Title
std::construct_at should support arrays
Status
new
Section
[specialized.construct]
Submitter
Jonathan Wakely

Created on 2020-04-29.00:00:00 last changed 1 month ago

Messages

Date: 2021-12-11.14:55:55

Proposed resolution:

This wording is relative to N4901.

  1. Modify [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 ([expr.context]) and is_unbounded_array_v<T> is false.

    -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;
    
Date: 2021-12-15.00:00:00

[ 2021-12-07; Zhihao Yuan comments and provides improved wording ]

The previous PR allows constructing arbitrary number of elements when T is an array of unknown bound:

extern int a[];
std::construct_at(&a, 0, 1, 2);

and leads to a UB.

Date: 2021-01-15.00:00:00

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

Previous resolution [SUPERSEDED]:

This wording is relative to N4878.

  1. Modify [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;
    
Date: 2020-05-15.00:00:00

[ 2020-05-09; Reflector prioritization ]

Set priority to 2 after reflector discussions.

Date: 2020-04-29.00:00:00

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.

History
Date User Action Args
2021-12-11 14:55:55adminsetmessages: + msg12252
2021-01-17 15:14:36adminsetmessages: + msg11663
2021-01-17 15:14:36adminsetmessages: + msg11662
2020-05-09 19:39:43adminsetmessages: + msg11278
2020-04-29 00:00:00admincreate