| Doc. no. | J16/01-0005 = WG21 N1291 |
| Date: | 20 Mar 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: Review 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: 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"]
Section: 22.2.1.5 [lib.locale.codecvt] Status: Open 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:
If
do_out(state, from, from_end, from_next, to, to_lim, to_next)would succeed (return value would be ok), and if from != from_end, thendo_out(state, from, from + 1, from_next, to, to_end, to_next)must also succeed. Ifdo_in(state, from, from_end, from_next, to, to_lim, to_next)would succeed, and if to != to_lim, thendo_in(state, from, from_end, from_next, to, to + 1, to_next)must also succeed. [Footnote: Informally, this means that every codecvt 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.
[Pre-Copenhagen: Matt provided wording.]
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 a formatted input function (27.6.1.2 [lib.istream.formatted]). 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 a formatted input function (27.6.1.2 [lib.istream.formatted]). 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:
[ pre-Copenhagen: Howard provided wording. ]
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: 23.1.2 [lib.associative.reqmts] Status: Ready Submitter: AFNOR Date: 7 Oct 1998
Set::iterator is described as implementation-defined with a reference to the container requirement; the container requirement says that const_iterator is an iterator pointing to const T and iterator an iterator pointing to T.
23.1.2 paragraph 2 implies that the keys should not be modified to break the ordering of elements. But that is not clearly specified. Especially considering that the current standard requires that iterator for associative containers be different from const_iterator. Set, for example, has the following:
typedef implementation defined iterator;
// See _lib.container.requirements_
23.1 [lib.container.requirements] actually requires that iterator type pointing to T (table 65). Disallowing user modification of keys by changing the standard to require an iterator for associative container to be the same as const_iterator would be overkill since that will unnecessarily significantly restrict the usage of associative container. A class to be used as elements of set, for example, can no longer be modified easily without either redesigning the class (using mutable on fields that have nothing to do with ordering), or using const_cast, which defeats requiring iterator to be const_iterator. The proposed solution goes in line with trusting user knows what he is doing.
Other Options Evaluated:
Option A. In 23.1.2 [lib.associative.reqmts], paragraph 2, after first sentence, and before "In addition,...", add one line:
Modification of keys shall not change their strict weak ordering.
Option B. Add three new sentences to 23.1.2 [lib.associative.reqmts]:
At the end of paragraph 5: "Keys in an associative container are immutable." At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators. It is unspecified whether or not iterator and const_iterator are the same type."
Option C. To 23.1.2 [lib.associative.reqmts], paragraph 3, which currently reads:
The phrase ``equivalence of keys'' means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 in the same container are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false.
add the following:
For any two keys k1 and k2 in the same container, comp(k1, k2) shall return the same value whenever it is evaluated. [Note: If k2 is removed from the container and later reinserted, comp(k1, k2) must still return a consistent value but this value may be different than it was the first time k1 and k2 were in the same container. This is intended to allow usage like a string key that contains a filename, where comp compares file contents; if k2 is removed, the file is changed, and the same k2 (filename) is reinserted, comp(k1, k2) must again return a consistent value but this value may be different than it was the previous time k2 was in the container.]
Proposed resolution:
Add the following to 23.1.2 [lib.associative.reqmts] at the indicated location:
At the end of paragraph 3: "For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value."
At the end of paragraph 5: "Keys in an associative container are immutable."
At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators. It is unspecified whether or not iterator and const_iterator are the same type."
Rationale:
Several arguments were advanced for and against allowing set elements to be mutable as long as the ordering was not effected. The argument which swayed the LWG was one of safety; if elements were mutable, there would be no compile-time way to detect of a simple user oversight which caused ordering to be modified. There was a report that this had actually happened in practice, and had been painful to diagnose. If users need to modify elements, it is possible to use mutable members or const_cast.
Simply requiring that keys be immutable is not sufficient, because the comparison object may indirectly (via pointers) operate on values outside of the keys.
The types iterator and const_iterator are permitted to be different types to allow for potential future work in which some member functions might be overloaded between the two types. No such member functions exist now, and the LWG believes that user functionality will not be impaired by permitting the two types to be the same. A function that operates on both iterator types can be defined for const_iterator alone, and can rely on the automatic conversion from iterator to const_iterator.
[Tokyo: The LWG crafted the proposed resolution and rationale.]
Section: 20.3.6 [lib.binders] Status: Open 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.]
[Tokyo: not discussed.]
Section: 24.5.3.5 [lib.istreambuf.iterator::equal] Status: Open Submitter: Nathan Myers Date: 15 Oct 1998
The member istreambuf_iterator<>::equal is specified to be unnecessarily inefficient. While this does not affect the efficiency of conforming implementations of iostreams, because they can "reach into" the iterators and bypass this function, it does affect users who use istreambuf_iterators.
The inefficiency results from a too-scrupulous definition, which requires a "true" result if neither iterator is at eof. In practice these iterators can only usefully be compared with the "eof" value, so the extra test implied provides no benefit, but slows down users' code.
The solution is to weaken the requirement on the function to return true only if both iterators are at eof.
Proposed resolution:
Replace 24.5.3.5 [lib.istreambuf.iterator::equal], paragraph 1,
-1- Returns: true if and only if both iterators are at end-of-stream, or neither is at end-of-stream, regardless of what streambuf object they use.
with
-1- Returns: true if and only if both iterators are at end-of-stream, regardless of what streambuf object they use.
[Toronto: most people saw no compelling reason to make this change. There was some argument that the standard already permits this behavior, on the grounds that it is illegal to have two different istreambuf_iterators into the same stream. A possible counterexample: "istreambuf_iterator i(cin); assert(i == i);". The standard currently requires that the assertion succeeds. (Assuming that we haven't reached eof on standard input, of course.)]
Section: 27.6.2.5.2 [lib.ostream.inserters.arithmetic] Status: Review 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: 27.6.1.2.2 [lib.istream.formatted.arithmetic] Status: Ready Submitter: Matt Austern Date: 20 Nov 1998
Formatted input is defined for the types short, unsigned short, int, unsigned int, long, unsigned long, float, double, long double, bool, and void*. According to section 27.6.1.2.2, formatted input of a value x is done as if by the following code fragment:
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; use_facet< numget >(loc).get(*this, 0, *this, err, val); setstate(err);
According to section 22.2.2.1.1 [lib.facet.num.get.members], however, num_get<>::get() is only overloaded for the types bool, long, unsigned short, unsigned int, unsigned long, unsigned long, float, double, long double, and void*. Comparing the lists from the two sections, we find that 27.6.1.2.2 is using a nonexistent function for types short and int.
