std::tuple_size<std::tuple>

 

C++

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Technical Specifications

 

Utilities library

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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

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piecewise_construct (C++11)

integer_sequence (C++14)

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swap

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std::tuple

Member functions

tuple::tuple

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tuple::swap

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make_tuple

tie

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tuple_cat

operator==operator!=operator<operator<=operator>operator>=

std::swap

get

Helper classes

tuple_size

tuple_element

uses_allocator

ignore

 

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 templates (3-5) are available when either of the headers <array> or <utility> are included.

All specializations of std::tuple_size satisfy UnaryTypeTrait with BaseCharacteristic std::integral_constant<std::size_t, N> for some N.

The cv-T templates (3-5) are SFINAE-friendly: if std::tuple_size<T>::value is ill-formed when treated as an unevaluated operand, (3-5) do not provide the member value. Access checking is performed as if in a context unrelated to tuple_size and T. Only the validity of the immediate context of the expression is considered.

(since C++17)

Helper variable template

template< class T > inline 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>

Example

Run this code

#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

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)