std::tuple_element<std::tuple>

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

make_from_tuple (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Elementary string conversions

to_chars (C++17)

from_chars (C++17)

chars_format (C++17)

Type operations

declval (C++11)

as_const (C++17)

 

std::tuple

Member functions

tuple::tuple

tuple::operator=

tuple::swap

Non-member functions

make_tuple

tie

forward_as_tuple

tuple_cat

operator==operator!=operator<operator<=operator>operator>=

std::swap

get

Helper classes

tuple_size

tuple_element

uses_allocator

ignore

 

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

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

See also

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