std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::find

 

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std::unordered_map

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unordered_map::unordered_map unordered_map::~unordered_map unordered_map::operator= unordered_map::get_allocator Iterators unordered_map::beginunordered_map::cbegin unordered_map::endunordered_map::cend Capacity unordered_map::size unordered_map::max_size unordered_map::empty Modifiers unordered_map::clear unordered_map::erase unordered_map::swap unordered_map::extract (C++17) unordered_map::merge (C++17) unordered_map::insert unordered_map::insert_range (C++23) unordered_map::insert_or_assign (C++17)   unordered_map::emplace unordered_map::emplace_hint unordered_map::try_emplace (C++17)	Lookup unordered_map::at unordered_map::operator[] unordered_map::count unordered_map::find unordered_map::contains (C++20) unordered_map::equal_range Bucket interface unordered_map::begin(size_type)unordered_map::cbegin(size_type) unordered_map::end(size_type)unordered_map::cend(size_type) unordered_map::bucket_count unordered_map::max_bucket_count unordered_map::bucket_size unordered_map::bucket Hash policy unordered_map::load_factor unordered_map::max_load_factor unordered_map::rehash unordered_map::reserve Observers unordered_map::hash_function unordered_map::key_eq
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operator==operator!= (until C++20)	std::swap(std::unordered_map) erase_if(std::unordered_map) (C++20)
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iterator find(const Key& key );	(1)	(since C++11)
const_iterator find(const Key& key )const;	(2)	(since C++11)
template<class K > iterator find(const K& x );	(3)	(since C++20)
template<class K > const_iterator find(const K& x )const;	(4)	(since C++20)

1,2) Finds an element with key equivalent to key.

3,4) Finds an element with key that compares equivalent to the value x. This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.

Parameters

key	-	key value of the element to search for
x	-	a value of any type that can be transparently compared with a key

Return value

An iterator to the requested element. If no such element is found, past-the-end (see end()) iterator is returned.

Complexity

Constant on average, worst case linear in the size of the container.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_generic_unordered_lookup	201811L	(C++20)	Heterogeneous comparison lookup in unordered associative containers; overloads (3,4)

Example

Run this code

#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()(std::stringconst& 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 b true true true

See also

at	access specified element with bounds checking (public member function)
operator[]	access or insert specified element (public member function)
count	returns the number of elements matching specific key (public member function)
equal_range	returns range of elements matching a specific key (public member function)