std::tuple_elementstd::tuple - cppreference.com (original) (raw)

Provides compile-time indexed access to the types of the elements of the tuple.

[edit] Member types

[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

#include <boost/type_index.hpp> #include #include #include #include #include   template<typename TupleLike, std::size_t I = 0> void printTypes() { if constexpr (I == 0) std::cout << boost::typeindex::type_id_with_cvr() << '\n';   if constexpr (I < std::tuple_size_v) { using SelectedType = std::tuple_element_t<I, TupleLike>;   std::cout << " The type at index " << I << " is: " << boost::typeindex::type_id_with_cvr() << '\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<std::tuple_element_t<1, MyPair>, bool&&>);   int main() { printTypes(); printTypes(); }

Possible output:

std::tuple<int, long&, char const&, bool&&, std::__cxx11::basic_string<char, std::char_traits, std::allocator >, 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, std::allocator > The type at index 5 is: MyStruct volatile std::pair<char, bool&&> The type at index 0 is: char The type at index 1 is: bool&&

[edit] See also