| Doc. no. | J16/01-0024 = WG21 N1310 |
| Date: | 10 May 2001 |
| Project: | Programming Language C++ |
| Reply to: | Matt Austern <austern@research.att.com> |
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.
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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.
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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.
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).h The intent of this LWG process is that only issues which are truly defects in the Standard move to the formal ISO DR status.
Section: 22.2.2.1.2 [lib.facet.num.get.virtuals] Status: Open Submitter: Nathan Myers Date: 6 Aug 1998
The current description of numeric input does not account for the possibility of overflow. This is an implicit result of changing the description to rely on the definition of scanf() (which fails to report overflow), and conflicts with the documented behavior of traditional and current implementations.
Users expect, when reading a character sequence that results in a value unrepresentable in the specified type, to have an error reported. The standard as written does not permit this.
Further comments from Dietmar:
I don't feel comfortable with the proposed resolution to issue 23: It kind of simplifies the issue to much. Here is what is going on:
Currently, the behavior of numeric overflow is rather counter intuitive and hard to trace, so I will describe it briefly:
Now the proposed resolution results in not modifying the value passed as last argument if an overflow is encountered but failbit is set. Checking errno for ERANGE still allows for detection of an overflow but not what the sign was.
Actually, my problem is not that much with the sign but this is at least making things worse... My problem is more that it is still necessary to check errno for the error description. Thus, I propose the following resolution:
Change paragraph 11 from
-11- Stage 3: The result of stage 2 processing can be one of
- A sequence of chars has been accumulated in stage 2 that is converted (according to the rules of scanf) to a value of the type of val. This value is stored in val and ios_base::goodbit is stored in err.
- The sequence of chars accumulated in stage 2 would have caused scanf to report an input failure. ios_base::failbit is assigned to err.
to become
-11- Stage 3: The result of stage 2 processing can be one of
- A sequence of chars has been accumulated in stage 2 that is converted (according to the rules of scanf) to a value of the type of val. This value is stored in val. If the conversion reported an overflow error for the type of val (ie. errno would be set to ERANGE by the used conversion function) then ios_base::failbit is stored in err, otherwise ios_base::goodbit is stored in err.
- The sequence of chars accumulated in stage 2 would have caused scanf to report an input failure. ios_base::failbit is assigned to err.
With this definition, overflow can be detected easily by storing a value different from the maximum value in val and checking whether this value was modified in case failbit is set: If it was, there was an overflow error, otherwise some other input error occurred (under the conditions for the second bullet val is not changed).
Proposed resolution:
In 22.2.2.1.2 [lib.facet.num.get.virtuals], paragraph 11, second bullet item, change
The sequence of chars accumulated in stage 2 would have caused scanf to report an input failure.
to
The sequence of chars accumulated in stage 2 would have caused scanf to report an input failure, or the value of the sequence cannot be represented in the type of _val_.
[post-Toronto: "cannot be represented" is probably wrong: infinity can be represented on an IEC559 platform, but 0.1 cannot be represented exactly. However, the alternate proposal may be wrong as well. It's not clear whether overflow (and underflow?) should always be treated as errors. This issue requires much more thought]
Section: 27 [lib.input.output] Status: Open Submitter: Nathan Myers Date: 6 Aug 1998
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. ]
Section: 27.4.2.4 [lib.ios.members.static] Status: Ready Submitter: Matt Austern Date: 21 Jun 1998
Two problems
(1) 27.4.2.4 doesn't say what ios_base::sync_with_stdio(f) returns. Does it return f, or does it return the previous synchronization state? My guess is the latter, but the standard doesn't say so.
(2) 27.4.2.4 doesn't say what it means for streams to be synchronized with stdio. Again, of course, I can make some guesses. (And I'm unhappy about the performance implications of those guesses, but that's another matter.)
Proposed resolution:
Change the following sentence in 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.
Add the following immediately after 27.4.2.4 [lib.ios.members.static], paragraph 2:
When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by
fputc(f, c);is the same as the effect of
str.rdbuf()->sputc(c);for any sequence of characters; the effect of extracting a character c by
c = fgetc(f);is the same as the effect of:
c = str.rdbuf()->sbumpc(c);for any sequences of characters; and the effect of pushing back a character c by
ungetc(c, f);is the same as the effect of
str.rdbuf()->sputbackc(c);for any sequence of characters. [Footnote: This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream. In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer. --End Footnote]
[pre-Copenhagen: PJP and Matt contributed the definition of "synchronization"]
[post-Copenhagen: proposed resolution was revised slightly: text was added in the non-normative footnote to say that operations on the two streams can be mixed arbitrarily.]
