| 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 fine that far but consider this code:
std::basic_stringbuf<char> sbuf("hello, world", std::ios_base::openmode(0));
std::cout << "'" << sbuf.str() << "'\n";
Paragraph 3 of 27.7.1.1 basically says that in this case neither the output sequence nor the input sequence is initialized and paragraph 2 of 27.7.1.2 basically says that str() either returns the input or the output sequence. None of them is initialized, ie. both are empty, in which case the return from str() is defined to be basic_string<cT>().
However, probably only test cases in some testsuites will detect this "problem"...
Proposed resolution:
Remove 27.7.1.1 paragraph 4.
Rationale:
We could fix 27.7.1.1 paragraph 4, but there would be no point. If we fixed it, it would say just the same thing as text that's already in the standard.
Section: 25.2.8 [lib.alg.unique] Status: Open 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.
[Toronto: This is related to issue 202. We can't specify unique's complexity until we decide what unique is supposed to do.]
Section: 25.1.5 [lib.alg.adjacent.find] Status: Review Submitter: Angelika Langer Date: May 15 2000
The complexity section of adjacent_find is defective:
template <class ForwardIterator> ForwardIterator adjacent_find(ForwardIterator first, ForwardIterator last BinaryPredicate pred);-1- Returns: The first iterator i such that both i and i + 1 are in the range [first, last) for which the following corresponding conditions hold: *i == *(i + 1), pred(*i, *(i + 1)) != false. Returns last if no such iterator is found.
-2- Complexity: Exactly find(first, last, value) - first applications of the corresponding predicate.
In the Complexity section, it is not defined what "value" is supposed to mean. My best guess is that "value" means an object for which one of the conditions pred(*i,value) or pred(value,*i) is true, where i is the iterator defined in the Returns section. However, the value type of the input sequence need not be equality-comparable and for this reason the term find(first, last, value) - first is meaningless.
A term such as find_if(first, last, bind2nd(pred,*i)) - first or find_if(first, last, bind1st(pred,*i)) - first might come closer to the intended specification. Binders can only be applied to function objects that have the function call operator declared const, which is not required of predicates because they can have non-const data members. For this reason, a specification using a binder could only be an "as-if" specification.
Proposed resolution:
Change the complexity section in 25.1.5 [lib.alg.adjacent.find] to:
For a nonempty range, exactly min((i - first) + 1, (last - first) - 1) applications of the corresponding predicate, where i is adjacent_find's return value.
[Copenhagen: the original resolution specified an upper bound. The LWG preferred an exact count.]
Section: 25.2.8 [lib.alg.unique] Status: Ready 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(), 26.4.1 [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:
In 25.2.8 change:
-4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap.
to:
-4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap. The expression *result = *first is valid.
Rationale:
Creating temporary copies is unavoidable, since the arguments may be input iterators; this implies that the value type must be copy constructible. However, we don't need to say this explicitly; it's already implied by table 72 in 24.1.1. We don't precisely want to say that the input iterator's value type T must be assignable, because we never quite use that property. We assign through the output iterator. The output iterator might have a different value type, or no value type; it might not use T's assignment operator. If it's an ostream_iterator, for example, then we'll use T's operator<< but not its assignment operator.
Section: 25.2.3 [lib.alg.transform], 26.4 [lib.numeric.ops] Status: Ready 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 1.9 [intro.execution] as:
-7- Accessing an object designated by a volatile lvalue (basic.lval), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.
As a consequence, the function call operator of a function object supplied to any of the algorithms listed above cannot modify data members, cannot invoke any function that has a side effect, and cannot even create and modify temporary objects. It is difficult to imagine a function object that is still useful under these severe limitations. For instance, any non-trivial transformator supplied to transform() might involve creation and modification of temporaries, which is prohibited according to the current wording of the standard.
On the other hand, popular implementations of these algorithms exhibit uniform and predictable behavior when invoked with a side-effect-producing function objects. It looks like the strong requirement is not needed for efficient implementation of these algorithms.
The requirement of side-effect-free function objects could be replaced by a more relaxed basic requirement (which would hold for all function objects supplied to any algorithm in the standard library):
A function objects supplied to an algorithm shall not invalidate any iterator or sequence that is used by the algorithm. Invalidation of the sequence includes destruction of the sorting order if the algorithm relies on the sorting order (see section 25.3 - Sorting and related operations [lib.alg.sorting]).
I can't judge whether it is intended that the function objects supplied to transform(), accumulate(), inner_product(), partial_sum(), or adjacent_difference() shall not modify sequence elements through dereferenced iterators.
It is debatable whether this issue is a defect or a change request. Since the consequences for user-supplied function objects are drastic and limit the usefulness of the algorithms significantly I would consider it a defect.
Proposed resolution:
Things to notice about these changes:
Change 25.2.3/2 from:
-2- Requires: op and binary_op shall not have any side effects.
to:
-2- Requires: in the ranges [first1, last1], [first2, first2 + (last1 - first1)] and [result, result + (last1- first1)], op and binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 25.2.3/2 from:
-2- Requires: op and binary_op shall not have any side effects.
to:
-2- Requires: op and binary_op shall not invalidate iterators or subranges, or modify elements in the ranges [first1, last1], [first2, first2 + (last1 - first1)], and [result, result + (last1 - first1)]. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.1/2 from:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op shall not cause side effects.
to:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the range [first, last], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of a fully closed range is intentional --end footnote]
Change 26.4.2/2 from:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op1 and binary_op2 shall not cause side effects.
to:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the ranges [first, last] and [first2, first2 + (last - first)], binary_op1 and binary_op2 shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.3/4 from:
-4- Requires: binary_op is expected not to have any side effects.
to:
-4- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.4/2 from:
-2- Requires: binary_op shall not have any side effects.
to:
-2- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
[Toronto: Dave Abrahams supplied wording.]
[Copenhagen: Proposed resolution was modified slightly. Matt added footnotes pointing out that the use of closed ranges was intentional.]
Section: 23.2.4.3 [lib.vector.modifiers] Status: Open Submitter: Lisa Lippincott Date: 06 June 2000
Paragraph 2 of 23.2.4.3 [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:
[Toronto: It's agreed that there is a defect in complexity of multi-element insert for vector and deque. For vector, the complexity should probably be something along the lines of c1 * N + c2 * distance(i, end()). However, there is some concern about whether it is reasonable to amortize away the copies that we get from a reallocation whenever we exceed the vector's capacity. For deque, the situation is somewhat less clear. Deque is notoriously complicated, and we may not want to impose complexity requirements that would imply any implementation technique more complicated than a while loop whose body is a single-element insert.]
Section: 23.2.2.4 [lib.list.ops] Status: Ready Submitter: Brian Parker 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:
Add a footnote to 23.2.2.4 [lib.list.ops], paragraph 1:
[Footnote: As specified in 20.1.5 [lib.allocator.requirements], paragraphs 4-5, the semantics described in this clause applies only to the case where allocators compare equal. --end footnote]
In 23.2.2.4 [lib.list.ops], replace paragraph 4 with:
Effects: Inserts the contents of x before position and x becomes empty. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
In 23.2.2.4 [lib.list.ops], replace paragraph 7 with:
Effects: Inserts an element pointed to by i from list x before position and removes the element from x. The result is unchanged if position == i or position == ++i. Pointers and references to *i continue to refer to this same element but as a member of *this. Iterators to *i (including i itself) continue to refer to the same element, but now behave as iterators into *this, not into x.
In 23.2.2.4 [lib.list.ops], replace paragraph 12 with:
Requires: [first, last) is a valid range in x. The result is undefined if position is an iterator in the range [first, last). Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
[pre-Copenhagen: Howard provided wording.]
Rationale:
The original proposed resolution said that iterators and references would remain "valid". The new proposed resolution clarifies what that means. Note that this only applies to the case of equal allocators. From 20.1.5 [lib.allocator.requirements] paragraph 4, the behavior of list when allocators compare nonequal is outside the scope of the standard.
Section: 26.3.2.1 [lib.valarray.cons], 26.3.2.2 [lib.valarray.assign] Status: Open 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.
[Toronto: it is agreed that there is a defect. A full discussion, and an attempt at fixing the defect, should wait until we can hear from valarray experts.]
Section: 19.1 [lib.std.exceptions] Status: Open 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:
There may be no cure for (1) other than changing the interface to out_of_range, though one could reasonably argue that (1) is not a defect. Personally I don't care that much if out-of-memory is reported when I only have 20 bytes left, in the case when out_of_range would have been reported. People who use exception-specifications might care a lot, though.
There is a cure for (2), but it isn't completely obvious. I think a note for implementors should be made in the standard. Avoiding possible termination in this case shouldn't be left up to chance. The cure is to use a reference-counted "string" implementation in the exception object. I am not necessarily referring to a std::string here; any simple reference-counting scheme for a NTBS would do.
Further discussion, in email:
...I'm not so concerned about (1). After all, a library implementation can add const char* constructors as an extension, and users don't need to avail themselves of the standard exceptions, though this is a lame position to be forced into. FWIW, std::exception and std::bad_alloc don't require a temporary basic_string.
...I don't think the fixed-size buffer is a solution to the problem,
strictly speaking, because you can't satisfy the postcondition
strcmp(what(), what_arg.c_str()) == 0
For all values of what_arg (i.e. very long values). That means that
the only truly conforming solution requires a dynamic allocation.