Proposed resolution:
In 27.6.1.2.2 [lib.istream.formatted.arithmetic] Arithmetic Extractors, remove the two lines (1st and 3rd) which read:
operator>>(short& val); ... operator>>(int& val);
And add the following at the end of that section (27.6.1.2.2) :
operator>>(short& val);The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet< numget >(loc).get(*this, 0, *this, err, lval); if (err == 0 && (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval)) err = ios_base::failbit; setstate(err);operator>>(int& val);The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet< numget >(loc).get(*this, 0, *this, err, lval); if (err == 0 && (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval)) err = ios_base::failbit; setstate(err);
[Post-Tokyo: PJP provided the above wording.]
Section: 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. One could argue that there is a consistent status quo as far as instantiations go, but one can't argue that in the case of specializations. The standard must either (1) give library implementors license to provide explicit specializations of any library template; or (2) give a complete list of exactly which specializations must be provided, and forbid library implementors from providing any specializations not on that list. At present the standard does neither.
Proposed resolution:
Append to 17.4.3.1 [lib.reserved.names] paragraph 1:
A program may manually instantiate any templates in the standard library only if the declaration depends on a user-defined name of external linkage and the instantiation meets the standard library requirements for the original template.
[Post-Tokyo: Judy Ward provided the above wording.]
[Toronto: The LWG is concerned about the scope of this proposed resolution: manually instantiating standard library templates is a common method for reducing compilation times. One possible alternative is a core change: allow (and ignore) manual instantiation requests when there is an explicit specialization. Another possible alternative is requiring that library implementors provide a list of specializations and explicit instantiations as part of their documentation. Judy has volunteered to provide wording for the latter alternative.]
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: 27.6.1.3 [lib.istream.unformatted] Status: Ready Submitter: Howard Hinnant Date: 6 Mar 1999
I may be misunderstanding the intent, but should not seekg set only the input stream and seekp set only the output stream? The description seems to say that each should set both input and output streams. If that's really the intent, I withdraw this proposal.
Proposed resolution:
In section 27.6.1.3 change:
basic_istream<charT,traits>& seekg(pos_type pos); Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).
To:
basic_istream<charT,traits>& seekg(pos_type pos); Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::in).
In section 27.6.1.3 change:
basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir); Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).
To:
basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir); Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::in).
In section 27.6.2.4, paragraph 2 change:
-2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).
To:
-2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::out).
In section 27.6.2.4, paragraph 4 change:
-4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).
To:
-4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::out).
[Dublin: Dietmar Kühl thinks this is probably correct, but would like the opinion of more iostream experts before taking action.]
[Tokyo: Reviewed by the LWG. PJP noted that although his docs are incorrect, his implementation already implements the Proposed Resolution.]
[Post-Tokyo: Matt Austern comments:
Is it a problem with basic_istream and basic_ostream, or is it a problem
with basic_stringbuf?
We could resolve the issue either by changing basic_istream and
basic_ostream, or by changing basic_stringbuf. I prefer the latter
change (or maybe both changes): I don't see any reason for the standard to
require that std::stringbuf s(std::string("foo"), std::ios_base::in);
s.pubseekoff(0, std::ios_base::beg); must fail.
This requirement is a bit weird. There's no similar requirement
for basic_streambuf<>::seekpos, or for basic_filebuf<>::seekoff or
basic_filebuf<>::seekpos.]
Section: 22.2.1.3.2 [lib.facet.ctype.char.members] Status: Ready Submitter: Dietmar Kühl Date: 20 Jul 1999
The description of the array version of narrow() (in paragraph 11) is flawed: There is no member do_narrow() which takes only three arguments because in addition to the range a default character is needed.
Additionally, for both widen and narrow we have two signatures followed by a Returns clause that only addresses one of them.
Proposed resolution:
Change the returns clause in 22.2.1.3.2 [lib.facet.ctype.char.members] paragraph 10 from:
Returns: do_widen(low, high, to).
to:
Returns: do_widen(c) or do_widen(low, high, to), respectively.
Change 22.2.1.3.2 [lib.facet.ctype.char.members] paragraph 10 and 11 from:
char narrow(char c, char /*dfault*/) const;
const char* narrow(const char* low, const char* high,
char /*dfault*/, char* to) const;
Returns: do_narrow(low, high, to).
to:
char narrow(char c, char dfault) const;
const char* narrow(const char* low, const char* high,
char dfault, char* to) const;
Returns: do_narrow(c, dfault) or
do_narrow(low, high, dfault, to), respectively.
[Kona: 1) the problem occurs in additional places, 2) a user defined version could be different.]
[Post-Tokyo: Dietmar provided the above wording at the request of the LWG. He could find no other places the problem occurred. He asks for clarification of the Kona "a user defined version..." comment above. Perhaps it was a circuitous way of saying "dfault" needed to be uncommented?]
[Post-Toronto: the issues list maintainer has merged in the proposed resolution from issue 207, which addresses the same paragraphs.]
Section: 27.6.2.1 [lib.ostream] Status: Ready Submitter: Dietmar Kühl Date: 20 Jul 1999
Paragraph 2 explicitly states that none of the basic_ostream functions falling into one of the groups "formatted output functions" and "unformatted output functions" calls any stream buffer function which might call a virtual function other than overflow(). Basically this is fine but this implies that sputn() (this function would call the virtual function xsputn()) is never called by any of the standard output functions. Is this really intended? At minimum it would be convenient to call xsputn() for strings... Also, the statement that overflow() is the only virtual member of basic_streambuf called is in conflict with the definition of flush() which calls rdbuf()->pubsync() and thereby the virtual function sync() (flush() is listed under "unformatted output functions").
In addition, I guess that the sentence starting with "They may use other public members of basic_ostream ..." probably was intended to start with "They may use other public members of basic_streamuf..." although the problem with the virtual members exists in both cases.
I see two obvious resolutions:
Proposed resolution:
Change the last sentence of 27.6.2.1 (lib.ostream) paragraph 2 from:
They may use other public members of basic_ostream except that they do not invoke any virtual members of rdbuf() except overflow().
to:
They may use other public members of basic_ostream except that they shall not invoke any virtual members of rdbuf() except overflow(), xsputn(), and sync().
[Kona: the LWG believes this is a problem. Wish to ask Jerry or PJP why the standard is written this way.]
[Post-Tokyo: Dietmar supplied wording at the request of the LWG. He comments: The rules can be made a little bit more specific if necessary be explicitly spelling out what virtuals are allowed to be called from what functions and eg to state specifically that flush() is allowed to call sync() while other functions are not.]
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.]
