std::tuple_size<std::pair>
Min standard notice:
Header: <utility>
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.
# Declarations
template< class T1, class T2 >
struct tuple_size<std::pair<T1, T2>>
: std::integral_constant<std::size_t, 2> { };
(since C++11)
# 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));
}
# Defect reports
| 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 |