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| Unordered associative | ||||
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| Deduction guides(C++17) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
iterator find(const Key& key); | (1) | (since C++11) (constexpr since C++26) |
const_iterator find(const Key& key)const; | (2) | (since C++11) (constexpr since C++26) |
template<class K> iterator find(const K& x); | (3) | (since C++20) (constexpr since C++26) |
template<class K> const_iterator find(const K& x)const; | (4) | (since C++20) (constexpr since C++26) |
Hash andKeyEqual are bothtransparent. This assumes that suchHash is callable with bothK andKey type, and that theKeyEqual is transparent, which, together, allows calling this function without constructing an instance ofKey.Contents |
| key | - | key value of the element to search for |
| x | - | a value of any type that can be transparently compared with a key |
An iterator to the requested element. If no such element is found, past-the-end (seeend()) iterator is returned.
Constant on average, worst case linear in the size of the container.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_generic_unordered_lookup | 201811L | (C++20) | Heterogeneous comparison lookup inunordered associative containers; overloads(3,4) |
#include <cstddef>#include <functional>#include <iostream>#include <string>#include <string_view>#include <unordered_map> usingnamespace std::literals; struct string_hash{using hash_type=std::hash<std::string_view>;using is_transparent=void; std::size_t operator()(constchar* str)const{return hash_type{}(str);}std::size_t operator()(std::string_view str)const{return hash_type{}(str);}std::size_t operator()(conststd::string& str)const{return hash_type{}(str);}}; int main(){// simple comparison demostd::unordered_map<int,char> example{{1,'a'},{2,'b'}}; if(auto search= example.find(2); search!= example.end())std::cout<<"Found "<< search->first<<' '<< search->second<<'\n';elsestd::cout<<"Not found\n"; // C++20 demo: Heterogeneous lookup for unordered containers (transparent hashing)std::unordered_map<std::string, size_t, string_hash,std::equal_to<>> map{{"one"s,1}};std::cout<<std::boolalpha<<(map.find("one")!= map.end())<<'\n'<<(map.find("one"s)!= map.end())<<'\n'<<(map.find("one"sv)!= map.end())<<'\n';}
Output:
Found 2 btruetruetrue
| access specified element with bounds checking (public member function)[edit] | |
| access or insert specified element (public member function)[edit] | |
| returns the number of elements matching specific key (public member function)[edit] | |
| returns range of elements matching a specific key (public member function)[edit] |