Section: 27.8.1.4 [lib.filebuf.virtuals] Status: Ready Submitter: Dietmar Kühl Date: 20 Jul 1999
Overridden virtual functions, seekpos()
In 27.8.1.1 [lib.filebuf] paragraph 3, it is stated that a joint input and output position is maintained by basic_filebuf. Still, the description of seekpos() seems to talk about different file positions. In particular, it is unclear (at least to me) what is supposed to happen to the output buffer (if there is one) if only the input position is changed. The standard seems to mandate that the output buffer is kept and processed as if there was no positioning of the output position (by changing the input position). Of course, this can be exactly what you want if the flag ios_base::ate is set. However, I think, the standard should say something like this:
Plus the appropriate error handling, that is...
Proposed resolution:
Change the unnumbered paragraph in 27.8.1.4 (lib.filebuf.virtuals) before paragraph 14 from:
pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);
Alters the file position, if possible, to correspond to the position stored in sp (as described below).
- if (which&ios_base::in)!=0, set the file position to sp, then update the input sequence
- if (which&ios_base::out)!=0, then update the output sequence, write any unshift sequence, and set the file position to sp.
to:
pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);
Alters the file position, if possible, to correspond to the position stored in sp (as described below). Altering the file position performs as follows:
1. if (om & ios_base::out)!=0, then update the output sequence and write any unshift sequence;
2. set the file position to sp;
3. if (om & ios_base::in)!=0, then update the input sequence;
where om is the open mode passed to the last call to open(). The operation fails if is_open() returns false.
[Kona: Dietmar is working on a proposed resolution.]
[Post-Tokyo: Dietmar supplied the above 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".]
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: Review 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: 27.6.3 [lib.std.manip] Status: Ready Submitter: Andy Sawyer Date: 7 Jul 1999
27.6.3 [lib.std.manip] paragraph 3 says (clause numbering added for exposition):
Returns: An object s of unspecified type such that if [1] out is an (instance of) basic_ostream then the expression out<<s behaves as if f(s) were called, and if [2] in is an (instance of) basic_istream then the expression in>>s behaves as if f(s) were called. Where f can be defined as: ios_base& f(ios_base& str, ios_base::fmtflags mask) { // reset specified flags str.setf(ios_base::fmtflags(0), mask); return str; } [3] The expression out<<s has type ostream& and value out. [4] The expression in>>s has type istream& and value in.
Given the definitions [1] and [2] for out and in, surely [3] should read: "The expression out << s has type basic_ostream& ..." and [4] should read: "The expression in >> s has type basic_istream& ..."
If the wording in the standard is correct, I can see no way of implementing any of the manipulators so that they will work with wide character streams.
e.g. wcout << setbase( 16 );
Must have value 'wcout' (which makes sense) and type 'ostream&' (which doesn't).
The same "cut'n'paste" type also seems to occur in Paras 4,5,7 and 8. In addition, Para 6 [setfill] has a similar error, but relates only to ostreams.
I'd be happier if there was a better way of saying this, to make it clear that the value of the expression is "the same specialization of basic_ostream as out"&
Proposed resolution:
Replace section 27.6.3 [lib.std.manip] except paragraph 1 with the following:
2- The type designated smanip in each of the following function descriptions is implementation-specified and may be different for each function.
smanip resetiosflags(ios_base::fmtflags mask);
-3- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:*
[Footnote: The expression cin >> resetiosflags(ios_base::skipws) clears ios_base::skipws in the format flags stored in the basic_istream<charT,traits> object cin (the same as cin >> noskipws), and the expression cout << resetiosflags(ios_base::showbase) clears ios_base::showbase in the format flags stored in the basic_ostream<charT,traits> object cout (the same as cout << noshowbase). --- end footnote]
ios_base& f(ios_base& str, ios_base::fmtflags mask)
{
// reset specified flags
str.setf(ios_base::fmtflags(0), mask);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setiosflags(ios_base::fmtflags mask);
-4- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:
ios_base& f(ios_base& str, ios_base::fmtflags mask)
{
// set specified flags
str.setf(mask);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setbase(int base);
-5- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, base) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, base) were called. The function f can be defined as:
ios_base& f(ios_base& str, int base)
{
// set basefield
str.setf(base == 8 ? ios_base::oct :
base == 10 ? ios_base::dec :
base == 16 ? ios_base::hex :
ios_base::fmtflags(0), ios_base::basefield);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setfill(char_type c);
-6- Returns: An object s of unspecified type such that if out is (or is derived from) basic_ostream<charT,traits> and c has type charT then the expression out<<s behaves as if f(s, c) were called. The function f can be defined as:
template<class charT, class traits>
basic_ios<charT,traits>& f(basic_ios<charT,traits>& str, charT c)
{
// set fill character
str.fill(c);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out.
smanip setprecision(int n);
-7- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
ios_base& f(ios_base& str, int n)
{
// set precision
str.precision(n);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in
.
smanip setw(int n);
-8- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
ios_base& f(ios_base& str, int n)
{
// set width
str.width(n);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
[Kona: Andy Sawyer and Beman Dawes will work to improve the wording of the proposed resolution.]
[Tokyo - The LWG noted that issue 216 involves the same paragraphs.]
[Post-Tokyo: The issues list maintainer combined the proposed resolution of this issue with the proposed resolution for issue 216 as they both involved the same paragraphs, and were so intertwined that dealing with them separately appear fraught with error. The full text was supplied by Bill Plauger; it was cross checked against changes supplied by Andy Sawyer. It should be further checked by the LWG.]
Section: 18.2.1.5 [lib.numeric.special] Status: Ready Submitter: Gabriel Dos Reis Date: 21 Jul 1999
bools are defined by the standard to be of integer types, as per 3.9.1 [basic.fundamental] paragraph 7. However "integer types" seems to have a special meaning for the author of 18.2. The net effect is an unclear and confusing specification for numeric_limits<bool> as evidenced below.
18.2.1.2/7 says numeric_limits<>::digits is, for built-in integer types, the number of non-sign bits in the representation.
4.5/4 states that a bool promotes to int ; whereas 4.12/1 says any non zero arithmetical value converts to true.
I don't think it makes sense at all to require numeric_limits<bool>::digits and numeric_limits<bool>::digits10 to be meaningful.
The standard defines what constitutes a signed (resp. unsigned) integer types. It doesn't categorize bool as being signed or unsigned. And the set of values of bool type has only two elements.
I don't think it makes sense to require numeric_limits<bool>::is_signed to be meaningful.
18.2.1.2/18 for numeric_limits<integer_type>::radix says:
For integer types, specifies the base of the representation.186)
This disposition is at best misleading and confusing for the standard requires a "pure binary numeration system" for integer types as per 3.9.1/7
The footnote 186) says: "Distinguishes types with base other than 2 (e.g BCD)." This also erroneous as the standard never defines any integer types with base representation other than 2.
Furthermore, numeric_limits<bool>::is_modulo and numeric_limits<bool>::is_signed have similar problems.
