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std::unordered_set<Key,Hash,KeyEqual,Allocator>::find

iterator find( const Key& key );	(1)
const_iterator find( const Key& key ) const;	(2)
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

Iterator to an element with key equivalent to key. 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	Comment
`__cpp_lib_generic_unordered_lookup`	201811L	(C++20)	Heterogeneous comparison lookup in unordered associative containers; overloads (3,4)

Example

#include <cstddef>
#include <functional>
#include <iostream>
#include <string>
#include <string_view>
#include <source_location>
#include <unordered_set>
 
using namespace std::literals;
 
namespace logger { bool enabled{false}; }
 
inline void who(const std::source_location sloc = std::source_location::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 (transparent 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