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

[Patrick Palka <ppalka@redhat.com>]

On Tue, 23 Aug 2022, Jonathan Wakely wrote:

> 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.

Makes sense.

> 
> > @@ -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.

Agreed on all points!  Thanks a lot.