Proposed resolution:
[Toronto: some LWG members thought this was merely a QoI issue, but most believed that it was at least a borderline defect. There was more support for nonnormative advice to implementors than for a normative change.]
Section: 20.1.5 [lib.allocator.requirements] Status: Open 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 Hinnant writes, in lib-7757:
I think I can prove that this is not provable by Table 32. And I agree it needs to be true except for the "and only if". If x1 != x2, I see no reason why it can't be true that Y(x1) == Y(x2). Admittedly I can't think of a practical instance where this would happen, or be valuable. But I also don't see a need to add that extra restriction. I think we only need:
if (x1 == x2) then Y(x1) == Y(x2)
If we decide that == on allocators is transitive, then I think I can prove the above. But I don't think == is necessarily transitive on allocators. That is:
Given x1 == x2 and x2 == x3, this does not mean x1 == x3.
Example:
x1 can deallocate pointers from: x1, x2, x3
x2 can deallocate pointers from: x1, x2, x4
x3 can deallocate pointers from: x1, x3
x4 can deallocate pointers from: x2, x4x1 == x2, and x2 == x4, but x1 != x4
Proposed resolution:
[Toronto: LWG members offered multiple opinions. One opinion is that it should not be required that x1 == x2 implies Y(x1) == Y(x2), and that it should not even be required that X(x1) == x1. Another opinion is that the second line from the bottom in table 32 already implies the desired property. This issue should be considered in light of other issues related to allocator instances.]
Section: 21.3.4 [lib.string.access] Status: Ready Submitter: Chris Newton Date: 27 Aug 2000
Paraphrased from a message that Chris Newton posted to comp.std.c++:
The standard's description of basic_string<>::operator[] seems to violate const correctness.
The standard (21.3.4/1) says that "If pos < size(), returns data()[pos]." The types don't work. The return value of data() is const charT*, but operator[] has a non-const version whose return type is reference.
Proposed resolution:
In section 21.3.4, paragraph 1, change "data()[pos]" to "*(begin() + pos)".
Section: 23.1.2 [lib.associative.reqmts] Status: Ready 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:
In Table 69, in section 23.1.2, change the complexity clause for insertion of a range from "N log(size() + N) (N is the distance from i to j) in general; linear if [i, j) is sorted according to value_comp()" to "N log(size() + N), where N is the distance from i to j".
[Copenhagen: Minor fix in proposed resolution: fixed unbalanced parens in the revised wording.]
Rationale:
Testing for valid insertions could be less efficient than simply inserting the elements when the range is not both sorted and between two adjacent existing elements; this could be a QOI issue.
The LWG considered two other options: (a) specifying that the complexity was linear if [i, j) is sorted according to value_comp() and between two adjacent existing elements; or (b) changing to Klog(size() + N) + (N - K) (N is the distance from i to j and K is the number of elements which do not insert immediately after the previous element from [i, j) including the first). The LWG felt that, since we can't guarantee linear time complexity whenever the range to be inserted is sorted, it's more trouble than it's worth to say that it's linear in some special cases.
Section: 18.6.2.1 [lib.bad.exception] Status: Ready Submitter: Martin Sebor Date: 24 Sep 2000
The synopsis for std::bad_exception lists the function ~bad_exception() but there is no description of what the function does (the Effects clause is missing).
Proposed resolution:
Remove the destructor from the class synopses of bad_alloc (18.4.2.1 [lib.bad.alloc]), bad_cast (18.5.2 [lib.bad.cast]), bad_typeid (18.5.3 [lib.bad.typeid]), and bad_exception (18.6.2.1 [lib.bad.exception]).
Rationale:
This is a general problem with the exception classes in clause 18. The proposed resolution is to remove the destructors from the class synopses, rather than to document the destructors' behavior, because removing them is more consistent with how exception classes are described in clause 19.
Section: D.7.1.3 [depr.strstreambuf.virtuals] Status: Ready Submitter: Martin Sebor Date: 5 Oct 2000
It appears that the interaction of the strstreambuf members overflow() and seekoff() can lead to undefined behavior in cases where defined behavior could reasonably be expected. The following program demonstrates this behavior:
#include <strstream>
int main ()
{
std::strstreambuf sb;
sb.sputc ('c');
sb.pubseekoff (-1, std::ios::end, std::ios::in);
return !('c' == sb.sgetc ());
}
D.7.1.1, p1 initializes strstreambuf with a call to basic_streambuf<>(), which in turn sets all pointers to 0 in 27.5.2.1, p1.
27.5.2.2.5, p1 says that basic_streambuf<>::sputc(c) calls overflow(traits::to_int_type(c)) if a write position isn't available (it isn't due to the above).
D.7.1.3, p3 says that strstreambuf::overflow(off, ..., ios::in) makes at least one write position available (i.e., it allows the function to make any positive number of write positions available).
D.7.1.3, p13 computes newoff = seekhigh - eback(). In D.7.1, p4 we see seekhigh = epptr() ? epptr() : egptr(), or seekhigh = epptr() in this case. newoff is then epptr() - eback().
D.7.1.4, p14 sets gptr() so that gptr() == eback() + newoff + off, or gptr() == epptr() + off holds.
If strstreambuf::overflow() made exactly one write position available then gptr() will be set to just before epptr(), and the program will return 0. Buf if the function made more than one write position available, epptr() and gptr() will both point past pptr() and the behavior of the program is undefined.
Proposed resolution:
Change the last sentence of D.7.1 [depr.strstreambuf] paragraph 4 from
Otherwise, seeklow equals gbeg and seekhigh is either pend, if pend is not a null pointer, or gend.
to become
Otherwise, seeklow equals gbeg and seekhigh is either gend if 0 == pptr(), or pbase() + max where max is the maximum value of pptr() - pbase() ever reached for this stream.
[ pre-Copenhagen: Dietmar provided wording for proposed resolution. ]
[ post-Copenhagen: Fixed a typo: proposed resolution said to fix 4.7.1, not D.7.1. ]
Rationale:
Note that this proposed resolution does not require an increase in the layout of strstreambuf to maintain max: If overflow() is implemented to make exactly one write position available, max == epptr() - pbase() always holds. However, if overflow() makes more than one write position available, the number of additional character (or some equivalent) has to be stored somewhere.
Section: 25.3.3 [lib.alg.binary.search] Status: Review Submitter: Matt Austern Date: 18 Oct 2000
Each of the four binary search algorithms (lower_bound, upper_bound, equal_range, binary_search) has a form that allows the user to pass a comparison function object. According to 25.3, paragraph 2, that comparison function object has to be a strict weak ordering.
This requirement is slightly too strict. Suppose we are searching through a sequence containing objects of type X, where X is some large record with an integer key. We might reasonably want to look up a record by key, in which case we would want to write something like this:
struct key_comp {
bool operator()(const X& x, int n) const {
return x.key() < n;
}
}
std::lower_bound(first, last, 47, key_comp());
key_comp is not a strict weak ordering, but there is no reason to prohibit its use in lower_bound.
There's no difficulty in implementing lower_bound so that it allows the use of something like key_comp. (It will probably work unless an implementor takes special pains to forbid it.) What's difficult is formulating language in the standard to specify what kind of comparison function is acceptable. We need a notion that's slightly more general than that of a strict weak ordering, one that can encompass a comparison function that involves different types. Expressing that notion may be complicated.
Additional questions raised at the Toronto meeting:
Additional discussion from Copenhagen:
Proposed resolution:
Change 25.3 [lib.alg.sorting] paragraph 3 from:
3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For the algorithms to work correctly, comp has to induce a strict weak ordering on the values.
to:
3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For algorithms not described in lib.alg.binary.search (25.3.3) to work correctly, comp has to induce a strict weak ordering on the values.
Add the following paragraph after 25.3 [lib.alg.sorting] paragraph 5:
-6- A sequence [start, finish) is partitioned with respect to an expression f(e) if there exists a non-negative integer n such that for all 0 <= i < distance(start, finish), f(*(begin+i)) is true if and only if i < n.
Change 25.3.3 [lib.alg.binary.search] paragraph 1 from:
-1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is in order according to the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.
to:
-1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is partitioned with respect to an expression formed by binding the search key to an argument of the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.
Change 25.3.3.1 [lib.lower.bound] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expression e < value or comp(e, value)
Remove 25.3.3.1 [lib.lower.bound] paragraph 2:
-2- Effects: Finds the first position into which value can be inserted without violating the ordering.
Change 25.3.3.2 [lib.upper.bound] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expression !(value < e) or !comp(value, e)
Remove 25.3.3.2 [lib.upper.bound] paragraph 2:
-2- Effects: Finds the furthermost position into which value can be inserted without violating the ordering.
Change 25.3.3.3 [lib.equal.range] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e).
Optionally add the following to the end of the proposed text above, which allows library implementors to make a small optimization at the cost of slightly complexifying the standard text. The idea is that we want to ensure that the partition point which defines the upper_bound is no earlier in the sequence than the partion point which defines the lower_bound, so that the implementor can do one of the searches over a subrange:
Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)
Note also that if we don't add the above, the result of equal_range() might be an invalid range.
