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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

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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)...));