std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::find
iterator find( const Key& key ); |
(1) | |
const_iterator find( const Key& key ) const; |
(2) | |
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
Iterator to an element with key equivalent to key. 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 testing macro: __cpp_lib_generic_unordered_lookup (for overloads (3,4)).
Example
#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';
}
Output:
Found 2 b true true true
See also
|
(C++11)
|
returns the number of elements matching specific key (public member function) |
|
(C++11)
|
returns range of elements matching a specific key (public member function) |
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