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# std::tuple_element<std::tuple>

From cppreference.com
< cpp‎ | utility‎ | tuple

Defined in header `<tuple>` |  |
template< std::size_t I, class T > class tuple_element; /* undefined */ | (1) | (since C++11)
template< std::size_t I, class... Types > class tuple_element< I, tuple<Types...> >; | (2) | (since C++11)
Defined in header `<tuple>` |  |
Defined in header `<array>` Defined in header `<utility>` |  | (since C++17) (since C++17)
template< std::size_t I, class T > class tuple_element< I, const T > { typedef typename std::add_const<typename std::tuple_element<I, T>::type>::type type; }; | (3) | (since C++11)
template< std::size_t I, class T > class tuple_element< I, volatile T > { typedef typename std::add_volatile<typename std::tuple_element<I, T>::type>::type type; }; | (4) | (since C++11)
template< std::size_t I, class T > class tuple_element< I, const volatile T > { typedef typename std::add_cv<typename std::tuple_element<I, T>::type>::type type; }; | (5) | (since C++11)

Provides compile-time indexed access to the types of the elements of the tuple.

### [edit] Member types

Member type | Definition
type | the type of `I`th element of the tuple, where `I` is in `[0, sizeof...(Types))`

### [edit] Helper types

template <std::size_t I, class T> using tuple_element_t = typename tuple_element<I, T>::type; |  | (since C++14)

### [edit] Possible implementation

    template< std::size_t I, class T >
    struct tuple_element;

    // recursive case
    template< std::size_t I, class Head, class... Tail >
    struct tuple_element<I, std::tuple<Head, Tail...>>
        : std::tuple_element<I-1, std::tuple<Tail...>> { };

    // base case
    template< class Head, class... Tail >
    struct tuple_element<0, std::tuple<Head, Tail...>> {
       typedef Head type;
    };

### [edit] Example

Run this code

    #include <iostream>
    #include <tuple>

    template <class... Args>
    struct type_list
    {
       template <std::size_t N>
       using type = typename std::tuple_element<N, std::tuple<Args...>>::type;
    };

    int main()
    {
       std::cout << std::boolalpha;
       type_list<int, char, bool>::type<2> x = true;
       std::cout << x << '\n';
    }

Output:

    true

### [edit] See also

std::tuple_element<std::array> | obtains the type of the elements of `array` (class template specialization) [edit]
std::tuple_element<std::pair> (C++11) | obtains the type of the elements of `pair` (class template specialization) [edit]

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