Proposed resolution:
Append to the end of 18.2.1.5 [lib.numeric.special]:
The specialization for bool shall be provided as follows:
namespace std { template<> class numeric_limits<bool> { public: static const bool is_specialized = true; static bool min() throw() { return false; } static bool max() throw() { return true; } static const int digits = 1; static const int digits10 = 0; static const bool is_signed = false; static const bool is_integer = true; static const bool is_exact = true; static const int radix = 2; static bool epsilon() throw() { return 0; } static bool round_error() throw() { return 0; } static const int min_exponent = 0; static const int min_exponent10 = 0; static const int max_exponent = 0; static const int max_exponent10 = 0; static const bool has_infinity = false; static const bool has_quiet_NaN = false; static const bool has_signaling_NaN = false; static const float_denorm_style has_denorm = denorm_absent; static const bool has_denorm_loss = false; static bool infinity() throw() { return 0; } static bool quiet_NaN() throw() { return 0; } static bool signaling_NaN() throw() { return 0; } static bool denorm_min() throw() { return 0; } static const bool is_iec559 = false; static const bool is_bounded = true; static const bool is_modulo = false; static const bool traps = false; static const bool tinyness_before = false; static const float_round_style round_style = round_toward_zero; }; }
[Tokyo: The LWG desires wording that specifies exact values rather than more general wording in the original proposed resolution.]
[Post-Tokyo: At the request of the LWG in Tokyo, Nico Josuttis provided the above wording.]
Section: 20.3 [lib.function.objects] Status: Ready Submitter: UK Panel Date: 26 Jul 1999
Paragraph 4 of 20.3 [lib.function.objects] says:
[Example: To negate every element of a: transform(a.begin(), a.end(), a.begin(), negate<double>()); The corresponding functions will inline the addition and the negation. end example]
(Note: The "addition" referred to in the above is in para 3) we can find no other wording, except this (non-normative) example which suggests that any "inlining" will take place in this case.
Indeed both:
17.4.4.3 Global Functions [lib.global.functions] 1 It is unspecified whether any global functions in the C++ Standard Library are defined as inline (7.1.2).
and
17.4.4.4 Member Functions [lib.member.functions] 1 It is unspecified whether any member functions in the C++ Standard Library are defined as inline (7.1.2).
take care to state that this may indeed NOT be the case.
Thus the example "mandates" behavior that is explicitly not required elsewhere.
Proposed resolution:
In 20.3 [lib.function.objects] paragraph 1, remove the sentence:
They are important for the effective use of the library.
Remove 20.3 [lib.function.objects] paragraph 2, which reads:
Using function objects together with function templates increases the expressive power of the library as well as making the resulting code much more efficient.
In 20.3 [lib.function.objects] paragraph 4, remove the sentence:
The corresponding functions will inline the addition and the negation.
[Kona: The LWG agreed there was a defect.]
[Tokyo: The LWG crafted the proposed resolution.]
Section: 23.3.5.2 [lib.bitset.members] Status: Ready Submitter: Darin Adler Date: 13 Aug 1999
In section 23.3.5.2 [lib.bitset.members], paragraph 13 defines the bitset::set operation to take a second parameter of type int. The function tests whether this value is non-zero to determine whether to set the bit to true or false. The type of this second parameter should be bool. For one thing, the intent is to specify a Boolean value. For another, the result type from test() is bool. In addition, it's possible to slice an integer that's larger than an int. This can't happen with bool, since conversion to bool has the semantic of translating 0 to false and any non-zero value to true.
Proposed resolution:
In 23.3.5 [lib.template.bitset] Para 1 Replace:
bitset<N>& set(size_t pos, int val = true );
With:
bitset<N>& set(size_t pos, bool val = true );
In 23.3.5.2 [lib.bitset.members] Para 12(.5) Replace:
bitset<N>& set(size_t pos, int val = 1 );
With:
bitset<N>& set(size_t pos, bool val = true );
[Kona: The LWG agrees with the description. Andy Sawyers will work on better P/R wording.]
[Post-Tokyo: Andy provided the above wording.]
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: Review 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).]
[Toronto: The LWG agrees with the general intent of the proposed resolution, but had some quibbles about the wording. Andy Sawyer has volunteered to provide revised 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.]
[pre-Copenhagen: Andy Koenig pointed out that it is possible to rewrite reverse_iterator so that it no longer makes this assumption.]
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.
3. Change the effects to:
Effects: Eliminates all but the first element e from every consecutive group of elements referred to by the iterator i in the range [first, last) for which the following corresponding conditions hold: e == *i or pred(e,*i) != false.
If we adopt (2), we also need to decide whether pred(*i, *(i - 1)) is really what we meant, or whether pred(*(i - 1), i) is more appropriate.
A LWG member, Nico Josuttis, comments:
First, I agree that the current wording is simply wrong. However, to follow all [known] current implementations I propose [option 3 above].
[ Tokyo: The issue was discussed at length without reaching consensus. Straw vote: Option 1 - preferred by 2 people. Option 2 - preferred by 0 people. Option 3 - preferred by 3 people. Many abstentions. ]
Section: 23.3.3 [lib.set], 23.3.4 [lib.multiset] Status: Ready Submitter: Judy Ward Date: 28 Feb 2000
The specification for the associative container requirements in Table 69 state that the find member function should "return iterator; const_iterator for constant a". The map and multimap container descriptions have two overloaded versions of find, but set and multiset do not, all they have is:
iterator find(const key_type & x) const;
Proposed resolution:
Change the prototypes for find(), lower_bound(), upper_bound(), and equal_range() in section 23.3.3 [lib.set] and section 23.3.4 [lib.multiset] to each have two overloads:
iterator find(const key_type & x); const_iterator find(const key_type & x) const;iterator lower_bound(const key_type & x); const_iterator lower_bound(const key_type & x) const;iterator upper_bound(const key_type & x); const_iterator upper_bound(const key_type & x) const;pair<iterator, iterator> equal_range(const key_type & x); pair<const_iterator, const_iterator> equal_range(const key_type & x) const;
[Tokyo: At the request of the LWG, Judy Ward provided wording extending the proposed resolution to lower_bound, upper_bound, and equal_range.]
Section: 22.2.2.1.2 [lib.facet.num.get.virtuals] Status: Ready Submitter: Matt Austern Date: 14 Mar 2000
Stage 2 processing of numeric conversion is broken.
Table 55 in 22.2.2.1.2 says that when basefield is 0 the integral conversion specifier is %i. A %i specifier determines a number's base by its prefix (0 for octal, 0x for hex), so the intention is clearly that a 0x prefix is allowed. Paragraph 8 in the same section, however, describes very precisely how characters are processed. (It must be done "as if" by a specified code fragment.) That description does not allow a 0x prefix to be recognized.
