Re: [Patch] Forward triviality in variant

[Jonathan Wakely <jwakely at redhat dot com>]

On 01/06/17 18:21 +0300, Ville Voutilainen wrote:
> On 1 June 2017 at 18:13, Jonathan Wakely <jwakely@redhat.com> wrote:
> > On 30/05/17 02:16 -0700, Tim Shen via libstdc++ wrote:
> > > diff --git a/libstdc++-v3/include/std/variant
> > > b/libstdc++-v3/include/std/variant
> > > index b9824a5182c..f81b815af09 100644
> > > --- a/libstdc++-v3/include/std/variant
> > > +++ b/libstdc++-v3/include/std/variant
> > > @@ -290,6 +290,53 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >           __ref_cast<_Tp>(__t));
> > >     }
> > >
> > > +  template<typename... _Types>
> > > +    struct _Traits
> > > +    {
> > > +      static constexpr bool is_default_constructible_v =
> > > +          is_default_constructible_v<typename _Nth_type<0,
> > > _Types...>::type>;
> > > +      static constexpr bool is_copy_constructible_v =
> > > +          __and_<is_copy_constructible<_Types>...>::value;
> > > +      static constexpr bool is_move_constructible_v =
> > > +          __and_<is_move_constructible<_Types>...>::value;
> > > +      static constexpr bool is_copy_assignable_v =
> > > +          is_copy_constructible_v && is_move_constructible_v
> > > +          && __and_<is_copy_assignable<_Types>...>::value;
> > > +      static constexpr bool is_move_assignable_v =
> > > +          is_move_constructible_v
> > > +          && __and_<is_move_assignable<_Types>...>::value;
> >
> >
> > It seems strange to me that these ones end with _v but the following
> > ones don't. Could we make them all have no _v suffix?
>
> Seems to me worth considering to rather make all of them have a _v suffix. :)
> > > +      static constexpr bool is_dtor_trivial =
> > > +          __and_<is_trivially_destructible<_Types>...>::value;
>
>
> They all seem to be shortcuts for something::value, so it seems to me
> logical to have
> them all be _v.

The _v suffixes in the standard are there to distinguish std::foo from
std::foo_v, but we don't have that problem.

__variant::_Traits<T...>::foo is a unique name, we don't need the
suffix, it's just noise.