如何获取整数参数包中的第i个整数？

Question

如何获取整数参数包中的第i个整数？例如

template<int... Is>
struct A
{
    enum { CONSTANT_0 = Is[0] }; //Assume the sizeof...(Is) > the index requested
};

Answer 1

像那样:

template <size_t I, int N, int... R>
struct pick : pick <I - 1, R...> { };

template <int N, int... R>
struct pick <0, N, R...> : std::integral_constant <int, N> { };

以便

pick <3, 1, 2, 3, 4, 5, 6>::value

等于4,和

template<int... Is>
struct A
{
    enum { CONSTANT_0 = pick <0, Is...>::value };
};

是你如何在你的情况下使用它.

其他方式:

template <size_t I, int... N>
using pick = typename std::tuple_element <I,
   std::tuple <std::integral_constant <int, N>...>
>::type;