Change 25.3.3.3 [lib.equal.range] paragraph 2 from:
-2- Effects: Finds the largest subrange [i, j) such that the value can be inserted at any iterator k in it without violating the ordering. k satisfies the corresponding conditions: !(*k < value) && !(value < *k) or comp(*k, value) == false && comp(value, *k) == false.
to:
-2- Returns:
make_pair(lower_bound(first, last, value),
upper_bound(first, last, value))
or
make_pair(lower_bound(first, last, value, comp),
upper_bound(first, last, value, comp))
Note that the original text did not say whether the first element of the return value was the beginning or end of the range, or something else altogether. The proposed text is both more precise and general enough to accomodate heterogeneous comparisons.
Change 25.3.3.3 [lib.binary.search] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)
[Dave Abrahams provided this wording]
Section: 27.6.1.5 [lib.iostreamclass] Status: Ready Submitter: Martin Sebor Date: 02 Nov 2000
Class template basic_iostream has no typedefs. The typedefs it inherits from its base classes can't be used, since (for example) basic_iostream<T>::traits_type is ambiguous.
Proposed resolution:
Add the following to basic_iostream's class synopsis in 27.6.1.5 [lib.iostreamclass], immediately after public:
// types: typedef charT char_type; typedef typename traits::int_type int_type; typedef typename traits::pos_type pos_type; typedef typename traits::off_type off_type; typedef traits traits_type;
Section: 27.4.4.3 [lib.iostate.flags] Status: Ready Submitter: Martin Sebor Date: 02 Nov 2000
27.4.4.3, p4 says about the postcondition of the function: If rdbuf()!=0 then state == rdstate(); otherwise rdstate()==state|ios_base::badbit.
The expression on the right-hand-side of the operator==() needs to be parenthesized in order for the whole expression to ever evaluate to anything but non-zero.
Proposed resolution:
Add parentheses like so: rdstate()==(state|ios_base::badbit).
Section: 27 [lib.input.output] Status: Ready Submitter: Martin Sebor Date: 02 Nov 2000
27.5.2.4.2, p4, and 27.8.1.6, p2, 27.8.1.7, p3, 27.8.1.9, p2, 27.8.1.10, p3 refer to in and/or out w/o ios_base:: qualification. That's incorrect since the names are members of a dependent base class (14.6.2 [temp.dep]) and thus not visible.
Proposed resolution:
Qualify the names with the name of the class of which they are members, i.e., ios_base.
Section: 20.1.5 [lib.allocator.requirements] Status: Review Submitter: Martin Sebor Date: 02 Nov 2000
I see that table 31 in 20.1.5, p3 allows T in std::allocator<T> to be of any type. But the synopsis in 20.4.1 calls for allocator<>::address() to be overloaded on reference and const_reference, which is ill-formed for all T = const U. In other words, this won't work:
template class std::allocator<const int>;
The obvious solution is to disallow specializations of allocators on const types. However, while containers' elements are required to be assignable (which rules out specializations on const T's), I think that allocators might perhaps be potentially useful for const values in other contexts. So if allocators are to allow const types a partial specialization of std::allocator<const T> would probably have to be provided.
Proposed resolution:
Change the text in row 1, column 2 of table 32 in 20.1.5, p3 from
any type
to
any non-const, non-volatile, non-reference type
Rationale:
Two resolutions were originally proposed: one that partially specialized std::allocator for const types, and one that said an allocator's value type may not be const. The LWG chose the second. The first wouldn't be appropriate, because allocators are intended for use by containers, and const value types don't work in containers. Encouraging the use of allocators with const value types would only lead to unsafe code.
The original text for proposed resolution 2 was modified so that it also forbids volatile types and reference types.
Section: 22.2.2.1.1 [lib.facet.num.get.members] Status: Ready Submitter: Matt Austern Date: 02 Nov 2000
In 22.2.2.1.1, we have a list of overloads for num_get<>::get(). There are eight overloads, all of which are identical except for the last parameter. The overloads are:
There is a similar list, in 22.2.2.1.2, of overloads for num_get<>::do_get(). In this list, the last parameter has the types:
These two lists are not identical. They should be, since get is supposed to call do_get with exactly the arguments it was given.
Proposed resolution:
In 22.2.2.1.1 [lib.facet.num.get.members], change
iter_type get(iter_type in, iter_type end, ios_base& str,
ios_base::iostate& err, short& val) const;
to
iter_type get(iter_type in, iter_type end, ios_base& str,
ios_base::iostate& err, float& val) const;
Section: 23.1 [lib.container.requirements] Status: Review Submitter: Peter Dimov Date: 07 Nov 2000
23.1/3 states that the objects stored in a container must be Assignable. 23.3.1 [lib.map], paragraph 2, states that map satisfies all requirements for a container, while in the same time defining value_type as pair<const Key, T> - a type that is not Assignable.
It should be noted that there exists a valid and non-contradictory interpretation of the current text. The wording in 23.1/3 avoids mentioning value_type, referring instead to "objects stored in a container." One might argue that map does not store objects of type map::value_type, but of map::mapped_type instead, and that the Assignable requirement applies to map::mapped_type, not map::value_type.
However, this makes map a special case (other containers store objects of type value_type) and the Assignable requirement is needlessly restrictive in general.
For example, the proposed resolution of active library issue 103 is to make set::iterator a constant iterator; this means that no set operations can exploit the fact that the stored objects are Assignable.
This is related to, but slightly broader than, closed issue 140.
Proposed resolution:
23.1/3: Strike the trailing part of the sentence:
, and the additional requirements of Assignable types from 23.1/3
so that it reads:
-3- The type of objects stored in these components must meet the requirements of CopyConstructible types (lib.copyconstructible).
23.1/4: Modify to make clear that this requirement is not for all containers. Change to:
-4- Table 64 defines the Assignable requirement. Some containers require this property of the types to be stored in the container. T is the type used to instantiate the container. t is a value of T, and u is a value of (possibly const) T.
23.1, Table 65: in the first row, change "T is Assignable" to "T is CopyConstructible".
23.2.1/2: Add sentence for Assignable requirement. Change to:
-2- A deque satisfies all of the requirements of a container and of a reversible container (given in tables in lib.container.requirements) and of a sequence, including the optional sequence requirements (lib.sequence.reqmts). In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on deque that are not described in one of these tables or for operations where there is additional semantic information.
23.2.2/2: Add Assignable requirement to specific methods of list. Change to:
-2- A list satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the the optional sequence requirements (lib.sequence.reqmts). The exceptions are the operator[] and at member functions, which are not provided. [Footnote: These member functions are only provided by containers whose iterators are random access iterators. --- end foonote]
list does not require the stored type T to be Assignable unless the following methods are instantiated: [Footnote: Implementors are permitted but not required to take advantage of T's Assignable properties for these methods. -- end foonote]
list<T,Allocator>& operator=(const list<T,Allocator>& x ); template <class InputIterator> void assign(InputIterator first, InputIterator last); void assign(size_type n, const T& t);Descriptions are provided here only for operations on list that are not described in one of these tables or for operations where there is additional semantic information.
23.2.4/2: Add sentence for Assignable requirement. Change to:
-2- A vector satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the optional sequence requirements (lib.sequence.reqmts). The exceptions are the push_front and pop_front member functions, which are not provided. In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on vector that are not described in one of these tables or for operations where there is additional semantic information.
Rationale:
list, set, multiset, map, multimap are able to store non-Assignables. However, there is some concern about list<T>: although in general there's no reason for T to be Assignable, some implementations of the member functions operator= and assign do rely on that requirement. The LWG does not want to forbid such implementations.
Note that the type stored in a standard container must still satisfy the requirements of the container's allocator; this rules out, for example, such types as "const int". See issue 274 for more details.
Section: 23.2.2.4 [lib.list.ops] Status: Review Submitter: P.J. Plauger Date: 27 Nov 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.
But what does the C++ Standard mean by "invalidate"? You can still dereference the iterator to a spliced list element, but you'd better not use it to delimit a range within the original list. For the latter operation, it has definitely lost some of its validity.
If we accept the proposed resolution to issue 250, then we'd better clarify that a "valid" iterator need no longer designate an element within the same container as it once did. We then have to clarify what we mean by invalidating a past-the-end iterator, as when a vector or string grows by reallocation. Clearly, such an iterator has a different kind of validity. Perhaps we should introduce separate terms for the two kinds of "validity."
Proposed resolution:
Add the following text to the end of section 24.1 [lib.iterator.requirements], after paragraph 5:
Invalidating an iterator means modifying it such that it may have a singular value. [Footnote: This definition applies to pointers, since pointers are iterators. The effect of dereferencing an iterator that has been invalidated is undefined.]
[post-Copenhagen: Matt provided wording.]
Section: 24.4.1 [lib.reverse.iterators] Status: Open Submitter: Steve Cleary Date: 27 Nov 2000
This came from an email from Steve Cleary to Fergus in reference to issue 179. The library working group briefly discussed this in Toronto and believed it should be a separate issue. There was also some reservations about whether this was a worthwhile problem to fix.
