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.


[edit] Parameters

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

[edit] Return value

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

[edit] Complexity

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 in unordered associative containers; overloads (3,4)

[edit] Example

#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_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
        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()
    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';
        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
std::size_t string_hash::operator()(const std::string&) const
std::size_t string_hash::operator()(std::string_view) const

[edit] See also

returns the number of elements matching specific key
(public member function) [edit]
returns range of elements matching a specific key
(public member function) [edit]