NAME¶

std::tuple_size - std::tuple_size

Synopsis¶

Defined in header <utility>
template< class T1, class T2 >

struct tuple_size<std::pair<T1, T2>> (since C++11)

: std::integral_constant<std::size_t, 2> { };

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.

Member constants¶

value the constant value 2
[static] (public static member constant)

Member functions¶

operator std::size_t converts the object to std::size_t, returns value
(public member function)
operator() returns value
(C++14) (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 required
be derived from integral_constant

See also¶

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