std::tuple_element<std::pair>
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tuple_element<std::pair>
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Deduction guides (C++17)
Defined in header
<utility>
template< std::size_t I, class T1, class T2 >
struct tuple_element<I, std::pair <T1, 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.
[edit] Member types
Member type
Definition
type
T1 if I == 0T2 if I == 1
[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
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
[edit] 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) |
[edit] See also
Structured binding (C++17)
binds the specified names to sub-objects or tuple elements of the initializer[edit]
obtains the type of the iterator or the sentinel of a std::ranges::subrange
(class template specialization) [edit]
(class template specialization) [edit]