Section: 22.2.1.5 [lib.locale.codecvt] Status: Review Submitter: Matt Austern Date: 25 Sep 1998
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 [lib.locale.codecvt.virtuals], paragraph 11) and basic_filebuf::underflow (27.8.1.4 [lib.filebuf.virtuals], paragraph 3) suggests that it must always be possible to pick off internal characters one at a time from a sequence of external characters. However, this is never explicitly stated one way or the other.
This issue seems (and is) quite technical, but it is important if we expect users to provide their own encoding facets. This is an area where the standard library calls user-supplied code, so a well-defined set of requirements for the user-supplied code is crucial. Users must be aware of the assumptions that the library makes. This issue affects positioning operations on basic_filebuf, unbuffered input, and several of codecvt's member functions.
Proposed resolution:
Add the following text as a new paragraph, following 22.2.1.5.2 [lib.locale.codecvt.virtuals] paragraph 2:
A codecvt facet that is used by basic_filebuf (27.8 [lib.file.streams]) must have the property that if
do_out(state, from, from_end, from_next, to, to_lim, to_next)would succeed (return value would be ok), where from != from_end, thendo_out(state, from, from + 1, from_next, to, to_end, to_next)must also succeed, and that ifdo_in(state, from, from_end, from_next, to, to_lim, to_next)would succeed, where to != to_lim, thendo_in(state, from, from_end, from_next, to, to + 1, to_next)must also succeed. [Footnote: Informally, this means that basic_filebuf assumes that the mapping from internal to external characters is 1 to N: a codecvt that is used by basic_filebuf must be able to translate characters one internal character at a time. --End Footnote]
Rationale:
The proposed resoluion says that conversions can be performed one internal character at a time. This rules out some encodings that would otherwise be legal. The alternative answer would mean there would be some internal positions that do not correspond to any external file position.
An example of an encoding that this rules out is one where the internT and externT are of the same type, and where the internal sequence c1 c2 corresponds to the external sequence c2 c1.
It was generally agreed that basic_filebuf relies on this property: it was designed under the assumption that the external-to-internal mapping is N-to-1, and it is not clear that basic_filebuf is implementable without that restriction.
The proposed resolution is expressed as a restriction on codecvt when used by basic_filebuf, rather than a blanket restriction on all codecvt facets, because basic_filebuf is the only other part of the library that uses codecvt. If a user wants to define a codecvt facet that implements a more general N-to-M mapping, there is no reason to prohibit it, so long as the user does not expect basic_filebuf to be able to use it.
Section: 21.3.7.9 [lib.string.io] Status: Review 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, replace:
Effects: Begins by constructing a sentry object k as if k were constructed by typename basic_istream<charT,traits>::sentry k( is). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:
with:
Effects: Behaves as an unformatted input function (27.6.1.2 [lib.istream.formatted]). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:
In 21.3.7.9 [lib.string.io], paragraph 6, replace
Effects: Begins by constructing a sentry object k as if by typename basic_istream<charT,traits>::sentry k( is, true). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c) until any of the following occurs:
with:
Effects: Behaves as an unformatted input function (27.6.1.2 [lib.istream.formatted]). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c) until any of the following occurs:
Rationale:
The real issue here is whether or not these string input functions perform formatted input. If they do, then they get their characters from a streambuf, rather than by calling an istream's member functions, and a streambuf signals failure either by returning eof or by throwing an exception. The proposed resolution makes it clear that these two functions do perform formatted input.
Section: 25 [lib.algorithms] Status: Open Submitter: Nico Josuttis Date: 29 Sep 1998
The standard does not state, how often a function object is copied, called, or the order of calls inside an algorithm. This may lead to surprising/buggy behavior. Consider the following example:
class Nth { // function object that returns true for the nth element
private:
int nth; // element to return true for
int count; // element counter
public:
Nth (int n) : nth(n), count(0) {
}
bool operator() (int) {
return ++count == nth;
}
};
....
// remove third element
list<int>::iterator pos;
pos = remove_if(coll.begin(),coll.end(), // range
Nth(3)), // remove criterion
coll.erase(pos,coll.end());
This call, in fact removes the 3rd AND the 6th element. This happens because the usual implementation of the algorithm copies the function object internally:
template <class ForwIter, class Predicate>
ForwIter std::remove_if(ForwIter beg, ForwIter end, Predicate op)
{
beg = find_if(beg, end, op);
if (beg == end) {
return beg;
}
else {
ForwIter next = beg;
return remove_copy_if(++next, end, beg, op);
}
}
The algorithm uses find_if() to find the first element that should be removed. However, it then uses a copy of the passed function object to process the resulting elements (if any). Here, Nth is used again and removes also the sixth element. This behavior compromises the advantage of function objects being able to have a state. Without any cost it could be avoided (just implement it directly instead of calling find_if()).