Steve said: "Fixing reverse_iterator. std::reverse_iterator can (and should) be changed to preserve these additional requirements." He also said in email that it can be done without breaking user's code: "If you take a look at my suggested solution, reverse_iterator doesn't have to take two parameters; there is no danger of breaking existing code, except someone taking the address of one of the reverse_iterator global operator functions, and I have to doubt if anyone has ever done that. . . But, just in case they have, you can leave the old global functions in as well -- they won't interfere with the two-template-argument functions. With that, I don't see how any user code could break."
Proposed resolution:
Section: 24.4.1.1 [lib.reverse.iterator] add/change the following declarations:
A) Add a templated assignment operator, after the same manner
as the templated copy constructor, i.e.:
template < class U >
reverse_iterator < Iterator >& operator=(const reverse_iterator< U >& u);
B) Make all global functions (except the operator+) have
two template parameters instead of one, that is, for
operator ==, !=, <, >, <=, >=, - replace:
template < class Iterator >
typename reverse_iterator< Iterator >::difference_type operator-(
const reverse_iterator< Iterator >& x,
const reverse_iterator< Iterator >& y);
with:
template < class Iterator1, class Iterator2 >
typename reverse_iterator < Iterator1 >::difference_type operator-(
const reverse_iterator < Iterator1 > & x,
const reverse_iterator < Iterator2 > & y);
Also make the addition/changes for these signatures in 24.4.1.3 [lib.reverse.iter.ops].
[ Copenhagen: The LWG is concerned that the proposed resolution introduces new overloads. Experience shows that introducing overloads is always risky, and that it would be inappropriate to make this change without implementation experience. It may be desirable to provide this feature in a different way. ]
Section: 25.3.7 [lib.alg.min.max] Status: Ready Submitter: Martin Sebor Date: 02 Dec 2000
The requirements in 25.3.7, p1 and 4 call for T to satisfy the requirements of LessThanComparable (20.1.2 [lib.lessthancomparable]) and CopyConstructible (20.1.3 [lib.copyconstructible]). Since the functions take and return their arguments and result by const reference, I believe the CopyConstructible requirement is unnecessary.
Proposed resolution:
Remove the CopyConstructible requirement. Specifically, replace 25.3.7, p1 with
-1- Requires: Type T is LessThanComparable (20.1.2 [lib.lessthancomparable]).
and replace 25.3.7, p4 with
-4- Requires: Type T is LessThanComparable (20.1.2 [lib.lessthancomparable]).
Section: 22.2.2.2.2 [lib.facet.num.put.virtuals] Status: Open Submitter: Howard Hinnant Date: 5 Dec 2000
Paragraph 16 mistakenly singles out integral types for inserting thousands_sep() characters. This conflicts with the syntax for floating point numbers described under 22.2.3.1/2.
Proposed resolution:
Change paragraph 16 from:
For integral types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 22.2.3.1.2 [lib.facet.numpunct.virtuals].
To:
For arithmetic types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 22.2.3.1.2 [lib.facet.numpunct.virtuals].
[ Copenhagen: Opinions were divided about whether this is actually an inconsistency, but at best it seems to have been unintentional. This is only an issue for floating-point output: The standard is unambiguous that implementations must parse thousands_sep characters when performing floating-point. The standard is also unambiguous that this requirement does not apply to the "C" locale. ]
[ A survey of existing practice is needed; it is believed that some implementations do insert thousands_sep characters for floating-point output and others doing. ]
Section: 25.2.4 [lib.alg.replace] Status: Open Submitter: Martin Sebor Date: 15 Dec 2000
The requirements in 25.2.4 [lib.alg.replace], p1 that T to be Assignable (23.1 [lib.container.requirements]) is not necessary or sufficient for either of the algorithms. The algorithms require that std::iterator_traits<ForwardIterator>::value_type be Assignable and that both std::iterator_traits<ForwardIterator>::value_type and be EqualityComparable (20.1.1 [lib.equalitycomparable]) with respect to one another.
Note that a similar problem occurs in several other places in section 25 as well (e.g., 25.1.6 [lib.alg.count], or 25.2.5 [lib.alg.fill]) so what really needs to happen is for all those places to be identified and corrected. The proposed resolution below addresses only 25.2.4.
Proposed resolution:
Change 25.2.4, p1 from
-1- Requires:Type T is Assignable (23.1 [lib.container.requirements]) (and, for replace(), EqualityComparable (20.1.1 [lib.equalitycomparable])).
to
-1- Requires:Type std::iterator_traits<ForwardIterator>::value_type is Assignable (23.1 [lib.container.requirements]), the type T is convertible tostd::iterator_traits<ForwardIterator>::value_type, (and, for replace(), types std::iterator_traits<ForwardIterator>::value_type and T are EqualityComparable (20.1.1 [lib.equalitycomparable]) with respect to one another).
[ The LWG agrees with the general idea of the proposed resolution, but not with the specific wording. (There is no definition in the standard of what it means for one type to be EqualityComparable to another.) Jeremy will provide new wording, and will review clause 25 for similar issues. ]
Section: 20.3.7 [lib.function.pointer.adaptors] Status: Ready Submitter: Martin Sebor Date: 26 Dec 2000
The example in 20.3.7 [lib.function.pointer.adaptors], p6 shows how to use the C library function strcmp() with the function pointer adapter ptr_fun(). But since it's unspecified whether the C library functions have extern "C" or extern "C++" linkage [17.4.2.2 [lib.using.linkage]], and since function pointers with different the language linkage specifications (7.5 [dcl.link]) are incompatible, whether this example is well-formed is unspecified.
Proposed resolution:
Replace the code snippet in the following text
-6- [Example:
replace_if(v.begin(), v.end(), not1(bind2nd(ptr_fun(strcmp), "C")), "C++");
with
-6- [Example:
int compare(const char*, const char*); replace_if(v.begin(), v.end(), not1(bind2nd(ptr_fun(compare), "abc")), "def");
[Copenhagen: Minor change in the proposed resolution. Since this issue deals in part with C and C++ linkage, it was believed to be too confusing for the strings in the example to be "C" and "C++". ]
Section: 27.8.1.6 [lib.ifstream.cons] Status: Ready Submitter: Martin Sebor Date: 31 Dec 2000
27.8.1.6 [lib.ifstream.cons], p2, 27.8.1.9 [lib.ofstream.cons], p2, and 27.8.1.12 [lib.fstream.cons], p2 say about the effects of each constructor:
... If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure).
The parenthetical note doesn't apply since the ctors cannot throw an exception due to the requirement in 27.4.4.1 [lib.basic.ios.cons], p3 that exceptions() be initialized to ios_base::goodbit.
Proposed resolution:
Strike the parenthetical note from the Effects clause in each of the paragraphs mentioned above.
Section: 25.4 [lib.alg.c.library] Status: Ready Submitter: Judy Ward Date: 30 Dec 2000
The <cstdlib> header file contains prototypes for bsearch and qsort (C++ Standard section 25.4 paragraphs 3 and 4) and other prototypes (C++ Standard section 21.4 paragraph 1 table 49) that require the typedef size_t. Yet size_t is not listed in the <cstdlib> synopsis table 78 in section 25.4.
Proposed resolution:
Add the type size_t to Table 78 (section 25.4) and add the type size_t <cstdlib> to Table 97 (section C.2).
Rationale:
Since size_t is in <stdlib.h>, it must also be in <cstdlib>.
Section: 19.3 [lib.errno] Status: Ready Submitter: Judy Ward Date: 30 Dec 2000
ISO/IEC 9899:1990/Amendment1:1994 Section 4.3 States: "The list of macros defined in <errno.h> is adjusted to include a new macro, EILSEQ"
ISO/IEC 14882:1998(E) section 19.3 does not refer to the above amendment.
Proposed resolution:
Update Table 26 (section 19.3) "Header <cerrno> synopsis" and Table 95 (section C.2) "Standard Macros" to include EILSEQ.
Section: 25.1.1 [lib.alg.foreach] Status: Open Submitter: Angelika Langer Date: 03 Jan 2001
The specification of the for_each algorithm does not have a "Requires" section, which means that there are no restrictions imposed on the function object whatsoever. In essence it means that I can provide any function object with arbitrary side effects and I can still expect a predictable result. In particular I can expect that the function object is applied exactly last - first times, which is promised in the "Complexity" section.
I don't see how any implementation can give such a guarantee without imposing requirements on the function object.
Just as an example: consider a function object that removes elements from the input sequence. In that case, what does the complexity guarantee (applies f exactly last - first times) mean?
One can argue that this is obviously a nonsensical application and a theoretical case, which unfortunately it isn't. I have seen programmers shooting themselves in the foot this way, and they did not understand that there are restrictions even if the description of the algorithm does not say so.
Proposed resolution:
Add a "Requires" section to section 25.1.1 similar to those proposed for transform and the numeric algorithms (see issue 242):
-2- Requires: In the range [first, last], f shall not invalidate iterators or subranges.
[Copenhagen: The LWG agrees that a function object passed to an algorithm should not invalidate iterators in the range that the algorithm is operating on. The LWG believes that this should be a blanket statement in Clause 25, not just a special requirement for for_each. ]
Section: 25.3.5 [lib.alg.set.operations] Status: Open Submitter: Matt Austern Date: 03 Jan 2001
The standard library contains four algorithms that compute set operations on sorted ranges: set_union, set_intersection, set_difference, and set_symmetric_difference. Each of these algorithms takes two sorted ranges as inputs, and writes the output of the appropriate set operation to an output range. The elements in the output range are sorted.
