std::tuple_element<std::pair> (3) - Linux Manuals

std::tuple_element<std::pair>: std::tuple_element<std::pair>

NAME

std::tuple_element<std::pair> - std::tuple_element<std::pair>

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, 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 primary template made ill-formed (hard error)