Proposed resolution:
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.]
Section: 23.2.5 [lib.vector.bool] Status: Open Submitter: AFNOR Date: 7 Oct 1998
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> may be 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.]
Section: 24.1.1 [lib.input.iterators] Status: Open Submitter: AFNOR Date: 7 Oct 1998
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.]
Section: 20.3.6 [lib.binders] Status: Ready Submitter: Bjarne Stroustrup Date: 7 Oct 1998
There are no versions of binders that apply to non-const elements of a sequence. This makes examples like for_each() using bind2nd() on page 521 of "The C++ Programming Language (3rd)" non-conforming. Suitable versions of the binders need to be added.
Further discussion from Nico:
What is probably meant here is shown in the following example:
class Elem {
public:
void print (int i) const { }
void modify (int i) { }
};
int main()
{
vector<Elem> coll(2);
for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::print),42)); // OK
for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::modify),42)); // ERROR
}
The error results from the fact that bind2nd() passes its first argument (the argument of the sequence) as constant reference. See the following typical implementation:
template <class Operation> class binder2nd : public unary_function<typename Operation::first_argument_type, typename Operation::result_type> { protected: Operation op; typename Operation::second_argument_type value; public: binder2nd(const Operation& o, const typename Operation::second_argument_type& v) : op(o), value(v) {}typename Operation::result_type operator()(const typename Operation::first_argument_type& x) const { return op(x, value); } };
The solution is to overload operator () of bind2nd for non-constant arguments:
template <class Operation> class binder2nd : public unary_function<typename Operation::first_argument_type, typename Operation::result_type> { protected: Operation op; typename Operation::second_argument_type value; public: binder2nd(const Operation& o, const typename Operation::second_argument_type& v) : op(o), value(v) {}typename Operation::result_type operator()(const typename Operation::first_argument_type& x) const { return op(x, value); } typename Operation::result_type operator()(typename Operation::first_argument_type& x) const { return op(x, value); } };
Proposed resolution:
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: NAD - 5. Accept proposed resolution - 3. Leave open - 6.]
[Copenhagen: It was generally agreed that this was a defect. Strap poll: NAD - 0. Accept proposed resolution - 10. Leave open - 1.]
Section: 27.6.2.5.2 [lib.ostream.inserters.arithmetic] Status: Ready Submitter: Matt Austern Date: 20 Nov 1998
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 the formatting conversion occurs as if it performed the following code fragment:
ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned short>(val)) : static_cast<long>(val)). failed();When val is of type int the formatting conversion occurs as if it performed the following code fragment:
ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned int>(val)) : static_cast<long>(val)). failed();When val is of type unsigned short or unsigned int the formatting conversion occurs as if it performed the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), static_cast<unsigned long>(val)). failed();When val is of type float the formatting conversion occurs as if it performed the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), static_cast<double>(val)). failed();
[post-Toronto: This differs from the previous proposed resolution; PJP provided the new wording. The differences are in signed short and int output.]
Rationale:
The original proposed resolution was to cast int and short to long, unsigned int and unsigned short to unsigned long, and float to double, thus ensuring that we don't try to use nonexistent num_put<> member functions. The current proposed resolution is more complicated, but gives more expected results for hex and octal output of signed short and signed int. (On a system with 16-bit short, for example, printing short(-1) in hex format should yield 0xffff.)
Section: 17.4.3.1 [lib.reserved.names] Status: Open 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 to explicit instantiations and to specializations, 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.
In Copenhagen, core working group decided on a proposed resolution to core issue 259. Under the proposed resolution, it will be legal for a translation unit to contain both a specialization and an explicit instantiation of the same template, provided that the specialization comes first. In such a case, the explicit instantiation will be ignored. Further discussion of library issue 120 assumes that the core 259 resolution will be adopted.
Proposed resolution:
Option 1.
Append to 17.4.3.1 [lib.reserved.names] paragraph 1:
A program may explicitly 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.
Option 2.
