Re: [PATCH 2/3] libstdc++: Implement std::pair/tuple/misc enhancements from P2321R2

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 23 Aug 2022 at 02:36, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
> --- a/libstdc++-v3/include/bits/stl_pair.h
> +++ b/libstdc++-v3/include/bits/stl_pair.h
> @@ -212,6 +212,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         swap(second, __p.second);
>        }
>
> +#if __cplusplus > 202002L
> +      /// Swap the first members and then the second members.
> +      constexpr void
> +      swap(const pair& __p) const
> +      noexcept(__and_<__is_nothrow_swappable<const _T1>,
> +                     __is_nothrow_swappable<const _T2>>::value)

This could use __and_v (which is just __and_::value today, but could
theoretically be optimized to use a requires expression and avoid
instantiating __and_ one day).

Is consistency with the C++11 overload more important? I *hope* we
won't need to make many changes to these noexcept-specifiers, so the
maintenance burden of using __ad_::value in one and __and_v in the
other shouldn't be too high.

> @@ -710,6 +792,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      noexcept(noexcept(__x.swap(__y)))
>      { __x.swap(__y); }
>
> +#if __cplusplus > 202002L
> +  template<typename _T1, typename _T2>
> +    requires is_swappable<const _T1>::value && is_swappable<const _T2>::value

is_swappable_v instead of ::value here ... this is already using a
requires-clause and so is substantially different to the old overload
anyway.



> +
>        // tuple swap
>        _GLIBCXX20_CONSTEXPR
>        void
>        swap(tuple& __in)
>        noexcept(__and_<__is_nothrow_swappable<_Elements>...>::value)
>        { _Inherited::_M_swap(__in); }
> +
> +#if __cplusplus > 202002L
> +      constexpr void
> +      swap(const tuple& __in) const
> +      noexcept(__and_<__is_nothrow_swappable<const _Elements>...>::value)

__and_v ?




>        _GLIBCXX20_CONSTEXPR
>        void
>        swap(tuple& __in)
>        noexcept(__and_<__is_nothrow_swappable<_T1>,
>                       __is_nothrow_swappable<_T2>>::value)
>        { _Inherited::_M_swap(__in); }
> +
> +#if __cplusplus > 202002L
> +      constexpr void
> +      swap(const tuple& __in) const
> +      noexcept(__and_<__is_nothrow_swappable<const _T1>,
> +                     __is_nothrow_swappable<const _T2>>::value)

__and_v ?


Thanks for doing this, those changes looked tedious to implement and test!

If you agree with the suggestions to use _v variable templates, this
is OK for trunk with those changes. I am willing to be persuaded to
not use the variable templates if there's a good reason I've missed.