Variable template template?

Question

Say you have a tuple type and you want to extract its template parameter package to create an instance of another template. If this is a type template, then I might have a utility like this:

template < typename Tuple, template <typename...> typename What > struct PutTupleInT; template < typename... Types, template <typename...> typename What > struct PutTupleInT<std::tuple<Types...>, What> { using Result = What<Types...>; }; 

But what if the desired template is a variable template? While template <typename...> typename What is a placeholder for a type template, what is a placeholder for a variable template?

I tried the following for clang-4.0.0 (the only compiler that now supports non-Pig type template parameters with automatic type), but this failed. Actually, I'm not sure if this is the correct syntax for C ++ 17.

 template < typename Tuple, template <typename...> auto What > struct PutTupleInV; template < typename... Types, template <typename...> auto What > struct PutTupleInV<std::tuple<Types...>, What> { static constexpr auto value = What<Types...>; }; 

Answer 1

I do not think you can do this. Quote from N4606:

§14.3.3 [temp.arg.template] / 1

The template argument for the template template must be the class template name or alias template expressed as an ID expression.

The variable template does not meet this requirement.

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You can trick a little and use a proxy type to select a template:

 template < typename Tuple, class Proxy> struct PutTupleInTV; template < typename... Types, class Proxy> struct PutTupleInTV<std::tuple<Types...>, Proxy> { static constexpr auto value = Proxy::template value<Types...>; }; 

and then for

 template<typename...> struct foo{}; template<typename... Ts> constexpr foo<Ts...> foo_v{}; struct use_foo { template<typename... Ts> static constexpr auto value = foo_v<Ts...>; }; 

could you say

 PutTupleInTV<tup, use_foo>::value 

live demonstration

Answer 2

PutTupleInTV does not match the name PutTupleInV. You do not specialize in the PutTupleInV template, but use special syntax to create something new called PutTupleInTV.