NAME¶

std::unordered_set::find - std::unordered_set::find

Synopsis¶

iterator find( const Key& key ); (1) (since C++11)
const_iterator find( const Key& key ) const; (2) (since C++11)
template< class K > (3) (since C++20)
iterator find( const K& x );
template< class K > (4) (since C++20)
const_iterator find( const K& x ) 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 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
Heterogeneous comparison lookup
__cpp_lib_generic_unordered_lookup 201811L (C++20) in unordered associative
containers; overloads (3,4)

Example¶

// Run this code

#include <cstddef>
#include <functional>
#include <iostream>
#include <source_location>
#include <string>
#include <string_view>
#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¶

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)