| Doc. no. | N1684=04-0124 |
| Date: | 10 Sep 2004 |
| Project: | Programming Language C++ |
| Reply to: | Matt Austern <[email protected]> |
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, Ready, and Review. See Library Defect Reports List for issues considered defects and 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 official version of this document see http://www.open-std.org/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 identifies 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.
WP - (Working Paper) - The proposed resolution has not been accepted as a Technical Corrigendum, but the full WG21 committee has voted to apply the Defect Report's Proposed Resolution to the working paper.
RR - (Record of Response) - The full WG21 committee has determined that this issue is not a defect in the Standard. Action on this issue is thus complete and no further action is possible under ISO rules.
Future - In addition to the regular status, the LWG believes that this issue should be revisited at the next revision of the standard. It is usually paired with NAD.
Issues are always given the status of New when they first appear on the issues list. They may progress to Open or Review while the LWG is actively working on them. When the LWG has reached consensus on the disposition of an issue, the status will then change to Dup, NAD, or Ready as appropriate. Once the full J16 committee votes to forward Ready issues to the Project Editor, they are given the status of Defect Report ( DR). These in turn may become the basis for Technical Corrigenda (TC), or are closed without action other than a Record of Response (RR ). The intent of this LWG process is that only issues which are truly defects in the Standard move to the formal ISO DR status.
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:
Further discussion from Redmond:
The basic problem is that we've defined our behavior, including our error-reporting behavior, in terms of C90. However, C90's method of reporting overflow in scanf is not technically an "input error". The strto_* functions are more precise.
There was general consensus that failbit should be set upon overflow. We considered three options based on this:
Straw poll: (1) 5; (2) 0; (3) 8.
Further discussion from Santa Cruz:
There was some discussion of what the intent of our error reporting mechanism was. There was general agreement on the following principles:
The crux of the disagreement was that some people, but not all, believed that the design was also based on a fourth principle: whenever converstion fails and failbit is set, nothing is to be extracted and the value of the variable being extracted into is guaranteed to be unchanged.
Some people believe that upon overflow, an implementation should "extract" a special value that allows the user to tell that it was overflow instead of some other kind of error. Straw poll: 1 person believed the standard should require that, 2 thought it should forbid it, and 6 thought the standard should allow but not require it.
Proposed resolution:
typo: 22.2.2.2.2 [lib.facet.num.put.virtuals], para 2, bullet 3. Strike "in." from the end.
Change 22.2.2.2 [lib.locale.nm.put], para 11, bullet 2 from:
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:
The sequence of chars accumulated in stage 2 would have caused scanf to report an input failure or to store a value outside the range representable by val. ios_base::failbit is assigned to err.
[PJP provided wording. this treats overflow or underflow the same as an ill-formed field. It's not exactly the consensus from Santa Cruz, but he thinks it's the simplest and most robust rule and that it corresponds to widespread common practice.]
[Kona: Wording here still isn't quite right, partly because it refers to scanf and the scanf description of error conditions is murky. The LWG had to do a very close reading of scanf in an attempt to figure out what this proposed resolution means. General agreement that the correct solution: (1) should not refer to scanf behavior, (2) should not set errno, (3) should allow users who care to figure out what kind of error happened. Martin will provide wording, Howard may help.]
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: 23.1.2 [lib.associative.reqmts], 23.1.1 [lib.sequence.reqmts] Status: Review Submitter: Andrew Koenig Date: 2 Mar 1999
Table 67 (23.1.1) says that container::erase(iterator) returns an iterator. Table 69 (23.1.2) says that in addition to this requirement, associative containers also say that container::erase(iterator) returns void. That's not an addition; it's a change to the requirements, which has the effect of making associative containers fail to meet the requirements for containers.
Proposed resolution:
In 23.1.2 [lib.associative.reqmts], in Table 69 Associative container requirements, change the return type of a.erase(q) from void to iterator. Change the assertion/not/pre/post-condition from "erases the element pointed to by q" to "erases the element pointed to by q. Returns an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, a.end() is returned."
In 23.1.2 [lib.associative.reqmts], in Table 69 Associative container requirements, change the return type of a.erase(q1, q2) from void to iterator. Change the assertion/not/pre/post-condition from "erases the elements in the range [q1, q2)" to "erases the elements in the range [q1, q2). Returns an iterator pointing to the element immediately following q2 prior to any elements being erased. If no such element exists, a.end() is returned."
In 23.3.1 [lib.map], in the map class synopsis; and in 23.3.2 [lib.multimap], in the multimap class synopsis; and in 23.3.3 [lib.set], in the set class synopsis; and in 23.3.4 [lib.multiset], in the multiset class synopsis: change the signature of the first erase overload to
iterator erase(iterator position);
[Pre-Kona: reopened at the request of Howard Hinnant]
[Post-Kona: the LWG agrees the return type should be iterator, not void. (Alex Stepanov agrees too.) Matt provided wording.]
[ Sydney: the proposed wording went in the right direction, but it wasn't good enough. We want to return an iterator from the range form of erase as well as the single-iterator form. Also, the wording is slightly different from the wording we have for sequences; there's no good reason for having a difference. Matt provided new wording, which we will review at the next meeting. ]
Section: 20.1.5 [lib.allocator.requirements], 23.1 [lib.container.requirements] Status: Open Submitter: Andy Sawyer Date: 21 Oct 1999
Must the value returned by max_size() be unchanged from call to call?
Must the value returned from max_size() be meaningful?
Possible meanings identified in lib-6827:
1) The largest container the implementation can support given "best case"
conditions - i.e. assume the run-time platform is "configured to the max", and
no overhead from the program itself. This may possibly be determined at the
point the library is written, but certainly no later than compile
time.
2) The largest container the program could create, given "best
case" conditions - i.e. same platform assumptions as (1), but take into account
any overhead for executing the program itself. (or, roughly
"storage=storage-sizeof(program)"). This does NOT include any resource allocated
by the program. This may (or may not) be determinable at compile time.
3)
The largest container the current execution of the program could create, given
knowledge of the actual run-time platform, but again, not taking into account
any currently allocated resource. This is probably best determined at program
start-up.
4) The largest container the current execution program could
create at the point max_size() is called (or more correctly at the point
max_size() returns :-), given it's current environment (i.e. taking into account
the actual currently available resources). This, obviously, has to be determined
dynamically each time max_size() is called.
Proposed resolution:
Change 20.1.5 [lib.allocator.requirements]
table 32 max_size() wording from:
the
largest value that can meaningfully be passed to
X::allocate
to:
the value of the largest
constant expression (5.19 [expr.const])
that could ever meaningfully be passed to X::allocate
Change 23.1 [lib.container.requirements]
table 65 max_size() wording from:
size()
of the largest possible container.
to:
the
value of the largest constant expression (5.19 [expr.const])
that could ever meaningfully be returned by X::size().
[Kona: The LWG informally discussed this and asked Andy Sawyer to submit an issue.]
[Tokyo: The LWG believes (1) above is the intended meaning.]
[Post-Tokyo: Beman Dawes supplied the above resolution at the request of the LWG. 21.3.3 [lib.string.capacity] was not changed because it references max_size() in 23.1. The term "compile-time" was avoided because it is not defined anywhere in the standard (even though it is used several places in the library clauses).]
[Copenhagen: Exactly what max_size means is still unclear. It may have a different meaning as a container member function than as an allocator member function. For the latter, it is probably best thought of as an architectural limit. Nathan will provide new wording.]
Section: 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: 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.
Additional comments from Nathan:
The vote [in Redmond] was on
whether to elaborately specify the use of the hint, or to require behavior only
if the value could be inserted adjacent to the hint. I would like to ensure that
we have a chance to vote for a deterministic treatment: "before, if possible,
otherwise after, otherwise anywhere appropriate", as an alternative to the
proposed "before or after, if possible, otherwise [...]".
Proposed resolution:
In table 69 "Associative Container Requirements" in 23.1.2 [lib.associative.reqmts], in the row for a.insert(p, t), change
iterator p is a hint pointing to where the insert should start to search.
to
insertion adjacent to iterator p is preferred if more than one insertion point is valid.
and change
logarithmic in general, but amortized constant if t is inserted right after p.
to
logarithmic in general, but amortized constant if t is inserted adjacent to iterator 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. The proposed resolution was that the new element should always be inserted as close to the hint as possible. So, for example, if there is a subsequence of equivalent values, then providing a.begin() as the hint means that the new element should be inserted before the subsequence even if a.begin() is far away. JC van Winkel supplied precise wording for this proposed resolution, and also for an alternative resolution in which hints are only used when they are adjacent to the insertion point.]
[Copenhagen: the LWG agreed to the original proposed resolution, in which an insertion hint would be used even when it is far from the insertion point. This was contingent on seeing a reference implementation showing that it is possible to implement this requirement without loss of efficiency. John Potter provided such a reference implementation.]
[Redmond: The LWG was reluctant to adopt the proposal that emerged from Copenhagen: it seemed excessively complicated, and went beyond fixing the defect that we identified in Toronto. PJP provided the new wording described in this issue. Nathan agrees that we shouldn't adopt the more detailed semantics, and notes: "we know that you can do it efficiently enough with a red-black tree, but there are other (perhaps better) balanced tree techniques that might differ enough to make the detailed semantics hard to satisfy."]
[Curaçao: Nathan should give us the alternative wording he suggests so the LWG can decide between the two options.]
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: 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.
Further discussion, from Redmond:
The most important progress we made at the Redmond meeting was realizing that there are two separable issues here: the const string& constructor, and the copy constructor. If a user writes something like throw std::out_of_range("foo"), the const string& constructor is invoked before anything gets thrown. The copy constructor is potentially invoked during stack unwinding.
The copy constructor is a more serious problem, becuase failure during stack unwinding invokes terminate. The copy constructor must be nothrow. Curaçao: Howard thinks this requirement is already present.
The fundamental problem is that it's difficult to get the nothrow requirement to work well with the requirement that the exception objects store a string of unbounded size, particularly if you also try to make the const string& constructor nothrow. Options discussed include:
(Not all of these options are mutually exclusive.)
Proposed resolution:
Rationale:
Throwing a bad_alloc while trying to construct a message for another exception-derived class is not necessarily a bad thing. And the bad_alloc constructor already has a no throw spec on it (18.4.2.1).
The copy constructors of all exception-derived classes already have a no throw spec. Reference 18.6.1, 19.1 and 15.4/13.
Future:
All involved would like to see const char* constructors added, but this should probably be done for C++0X as opposed to a DR.
I believe the no throw specs currently decorating these functions could be improved by some kind of static no throw spec checking mechanism (in a future C++ language). As they stand, the copy constructors might fail via a call to unexpected. I think what is intended here is that the copy constructors can't fail.
[Pre-Sydney: reopened at the request of Howard Hinnant.]
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: 24.4.1 [lib.reverse.iterators] Status: Open Submitter: Steve Cleary Date: 27 Nov 2000
This came from an email from Steve Cleary to Fergus in reference to issue 179. The library working group briefly discussed this in Toronto and believed it should be a separate issue. There was also some reservations about whether this was a worthwhile problem to fix.
Steve said: "Fixing reverse_iterator. std::reverse_iterator can (and should) be changed to preserve these additional requirements." He also said in email that it can be done without breaking user's code: "If you take a look at my suggested solution, reverse_iterator doesn't have to take two parameters; there is no danger of breaking existing code, except someone taking the address of one of the reverse_iterator global operator functions, and I have to doubt if anyone has ever done that. . . But, just in case they have, you can leave the old global functions in as well -- they won't interfere with the two-template-argument functions. With that, I don't see how any user code could break."
Proposed resolution:
Section: 24.4.1.1 [lib.reverse.iterator] add/change the following declarations:
A) Add a templated assignment operator, after the same manner
as the templated copy constructor, i.e.:
template < class U >
reverse_iterator < Iterator >& operator=(const reverse_iterator< U >& u);
B) Make all global functions (except the operator+) have
two template parameters instead of one, that is, for
operator ==, !=, <, >, <=, >=, - replace:
template < class Iterator >
typename reverse_iterator< Iterator >::difference_type operator-(
const reverse_iterator< Iterator >& x,
const reverse_iterator< Iterator >& y);
with:
template < class Iterator1, class Iterator2 >
typename reverse_iterator < Iterator1 >::difference_type operator-(
const reverse_iterator < Iterator1 > & x,
const reverse_iterator < Iterator2 > & y);
Also make the addition/changes for these signatures in 24.4.1.3 [lib.reverse.iter.ops].
[ Copenhagen: The LWG is concerned that the proposed resolution introduces new overloads. Experience shows that introducing overloads is always risky, and that it would be inappropriate to make this change without implementation experience. It may be desirable to provide this feature in a different way. ]
Section: 25.1.1 [lib.alg.foreach] Status: Open Submitter: Angelika Langer Date: 03 Jan 2001
The specification of the for_each algorithm does not have a "Requires" section, which means that there are no restrictions imposed on the function object whatsoever. In essence it means that I can provide any function object with arbitrary side effects and I can still expect a predictable result. In particular I can expect that the function object is applied exactly last - first times, which is promised in the "Complexity" section.
I don't see how any implementation can give such a guarantee without imposing requirements on the function object.
Just as an example: consider a function object that removes elements from the input sequence. In that case, what does the complexity guarantee (applies f exactly last - first times) mean?
One can argue that this is obviously a nonsensical application and a theoretical case, which unfortunately it isn't. I have seen programmers shooting themselves in the foot this way, and they did not understand that there are restrictions even if the description of the algorithm does not say so.
Proposed resolution:
Add a "Requires" section to section 25.1.1 similar to those proposed for transform and the numeric algorithms (see issue 242):
-2- Requires: In the range [first, last], f shall not invalidate iterators or subranges.
[Copenhagen: The LWG agrees that a function object passed to an algorithm should not invalidate iterators in the range that the algorithm is operating on. The LWG believes that this should be a blanket statement in Clause 25, not just a special requirement for for_each. ]
Section: 17.4.3.1.1 [lib.macro.names] Status: Open Submitter: James Kanze Date: 11 Jan 2001
Paragraph 2 of 17.4.3.1.1 [lib.macro.names] reads: "A translation unit that includes a header shall not contain any macros that define names declared in that header." As I read this, it would mean that the following program is legal:
#define npos 3.14 #include <sstream>
since npos is not defined in <sstream>. It is, however, defined in <string>, and it is hard to imagine an implementation in which <sstream> didn't include <string>.
I think that this phrase was probably formulated before it was decided that a standard header may freely include other standard headers. The phrase would be perfectly appropriate for C, for example. In light of 17.4.4.1 [lib.res.on.headers] paragraph 1, however, it isn't stringent enough.
Proposed resolution:
In paragraph 2 of 17.4.3.1.1 [lib.macro.names], change "A translation unit that includes a header shall not contain any macros that define names declared in that header." to "A translation unit that includes a header shall not contain any macros that define names declared in any standard header."
[Copenhagen: the general idea is clearly correct, but there is concern about making sure that the two paragraphs in 17.4.3.1.1 [lib.macro.names] remain consistent. Nathan will provide new wording.]
Section: 24.1.4 [lib.bidirectional.iterators], 24.1.5 [lib.random.access.iterators] Status: Open Submitter: John Potter Date: 22 Jan 2001
In section 24.1.4 [lib.bidirectional.iterators], Table 75 gives the return type of *r-- as convertible to T. This is not consistent with Table 74 which gives the return type of *r++ as T&. *r++ = t is valid while *r-- = t is invalid.
In section 24.1.5 [lib.random.access.iterators], Table 76 gives the return type of a[n] as convertible to T. This is not consistent with the semantics of *(a + n) which returns T& by Table 74. *(a + n) = t is valid while a[n] = t is invalid.
Discussion from the Copenhagen meeting: the first part is uncontroversial. The second part, operator[] for Random Access Iterators, requires more thought. There are reasonable arguments on both sides. Return by value from operator[] enables some potentially useful iterators, e.g. a random access "iota iterator" (a.k.a "counting iterator" or "int iterator"). There isn't any obvious way to do this with return-by-reference, since the reference would be to a temporary. On the other hand, reverse_iterator takes an arbitrary Random Access Iterator as template argument, and its operator[] returns by reference. If we decided that the return type in Table 76 was correct, we would have to change reverse_iterator. This change would probably affect user code.
History: the contradiction between reverse_iterator and the Random Access Iterator requirements has been present from an early stage. In both the STL proposal adopted by the committee (N0527==94-0140) and the STL technical report (HPL-95-11 (R.1), by Stepanov and Lee), the Random Access Iterator requirements say that operator[]'s return value is "convertible to T". In N0527 reverse_iterator's operator[] returns by value, but in HPL-95-11 (R.1), and in the STL implementation that HP released to the public, reverse_iterator's operator[] returns by reference. In 1995, the standard was amended to reflect the contents of HPL-95-11 (R.1). The original intent for operator[] is unclear.
In the long term it may be desirable to add more fine-grained iterator requirements, so that access method and traversal strategy can be decoupled. (See "Improved Iterator Categories and Requirements", N1297 = 01-0011, by Jeremy Siek.) Any decisions about issue 299 should keep this possibility in mind.
Further discussion: I propose a compromise between John Potter's resolution, which requires T& as the return type of a[n], and the current wording, which requires convertible to T. The compromise is to keep the convertible to T for the return type of the expression a[n], but to also add a[n] = t as a valid expression. This compromise "saves" the common case uses of random access iterators, while at the same time allowing iterators such as counting iterator and caching file iterators to remain random access iterators (iterators where the lifetime of the object returned by operator*() is tied to the lifetime of the iterator).
Note that the compromise resolution necessitates a change to reverse_iterator. It would need to use a proxy to support a[n] = t.
Note also there is one kind of mutable random access iterator that will no longer meet the new requirements. Currently, iterators that return an r-value from operator[] meet the requirements for a mutable random access iterartor, even though the expression a[n] = t will only modify a temporary that goes away. With this proposed resolution, a[n] = t will be required to have the same operational semantics as *(a + n) = t.
Proposed resolution:
In section 24.1.4 [lib.bidirectdional.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 operational semantics for a[n] to " the r-value of a[n] is equivalent to the r-value of *(a + n)". Add a new row in the table for the expression a[n] = t with a return type of convertible to T and operational semantics of *(a + n) = t.
Section: 27.6 [lib.iostream.format] Status: Open Submitter: Martin Sebor Date: 19 Mar 2001
The descriptions of the constructors of basic_istream<>::sentry (27.6.1.1.2 [lib.istream::sentry]) and basic_ostream<>::sentry (27.6.2.3 [lib.ostream::sentry]) do not explain what the functions do in case an exception is thrown while they execute. Some current implementations allow all exceptions to propagate, others catch them and set ios_base::badbit instead, still others catch some but let others propagate.