Very roughly, stage 2 processing reads a char_type ct. It converts ct to a char, not by using narrow but by looking it up in a translation table that was created by widening the string literal "0123456789abcdefABCDEF+-". The character "x" is not found in that table, so it can't be recognized by stage 2 processing.
Proposed resolution:
In 22.2.2.1.2 paragraph 8, replace the line:
static const char src[] = "0123456789abcdefABCDEF+-";
with the line:
static const char src[] = "0123456789abcdefxABCDEFX+-";
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. Possible solutions discussed by the LWG include: (1) allowing users to add overloads within namespace std, provided that the user-defined overload has the same semantics as the function being overloaded. (2) Specifying a special set of names, such as swap and abs which library components must refer to without qualification; all names not on this list must be qualified by std::. (There are many possible variations on this issue. For example, we might say that a function f should always be unqualified within g whenever g's description in the standard says that it calls f.) (3) Asking CWG to revisit partial specialization of function template. ]
Section: 22.2 [lib.locale.categories] Status: Review 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
virtual catalog do_open(const basic_string<char>&, const locale&) const;
virtual string_type do_get(catalog, int set, int msgid,
const string_type& dfault) const;
virtual void do_close(catalog) const;
};
}
Remove section 22.2.1.4 [lib.locale.ctype.byname.special] completely (because in this case only those members are defined to be virtual which are defined to be virtual in 'ctype<cT>'.)
[Post-Tokyo: Dietmar Kühl submitted this issue at the request of the LWG to solve the underlying problems raised by issue 138.]
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: Review 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: Open 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"
[post-Toronto: Matt supplied the proposed resolution. It makes more sense to have "if (flags & floatfield) == fixed" than to have "if (flags & floatfield) != 0" because the latter would mean there was no way to specify %f format with a precision of zero, which is meaningful. (%e or %g format with a precision of zero is not meaningful, and printf will treat them the same way as if the precision was 1.)]
Section: 17.4.3.1 [lib.reserved.names] Status: Review 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: Open 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:
OPTION 1:
General Idea:
Point out that in insert(p,t), the iterator p will (if possible)
be used to insert t just before p or just after p. If this is
not possible, the hint is ignored.
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, insert t right after p if possible; otherwise, p is ignored.
complexity:
Change:
right after p
To:
right before or right after 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, insert t right after p if possible; otherwise, p is ignored.
complexity:
logarithmic in general, but amortized constant if t is inserted right before or right after p.
OPTION 2
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 is 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.]
Section: 20.4.1.1 [lib.allocator.members] Status: Ready Submitter: Dietmar Kühl Date: 24 Apr 2000
In paragraphs 12 and 13 the effects of construct() and destruct() are described as returns but the functions actually return void.
Proposed resolution:
Substitute "Returns" by "Effect".
Section: 24.4.1.1 [lib.reverse.iterator] Status: Review 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: 23.2.2.1 [lib.list.cons] Status: Ready Submitter: Dietmar Kühl Date: 24 Apr 2000
The complexity specification in paragraph 6 says that the complexity is linear in first - last. Even if operator-() is defined on iterators this term is in general undefined because it would have to be last - first.
Proposed resolution:
Change paragraph 6 from
Linear in first - last.
to become
Linear in distance(first, last).
Section: 27.7.1.1 [lib.stringbuf.cons] Status: Review 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, at most (last - first) - 1 comparisons."
Section: 25.2.8 [lib.alg.unique] Status: Review 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: Review 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.
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)].
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.
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.
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.
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.
[Toronto: Dave Abrahams supplied wording.]
Section: 27.6.1.3 [lib.istream.unformatted] Status: Ready Submitter: Martin Sebor Date: May 15 2000
basic_istream<>::get(), and basic_istream<>::getline(), are unclear with respect to the behavior and side-effects of the named functions in case of an error.
27.6.1.3, p1 states that "... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input..." It is not clear from this (or the rest of the paragraph) what precisely the behavior should be when the sentry ctor exits by throwing an exception or when the sentry object returns false. In particular, what is the number of characters extracted that gcount() returns supposed to be?
27.6.1.3 p8 and p19 say about the effects of get() and getline(): "... In any case, it then stores a null character (using charT()) into the next successive location of the array." Is not clear whether this sentence applies if either of the conditions above holds (i.e., when sentry fails).
Proposed resolution:
Add to 27.6.1.3, p1 after the sentence
"... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input."
the following
"Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array."
Rationale:
Although the general philosophy of the input functions is that the argument should not be modified upon failure, getline historically added a terminating null unconditionally. Most implementations still do that. Earlier versions of the draft standard had language that made this an unambiguous requirement; those words were moved to a place where their context made them less clear. See Jerry Schwarz's message c++std-lib-7618.
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: 22.2.5 [lib.category.time] Status: Ready Submitter: Martin Sebor Date: 22 June 2000
There is no requirement that any of time_get member functions set ios::eofbit when they reach the end iterator while parsing their input. Since members of both the num_get and money_get facets are required to do so (22.2.2.1.2, and 22.2.6.1.2, respectively), time_get members should follow the same requirement for consistency.
Proposed resolution:
Add paragraph 2 to section 22.2.5.1 with the following text:
If the end iterator is reached during parsing by any of the get() member functions, the member sets ios_base::eofbit in err.
Rationale:
Two alternative resolutions were proposed. The LWG chose this one because it was more consistent with the way eof is described for other input facets.
Section: 23.2.2.4 [lib.list.ops] Status: Review 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: 27.7.1 [lib.stringbuf] Status: Ready Submitter: Martin Sebor Date: 28 Jul 2000
The synopsis for the template class basic_stringbuf doesn't list a typedef for the template parameter Allocator. This makes it impossible to determine the type of the allocator at compile time. It's also inconsistent with all other template classes in the library that do provide a typedef for the Allocator parameter.
Proposed resolution:
Add to the synopses of the class templates basic_stringbuf (27.7.1), basic_istringstream (27.7.2), basic_ostringstream (27.7.3), and basic_stringstream (27.7.4) the typedef:
typedef Allocator allocator_type;
Section: 27.7 [lib.string.streams] Status: Ready Submitter: Martin Sebor Date: 28 Jul 2000
27.7.2.2, p1 uses a C-style cast rather than the more appropriate const_cast<> in the Returns clause for basic_istringstream<>::rdbuf(). The same C-style cast is being used in 27.7.3.2, p1, D.7.2.2, p1, and D.7.3.2, p1, and perhaps elsewhere. 27.7.6, p1 and D.7.2.2, p1 are missing the cast altogether.
C-style casts have not been deprecated, so the first part of this issue is stylistic rather than a matter of correctness.
