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Subject: Re: template template aliases?
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On 30 January 2017 at 19:24, Eric Niebler <eric.niebler@gmail.com> wrote:
> Crazy thought.
>
> Why not permit template template aliases?
>
>   template <class A>
>   template <class B>
>   using Foo = pair<A, B>;
>
> Foo<A> is a unary template such that Foo<A><B> is pair<A,B>.
>
> Why?
>
> Higher order template metaprogramming sometimes requires passing a template
> to a template. For example:
>
>   // with C++17 fold expressions
>   template <template <class Ts> class Pred, class...Ts>
>   constexpr bool all_of = (... && Pred<Ts>::value);
>
>   static_assert(all_of<std::is_scalar, int, short, float>);
>
> Q: How would you use all_of to check that all types in a pack are
> convertible to bool?
> A: With a "bind"-like template template alias:
>
>   template <template<class...> class Fn, class...As>
>   template <class...Bs>
>   using bind_back = Fn<Bs..., As...>;
>
>   template <template<class...> class Fn>
>   template <class T>
>   using unary = Fn<T>;
>
>   static_assert(all_of<unary<bind_back<std::is_convertible, bool>>, int,
> short, float>);
>
> What would this language feature break?


It mixes the water about whether an alias is a template (currently it
isn't, it's replaced immediately and
cannot be specialized) or not. That doesn't mean the whole idea is
nuts, but it's probably
something to think about.

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