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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
       compile-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 <tuple>
	#include <iostream>
	#include <string>

	namespace detail {

	template <std::size_t>
	struct index_tag { explicit constexpr 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				   er-
       ror)

See also
	  Structured  binding (C++17)		     binds the specified names
       to sub-objects
						    or tuple elements  of  the
       initializer
	  std::tuple_element<std::tuple>	     obtains  the  type	of the
       specified element
	  (C++11)				    (class  template  special-
       ization)
	  std::tuple_element<std::array>	     obtains  the  type	of the
       elements	of array
	  (C++11)				    (class  template  special-
       ization)
	  std::tuple_element<std::ranges::subrange>  obtains  the  type	of the
       iterator	or the
	  (C++20)					sentinel     of	     a
       std::ranges::subrange
						    (class  template  special-
       ization)
	  std::tuple_size<std::pair>		    obtains the	size of	a pair
	  (C++11)				    (class  template  special-
       ization)

http://cppreference.com		  2022.07.31		 std::tuple_element(3)

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