# std::tuple_size<std::tuple>

From cppreference.com
< cpp‎ | utility‎ | 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>` |  | (since C++17)
Defined in header `<utility>` |  | (since C++17)
Defined in header `<ranges>` |  | (since C++20)
Defined in header `<span>` |  | (since C++20)
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. This allows

    #include <utility>
    struct X { int a, b; };
    const auto [x, y] = X(); // decomposition declaration first attempts tuple_size<const X>
                             // which attempts to use tuple_size<X>::value, and that's OK

(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)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/tuple/tuple_size&oldid=110114"