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