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| Member functions | ||||
| Non-member functions | ||||
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) | ||||
| Helper concepts | ||||
(C++23) | ||||
| Helper classes | ||||
tuple_element<std::tuple> | ||||
(C++23) | ||||
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| Deduction guides(C++17) |
Defined in header <tuple> | ||
template<std::size_t I,class...Types> struct tuple_element< I,std::tuple<Types...>>; | (since C++11) | |
Provides compile-time indexed access to the types of the elements of the tuple.
Contents |
| Type | Definition |
| type | the type ofIth element of the tuple, whereI is in[0, sizeof...(Types)) |
template<std::size_t I,class T>struct tuple_element; #ifndef __cpp_pack_indexing// recursive casetemplate<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 casetemplate<class Head,class...Tail>struct tuple_element<0,std::tuple<Head, Tail...>>{using type= Head;}; #else// C++26 implementation using pack indexingtemplate<std::size_t I,class...Ts>struct tuple_element<I,std::tuple<Ts...>>{using type= Ts...[I];};#endif |
#include <boost/type_index.hpp>#include <cstddef>#include <iostream>#include <string>#include <tuple>#include <utility> template<typename TupleLike,std::size_t I=0>void printTypes(){ifconstexpr(I==0)std::cout<< boost::typeindex::type_id_with_cvr<TupleLike>()<<'\n'; ifconstexpr(I<std::tuple_size_v<TupleLike>){using SelectedType=std::tuple_element_t<I, TupleLike>; std::cout<<" The type at index "<< I<<" is: "<< boost::typeindex::type_id_with_cvr<SelectedType>()<<'\n'; printTypes<TupleLike, I+1>();}} struct MyStruct{}; using MyTuple=std::tuple<int,long&,constchar&,bool&&,std::string,volatile MyStruct>; using MyPair=std::pair<char,bool&&>; static_assert(std::is_same_v<std::tuple_element_t<0, MyPair>,char>);static_assert(std::is_same_v<std::tuple_element_t<1, MyPair>,bool&&>); int main(){ printTypes<MyTuple>(); printTypes<MyPair>();}
Possible output:
std::tuple<int, long&, char const&, bool&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, MyStruct volatile> The type at index 0 is: int The type at index 1 is: long& The type at index 2 is: char const& The type at index 3 is: bool&& The type at index 4 is: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > The type at index 5 is: MyStruct volatilestd::pair<char, bool&&> The type at index 0 is: char The type at index 1 is: bool&&
| Structured binding(C++17) | binds the specified names to sub-objects or tuple elements of the initializer[edit] |
(C++11) | obtains the element types of a tuple-like type (class template)[edit] |
