2975. Missing case for pair construction in scoped and polymorphic allocators

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

Priority: 3

View all other issues in [mem.poly.allocator.mem].

View all issues with C++20 status.

Discussion:

scoped_allocator_adaptor ([allocator.adaptor.syn]) and polymorphic_allocator ([mem.poly.allocator.class]) have identical families of members named construct:

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 perform uses_allocator construction, and therefore need special handling for pair constructions since pair doesn't specialize uses_allocator (tuple gets all of that magic and pair is left out in the cold). Presumably, the intent is that the construct overloads whose first argument is a pointer to pair capture all pair constructions. This is not the case: invoking construct with a pair pointer and a non-constant lvalue pair resolves to the first overload when it is viable: it's a better match than the pair-pointer-and-const-lvalue-pair overload. The first overload notably does not properly perform piecewise uses_allocator construction for pairs 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Previous resolution [SUPERSEDED]: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 of T with allocator resource() (see 20.2.8.2 [allocator.uses.construction]) and constructor arguments std::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 a T object in the storage whose address is represented by p by uses-allocator construction with allocator resource() and constructor arguments std::forward<Args>(args)....

   -7- Throws: Nothing unless the constructor for T throws.

   -?- Remarks: This function shall not participate in overload resolution unless T is not a specialization of pair.

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 unless T is not a specialization of pair.

[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 to N4659.

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 of T with allocator resource() (see 20.2.8.2 [allocator.uses.construction]) and constructor arguments std::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 a T object in the storage whose address is represented by p by uses-allocator construction with allocator resource() and constructor arguments std::forward<Args>(args)....

   -7- Throws: Nothing unless the constructor for T throws.

   -?- Remarks: This function shall not participate in overload resolution if T is a specialization of pair.

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 if T is a specialization of pair.