In light of the resolution to core issue 259, no normative changes in the library clauses are necessary. Add the following non-normative note to the end of 17.4.3.1 [lib.reserved.names] paragraph 1:
[Note: A program may explicitly instantiate standard library templates, even when an explicit instantiation does not depend on a user-defined name. --end note]
[Copenhagen: LWG discussed three options. (A) Users may not explicitly instantiate standard library templates, except on user-defined types. Consequence: library implementors may freely specialize or instantiate templates. (B) It is implementation defined whether users may explicitly instantiate standard library templates on non-user-defined types. Consequence: library implementors may freely specialize or instantiate templates, but must document the templates they have explicitly instantiated. (C) Users may explicitly instantiate any standard library template. Consequence: library implementors may freely specialize templates, but may not explicitly instantiate them. This is a serious burden for implementors; one way they can manage it is by defining the standard template as a wrapper, and putting all of the real work in an internal helper class/function. ]
[Straw poll (first number is favor, second is strongly oppose): A - 4, 0; B - 0, 9; C - 9, 1. Proposed resolution 1, above, is option A. (It is the original proposed resolution.) Proposed resolution 2, above, is option C. Because there was no support for option B, no wording is provided.]
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 77Rationale), and a complete design review is needed before making piecemeal changes. Robert Klarer will work on formulating the issues.]
Section: 27.6.2.5.4 [lib.ostream.inserters.character] Status: Review Submitter: Dietmar Kühl Date: 20 Jul 1999
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((const char*)s) if the second argument is of type const charT*
- char_traits<char>::length(s) if the second argument is of type const char*, const signed char*, or const unsigned char* and and charT is not 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.]
[Toronto: The original proposed resolution involved char_traits<signed char> and char_traits<unsigned char>. There was strong opposition to requiring that library implementors provide those specializations of char_traits.]
[Copenhagen: This still isn't quite right: proposed resolution text got garbled when the signed char/unsigned char specializations were removed. Dietmar will provide revised wording.]
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;
}
Comment from John Potter:
In case nobody has noticed, accepting it will break reverse_iterator.
The fix is to make the comparison operators templated on two types.
template <class Iterator1, class Iterator2> bool operator== (reverse_iterator<Iterator1> const& x, reverse_iterator<Iterator2> const& y);Obviously: return x.base() == y.base();
Currently, no reverse_iterator to const_reverse_iterator compares are valid.
BTW, I think the issue is in support of bad code. Compares should be between two iterators of the same type. All std::algorithms require the begin and end iterators to be of the same type.
Proposed resolution:
In section 23.1 [lib.container.requirements] after paragraph 7 add:
It is possible to mix iterators and const_iterators in iterator comparison and iterator difference operations.
[Post-Tokyo: Judy supplied the above wording at the request of the LWG.]
[post-Toronto: Judy supplied a new proposed resolution. The old version did not include the words "and iterator difference".]
[Copenhagen: There was some concern that "it is possible to mix" might be too informal. Howard and Dave will provide new wording, which will involve a list of expressions that are guaranteed to be valid.]
Rationale:
The LWG believes it is clear that the above wording applies only to the nested types X::iterator and X::const_iterator, where X is a container. There is no requirement that X::reverse_iterator and X::const_reverse_iterator can be mixed. If mixing them is considered important, that's a separate issue. (Issue 280.)
Section: 17 [lib.library] Status: Ready Submitter: Al Stevens Date: 15 Aug 1999
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. There there can't be another size_t or ptrdiff_t meant anyway because, according to 17.4.3.1.4 [lib.extern.types],
For each type T from the Standard C library, the types ::T and std::T are reserved to the implementation and, when defined, ::T shall be identical to std::T.
The issue is treated as a Defect Report to make explicit the Project Editor's authority to make this change.
[Post-Tokyo: Nico Josuttis provided the above wording at the request of the LWG.]
[Toronto: This is tangentially related to issue 229, but only tangentially: the intent of this issue is to address use of the name size_t in contexts outside of namespace std, such as in the description of ::operator new. The proposed changes should be reviewed to make sure they are correct.]
[pre-Copenhagen: Nico has reviewed the changes and believes them to be correct.]
Section: 25.2.2 [lib.alg.swap] Status: Open Submitter: Andrew Koenig Date: 14 Aug 1999
The description of iter_swap in 25.2.2 paragraph 7,says that it ``exchanges the values''
of the objects to which two iterators refer.
What it doesn't say is whether it does so using swap or using the assignment operator and copy constructor.
This question is an important one to answer, because swap is specialized to work efficiently for standard containers.
For example:
vector<int> v1, v2; iter_swap(&v1, &v2);
Is this call to iter_swap equivalent to calling swap(v1, v2)? Or is it equivalent to
{
vector<int> temp = v1;
v1 = v2;
v2 = temp;
}
The first alternative is O(1); the second is O(n).
