std::tuple_element

 

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation(C++20) bitset hash (C++11) Relational operators(deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

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, constvolatile 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:

std::tuple_element<std::tuple> (C++11)	obtains the type of the specified element (class template specialization)
std::tuple_element<std::pair> (C++11)	obtains the type of the elements of pair (class template specialization)
std::tuple_element<std::array> (C++11)	obtains the type of the elements of array (class template specialization)
std::tuple_element<std::ranges::subrange> (C++20)	obtains the type of the iterator or the sentinel of a std::ranges::subrange (class template specialization)
std::tuple_element<std::complex> (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 Ith 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
tuple_size (C++11)	obtains the number of elements of a tuple-like type (class template)
tuple_cat (C++11)	creates a tuple by concatenating any number of tuples (function template)