The text also mentions that the functions may call setstate(failbit) (without actually saying on what object, but presumably the stream argument is meant). That may have been fine for basic_istream<>::sentry prior to issue 195, since the function performs an input operation which may fail. However, issue 195 amends 27.6.1.1.2 [lib.istream::sentry], p2 to clarify that the function should actually call setstate(failbit | eofbit), so the sentence in p3 is redundant or even somewhat contradictory.
The same sentence that appears in 27.6.2.3 [lib.ostream::sentry], p3 doesn't seem to be very meaningful for basic_istream<>::sentry which performs no input. It is actually rather misleading since it would appear to guide library implementers to calling setstate(failbit) when os.tie()->flush(), the only called function, throws an exception (typically, it's badbit that's set in response to such an event).
Additional comments from Martin, who isn't comfortable with the current proposed resolution (see c++std-lib-11530)
The istream::sentry ctor says nothing about how the function deals with exemptions (27.6.1.1.2, p1 says that the class is responsible for doing "exception safe"(*) prefix and suffix operations but it doesn't explain what level of exception safety the class promises to provide). The mockup example of a "typical implementation of the sentry ctor" given in 27.6.1.1.2, p6, removed in ISO/IEC 14882:2003, doesn't show exception handling, either. Since the ctor is not classified as a formatted or unformatted input function, the text in 27.6.1.1, p1 through p4 does not apply. All this would seem to suggest that the sentry ctor should not catch or in any way handle exceptions thrown from any functions it may call. Thus, the typical implementation of an istream extractor may look something like [1].
The problem with [1] is that while it correctly sets ios::badbit if an exception is thrown from one of the functions called from the sentry ctor, if the sentry ctor reaches EOF while extracting whitespace from a stream that has eofbit or failbit set in exceptions(), it will cause an ios::failure to be thrown, which will in turn cause the extractor to set ios::badbit.
The only straightforward way to prevent this behavior is to move the definition of the sentry object in the extractor above the try block (as suggested by the example in 22.2.8, p9 and also indirectly supported by 27.6.1.3, p1). See [2]. But such an implementation will allow exceptions thrown from functions called from the ctor to freely propagate to the caller regardless of the setting of ios::badbit in the stream object's exceptions().
So since neither [1] nor [2] behaves as expected, the only possible solution is to have the sentry ctor catch exceptions thrown from called functions, set badbit, and propagate those exceptions if badbit is also set in exceptions(). (Another solution exists that deals with both kinds of sentries, but the code is non-obvious and cumbersome -- see [3].)
Please note that, as the issue points out, current libraries do not behave consistently, suggesting that implementors are not quite clear on the exception handling in istream::sentry, despite the fact that some LWG members might feel otherwise. (As documented by the parenthetical comment here: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1480.html#309)
Also please note that those LWG members who in Copenhagen felt that "a sentry's constructor should not catch exceptions, because sentries should only be used within (un)formatted input functions and that exception handling is the responsibility of those functions, not of the sentries," as noted here http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2001/n1310.html#309 would in effect be either arguing for the behavior described in [1] or for extractors implemented along the lines of [3].
The original proposed resolution (Revision 25 of the issues list) clarifies the role of the sentry ctor WRT exception handling by making it clear that extractors (both library or user-defined) should be implemented along the lines of [2] (as opposed to [1]) and that no exception thrown from the callees should propagate out of either function unless badbit is also set in exceptions().
[1] Extractor that catches exceptions thrown from sentry:
struct S { long i; };
istream& operator>> (istream &strm, S &s)
{
ios::iostate err = ios::goodbit;
try {
const istream::sentry guard (strm, false);
if (guard) {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
}
catch (...) {
bool rethrow;
try {
strm.setstate (ios::badbit);
rethrow = false;
}
catch (...) {
rethrow = true;
}
if (rethrow)
throw;
}
if (err)
strm.setstate (err);
return strm;
}
[2] Extractor that propagates exceptions thrown from sentry:
istream& operator>> (istream &strm, S &s)
{
istream::sentry guard (strm, false);
if (guard) {
ios::iostate err = ios::goodbit;
try {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
catch (...) {
bool rethrow;
try {
strm.setstate (ios::badbit);
rethrow = false;
}
catch (...) {
rethrow = true;
}
if (rethrow)
throw;
}
if (err)
strm.setstate (err);
}
return strm;
}
[3] Extractor that catches exceptions thrown from sentry but doesn't set badbit if the exception was thrown as a result of a call to strm.clear().
istream& operator>> (istream &strm, S &s)
{
const ios::iostate state = strm.rdstate ();
const ios::iostate except = strm.exceptions ();
ios::iostate err = std::ios::goodbit;
bool thrown = true;
try {
const istream::sentry guard (strm, false);
thrown = false;
if (guard) {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
}
catch (...) {
if (thrown && state & except)
throw;
try {
strm.setstate (ios::badbit);
thrown = false;
}
catch (...) {
thrown = true;
}
if (thrown)
throw;
}
if (err)
strm.setstate (err);
return strm;
}
Proposed resolution:
Remove the last sentence of 27.6.1.1.2 [lib.istream::sentry] p5 (but not the footnote, which should be moved to the preceding sentence).
Remove the last sentence of 27.6.2.3 [lib.ostream::sentry] p3 (but not the footnote, which should be moved to the preceding sentence).
Rationale:
The LWG feels that no clarification of EH policy is necessary: the standard is precise about which operations sentry's constructor performs, and about which of those operations can throw. However, the sentence at the end should be removed because it's redundant.
Section: 27.6.1.3 [lib.istream.unformatted] Status: Open Submitter: Howard Hinnant Date: 09 Oct 201
I think we have a defect.
According to lwg issue 60 which is now a dr, the description of seekg in 27.6.1.3 [lib.istream.unformatted] paragraph 38 now looks like:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail() != true, executes rdbuf()>pubseekpos( pos).
And according to lwg issue 243 which is also now a dr, 27.6.1.3, paragraph 1 looks like:
Each unformatted input function begins execution by constructing an object of class sentry with the default argument noskipws (second) argument true. If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input. 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. If an exception is thrown during input then ios::badbit is turned on in *this'ss error state. If (exception()&badbit)!= 0 then the exception is rethrown. It also counts the number of characters extracted. If no exception has been thrown it ends by storing the count in a member object and returning the value specified. In any event the sentry object is destroyed before leaving the unformatted input function.
And finally 27.6.1.1.2/5 says this about sentry:
If, after any preparation is completed, is.good() is true, ok_ != false otherwise, ok_ == false.
So although the seekg paragraph says that the operation proceeds if !fail(), the behavior of unformatted functions says the operation proceeds only if good(). The two statements are contradictory when only eofbit is set. I don't think the current text is clear which condition should be respected.
Further discussion from Redmond:
PJP: It doesn't seem quite right to say that seekg is "unformatted". That makes specific claims about sentry that aren't quite appropriate for seeking, which has less fragile failure modes than actual input. If we do really mean that it's unformatted input, it should behave the same way as other unformatted input. On the other hand, "principle of least surprise" is that seeking from EOF ought to be OK.
Dietmar: nothing should depend on eofbit. Eofbit should only be examined by the user to determine why something failed.
[Taken from c++std-lib-8873, c++std-lib-8874, c++std-lib-8876]
Proposed resolution:
[Santa Cruz: On the one hand, it would clearly be silly to seek to a non-EOF position without resetting eofbit. On the other hand, having seek clear eofbit explicitly would set a major precedent: there is currently no place where any of the flags are reset without the user explicitly asking for them to be. This is the tip of a general problem, that the various flags are stickier than many users might expect. Bill, Gaby, and Howard will discuss this issue and propose a resolution.]
Section: 22.2.1 [lib.category.ctype] Status: Open Submitter: Matt Austern Date: 23 Jan 2002
What should the following program print?
#include <locale>
#include <iostream>
class my_ctype : public std::ctype<char>
{
typedef std::ctype<char> base;
public:
my_ctype(std::size_t refs = 0) : base(my_table, false, refs)
{
std::copy(base::classic_table(), base::classic_table() + base::table_size,
my_table);
my_table[(unsigned char) '_'] = (base::mask) (base::print | base::space);
}
private:
mask my_table[base::table_size];
};
int main()
{
my_ctype ct;
std::cout << "isspace: " << ct.is(std::ctype_base::space, '_') << " "
<< "isalpha: " << ct.is(std::ctype_base::alpha, '_') << std::endl;
}
The goal is to create a facet where '_' is treated as whitespace.
On gcc 3.0, this program prints "isspace: 1 isalpha: 0". On Microsoft C++ it prints "isspace: 1 isalpha: 1".
I believe that both implementations are legal, and the standard does not give enough guidance for users to be able to use std::ctype's protected interface portably.
The above program assumes that ctype_base::mask enumerators like space and print are disjoint, and that the way to say that a character is both a space and a printing character is to or those two enumerators together. This is suggested by the "exposition only" values in 22.2.1 [lib.category.ctype], but it is nowhere specified in normative text. An alternative interpretation is that the more specific categories subsume the less specific. The above program gives the results it does on the Microsoft compiler because, on that compiler, print has all the bits set for each specific printing character class.
From the point of view of std::ctype's public interface, there's no important difference between these two techniques. From the point of view of the protected interface, there is. If I'm defining a facet that inherits from std::ctype<char>, I'm the one who defines the value that table()['a'] returns. I need to know what combination of mask values I should use. This isn't so very esoteric: it's exactly why std::ctype has a protected interface. If we care about users being able to write their own ctype facets, we have to give them a portable way to do it.
Related reflector messages: lib-9224, lib-9226, lib-9229, lib-9270, lib-9272, lib-9273, lib-9274, lib-9277, lib-9279.
Issue 339 is related, but not identical. The proposed resolution if issue 339 says that ctype_base::mask must be a bitmask type. It does not say that the ctype_base::mask elements are bitmask elements, so it doesn't directly affect this issue.
More comments from Benjamin Kosnik, who believes that that C99 compatibility essentially requires what we're calling option 1 below.
I think the C99 standard is clear, that isspace -> !isalpha.
--------
#include <locale>
#include <iostream>
class my_ctype : public std::ctype<char>
{
private:
typedef std::ctype<char> base;
mask my_table[base::table_size];
public:
my_ctype(std::size_t refs = 0) : base(my_table, false, refs)
{
std::copy(base::classic_table(), base::classic_table() + base::table_size,
my_table);
mask both = base::print | base::space;
my_table[static_cast<mask>('_')] = both;
}
};
int main()
{
using namespace std;
my_ctype ct;
cout << "isspace: " << ct.is(ctype_base::space, '_') << endl;
cout << "isprint: " << ct.is(ctype_base::print, '_') << endl;
// ISO C99, isalpha iff upper | lower set, and !space.
// 7.5, p 193
// -> looks like g++ behavior is correct.
// 356 -> bitmask elements are required for ctype_base
// 339 -> bitmask type required for mask
cout << "isalpha: " << ct.is(ctype_base::alpha, '_') << endl;
}
Proposed resolution:
Informally, we have three choices:
Either of the first two options is just as good from the standpoint of portability. Either one will require some implementations to change.
[ More discussion is needed. Nobody likes option 3. Options 1 and 2 are both controversial, 2 perhaps less so. Benjamin thinks that option 1 is required for C99 compatibility. ]
Section: 20.3.6.2 [lib.bind.1st] Status: Open Submitter: Andrew Demkin Date: 26 Apr 2002
The definition of bind1st() (20.3.6.2 [lib.bind.1st]) can result in the construction of an unsafe binding between incompatible pointer types. For example, given a function whose first parameter type is 'pointer to T', it's possible without error to bind an argument of type 'pointer to U' when U does not derive from T:
foo(T*, int);
struct T {};
struct U {};
U u;
int* p;
int* q;
for_each(p, q, bind1st(ptr_fun(foo), &u)); // unsafe binding
The definition of bind1st() includes a functional-style conversion to map its argument to the expected argument type of the bound function (see below):
typename Operation::first_argument_type(x)
A functional-style conversion (5.2.3 [expr.type.conv]) is defined to be semantically equivalent to an explicit cast expression (5.4 [expr.cast]), which may (according to 5.4, paragraph 5) be interpreted as a reinterpret_cast, thus masking the error.
The problem and proposed change also apply to 20.3.6.4 [lib.bind.2nd].
Proposed resolution:
The simplest and most localized change to prevent such errors is to require bind1st() use a static_cast expression rather than the functional-style conversion; that is, have bind1st() return:
binder1st<Operation>( op,
static_cast<typename Operation::first_argument_type>(x)).
A more agressive solution is to change the semantics of functional-style conversions to not permit a reinterpret_cast. For contexts that require the semantics of reinterpret_cast, the language may want to require the use of an explicit cast expression such as '(T) x' or 'reinterpret_cast<T>(x)' and limit the behavior of the functional notation to match statically-checked and standard conversions (as defined by 5.2.9 and 4.10, etc.). Although changing the semantics of functional-style conversions may seem drastic and does have language-wide ramifications, it has the benefit of better unifying the conversion rules for user defined types and built-in types, which can be especially important for generic template programming.
[Santa Cruz: it's clear that a function-style cast is wrong. Maybe a static cast would be better, or maybe no cast at all. Jeremy will check with the original author of this part of the Standard and will see what the original intent was.]
Section: 27 [lib.input.output] Status: Open Submitter: Walter Brown, Marc Paterno Date: 10 May 2002
The following member functions are declared const, yet return non-const pointers. We believe they are should be changed, because they allow code that may surprise the user. See document N1360 for details and rationale.
[Santa Cruz: the real issue is that we've got const member functions that return pointers to non-const, and N1360 proposes replacing them by overloaded pairs. There isn't a consensus about whether this is a real issue, since we've never said what our constness policy is for iostreams. N1360 relies on a distinction between physical constness and logical constness; that distinction, or those terms, does not appear in the standard.]
Proposed resolution:
In 27.4.4 and 27.4.4.2
Replace
basic_ostream<charT,traits>* tie() const;
with
basic_ostream<charT,traits>* tie(); const basic_ostream<charT,traits>* tie() const;
and replace
basic_streambuf<charT,traits>* rdbuf() const;
with
basic_streambuf<charT,traits>* rdbuf(); const basic_streambuf<charT,traits>* rdbuf() const;
In 27.5.2 and 27.5.2.3.1
Replace
char_type* eback() const;
with
char_type* eback(); const char_type* eback() const;
Replace
char_type gptr() const;
with
char_type* gptr(); const char_type* gptr() const;
Replace
char_type* egptr() const;
with
char_type* egptr(); const char_type* egptr() const;
In 27.5.2 and 27.5.2.3.2
Replace
char_type* pbase() const;
with
char_type* pbase(); const char_type* pbase() const;
Replace
char_type* pptr() const;
with
char_type* pptr(); const char_type* pptr() const;
Replace
char_type* epptr() const;
with
char_type* epptr(); const char_type* epptr() const;
In 27.7.2, 27.7.2.2, 27.7.3 27.7.3.2, 27.7.4, and 27.7.6
Replace
basic_stringbuf<charT,traits,Allocator>* rdbuf() const;
with
basic_stringbuf<charT,traits,Allocator>* rdbuf(); const basic_stringbuf<charT,traits,Allocator>* rdbuf() const;
In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13
Replace
basic_filebuf<charT,traits>* rdbuf() const;
with
basic_filebuf<charT,traits>* rdbuf(); const basic_filebuf<charT,traits>* rdbuf() const;
Section: 21.3.5.6 [lib.string::replace] Status: Open Submitter: Beman Dawes Date: 3 Jun 2002
21.3.5.6 [lib.string::replace] basic_string::replace, second signature, given in paragraph 1, has two "Throws" paragraphs (3 and 5).
In addition, the second "Throws" paragraph (5) includes specification (beginning with "Otherwise, the function replaces ...") that should be part of the "Effects" paragraph.
Proposed resolution:
[This is a typo that escalated. It's clear that what's in the Standard is wrong. It's less clear what the fix ought to be. Someone who understands string replace well needs to work on this.]
Section: 27.3 [lib.iostream.objects] Status: Review Submitter: Ruslan Abdikeev Date: 8 Jul 2002
Is it safe to use standard iostream objects from constructors of static objects? Are standard iostream objects constructed and are their associations established at that time?
Surpisingly enough, Standard does NOT require that.
27.3/2 [lib.iostream.objects] guarantees that standard iostream objects are constructed and their associations are established before the body of main() begins execution. It also refers to ios_base::Init class as the panacea for constructors of static objects.
However, there's nothing in 27.3 [lib.iostream.objects], in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init], that would require implementations to allow access to standard iostream objects from constructors of static objects.
Details:
Core text refers to some magic object ios_base::Init, which will be discussed below:
"The [standard iostream] objects are constructed, and their associations are established at some time prior to or during first time an object of class basic_ios<charT,traits>::Init is constructed, and in any case before the body of main begins execution." (27.3/2 [lib.iostream.objects])
The first non-normative footnote encourages implementations to initialize standard iostream objects earlier than required.
However, the second non-normative footnote makes an explicit and unsupported claim:
"Constructors and destructors for static objects can access these [standard iostream] objects to read input from stdin or write output to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])
The only bit of magic is related to that ios_base::Init class. AFAIK, the rationale behind ios_base::Init was to bring an instance of this class to each translation unit which #included <iostream> or related header. Such an inclusion would support the claim of footnote quoted above, because in order to use some standard iostream object it is necessary to #include <iostream>.
However, while Standard explicitly describes ios_base::Init as an appropriate class for doing the trick, I failed to found a mention of an _instance_ of ios_base::Init in Standard.
Proposed resolution:
Add to [lib.iostream.objects], p2, immediately before the last sentence of the paragraph, the following two sentences:
It is implementation-defined whether the header <iostream> defines an ios_base::Init object or not. If it does not, an implementation must specify the means of achieving safe access to the standard objects for input and output during program startup.
[Santa Cruz: The LWG is leaning toward NAD. There isn't any normative wording saying that the Init scheme will be used, but that is probably intentional. Implementers use dirty tricks for iostream initialization, and doing it portably is somewhere between difficult and impossible. Too much constraint in this area is dangerous, and if we are to make any changes it would probably be more appropriate for them to be nonnormative. Summer '04 mid-meeting mailing: Martin provided wording for resolution and rationale.]
Rationale:
The original proposed resolution unconditionally required implementations to define an ios_base::Init object of some implementation-defined name in the header <iostream>. That's an overspecification. First, defining the object may be unnecessary and even detrimental to performance if an implementation can guarantee that the 8 standard iostream objects will be initialized before any other user-defined object in a program. Second, there is no need to require implementations to document the name of the object.