A LWG member, Dave Abrahams, comments:
Not an objection necessarily, but I want to point out the cost of that requirement:
iter_swap(list<T>::iterator, list<T>::iterator)
can currently be specialized to be more efficient than iter_swap(T*,T*) for many T (by using splicing). Your proposal would make that optimization illegal.
[Kona: The LWG notes the original need for iter_swap was proxy iterators which are no longer permitted.]
Proposed resolution:
Change the effect clause of iter_swap in 25.2.2 paragraph 7 from:
Exchanges the values pointed to by the two iterators a and b.
to
swap(*a, *b).
[post-Toronto: The LWG is concerned about possible overspecification: there may be cases, such as Dave Abrahams's example above, and such as vector<bool>'s iterators, where it makes more sense for iter_swap to do something other than swap. If performance is a concern, it may be better to have explicit complexity requirements than to say how iter_swap should be implemented.]
Section: 20.1.5 [lib.allocator.requirements], 23.1 [lib.container.requirements] Status: Open Submitter: Andy Sawyer Date: 21 Oct 1999
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).]
[Copenhagen: Exactly what max_size means is still unclear. It may have a different meaning as a container member function than as an allocator member function. For the latter, it is probably best thought of as an architectural limit. Nathan will provide new wording.]
Section: 24.1 [lib.iterator.requirements] Status: Review Submitter: Beman Dawes Date: 3 Nov 1999
Is a pointer or reference obtained from an iterator still valid after destruction of the iterator?
Is a pointer or reference obtained from an iterator still valid after the value of the iterator changes?
#include <iostream>
#include <vector>
#include <iterator>
int main()
{
typedef std::vector<int> vec_t;
vec_t v;
v.push_back( 1 );
// Is a pointer or reference obtained from an iterator still
// valid after destruction of the iterator?
int * p = &*v.begin();
std::cout << *p << '\n'; // OK?
// Is a pointer or reference obtained from an iterator still
// valid after the value of the iterator changes?
vec_t::iterator iter( v.begin() );
p = &*iter++;
std::cout << *p << '\n'; // OK?
return 0;
}
The standard doesn't appear to directly address these questions. The standard needs to be clarified. At least two real-world cases have been reported where library implementors wasted considerable effort because of the lack of clarity in the standard. The question is important because requiring pointers and references to remain valid has the effect for practical purposes of prohibiting iterators from pointing to cached rather than actual elements of containers.
The standard itself assumes that pointers and references obtained from an iterator are still valid after iterator destruction or change. The definition of reverse_iterator::operator*(), 24.4.1.3.3 [lib.reverse.iter.op.star], which returns a reference, defines effects:
Iterator tmp = current; return *--tmp;
The definition of reverse_iterator::operator->(), 24.4.1.3.4 [lib.reverse.iter.opref], which returns a pointer, defines effects:
return &(operator*());
Because the standard itself assumes pointers and references remain valid after iterator destruction or change, the standard should say so explicitly. This will also reduce the chance of user code breaking unexpectedly when porting to a different standard library implementation.
Proposed resolution:
Add a new paragraph to 24.1 [lib.iterator.requirements]:
Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.
Replace paragraph 1 of 24.4.1.3.3 [lib.reverse.iter.op.star] with:
Effects:
this->tmp = current; --this->tmp; return *this->tmp;[Note: This operation must use an auxiliary member variable, rather than a temporary variable, to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.1 [lib.iterator.requirements].) The name of this member variable is shown for exposition only. --end note]
[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.]
[Pre-Toronto: Steve Cleary pointed out the no-invalidation assumption by reverse_iterator. The issue and proposed resolution was reformulated yet again to reflect this reality.]
[Copenhagen: Andy Koenig pointed out that it is possible to rewrite reverse_iterator so that it no longer makes this assumption. However, this issue is related to issue 299. If we decide it is intentional that p[n] may return by value instead of reference when p is a Random Access Iterator, then other changes in reverse_iterator will be necessary.]
Section: 24.1.3 [lib.forward.iterators] Status: Open Submitter: Matt Austern Date: 19 Nov 1999
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.]
Section: 18.2.1 [lib.limits] Status: Open 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.
