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3436. std::construct_at should support arrays

Section: 27.11.8 [specialized.construct] Status: New Submitter: Jonathan Wakely Opened: 2020-04-29 Last modified: 2021-12-11

Priority: 2

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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]

Previous resolution [SUPERSEDED]:

This wording is relative to N4878.

1. Modify 27.11.8 [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;
   

[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.

Proposed resolution:

This wording is relative to N4901.

1. Modify 27.11.8 [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 (7.2.3 [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;