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3321. uninitialized_construct_using_allocator should use construct_at

Section: 20.2.8.2 [allocator.uses.construction] Status: C++20 Submitter: United States Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 0

View all other issues in [allocator.uses.construction].

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Discussion:

Addresses US 213

uninitialized_construct_using_allocator should use construct_at instead of operator new

Proposed change:

Effects: Equivalent to:

return ::new(static_cast<void*>(p))
construct_at(p,
T(make_obj_using_allocator<T>(alloc,
std::forward<Args>(args)...)));

Tim Song:

The proposed wording in the NB comment is incorrect, because it prevents guaranteed elision.

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 20.2.8.2 [allocator.uses.construction] as indicated:

   template<class T, class Alloc, class... Args>
     constexpr T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);
   

   -17- Effects: Equivalent to:

   return ::new(static_cast<void*>(p))
     T(make_obj_using_allocator<T>(apply([&](auto&&...xs) {
            return construct_at(p, std::forward<decltype(xs)>(xs)...);
        }, uses_allocator_construction_args<T>(alloc, std::forward<Args>(args)...));