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2975. Missing case forpair construction in scoped and polymorphic allocators

Section: 20.5.3.3[mem.poly.allocator.mem], 20.6.4[allocator.adaptor.members]Status:C++20Submitter: Casey CarterOpened: 2017-06-13Last modified: 2021-02-25

Priority:3

View all otherissues in [mem.poly.allocator.mem].

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Discussion:

scoped_allocator_adaptor ([allocator.adaptor.syn]) andpolymorphic_allocator ([mem.poly.allocator.class]) have identical families of members namedconstruct:

template <class T, class... Args>  void construct(T* p, Args&&... args);template <class T1, class T2, class... Args1, class... Args2>  void construct(pair<T1,T2>* p, piecewise_construct_t,                 tuple<Args1...> x, tuple<Args2...> y);template <class T1, class T2>  void construct(pair<T1,T2>* p);template <class T1, class T2, class U, class V>  void construct(pair<T1,T2>* p, U&& x, V&& y);template <class T1, class T2, class U, class V>  void construct(pair<T1,T2>* p, const pair<U, V>& pr);template <class T1, class T2, class U, class V>  void construct(pair<T1,T2>* p, pair<U, V>&& pr);

Both allocators performuses_allocator construction, and therefore need special handling forpair constructions sincepair doesn't specializeuses_allocator (tuple gets all of that magic andpair is left out in the cold). Presumably, the intent is that theconstruct overloads whose first argument is a pointer topair capture allpair constructions. This is not the case: invokingconstruct with apair pointer and a non-constant lvaluepair resolves to thefirst overload when it is viable: it's a better match than thepair-pointer-and-const-lvalue-pair overload. The first overload notably does not properly perform piecewiseuses_allocator construction forpairs as intended.

[2017-07 Toronto Monday issue prioritization]

Priority 2; Marshall to work with Casey to reduce the negations in the wording.

Previous resolution [SUPERSEDED]:

1. Modify 20.5.3.3[mem.poly.allocator.mem] as indicated:

   template <class T, class... Args>  void construct(T* p, Args&&... args);

   -5-Requires: Uses-allocator construction ofT with allocatorresource() (see 20.2.8.2[allocator.uses.construction]) and constructor argumentsstd::forward<Args>(args)... is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators.—end note]

   -6-Effects: Construct aT object in the storage whose address is represented byp by uses-allocator construction with allocatorresource() and constructor argumentsstd::forward<Args>(args)....

   -7-Throws: Nothing unless the constructor forT throws.

   -?-Remarks: This function shall not participate in overload resolution unlessT is not a specialization ofpair.

2. Modify 20.6.4[allocator.adaptor.members] as indicated:

   template <class T, class... Args>  void construct(T* p, Args&&... args);

   -9-Effects: […]

   -?-Remarks: This function shall not participate in overload resolution unlessT is not a specialization ofpair.

[2017-11-02 Marshall and Casey provide updated wording]

[2017-11 Albuquerque Wednesday issue processing]

Move to Ready.

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative toN4659.

1. Modify 20.5.3.3[mem.poly.allocator.mem] as indicated:

   template <class T, class... Args>  void construct(T* p, Args&&... args);

   -5-Requires: Uses-allocator construction ofT with allocatorresource() (see 20.2.8.2[allocator.uses.construction]) and constructor argumentsstd::forward<Args>(args)... is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators.—end note]

   -6-Effects: Construct aT object in the storage whose address is represented byp by uses-allocator construction with allocatorresource() and constructor argumentsstd::forward<Args>(args)....

   -7-Throws: Nothing unless the constructor forT throws.

   -?-Remarks: This function shall not participate in overload resolution ifT is a specialization ofpair.

2. Modify 20.6.4[allocator.adaptor.members] as indicated:

   template <class T, class... Args>  void construct(T* p, Args&&... args);

   -9-Effects: […]

   -?-Remarks: This function shall not participate in overload resolution ifT is a specialization ofpair.