The new proposed resolution specifies that implementations may (but need not) define an ios_base::Init object, while requiring them to document whether they do or not, and if not, to document how portable programs achieve safe access to the 8 standard iostream objects during program startup (3.6)(*). The intent is that if an implementation documents that <iostream> defines an ios_base::Init object, it implies that the header must be #included before any references to the standard iostream objects. Otherwise, if an implementation does not define an ios_base::Init object in <iostream> it must either assure and document that the standard iostream objects are safely accessible at startup, or specify what a portable program must do to safely access them (e.g., it may require that a program define an ios_base::Init object before doing so, or that it call ios::sync_with_stdio(), etc.).
(*) Note that the term startup is broader than the term "Constructors and destructors for static objects" used in Footnote 265 since the former includes other functions besides constructors and destructors, including the following example:
int foo () { return (std::cout << "foo()\n").rdstate (); }
int i = foo ();
int main () { return i; }
Section: 23.1 [lib.container.requirements] Status: Open Submitter: Frank Compagner Date: 20 Jul 2002
The requirements for multiset and multimap containers (23.1 [lib.containers.requirements], 23.1.2 [lib.associative.reqmnts], 23.3.2 [lib.multimap] and 23.3.4 [lib.multiset]) make no mention of the stability of the required (mutating) member functions. It appears the standard allows these functions to reorder equivalent elements of the container at will, yet the pervasive red-black tree implementation appears to provide stable behaviour.
This is of most concern when considering the behaviour of erase(). A stability requirement would guarantee the correct working of the following 'idiom' that removes elements based on a certain predicate function.
multimap<int, int> m;
multimap<int, int>::iterator i = m.begin();
while (i != m.end()) {
if (pred(i))
m.erase (i++);
else
++i;
}
Although clause 23.1.2/8 guarantees that i remains a valid iterator througout this loop, absence of the stability requirement could potentially result in elements being skipped. This would make this code incorrect, and, furthermore, means that there is no way of erasing these elements without iterating first over the entire container, and second over the elements to be erased. This would be unfortunate, and have a negative impact on both performance and code simplicity.
If the stability requirement is intended, it should be made explicit (probably through an extra paragraph in clause 23.1.2).
If it turns out stability cannot be guaranteed, i'd argue that a remark or footnote is called for (also somewhere in clause 23.1.2) to warn against relying on stable behaviour (as demonstrated by the code above). If most implementations will display stable behaviour, any problems emerging on an implementation without stable behaviour will be hard to track down by users. This would also make the need for an erase_if() member function that much greater.
This issue is somewhat related to LWG issue 130.
[Santa Cruz: More people need to look at this. Much user code may assume stability. On the other hand, it seems drastic to add a new requirement now.]
Proposed resolution:
Section: 27.7.1.3 [lib.stringbuf.virtuals] Status: Open Submitter: Ray Lischner Date: 14 Aug 2002
In Section 27.7.1.3 [lib.stringbuf.virtuals], Table 90, the implication is that the four conditions should be mutually exclusive, but they are not. The first two cases, as written, are subcases of the third. I think it would be clearer if the conditions were rewritten as follows:
(which & (ios_base::in|ios_base::out)) == ios_base::in
(which & (ios_base::in|ios_base::out)) == ios_base::out
(which & (ios_base::in|ios_base::out)) == (ios_base::in|ios_base::out) and way == either ios_base::beg or ios_base::end
Otherwise
As written, it is unclear what should be the result if cases 1 & 2 are true, but case 3 is false, e.g.,
seekoff(0, ios_base::cur, ios_base::in | ios_base::out)
[Santa Cruz: The ambiguity seems real. We need to do a survey of implementations before we decide on a solution.]
Proposed resolution:
Section: 22.1.1 [lib.locale] Status: Review Submitter: Martin Sebor Date: 6 Sep 2002
I think there is a problem with 22.1.1, p6 which says that
-6- An instance of locale is immutable; once a facet reference
is obtained from it, that reference remains usable as long
as the locale value itself exists.
and 22.1.1.2, p4:
const locale& operator=(const locale& other) throw();
-4- Effects: Creates a copy of other, replacing the current value.
How can a reference to a facet obtained from a locale object remain valid after an assignment that clearly must replace all the facets in the locale object? Imagine a program such as this
std::locale loc ("de_DE");
const std::ctype<char> &r0 = std::use_facet<std::ctype<char> >(loc);
loc = std::locale ("en_US");
const std::ctype<char> &r1 = std::use_facet<std::ctype<char> >(loc);
Is r0 really supposed to be preserved and destroyed only when loc goes out of scope?
Proposed resolution:
[Summer '04 mid-meeting mailing: Martin and Dietmar believe this is a duplicate of issue 31 and recommend that it be closed. ]
Section: 22.2.1.5 [lib.locale.codecvt] Status: Open Submitter: Martin Sebor Date: 30 Aug 2002
It seems that the descriptions of codecvt do_in() and do_out() leave sufficient room for interpretation so that two implementations of codecvt may not work correctly with the same filebuf. Specifically, the following seems less than adequately specified:
Finally, the conditions described at the end of 22.2.1.5.2 [lib.locale.codecvt.virtuals], p4 don't seem to be possible:
"A return value of partial, if (from_next == from_end), indicates that either the destination sequence has not absorbed all the available destination elements, or that additional source elements are needed before another destination element can be produced."
If the value is partial, it's not clear to me that (from_next ==from_end) could ever hold if there isn't enough room in the destination buffer. In order for (from_next==from_end) to hold, all characters in that range must have been successfully converted (according to 22.2.1.5.2 [lib.locale.codecvt.virtuals], p2) and since there are no further source characters to convert, no more room in the destination buffer can be needed.
It's also not clear to me that (from_next==from_end) could ever hold if additional source elements are needed to produce another destination character (not element as incorrectly stated in the text). partial is returned if "not all source characters have been converted" according to Table 53, which also implies that (from_next==from) does NOT hold.
Could it be that the intended qualifying condition was actually (from_next != from_end), i.e., that the sentence was supposed to read
"A return value of partial, if (from_next != from_end),..."
which would make perfect sense, since, as far as I understand it, partial can only occur if (from_next != from_end)?
Proposed resolution:
To address these issues, I propose that paragraphs 2, 3, and 4 be rewritten as follows. The proposal incorporates the accepted resolution of lwg issue 19.
-2- Effects: Converts characters in the range of source elements
[from, from_end), placing the results in sequential positions
starting at destination to. Converts no more than (from_end from)
source elements, and stores no more than (to_limit to)
destination elements.
Stops if it encounters a sequence of source elements it cannot
convert to a valid destination character. It always leaves the
from_next and to_next pointers pointing one beyond the last
element successfully converted.
[Note: If returns noconv, internT and externT are the same type
and the converted sequence is identical to the input sequence
[from, from_next). to_next is set equal to to, the value of
state is unchanged, and there are no changes to the values in
[to, to_limit). --end note]
-3- Notes: Its operations on state are unspecified.
[Note: This argument can be used, for example, to maintain shift
state, to specify conversion options (such as count only), or to
identify a cache of seek offsets. --end note]
-4- Returns: An enumeration value, as summarized in Table 53:
Table 53 -- do_in/do_out result values
Value Meaning
+---------+----------------------------------------------------+
| ok | successfully completed the conversion of all |
| | complete characters in the source range |
+---------+----------------------------------------------------+
| partial | the characters in the source range would, after |
| | conversion, require space greater than that |
| | available in the destination range |
+---------+----------------------------------------------------+
| error | encountered either a sequence of elements in the |
| | source range forming a valid source character that |
| | could not be converted to a destination character, |
| | or a sequence of elements in the source range that |
| | could not possibly form a valid source character |
+---------+----------------------------------------------------+
| noconv | internT and externT are the same type, and input |
| | sequence is identical to converted sequence |
+---------+----------------------------------------------------+
A return value of partial, i.e., if (from_next != from_end),
indicates that either the destination sequence has not absorbed
all the available destination elements, or that additional
source elements are needed before another destination character
can be produced.
[Santa Cruz: The LWG agrees that this is an important issue and that this general direction is probably correct. Dietmar, Howard, PJP, and Matt will review this wording.]
[Kona: this isn't quite right. (a) the description of noconv is too vague, both in the existing standard and in the current proposed resolution; (b) the description of what noconv means should be normative; (c) the phrase "partial, i.e. if from_next != from_end" isn't quite right, because those are two separate cases, it's possible to get partial either form insufficient input or from insufficient space in the output buffer. The big problem is that the standard is written with the assumption of 1->N conversion in mind, not M->N. Bill, Howard, and Martin will provide new wording. ]
Section: 25.3.3.3 [lib.equal.range] Status: Open Submitter: Hans Bos Date: 18 Oct 2002
Section 25.3.3.3 [lib.equal.range] states that at most 2 * log(last - first) + 1 comparisons are allowed for equal_range.
It is not possible to implement equal_range with these constraints.
In a range of one element as in:
int x = 1;
equal_range(&x, &x + 1, 1)
it is easy to see that at least 2 comparison operations are needed.
For this case at most 2 * log(1) + 1 = 1 comparison is allowed.
I have checked a few libraries and they all use the same (nonconforming) algorithm for equal_range that has a complexity of
2* log(distance(first, last)) + 2.
I guess this is the algorithm that the standard assumes for equal_range.
It is easy to see that 2 * log(distance) + 2 comparisons are enough since equal range can be implemented with lower_bound and upper_bound (both log(distance) + 1).
I think it is better to require something like 2log(distance) + O(1) (or even logarithmic as multiset::equal_range). Then an implementation has more room to optimize for certain cases (e.g. have log(distance) characteristics when at most match is found in the range but 2log(distance) + 4 for the worst case).
[Santa Cruz: The issue is real, but of greater scope than just equal_range: it affects all of the binary search algorithms. What is the complexity supposed to be for ranges of 0 or 1 elements? What base are we using for the logarithm? Are these bounds supposed to be exact, or asymptotic? (If the latter, of course, then none of the other questions matter.)]
Proposed resolution:
Section: 17 [lib.library] Status: Open Submitter: Matt Austern Date: 23 Oct 2002
Many function templates have parameters that are passed by value; a typical example is find_if's pred parameter in 25.1.2 [lib.alg.find]. Are the corresponding template parameters (Predicate in this case) implicitly required to be CopyConstructible, or does that need to be spelled out explicitly?
This isn't quite as silly a question as it might seem to be at first sight. If you call find_if in such a way that template argument deduction applies, then of course you'll get call by value and you need to provide a copy constructor. If you explicitly provide the template arguments, however, you can force call by reference by writing something like find_if<my_iterator, my_predicate&>. The question is whether implementation are required to accept this, or whether this is ill-formed because my_predicate& is not CopyConstructible.
The scope of this problem, if it is a problem, is unknown. Function object arguments to generic algorithms in clauses 25 [lib.algorithms] and 26 [lib.numerics] are obvious examples. A review of the whole library is necessary.
Proposed resolution:
[ This is really two issues. First, predicates are typically passed by value but we don't say they must be Copy Constructible. They should be. Second: is specialization allowed to transform value arguments into references? References aren't copy constructible, so this should not be allowed. ]
Section: 24.4.1.3.11 [lib.reverse.iter.opindex] Status: Review Submitter: Matt Austern Date: 23 Oct 2002
In 24.4.1.3.11 [lib.reverse.iter.opindex], reverse_iterator<>::operator[] is specified as having a return type of reverse_iterator::reference, which is the same as iterator_traits<Iterator>::reference. (Where Iterator is the underlying iterator type.)
The trouble is that Iterator's own operator[] doesn't necessarily have a return type of iterator_traits<Iterator>::reference. Its return type is merely required to be convertible to Iterator's value type. The return type specified for reverse_iterator's operator[] would thus appear to be impossible.
With the resolution of issue 299, the type of a[n] will continue to be required (for random access iterators) to be convertible to the value type, and also a[n] = t will be a valid expression. Implementations of reverse_iterator will likely need to return a proxy from operator[] to meet these requirements. As mentioned in the comment from Dave Abrahams, the simplest way to specify that reverse_iterator meet this requirement to just mandate it and leave the return type of operator[] unspecified.
Proposed resolution:
In 24.4.1.2 [lib.reverse.iter.requirements] change:
reference operator[](difference_type n) const;
to:
implementation defined operator[](difference_type n) const; // see lib.random.access.iterators
[ Comments from Dave Abrahams: IMO we should resolve 386 by just saying that the return type of reverse_iterator's operator[] is unspecified, allowing the random access iterator requirements to impose an appropriate return type. If we accept 299's proposed resolution (and I think we should), the return type will be readable and writable, which is about as good as we can do. ]
Section: 26.2 [lib.complex.numbers] Status: Open Submitter: Gabriel Dos Reis Date: 8 Nov 2002
The absence of explicit description of std::complex<T> layout makes it imposible to reuse existing software developed in traditional languages like Fortran or C with unambigous and commonly accepted layout assumptions. There ought to be a way for practitioners to predict with confidence the layout of std::complex<T> whenever T is a numerical datatype. The absence of ways to access individual parts of a std::complex<T> object as lvalues unduly promotes severe pessimizations. For example, the only way to change, independently, the real and imaginary parts is to write something like
complex<T> z; // ... // set the real part to r z = complex<T>(r, z.imag()); // ... // set the imaginary part to i z = complex<T>(z.real(), i);
At this point, it seems appropriate to recall that a complex number is, in effect, just a pair of numbers with no particular invariant to maintain. Existing practice in numerical computations has it that a complex number datatype is usually represented by Cartesian coordinates. Therefore the over-encapsulation put in the specification of std::complex<> is not justified.
Proposed resolution:
Add the following requirements to 26.2 [lib.complex.numbers] as 26.2/4:
If z is an lvalue expression of type cv std::complex<T> then
- the expression reinterpret_cast<cv T(&)[2]>(z) is well-formed; and
- reinterpret_cast<cvT(&)[2]>(z)[0]designates the real part of z; and
- reinterpret_cast<cvT(&)[2]>(z)[1]designates the imaginary part of z.
Moreover, if a is an expression of pointer type cv complex<T>* and the expression a[i] is well-defined for an integer expression i then:
- reinterpret_cast<cvT*>(a)[2+i] designates the real part of a[i]; and
- reinterpret_cast<cv T*>(a)[2+i+1] designates the imaginary part of a[i].
In the header synopsis in 26.2.1 [lib.complex.synopsis], replace
template<class T> T real(const complex<T>&); template<class T> T imag(const complex<T>&);
with
template<class T> const T& real(const complex<T>&); template<class T> T& real( complex<T>&); template<class T> const T& imag(const complex<T>&); template<class T> T& imag( complex<T>&);
In 26.2.7 [lib.complex.value.ops] paragraph 1, change
template<class T> T real(const complex<T>&);
to
template<class T> const T& real(const complex<T>&); template<class T> T& real( complex<T>&);
and change the Returns clause to "Returns: The real part of x
.In 26.2.7 [lib.complex.value.ops] paragraph 2, change
template<class T> T imag(const complex<T>&);
to
template<class T> const T& imag(const complex<T>&); template<class T> T& imag( complex<T>&);
and change the Returns clause to "Returns: The imaginary part of x
.[Kona: The layout guarantee is absolutely necessary for C compatibility. However, there was disagreement about the other part of this proposal: retrieving elements of the complex number as lvalues. An alternative: continue to have real() and imag() return rvalues, but add set_real() and set_imag(). Straw poll: return lvalues - 2, add setter functions - 5. Related issue: do we want reinterpret_cast as the interface for converting a complex to an array of two reals, or do we want to provide a more explicit way of doing it? Howard will try to resolve this issue for the next meeting.]
[pre-Sydney: Howard summarized the options in n1589.]
Rationale:
The LWG believes that C99 compatibility would be enough justification for this change even without other considerations. All existing implementations already have the layout proposed here.
Section: 27.6.2.5.1 [lib.ostream.formatted.reqmts] Status: Open Submitter: Martin Sebor Date: 27 Dec 2002
There is a contradiction in Formatted output about what bit is supposed to be set if the formatting fails. On sentence says it's badbit and another that it's failbit.
27.6.2.5.1, p1 says in the Common Requirements on Formatted output functions:
... If the generation fails, then the formatted output function
does setstate(ios::failbit), which might throw an exception.
27.6.2.5.2, p1 goes on to say this about Arithmetic Inserters:
... 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();
... If failed is true then does setstate(badbit) ...
The original intent of the text, according to Jerry Schwarz (see c++std-lib-10500), is captured in the following paragraph:
In general "badbit" should mean that the stream is unusable because of some underlying failure, such as disk full or socket closure; "failbit" should mean that the requested formatting wasn't possible because of some inconsistency such as negative widths. So typically if you clear badbit and try to output something else you'll fail again, but if you clear failbit and try to output something else you'll succeed.
In the case of the arithmetic inserters, since num_put cannot report failure by any means other than exceptions (in response to which the stream must set badbit, which prevents the kind of recoverable error reporting mentioned above), the only other detectable failure is if the iterator returned from num_put returns true from failed().
Since that can only happen (at least with the required iostream specializations) under such conditions as the underlying failure referred to above (e.g., disk full), setting badbit would seem to be the appropriate response (indeed, it is required in 27.6.2.5.2, p1). It follows that failbit can never be directly set by the arithmetic (it can only be set by the sentry object under some unspecified conditions).
The situation is different for other formatted output functions which can fail as a result of the streambuf functions failing (they may do so by means other than exceptions), and which are then required to set failbit.
The contradiction, then, is that ostream::operator<<(int) will set badbit if the disk is full, while operator<<(ostream&, char) will set failbit under the same conditions. To make the behavior consistent, the Common requirements sections for the Formatted output functions should be changed as proposed below.
Proposed resolution:
[Kona: There's agreement that this is a real issue. What we decided at Kona: 1. An error from the buffer (which can be detected either directly from streambuf's member functions or by examining a streambuf_iterator) should always result in badbit getting set. 2. There should never be a circumstance where failbit gets set. That represents a formatting error, and there are no circumstances under which the output facets are specified as signaling a formatting error. (Even more so for string output that for numeric because there's nothing to format.) If we ever decide to make it possible for formatting errors to exist then the facets can signal the error directly, and that should go in clause 22, not clause 27. 3. The phrase "if generation fails" is unclear and should be eliminated. It's not clear whether it's intended to mean a buffer error (e.g. a full disk), a formatting error, or something else. Most people thought it was supposed to refer to buffer errors; if so, we should say so. Martin will provide wording.]