Proposed resolution:
In 27.7.2.2, p1 replace
-1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In 27.7.3.2, p1 replace
-1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In 27.7.6, p1, replace
-1- Returns: &sb
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In D.7.2.2, p1 replace
-2- Returns: &sb.
with
-2- Returns: const_cast<strstreambuf*>(&sb).
Section: 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: 27.4.4.2 [lib.basic.ios.members] Status: Ready Submitter: Martin Sebor Date: 21 Aug 2000
27.4.4.2, p17 says
-17- Before copying any parts of rhs, calls each registered callback pair (fn,index) as (*fn)(erase_event,*this,index). After all parts but exceptions() have been replaced, calls each callback pair that was copied from rhs as (*fn)(copy_event,*this,index).
The name copy_event isn't defined anywhere. The intended name was copyfmt_event.
Proposed resolution:
Replace copy_event with copyfmt_event in the named paragraph.
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: Review 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: 24.5.1.2 [lib.istream.iterator.ops] Status: Ready Submitter: Martin Sebor Date: 27 Aug 2000
The synopsis of istream_iterator::operator++(int) in 24.5.1 shows it as returning the iterator by value. 24.5.1.2, p5 shows the same operator as returning the iterator by reference. That's incorrect given the Effects clause below (since a temporary is returned). The `&' is probably just a typo.
Proposed resolution:
Change the declaration in 24.5.1.2, p5 from
istream_iterator<T,charT,traits,Distance>& operator++(int);
to
istream_iterator<T,charT,traits,Distance> operator++(int);
(that is, remove the `&').
Section: 24.5.1.2 [lib.istream.iterator.ops] Status: Ready Submitter: Martin Sebor Date: 27 Aug 2000
24.5.1, p3 lists the synopsis for
template <class T, class charT, class traits, class Distance>
bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
const istream_iterator<T,charT,traits,Distance>& y);
but there is no description of what the operator does (i.e., no Effects or Returns clause) in 24.5.1.2.
Proposed resolution:
Add paragraph 7 to the end of section 24.5.1.2 with the following text:
template <class T, class charT, class traits, class Distance>
bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
const istream_iterator<T,charT,traits,Distance>& y);
-7- Returns: !(x == y).
Section: 17.3.2.1.2 [lib.bitmask.types] Status: Ready Submitter: Beman Dawes Date: 03 Sep 2000
The ~ operation should be applied after the cast to int_type.
Proposed resolution:
Change 17.3.2.1.2 [lib.bitmask.types] operator~ from:
bitmask operator~ ( bitmask X )
{ return static_cast< bitmask>(static_cast<int_type>(~ X)); }
to:
bitmask operator~ ( bitmask X )
{ return static_cast< bitmask>(~static_cast<int_type>(X)); }
Section: 21.3 [lib.basic.string] Status: Ready Submitter: Kevlin Henney Date: 04 Sep 2000
The note in paragraph 6 suggests that the invalidation rules for references, pointers, and iterators in paragraph 5 permit a reference- counted implementation (actually, according to paragraph 6, they permit a "reference counted implementation", but this is a minor editorial fix).
However, the last sub-bullet is so worded as to make a reference-counted implementation unviable. In the following example none of the conditions for iterator invalidation are satisfied:
// first example: "*******************" should be printed twice
string original = "some arbitrary text", copy = original;
const string & alias = original;
string::const_iterator i = alias.begin(), e = alias.end();
for(string::iterator j = original.begin(); j != original.end(); ++j)
*j = '*';
while(i != e)
cout << *i++;
cout << endl;
cout << original << endl;
Similarly, in the following example:
// second example: "some arbitrary text" should be printed out
string original = "some arbitrary text", copy = original;
const string & alias = original;
string::const_iterator i = alias.begin();
original.begin();
while(i != alias.end())
cout << *i++;
I have tested this on three string implementations, two of which were reference counted. The reference-counted implementations gave "surprising behavior" because they invalidated iterators on the first call to non-const begin since construction. The current wording does not permit such invalidation because it does not take into account the first call since construction, only the first call since various member and non-member function calls.
Proposed resolution:
Change the following sentence in 21.3 paragraph 5 from
Subsequent to any of the above uses except the forms of insert() and erase() which return iterators, the first call to non-const member functions operator[](), at(), begin(), rbegin(), end(), or rend().
to
Following construction or any of the above uses, except the forms of insert() and erase() which return iterators, the first call to non- const member functions operator[](), at(), begin(), rbegin(), end(), or rend().
Section: 23.1.2 [lib.associative.reqmts] Status: Review 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".
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: 20.2.2 [lib.pairs] Status: Ready Submitter: Martin Sebor Date: 11 Sep 2000
I don't see any requirements on the types of the elements of the std::pair container in 20.2.2. From the descriptions of the member functions it appears that they must at least satisfy the requirements of 20.1.3 [lib.copyconstructible] and 20.1.4 [lib.default.con.req], and in the case of the [in]equality operators also the requirements of 20.1.1 [lib.equalitycomparable] and 20.1.2 [lib.lessthancomparable].
I believe that the the CopyConstructible requirement is unnecessary in the case of 20.2.2, p2.
Proposed resolution:
Change the Effects clause in 20.2.2, p2 from
-2- Effects: Initializes its members as if implemented: pair() : first(T1()), second(T2()) {}
to
-2- Effects: Initializes its members as if implemented: pair() : first(), second() {}
Rationale:
The existing specification of pair's constructor appears to be a historical artifact: there was concern that pair's members be properly zero-initialized when they are built-in types. At one time there was uncertainty about whether they would be zero-initialized if the default constructor was written the obvious way. The core language was clarified some time ago, however, and there is no longer any doubt that the straightforward implementation is correct.
Section: 18.6.2.1 [lib.bad.exception] Status: Review 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: Review 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 4.7.1 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. ]
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: 22.1.1 [lib.locale] Status: Ready Submitter: Martin Sebor Date: 5 Oct 2000
The synopsis of the class std::locale in 22.1.1 contains two typos: the semicolons after the declarations of the default ctor locale::locale() and the copy ctor locale::locale(const locale&) are missing.
Proposed resolution:
Add the missing semicolons, i.e., change
// construct/copy/destroy:
locale() throw()
locale(const locale& other) throw()
in the synopsis in 22.1.1 to
// construct/copy/destroy:
locale() throw();
locale(const locale& other) throw();
Section: 25.3.3 [lib.alg.binary.search] Status: Open 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.
Here is a first attempt: the comparison function comp must be equivalent to a comparison function of the form C(pi(x), y), and [first, last) must be sorted in ascending order by the comparison function C(pi(x), pi(y)), where U is a synonym for iterator_traits<ForwardIterator>::value_type, x is a value of type U, y is a value of type T, C is a strict weak ordering whose value type is T, and pi is a homomorphism from U to T.
