std::tuple_size<std::pair>
From cppreference.com
| 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 |
(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
a |
| obtains the size of a std::ranges::subrange (class template specialization) | |
obtains the type of the elements of pair (class template specialization) |