iterator find( const Key& key );
const_iterator find( const Key& key ) const;
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.


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.


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


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


#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';
        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';


Found 2 b

See also

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

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