std:: tuple_size <std::pair>
|
Defined in header
<utility>
|
||
|
template
<
class
T1,
class
T2
>
struct
tuple_size
<
std::
pair
<
T1, T2
>>
|
(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
#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) |
|
|
(C++11)
|
obtains the type of the elements of
pair
(class template specialization) |