Rationale:
Section: 23.3.5.1 [lib.bitset.cons] Status: Open Submitter: Martin Sebor Date: 5 Jan 2003
23.3.5.1, p6 [lib.bitset.cons] talks about a generic character having the value of 0 or 1 but there is no definition of what that means for charT other than char and wchar_t. And even for those two types, the values 0 and 1 are not actually what is intended -- the values '0' and '1' are. This, along with the converse problem in the description of to_string() in 23.3.5.2, p33, looks like a defect remotely related to DR 303.
http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#303
23.3.5.1:
-6- An element of the constructed string has value zero if the
corresponding character in str, beginning at position pos,
is 0. Otherwise, the element has the value one.
23.3.5.2:
-33- Effects: Constructs a string object of the appropriate
type and initializes it to a string of length N characters.
Each character is determined by the value of its
corresponding bit position in *this. Character position N
?- 1 corresponds to bit position zero. Subsequent decreasing
character positions correspond to increasing bit positions.
Bit value zero becomes the character 0, bit value one becomes
the character 1.
Also note the typo in 23.3.5.1, p6: the object under construction is a bitset, not a string.
Proposed resolution:
Change the constructor's function declaration immediately before 23.3.5.1 [lib.bitset.cons] p3 to:
template <class charT, class traits, class Allocator>
explicit
bitset(const basic_string<charT, traits, Allocator>& str,
typename basic_string<charT, traits, Allocator>::size_type pos = 0,
typename basic_string<charT, traits, Allocator>::size_type n =
basic_string<charT, traits, Allocator>::npos,
charT zero = charT('0'), charT one = charT('1'))
Change the first two sentences of 23.3.5.1 [lib.bitset.cons] p6 to: "An element of the constructed string has value 0 if the corresponding character in str, beginning at position pos, is zero. Otherwise, the element has the value 1.
Change the text of the second sentence in 23.3.5.1, p5 to read: "The function then throws invalid_argument if any of the rlen characters in str beginning at position pos is other than zero or one. The function uses traits::eq() to compare the character values."
Change the declaration of the to_string member function immediately before 23.3.5.2 [lib.bitset.members] p33 to:
template <class charT, class traits, class Allocator>
basic_string<charT, traits, Allocator>
to_string(charT zero = charT('0'), charT one = charT('1')) const;
Change the last sentence of 23.3.5.2 [lib.bitset.members] p33 to: "Bit value 0 becomes the character zero, bit value 1 becomes the character one.
Change 23.3.5.3 [lib.bitset.operators] p8 to:
Returns:
os << x.template to_string<charT,traits,allocator<charT> >(
use_facet<ctype<charT> >(os.getloc()).widen('0'),
use_facet<ctype<charT> >(os.getloc()).widen('1'));
Rationale:
There is a real problem here: we need the character values of '0' and '1', and we have no way to get them since strings don't have imbued locales. In principle the "right" solution would be to provide an extra object, either a ctype facet or a full locale, which would be used to widen '0' and '1'. However, there was some discomfort about using such a heavyweight mechanism. The proposed resolution allows those users who care about this issue to get it right.
We fix the inserter to use the new arguments. Note that we already fixed the analogous problem with the extractor in issue 303.
Section: 27.6.2.3 [lib.ostream::sentry] Status: Open Submitter: Martin Sebor Date: 5 Jan 2003
17.4.4.8, p3 prohibits library dtors from throwing exceptions.
27.6.2.3, p4 says this about the ostream::sentry dtor:
-4- If ((os.flags() & ios_base::unitbuf) && !uncaught_exception())
is true, calls os.flush().
27.6.2.6, p7 that describes ostream::flush() says:
-7- If rdbuf() is not a null pointer, calls rdbuf()->pubsync().
If that function returns ?-1 calls setstate(badbit) (which
may throw ios_base::failure (27.4.4.3)).
That seems like a defect, since both pubsync() and setstate() can throw an exception.
Proposed resolution:
[ The contradiction is real. Clause 17 says destructors may never throw exceptions, and clause 27 specifies a destructor that does throw. In principle we might change either one. We're leaning toward changing clause 17: putting in an "unless otherwise specified" clause, and then putting in a footnote saying the sentry destructor is the only one that can throw. PJP suggests specifying that sentry::~sentry() should internally catch any exceptions it might cause. ]
Section: 27.6.2.3 [lib.ostream::sentry] Status: Open Submitter: Martin Sebor Date: 5 Jan 2003
While reviewing unformatted input member functions of istream for their behavior when they encounter end-of-file during input I found that the requirements vary, sometimes unexpectedly, and in more than one case even contradict established practice (GNU libstdc++ 3.2, IBM VAC++ 6.0, STLPort 4.5, SunPro 5.3, HP aCC 5.38, Rogue Wave libstd 3.1, and Classic Iostreams).
The following unformatted input member functions set eofbit if they encounter an end-of-file (this is the expected behavior, and also the behavior of all major implementations):
basic_istream<charT, traits>&
get (char_type*, streamsize, char_type);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
get (char_type*, streamsize);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
getline (char_type*, streamsize, char_type);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
getline (char_type*, streamsize);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
ignore (int, int_type);
basic_istream<charT, traits>&
read (char_type*, streamsize);
Also sets failbit if it encounters end-of-file.
streamsize readsome (char_type*, streamsize);
The following unformated input member functions set failbit but not eofbit if they encounter an end-of-file (I find this odd since the functions make it impossible to distinguish a general failure from a failure due to end-of-file; the requirement is also in conflict with all major implementation which set both eofbit and failbit):
int_type get();
basic_istream<charT, traits>&
get (char_type&);
These functions only set failbit of they extract no characters, otherwise they don't set any bits, even on failure (I find this inconsistency quite unexpected; the requirement is also in conflict with all major implementations which set eofbit whenever they encounter end-of-file):
basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&, char_type);
basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&);
This function sets no bits (all implementations except for STLport and Classic Iostreams set eofbit when they encounter end-of-file):
int_type peek ();
Proposed resolution:
Informally, what we want is a global statement of intent saying that eofbit gets set if we trip across EOF, and then we can take away the specific wording for individual functions. A full review is necessary. The wording currently in the standard is a mishmash, and changing it on an individual basis wouldn't make things better. Dietmar will do this work.
Section: 20.1.5 [lib.allocator.requirements] Status: Open Submitter: Markus Mauhart Date: 27 Feb 2003
I think that in par2 of 20.1.5 [lib.allocator.requirements] the last two lines of table 32 contain two incorrect type casts. The lines are ...
a.construct(p,t) Effect: new((void*)p) T(t) a.destroy(p) Effect: ((T*)p)?->~T()
.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:
alloc<T> a ;// an allocator for T
alloc<T>::pointer p ;// random access iterator
// (may be different from T*)
alloc<T>::reference r = *p;// T&
T const& t ;
For that two type casts ("(void*)p" and "(T*)p") to be well-formed this would require then conversions to T* and void* for all alloc<T>::pointer, so it would implicitely introduce extra requirements for alloc<T>::pointer, additionally to the only current requirement (being a random access iterator).
Proposed resolution:
"(void*)p" should be replaced with "(void*)&*p" and that "((T*)p)?->" should be replaced with "(*p)." or with "(&*p)->".
Note: Actually I would prefer to replace "((T*)p)?->dtor_name" with "p?->dtor_name", but AFAICS this is not possible cause of an omission in 13.5.6 [over.ref] (for which I have filed another DR on 29.11.2002).
[Kona: The LWG thinks this is somewhere on the border between Open and NAD. The intend is clear: construct constructs an object at the location p. It's reading too much into the description to think that literally calling new is required. Tweaking this description is low priority until we can do a thorough review of allocators, and, in particular, allocators with non-default pointer types.]
Section: 25.4 [lib.alg.c.library] Status: Ready Submitter: Ray Lischner Date: 08 Apr 2003
Section 25.4 [lib.alg.c.library] describes bsearch and qsort, from the C standard library. Paragraph 4 does not list any restrictions on qsort, but it should limit the base parameter to point to POD. Presumably, qsort sorts the array by copying bytes, which requires POD.
Proposed resolution:
In 25.4 [lib.alg.c.library] paragraph 4, just after the declarations and before the nonnormative note, add these words: "both of which have the same behavior as the original declaration. The behavior is undefined unless the objects in the array pointed to by base are of POD type."
[Something along these lines is clearly necessary. Matt provided wording.]
Section: 23.2.4.3 [lib.vector.modifiers] Status: Review Submitter: Dave Abrahams Date: 27 Apr 2003
There is a possible defect in the standard: the standard text was never intended to prevent arbitrary ForwardIterators, whose operations may throw exceptions, from being passed, and it also wasn't intended to require a temporary buffer in the case where ForwardIterators were passed (and I think most implementations don't use one). As is, the standard appears to impose requirements that aren't met by any existing implementation.
Proposed resolution:
Replace 23.2.4.2 [lib.vector.capacity] paragraph 1 with:
1- Notes: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor or assignment operator of T or by any InputIterator operation there are no effects.
[We probably need to say something similar for deque.]
Section: 24.1 [lib.iterator.requirements] Status: Open Submitter: Nathan Myers Date: 3 June 2003
I've been discussing iterator semantics with Dave Abrahams, and a surprise has popped up. I don't think this has been discussed before.
24.1 [lib.iterator.requirements] says that the only operation that can be performed on "singular" iterator values is to assign a non-singular value to them. (It doesn't say they can be destroyed, and that's probably a defect.) Some implementations have taken this to imply that there is no need to initialize the data member of a reverse_iterator<> in the default constructor. As a result, code like
std::vector<std::reverse_iterator<char*> > v(7); v.reserve(1000);
invokes undefined behavior, because it must default-initialize the vector elements, and then copy them to other storage. Of course many other vector operations on these adapters are also left undefined, and which those are is not reliably deducible from the standard.
I don't think that 24.1 was meant to make standard-library iterator types unsafe. Rather, it was meant to restrict what operations may be performed by functions which take general user- and standard iterators as arguments, so that raw pointers would qualify as iterators. However, this is not clear in the text, others have come to the opposite conclusion.
One question is whether the standard iterator adaptors have defined copy semantics. Another is whether they have defined destructor semantics: is
{ std::vector<std::reverse_iterator<char*> > v(7); }
undefined too?
Note this is not a question of whether algorithms are allowed to rely on copy semantics for arbitrary iterators, just whether the types we actually supply support those operations. I believe the resolution must be expressed in terms of the semantics of the adapter's argument type. It should make clear that, e.g., the reverse_iterator<T> constructor is actually required to execute T(), and so copying is defined if the result of T() is copyable.
Issue 235, which defines reverse_iterator's default constructor more precisely, has some relevance to this issue. However, it is not the whole story.
The issue was whether
reverse_iterator() { }
is allowed, vs.
reverse_iterator() : current() { }
The difference is when T is char*, where the first leaves the member uninitialized, and possibly equal to an existing pointer value, or (on some targets) may result in a hardware trap when copied.
8.5 paragraph 5 seems to make clear that the second is required to satisfy DR 235, at least for non-class Iterator argument types.
But that only takes care of reverse_iterator, and doesn't establish a policy for all iterators. (The reverse iterator adapter was just an example.) In particular, does my function
template <typename Iterator> void f() { std::vector<Iterator> v(7); }
evoke undefined behavior for some conforming iterator definitions? I think it does, now, because vector<> will destroy those singular iterator values, and that's explicitly disallowed.
24.1 shouldn't give blanket permission to copy all singular iterators, because then pointers wouldn't qualify as iterators. However, it should allow copying of that subset of singular iterator values that are default-initialized, and it should explicitly allow destroying any iterator value, singular or not, default-initialized or not.
Related issue: 407
Proposed resolution:
[ We don't want to require all singular iterators to be copyable, because that is not the case for pointers. However, default construction may be a special case. Issue: is it really default construction we want to talk about, or is it something like value initialization? We need to check with core to see whether default constructed pointers are required to be copyable; if not, it would be wrong to impose so strict a requirement for iterators. ]
Section: 27.8.1.7 [lib.ifstream.members], 27.8.1.10 [lib.ofstream.members] Status: Review Submitter: Nathan Myers Date: 3 June 2003
A strict reading of 27.8.1 [lib.fstreams] shows that opening or closing a basic_[io]fstream does not affect the error bits. This means, for example, that if you read through a file up to EOF, and then close the stream and reopen it at the beginning of the file, the EOF bit in the stream's error state is still set. This is counterintuitive.
The LWG considered this issue once before, as issue 22, and put in a footnote to clarify that the strict reading was indeed correct. We did that because we believed the standard was unambiguous and consistent, and that we should not make architectural changes in a TC. Now that we're working on a new revision of the language, those considerations no longer apply.
Proposed resolution:
Change 27.8.1.7 [lib.ifstream.members], para. 3 from:
Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)).
to:
Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().
Change 27.8.1.10 [lib.ofstream.members], para. 3 from:
Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)).
to:
Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().
Change 27.8.1.13 [lib.fstream.members], para. 3 from:
Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) )
to:
Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) ), else calls clear().
[Kona: the LWG agrees this is a good idea. Post-Kona: Bill provided wording. He suggests having open, not close, clear the error flags.]
[Post-Sydney: Howard provided a new proposed resolution. The old one didn't make sense because it proposed to fix this at the level of basic_filebuf, which doesn't have access to the stream's error state. Howard's proposed resolution fixes this at the level of the three fstream class template instead.]
Section: 23.2.3.1 [lib.queue], 23.2.3.3 [lib.stack] Status: Ready Submitter: Hans Bos Date: 7 Jun 2003
Sections 23.2.3.1 [lib.queue] and 23.2.3.3 [lib.stack] list comparison operators (==, !=, <, <=, >, =>) for queue and stack. Only the semantics for queue::operator== (23.2.3.1 [lib.queue] par2) and queue::operator< (23.2.3.1 [lib.queue] par3) are defined.
Proposed resolution:
Add the following new paragraphs after 23.2.3.1 [lib.queue] paragraph 3:
operator!=Returns: x.c != y.c
operator>Returns: x.c > y.c
operator<=Returns: x.c <= y.c
operator>=Returns: x.c >= y.c
Add the following paragraphs at the end of 23.2.3.3 [lib.stack]:
operator==Returns: x.c == y.c
operator<Returns: x.c < y.c
operator!=Returns: x.c != y.c
operator>Returns: x.c > y.c
operator<=Returns: x.c <= y.c
operator>=Returns: x.c >= y.c
[Kona: Matt provided wording.]
Rationale:
There isn't any real doubt about what these operators are supposed to do, but we ought to spell it out.Section: 27.4.4.3 [lib.iostate.flags] Status: Ready Submitter: Martin Sebor Date: 10 Jul 2003
The Effects clause in 27.4.4.3 [lib.iostate.flags] paragraph 5 says that the function only throws if the respective bits are already set prior to the function call. That's obviously not the intent. The typo ought to be corrected and the text reworded as: "If (state & exceptions()) == 0, returns. ..."
Proposed resolution:
In 27.4.4.3 [lib.iostate.flags] paragraph 5, replace "If (rdstate() & exceptions()) == 0" with "If ((state | (rdbuf() ? goodbit : badbit)) & exceptions()) == 0".
[Kona: the original proposed resolution wasn't quite right. We really do mean rdstate(); the ambiguity is that the wording in the standard doesn't make it clear whether we mean rdstate() before setting the new state, or rdsate() after setting it. We intend the latter, of course. Post-Kona: Martin provided wording.]
Section: 27.6.1.2.3 [lib.istream::extractors] Status: Review Submitter: Bo Persson Date: 13 Jul 2003
The second sentence of the proposed resolution says:
"If it inserted no characters because it caught an exception thrown while extracting characters from sb and ..."
However, we are not extracting from sb, but extracting from the basic_istream (*this) and inserting into sb. I can't really tell if "extracting" or "sb" is a typo.
[ Sydney: Definitely a real issue. We are, indeed, extracting characters from an istream and not from sb. The problem was there in the FDIS and wasn't fixed by issue 64. Probably what was intended was to have *this instead of sb. We're talking about the exception flag state of a basic_istream object, and there's only one basic_istream object in this discussion, so that would be a consistent interpretation. (But we need to be careful: the exception policy of this member function must be consistent with that of other extractors.) PJP will provide wording. ]
Proposed resolution:
Change the sentence from:
If it inserted no characters because it caught an exception thrown while extracting characters from sb and failbit is on in exceptions(), then the caught exception is rethrown.
to:
If it inserted no characters because it caught an exception thrown while extracting characters from *this and failbit is on in exceptions(), then the caught exception is rethrown.
Section: 27.6.1.4 [lib.istream.manip] Status: Ready Submitter: Martin Sebor Date: 18 Sep 2003
According to 27.6.1.4, the ws() manipulator is not required to construct the sentry object. The manipulator is also not a member function so the text in 27.6.1, p1 through 4 that describes the exception policy for istream member functions does not apply. That seems inconsistent with the rest of extractors and all the other input functions (i.e., ws will not cause a tied stream to be flushed before extraction, it doesn't check the stream's exceptions or catch exceptions thrown during input, and it doesn't affect the stream's gcount).
Proposed resolution:
Add to 27.6.1.4 [lib.istream.manip], immediately before the first sentence of paragraph 1, the following text:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to is.gcount(). After constructing a sentry object...
[Post-Kona: Martin provided wording]
Section: 18.2.2 [lib.c.limits] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
Given two overloads of the function foo(), one taking an argument of type int
and the other taking a long, which one will the call foo(LONG_MAX) resolve to?
The expected answer should be foo(long), but whether that is true depends on the
#defintion of the LONG_MAX macro, specifically its type. This issue is about the
fact that the type of these macros is not actually required to be the same as
the the type each respective limit.
Section 18.2.2 of the C++ Standard does
not specify the exact types of the XXX_MIN and XXX_MAX macros #defined in the
<climits> and <limits.h> headers such as INT_MAX and LONG_MAX and
instead defers to the C standard.
Section 5.2.4.2.1, p1 of the C standard
specifies that "The values [of these constants] shall be replaced by constant
expressions suitable for use in #if preprocessing directives. Moreover, except
for CHAR_BIT and MB_LEN_MAX, the following shall be replaced by expressions that
have the same type as would an expression that is an object of the corresponding
type converted according to the integer promotions."
The "corresponding type
converted according to the integer promotions" for LONG_MAX is, according to
6.4.4.1, p5 of the C standard, the type of long converted to the first of the
following set of types that can represent it: int, long int, long long int. So
on an implementation where (sizeof(long) == sizeof(int)) this type is actually
int, while on an implementation where (sizeof(long) > sizeof(int)) holds this
type will be long.
This is not an issue in C since the type of the macro
cannot be detected by any conforming C program, but it presents a portability
problem in C++ where the actual type is easily detectable by overload
resolution.
Proposed resolution:
[Kona: the LWG does not believe this is a defect. The C macro definitions are what they are; we've got a better mechanism, std::numeric_limits, that is specified more precisely than the C limit macros. At most we should add a nonnormative note recommending that users who care about the exact types of limit quantities should use <limits> instead of <climits>.]
