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std::tuple_size<std::pair>

Defined in header <utility> 
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

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 <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:

3
2

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

See also

Structured binding (C++17) binds the specified names to sub-objects or tuple elements of the initializer
(C++11)
obtains the number of elements of a tuple-like type
(class template)
(C++11)
obtains the size of an array
(class template specialization)
(C++11)
obtains the size of tuple at compile time
(class template specialization)
(C++20)
obtains the number of components of a std::ranges::subrange
(class template specialization)
(C++11)
obtains the type of the elements of pair
(class template specialization)

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