This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Relax std::move_if_noexcept for std::pair


On 12/20/18 9:04 AM, Ville Voutilainen wrote:
On Thu, 20 Dec 2018 at 08:29, François Dumont <frs.dumont@gmail.com> wrote:
Hi

      I eventually find out what was the problem with the
std::move_if_noexcept within associative containers.

      The std::pair move default constructor might not move both first
and second member. If any is not moveable it will just copy it. And then
..as it should..

the noexcept qualification of the copy constructor will participate in
the noexcept qualification of the std::pair move constructor. So
std::move_if_noexcept can eventually decide to not use move because a
_copy_ constructor not noexcept qualified.
..and again, as it should.

      This is why I am partially specializing __move_if_noexcept_cond. As
there doesn't seem to exist any Standard meta function to find out if
move will take place I resort using std::is_const as in this case for
sure the compiler won't call the move constructor.
That seems wrong; just because a type is or is not const has nothing
to do whether
it's nothrow_move_constructible.

Indeed, I am not changing that.



I don't understand what problem this is solving, and how it's not
introducing new problems.

The problem I am trying to solve is shown by the tests I have adapted. Allow more move semantic in associative container where key are stored as const.

But if I make counter_type copy constructor noexcept then I also get the move on the pair.second instance, great. I am just surprise to have to make a copy constructor noexcept to have std::move_if_noexcept work as I expect.

I think I just need to understand why we need std::move_if_noexcept in unordered containers or even rb_tree. Couldn't we just use std::move ? I don't understand what we are trying to avoid with this noexcept check.

François



Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]