Section: 22.2.1.1.2 [lib.locale.ctype.virtuals] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
The Effects and Returns clauses of the do_widen() member function of the ctype facet fail to specify the behavior of the function on failure. That the function may not be able to simply cast the narrow character argument to the type of the result since doing so may yield the wrong value for some wchar_t encodings. Popular implementations of ctype<wchar_t> that use mbtowc() and UTF-8 as the native encoding (e.g., GNU glibc) will fail when the argument's MSB is set. There is no way for the the rest of locale and iostream to reliably detect this failure.
Proposed resolution:
[Kona: This is a real problem. Widening can fail. It's unclear what the solution should be. Returning WEOF works for the wchar_t specialization, but not in general. One option might be to add a default, like narrow. But that's an incompatible change. Using traits::eof might seem like a good idea, but facets don't have access to traits (a recurring problem). We could have widen throw an exception, but that's a scary option; existing library components aren't written with the assumption that widen can throw.]
Section: 27.4.2.1.6 [lib.ios::Init] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
The dtor of the ios_base::Init object is supposed to call flush() on the 6 standard iostream objects cout, cerr, clog, wcout, wcerr, and wclog. This call may cause an exception to be thrown.
17.4.4.8, p3 prohibits all library destructors from throwing exceptions.
The question is: What should this dtor do if one or more of these calls to flush() ends up throwing an exception? This can happen quite easily if one of the facets installed in the locale imbued in the iostream object throws.
Proposed resolution:
[Kona: We probably can't do much better than what we've got, so the LWG is leaning toward NAD. At the point where the standard stream objects are being cleaned up, the usual error reporting mechanism are all unavailable. And exception from flush at this point will definitely cause problems. A quality implementation might reasonably swallow the exception, or call abort, or do something even more drastic.]
Section: 27.6.1.1.2 [lib.istream::sentry] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
27.6.1.1.2, p2 says that istream::sentry ctor prepares for input if is.good() is true. p4 then goes on to say that the ctor sets the sentry::ok_ member to true if the stream state is good after any preparation. 27.6.1.2.1, p1 then says that a formatted input function endeavors to obtain the requested input if the sentry's operator bool() returns true. Given these requirements, no formatted extractor should ever set failbit if the initial stream rdstate() == eofbit. That is contrary to the behavior of all implementations I tested. The program below prints out eof = 1, fail = 0 eof = 1, fail = 1 on all of them.
#include <sstream>
#include <cstdio>
int main()
{
std::istringstream strm ("1");
int i = 0;
strm >> i;
std::printf ("eof = %d, fail = %d\n",
!!strm.eof (), !!strm.fail ());
strm >> i;
std::printf ("eof = %d, fail = %d\n",
!!strm.eof (), !!strm.fail ());
}
Comments from Jerry Schwarz (c++std-lib-11373):
Jerry Schwarz wrote:
I don't know where (if anywhere) it says it in the standard, but the
formatted extractors are supposed to set failbit if they don't extract any
characters. If they didn't then simple loops like
while (cin >> x);
would loop forever.
Further comments from Martin Sebor:
The question
is which part of the extraction should prevent this from happening by setting
failbit when eofbit is already set. It could either be the sentry object or the
extractor. It seems that most implementations have chosen to set failbit in the
sentry [...] so that's the text that will need to be corrected.
Proposed resolution:
Kona: Possibly NAD. If eofbit is set then good() will return false. We then set ok to false. We believe that the sentry's constructor should always set failbit when ok is false, and we also think the standard already says that. Possibly it could be clearer.
Section: 27.5.2.1 [lib.streambuf.cons] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
The reflector thread starting with c++std-lib-11346 notes that the class template basic_streambuf, along with basic_stringbuf and basic_filebuf, is copy-constructible but that the semantics of the copy constructors are not defined anywhere. Further, different implementations behave differently in this respect: some prevent copy construction of objects of these types by declaring their copy ctors and assignment operators private, others exhibit undefined behavior, while others still give these operations well-defined semantics.
Note that this problem doesn't seem to be isolated to just the three types mentioned above. A number of other types in the library section of the standard provide a compiler-generated copy ctor and assignment operator yet fail to specify their semantics. It's believed that the only types for which this is actually a problem (i.e. types where the compiler-generated default may be inappropriate and may not have been intended) are locale facets. See issue 439.
Proposed resolution:
27.5.2 [lib.streambuf]: Add into the synopsis, public section, just above the destructor declaration:
basic_streambuf(const basic_streambuf& sb); basic_streambuf& operator=(const basic_streambuf& sb);
Insert after 27.5.2.1, paragraph 2:
basic_streambuf(const basic_streambuf& sb);Constructs a copy of sb.
Postcondtions:
eback() == sb.eback() gptr() == sb.gptr() egptr() == sb.egptr() pbase() == sb.pbase() pptr() == sb.pptr() epptr() == sb.epptr() getloc() == sb.getloc()basic_streambuf& operator=(const basic_streambuf& sb);Assigns the data members of sb to this.
Postcondtions:
eback() == sb.eback() gptr() == sb.gptr() egptr() == sb.egptr() pbase() == sb.pbase() pptr() == sb.pptr() epptr() == sb.epptr() getloc() == sb.getloc()Returns: *this.
27.7.1 [lib.stringbuf]:
Option A:Option B:Insert into the basic_stringbuf synopsis in the private section:
basic_stringbuf(const basic_stringbuf&); // not defined basic_stringbuf& operator=(const basic_stringbuf&); // not defined
Insert into the basic_stringbuf synopsis in the public section:
basic_stringbuf(const basic_stringbuf& sb); basic_stringbuf& operator=(const basic_stringbuf& sb);27.7.1.1, insert after paragraph 4:
basic_stringbuf(const basic_stringbuf& sb);Constructs an independent copy of sb as if with sb.str(), and with the openmode that sb was constructed with.
Postcondtions:
str() == sb.str() gptr() - eback() == sb.gptr() - sb.eback() egptr() - eback() == sb.egptr() - sb.eback() pptr() - pbase() == sb.pptr() - sb.pbase() getloc() == sb.getloc()Note: The only requirement on epptr() is that it point beyond the initialized range if an output sequence exists. There is no requirement that epptr() - pbase() == sb.epptr() - sb.pbase().
basic_stringbuf& operator=(const basic_stringbuf& sb);After assignment the basic_stringbuf has the same state as if it were initially copy constructed from sb, except that the basic_stringbuf is allowed to retain any excess capacity it might have, which may in turn effect the value of epptr().
27.8.1.1 [lib.filebuf]
Insert at the bottom of the basic_filebuf synopsis:
private: basic_filebuf(const basic_filebuf&); // not defined basic_filebuf& operator=(const basic_filebuf&); // not defined
[Kona: this is an issue for basic_streambuf itself and for its derived classes. We are leaning toward allowing basic_streambuf to be copyable, and specifying its precise semantics. (Probably the obvious: copying the buffer pointers.) We are less sure whether the streambuf derived classes should be copyable. Howard will write up a proposal.]
[Sydney: Dietmar presented a new argument against basic_streambuf being copyable: it can lead to an encapsulation violation. Filebuf inherits from streambuf. Now suppose you inhert a my_hijacking_buf from streambuf. You can copy the streambuf portion of a filebuf to a my_hijacking_buf, giving you access to the pointers into the filebuf's internal buffer. Perhaps not a very strong argument, but it was strong enough to make people nervous. There was weak preference for having streambuf not be copyable. There was weak preference for having stringbuf not be copyable even if streambuf is. Move this issue to open for now. ]
Rationale:
27.5.2 [lib.streambuf]: The proposed basic_streambuf copy constructor and assignment operator are the same as currently implied by the lack of declarations: public and simply copies the data members. This resolution is not a change but a clarification of the current standard.
27.7.1 [lib.stringbuf]: There are two reasonable options: A) Make basic_stringbuf not copyable. This is likely the status-quo of current implementations. B) Reasonable copy semantics of basic_stringbuf can be defined and implemented. A copyable basic_streambuf is arguably more useful than a non-copyable one. This should be considered as new functionality and not the fixing of a defect. If option B is chosen, ramifications from issue 432 are taken into account.
27.8.1.1 [lib.filebuf]: There are no reasonable copy semantics for basic_filebuf.
Section: 17.4.3.1 [lib.reserved.names] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
It has been suggested that 17.4.3.1, p1 may or may not allow programs to explicitly specialize members of standard templates on user-defined types. The answer to the question might have an impact where library requirements are given using the "as if" rule. I.e., if programs are allowed to specialize member functions they will be able to detect an implementation's strict conformance to Effects clauses that describe the behavior of the function in terms of the other member function (the one explicitly specialized by the program) by relying on the "as if" rule.
Proposed resolution:
Add the following sentence immediately after the text of 17.4.3.1 [lib.reserved.names], p1:
The behavior of a program that declares explicit specializations of any members of class templates or explicit specializations of any member templates of classes or class templates defined in this library is undefined.
[Kona: straw poll was 6-1 that user programs should not be allowed to specialize individual member functions of standard library class templates, and that doing so invokes undefined behavior. Post-Kona: Martin provided wording.]
[Sydney: The LWG agrees that the standard shouldn't permit users to specialize individual member functions unless they specialize the whole class, but we're not sure these words say what we want them to; they could be read as prohibiting the specialization of any standard library class templates. We need to consult with CWG to make sure we use the right wording.]
Section: 27 [lib.input.output] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
A third party test suite tries to exercise istream::ignore(N) with a negative value of N and expects that the implementation will treat N as if it were 0. Our implementation asserts that (N >= 0) holds and aborts the test.
I can't find anything in section 27 that prohibits such values but I don't see what the effects of such calls should be, either (this applies to a number of unformatted input functions as well as some member functions of the basic_streambuf template).
Proposed resolution:
I propose that we add to each function in clause 27 that takes an argument, say N, of type streamsize a Requires clause saying that "N >= 0." The intent is to allow negative streamsize values in calls to precision() and width() but disallow it in calls to streambuf::sgetn(), istream::ignore(), or ostream::write().
[Kona: The LWG agreed that this is probably what we want. However, we need a review to find all places where functions in clause 27 take arguments of type streamsize that shouldn't be allowed to go negative. Martin will do that review.]
Section: 17.3.1.1 [lib.structure.summary] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
The text in 17.3.1.1, p1 says:
"Paragraphs labelled "Note(s):" or
"Example(s):" are informative, other paragraphs are normative."
The library
section makes heavy use of paragraphs labeled "Notes(s)," some of which are
clearly intended to be normative (see list 1), while some others are not (see
list 2). There are also those where the intent is not so clear (see list 3).
List 1 -- Examples of (presumably) normative Notes:
20.4.1.1, p3,
20.4.1.1, p10, 21.3.1, p11, 22.1.1.2, p11, 23.2.1.3, p2, 25.3.7, p3, 26.2.6,
p14a, 27.5.2.4.3, p7.
List 2 -- Examples of (presumably) informative Notes:
18.4.1.3, p3, 21.3.5.6, p14, 22.2.1.5.2, p3, 25.1.1, p4, 26.2.5, p1,
27.4.2.5, p6.
List 3 -- Examples of Notes that are not clearly either
normative or informative:
22.1.1.2, p8, 22.1.1.5, p6, 27.5.2.4.5, p4.
None of these lists is meant to be exhaustive.
Proposed resolution:
[Definitely a real problem. The big problem is there's material that doesn't quite fit any of the named paragraph categories (e.g. Effects). Either we need a new kind of named paragraph, or we need to put more material in unnamed paragraphs jsut after the signature. We need to talk to the Project Editor about how to do this. ]
Section: 20.4.3 [lib.temporary.buffer] Status: Ready Submitter: Martin Sebor Date: 18 Sep 2003
The standard is not clear about the requirements on the value returned from a call to get_temporary_buffer(0). In particular, it fails to specify whether the call should return a distinct pointer each time it is called (like operator new), or whether the value is unspecified (as if returned by malloc). The standard also fails to mention what the required behavior is when the argument is less than 0.
Proposed resolution:
Change 20.4.3 [lib.temporary.buffer] paragraph 2 from "...or a pair of 0 values if no storage can be obtained" to "...or a pair of 0 values if no storage can be obtained or if n <= 0."
[Kona: Matt provided wording]
Section: 25.1.9 [lib.alg.search], 25.2.5 [lib.alg.fill], 25.2.6 [lib.alg.generate] Status: Ready Submitter: Martin Sebor Date: 18 Sep 2003
The complexity requirements for these function templates are incorrect (or don't even make sense) for negative n:
25.1.9, p7 (search_n):
Complexity: At most (last1 - first1) * count
applications of the corresponding predicate.
25.2.5, p3 (fill_n):
Complexity: Exactly last - first (or n)
assignments.
25.2.6, p3 (generate_n):
Complexity: Exactly last - first (or n)
assignments.
In addition, the Requirements or the Effects clauses for the latter two templates don't say anything about the behavior when n is negative.
Proposed resolution:
Change 25.1.9, p7 to
Complexity: At most (last1 - first1) * count applications of the corresponding predicate if count is positive, or 0 otherwise.
Change 25.2.5, p2 to
Effects: Assigns value through all the iterators in the range [first, last), or [first, first + n) if n is positive, none otherwise.
Change 25.2.5, p3 to:
Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.
Change 25.2.6, p1 to (notice the correction for the misspelled "through"):
Effects: Invokes the function object genand assigns the return value of gen through all the iterators in the range [first, last), or [first, first + n) if n is positive, or [first, first) otherwise.
Change 25.2.6, p3 to:
Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.
Rationale:
Informally, we want to say that whenever we see a negative number we treat it the same as if it were zero. We believe the above changes do that (although they may not be the minimal way of saying so). The LWG considered and rejected the alternative of saying that negative numbers are undefined behavior.
Section: 22.2.2.1.2 [lib.facet.num.get.virtuals] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
The requirements specified in Stage 2 and reiterated in the rationale of DR 221 (and echoed again in DR 303) specify that num_get<charT>:: do_get() compares characters on the stream against the widened elements of "012...abc...ABCX+-"
An implementation is required to allow programs to instantiate the num_get template on any charT that satisfies the requirements on a user-defined character type. These requirements do not include the ability of the character type to be equality comparable (the char_traits template must be used to perform tests for equality). Hence, the num_get template cannot be implemented to support any arbitrary character type. The num_get template must either make the assumption that the character type is equality-comparable (as some popular implementations do), or it may use char_traits<charT> to do the comparisons (some other popular implementations do that). This diversity of approaches makes it difficult to write portable programs that attempt to instantiate the num_get template on user-defined types.
Proposed resolution:
[Kona: the heart of the problem is that we're theoretically supposed to use traits classes for all fundamental character operations like assignment and comparison, but facets don't have traits parameters. This is a fundamental design flaw and it appears all over the place, not just in this one place. It's not clear what the correct solution is, but a thorough review of facets and traits is in order. The LWG considered and rejected the possibility of changing numeric facets to use narrowing instead of widening. This may be a good idea for other reasons (see issue 459), but it doesn't solve the problem raised by this issue. Whether we use widen or narrow the num_get facet still has no idea which traits class the user wants to use for the comparison, because only streams, not facets, are passed traits classes. The standard does not require that two different traits classes with the same char_type must necessarily have the same behavior.]
Informally, one possibility: require that some of the basic character operations, such as eq, lt, and assign, must behave the same way for all traits classes with the same char_type. If we accept that limitation on traits classes, then the facet could reasonably be required to use char_traits<charT>
.Section: 26.3.2.4 [lib.valarray.sub] Status: Open Submitter: Martin Sebor Date: 18 Sep 2003
The standard fails to specify the behavior of valarray::operator[](slice) and other valarray subset operations when they are passed an "invalid" slice object, i.e., either a slice that doesn't make sense at all (e.g., slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray object (e.g., slice (2, 1, 1) for a valarray of size 1).
Proposed resolution:
[Kona: the LWG believes that invalid slices should invoke undefined behavior. Valarrays are supposed to be designed for high performance, so we don't want to require specific checking. We need wording to express this decision.]
Section: 20.1.5 [lib.allocator.requirements], 25 [lib.algorithms] Status: Open Submitter: Matt Austern Date: 20 Sep 2003
Clause 20.1.5 [lib.allocator.requirements] paragraph 4 says that implementations are permitted to supply containers that are unable to cope with allocator instances and that container implementations may assume that all instances of an allocator type compare equal. We gave implementers this latitude as a temporary hack, and eventually we want to get rid of it. What happens when we're dealing with allocators that don't compare equal?
In particular: suppose that v1 and v2 are both objects of type vector<int, my_alloc> and that v1.get_allocator() != v2.get_allocator(). What happens if we write v1.swap(v2)? Informally, three possibilities:
1. This operation is illegal. Perhaps we could say that an implementation is required to check and to throw an exception, or perhaps we could say it's undefined behavior.
2. The operation performs a slow swap (i.e. using three invocations of operator=, leaving each allocator with its original container. This would be an O(N) operation.
3. The operation swaps both the vectors' contents and their allocators. This would be an O(1) operation. That is:
my_alloc a1(...);
my_alloc a2(...);
assert(a1 != a2);
vector<int, my_alloc> v1(a1);
vector<int, my_alloc> v2(a2);
assert(a1 == v1.get_allocator());
assert(a2 == v2.get_allocator());
v1.swap(v2);
assert(a1 == v2.get_allocator());
assert(a2 == v1.get_allocator());
Proposed resolution:
[Kona: This is part of a general problem. We need a paper saying how to deal with unequal allocators in general.]
[pre-Sydney: Howard argues for option 3 in n1599.]
Section: 27.7.1.3 [lib.stringbuf.virtuals] Status: Ready Submitter: Christian W Brock Date: 24 Sep 2003
27.7.1.3 par 8 says:
Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).
The sentences "plus one additional write position." and especially "(as does the write end pointer epptr())" COULD by interpreted (and is interpreted by at least my library vendor) as:
post-condition: epptr() == pptr()+1
This WOULD force sputc() to call the virtual overflow() each time.
The proposed change also affects Defect Report 169.
Proposed resolution:
27.7.1.1/2 Change:
2- Notes: The function allocates no array object.
to:
2- Postcondition: str() == "".
27.7.1.1/3 Change:
-3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which . Then copies the content of str into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to which. If which & ios_base::out is true, initializes the output sequence with the underlying sequence. If which & ios_base::in is true, initializes the input sequence with the underlying sequence.
to:
-3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which. Then copies the content of str into the basic_stringbuf underlying character sequence. If which & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (which & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If which & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.