The ordinary mathematical definitions are generalized so that they apply to ranges containing multiple copies of a given element. Two elements are considered to be "the same" if, according to an ordering relation provided by the user, neither one is less than the other. So, for example, if one input range contains five copies of an element and another contains three, the output range of set_union will contain five copies, the output range of set_intersection will contain three, the output range of set_difference will contain two, and the output range of set_symmetric_difference will contain two.
Because two elements can be "the same" for the purposes of these set algorithms, without being identical in other respects (consider, for example, strings under case-insensitive comparison), this raises a number of unanswered questions:
The standard should either answer these questions, or explicitly say that the answers are unspecified. I prefer the former option, since, as far as I know, all existing implementations behave the same way.
Proposed resolution:
[The LWG agrees that the standard should answer these questions. Matt will provide wording.]
Section: 27.4.4.2 [lib.basic.ios.members] Status: Ready Submitter: Martin Sebor Date: 05 Jan 2001
The Effects clause of the member function copyfmt() in 27.4.4.2, p15 doesn't consider the case where the left-hand side argument is identical to the argument on the right-hand side, that is (this == &rhs). If the two arguments are identical there is no need to copy any of the data members or call any callbacks registered with register_callback(). Also, as Howard Hinnant points out in message c++std-lib-8149 it appears to be incorrect to allow the object to fire erase_event followed by copyfmt_event since the callback handling the latter event may inadvertently attempt to access memory freed by the former.
Proposed resolution:
Change the Effects clause in 27.4.4.2, p15 from
-15- Effects:Assigns to the member objects of *this the corresponding member objects of rhs, except that...
to
-15- Effects:If (this == &rhs) does nothing. Otherwise assigns to the member objects of *this the corresponding member objects of rhs, except that...
Section: 17.4.3.1.1 [lib.macro.names] Status: Open Submitter: James Kanze Date: 11 Jan 2001
Paragraph 2 of 17.4.3.1.1 [lib.macro.names] reads: "A translation unit that includes a header shall not contain any macros that define names declared in that header." As I read this, it would mean that the following program is legal:
#define npos 3.14 #include <sstream>
since npos is not defined in <sstream>. It is, however, defined in <string>, and it is hard to imagine an implementation in which <sstream> didn't include <string>.
I think that this phrase was probably formulated before it was decided that a standard header may freely include other standard headers. The phrase would be perfectly appropriate for C, for example. In light of 17.4.4.1 [lib.res.on.headers] paragraph 1, however, it isn't stringent enough.
Proposed resolution:
In paragraph 2 of 17.4.3.1.1 [lib.macro.names], change "A translation unit that includes a header shall not contain any macros that define names declared in that header." to "A translation unit that includes a header shall not contain any macros that define names declared in any standard header."
[Copenhagen: the general idea is clearly correct, but there is concern about making sure that the two paragraphs in 17.4.3.1.1 [lib.macro.names] remain consistent. Nathan will provide new wording.]
Section: 26.5 [lib.c.math] Status: Ready Submitter: Jens Maurer Date: 12 Jan 2001
Table 80 lists the contents of the <cmath> header. It does not list abs(). However, 26.5, paragraph 6, which lists added signatures present in <cmath>, does say that several overloads of abs() should be defined in <cmath>.
Proposed resolution:
Add abs to Table 80. Also, remove the parenthetical list of functions "(abs(), div(), rand(), srand())" from 26.5 [lib.c.math], paragraph 1.
[Copenhagen: Modified proposed resolution so that it also gets rid of that vestigial list of functions in paragraph 1.]
Section: 20.2.2 [lib.pairs] Status: Review Submitter: Martin Sebor Date: 14 Jan 2001
The synopsis of the header <utility> in 20.2 [lib.utility] lists the complete set of equality and relational operators for pair but the section describing the template and the operators only describes operator==() and operator<(), and it fails to mention any requirements on the template arguments. The remaining operators are not mentioned at all.
Proposed resolution:
Add the following after 20.2.2 [lib.pairs], paragraph 5:
template <class T1, class T2>
bool operator!=(const pair<T1, T2>& x, const pair<T1, T2>& y);Requires: Types T1 and T2 are EqualityComparable (20.1.1 [lib.equalitycomparable]).
Returns: !(x == y).
Add the following after 20.2.2 [lib.pairs], paragraph 6:
template <class T1, class T2>
bool operator>(const pair<T1, T2>& x, const pair<T1, T2>& y);Requires: Types T1 and T2 are LessThanComparable (20.1.2 [lib.lessthancomparable]).
Returns: y < x.
template <class T1, class T2>
bool operator<=(const pair<T1, T2>& x, const pair<T1, T2>& y);Requires: Types T1 and T2 are LessThanComparable (20.1.2 [lib.lessthancomparable]).
Returns: !(y < x).
template <class T1, class T2>
bool operator>=(const pair<T1, T2>& x, const pair<T1, T2>& y);Requires: Types T1 and T2 are LessThanComparable (20.1.2 [lib.lessthancomparable]).
Returns: !(x < y).
[post-Copenhagen: modified proposed resolution so that it does not create a new section 20.2.2.1. That would violate ISO rules: we cannot have 20.2.2.1 unless we also have 20.2.2.2.]
Section: 20.3.8 [lib.member.pointer.adaptors] Status: Ready Submitter: Martin Sebor Date: 6 Jan 2001
The class templates const_mem_fun_t in 20.3.8, p8 and const_mem_fun1_t in 20.3.8, p9 derive from unary_function<T*, S>, and binary_function<T*, A, S>, respectively. Consequently, their argument_type, and first_argument_type members, respectively, are both defined to be T* (non-const). However, their function call member operator takes a const T* argument. It is my opinion that argument_type should be const T* instead, so that one can easily refer to it in generic code. The example below derived from existing code fails to compile due to the discrepancy:
template <class T>
void foo (typename T::argument_type arg) // #1
{
typename T::result_type (T::*pf) (typename
T::argument_type)
const = // #2
&T::operator();
}
struct X { /* ... */ };
int main ()
{
const X x;
foo<std::const_mem_fun_t<void, X>
>(&x);
// #3
}
#1 foo() takes a plain unqualified X* as an argument
#2 the type of the pointer is incompatible with the type of the member
function
#3 the address of a constant being passed to a function taking a non-const
X*
Proposed resolution:
Replace the top portion of the definition of the class template const_mem_fun_t in 20.3.8, p8
template <class S, class T> class const_mem_fun_t
: public
unary_function<T*, S> {
with
template <class S, class T> class const_mem_fun_t
: public
unary_function<const T*, S> {
Also replace the top portion of the definition of the class template const_mem_fun1_t in 20.3.8, p9
template <class S, class T, class A> class const_mem_fun1_t
: public
binary_function<T*, A, S> {
with
template <class S, class T, class A> class const_mem_fun1_t
: public
binary_function<const T*, A, S> {
Rationale:
This is simply a contradiction: the argument_type typedef, and the argument type itself, are not the same.
Section: 18.4.1.2 [lib.new.delete.array] Status: Ready Submitter: John A. Pedretti Date: 10 Jan 2001
The default behavior of operator delete[] described in 18.4.1.2, p12 - namely that for non-null value of ptr, the operator reclaims storage allocated by the earlier call to the default operator new[] - is not correct in all cases. Since the specified operator new[] default behavior is to call operator new (18.4.1.2, p4, p8), which can be replaced, along with operator delete, by the user, to implement their own memory management, the specified default behavior of operator delete[] must be to call operator delete.
Proposed resolution:
Change 18.4.1.2, p12 from
-12- Default behavior:
- For a null value of ptr , does nothing.
- Any other value of ptr shall be a value returned earlier by a call to the default operator new[](std::size_t). [Footnote: The value must not have been invalidated by an intervening call to operator delete[](void*) (17.4.3.7 [lib.res.on.arguments]). --- end footnote] For such a non-null value of ptr , reclaims storage allocated by the earlier call to the default operator new[].
to
-12- Default behavior: Calls operator delete(ptr) or operator delete(ptr, std::nothrow) respectively.
and expunge paragraph 13.
Section: 24.1.4 [lib.bidirectional.iterators], 24.1.5 [lib.random.access.iterators] Status: Open Submitter: John Potter Date: 22 Jan 2001
In section 24.1.4 [lib.bidirectional.iterators], Table 75 gives the return type of *r-- as convertible to T. This is not consistent with Table 74 which gives the return type of *r++ as T&. *r++ = t is valid while *r-- = t is invalid.
In section 24.1.5 [lib.random.access.iterators], Table 76 gives the return type of a[n] as convertible to T. This is not consistent with the semantics of *(a + n) which returns T& by Table 74. *(a + n) = t is valid while a[n] = t is invalid.
Discussion from the Copenhagen meeting: the first part is uncontroversial. The second part, operator[] for Random Access Iterators, requires more thought. There are reasonable arguments on both sides. Return by value from operator[] enables some potentially useful iterators, e.g. a random access "iota iterator" (a.k.a "counting iterator" or "int iterator"). There isn't any obvious way to do this with return-by-reference, since the reference would be to a temporary. On the other hand, reverse_iterator takes an arbitrary Random Access Iterator as template argument, and its operator[] returns by reference. If we decided that the return type in Table 76 was correct, we would have to change reverse_iterator. This change would probably affect user code.
