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 <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, index_tag<0>)
     {
         return pair.first;
     }
     template<class T, class U>
     constexpr U get_val_dispatch(std::pair<T, U> const& pair, index_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 retroactively 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 initializer
   std::tuple_element<std::tuple> obtains the type of the specified element
   (C++11) (class template specialization)
   std::tuple_element<std::array> obtains the type of the elements of array
   (C++11) (class template specialization)
   std::tuple_element<std::ranges::subrange> obtains the type of the iterator or the
   (C++20) sentinel of a std::ranges::subrange
                        (class template specialization)
   std::tuple_size<std::pair> obtains the size of a pair
   (C++11) (class template specialization)