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std::vector and std::deque
should conditionally require Cpp17CopyInsertable in their preconditionsSection: 24.3.11.3 [vector.capacity], 24.3.11.5 [vector.modifiers], 24.3.8.3 [deque.capacity], 24.3.8.4 [deque.modifiers] Status: New Submitter: Jiang An Opened: 2022-08-24 Last modified: 2022-11-06
Priority: 3
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Discussion:
This issue is raised from editorial issue #5776.
In order to achieve strong exception safety, some operations ofstd:vector and
std::deque may use copy insertion for relocation of old elements, if move construction
of its element type is potentially throwing and copy insertion is available. However, currently
only Cpp17MoveInsertable is mentioned in many of their Preconditions (e.g. those of
insert for rvalues), which seemly fails to cover the cases in which copy insertion is
formally invalid but the semantic requirements of Cpp17CopyInsertable are not met.
Perhaps we should create a new named requirement for these operations, which is equivalent to
Cpp17CopyInsertable when !is_nothrow_move_constructible_v<T> &&
is_copy_constructible_v<T> is true, and equivalent to Cpp17MoveInsertable
otherwise.
[2022-09-23; Reflector poll]
Set priority to 3 after reflector poll.
Jonathan: I think the point (which LWG 3758 fails to explain clearly) is that today's implementations
sometimes use copy insertion when move insertion is syntactically valid, but is potentially-throwing.
But the preconditions don't require copy insertion. If vector::resize(size_type) decides to
use copy construction, because move construction might throw and the type is copy constructible (which
is implied to be permitted by the Remarks), do we require Cpp17CopyInsertable's semantic
requirement that the new value is equivalent to the one we copied? We don't say so.
tl;dr The user is trying to resize a vector and the value type is Cpp17MoveInsertable into the vector,
but the implementation decides to copy not move. What are the preconditions on the user's type?
[2022-11-06; Daniel comments]
This issue has considerable overlap with LWG 2158.
Proposed resolution: