std::tuple_elementstd::pair - cppreference.com (original) (raw)

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.

[edit] Member types

[edit] 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; };

[edit] Example

#include #include #include   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{}); }   int main() { auto var = std::make_pair(1, std::string{"one"});   std::cout << get_val<0>(var) << " = " << get_val<1>(var); }

Output:

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also