Re: [gil] Review of move assignment operator for image class

From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2020-04-02 15:54:13

On Mon, 30 Mar 2020 at 20:38, Mateusz Loskot <mateusz_at_[hidden]> wrote:
>
> image& operator=(image&& img)
> {
>     if (this == std::addressof(img))
>         return *this;
>
>     auto const exchange_memory = [](image& lhs, image& rhs)
>     {
>         lhs._memory = boost::exchange(rhs._memory, nullptr);
>         lhs._align_in_bytes = boost::exchange(rhs._align_in_bytes, 0);
>         lhs._allocated_bytes = boost::exchange(rhs._allocated_bytes, 0);
>         lhs._view = boost::exchange(rhs._view, image::view_t{});
>     };
>
>     constexpr bool pocma =
> std::allocator_traits<Alloc>::propagate_on_container_move_assignment::value;
>     if (pocma)
>     {

This obviously is not good as non-constexpr will require _Alloc::operator=
but there is no `if constexpr` in C++11 to discard instantiation.

Is there any BOOST_IF_CONSTEXPR magic for such a situation?
Since we are stuck at C++11, we can't use hana::eval_if
and looks like we only can do the old-school tag dispatching.

p.s. Sorry if my initial C++ question w/o strict reference to Boost
was off-topic,
or perhaps I made a non-commentable terrible C++ blunder :)

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net