# std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::find

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< cpp‎ | container‎ | unordered map

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`.

### 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
--- | --- | --- | ---
`__cpp_lib_generic_unordered_lookup` | `201811L` | (C++20) | Heterogeneous comparison lookup in unordered associative containers; overloads (3,4)

### Example

Run this code

    #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';
        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';
    }

Output:

    Found 2 b
    true
    true
    true

### See also

at | access specified element with bounds checking (public member function)
operator[] | access or insert specified element (public member function)
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)

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