std::tuple_element
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(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)
struct tuple_element; // not defined
template< std::size_t I, class T >
(2)
(since C++11)
struct tuple_element< I, const T > {
using type = typename
std::add_const <typename std::tuple_element<I, T>::type>::type;
template< std::size_t I, class T >
(3)
(since C++11) struct tuple_element< I, volatile T > {
using type = typename
std::add_volatile <typename std::tuple_element<I, T>::type>::type;
(deprecated in C++20)
template< std::size_t I, class T >
(4)
(since C++11) struct tuple_element< I, const volatile T > {
using type = typename
std::add_cv <typename std::tuple_element<I, T>::type>::type;
(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.
Contents
[edit] Specializations
The standard library provides following specializations for standard library types:
obtains the type of the iterator or the sentinel of a std::ranges::subrange
(class template specialization) [edit]
(class template specialization) [edit]
(C++26)
(class template specialization) [edit]
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.
[edit] 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)
[edit] 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)
using tuple_element_t = typename tuple_element<I, T>::type;
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_tuple_element_t |
201402L |
(C++14) | std::tuple_element_t
|
[edit] 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
[edit] 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 |
[edit] See also
Structured binding (C++17)
binds the specified names to sub-objects or tuple elements of the initializer[edit]