| 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> | ||
(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) |
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