History: the contradiction between reverse_iterator and the Random Access Iterator requirements has been present from an early stage. In both the STL proposal adopted by the committee (N0527==94-0140) and the STL technical report (HPL-95-11 (R.1), by Stepanov and Lee), the Random Access Iterator requirements say that operator[]'s return value is "convertible to T". In N0527 reverse_iterator's operator[] returns by value, but in HPL-95-11 (R.1), and in the STL implementation that HP released to the public, reverse_iterator's operator[] returns by reference. In 1995, the standard was amended to reflect the contents of HPL-95-11 (R.1). The original intent for operator[] is unclear.
In the long term it may be desirable to add more fine-grained iterator requirements, so that access method and traversal strategy can be decoupled. (See "Improved Iterator Categories and Requirements", N1297 = 01-0011, by Jeremy Siek.) Any decisions about issue 299 should keep this possibility in mind.
Proposed resolution:
In section 24.1.4 [lib.bidirectional.iterators], change the return type in table 75 from "convertible to T" to T&.
In section 24.1.5 [lib.random.access.iterators], change the return type in table 76 from "convertible to T" to T&.
Section: 23.2.2.4 [lib.list.ops] Status: Open Submitter: John Pedretti Date: 23 Jan 2001
The "Effects" clause for list::merge() (23.2.2.4, p23) appears to be incomplete: it doesn't cover the case where the argument list is identical to *this (i.e., this == &x). The requirement in the note in p24 (below) is that x be empty after the merge which is surely unintended in this case.
Proposed resolution:
Change 23.2.2.4, p23 to:
Effects: If &x == this, does nothing; otherwise, merges the argument list into the list.
[Copenhagen: The proposed resolution does not fix all of the problems in 23.2.2.4 [lib.list.ops], p22-25. Three different paragraphs (23, 24, 25) describe the effects of merge. Changing p23, without changing the other two, appears to introduce contradictions. Additionally, "merges the argument list into the list" is excessively vague.]
Section: 21.3.1 [lib.string.cons] Status: Ready Submitter: Martin Sebor Date: 27 Jan 2001
The effects clause for the basic_string template ctor in 21.3.1, p15 leaves out the third argument of type Allocator. I believe this to be a mistake.
Proposed resolution:
Replace
-15- Effects: If InputIterator is an integral type, equivalent to
basic_string(static_cast<size_type>(begin), static_cast<value_type>(end))
with
-15- Effects: If InputIterator is an integral type, equivalent to
basic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)
Section: 23.3.5.3 [lib.bitset.operators] Status: Ready Submitter: Matt Austern Date: 5 Feb 2001
In 23.3.5.3, we are told that bitset's input operator "Extracts up to N (single-byte) characters from is.", where is is a stream of type basic_istream<charT, traits>.
The standard does not say what it means to extract single byte characters from a stream whose character type, charT, is in general not a single-byte character type. Existing implementations differ.
A reasonable solution will probably involve widen() and/or narrow(), since they are the supplied mechanism for a single character between char and arbitrary charT.
Narrowing the input characters is not the same as widening the literals '0' and '1', because there may be some locales in which more than one wide character maps to the narrow character '0'. Narrowing means that alternate representations may be used for bitset input, widening means that they may not be.
Note that for numeric input, num_get<> (22.2.2.1.2/8) compares input characters to widened version of narrow character literals.
From Pete Becker, in c++std-lib-8224:
Different writing systems can have different representations for the digits that represent 0 and 1. For example, in the Unicode representation of the Devanagari script (used in many of the Indic languages) the digit 0 is 0x0966, and the digit 1 is 0x0967. Calling narrow would translate those into '0' and '1'. But Unicode also provides the ASCII values 0x0030 and 0x0031 for for the Latin representations of '0' and '1', as well as code points for the same numeric values in several other scripts (Tamil has no character for 0, but does have the digits 1-9), and any of these values would also be narrowed to '0' and '1'.
...
It's fairly common to intermix both native and Latin representations of numbers in a document. So I think the rule has to be that if a wide character represents a digit whose value is 0 then the bit should be cleared; if it represents a digit whose value is 1 then the bit should be set; otherwise throw an exception. So in a Devanagari locale, both 0x0966 and 0x0030 would clear the bit, and both 0x0967 and 0x0031 would set it. Widen can't do that. It would pick one of those two values, and exclude the other one.
From Jens Maurer, in c++std-lib-8233:
Whatever we decide, I would find it most surprising if bitset conversion worked differently from int conversion with regard to alternate local representations of numbers.
Thus, I think the options are:
- Have a new defect issue for 22.2.2.1.2/8 so that it will require the use of narrow().
- Have a defect issue for bitset() which describes clearly that widen() is to be used.
Proposed resolution:
Replace the first two sentences of paragraph 5 with:
Extracts up to N characters from is. Stores these characters in a temporary object str of type basic_string<charT, traits>, then evaluates the expression x = bitset<N>(str).
Replace the third bullet item in paragraph 5 with:
Rationale:
Input for bitset should work the same way as numeric input. Using widen does mean that alternative digit representations will not be recognized, but this was a known consequence of the design choice.
Section: 24.1 [lib.iterator.requirements] Status: Open Submitter: Dave Abrahams Date: 5 Feb 2001
We all "know" that input iterators are allowed to produce values when dereferenced of which there is no other in-memory copy.
But: Table 72, with a careful reading, seems to imply that this can only be the case if the value_type has no members (e.g. is a built-in type).
The problem occurs in the following entry:
a->m pre: (*a).m is well-defined
Equivalent to (*a).m
*a.m can be well-defined if *a is not a reference type, but since operator->() must return a pointer for a->m to be well-formed, it needs something to return a pointer to. This seems to indicate that *a must be buffered somewhere to make a legal input iterator.
I don't think this was intentional.
Proposed resolution:
[Copenhagen: the two obvious possibilities are to keep the operator-> requirement for Input Iterators, and put in a non-normative note describing how it can be implemented with proxies, or else moving the operator-> requirement from Input Iterator to Forward Iterator. If we do the former we'll also have to change istreambuf_iterator, because it has no operator->. A straw poll showed roughly equal support for the two options.]
Section: 22.2.1.5.2 [lib.locale.codecvt.virtuals] Status: Review Submitter: Howard Hinnant Date: 24 Jan 2001
22.2.1.5/3 introduces codecvt in part with:
codecvt<wchar_t,char,mbstate_t> converts between the native character sets for tiny and wide characters. Instantiations on mbstate_t perform conversion between encodings known to the library implementor.
But 22.2.1.5.2/10 describes do_length in part with:
... codecvt<wchar_t, char, mbstate_t> ... return(s) the lesser of max and (from_end-from).
The semantics of do_in and do_length are linked. What one does must be consistent with what the other does. 22.2.1.5/3 leads me to believe that the vendor is allowed to choose the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in performs so that it makes his customers happy on a given platform. But 22.2.1.5.2/10 explicitly says what codecvt<wchar_t,char,mbstate_t>::do_length must return. And thus indirectly specifies the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in must perform. I believe that this is not what was intended and is a defect.
Discussion from the -lib reflector:
This proposal would have the effect of making the semantics of
all of the virtual functions in codecvt<wchar_t, char,
mbstate_t> implementation specified. Is that what we want, or
do we want to mandate specific behavior for the base class virtuals
and leave the implementation specified behavior for the codecvt_byname
derived class? The tradeoff is that former allows implementors to
write a base class that actually does something useful, while the
latter gives users a way to get known and specified---albeit
useless---behavior, and is consistent with the way the standard
handles other facets. It is not clear what the original intention
was.
Nathan has suggest a compromise: a character that is a widened version of the characters in the basic execution character set must be converted to a one-byte sequence, but there is no such requirement for characters that are not part of the basic execution character set.
Proposed resolution:
Change 22.2.1.5.2/10 from:
-10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiations required in Table 51 (21.1.1.1.1), namely codecvt<wchar_t, char, mbstate_t> and codecvt<char, char, mbstate_t>, return the lesser of max and (from_end-from).
to:
-10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiation codecvt<char, char, mbstate_t> returns the lesser of max and (from_end-from).
[Copenhagen: straw poll was 3-1 in favor, with many abstentions. Nathan would like to see more guarantees than are in the proposed resolution. He will discuss this issue with the other people who care about it.]
Section: 18.1 [lib.support.types] Status: Ready Submitter: Steve Clamage Date: 21 Feb 2001
Spliced together from reflector messages c++std-lib-8294 and -8295:
18.1, paragraph 5, reads: "The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9). The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."
For the POD requirement, it doesn't say "no diagnostic required" or "undefined behavior". I read 1.4 [intro.compliance], paragraph 1, to mean that a diagnostic is required. It's not clear whether this requirement was intended. While it's possible to provide such a diagnostic, the extra complication doesn't seem to add any value.
Proposed resolution:
Change 18.1, paragraph 5, to "If type is not a POD structure or a POD union the results are undefined."
[Copenhagen: straw poll was 7-4 in favor. It was generally agreed that requiring a diagnostic was inadvertent, but some LWG members thought that diagnostics should be required whenever possible.]