[post-Toronto: The opinion of the LWG is that the wording in the standard, as well as the wording of the proposed resolution, is flawed. The term "arithmetic types" is well defined in C and C++, and it is not clear that the term is being used correctly. It is also not clear that the term "implementation dependent" has any useful meaning in this context. The biggest problem is that numeric_limits seems to be intended both for built-in types and for user-defined types, and the standard doesn't make it clear how numeric_limits applies to each of those cases. A wholesale review of numeric_limits is needed. A paper would be welcome.]
Section: 25.2.8 [lib.alg.unique] Status: Open Submitter: Andrew Koenig Date: 13 Jan 2000
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, and change pred(*i, *(i - 1)) to pred(*(i - 1), i).
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.
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. ]
[Copenhagen: There was some support for all options. The option with the least support was 1 (one person in favor), and the option with the most support was 2 (seven in favor). One person was strongly opposed to option 1, and one person was strongly opposed to the variation on option 2 in which the order of arguments would remain pred(*i, *(i - 1)).]
Section: 17.4.4.3 [lib.global.functions] Status: Open Submitter: Dave Abrahams Date: 01 Apr 2000
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 (3.4.2 [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.]
[Toronto: The LWG is not sure if this is a defect in the standard. Most LWG members believe that an implementation of std::unique like the one quoted in this issue is already illegal, since, under certain circumstances, its semantics are not those specified in the standard. The standard's description of unique does not say that overloading adjacent_find should have any effect.]
Section: 17.4.3.1 [lib.reserved.names] Status: Open Submitter: Dave Abrahams Date: 01 Apr 2000
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.]
[Toronto: Dave Abrahams and Peter Dimov have proposed a resolution that involves core changes: it would add partial specialization of function template. The Core Working Group is reluctant to add partial specialization of function templates. It is viewed as a large change, CWG believes that proposal presented leaves some syntactic issues unanswered; if the CWG does add partial specialization of function templates, it wishes to develop its own proposal. The LWG continues to believe that there is a serious problem: there is no good way for users to force the library to use user specializations of generic standard library functions, and in certain cases (e.g. transcendental functions called by valarray and complex) this is important. Koenig lookup isn't adequate, since names within the library must be qualified with std (see issue 225), specialization doesn't work (we don't have partial specialization of function templates), and users aren't permitted to add overloads within namespace std. ]
[Copenhagen: Discussed at length, with no consensus. Relevant papers in the pre-Copenhagen mailing: N1289, N1295, N1296. Discussion focused on four options. (1) Relax restrictions on overloads within namespace std. (2) Mandate that the standard library use unqualified calls for swap and possibly other functions. (3) Introduce helper class templates for swap and possibly other functions. (4) Introduce partial specialization of function templates. Every option had both support and opposition. Straw poll (first number is support, second is strongly opposed): (1) 6, 4; (2) 6, 7; (3) 3, 8; (4) 4, 4.]
Section: 22.2 [lib.locale.categories] Status: Ready Submitter: Dietmar Kühl Date: 20 Apr 2000
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 'ctype_byname': In 'ctype<char>' the method 'do_is()' is not virtual but it is made virtual in both 'ctype<cT>' and 'ctype_byname<cT>'. Thus, a class derived from 'ctype_byname<char>' can tell whether this class is specialized or not under the current specification: Without the specialization, 'do_is()' is virtual while with specialization it is not virtual.
Proposed resolution:
Change section 22.2.1.2 (lib.locale.ctype.byname) to become:
namespace std {
template <class charT>
class ctype_byname : public ctype<charT> {
public:
typedef ctype<charT>::mask mask;
explicit ctype_byname(const char*, size_t refs = 0);
protected:
~ctype_byname(); // virtual
};
}
Change section 22.2.1.6 (lib.locale.codecvt.byname) to become:
namespace std {
template <class internT, class externT, class stateT>
class codecvt_byname : public codecvt<internT, externT, stateT> {
public:
explicit codecvt_byname(const char*, size_t refs = 0);
protected:
~codecvt_byname(); // virtual
};
}
Change section 22.2.3.2 (lib.locale.numpunct.byname) to become:
namespace std {
template <class charT>
class numpunct_byname : public numpunct<charT> {
// this class is specialized for char and wchar_t.
