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std::unordered_set::find(3)    C++ Standard Libary   std::unordered_set::find(3)

NAME
     std::unordered_set::find - std::unordered_set::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_transpar-
     ent 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 compar-
     ison lookup
	__cpp_lib_generic_unordered_lookup 201811L (C++20) in unordered associa-
     tive
							   containers; overloads
     (3,4)

Example
     // Run this code

      #include <cstddef>
      #include <functional>
      #include <iostream>
      #include <source_location>
      #include <string>
      #include <string_view>
      #include <unordered_set>

      using namespace std::literals;

      namespace logger { bool enabled{false}; }

      inline   void  who(const	std::source_location  sloc  =  std::source_loca-
     tion::current())
      {
	  if (logger::enabled)
	      std::cout << sloc.function_name() << '\n';
      }

      struct string_hash // C++20's transparent hashing
      {
	  using hash_type = std::hash<std::string_view>;
	  using is_transparent = void;

	  std::size_t operator()(const char* str) const
	  {
	      who();
	      return hash_type{}(str);
	  }
	  std::size_t operator()(std::string_view str) const
	  {
	      who();
	      return hash_type{}(str);
	  }
	  std::size_t operator()(std::string const& str) const
	  {
	      who();
	      return hash_type{}(str);
	  }
      };

      int main()
      {
	  std::unordered_set<int> example{1, 2, -10};

	  std::cout << "Simple comparison demo:\n" << std::boolalpha;
	  if (auto search = example.find(2); search != example.end())
	      std::cout << "Found " << *search << '\n';
	  else
	      std::cout << "Not found\n";

	  std::unordered_set<std::string,     string_hash,	std::equal_to<>>
     set{"one"s, "two"s};

	  logger::enabled = true;
	  std::cout << "Heterogeneous lookup for unordered containers (transpar-
     ent hashing):\n"
		    << (set.find("one")   != set.end()) << '\n'
		    << (set.find("one"s)  != set.end()) << '\n'
		    << (set.find("one"sv) != set.end()) << '\n';
      }

Possible output:
      Simple comparison demo:
      Found 2
      Heterogeneous lookup for unordered containers (transparent hashing):
      std::size_t string_hash::operator()(const char*) const
      true
      std::size_t string_hash::operator()(const std::string&) const
      true
      std::size_t string_hash::operator()(std::string_view) const
      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)

http://cppreference.com 	   2024.06.10	     std::unordered_set::find(3)
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