In this notation, the existing language refers to the special case where T and U are the same type and where pi is the identity function.
Additional questions raised at the Toronto meeting:
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: New 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: New 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: New 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: New 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:
Proposed resolution 1
Add the following definition of a partial specialization immediately below the definition of the primary template in 20.4.1:
template <class T>
class allocator<const T> {
public:
typedef size_t size_type;
typedef ptrdiff_t difference_type;
typedef const T* pointer;
typedef const T* const_pointer;
typedef const T& reference;
typedef const T& const_reference;
typedef const T value_type;
template <class U> struct rebind { typedef allocator<U> other; };
allocator() throw();
allocator(const allocator&) throw();
template <class U> allocator(const allocator<U>&) throw();
~allocator() throw();
const_pointer address(const_reference x) const;
const_pointer allocate(size_type, allocator<void>::const_pointer hint = 0);
void deallocate(const_pointer p, size_type n);
size_type max_size() const throw();
void construct(const_pointer p, const_reference val);
void destroy(const_pointer p);
};
Proposed resolution 2
Change the text in row 1, column 2 of table 32 in 20.1.5, p3 from
any type
to
any non-const type
Section: 22.2.2.1.1 [lib.facet.num.get.members] Status: New 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: New 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:
Remove the requirement that the objects stored in a container be Assignable from 23.1/3 and reintroduce it on a case by case basis (for vector and deque.)
Rationale:
list, set, multiset, map, multimap are able to store non-Assignables.
Section: 20.1.5 [lib.allocator.requirements] Status: New Submitter: Matt Austern Date: 07 Nov 2000
In 20.1.5, paragraph 5, the standard says that "Implementors are encouraged to supply libraries that can accept allocators that encapsulate more general memory models and that support non-equal instances." This is intended as normative encouragement to standard library implementors. However, it is possible to interpret this sentence as applying to nonstandard third-party libraries.
Proposed resolution:
In 20.1.5, paragraph 5, change "Implementors" to "Implementors of the library described in this International Standard".
Section: 23.2.2.4 [lib.list.ops] Status: New 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:
Section: 23.1 [lib.container.requirements] Status: New 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 believes it should be a separate issue.
Steve said: "We may want to state that the const/non-const iterators must have the same difference type, size_type, and category."
(Comment from Judy) I'm not sure if the above sentence should be true for all const and non-const iterators in a particular container, or if it means the container's iterator can't be compared with the container's const_iterator unless the above it true. I suspect the former.
Proposed resolution:
In Section: 23.1 [lib.container.requirements], table 65, in the assertion/note pre/post condition for X::const_iterator, add the following:
typeid(X::const_iterator::difference_type) == typeid(X::iterator::difference_type)
typeid(X::const_iterator::size_type) == typeid(X::iterator::size_type)
typeid(X::const_iterator::category) == typeid(X::iterator::category)
Section: 24.4.1 [lib.reverse.iterators] Status: New 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].
Section: 25.3.7 [lib.alg.min.max] Status: New 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: New 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].
Section: 25.2.4 [lib.alg.replace] Status: New 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).
Section: 20.3.7 [lib.function.pointer.adaptors] Status: New 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), "C")), "C++");
Section: 27.8.1.6 [lib.ifstream.cons] Status: New 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: New 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).
Section: 27.4.2.2 [lib.fmtflags.state] Status: New Submitter: Judy Ward Date: 30 Dec 2000
The Effects clause for ios_base::setf(fmtflags fmtfl) says "Sets fmtfl in flags()". What happens if the user first calls ios_base::scientific and then calls ios_base::fixed or vice-versa? This is an issue for all of the conflicting flags, i.e. ios_base::left and ios_base::right or ios_base::dec, ios_base::hex and ios_base::oct.
I see three possible solutions:
Most existing implementations that I tried seem to conform to resolution #3, except that when using the iomanip manipulator hex or oct then that always overrides dec, but calling setf(ios_base::hex) doesn't.
There is a sort of related issue, which is that although the ios_base constructor says that each ios_base member has an indeterminate value after construction, all the existing implementations I tried explicitly set ios_base::dec.
Proposed resolution:
Section: 19.3 [lib.errno] Status: New 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: 26.5 [lib.c.math] Status: New Submitter: Judy Ward Date: 30 Dec 2000
In ISO/IEC 9899:1990 Programming Languages C we find the following concerning <math.h>:
7.13.4 Mathematics <math.h>
The names of all existing functions declared in the <math.h> header, suffixed with f or l, are reserved respectively for corresponding functions with float and long double arguments are return values.
For example, float sinf(float) is reserved.
In the C99 standard, <math.h> must contain declarations for these functions.
So, is it acceptable for an implementor to add these prototypes to the C++ versions of the math headers? Are they required?
Proposed resolution:
Add these Functions to Table 80, section 26.5 and to Table 99, section C.2:
acosf asinf atanf atan2f ceilf cosf coshf
expf fabsf floorf fmodf frexpf ldexpf
logf log10f modff powf sinf sinhf sqrtf
tanf tanhf
acosl asinl atanl atan2l ceill cosl coshl
expl fabsl floorl fmodl frexpl ldexpl
logl log10l modfl powl sinl sinhl sqrtl
tanl tanhl
There should probably be a note saying that these functions are optional and, if supplied, should match the description in the 1999 version of the C standard. In the next round of C++ standardization they can then become mandatory.
Section: 25.1.1 [lib.alg.foreach] Status: New 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.
Rationale:
This is a minimum requirement that frees library implementations from the duty to invoke the function exactly last - first times when the function object modifies the input range. It imposes restrictions on the user, however. The committee will have to decide whether the restrictions are too severe.
Section: 25.3.5 [lib.alg.set.operations] Status: New 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:
Section: 27.4.4.2 [lib.basic.ios.members] Status: New 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: 25.2.3 [lib.alg.transform] Status: New Submitter: Angelika Langer Date: 04 Jan 2001
This issue is related to issue 242. In case that the resolution proposed for issue 242 is accepted, we have have the following situation: The 4 numeric algorithms (accumulate and consorts) as well as transform would allow a certain category of side effects. The numeric algorithms specify that they invoke the functor "for every iterator i in the range [first, last) in order". transform, in contrast, would not give any guarantee regarding order of invocation of the functor, which means that the functor can be invoked in any arbitrary order.
Why would that be a problem? Consider an example: say the transformator that is a simple enumerator ( or more generally speaking, "is order-sensitive" ). Since a standard compliant implementation of transform is free to invoke the enumerator in no definite order, the result could be a garbled enumeration. Strictly speaking this is not a problem, but it is certainly at odds with the prevalent understanding of transform as an algorithms that assigns "a new _corresponding_ value" to the output elements.
