std::tuple_size<std::pair>

From cppreference.com
Defined in header <utility>
template< class T1, class T2 >
struct tuple_size<std::pair<T1, T2>>;
(since C++11)

The partial specialization of std::tuple_size for pairs provides a compile-time way to obtain the number of elements in a pair, which is always 2, using tuple-like syntax.

Members

value
[static]
integral constant with value 2
(public static member constant)

[edit] Example

#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));
}

Output:

3
2

[edit] See also

obtains the size of tuple at compile time
(class template specialization)
obtains the size of an array
(class template specialization)
obtains the type of the elements of pair
(class template specialization)