std::tuple_size<std::pair>

Defined in header `<utility>`
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)

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.

Inherited from std::integral_constant

Member constants

value

[static]

the constant value 2
(public static member constant)

Member functions

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

Member types

Type	Definition
`value_type`	`std::size_t`
`type`	`std::integral_constant<std::size_t, value>`

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

tuple_size	obtains 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)