All implementations that I know of invoke the transformator in definite order, namely starting from first and proceeding to last - 1. Unless there is an optimization conceivable that takes advantage of the indefinite order I would suggest to specify the order, because it eliminate the uncertainty that users would otherwise have regarding the order of execution of their potentially order-sensitive function objects.
Proposed resolution:
In section 25.2.3 - Transform [lib.alg.transform] change:
-1- Effects: Assigns through every iterator i in the range [result, result + (last1 - first1)) a new corresponding value equal to op(*(first1 + (i - result)) or binary_op(*(first1 + (i - result), *(first2 + (i - result))).
to:
-1- Effects: Computes values by invoking the operation op or binary_op for every iterator in the range [first1, last1) in order. Assigns through every iterator i in the range [result, result + (last1 - first1)) a new corresponding value equal to op(*(first1 + (i - result)) or binary_op(*(first1 + (i - result), *(first2 + (i - result))).
Section: 17.4.3.1.1 [lib.macro.names] Status: New 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."
Section: 26.5 [lib.c.math] Status: New 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.
Section: 20.2.2 [lib.pairs] Status: New 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:
Since it is a convention used throughout the rest of the document to group non-member functions and templates provided as part of the interface of a class or a template into their own separate section, I propose to add a separate subsection for non-member pair function templates after 20.2.2, p4, containing the current paragraphs 5 through 8 as well as the descriptions of the missing functions. That is, add after
template<class U, class V> pair(const pair<U, V>&p);
-4- Effects:Initializes members from the corresponding members of the argument, performing implicit conversions as needed.
the following:
20.2.2.1 - pair non-member functions [lib.pairs.nonmembers]
template <class T1, class T2>
bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y);
-1- Requires:Types T1 and T2 are EqualityComparable (20.1.1 [lib.equalitycomparable]).
-2- Returns: x.first == y.first && x.second == y.second.
template <class T1, class T2>
bool operator!=(const pair<T1, T2>& x, const pair<T1, T2>& y);
-3- Requires: Types T1 and T2 are EqualityComparable (20.1.1 [lib.equalitycomparable]).
-4- Returns: !(x == y).
template <class T1, class T2>
bool operator<(const pair<T1, T2>& x, const pair<T1, T2>& y);
-5- Requires: Types T1 and T2 are LessThanComparable (20.1.2 [lib.lessthancomparable]).
-6- Returns: x.first < y.first || (!(y.first < x.first) && x.second < y.second).
template <class T1, class T2>
bool operator<=(const pair<T1, T2>& x, const pair<T1, T2>& y);
-7- Requires: Types T1 and T2 are LessThanComparable (20.1.2 [lib.lessthancomparable]).
-8- Returns: !(y < x).
template <class T1, class T2>
bool operator>=(const pair<T1, T2>& x, const pair<T1, T2>& y);
-9- Requires: Types T1 and T2 are LessThanComparable (20.1.2 [lib.lessthancomparable]).
-10- Returns: !(x < y).
template <class T1, class T2>
bool operator>(const pair<T1, T2>& x, const pair<T1, T2>& y);
-11- Requires: Types T1 and T2 are LessThanComparable (20.1.2 [lib.lessthancomparable]).
-12- Returns: y < x.
template <class T1, class T2>
pair<T1, T2> make_pair(const T1& x, const T2& y);
-- Returns: pair<T1, T2>(x, y).
-12- [Example: In place of:
return pair<int, double>(5, 3.1415926); // explicit types
a C++ program may contain:
return make_pair(5, 3.1415926); // types are deduced
--- end example]
Note: Since the accepted resolution of the library issue 181 changes the signature of make_pair, it must be reflected in the resolution of this issue should it too be accepted.
Section: 20.3.8 [lib.member.pointer.adaptors] Status: New 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> {
Section: 18.4.1.2 [lib.new.delete.array] Status: New 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: New 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.
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: New 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.
Section: 21.3.1 [lib.string.cons] Status: New 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: 22.2.1.5.2 [lib.locale.codecvt.virtuals] Status: New Submitter: Gregory Bumgardner Date: 25 Jan 2001
The effects of codecvt<>::do_length() are described in 22.2.1.5.2, paragraph 10. As implied by that paragraph, and clarified in issue 75, codecvt<>::do_length() must process the source data and update the stateT argument just as if the data had been processed by codecvt<>::in(). However, the standard does not specify how do_length() would report a translation failure, should the source sequence contain untranslatable or illegal character sequences.
The other conversion methods return an "error" result value to indicate that an untranslatable character has been encountered, but do_length() already has a return value (the number of source characters that have been processed by the method).
Proposed resolution:
This issue cannot be resolved without modifying the interface. An exception cannot be used, as there would be no way to determine how many characters have been processed and the state object would be left in an indeterminate state.
A source compatible solution involves adding a fifth argument to length() and do_length() that could be used to return position of the offending character sequence. This argument would have a default value that would allow it to be ignored:
int length(stateT& state,
const externT* from,
const externT* from_end,
size_t max,
const externT** from_next = 0);
virtual
int do_length(stateT& state,
const externT* from,
const externT* from_end,
size_t max,
const externT** from_next);
Then an exception could be used to report any translation errors and the from_next argument, if used, could then be used to retrieve the location of the offending character sequence.
Section: 23.3.5.3 [lib.bitset.operators] Status: New 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:
Alternative A:
Replace the first sentence of paragraph 5 with:
Extracts up to N characters from is, converting each character c to char as if by calling is.narrow(c, ' ').
Alternative B:
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:
- the next input character is neither is.widen(0) nor is.widen(1) (in which case the input character is not extracted).
Section: 24.1 [lib.iterator.requirements] Status: New 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:
An sketch of one possible solution: one can return a proxy containing an instance of the value_type if the proxy has an operator->(), since it is required that operator->() be called again on the result if defined (see 13.3.3.1 [over.best.ics] paragraph 8, and footnote 120). This is quirky and not obviously intentional, but workable.
Section: 22.2.1.5.2 [lib.locale.codecvt.virtuals] Status: New 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).
Section: 18.1 [lib.support.types] Status: New 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."
Section: 23.2.3 [lib.container.adaptors] Status: New 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);
}
Section: 27 [lib.input.output] Status: New 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>, <cstdio> and <cwchar> from Table 82.
Section: 27.6 [lib.iostream.format] Status: New 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 27.06.02.01.03, 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 27.06.02.01.03, 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.)
Section: 17.4.1.2 [lib.headers], 19.3 [lib.errno] Status: New 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 is 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:
Section: 27.6.2.1 [lib.ostream] Status: New 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:
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