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4147. Precondition on inplace_vector::emplace

Section: 23.2.4 [sequence.reqmts] Status: WP Submitter: Arthur O'Dwyer Opened: 2024-08-26 Last modified: 2024-11-28

Priority: Not Prioritized

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Discussion:

Inserting into the middle of an inplace_vector, just like inserting into the middle of a vector or deque, requires that we construct the new element out-of-line, shift down the trailing elements (Cpp17MoveAssignable), and then move-construct the new element into place (Cpp17MoveInsertable). P0843R14 failed to make this change, but it should have.

[2024-09-18; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   a.emplace(p, args)
   

   -19- Result: iterator.

   -20- Preconditions: T is Cpp17EmplaceConstructible into X from args. For vector, inplace_vector, and deque, T is also Cpp17MoveInsertable into X and Cpp17MoveAssignable.

   -21- Effects: Inserts an object of type T constructed with std::forward<Args>(args)... before p.

   [Note 1: args can directly or indirectly refer to a value in a. — end note]

   -22- Returns: An iterator that points to the new element constructed from args into a.