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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>{ | (2) | (since C++11) |
template<std::size_t I,class T> struct tuple_element< I,volatile T>{ | (3) | (since C++11) (deprecated in C++20) |
template<std::size_t I,class T> struct tuple_element< I,constvolatile T>{ | (4) | (since C++11) (deprecated in C++20) |
Provides compile-time indexed access to the types of the elements of atuple-like type.
| (since C++17) |
Contents |
The standard library provides following specializations for standard library types:
| obtains the type of the specified element (class template specialization)[edit] | |
obtains the type of the elements ofpair(class template specialization)[edit] | |
obtains the type of the elements ofarray(class template specialization)[edit] | |
| obtains the type of the iterator or the sentinel of astd::ranges::subrange (class template specialization)[edit] | |
| obtains the underlying real and imaginary number type of astd::complex (class template specialization)[edit] |
Users may specializestd::tuple_element for program-defined types to make them tuple-like.
In normal cases where theget functions returns reference members or reference to subobjects, only specializations for cv-unqualified types are needed to be customized.
| Member type | Definition |
| type | for a standard specialization, the type ofIth element of thetuple-like typeT, whereI is in[0, std::tuple_size<T>::value) |
Defined in header <tuple> | ||
template<std::size_t I,class T> using tuple_element_t=typename tuple_element<I, T>::type; | (since C++14) | |
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_tuple_element_t | 201402L | (C++14) | std::tuple_element_t |
#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
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 |
| Structured binding(C++17) | binds the specified names to sub-objects or tuple elements of the initializer[edit] |
(C++11) | obtains the number of elements of a tuple-like type (class template)[edit] |
(C++11) | creates atuple by concatenating any number of tuples(function template)[edit] |
