# std::tuple_element<std::pair>

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< cpp‎ | utility‎ | pair

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

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)

### See also

tuple_element | obtains the type of the specified element (class template specialization)
std::tuple_element<std::array> | obtains the type of the elements of `array` (class template specialization)
std::tuple_size<std::pair> (C++11) | obtains the size of a `pair` (class template specialization)

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