std::unordered_set<Key,Hash,KeyEqual,Allocator>::find

 

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std::unordered_set

Member types
Member functions
unordered_set::unordered_set unordered_set::~unordered_set unordered_set::operator= unordered_set::get_allocator Iterators unordered_set::beginunordered_set::cbegin unordered_set::endunordered_set::cend Capacity unordered_set::size unordered_set::max_size unordered_set::empty Modifiers unordered_set::clear unordered_set::erase unordered_set::swap unordered_set::extract (C++17) unordered_set::merge (C++17) unordered_set::insert unordered_set::insert_range (C++23)   unordered_set::emplace unordered_set::emplace_hint	Lookup unordered_set::count unordered_set::find unordered_set::contains (C++20) unordered_set::equal_range Bucket interface unordered_set::begin(size_type)unordered_set::cbegin(size_type) unordered_set::end(size_type)unordered_set::cend(size_type) unordered_set::bucket_count unordered_set::max_bucket_count unordered_set::bucket_size unordered_set::bucket Hash policy unordered_set::load_factor unordered_set::max_load_factor unordered_set::rehash unordered_set::reserve Observers unordered_set::hash_function unordered_set::key_eq
Non-member functions
operator==operator!= (until C++20)	std::swap(std::unordered_set) erase_if(std::unordered_set) (C++20)
Deduction guides (C++17)

 

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 <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)