std::tuple_size<std::pair> - cppreference.com

From cppreference.com


`template< class T1, class T2 > struct tuple_size<std::pair<T1, T2>> : std::integral_constant<std::size_t, 2> { };`		(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.

Inherited from std::integral_constant

Member constants

the constant value 2
(public static member constant)

Member functions

	converts the object to `std::size_t`, returns `value` (public member function)
	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 <tuple>
#include <utility>

template<class T>
void test([[maybe_unused]]T t)
{
    [[maybe_unused]]
    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:

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2313	C++11	specializations for `pair` were not required to be derived from `integral_constant`	required