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| Member functions | ||||
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| Non-member functions | ||||
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) | ||||
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| Helper classes | ||||
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tuple_element<std::pair> (C++11) | ||||
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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) | |
The partial specializations ofstd::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 ifI >= 2.
Contents |
| Member type | Definition |
type | T1 ifI == 0T2 ifI == 1 |
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;}; |
#include <iostream>#include <string>#include <tuple> namespace detail{template<std::size_t>struct index_tag{constexprexplicit 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>autoconstexpr get_val(std::pair<T, U>const& pair)->typenamestd::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
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) |
| Structured binding(C++17) | binds the specified names to sub-objects or tuple elements of the initializer[edit] |
| obtains the type of the specified element (class template specialization)[edit] | |
obtains the type of the elements ofarray(class template specialization)[edit] | |
| obtains the type of the iterator or the sentinel of astd::ranges::subrange (class template specialization)[edit] | |
(C++11) | obtains the size of apair(class template specialization)[edit] |
