std::tuple_size<std::pair>

 

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std::pair

Member functions

pair::pair

pair::operator=

pair::swap (C++11)

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::pair) (C++11)

get(std::pair) (C++11)

Helper classes

tuple_size<std::pair> (C++11)

tuple_element<std::pair> (C++11)

basic_common_reference<std::pair> (C++23)

common_type<std::pair> (C++23)

formatter<std::pair> (C++23)

piecewise_construct_t (C++11)

Deduction guides(C++17)

 

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

Run this code

#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
tuple_size (C++11)	obtains the number of elements of a tuple-like type (class template)
std::tuple_size<std::array> (C++11)	obtains the size of an array (class template specialization)
std::tuple_size<std::tuple> (C++11)	obtains the size of a tuple (class template specialization)
std::tuple_size<std::ranges::subrange> (C++20)	obtains the size of a std::ranges::subrange (class template specialization)
std::tuple_element<std::pair> (C++11)	obtains the type of the elements of pair (class template specialization)