27.7.1.2/1 Change:
-1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the buffer is only created in input mode, the underlying character sequence is equal to the input sequence; otherwise, it is equal to the output sequence. In case of an empty underlying character sequence, the function returns basic_string<charT,traits,Allocator>().
to:
-1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the basic_stringbuf was created only in input mode, the resultant basic_string contains the character sequence in the range [eback(), egptr()). If the basic_stringbuf was created with (which & ios_base::out) being true then the resultant basic_string contains the character sequence in the range [pbase(), high_mark) where high_mark represents the position one past the highest initialized character in the buffer. Characters can be initialized either through writing to the stream, or by constructing the basic_stringbuf with a basic_string, or by calling the str(basic_string) member function. In the case of calling the str(basic_string) member function, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_string). Otherwise the basic_stringbuf has been created in neither input nor output mode and a zero length basic_string is returned.
27.7.1.2/2 Change:
-2- Effects: If the basic_stringbuf's underlying character sequence is not empty, deallocates it. Then copies the content of s into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to the mode stored when creating the basic_stringbuf object. If (mode&ios_base::out) is true, then initializes the output sequence with the underlying sequence. If (mode&ios_base::in) is true, then initializes the input sequence with the underlying sequence.
to:
-2- Effects: Copies the content of s into the basic_stringbuf underlying character sequence. If mode & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (mode & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If mode & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.
Remove 27.2.1.2/3. (Same rationale as issue 238: incorrect and unnecessary.)
27.7.1.3/1 Change:
1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof().
to:
1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof(). Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.
27.7.1.3/9 Change:
-9- Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).
to:
-9- The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position.
27.7.1.3/12 Change:
-12- _ If (newoff + off) < 0, or (xend - xbeg) < (newoff + off), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .
to:
-12- _ If (newoff + off) < 0, or if (newoff + off) refers to an uninitialized character (as defined in 27.7.1.2 [lib.stringbuf.members] paragraph 1), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .
[post-Kona: Howard provided wording. At Kona the LWG agreed that something along these lines was a good idea, but the original proposed resolution didn't say enough about the effect of various member functions on the underlying character sequences.]
Rationale:
The current basic_stringbuf description is over-constrained in such a way as to prohibit vendors from making this the high-performance in-memory stream it was meant to be. The fundamental problem is that the pointers: eback(), gptr(), egptr(), pbase(), pptr(), epptr() are observable from a derived client, and the current description restricts the range [pbase(), epptr()) from being grown geometrically. This change allows, but does not require, geometric growth of this range.
Backwards compatibility issues: These changes will break code that derives from basic_stringbuf, observes epptr(), and depends upon [pbase(), epptr()) growing by one character on each call to overflow() (i.e. test suites). Otherwise there are no backwards compatibility issues.
27.7.1.1/2: The non-normative note is non-binding, and if it were binding, would be over specification. The recommended change focuses on the important observable fact.
27.7.1.1/3: This change does two things: 1. It describes exactly what must happen in terms of the sequences. The terms "input sequence" and "output sequence" are not well defined. 2. It introduces a common extension: open with app or ate mode. I concur with issue 238 that paragraph 4 is both wrong and unnecessary.
27.7.1.2/1: This change is the crux of the efficiency issue. The resultant basic_string is not dependent upon epptr(), and thus implementors are free to grow the underlying buffer geometrically during overflow() *and* place epptr() at the end of that buffer.
27.7.1.2/2: Made consistent with the proposed 27.7.1.1/3.
27.7.1.3/1: Clarifies that characters written to the stream beyond the initially specified string are available for reading in an i/o basic_streambuf.
27.7.1.3/9: Made normative by removing "Notes:", and removed the trailing parenthetical comment concerning epptr().
27.7.1.3/12: Restricting the positioning to [xbeg, xend) is no longer allowable since [pbase(), epptr()) may now contain uninitialized characters. Positioning is only allowable over the initialized range.
Section: 23.3.5.2 [lib.bitset.members] Status: Review Submitter: Martin Sebor Date: 15 Oct 2003
It has been pointed out a number of times that the bitset to_string() member function template is tedious to use since callers must explicitly specify the entire template argument list (3 arguments). At least two implementations provide a number of overloads of this template to make it easier to use.
Proposed resolution:
In order to allow callers to specify no template arguments at all, just the first one (charT), or the first 2 (charT and traits), in addition to all three template arguments, add the following three overloads to both the interface (declarations only) of the class template bitset as well as to section 23.3.5.2, immediately after p34, the Returns clause of the existing to_string() member function template:
template <class charT, class traits>
basic_string<charT, traits, allocator<charT> >
to_string () const;
-34.1- Returns: to_string<charT, traits, allocator<charT> >().
template <class charT>
basic_string<charT, char_traits<charT>, allocator<charT> >
to_string () const;
-34.2- Returns: to_string<charT, char_traits<charT>, allocator<charT> >().
basic_string<char, char_traits<char>, allocator<char> >
to_string () const;
-34.3- Returns: to_string<char, char_traits<char>, allocator<char> >().
[Kona: the LWG agrees that this is an improvement over the status quo. Dietmar thought about an alternative using a proxy object but now believes that the proposed resolution above is the right choice. ]
Section: 21.3.7.9 [lib.string.io] Status: Ready Submitter: Martin Sebor Date: 15 Oct 2003
It has been pointed out that the proposed resolution in DR 25 may not be
quite up to snuff:
http://gcc.gnu.org/ml/libstdc++/2003-09/msg00147.html
http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#25
It looks like Petur is right. The complete corrected text is copied below. I think we may have have been confused by the reference to 22.2.2.2.2 and the subsequent description of `n' which actually talks about the second argument to sputn(), not about the number of fill characters to pad with.
So the question is: was the original text correct? If the intent was to follow classic iostreams then it most likely wasn't, since setting width() to less than the length of the string doesn't truncate it on output. This is also the behavior of most implementations (except for SGI's standard iostreams where the operator does truncate).
Proposed resolution:
Change the text in 21.3.7.9, p4 from
If bool(k) is true, inserts characters as if by calling os.rdbuf()->sputn(str.data(), n), padding as described in stage 3 of lib.facet.num.put.virtuals, where n is the larger of os.width() and str.size();
to
If bool(k) is true, determines padding as described in lib.facet.num.put.virtuals, and then inserts the resulting sequence of characters seq as if by calling os.rdbuf()->sputn(seq, n), where n is the larger of os.width() and str.size();
[Kona: it appears that neither the original wording, DR25, nor the proposed resolution, is quite what we want. We want to say that the string will be output, padded to os.width() if necessary. We don't want to duplicate the padding rules in clause 22, because they're complicated, but we need to be careful because they weren't quite written with quite this case in mind. We need to say what the character sequence is, and then defer to clause 22. Post-Kona: Benjamin provided wording.]
Section: 23.1.1 [lib.sequence.reqmts] Status: Review Submitter: Howard Hinnant Date: 20 Oct 2003
Section 23.1.1 [lib.sequence.reqmts], paragraphs 9-11, fixed up the problem noticed with statements like:
vector<int> v(10, 1);
The intent of the above statement was to construct with:
vector(size_type, const value_type&);
but early implementations failed to compile as they bound to:
template <class InputIterator> vector(InputIterator f, InputIterator l);
instead.
Paragraphs 9-11 say that if InputIterator is an integral type, then the member template constructor will have the same effect as:
vector<static_cast<size_type>(f), static_cast<value_type>(l));
(and similarly for the other member template functions of sequences).
There is also a note that describes one implementation technique:
One way that sequence implementors can satisfy this requirement is to specialize the member template for every integral type.
This might look something like:
template <class T>
struct vector
{
typedef unsigned size_type;
explicit vector(size_type) {}
vector(size_type, const T&) {}
template <class I>
vector(I, I);
// ...
};
template <class T>
template <class I>
vector<T>::vector(I, I) { ... }
template <>
template <>
vector<int>::vector(int, int) { ... }
template <>
template <>
vector<int>::vector(unsigned, unsigned) { ... }
// ...
Label this solution 'A'.
The standard also says:
Less cumbersome implementation techniques also exist.
A popular technique is to not specialize as above, but instead catch every call with the member template, detect the type of InputIterator, and then redirect to the correct logic. Something like:
template <class T>
template <class I>
vector<T>::vector(I f, I l)
{
choose_init(f, l, int2type<is_integral<I>::value>());
}
template <class T>
template <class I>
vector<T>::choose_init(I f, I l, int2type<false>)
{
// construct with iterators
}
template <class T>
template <class I>
vector<T>::choose_init(I f, I l, int2type<true>)
{
size_type sz = static_cast<size_type>(f);
value_type v = static_cast<value_type>(l);
// construct with sz,v
}
Label this solution 'B'.
Both of these solutions solve the case the standard specifically mentions:
vector<int> v(10, 1); // ok, vector size 10, initialized to 1
However, (and here is the problem), the two solutions have different behavior in some cases where the value_type of the sequence is not an integral type. For example consider:
pair<char, char> p('a', 'b');
vector<vector<pair<char, char> > > d('a', 'b');
The second line of this snippet is likely an error. Solution A catches the error and refuses to compile. The reason is that there is no specialization of the member template constructor that looks like:
template <>
template <>
vector<vector<pair<char, char> > >::vector(char, char) { ... }
So the expression binds to the unspecialized member template constructor, and then fails (compile time) because char is not an InputIterator.
Solution B compiles the above example though. 'a' is casted to an unsigned integral type and used to size the outer vector. 'b' is static casted to the inner vector using it's explicit constructor:
explicit vector(size_type n);
and so you end up with a static_cast<size_type>('a') by static_cast<size_type>('b') matrix.
It is certainly possible that this is what the coder intended. But the explicit qualifier on the inner vector has been thwarted at any rate.
The standard is not clear whether the expression:
vector<vector<pair<char, char> > > d('a', 'b');
(and similar expressions) are:
My preference is listed in the order presented.
There are still other techniques for implementing the requirements of paragraphs 9-11, namely the "restricted template technique" (e.g. enable_if). This technique is the most compact and easy way of coding the requirements, and has the behavior of #2 (rejects the above expression).
Choosing 1 would allow all implementation techniques I'm aware of. Choosing 2 would allow only solution 'A' and the enable_if technique. Choosing 3 would allow only solution 'B'.
Possible wording for a future standard if we wanted to actively reject the expression above would be to change "static_cast" in paragraphs 9-11 to "implicit_cast" where that is defined by:
template <class T, class U>
inline
T implicit_cast(const U& u)
{
return u;
}
Proposed resolution:
Replace 23.1.1 [lib.sequence.reqmts] paragraphs 9 - 11 with:For every sequence defined in this clause and in clause lib.strings:
If the constructor
template <class InputIterator>
X(InputIterator f, InputIterator l,
const allocator_type& a = allocator_type())
is called with a type InputIterator that does not qualify as an input iterator, then the constructor will behave as if the overloaded constructor:
X(size_type, const value_type& = value_type(),
const allocator_type& = allocator_type())
were called instead, with the arguments f, l and a, respectively.
If the member functions of the forms:
template <class InputIterator> // such as insert()
rt fx1(iterator p, InputIterator f, InputIterator l);
template <class InputIterator> // such as append(), assign()
rt fx2(InputIterator f, InputIterator l);
template <class InputIterator> // such as replace()
rt fx3(iterator i1, iterator i2, InputIterator f, InputIterator l);
are called with a type InputIterator that does not qualify as an input iterator, then these functions will behave as if the overloaded member functions:
rt fx1(iterator, size_type, const value_type&);
rt fx2(size_type, const value_type&);
rt fx3(iterator, iterator, size_type, const value_type&);
were called instead, with the same arguments.
In the previous paragraph the alternative binding will fail if f is not implicitly convertible to X::size_type or if l is not implicitly convertible to X::value_type.
The extent to which an implementation determines that a type cannot be an input iterator is unspecified, except that as a minimum integral types shall not qualify as input iterators.
[ Kona: agreed that the current standard requires v('a', 'b') to be accepted, and also agreed that this is surprising behavior. The LWG considered several options, including something like implicit_cast, which doesn't appear to be quite what we want. We considered Howards three options: allow acceptance or rejection, require rejection as a compile time error, and require acceptance. By straw poll (1-6-1), we chose to require a compile time error. Post-Kona: Howard provided wording. ]
[ Sydney: The LWG agreed with this general direction, but there was some discomfort with the wording in the original proposed resolution. Howard submitted new wording, and we will review this again in Redmond. ]
Rationale:
The proposed resolution fixes:
vector<int> v(10, 1);
since as integral types 10 and 1 must be disqualified as input iterators and therefore the (size,value) constructor is called (as if).
The proposed resolution breaks:
vector<vector<T> > v(10, 1);
because the integral type 1 is not *implicitly* convertible to vector<T>. The wording above requires a diagnostic.
The proposed resolution leaves the behavior of the following code unspecified.
struct A
{
operator int () const {return 10;}
};
struct B
{
B(A) {}
};
vector<B> v(A(), A());
The implementation may or may not detect that A is not an input iterator and employee the (size,value) constructor. Note though that in the above example if the B(A) constructor is qualified explicit, then the implementation must reject the constructor as A is no longer implicitly convertible to B.
Section: 27.4.3 [lib.fpos] Status: Ready Submitter: Vincent Leloup Date: 17 Nov 2003
In section 27.4.3.1 [lib.fpos.members] fpos<stateT>::state() is declared non const, but in section 27.4.3 [lib.fpos] it is declared const.
Proposed resolution:
In section 27.4.3.1 [lib.fpos.members], change the declaration of fpos<stateT>::state() to const.
Section: 27.6.2.3 [lib.ostream::sentry] Status: Ready Submitter: Vincent Leloup Date: 18 Nov 2003
In section 27.6.2.3 [lib.ostream::sentry] paragraph 4, in description part basic_ostream<charT, traits>::sentry::operator bool() is declared as non const, but in section 27.6.2.3, in synopsis it is declared const.
Proposed resolution:
In section 27.6.2.3 [lib.ostream::sentry] paragraph 4, change the declaration of sentry::operator bool() to const.
Section: 27.8.1.3 [lib.filebuf.members] Status: Ready Submitter: Vincent Leloup Date: 20 Nov 2003
In section 27.8.1.3 [lib.filebuf.members] par6, in effects description of basic_filebuf<charT, traits>::close(), overflow(EOF) is used twice; should be overflow(traits::eof()).
Proposed resolution:
Change overflow(EOF) to overflow(traits::eof()).
Section: 27.8.1 [lib.fstreams] Status: Review Submitter: Vincent Leloup Date: 20 Nov 2003
27.8.1.7 [lib.ifstream.members] p1, 27.8.1.10 [lib.ofstream.members] p1, 27.8.1.13 [lib.fstream.members] p1 seems have same problem as exposed in LWG issue 252.
Proposed resolution:
[Sydney: Genuine defect. 27.8.1.13 needs a cast to cast away constness. The other two places are stylistic: we could change the C-style casts to const_cast. Post-Sydney: Howard provided wording. ]
Change 27.8.1.7/1 from:
Returns: (basic_filebuf<charT,traits>*)&sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Change 27.8.1.10/1 from:
Returns: (basic_filebuf<charT,traits>*)&sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Change 27.8.1.13/1 from:
Returns: &sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Section: 24.3.1 [lib.iterator.traits] Status: Review Submitter: Dave Abrahams Date: 9 Dec 2003
The standard places no restrictions at all on the reference type of input, output, or forward iterators (for forward iterators it only specifies that *x must be value_type& and doesn't mention the reference type). Bidirectional iterators' reference type is restricted only by implication, since the base iterator's reference type is used as the return type of reverse_iterator's operator*, which must be T& in order to be a conforming forward iterator.
Here's what I think we ought to be able to expect from an input or forward iterator's reference type R, where a is an iterator and V is its value_type
A mutable forward iterator ought to satisfy, for x of type V:
I think these requirements capture existing container iterators (including vector<bool>'s), but render istream_iterator invalid; its reference type would have to be changed to a constant reference.
(Jeremy Siek) During the discussion in Sydney, it was felt that a simpler long term solution for this was needed. The solution proposed was to require reference to be the same type as *a and pointer to be the same type as a->. Most iterators in the Standard Library already meet this requirement. Some iterators are output iterators, and do not need to meet the requirement, and others are only specified through the general iterator requirements (which will change with this resolution). The sole case where there is an explicit definition of the reference type that will need to change is istreambuf_iterator which returns charT from operator* but has a reference type of charT&. We propose changing the reference type of istreambuf_iterator to charT.
The other option for resolving the issue with pointer, mentioned in the note below, is to remove pointer altogether. I prefer placing requirements on pointer to removing it for two reasons. First, pointer will become useful for implementing iterator adaptors and in particular, reverse_iterator will become more well defined. Second, removing pointer is a rather drastic and publicly-visible action to take.
The proposed resolution technically enlarges the requirements for iterators, which means there are existing iterators (such as istreambuf_iterator, and potentially some programmer-defined iterators) that will no longer meet the requirements. Will this break existing code? The scenario in which it would is if an algorithm implementation (say in the Standard Library) is changed to rely on iterator_traits::reference, and then is used with one of the iterators that do not have an appropriately defined iterator_traits::reference.
The proposed resolution makes one other subtle change. Previously, it was required that output iterators have a difference_type and value_type of void, which means that a forward iterator could not be an output iterator. This is clearly a mistake, so I've changed the wording to say that those types may be void.
Proposed resolution:
In 24.3.1.1, after:
be defined as the iterator's difference type, value type and iterator category, respectively.
In 24.3.1.1, add:
In addition, the typesiterator_traits<Iterator>::reference iterator_traits<Iterator>::pointermust be defined as the iterator's reference and pointer types, that is, the same type as the type of *a and a->, respectively.
In 24.3.1.1, change:
In the case of an output iterator, the typesiterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_typeare both defined as void.
to:
In the case of an output iterator, the typesiterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_type iterator_traits<Iterator>::reference iterator_traits<Iterator>::pointermay be defined as void.
In 24.5.6, change:
typename traits::off_type, charT*, charT&>
to:
typename traits::off_type, charT*, charT>
[ Sydney: Agreed that this is under-specified, but it's not the only place where iterators are under-specified. We may need a more complete review. One nice simple rule that would solve this problem: *a is always reference by definition, and a-> is always pointer by definition. If we do that, then we'll also have to change a few existing iterators (e.g. istreambuf_iterator) so they conform to that rule. We need a review of how extensive those changes would be. Or instead of fixing pointer we could remove it. (Which would require changing reverse_iterator as part of that change, since the present reverse_iterator relies on the existence of pointer.) Jeremy will supply a more detailed analysis for Redmond. ]
Section: 24.1 [lib.iterator.requirements], 23.1 [lib.container.requirements] Status: Open Submitter: Andy Koenig Date: 16 Dec 2003
What requirements does the standard place on equality comparisons between iterators that refer to elements of different containers. For example, if v1 and v2 are empty vectors, is v1.end() == v2.end() allowed to yield true? Is it allowed to throw an exception?
