获取第N种可变参数模板模板？

Question

如何获取第N种可变参数模板模板？例如

template<typename... Args>
class MyClass
{
    Args[0] mA; // This is wrong. How to get the type?

};

Answer 1

你可以使用std::tuple:

#include<tuple>

template<typename... Args>
class MyClass
{
    typename std::tuple_element<0, std::tuple<Args...> >::type mA;
};

Answer 2

如果你想要一些不使用std::tuple它的东西

template<std::size_t N, typename T, typename... types>
struct get_Nth_type
{
    using type = typename get_Nth_type<N - 1, types...>::type;
};

template<typename T, typename... types>
struct get_Nth_type<0, T, types...>
{
    using type = T;
};

比

template<std::size_t N, typename... Args>
using get = typename get_Nth_type<N, Args...>::type;

template<typename... Args>
class MyClass
{
    get<0, Args...> mA;
};