public:
typedef charT char_type;
typedef basic_string<charT> string_type;
explicit numpunct_byname(const char*, size_t refs = 0);
protected:
~numpunct_byname(); // virtual
};
}
Change section 22.2.4.2 (lib.locale.collate.byname) to become:
namespace std {
template <class charT>
class collate_byname : public collate<charT> {
public:
typedef basic_string<charT> string_type;
explicit collate_byname(const char*, size_t refs = 0);
protected:
~collate_byname(); // virtual
};
}
Change section 22.2.5.2 (lib.locale.time.get.byname) to become:
namespace std {
template <class charT, class InputIterator = istreambuf_iterator<charT> >
class time_get_byname : public time_get<charT, InputIterator> {
public:
typedef time_base::dateorder dateorder;
typedef InputIterator iter_type
explicit time_get_byname(const char*, size_t refs = 0);
protected:
~time_get_byname(); // virtual
};
}
Change section 22.2.5.4 (lib.locale.time.put.byname) to become:
namespace std {
template <class charT, class OutputIterator = ostreambuf_iterator<charT> >
class time_put_byname : public time_put<charT, OutputIterator>
{
public:
typedef charT char_type;
typedef OutputIterator iter_type;
explicit time_put_byname(const char*, size_t refs = 0);
protected:
~time_put_byname(); // virtual
};
}"
Change section 22.2.6.4 (lib.locale.moneypunct.byname) to become:
namespace std {
template <class charT, bool Intl = false>
class moneypunct_byname : public moneypunct<charT, Intl> {
public:
typedef money_base::pattern pattern;
typedef basic_string<charT> string_type;
explicit moneypunct_byname(const char*, size_t refs = 0);
protected:
~moneypunct_byname(); // virtual
};
}
Change section 22.2.7.2 (lib.locale.messages.byname) to become:
namespace std {
template <class charT>
class messages_byname : public messages<charT> {
public:
typedef messages_base::catalog catalog;
typedef basic_string<charT> string_type;
explicit messages_byname(const char*, size_t refs = 0);
protected:
~messages_byname(); // virtual
};
}
Remove section 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.]
[Copenhagen: proposed resolution was revised slightly, to remove three last virtual functions from messages_byname.]
Section: 17.4.1.1 [lib.contents] Status: Open Submitter: Steve Clamage Date: 19 Apr 2000
Throughout the library chapters, the descriptions of library entities refer to other library entities without necessarily qualifying the names.
For example, section 25.2.2 "Swap" describes the effect of swap_ranges in terms of the unqualified name "swap". This section could reasonably be interpreted to mean that the library must be implemented so as to do a lookup of the unqualified name "swap", allowing users to override any ::std::swap function when Koenig lookup applies.
Although it would have been best to use explicit qualification with "::std::" throughout, too many lines in the standard would have to be adjusted to make that change in a Technical Corrigendum.
Issue 182, which addresses qualification of size_t, is a special case of this.
Proposed resolution:
To section 17.4.1.1 "Library contents" Add the following paragraph:
Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects section for library function F is described as calling library function G, the function ::std::G is meant.
[Post-Tokyo: Steve Clamage submitted this issue at the request of the LWG to solve a problem in the standard itself similar to the problem within implementations of library identified by issue 225. Any resolution of issue 225 should be coordinated with the resolution of this issue.]
[post-Toronto: Howard is undecided about whether it is appropriate for all standard library function names referred to in other standard library functions to be explicitly qualified by std: it is common advice that users should define global functions that operate on their class in the same namespace as the class, and this requires argument-dependent lookup if those functions are intended to be called by library code. Several LWG members are concerned that valarray appears to require argument-dependent lookup, but that the wording may not be clear enough to fall under "unless explicitly described otherwise".]
Section: 17 [lib.library] Status: Ready Submitter: Beman Dawes Date: 26 Apr 2000
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 23.1 [lib.container.requirements] table 65 for value_type: change "T is Assignable" to "T is CopyConstructible and Assignable"
In 23.1.2 [lib.associative.reqmts] table 69 X::key_type; change
"Key is Assignable" to "Key is
CopyConstructible and Assignable"
In 24.1.2 [lib.output.iterators] paragraph 1, change:
A class or a built-in type X satisfies the requirements of an output iterator if X is an Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:
to:
A class or a built-in type X satisfies the requirements of an output iterator if X is a CopyConstructible (20.1.3) and Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:
[Post-Tokyo: Beman Dawes submitted this issue at the request of the LWG. He asks that the 25.2.4 [lib.alg.replace] and 25.2.5 [lib.alg.fill] changes be studied carefully, as it is not clear that CopyConstructible is really a requirement and may be overspecification.]
Rationale:
The original proposed resolution also included changes to input iterator, fill, and replace. The LWG believes that those changes are not necessary. The LWG considered some blanket statement, where an Assignable type was also required to be Copy Constructible, but decided against this because fill and replace really don't require the Copy Constructible property.
