Re: Suggestions for convenience aliases in <type_traits>

[Jonathan Wakely <jwakely at redhat dot com>]

On 11/06/14 07:53 +0200, Daniel Krügler wrote:
> 2014-06-10 13:17 GMT+02:00 Jonathan Wakely <jwakely@redhat.com>:
> > On 10/06/14 12:47 +0200, Daniel Krügler wrote:
> > > > Is that reasonable?
> > >
> > > Personally I consider these as bad examples for alias templates and
> > > suggest to provide corresponding class templates instead (such __and,
> > > __or, __not).
> >
> > Do you mean like this?
> >
> >  template<typename _If, typename _Then, typename _Else>
> >    struct __conditional_
> >
> >    : conditional<_If::value, _Then, _Else>
> >    { };
> >
> > With that I lose the convenience of the C++14 '_t' aliases and have to
> > write 'typename' and '::type' every time I use it.
> >
> > Or do you mean like this:
> >
> >  template<typename _If, typename _Then, typename _Else>
> >    struct __conditional_t_
> >
> >    : conditional<_If::value, _Then, _Else>::type
> >    { };
> >
> > In that case doesn't it also evaluate the prediate immediately, and so
> > has no advantage over the alias template.
>
> [Apologies for my late response, but I was completely bound by other things]
>
> The latter. There is an advantage of the second form, because you
> combine it like this
>
> SomeTemplate<__conditional_t_<X, Y, Z>>
>
> this would not necessarily instantiate the __conditional_t_ class
> template (but it would for the alias). This situation becomes
> relevant, if this
>
> __conditional_t_<X, Y, Z>
>
> is part of a potentially unevaluated part of template parameters.

OK, thanks for the explanation. In the cases I was thinking of using
this there are usually no unevaluated parameters, but I agree that if
we are going to add it to <type_traits> then your suggestion is more
flexible and has advantages for other use cases  (e.g. nested uses of
std::conditional inside std::conditional).

So if we were to use that design, I think the '_t_' suffix is not
appropriate, but we could have:

 template<typename _If, typename _Then, typename _Else>
   struct __conditional_
   : conditional<_If::value, _Then, _Else>
   { };

And then an alias for the cases where it isn't potentially unevaluated
and immediate instantiation is OK:

 template<typename _If, typename _Then, typename _Else>
   using __conditional_t_ = typename
     __conditional_<_If::value, _Then, _Else>::type;