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std::tuple_element(3)	       C++ Standard Libary	   std::tuple_element(3)

NAME
     std::tuple_element - std::tuple_element

Synopsis
	Defined in header <utility>
	template< std::size_t I, class T1, class T2 >  (since C++11)
	struct tuple_element<I, std::pair<T1, T2>>;

	The partial specializations of std::tuple_element for pairs provide com-
     pile-time
	access to the types of the pair's elements, using tuple-like syntax. The
     program is
	ill-formed if I >= 2.

Member types
	Member type Definition
	type	    T1 if I == 0
		    T2 if I == 1

Possible implementation
	template<std::size_t I, typename T>
	struct tuple_element;

	template<std::size_t I, typename T1, typename T2>
	struct tuple_element<I, std::pair<T1, T2>>
	{
	    static_assert(I < 2, "std::pair has only 2 elements!");
	};

	template<typename T1, typename T2>
	struct tuple_element<0, std::pair<T1, T2>>
	{
	    using type = T1;
	};

	template<typename T1, typename T2>
	struct tuple_element<1, std::pair<T1, T2>>
	{
	    using type = T2;
	};

Example
     // Run this code

      #include <iostream>
      #include <string>
      #include <tuple>

      namespace detail
      {
	  template<std::size_t>
	  struct index_tag { constexpr explicit index_tag() = default; };

	  template<class T, class U>
	  constexpr   T   get_val_dispatch(std::pair<T,   U>  const&  pair,  in-
     dex_tag<0>)
	  {
	      return pair.first;
	  }

	  template<class T, class U>
	  constexpr  U	get_val_dispatch(std::pair<T,  U>   const&   pair,   in-
     dex_tag<1>)
	  {
	      return pair.second;
	  }
      } // namespace detail

      template<std::size_t N, class T, class U>
      auto constexpr get_val(std::pair<T, U> const& pair)
	  -> typename std::tuple_element<N, std::pair<T, U>>::type
      {
	  return detail::get_val_dispatch(pair, detail::index_tag<N>{});
      }

      int main()
      {
	  auto var = std::make_pair(1, std::string{"one"});

	  std::cout << get_val<0>(var) << " = " << get_val<1>(var);
      }

Output:
      1 = one

	Defect reports

	The  following	behavior-changing  defect  reports were applied retroac-
     tively to
	previously published C++ standards.

	   DR	 Applied to	      Behavior as published		 Correct
     behavior
	LWG 2974 C++11	    out-of-bounds index referred the undefined made ill-
     formed (hard
			    primary template			       error)

See also
	Structured binding (C++17)		  binds the specified  names  to
     sub-objects
						  or  tuple elements of the ini-
     tializer
	std::tuple_element<std::tuple>		  obtains the type of the speci-
     fied element
	(C++11) 				   (class  template  specializa-
     tion)
	std::tuple_element<std::array>		   obtains  the type of the ele-
     ments of array
	(C++11) 				   (class  template  specializa-
     tion)
	std::tuple_element<std::ranges::subrange> obtains the type of the itera-
     tor or the
	(C++20) 					sentinel       of      a
     std::ranges::subrange
						  (class  template   specializa-
     tion)
	std::tuple_size<std::pair>		  obtains the size of a pair
	(C++11) 				   (class  template  specializa-
     tion)

http://cppreference.com 	   2024.06.10		   std::tuple_element(3)
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