std::tuple_size<std::pair>
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tuple_size<std::pair>
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Defined in header
<utility>
template< class T1, class T2 >
(since C++11)
struct tuple_size<std::pair <T1, T2>>
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.
Contents
Inherited from std::integral_constant
Member constants
value
[static]
(public static member constant)
Member functions
operator()
(C++14)
(public member function)
Member types
Type
Definition
value_type
std::size_t
type
std::integral_constant <std::size_t, value>
[edit] 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
[edit] 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 |
[edit] See also
Structured binding (C++17)
binds the specified names to sub-objects or tuple elements of the initializer[edit]
(C++11)
a tuple
(class template specialization) [edit]