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std::tuple_element<std::tuple>

STD：：元组[医]元素<std：：tuple>

Defined in header ​​
template< std::size_t I, class T > class tuple_element; /* undefined */(1)(since C++11)
template< std::size_t I, class... Types > class tuple_element< I, tuple<Types...> >;(2)(since C++11)
Defined in header <tuple>​​
Defined in header <array> Defined in header <utility>​(since C++17)(since C++17)
template< std::size_t I, class T > class tuple_element< I, const T > { typedef typename std::add_const<typename std::tuple_element<I, T>::type>::type type; };(3)(since C++11)
template< std::size_t I, class T > class tuple_element< I, volatile T > { typedef typename std::add_volatile<typename std::tuple_element<I, T>::type>::type type; };(4)(since C++11)
template< std::size_t I, class T > class tuple_element< I, const volatile T > { typedef typename std::add_cv<typename std::tuple_element<I, T>::type>::type type; };(5)(since C++11)

提供对元组元素类型的编译时索引访问。

成员类型

Member typeDefinition
typethe type of Ith element of the tuple, where I is in [0, sizeof...(Types))

帮助者类型

template using tuple_element_t = typename tuple_element::type;​(since C++14)

可能的实施

模板<std：：size[医]TI，类T>结构元组[医]元素；//递归案例模板<std：：size[医]我，班长，班长...。尾>结构元组[医]元素<i，std：：tuple<head，tail...>>：std：：tuple[医]元素<i-1，std：：tuple<tail...>>{}；//基案例模板<class Head，class...。尾>结构元组[医]元素<0，std：：元组<头，尾…>>{冒枝子型；}；

*。

例

二次

#include <iostream>
#include <tuple>
 
template <class... Args>
struct type_list
{
   template <std::size_t N>
   using type = typename std::tuple_element<N, std::tuple<Args...>>::type;
};
 
int main()
{
   std::cout << std::boolalpha;
   type_list<int, char, bool>::type<2> x = true;
   std::cout << x << '\n';
}

二次

产出：

二次

true

二次

另见

std::tuple_elementobtains the type of the elements of array (class template specialization)
std::tuple_element<std::pair> (C++11)obtains the type of the elements of pair (class template specialization)

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http：//en.cppreference.com/w/cpp/实用程序/tuple/tuple[医]元素

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