std::unordered_map find() method

od C++20

// (1) Non const version
iterator find( const Key& key );

// (2) Const version
const_iterator find( const Key& key ) const;

// (3) Non const version
template< class K > iterator find( const K& x );

// (4) Const version
template< class K > const_iterator find( const K& x ) const;

od C++11

// (1) Non const version
iterator find( const Key& key );

// (2) Const version
const_iterator find( const Key& key ) 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 elements to count
• 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

Average case, constant - O(1).
Worst case, linear in size of the container - O(size()).

Exceptions

(none)

Notes

Feature testing macro: __cpp_lib_generic_unordered_lookup (for overload (3-4))

Example

Main.cpp

#include <cstddef>
#include <iostream>
#include <functional>
#include <string>
#include <string_view>
#include <unordered_map>
 
using namespace std::literals;
using std::size_t;
 
struct string_hash
{
    using hash_type = std::hash<std::string_view>;
    using is_transparent = void;
 
    size_t operator()(const char* str) const        { return hash_type{}(str); }
    size_t operator()(std::string_view str) const   { return hash_type{}(str); }
    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'}};
 
    auto search = example.find(2);
    if (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';
}

Possible output

Found 2 b
true
true
true