Section: 23.2.3 [lib.container.adaptors] Status: Ready Submitter: Howard Hinnant Date: 13 Mar 2001
From reflector message c++std-lib-8330. See also lib-8317.
The standard is currently inconsistent in 23.2.3.2 [lib.priority.queue] paragraph 1 and 23.2.3.3 [lib.stack] paragraph 1. 23.2.3.3/1, for example, says:
-1- Any sequence supporting operations back(), push_back() and pop_back() can be used to instantiate stack. In particular, vector (lib.vector), list (lib.list) and deque (lib.deque) can be used.
But this is false: vector<bool> can not be used, because the container adaptors return a T& rather than using the underlying container's reference type.
This is a contradiction that can be fixed by:
I propose 3. This does not preclude option 2 if we choose to do it later (see issue 96); the issues are independent. Option 3 offers a small step towards support for proxied containers. This small step fixes a current contradiction, is easy for vendors to implement, is already implemented in at least one popular lib, and does not break any code.
Proposed resolution:
Summary: Add reference and const_reference typedefs to queue, priority_queue and stack. Change return types of "value_type&" to "reference". Change return types of "const value_type&" to "const_reference". Details:
Change 23.2.3.1/1 from:
namespace std {
template <class T, class Container = deque<T> >
class queue {
public:
typedef typename Container::value_type value_type;
typedef typename Container::size_type size_type;
typedef Container container_type;
protected:
Container c;
public:
explicit queue(const Container& = Container());
bool empty() const { return c.empty(); }
size_type size() const { return c.size(); }
value_type& front() { return c.front(); }
const value_type& front() const { return c.front(); }
value_type& back() { return c.back(); }
const value_type& back() const { return c.back(); }
void push(const value_type& x) { c.push_back(x); }
void pop() { c.pop_front(); }
};
to:
namespace std {
template <class T, class Container = deque<T> >
class queue {
public:
typedef typename Container::value_type value_type;
typedef typename Container::reference reference;
typedef typename Container::const_reference const_reference;
typedef typename Container::value_type value_type;
typedef typename Container::size_type size_type;
typedef Container container_type;
protected:
Container c;
public:
explicit queue(const Container& = Container());
bool empty() const { return c.empty(); }
size_type size() const { return c.size(); }
reference front() { return c.front(); }
const_reference front() const { return c.front(); }
reference back() { return c.back(); }
const_reference back() const { return c.back(); }
void push(const value_type& x) { c.push_back(x); }
void pop() { c.pop_front(); }
};
Change 23.2.3.2/1 from:
namespace std {
template <class T, class Container = vector<T>,
class Compare = less<typename Container::value_type> >
class priority_queue {
public:
typedef typename Container::value_type value_type;
typedef typename Container::size_type size_type;
typedef Container container_type;
protected:
Container c;
Compare comp;
public:
explicit priority_queue(const Compare& x = Compare(),
const Container& = Container());
template <class InputIterator>
priority_queue(InputIterator first, InputIterator last,
const Compare& x = Compare(),
const Container& = Container());
bool empty() const { return c.empty(); }
size_type size() const { return c.size(); }
const value_type& top() const { return c.front(); }
void push(const value_type& x);
void pop();
};
// no equality is provided
}
to:
namespace std {
template <class T, class Container = vector<T>,
class Compare = less<typename Container::value_type> >
class priority_queue {
public:
typedef typename Container::value_type value_type;
typedef typename Container::reference reference;
typedef typename Container::const_reference const_reference;
typedef typename Container::size_type size_type;
typedef Container container_type;
protected:
Container c;
Compare comp;
public:
explicit priority_queue(const Compare& x = Compare(),
const Container& = Container());
template <class InputIterator>
priority_queue(InputIterator first, InputIterator last,
const Compare& x = Compare(),
const Container& = Container());
bool empty() const { return c.empty(); }
size_type size() const { return c.size(); }
const_reference top() const { return c.front(); }
void push(const value_type& x);
void pop();
};
// no equality is provided
}
And change 23.2.3.3/1 from:
namespace std {
template <class T, class Container = deque<T> >
class stack {
public:
typedef typename Container::value_type value_type;
typedef typename Container::size_type size_type;
typedef Container container_type;
protected:
Container c;
public:
explicit stack(const Container& = Container());
bool empty() const { return c.empty(); }
size_type size() const { return c.size(); }
value_type& top() { return c.back(); }
const value_type& top() const { return c.back(); }
void push(const value_type& x) { c.push_back(x); }
void pop() { c.pop_back(); }
};
template <class T, class Container>
bool operator==(const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator< (const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator!=(const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator> (const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator>=(const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator<=(const stack<T, Container>& x,
const stack<T, Container>& y);
}
to:
namespace std {
template <class T, class Container = deque<T> >
class stack {
public:
typedef typename Container::value_type value_type;
typedef typename Container::reference reference;
typedef typename Container::const_reference const_reference;
typedef typename Container::size_type size_type;
typedef Container container_type;
protected:
Container c;
public:
explicit stack(const Container& = Container());
bool empty() const { return c.empty(); }
size_type size() const { return c.size(); }
reference top() { return c.back(); }
const_reference top() const { return c.back(); }
void push(const value_type& x) { c.push_back(x); }
void pop() { c.pop_back(); }
};
template <class T, class Container>
bool operator==(const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator< (const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator!=(const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator> (const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator>=(const stack<T, Container>& x,
const stack<T, Container>& y);
template <class T, class Container>
bool operator<=(const stack<T, Container>& x,
const stack<T, Container>& y);
}
[Copenhagen: This change was discussed before the IS was released and it was deliberately not adopted. Nevertheless, the LWG believes (straw poll: 10-2) that it is a genuine defect.]
Section: 27 [lib.input.output] Status: Ready Submitter: Martin Sebor Date: 15 Mar 2001
Table 82 in section 27 mentions the header <cstdlib> for String streams (27.7 [lib.string.streams]) and the headers <cstdio> and <cwchar> for File streams (27.8 [lib.file.streams]). It's not clear why these headers are mentioned in this context since they do not define any of the library entities described by the subclauses. According to 17.4.1.1 [lib.contents], only such headers are to be listed in the summary.
Proposed resolution:
Remove <cstdlib> and <cwchar> from Table 82.
[Copenhagen: changed the proposed resolution slightly. The original proposed resolution also said to remove <cstdio> from Table 82. However, <cstdio> is mentioned several times within section 27.8 [lib.file.streams], including 27.8.2 [lib.c.files].]
Section: 27.6 [lib.iostream.format] Status: Open Submitter: Martin Sebor Date: 19 Mar 2001
The descriptions of the constructors of basic_istream<>::sentry (27.6.1.1.2 [lib.istream::sentry]) and basic_ostream<>::sentry (27.6.2.3 [lib.ostream::sentry]) do not explain what the functions do in case an exception is thrown while they execute. Some current implementations allow all exceptions to propagate, others catch them and set ios_base::badbit instead, still others catch some but let others propagate.
The text also mentions that the functions may call setstate(failbit) (without actually saying on what object, but presumably the stream argument is meant). That may have been fine for basic_istream<>::sentry prior to issue 195, since the function performs an input operation which may fail. However, issue 195 amends 27.6.1.1.2 [lib.istream::sentry], p2 to clarify that the function should actually call setstate(failbit | eofbit), so the sentence in p3 is redundant or even somewhat contradictory.
The same sentence that appears in 27.6.2.3 [lib.ostream::sentry], p3 doesn't seem to be very meaningful for basic_istream<>::sentry which performs no input. It is actually rather misleading since it would appear to guide library implementers to calling setstate(failbit) when os.tie()->flush(), the only called function, throws an exception (typically, it's badbit that's set in response to such an event).
Proposed resolution:
Add the following paragraph immediately after 27.6.1.1.2 [lib.istream::sentry], p5
If an exception is thrown during the preparation then ios::badbit is turned on* in is's error state.
[Footnote: This is done without causing an ios::failure to be thrown. --- end footnote]
If (is.exceptions() & ios_base::badbit)!= 0 then the exception is rethrown.
And strike the following sentence from 27.6.1.1.2 [lib.istream::sentry], p5
During preparation, the constructor may call setstate(failbit) (which may throw ios_base::failure (lib.iostate.flags))
Add the following paragraph immediately after 27.6.2.3 [lib.ostream::sentry], p3
If an exception is thrown during the preparation then ios::badbit is turned on* in os's error state.
[Footnote: This is done without causing an ios::failure to be thrown. --- end footnote]
If (os.exceptions() & ios_base::badbit)!= 0 then the exception is rethrown.
And strike the following sentence from 27.6.2.3 [lib.ostream::sentry], p3
During preparation, the constructor may call setstate(failbit) (which may throw ios_base::failure (lib.iostate.flags))
(Note that the removal of the two sentences means that the ctors will not be able to report the failure of any implementation-dependent operations referred to in footnotes 280 and 293, unless such operations throw an exception.)
[ Copenhagen: It was agreed that there was an issue here, but there was disagreement about the resolution. Some LWG members argued that a sentry's constructor should not catch exceptions, because sentries should only be used within (un)formatted input functions and that exception handling is the responsibility of those functions, not of the sentries. ]
Section: 17.4.1.2 [lib.headers], 19.3 [lib.errno] Status: Review Submitter: Steve Clamage Date: 21 Mar 2001
Exactly how should errno be declared in a conforming C++ header?