The standard appears to be silent on both questions.
Proposed resolution:
[Sydney: The intention is that comparing two iterators from different containers is undefined, but it's not clear if we say that, or even whether it's something we should be saying in clause 23 or in clause 24. Intuitively we might want to say that equality is defined only if one iterator is reachable from another, but figuring out how to say it in any sensible way is a bit tricky: reachability is defined in terms of equality, so we can't also define equality in terms of reachability. ]
Section: 24.1.5 [lib.random.access.iterators] Status: Ready Submitter: Dave Abrahams Date: 7 Jan 2004
Table 76, the random access iterator requirement table, says that the return type of a[n] must be "convertible to T". When an iterator's value_type T is an abstract class, nothing is convertible to T. Surely this isn't an intended restriction?
Proposed resolution:
Change the return type to "convertible to T const&".
Section: 18.1 [lib.support.types] Status: Ready Submitter: Pete Becker Date: 15 Jan 2004
Original text:
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."
Revised text:
"If type is not a POD structure or a POD union the results are undefined."
Looks to me like the revised text should have replaced only the second sentence. It doesn't make sense standing alone.
Proposed resolution:
Change 18.1, paragraph 5, to:
The macro offsetof accepts a restricted set of type arguments in this International Standard. If type is not a POD structure or a POD union the results are undefined. The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."
Section: 22.1.1.3 [lib.locale.members] Status: Review Submitter: Bill Plauger Date: 30 Jan 2004
template<class Facet> locale::combine(const locale&) const;
is obliged to create a locale that has no name. This is overspecification and overkill. The resulting locale should follow the usual rules -- it has a name if the locale argument has a name and Facet is one of the standard facets.
[ Sydney and post-Sydney (see c++std-lib-13439, c++std-lib-13440, c++std-lib-13443): agreed that it's overkill to say that the locale is obligated to be nameless. However, we also can't require it to have a name. At the moment, locale names are based on categories and not on individual facets. If a locale contains two different facets of different names from the same category, then this would not fit into existing naming schemes. We need to give implementations more freedom. Bill will provide wording. ]
Proposed resolution:
[Bill now recommends that this be closed as NAD.]
Rationale:
Section: 27.7.1.3 [lib.stringbuf.virtuals] Status: Review Submitter: Bill Plauger Date: 30 Jan 2004
pos_type basic_stringbuf::seekoff(off_type, ios_base::seekdir,
ios_base::openmode);
is obliged to fail if nothing has been inserted into the stream. This is unnecessary and undesirable. It should be permissible to seek to an effective offset of zero.
[ Sydney: Agreed that this is an annoying problem: seeking to zero should be legal. Bill will provide wording. ]
Proposed resolution:
Change the sentence from:
For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer, the positioning operation fails.
to:
For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails.
Section: 27.8.1.3 [lib.filebuf.members] Status: Review Submitter: Bill Plauger Date: 30 Jan 2004
basic_filebuf *basic_filebuf::open(const char *, ios_base::open_mode);
should be supplemented with the overload:
basic_filebuf *basic_filebuf::open(const wchar_t *, ios_base::open_mode);
Depending on the operating system, one of these forms is fundamental and the other requires an implementation-defined mapping to determine the actual filename.
[Sydney: Yes, we want to allow wchar_t filenames. Bill will provide wording.]
Proposed resolution:
Change from:
basic_filebuf<charT,traits>* open( const char* s, ios_base::openmode mode );Effects: If is_open() != false, returns a null pointer. Otherwise, initializes the filebuf as required. It then opens a file, if possible, whose name is the NTBS s ("as if" by calling std::fopen(s,modstr)).
to:
basic_filebuf<charT,traits>* open( const char* s, ios_base::openmode mode ); basic_filebuf<charT,traits>* open( const wchar_t* ws, ios_base::openmode mode );Effects: If is_open() != false, returns a null pointer. Otherwise, initializes the filebuf as required. It then opens a file, if possible, whose name is the NTBS s ("as if" by calling std::fopen(s,modstr)). For the second signature, the NTBS s is determined from the WCBS ws in an implementation-defined manner.
(NOTE: For a system that "naturally" represents a filename as a WCBS, the NTBS s in the first signature may instead be mapped to a WCBS; if so, it follows the same mapping rules as the first argument to open.) u
Section: 27.3 [lib.iostream.objects] Status: Review Submitter: Bill Plauger Date: 30 Jan 2004
Both cerr::tie() and wcerr::tie() are obliged to be null at program startup. This is overspecification and overkill. It is both traditional and useful to tie cerr to cout, to ensure that standard output is drained whenever an error message is written. This behavior should at least be permitted if not required. Same for wcerr::tie().
Proposed resolution:
Add to the description of cerr:
After the object cerr is initialized, cerr.tie() returns &cout. Its state is otherwise the same as required for basic_ios<char>::init (lib.basic.ios.cons).
Add to the description of wcerr:
After the object wcerr is initialized, wcerr.tie() returns &wcout. Its state is otherwise the same as required for basic_ios<wchar_t>::init (lib.basic.ios.cons).
[Sydney: straw poll (3-1): we should require, not just permit, cout and cerr to be tied on startup. Pre-Redmond: Bill will provide wording.]
Section: 17.4.1.2 [lib.headers] Status: Open Submitter: Bill Plauger Date: 30 Jan 2004
The C++ Standard effectively requires that the traditional C headers (of the form <xxx.h>) be defined in terms of the newer C++ headers (of the form <cxxx>). Clauses 17.4.1.2/4 and D.5 combine to require that:
The rules were left in this form despited repeated and heated objections from several compiler vendors. The C headers are often beyond the direct control of C++ implementors. In some organizations, it's all they can do to get a few #ifdef __cplusplus tests added. Third-party library vendors can perhaps wrap the C headers. But neither of these approaches supports the drastic restructuring required by the C++ Standard. As a result, it is still widespread practice to ignore this conformance requirement, nearly seven years after the committee last debated this topic. Instead, what is often implemented is:
The practical benefit for implementors with the second approach is that they can use existing C library headers, as they are pretty much obliged to do. The practical cost for programmers facing a mix of implementations is that they have to assume weaker rules:
There also exists the possibility of subtle differences due to Koenig lookup, but there are so few non-builtin types defined in the C headers that I've yet to see an example of any real problems in this area.
It is worth observing that the rate at which programmers fall afoul of these differences has remained small, at least as measured by newsgroup postings and our own bug reports. (By an overwhelming margin, the commonest problem is still that programmers include <string> and can't understand why the typename string isn't defined -- this a decade after the committee invented namespace std, nominally for the benefit of all programmers.)
We should accept the fact that we made a serious mistake and rectify it, however belatedly, by explicitly allowing either of the two schemes for declaring C names in headers.
[Sydney: This issue has been debated many times, and will certainly have to be discussed in full committee before any action can be taken. However, the preliminary sentiment of the LWG was in favor of the change. (6 yes, 0 no, 2 abstain) Robert Klarer suggests that we might also want to undeprecate the C-style .h headers.]
Proposed resolution:
Section: 23.3.5.1 [lib.bitset.cons] Status: Review Submitter: Dag Henriksson Date: 30 Jan 2004
The constructor from unsigned long says it initializes "the first M bit positions to the corresponding bit values in val. M is the smaller of N and the value CHAR_BIT * sizeof(unsigned long)."
Object-representation vs. value-representation strikes again. CHAR_BIT * sizeof (unsigned long) does not give us the number of bits an unsigned long uses to hold the value. Thus, the first M bit position above is not guaranteed to have any corresponding bit values in val.
Proposed resolution:
In 23.3.5.1 [lib.bitset.cons] paragraph 2, change "M is the smaller of N and the value CHAR_BIT * sizeof (unsigned long). (249)" to "M is the smaller of N and the number of bits in the value representation (section 3.9 [basic.types]) of unsigned long."
Section: 24.1.5 [lib.random.access.iterators] Status: Open Submitter: Daniel Frey Date: 27 Feb 2004
In 24.1.5 [lib.random.access.iterators], table 76 the operational semantics for the expression "r -= n" are defined as "return r += -n". This means, that the expression -n must be valid, which is not the case for unsigned types.
[ Sydney: Possibly not a real problem, since difference type is required to be a signed integer type. However, the wording in the standard may be less clear than we would like. ]
Proposed resolution:
To remove this limitation, I suggest to change the operational semantics for this column to:
{ Distance m = n; if (m >= 0) while (m--) --r; else
while (m++) ++r; return r; }
Section: 22.2.2.1.2 [lib.facet.num.get.virtuals] Status: Open Submitter: Martin Sebor Date: 16 Mar 2004
When parsing strings of wide-character digits, the standard requires the library to widen narrow-character "atoms" and compare the widened atoms against the characters that are being parsed. Simply narrowing the wide characters would be far simpler, and probably more efficient. The two choices are equivalent except in convoluted test cases, and many implementations already ignore the standard and use narrow instead of widen.
First, I disagree that using narrow() instead of widen() would necessarily have unfortunate performance implications. A possible implementation of narrow() that allows num_get to be implemented in a much simpler and arguably comparably efficient way as calling widen() allows, i.e. without making a virtual call to do_narrow every time, is as follows:
inline char ctype<wchar_t>::narrow (wchar_t wc, char dflt) const
{
const unsigned wi = unsigned (wc);
if (wi > UCHAR_MAX)
return typeid (*this) == typeid (ctype<wchar_t>) ?
dflt : do_narrow (wc, dflt);
if (narrow_ [wi] < 0) {
const char nc = do_narrow (wc, dflt);
if (nc == dflt)
return dflt;
narrow_ [wi] = nc;
}
return char (narrow_ [wi]);
}
Second, I don't think the change proposed in the issue (i.e., to use narrow() instead of widen() during Stage 2) would be at all drastic. Existing implementations with the exception of libstdc++ currently already use narrow() so the impact of the change on programs would presumably be isolated to just a single implementation. Further, since narrow() is not required to translate alternate wide digit representations such as those mentioned in issue 303 to their narrow equivalents (i.e., the portable source characters '0' through '9'), the change does not necessarily imply that these alternate digits would be treated as ordinary digits and accepted as part of numbers during parsing. In fact, the requirement in 22.2.1.1.2 [lib.locale.ctype.virtuals], p13 forbids narrow() to translate an alternate digit character, wc, to an ordinary digit in the basic source character set unless the expression (ctype<charT>::is(ctype_base::digit, wc) == true) holds. This in turn is prohibited by the C standard (7.25.2.1.5, 7.25.2.1.5, and 5.2.1, respectively) for charT of either char or wchar_t.
[Sydney: To a large extent this is a nonproblem. As long as you're only trafficking in char and wchar_t we're only dealing with a stable character set, so you don't really need either 'widen' or 'narrow': can just use literals. Finally, it's not even clear whether widen-vs-narrow is the right question; arguably we should be using codecvt instead.]
Proposed resolution:
Change stage 2 so that implementations are permitted to use either technique to perform the comparison:
Section: 27.8.1 [lib.fstreams] Status: New Submitter: Ben Hutchings Date: 1 Apr 2004
The second parameters of the non-default constructor and of the open member function for basic_fstream, named "mode", are optional according to the class declaration in 27.8.1.11 [lib.fstream]. The specifications of these members in 27.8.1.12 [lib.fstream.cons] and 27.8.1.13 lib.fstream.members] disagree with this, though the constructor declaration has the "explicit" function-specifier implying that it is intended to be callable with one argument.
Proposed resolution:
Section: 22.2.5.1 [lib.locale.time.get] Status: New Submitter: Bill Plauger Date: 23 Mar 2004
Template time_get currently contains difficult, if not impossible, requirements for do_date_order, do_get_time, and do_get_date. All require the implementation to scan a field generated by the %x or %X conversion specifier in strftime. Yes, do_date_order can always return no_order, but that doesn't help the other functions. The problem is that %x can be nearly anything, and it can vary widely with locales. It's horribly onerous to have to parse "third sunday after Michaelmas in the year of our Lord two thousand and three," but that's what we currently ask of do_get_date. More practically, it leads some people to think that if %x produces 10.2.04, we should know to look for dots as separators. Still not easy.
Note that this is the opposite effect from the intent stated in the footnote earlier in this subclause:
"In other words, user confirmation is required for reliable parsing of user-entered dates and times, but machine-generated formats can be parsed reliably. This allows parsers to be aggressive about interpreting user variations on standard formats."
We should give both implementers and users an easier and more reliable alternative: provide a (short) list of alternative delimiters and say what the default date order is for no_order. For backward compatibility, and maximum latitude, we can permit an implementation to parse whatever %x or %X generates, but we shouldn't require it.
Proposed resolution:
Section: 3.6.3 [basic.start.term], 18.3 [lib.support.start.term] Status: New Submitter: Bill Plauger Date: 23 Mar 2004
3.6.3 Termination spells out in detail the interleaving of static destructor calls and calls to functions registered with atexit. To match this behavior requires intimate cooperation between the code that calls destructors and the exit/atexit machinery. The former is tied tightly to the compiler; the latter is a primitive mechanism inherited from C that traditionally has nothing to do with static construction and destruction. The benefits of intermixing destructor calls with atexit handler calls is questionable at best, and very difficult to get right, particularly when mixing third-party C++ libraries with different third-party C++ compilers and C libraries supplied by still other parties.
I believe the right thing to do is defer all static destruction until after all atexit handlers are called. This is a change in behavior, but one that is likely visible only to perverse test suites. At the very least, we should permit deferred destruction even if we don't require it.
Proposed resolution:
Section: 20.4.5 [lib.auto.ptr] Status: New Submitter: Rani Sharoni Date: 7 Dec 2003
TC1 CWG DR #84 effectively made the template<class Y> operator auto_ptr<Y>() member of auto_ptr (20.4.5.3/4) obsolete.
The sole purpose of this obsolete conversion member is to enable copy initialization base from r-value derived (or any convertible types like cv-types) case:
#include <memory>
using std::auto_ptr;
struct B {};
struct D : B {};
auto_ptr<D> source();
int sink(auto_ptr<B>);
int x1 = sink( source() ); // #1 EDG - no suitable copy constructor
The excellent analysis of conversion operations that was given in the final auto_ptr proposal (http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/1997/N1128.pdf) explicitly specifies this case analysis (case 4). DR #84 makes the analysis wrong and actually comes to forbid the loophole that was exploited by the auto_ptr designers.
I didn't encounter any compliant compiler (e.g. EDG, GCC, BCC and VC) that ever allowed this case. This is probably because it requires 3 user defined conversions and in fact current compilers conform to DR #84.
I was surprised to discover that the obsolete conversion member actually has negative impact of the copy initialization base from l-value derived case:
auto_ptr<D> dp; int x2 = sink(dp); // #2 EDG - more than one user-defined conversion applies
I'm sure that the original intention was allowing this initialization using the template<class Y> auto_ptr(auto_ptr<Y>& a) constructor (20.4.5.1/4) but since in this copy initialization it's merely user defined conversion (UDC) and the obsolete conversion member is UDC with the same rank (for the early overloading stage) there is an ambiguity between them.
Removing the obsolete member will have impact on code that explicitly invokes it:
int y = sink(source().operator auto_ptr<B>());
IMHO no one ever wrote such awkward code and the reasonable workaround for #1 is:
int y = sink( auto_ptr<B>(source()) );
I was even more surprised to find out that after removing the obsolete conversion member the initialization was still ill-formed: int x3 = sink(dp); // #3 EDG - no suitable copy constructor
This copy initialization semantically requires copy constructor which means that both template conversion constructor and the auto_ptr_ref conversion member (20.4.5.3/3) are required which is what was explicitly forbidden in DR #84. This is a bit amusing case in which removing ambiguity results with no candidates.
I also found exception safety issue with auto_ptr related to auto_ptr_ref:
int f(auto_ptr<B>, std::string); auto_ptr<B> source2(); // string constructor throws while auto_ptr_ref // "holds" the pointer int x4 = f(source2(), "xyz"); // #4
The theoretic execution sequence that will cause a leak:
According to 20.4.5.3/3 and 20.4.5/2 the auto_ptr_ref conversion member returns auto_ptr_ref<Y> that holds *this and this is another defect since the type of *this is auto_ptr<X> where X might be different from Y. Several library vendors (e.g. SGI) implement auto_ptr_ref<Y> with Y* as member which is much more reasonable. Other vendor implemented auto_ptr_ref as defectively required and it results with awkward and catastrophic code: int oops = sink(auto_ptr<B>(source())); // warning recursive on all control paths
Dave Abrahams noticed that there is no specification saying that auto_ptr_ref copy constructor can't throw.
My proposal comes to solve all the above issues and significantly simplify auto_ptr implementation. One of the fundamental requirements from auto_ptr is that it can be constructed in an intuitive manner (i.e. like ordinary pointers) but with strict ownership semantics which yield that source auto_ptr in initialization must be non-const. My idea is to add additional constructor template with sole propose to generate ill-formed, diagnostic required, instance for const auto_ptr arguments during instantiation of declaration. This special constructor will not be instantiated for other types which is achievable using 14.8.2/2 (SFINAE). Having this constructor in hand makes the constructor template<class Y> auto_ptr(auto_ptr<Y> const&) legitimate since the actual argument can't be const yet non const r-value are acceptable.
This implementation technique makes the "private auxiliary class" auto_ptr_ref obsolete and I found out that modern C++ compilers (e.g. EDG, GCC and VC) consume the new implementation as expected and allow all intuitive initialization and assignment cases while rejecting illegal cases that involve const auto_ptr arguments.
The proposed auto_ptr interface:
namespace std {
template<class X> class auto_ptr {
public:
typedef X element_type;
// 20.4.5.1 construct/copy/destroy:
explicit auto_ptr(X* p=0) throw();
auto_ptr(auto_ptr&) throw();
template<class Y> auto_ptr(auto_ptr<Y> const&) throw();
auto_ptr& operator=(auto_ptr&) throw();
template<class Y> auto_ptr& operator=(auto_ptr<Y>) throw();
~auto_ptr() throw();
// 20.4.5.2 members:
X& operator*() const throw();
X* operator->() const throw();
X* get() const throw();
X* release() throw();
void reset(X* p=0) throw();
private:
template<class U>
auto_ptr(U& rhs, typename
unspecified_error_on_const_auto_ptr<U>::type = 0);
};
}
One compliant technique to implement the unspecified_error_on_const_auto_ptr helper class is using additional private auto_ptr member class template like the following:
template<typename T> struct unspecified_error_on_const_auto_ptr;
template<typename T>
struct unspecified_error_on_const_auto_ptr<auto_ptr<T> const>
{ typedef typename auto_ptr<T>::const_auto_ptr_is_not_allowed type; };
There are other techniques to implement this helper class that might work better for different compliers (i.e. better diagnostics) and therefore I suggest defining its semantic behavior without mandating any specific implementation. IMO, and I didn't found any compiler that thinks otherwise, 14.7.1/5 doesn't theoretically defeat the suggested technique but I suggest verifying this with core language experts.
