std::tuple_element<std::tuple>
From cppreference.com
Defined in header <tuple>
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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.
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[edit] Member types
Type | Definition |
type | the type of I th element of the tuple, where I is in [ 0, sizeof...(Types))
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[edit] Possible implementation
template<std::size_t I, class T> struct tuple_element; #ifndef __cpp_pack_indexing // recursive case template<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 case template<class Head, class... Tail> struct tuple_element<0, std::tuple<Head, Tail...>> { using type = Head; }; #else // C++26 implementation using pack indexing template<std::size_t I, class... Ts> struct tuple_element<I, std::tuple<Ts...>> { using type = Ts...[I]; }; #endif |
[edit] Example
Run this code
#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() { if constexpr (I == 0) std::cout << boost::typeindex::type_id_with_cvr<TupleLike>() << '\n'; if constexpr (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&, const char&, 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<