std::unordered_map::find(3)

NAME
std::unordered_map::find - std::unordered_map::find

Synopsis
   iterator find( const Key& key ); (1) (since C++11)
   const_iterator find( const Key& key ) const; (2) (since C++11)
   template< class K > (3) (since C++20)
   iterator find( const K& x );
   template< class K > (4) (since C++20)
   const_iterator find( const K& x ) const;
   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
                        Heterogeneous comparison lookup
   __cpp_lib_generic_unordered_lookup 201811L (C++20) 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>
 using namespace std::literals;
 struct string_hash
 {
     using hash_type = std::hash<std::string_view>;
     using is_transparent = void;
     std::size_t operator()(const char* str) const { return hash_type{}(str); }
     std::size_t operator()(std::string_view str) const { return hash_type{}(str); }
     std::size_t operator()(std::string const& str) const { return hash_type{}(str); }
 };
 int main()
 {
     // simple comparison demo
     std::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';
     else
         std::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
   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)