This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

3436. std::construct_at should support arrays

Section: 25.11.8 [specialized.construct] Status: New Submitter: Jonathan Wakely Opened: 2020-04-29 Last modified: 2021-01-17

Priority: 2

View all issues with New status.

Discussion:

std::construct_at is ill-formed for array types, because the type of the new-expression is T not T* so it cannot be converted to the return type.

In C++17 allocator_traits::construct did work for arrays, because it returns void so there is no ill-formed conversion. On the other hand, in C++17 allocator_traits::destroy didn't work for arrays, because p->~T() isn't valid.

In C++20 allocator_traits::destroy does work, because std::destroy_at treats arrays specially, but allocator_traits::construct no longer works because it uses std::construct_at.

It seems unnecessary and/or confusing to remove support for arrays in construct when we're adding it in destroy.

I suggest that std::construct_at should also handle arrays. It might be reasonable to restrict that support to the case where sizeof...(Args) == 0, if supporting parenthesized aggregate-initialization is not desirable in std::construct_at.

[2020-05-09; Reflector prioritization]

Set priority to 2 after reflector discussions.

[2021-01-16; Zhihao Yuan provides wording]

Proposed resolution:

This wording is relative to N4878.

  1. Modify 25.11.8 [specialized.construct] as indicated:

    template<class T, class... Args>
      constexpr T* construct_at(T* location, Args&&... args);
    
    namespace ranges {
      template<class T, class... Args>
        constexpr T* construct_at(T* location, Args&&... args);
    }
    

    -1- Constraints: The expression ::new (declval<void*>()) T(declval<Args>()...) is well-formed when treated as an unevaluated operand.

    -2- Effects: Equivalent to:

    returnauto ptr = ::new (voidify(*location)) T(std::forward<Args>(args)...);
    if constexpr (is_array_v<T>)
      return launder(location);
    else
      return ptr;