std::unordered_set<Key,Hash,KeyEqual,Allocator>::find
From cppreference.com
< cpp | container | unordered set
| 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-test macro | Value | Std | Comment |
|---|---|---|---|
__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 <source_location> #include <unordered_set> using namespace std::literals; namespace logger { bool enabled{false}; } inline void who(const std::source_location sloc = std::source_location::current()) { if (logger::enabled) std::cout << sloc.function_name() << '\n'; } struct string_hash // C++20's transparent hashing { using hash_type = std::hash<std::string_view>; using is_transparent = void; std::size_t operator()(const char* str) const { who(); return hash_type{}(str); } std::size_t operator()(std::string_view str) const { who(); return hash_type{}(str); } std::size_t operator()(std::string const& str) const { who(); return hash_type{}(str); } }; int main() { std::unordered_set<int> example = { 1, 2, -10 }; std::cout << "Simple comparison demo:\n" << std::boolalpha; if (auto search = example.find(2); search != example.end()) std::cout << "Found " << *search << '\n'; else std::cout << "Not found\n"; std::unordered_set<std::string, string_hash, std::equal_to<>> set{"one"s, "two"s}; logger::enabled = true; std::cout << "Heterogeneous lookup for unordered containers (transparent hashing):\n" << (set.find("one") != set.end()) << '\n' << (set.find("one"s) != set.end()) << '\n' << (set.find("one"sv) != set.end()) << '\n'; }
Possible output:
Simple comparison demo: Found 2 Heterogeneous lookup for unordered containers (transparent hashing): std::size_t string_hash::operator()(const char*) const true std::size_t string_hash::operator()(const std::string&) const true std::size_t string_hash::operator()(std::string_view) const 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) |