Std::tuple_element (3) Linux Manual Page

std::tuple_element<std::tuple> – std::tuple_element<std::tuple>

Synopsis

Defined in header<tuple>

    template <std::size_t I, class T>
    (1)(since C++ 11)

        class tuple_element; /* undefined */

template <std::size_t I, class... Types>
(2)(since C++ 11)

    class tuple_element<I, tuple<Types...>>;

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 <std::size_t I, class T>

                    class tuple_element<I, const T>
{

    typedef typename(3)(since C++ 11)

        std::add_const<typename std::tuple_element<I, T>::type>::type type;
};

template <std::size_t I, class T>

class tuple_element<I, volatile T>
{

    typedef typename(4)(since C++ 11)

        std::add_volatile<typename std::tuple_element<I, T>::type>::type type;
};

template <std::size_t I, class T>

class tuple_element<I, const volatile T>
{

    typedef typename(5)(since C++ 11)

        std::add_cv<typename std::tuple_element<I, T>::type>::type type;
};

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

Member types

Member type Definition
type the type of Ith element of the tuple, where I is in [0, sizeof…(Types))

Helper types

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

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;
  };

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

std::tuple_element (3) Linux Manual Page