获取元组元素类型的索引?

问题描述

如果我有一个具有不同元素类型的元组，例如

If I have a tuple with different element types like

std::tuple<T0, T1, T2, ...>

以及如何获取元素类型的索引?

And how to get the index of a element type?

template<class T, class Tuple>
struct Index
{
    enum {value = ?;}
};

谢谢.

推荐答案

template <class T, class Tuple>
struct Index;

template <class T, class... Types>
struct Index<T, std::tuple<T, Types...>> {
    static const std::size_t value = 0;
};

template <class T, class U, class... Types>
struct Index<T, std::tuple<U, Types...>> {
    static const std::size_t value = 1 + Index<T, std::tuple<Types...>>::value;
};

在 Coli 现场观看.

See it live at Coliru.

此实现返回给定类型第一次出现的索引.请求不在元组中的类型的索引会导致编译错误(并且相当难看).

This implementation returns the index of the first occurrence of a given type. Asking for the index of a type that is not in the tuple results in a compile error (and a fairly ugly one at that).

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