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std::tuple_size<std::tuple>

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>`
Defined in header `<array>` Defined in header `<utility>`		(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.

In addition to being available via inclusion of the <tuple> header, the (3), (4) and (5) templates are available when either of the headers <array> or <utility> are included.

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:

std::tuple_size<std::array>	obtains the size of an `array` (class template specialization)
std::tuple_size<std::pair> (C++11)	obtains the size of a `pair` (class template specialization)
std::get(std::tuple)	tuple accesses specified element (function template)
tuple_element	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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