[libcxx-dev] std::vector using copy constructor on existing elements during reserve()

[Eric Fiselier via libcxx-dev]

On Tue, Apr 16, 2019 at 2:25 PM Michael Narigon via libcxx-dev <
libcxx-dev at lists.llvm.org> wrote:

> std:vector is using the element copy constructor on existing elements
> during its resizing operation in reserve(). I would expect it to use the
> move constructor for efficiency. Is this intended behavior?
>

Yes, this behavior is mandated by the standard. The copy constructor is
used when the move constructor is not noexcept. From cppreference's
`std::move_if_noexcept` documentation:

std::vector::resize
> <https://en.cppreference.com/w/cpp/container/vector/resize>, which may
> have to allocate new storage and then move or copy elements from old
> storage to new storage.
> If an exception occurs during this operation, std::vector::resize
> <https://en.cppreference.com/w/cpp/container/vector/resize> undoes
> everything it did to this point, which is only possible if
> std::move_if_noexcept was used to decide whether to use move construction
> or copy construction. (unless copy constructor is

not available, in which case move constructor is used either way and the
> strong exception guarantee may be waived)

 https://en.cppreference.com/w/cpp/utility/move_if_noexcept

If you add `noexcept` to your move constructor, vector should call the move
constructor.




>
> This is with the libc++ delivered with Xcode 10.1.
>
> Attached is a sample program displaying the behavior.
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