std:: tuple_element <std::pair>

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::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 <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 primary template made ill-formed (hard error)

See also

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