The C standard says in 7.1.4 that it is unspecified whether errno is a macro or an identifier with external linkage. In some implementations it can be either, depending on compile-time options. (E.g., on Solaris in multi-threading mode, errno is a macro that expands to a function call, but is an extern int otherwise. "Unspecified" allows such variability.)
The C++ standard:
I find no other references to errno.
We should either explicitly say that errno must be a macro, even though it need not be a macro in C, or else explicitly leave it unspecified. We also need to say something about namespace std. A user who includes <cerrno> needs to know whether to write errno, or ::errno, or std::errno, or else <cerrno> is useless.
Two acceptable fixes:
errno must be a macro. This is trivially satisfied by adding
#define errno (::std::errno)
to the headers if errno is not already a macro. You then always
write errno without any scope qualification, and it always expands
to a correct reference. Since it is always a macro, you know to
avoid using errno as a local identifer.
errno is in the global namespace. This fix is inferior, because ::errno is not guaranteed to be well-formed.
[ This issue was first raised in 1999, but it slipped through the cracks. ]
Proposed resolution:
Change the Note in section 17.4.1.2p5 from
Note: the names defined as macros in C include the following: assert, errno, offsetof, setjmp, va_arg, va_end, and va_start.
to
Note: the names defined as macros in C include the following: assert, offsetof, setjmp, va_arg, va_end, and va_start.
In section 19.3, change paragraph 2 from
The contents are the same as the Standard C library header <errno.h>.
to
The contents are the same as the Standard C library header <errno.h>, except that errno shall be defined as a macro.
Section: 27.6.2.1 [lib.ostream] Status: Review Submitter: Andy Sawyer Date: 21 Mar 2001
In 27.6.2.1 [lib.ostream], the synopsis of class basic_ostream says:
// partial specializationss
template<class traits>
basic_ostream<char,traits>& operator<<( basic_ostream<char,traits>&,
const char * );
Problems:
Proposed resolution:
In the synopsis in 27.6.2.1 [lib.ostream], remove the // partial specializationss comment.
Section: 20 [lib.utilities] Status: Ready Submitter: Martin Sebor Date: 29 Mar 2001
Table 27 in section 20 lists the header <memory> (only) for Memory (lib.memory) but neglects to mention the headers <cstdlib> and <cstring> that are discussed in 20.4.6 [lib.c.malloc].
Proposed resolution:
Add <cstdlib> and <cstring> to Table 27, in the same row as <memory>.
Section: 23.2.2.4 [lib.list.ops] Status: New Submitter: Andy Sawyer Date: 1 May 2001
23.2.2.4 [lib.list.ops], Para 21 describes the complexity of list::unique as: "If the range (last - first) is not empty, exactly (last - first) -1 applications of the corresponding predicate, otherwise no applications of the predicate)".
"(last - first)" is not a range.
Proposed resolution:
Change the "range" from (last - first) to [first, last). Change the complexity from "(last - first) -1 applications of the corresponding predicate" to "distance(first,last)-1 applications of the corresponding predicate.
Section: 23.1.2 [lib.associative.reqmts] Status: New Submitter: Martin Sebor Date: 4 May 2001
Table 69 says this about a_uniq.insert(t):
inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair indicates whether the insertion takes place and the iterator component of the pair points to the element with key equivalent to the key of t.
The description should be more specific about exactly how the bool component indicates whether the insertion takes place.
Proposed resolution:
Change the text in question to
...The bool component of the returned pair is true if and only if the insertion takes place...
Section: 22 [lib.localization] Status: New Submitter: Martin Sebor Date: 4 May 2001
The localization section of the standard refers to specializations of the facet templates as instantiations even though the required facets are typically specialized rather than explicitly (or implicitly) instantiated. In the case of ctype<char> and ctype_byname<char> (and the wchar_t versions), these facets are actually required to be specialized. The terminology should be corrected to make it clear that the standard doesn't mandate explicit instantiation (the term specialization encompasses both explicit instantiations and specializations).
Proposed resolution:
In the following paragraphs, replace all occurrences of the word instantiation or instantiations with specialization or specializations, respectively:
22.1.1.1.1, p4, Table 52, 22.2.1.1, p2, 22.2.1.5, p3, 22.2.1.5.1, p5, 22.2.1.5.2, p10, 22.2.2, p2, 22.2.3.1, p1, 22.2.3.1.2, p1, p2 and p3, 22.2.4.1, p1, 22.2.4.1.2, p1, 22,2,5, p1, 22,2,6, p2, 22.2.6.3.2, p7, and Footnote 242.
And change the text in 22.1.1.1.1, p4 from
An implementation is required to provide those instantiations for facet templates identified as members of a category, and for those shown in Table 52:
to
An implementation is required to support those specializations...
Section: 22.2.3.2 [lib.locale.numpunct.byname] Status: New Submitter: Martin Sebor Date: 12 May 2001
The definition of the numpunct_byname template contains the following comment:
namespace std {
template <class charT>
class numpunct_byname : public numpunct<charT> {
// this class is specialized for char and wchar_t.
...
There is no documentation of the specializations and it seems conceivable that an implementation will not explicitly specialize the template at all, but simply provide the primary template.
Proposed resolution:
Remove the comment from the text in 22.2.3.2 and from the proposed resolution of library issue 228.
Section: 18.4.1.1 [lib.new.delete.single], 18.4.1.2 [lib.new.delete.array] Status: New Submitter: Beman Dawes Date: 15 May 2001
The standard specifies 17.3.1.3 [lib.structure.specifications] that "Required behavior" elements describe "the semantics of a function definition provided by either the implementation or a C++ program."
The standard specifies 17.3.1.3 [lib.structure.specifications] that "Requires" elements describe "the preconditions for calling the function."
In the sections noted below, the current wording specifies "Required Behavior" for what are actually preconditions, and thus should be specified as "Requires".
Proposed resolution:
In 18.4.1.1 [lib.new.delete.single] Para 12 Change:
Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...
to:
Requires: the value of ptr be null or the value returned by an earlier call ...
In 18.4.1.2 [lib.new.delete.array] Para 11 Change:
Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...
to:
Requires: the value of ptr be null or the value returned by an earlier call ...
Section: 23.2.2.1 [lib.list.cons] Status: New Submitter: Howard Hinnant Date: 17 May 2001
Section 23.2.2.1, paragraphs 6-8 specify that list assign (both forms) have the "effects" of a call to erase followed by a call to insert.
I would like to document that implementers have the freedom to implement assign by other methods, as long as the end result is the same and the exception guarantee is as good or better than the basic guarantee.
The motivation for this is to use T's assignment operator to recycle existing nodes in the list instead of erasing them and reallocating them with new values. It is also worth noting that, with careful coding, most common cases of assign (everything but assignment with true input iterators) can elevate the exception safety to strong if T's assignment has a nothrow guarantee (with no extra memory cost). Metrowerks does this. However I do not propose that this subtlety be standardized. It is a QoI issue.
Existing practise: Metrowerks and SGI recycle nodes, Dinkumware and Rogue Wave don't.
Proposed resolution:
Change 23.2.2.1/7 from:
Effects:
erase(begin(), end()); insert(begin(), first, last);
to:
Effects: Replaces the contents of the list with the range [first, last).
PostCondition: *this == list<T, Allocator>(first, last)
Notes: If an exception is thrown, the contents of the list are indeterminate.
Change 23.2.2.1/8 from:
Effects:
erase(begin(), end()); insert(begin(), n, t);
to:
Effects: Replaces the contents of the list with n copies of t.
PostCondition: *this == list<T, Allocator>(n, t)
Notes: If an exception is thrown, the contents of the list are self consistent but indeterminate.
Rationale:
Section: 22.2.2.1.2 [lib.facet.num.get.virtuals] Status: New Submitter: Kevin Djang Date: 17 May 2001
Section 22.2.2.1.2 at p7 states that "A length specifier is added to the conversion function, if needed, as indicated in Table 56." However, Table 56 uses the term "length modifier", not "length specifier".
Proposed resolution:
In 22.2.2.1.2 at p7, change the text "A length specifier is added ..." to be "A length modifier is added ..."
Rationale:
Section: 23.1 [lib.container.requirements] Status: New Submitter: Matt Austern Date: 17 May 2001
It's widely assumed that, if X is a container, iterator_traits<X::iterator>::value_type and iterator_traits<X::const_iterator>::value_type should both be X::value_type. However, this is nowhere stated. The language in Table 65 is not precise about the iterators' value types (it predates iterator_traits), and could even be interpreted as saying that iterator_traits<X::const_iterator>::value_type should be "const X::value_type".
Related issue: 279.
Proposed resolution:
In Table 65 ("Container Requirements"), change the return type for X::iterator to "iterator type whose value type is T". Change the return type for X::const_iterator to "constant iterator type whose value type is T".
Rationale:
This belongs as a container requirement, rather than an iterator requirement, because the whole notion of iterator/const_iterator pairs is specific to containers' iterator.
It is existing practice that (for example) iterator_traits<list<int>::const_iterator>::value_type is "int", rather than "const int". This is consistent with the way that const pointers are handled: the standard already requires that iterator_traits<const int*>::value_type is int.
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