| Defined in header <tuple> | ||
|---|---|---|
template< class T > class tuple_size; /*undefined*/ | (1) | (since C++11) |
template< class... Types >
class tuple_size< std::tuple<Types...> >
: public std::integral_constant<std::size_t, sizeof...(Types)> { }; | (2) | (since C++11) |
| Defined in header <tuple> | ||
(since C++17) (since C++17) | ||
template< class T >
class tuple_size<const T>
: public std::integral_constant<std::size_t, tuple_size<T>::value> { }; | (3) | (since C++11) |
template< class T >
class tuple_size< volatile T >
: public std::integral_constant<std::size_t, tuple_size<T>::value> { }; | (4) | (since C++11) |
template< class T >
class tuple_size< const volatile T >
: public std::integral_constant<std::size_t, tuple_size<T>::value> { }; | (5) | (since C++11) |
Provides access to the number of elements in a tuple as a compile-time constant expression.
Helper variable template
template< class T > constexpr std::size_t tuple_size_v = tuple_size<T>::value; | (since C++17) |
Inherited from std::integral_constant
Member constants
| value [static] | sizeof...(Types) (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> |
Notes
All specializations of std::tuple_size satisfy UnaryTypeTrait with BaseCharacteristic std::integral_constant<std::size_t, N> for some N.
Example
#include <iostream>
#include <tuple>
template <class T>
void test(T t)
{
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));
}Output:
3
See also
obtains the size of an array (class template specialization) | |
| (C++11) | obtains the size of a pair (class template specialization) |
| tuple accesses specified element (function template) | |
| obtains the type of the specified element (class template specialization) | |
| std::experimental::tuple_size_v
(library fundamentals TS) | variable template alias of std::tuple_size::value (variable template) |
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