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

Since C++11

1,2) Finds an element with key equivalent to key.

# Declarations

iterator find( const Key& key );

(since C++11)

const_iterator find( const Key& key ) const;

(since C++11)

template< class K >
iterator find( const K& x );

(since C++20)

template< class K >
const_iterator find( const K& x ) const;

(since C++20)

# Parameters

# Return value

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

# 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

#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';
}

# See also