W3cubDocs

/C++

std::tuple_element<std::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.

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

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

#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

See also

obtains the type of the elements of array
(class template specialization)
(C++11)
 obtains the type of the elements of pair
(class template specialization)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
http://en.cppreference.com/w/cpp/utility/tuple/tuple_element