Expand a type N times in template parameter

Question

I have the following problem:

template< std::size_t N >
class A
{
  std::function< std::size_t( /*std::size_t,....,std::size_t <- N-times*/) > foo;
};

As you can see above, I try to declare an std::function<...> foo as a member of a class A. Here, I want foo to have the return type std::size_t (which is no problem) and as input, I will pass N-times the type std::size_t but I don't know how. Is there any possibility?

Many thanks in advance.

Answer 1

You can use std::index_sequence:

template<std::size_t N, typename = std::make_index_sequence<N>>
struct A;

template<std::size_t N, std::size_t... S>
struct A<N, std::index_sequence<S...>> {
    std::function<std::size_t(decltype(S)...)> foo;
};

Live example

If you like, you could also define to what type it expands:

template<typename T, std::size_t N, typename = std::make_index_sequence<N>>
struct A;

template<typename T, std::size_t N, std::size_t... S>
struct A<T, N, std::index_sequence<S...>> {
    template<std::size_t>
    using type = T;

    std::function<std::size_t(decltype(S)...)> foo;
};

Answer 2

For arbitrary type and not just size_t, just write a helper alias:

template<class T, size_t>
using Type = T;

template<std::size_t... S>
struct AHelper<std::index_sequence<S...>> {
    std::function<size_t(Type<MyArbitraryTypeHere, S>...)> foo;
};