Variable template template?

Question

Say you have a tuple type and you want to extract its template parameter pack in order to instantiate another template. If that is a type template, then I can 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 the "placeholder" for a type template, then what is the "placeholder" for a variable template?

I've tried the following for clang-4.0.0 (the only compiler by now that supports non-type template parameters with auto type), but it failed. Actually I am not sure if this is a 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...>;
};

Can't you use `decltype(vartempl)` and extract the type of the variable template, that is what you are interested in actually? — skypjack, Oct 23 '16 at 09:59

score 6 · Accepted Answer · answered Oct 23 '16 at 09:20

I don't think you can do that. Quoting N4606:

§14.3.3 [temp.arg.template]/1

A template-argument for a template template-parameter shall be the name of a class template or an alias template, expressed as id-expression.

A variable template doesn't fit this requirement.

You could cheat a little and use a proxy type to select the 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...>;
};

you could say

PutTupleInTV<tup, use_foo>::value

live demo

The cheat actually doesn't help much, as it requires the variables to be named "value" and to be placed in a struct. — Vahagn, Oct 24 '16 at 15:57
@Vahagn yes, you'd have to make proxy structs yourself. I don't see a way around this (doesn't mean there's none, I just don't know of any) — krzaq, Oct 24 '16 at 15:59

score 0 · Answer 2 · answered Oct 23 '16 at 08:47

0

PutTupleInTV is not the same name as PutTupleInV. You are not specializing the template PutTupleInV but using specializing syntax to create something new, called PutTupleInTV.

answered Oct 23 '16 at 08:47

yeoman

1,671
12
15

Variable template template?

2 Answers2