Further changes in standard text:
Remove section 20.4.5.3
Change 20.4.5/2 to read something like: Initializing auto_ptr<X> from const auto_ptr<Y> will result with unspecified ill-formed declaration that will require unspecified diagnostic.
Change 20.4.5.1/4,5,6 to read:
template<class Y> auto_ptr(auto_ptr<Y> const& a) throw();
4 Requires: Y* can be implicitly converted to X*.
5 Effects: Calls const_cast<auto_ptr<Y>&>(a).release().
6 Postconditions: *this holds the pointer returned from a.release().
Change 20.4.5.1/10
template<class Y> auto_ptr& operator=(auto_ptr<Y> a) throw();
10 Requires: Y* can be implicitly converted to X*. The expression delete get() is well formed.
LWG TC DR #127 is obsolete.
Notice that the copy constructor and copy assignment operator should remain as before and accept non-const auto_ptr& since they have effect on the form of the implicitly declared copy constructor and copy assignment operator of class that contains auto_ptr as member per 12.8/5,10:
struct X {
// implicit X(X&)
// implicit X& operator=(X&)
auto_ptr<D> aptr_;
};
In most cases this indicates about sloppy programming but preserves the current auto_ptr behavior.
Dave Abrahams encouraged me to suggest fallback implementation in case that my suggestion that involves removing of auto_ptr_ref will not be accepted. In this case removing the obsolete conversion member to auto_ptr<Y> and 20.4.5.3/4,5 is still required in order to eliminate ambiguity in legal cases. The two constructors that I suggested will co exist with the current members but will make auto_ptr_ref obsolete in initialization contexts. auto_ptr_ref will be effective in assignment contexts as suggested in DR #127 and I can't see any serious exception safety issues in those cases (although it's possible to synthesize such). auto_ptr_ref<X> semantics will have to be revised to say that it strictly holds pointer of type X and not reference to an auto_ptr for the favor of cases in which auto_ptr_ref<Y> is constructed from auto_ptr<X> in which X is different from Y (i.e. assignment from r-value derived to base).
Proposed resolution:
Section: 23.2.4 [lib.vector], 23.3.1 [lib.map] Status: New Submitter: Thorsten Ottosen Date: 12 May 2004
To add slightly more convenience to vector<T> and map<Key,T> we should consider to add
Rationale:
Proposed resolution:
Section: 17.4.1.2 [lib.headers] Status: New Submitter: Steve Clamage Date: 3 Jun 2004
C header <iso646.h> defines macros for some operators, such as not_eq for !=.
Section 17.4.1.2 [lib.headers] "Headers" says that except as noted in clauses 18 through 27, the <cname> C++ header contents are the same as the C header <name.h>. In particular, table 12 lists <ciso646> as a C++ header.
I don't find any other mention of <ciso646>, or any mention of <iso646.h>, in clauses 17 thorough 27. That implies that the contents of <ciso646> are the same as C header <iso646.h>.
Annex C (informative, not normative) in [diff.header.iso646.h] C.2.2.2 "Header <iso646.h>" says that the alternative tokens are not defined as macros in <ciso646>, but does not mention the contents of <iso646.h>.
I don't find any normative text to support C.2.2.2.
Proposed resolution:
Add a section somewhere (clause 18? clause 22?) that says <iso646.h> and <ciso646> are empty for C++.
Section: 21.3.1 [lib.string.cons] Status: New Submitter: Daniel Frey Date: 10 Jun 2004
Today, my colleagues and me wasted a lot of time. After some time, I found the problem. It could be reduced to the following short example:
#include <string>
int main() { std::string( 0 ); }
The problem is that the tested compilers (GCC 2.95.2, GCC 3.3.1 and Comeau online) compile the above without errors or warnings! The programs (at least for the GCC) resulted in a SEGV.
I know that the standard explicitly states that the ctor of string requires a char* which is not zero. STLs could easily detect the above case with a private ctor for basic_string which takes a single 'int' argument. This would catch the above code at compile time and would not ambiguate any other legal ctors.
Proposed resolution:
Section: 21.1.3.1 [lib.char.traits.specializations.char] Status: New Submitter: Martin Sebor Date: 28 Jun 2004
Table 37 describes the requirements on Traits::compare() in terms of those on Traits::lt(). 21.1.3.1, p6 requires char_traits<char>::lt() to yield the same result as operator<(char, char).
Most, if not all, implementations of char_traits<char>::compare() call memcmp() for efficiency. However, the C standard requires both memcmp() and strcmp() to interpret characters under comparison as unsigned, regardless of the signedness of char. As a result, all these char_traits implementations fail to meet the requirement imposed by Table 37 on compare() when char is signed.
Read email thread starting with c++std-lib-13499 for more.
Proposed resolution:
Change 21.1.3.1, p6 from
The two-argument members assign, eq, and lt are defined identically
to the built-in operators =, ==, and < respectively.
to
The two-argument members assign and eq are defined identically
to the built-in operators = and == respectively. The two-argument
member lt is defined identically to the built-in operator < for
type unsigned char.
Section: 27.4.4.3 [lib.iostate.flags] Status: New Submitter: Martin Sebor Date: 28 Jun 2004
The program below is required to compile but when run it typically produces unexpected results due to the user-defined conversion from std::cout or any object derived from basic_ios to void*.
#include <cassert>
#include <iostream>
int main ()
{
assert (std::cin.tie () == std::cout);
// calls std::cout.ios::operator void*()
}
Proposed resolution:
Replace std::basic_ios<charT, traits>::operator void*() with another conversion operator to some unspecified type that is guaranteed not to be convertible to any other type except for bool (a pointer-to-member might be one such suitable type). In addition, make it clear that the pointer type need not be a pointer to a complete type and when non-null, the value need not be valid.
Specifically, change in [lib.ios] the signature of
operator void*() const;
to
operator unspecified_pointer_type () const;
and change [lib.iostate.flags], p1 from
operator void*() const;
to
operator unspecified_pointer_type() const;
-1- Returns: If fail() then a null pointer; otherwise some
non-null but not necessarily valid pointer to indicate
success.
-2- Note: The type named unspecified_pointer_type above is a pointer
to some unspecified, possibly incomplete type, that is guaranteed
not to be convertible to any other type except bool.(Footnote 1)
--
Footnote 1: A pointer-to-member might be one such suitable type.
Section: 23.2.5 [lib.vector.bool] Status: New Submitter: Martin Sebor Date: 28 Jun 2004
The overloads of relational operators for vector<bool> specified in [lib.vector.bool] are redundant (they are semantically identical to those provided for the vector primary template) and may even be diagnosed as ill-formed (refer to Daveed Vandevoorde's explanation in c++std-lib-13647).
Proposed resolution:
Remove all overloads of overloads of relational operators for vector<bool> from [lib.vector.bool].
Section: 23 [lib.containers] Status: New Submitter: Martin Sebor Date: 28 Jun 2004
The standard doesn't prohibit the destructors (or any other special functions) of containers' elements invoked from a member function of the container from "recursively" calling the same (or any other) member function on the same container object, potentially while the container is in an intermediate state, or even changing the state of the container object while it is being modified. This may result in some surprising (i.e., undefined) behavior.
Read email thread starting with c++std-lib-13637 for more.
Proposed resolution:
Add to Container Requirements the following new paragraph:
Unless otherwise specified, the behavior of a program that
invokes a container member function f from a member function
g of the container's value_type on a container object c that
called g from its mutating member function h, is undefined.
I.e., if v is an element of c, directly or indirectly calling
c.h() from v.g() called from c.f(), is undefined.
Section: 18.6.1 [lib.exception] Status: New Submitter: Martin Sebor Date: 28 Jun 2004
[lib.exception] specifies the following:
exception (const exception&) throw();
exception& operator= (const exception&) throw();
-4- Effects: Copies an exception object.
-5- Notes: The effects of calling what() after assignment
are implementation-defined.
First, does the Note only apply to the assignment operator? If so, what are the effects of calling what() on a copy of an object? Is the returned pointer supposed to point to an identical copy of the NTBS returned by what() called on the original object or not?
Second, is this Note intended to extend to all the derived classes in section 19? I.e., does the standard provide any guarantee for the effects of what() called on a copy of any of the derived class described in section 19?
Finally, if the answer to the first question is no, I believe it constitutes a defect since throwing an exception object typically implies invoking the copy ctor on the object. If the answer is yes, then I believe the standard ought to be clarified to spell out exactly what the effects are on the copy (i.e., after the copy ctor was called).
Proposed resolution:
Section: 25.3.3.3 [lib.equal.range] Status: New Submitter: Prateek R Karandikar Date: 29 Feb 1900
There is no "Returns:" clause for std::equal_range, which returns non-void.
Proposed resolution:
Section: 22.2.1.1 [lib.locale.ctype] Status: New Submitter: Martin Sebor Date: 1 Jul 2004
Most ctype member functions come in two forms: one that operates on a single character at a time and another form that operates on a range of characters. Both forms are typically described by a single Effects and/or Returns clause.
The Returns clause of each of the single-character non-virtual forms suggests that the function calls the corresponding single character virtual function, and that the array form calls the corresponding virtual array form. Neither of the two forms of each virtual member function is required to be implemented in terms of the other.
There are three problems:
1. One is that while the standard does suggest that each non-virtual member function calls the corresponding form of the virtual function, it doesn't actually explicitly require it.
Implementations that cache results from some of the virtual member functions for some or all values of their arguments might want to call the array form from the non-array form the first time to fill the cache and avoid any or most subsequent virtual calls. Programs that rely on each form of the virtual function being called from the corresponding non-virtual function will see unexpected behavior when using such implementations.
2. The second problem is that either form of each of the virtual functions can be overridden by a user-defined function in a derived class to return a value that is different from the one produced by the virtual function of the alternate form that has not been overriden.
Thus, it might be possible for, say, ctype::widen(c) to return one value, while for ctype::widen(&c, &c + 1, &wc) to set wc to another value. This is almost certainly not intended. Both forms of every function should be required to return the same result for the same character, otherwise the same program using an implementation that calls one form of the functions will behave differently than when using another implementation that calls the other form of the function "under the hood."
3. The last problem is that the standard text fails to specify whether one form of any of the virtual functions is permitted to be implemented in terms of the other form or not, and if so, whether it is required or permitted to call the overridden virtual function or not.
Thus, a program that overrides one of the virtual functions so that it calls the other form which then calls the base member might end up in an infinite loop if the called form of the base implementation of the function in turn calls the other form.
Proposed resolution:
To fix these problems I propose the following:
Add two paragraphs immediately after 22.2.1.1 [lib.locale.ctype], p2, with the following text:
-3- Each ctype non-virtual member function that comes in two forms,
one that takes a range of elements of char_type, and another
that takes just a single element of char_type, is required to
call the corresponding form of the virtual member function
with the same value of char_type to obtain the result. The
result for the same argument may be cached and returned from
subsequent calls to either form of the non-virtual member
function with that argument.
-4- For each ctype virtual member function that comes in two forms
(as explained above), the single element form is required to
produce the same result for a character c that the corresponding
array form produces for the array element with the same value as
c, and vice versa.
-5- It is unspecified whether the array form of each virtual member
function calls the single-element virtual overload of the same
function in a loop, or whether the single element form calls
the array form with an array of a single element with the value
of its argument, or whether neither form calls the other. In
any case, an implementation is not permitted to make calls from
one form of any virtual member function to the corresponding
other form that is overridden in a derived class.
Section: 27.6.2.5.4 [lib.ostream.inserters.character] Status: New Submitter: Martin Sebor Date: 1 Jul 2004
I think Footnote 297 is confused. The paragraph it applies to seems quite clear in that widen() is only called if the object is not a char stream (i.e., not basic_ostream<char>), so it's irrelevant what the value of widen(c) is otherwise.
Proposed resolution:
I propose to strike the Footnote.
Section: 25.1.1 [lib.alg.foreach] Status: New Submitter: Stephan T. Lavavej, Jaakko Jarvi Date: 9 Jul 2004
It is not clear whether the function object passed to for_each is allowed to modify the elements of the sequence being iterated over.
for_each is classified without explanation in [lib.alg.nonmodifying], "25.1 Non-modifying sequence operations". 'Non-modifying sequence operation' is never defined.
25(5) says: "If an algorithm's Effects section says that a value pointed to by any iterator passed as an argument is modified, then that algorithm has an additional type requirement: The type of that argument shall satisfy the requirements of a mutable iterator (24.1)."
for_each's Effects section does not mention whether arguments can be modified:
"Effects: Applies f to the result of dereferencing every iterator in the range [first, last), starting from first and proceeding to last - 1."
Every other algorithm in [lib.alg.nonmodifying] is "really" non-modifying in the sense that neither the algorithms themselves nor the function objects passed to the algorithms may modify the sequences or elements in any way. This DR affects only for_each.
We suspect that for_each's classification in "non-modifying sequence operations" means that the algorithm itself does not inherently modify the sequence or the elements in the sequence, but that the function object passed to it may modify the elements it operates on.
The original STL document by Stepanov and Lee explicitly prohibited the function object from modifying its argument. The "obvious" implementation of for_each found in several standard library implementations, however, does not impose this restriction. As a result, we suspect that the use of for_each with function objects that modify their arguments is wide-spread. If the restriction was reinstated, all such code would become non-conforming. Further, none of the other algorithms in the Standard could serve the purpose of for_each (transform does not guarantee the order in which its function object is called).
We suggest that the standard be clarified to explicitly allow the function object passed to for_each modify its argument.
Proposed resolution:
Add the following sentence to the Effects in 25.1.1 [lib.alg.foreach]:
"f may apply non-constant functions through the dereferenced iterators passed to it; if it does, the type of first shall satisfy the requirements of a mutable iterator (24.1)."
Section: 24.1.3 [lib.forward.iterators] Status: New Submitter: Dave Abrahams Date: 9 Jul 2004
24.1/3 says:
Forward iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified
The problem is that satisfying the requirements of output iterator means that you can always assign *something* into the result of dereferencing it. That makes almost all non-mutable forward iterators non-conforming. I think we need to sever the refinement relationship between forward iterator and output iterator.
Proposed resolution:
in 24.1/3, replace:
Forward iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified.
with
A forward iterator satisfies all the input iterator requirements. A mutable forward iterator satisfies all the output iterator requirements.
Section: 24.1.3 [lib.forward.iterators] Status: New Submitter: Dave Abrahams Date: 11 Jul 2004
The Forward Iterator requirements table contains the following:
expression return type operational precondition
semantics
========== ================== =========== ==========================
a->m U& if X is mutable, (*a).m pre: (*a).m is well-defined.
otherwise const U&
r->m U& (*r).m pre: (*r).m is well-defined.
The first line is exactly right. The second line is wrong. Basically it implies that the const-ness of the iterator affects the const-ness of referenced members. But Paragraph 11 of [lib.iterator.requirements] says:
In the following sections, a and b denote values of type const X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T, o denotes a value of some type that is writable to the output iterator.
AFAICT if we need the second line at all, it should read the same as the first line.
Related issue: 478
Proposed resolution:
Section: 24.1.3 [lib.forward.iterators] Status: New Submitter: Dave Abrahams Date: 11 Jul 2004
The Forward Iterator requirements table contains the following:
expression return type operational precondition
semantics
========== ================== =========== ==========================
a->m U& if X is mutable, (*a).m pre: (*a).m is well-defined.
otherwise const U&
r->m U& (*r).m pre: (*r).m is well-defined.
The second line may be unnecessary. Paragraph 11 of [lib.iterator.requirements] says:
In the following sections, a and b denote values of type const X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T, o denotes a value of some type that is writable to the output iterator.
Because operators can be overloaded on an iterator's const-ness, the current requirements allow iterators to make many of the operations specified using the identifiers a and b invalid for non-const iterators. Rather than expanding the tables, I think the right answer is to change
"const X"
to
"X or const X"
in paragraph 11 of [lib.iterator.requirements].
Related issue: 477
Proposed resolution:
Section: 23.1 [lib.container.requirements] Status: New Submitter: Herb Sutter Date: 1 Aug 2004
Nothing in the standard appears to make this program ill-formed:
struct C {
void* operator new( size_t s ) { return ::operator new( s ); }
// NOTE: this hides in-place and nothrow new
};
int main() {
vector<C> v;
v.push_back( C() );
}
Is that intentional? We should clarify whether or not we intended to require containers to support types that define their own special versions of operator new.
Proposed resolution:
Section: 20.3.1 [lib.base] Status: New Submitter: Joe Gottman Date: 19 Aug 2004
The classes std::unary_function and std::binary_function are both designed to be inherited from but contain no virtual functions. This makes it too easy for a novice programmer to write code like binary_function<int, int, int> *p = new plus<int>; delete p;
There are two common ways to prevent this source of undefined behavior: give the base class a public virtual destructor, or give it a protected nonvirtual destructor. Since unary_function and binary_function have no other virtual functions, (note in particular the absence of an operator()() ), it would cost too much to give them public virtual destructors. Therefore, they should be given protected nonvirtual destructors.
Proposed resolution:
Change Paragraph 20.3.1 of the Standard from
template <class Arg, class Result>
struct unary_function {
typedef Arg argument_type;
typedef Result result_type;
};
template <class Arg1, class Arg2, class Result>
struct binary_function {
typedef Arg1 first_argument_type;
typedef Arg2 second_argument_type;
typedef Result result_type;
};
to
template <class Arg, class Result>
struct unary_function {
typedef Arg argument_type;
typedef Result result_type;
protected:
~unary_function() {}
};
template <class Arg1, class Arg2, class Result>
struct binary_function {
typedef Arg1 first_argument_type;
typedef Arg2 second_argument_type;
typedef Result result_type;
protected:
~binary_function() {}
};
Section: 25.2.8 [lib.alg.unique] Status: New Submitter: Andrew Koenig Date: 30 Aug 2004
The standard says that unique(first, last) "eliminates all but the first element from every consecutive group of equal elements" in [first, last) and returns "the end of the resulting range". So a postcondition is that [first, result) is the same as the old [first, last) except that duplicates have been eliminated.
What postconditions are there on the range [result, last)? One might argue that the standard says nothing about those values, so they can be anything. One might also argue that the standard doesn't permit those values to be changed, so they must not be. Should the standard say something explicit one way or the other?
Proposed resolution:
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