except the forms of insert() and erase() which return iterators, the first call to non- const member functions operator[](), at(), begin(), rbegin(), end(), or rend().

264.  Associative container insert(i, j) complexity requirements are not feasible.

Section: 23.1.2 lib.associative.reqmts Status: New  Submitter: John Potter   Date: 07 Sep 2000

Table 69 requires linear time if [i, j) is sorted. Sorted is necessary but not sufficient. Consider inserting a sorted range of even integers into a set<int> containing the odd integers in the same range.

Related issue: 102

Proposed Resolution:

Testing for valid insertions could be less efficient than simply inserting the elements when the range is not both sorted and between two adjacent existing elements. This could be a QOI issue and I offer three resolutions.

A. Drop the linear requirement.

B. Change to: linear if [i, j) is sorted according to value_comp() and between two adjacent existing elements.

C. Change to: Klog(size() + N) + (N - K) (N is the distance from i to j and K is the number of elements which do not insert immediately after the previous element from [i, j) including the first)

Either A or C will resolve issue 102 also.

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