How to get N-th type from a tuple?

Question

I want to make a template where I can input an index and it will give me the type at that index. I know I can do this with decltype(std::get(tup)) but

Answer 1

You can use a class template and partial specializations to do what you want. (Note that std::tuple_element does almost the same like the other answer says):

#include <tuple>
#include <type_traits>

template <int N, typename... Ts>
struct get;

template <int N, typename T, typename... Ts>
struct get<N, std::tuple<T, Ts...>>
{
    using type = typename get<N - 1, std::tuple<Ts...>>::type;
};

template <typename T, typename... Ts>
struct get<0, std::tuple<T, Ts...>>
{
    using type = T;
};

int main()
{
    using var = std::tuple<int, bool, std::string>;
    using type = get<2, var>::type;

    static_assert(std::is_same<type, std::string>::value, ""); // works
}

Answer 2

That trait already exists, and it is called std::tuple_element.

Here is a live example that demonstrates its usage.