Section: 22.2.2.2.2 [lib.facet.num.put.virtuals] Status: Review Submitter: James Kanze, Stephen Clamage Date: 25 Apr 2000
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:
In 22.2.2.2.2 [lib.facet.num.put.virtuals], paragraph 11, change "if (flags & fixed) != 0" to "if (flags & floatfield) == fixed || (flags & floatfield) == scientific"
Rationale:
The floatfield determines whether numbers are formatted as if with %f, %e, or %g. If the fixed bit is set, it's %f, if scientific it's %e, and if both bits are set, or neither, it's %e.
Turning to the C standard, a precision of 0 is meaningful for %f and %e, but not for %g: for %g, precision 0 is taken to be the same as precision 1.
The proposed resolution has the effect that the output of the above program will be "1e+00".
Section: 17.4.3.1 [lib.reserved.names] Status: Ready Submitter: Peter Dimov Date: 18 Apr 2000
17.4.3.1/1 uses the term "depends" to limit the set of allowed specializations of standard templates to those that "depend on a user-defined name of external linkage."
This term, however, is not adequately defined, making it possible to construct a specialization that is, I believe, technically legal according to 17.4.3.1/1, but that specializes a standard template for a built-in type such as 'int'.
The following code demonstrates the problem:
#include <algorithm>template<class T> struct X { typedef T type; };namespace std { template<> void swap(::X<int>::type& i, ::X<int>::type& j); }
Proposed resolution:
Change "user-defined name" to "user-defined type".
Rationale:
This terminology is used in section 2.5.2 and 4.1.1 of The C++ Programming Language. It disallows the example in the issue, since the underlying type itself is not user-defined. The only possible problem I can see is for non-type templates, but there's no possible way for a user to come up with a specialization for bitset, for example, that might not have already been specialized by the implementor?
[Toronto: this may be related to issue 120.]
[post-Toronto: Judy provided the above proposed resolution and rationale.]
Section: 23.1.2 [lib.associative.reqmts] Status: Review 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:
General Idea (Andrew Koenig): t is inserted 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(), t); // inserts as first element of set of equal keys mm.insert(mm.end(), t); // inserts as last element of set of equal keys
These examples would allow t to be inserted at the beginning and end, respectively, of the set of elements with the same key as t.
assertion/note/pre/postcondition in table 69
Change:
iterator p is a hint pointing to where the insert should start to search.
To:
if t is inserted, p is used as follows: insert t right before p if possible; otherwise, if p is equal to a.end(), or if the key value of t is greater than the key value of *p, t is inserted just before a.lowerbound(the key value of t); otherwise, t is inserted right before a.upperbound(the key value of t).
complexity:
Change:
right after p
To:
right before p
Thus making:
assertion/note/pre/postcondition:
inserts t if and only if there is no element with key equivalent to the key of t in containers with unique keys; always inserts t in containers with equivalent keys. always returns the iterator pointing to the element with key equivalent to the key of t. if t is inserted, p is used as follows: insert t right before p if possible; otherwise, if p is equal to a.end(), or if the key value of t is greater than the key value of *p, t is inserted just before a.lowerbound(the key value of t); otherwise, t is inserted right before a.upperbound(the key value of t).
NON-NORMATIVE FOOTNOTE: | This gives the user a way of controlling the order | in which elements appear that have equal keys. Doing this is | particularly easy in two common cases:| mm.insert(mm.begin(), t); // inserts as first element of set of equal keys | mm.insert(mm.end(), t); // inserts as last element of set of equal keys
END-FOOTNOTE
complexity:
logarithmic in general, but amortized constant if t is inserted right before p.
[Toronto: there was general agreement that this is a real defect: when inserting an element x into a multiset that already contains several copies of x, there is no way to know whether the hint will be used. There was some support for an alternative resolution: we check on both sides of the hint (both before and after, in that order). If either is the correct location, the hint is used; otherwise it is not. This would be different from the original proposed resolution, because in the proposed resolution the hint will be used even if it is very far from the insertion point. JC van Winkel supplied precise wording for both options.]
[Copenhagen: the LWG looked at both options, and preferred the original. This preference is contingent on seeing a reference implementation showing that it is possible to implement this requirement without loss of efficiency.]
Section: 24.4.1.1 [lib.reverse.iterator] Status: Ready 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:
In section 24.4.1.3.1 [lib.reverse.iter.cons] add the following paragraph:
reverse_iterator()
Default initializes current. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a default constructed iterator of type Iterator.
[pre-Copenhagen: Dietmar provide wording for proposed resolution.]
Section: 27.7.1.1 [lib.stringbuf.cons] Status: Ready 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