std:: tuple_element

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Defined in header <tuple>
Defined in header <array>
Defined in header <utility>
Defined in header <ranges>
(since C++20)
Defined in header <complex>
(since C++26)
template < std:: size_t I, class T >
struct tuple_element ; // not defined
(1) (since C++11)
template < std:: size_t I, class T >

struct tuple_element < I, const T > {
using type = typename
std:: add_const < typename std :: tuple_element < I, T > :: type > :: type ;

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

struct tuple_element < I, volatile T > {
using type = typename
std:: add_volatile < typename std :: tuple_element < I, T > :: type > :: type ;

} ;
(3) (since C++11)
(deprecated in C++20)
template < std:: size_t I, class T >

struct tuple_element < I, const volatile T > {
using type = typename
std:: add_cv < typename std :: tuple_element < I, T > :: type > :: type ;

} ;
(4) (since C++11)
(deprecated in C++20)

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

1) The primary template is not defined. An explicit (full) or partial specialization is required to make a type tuple-like.
2-4) Specializations for cv-qualified types simply add corresponding cv-qualifiers by default.

std::tuple_element interacts with the core language: it can provide structured binding support in the tuple-like case.

(since C++17)

Specializations

The standard library provides following specializations for standard library types:

(C++11)
obtains the type of the specified element
(class template specialization)
(C++11)
obtains the type of the elements of pair
(class template specialization)
(C++11)
obtains the type of the elements of array
(class template specialization)
(C++20)
obtains the type of the iterator or the sentinel of a std::ranges::subrange
(class template specialization)
(C++26)
obtains the underlying real and imaginary number type of a std::complex
(class template specialization)

Users may specialize std::tuple_element for program-defined types to make them tuple-like.

In normal cases where the get functions returns reference members or reference to subobjects, only specializations for cv-unqualified types are needed to be customized.

Member types

Member type Definition
type for a standard specialization, the type of I th element of the tuple-like type T , where I is in [ ​ 0 ​ , std:: tuple_size < T > :: value )

Helper types

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

Notes

Feature-test macro Value Std Feature
__cpp_lib_tuple_element_t 201402L (C++14) std::tuple_element_t

Example

Run this code 
#include <array>
#include <cstddef>
#include <iostream>
#include <ranges>
#include <tuple>
#include <type_traits>
#include <utility>
 
template<typename T1, typename T2, typename T3>
struct Triple
{
    T1 t1;
    T2 t2;
    T3 t3;
};
 
// A specialization of std::tuple_element for program-defined type Triple:
template<std::size_t I, typename T1, typename T2, typename T3>
    struct std::tuple_element<I, Triple<T1, T2, T3>>
    { static_assert(false, "Invalid index"); }; 
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<0, Triple<T1, T2, T3>> { using type = T1; };
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<1, Triple<T1, T2, T3>> { using type = T2; };
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<2, Triple<T1, T2, T3>> { using type = T3; };
 
 
template<typename... Args> struct TripleTypes
{
    static_assert(3 == sizeof...(Args), "Expected exactly 3 type names");
    template<std::size_t N>
    using type = typename std::tuple_element_t<N, Triple<Args...>>;
};
 
int main()
{
    TripleTypes<char, int, float>::type<1> i{42};
    std::cout << i << '\n';
 
    using Tri = Triple<int, char, short>; //< Program-defined type
    static_assert(std::is_same_v<std::tuple_element_t<0, Tri>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Tri>, char> &&
                  std::is_same_v<std::tuple_element_t<2, Tri>, short>);
 
    using Tuple = std::tuple<int, char, short>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Tuple>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Tuple>, char> &&
                  std::is_same_v<std::tuple_element_t<2, Tuple>, short>);
 
    using Array3 = std::array<int, 3>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Array3>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Array3>, int> &&
                  std::is_same_v<std::tuple_element_t<2, Array3>, int>);
 
    using Pair = std::pair<Tuple, Tri>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Pair>, Tuple> &&
                  std::is_same_v<std::tuple_element_t<1, Pair>, Tri>);
 
    using Sub = std::ranges::subrange<int*, int*>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Sub>, int*> &&
                  std::is_same_v<std::tuple_element_t<1, Sub>, int*>);
}

Output: 

42

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2212 C++11 specializations for cv types were not required in some headers, which led to ambiguity required

See also

Structured binding (C++17) binds the specified names to sub-objects or tuple elements of the initializer
(C++11)
obtains the number of elements of a tuple-like type
(class template)
(C++11)
creates a tuple by concatenating any number of tuples
(function template)
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