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std::tuple_size<std::pair>

STD：：元组[医]大小<std：：ping>

Defined in header ​​
template< class T1, class T2 > struct tuple_size<std::pair<T1, T2>>;​(since C++11) (until C++14)
template <class T1, class T2> struct tuple_size<std::pair<T1, T2>> : std::integral_constant<std::size_t, 2> { };​(since C++14)

部分专业化std::tuple_sizeFORMIS提供了一种编译时方法，可以使用类似于元组的语法获取对中的元素数(始终为2)。

继承自STD：积分[医]常量

成员常数

value staticthe constant value 2 (public static member constant)

成员函数

operator std::size_tconverts the object to std::size_t, returns value (public member function)
operator() (C++14)returns value (public member function)

成员类型

TypeDefinition
value_typestd::size_t
typestd::integral_constant<std::size_t, value>

例

二次

#include <iostream>
#include <utility>
#include <tuple>
 
template<class T>
void test(T t)
{
    int a[std::tuple_size<T>::value]; // can be used at compile time
    std::cout << std::tuple_size<T>::value << '\n'; // or at run time
}
 
int main()
{
    test(std::make_tuple(1, 2, 3.14)
    test(std::make_pair(1, 3.14)
}

二次

产出：

二次

3
2

二次

另见

tuple_sizeobtains the size of tuple at compile time (class template specialization)
std::tuple_size<std::array>obtains the size of an 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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