NAME¶

std::unordered_set - std::unordered_set

Synopsis¶

Defined in header <unordered_set>
template<

class Key,
class Hash = std::hash<Key>, (1) (since C++11)
class KeyEqual = std::equal_to<Key>,
class Allocator = std::allocator<Key>

> class unordered_set;
namespace pmr {

template<
class Key,
class Hash = std::hash<Key>,
class Pred = std::equal_to<Key> (2) (since C++17)
> using unordered_set = std::unordered_set<Key, Hash, Pred,

std::pmr::polymorphic_allocator<Key>>;

}

std::unordered_set is an associative container that contains a set of unique objects
of type Key. Search, insertion, and removal have average constant-time complexity.

Internally, the elements are not sorted in any particular order, but organized into
buckets. Which bucket an element is placed into depends entirely on the hash of its
value. This allows fast access to individual elements, since once a hash is
computed, it refers to the exact bucket the element is placed into.

Container elements may not be modified (even by non const iterators) since
modification could change an element's hash and corrupt the container.

std::unordered_set meets the requirements of Container, AllocatorAwareContainer,
UnorderedAssociativeContainer.

Notes¶

* The swap functions do not invalidate any of the iterators inside the container,
but they do invalidate the iterator marking the end of the swap region.
* References and pointers to data stored in the container are only invalidated by
erasing that element, even when the corresponding iterator is invalidated.
* After container move assignment, unless elementwise move assignment is forced by
incompatible allocators, references, pointers, and iterators (other than the
past-the-end iterator) to moved-from container remain valid, but refer to
elements that are now in *this.

Template parameters¶

This section is incomplete
Reason: Add descriptions of the template parameters.

Member types¶

Member type Definition
key_type Key
value_type Key
size_type Unsigned integer type (usually std::size_t)
difference_type Signed integer type (usually std::ptrdiff_t)
hasher Hash
key_equal KeyEqual
allocator_type Allocator
reference value_type&
const_reference const value_type&
pointer std::allocator_traits<Allocator>::pointer
const_pointer std::allocator_traits<Allocator>::const_pointer
iterator Constant LegacyForwardIterator to value_type
const_iterator LegacyForwardIterator to const value_type
An iterator type whose category, value, difference, pointer
and
local_iterator reference types are the same as iterator. This iterator
can be used to iterate through a single bucket but not
across buckets
An iterator type whose category, value, difference, pointer
and
const_local_iterator reference types are the same as const_iterator. This
iterator
can be used to iterate through a single bucket but not
across buckets
node_type (since C++17) a specialization of node handle representing a container
node
type describing the result of inserting a node_type, a
specialization of

template<class Iter, class NodeType>
struct /*unspecified*/
insert_return_type {
(since C++17) Iter position;
bool inserted;
NodeType node;
};
instantiated with template arguments iterator and
node_type.

Member functions¶

constructor constructs the unordered_set
(public member function)
destructor destructs the unordered_set
(public member function)
operator= assigns values to the container
(public member function)
get_allocator returns the associated allocator
(public member function)

Iterators¶

begin returns an iterator to the beginning
cbegin (public member function)
end returns an iterator to the end
cend (public member function)

Capacity¶

empty checks whether the container is empty
(public member function)
size returns the number of elements
(public member function)
max_size returns the maximum possible number of elements
(public member function)

Modifiers¶

clear clears the contents
(public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)
insert_range inserts a range of elements
(C++23) (public member function)
emplace constructs element in-place
(public member function)
emplace_hint constructs elements in-place using a hint
(public member function)
erase erases elements
(public member function)
swap swaps the contents
(public member function)
extract extracts nodes from the container
(C++17) (public member function)
merge splices nodes from another container
(C++17) (public member function)

Lookup¶

count returns the number of elements matching specific key
(public member function)
find finds element with specific key
(public member function)
contains checks if the container contains element with specific key
(C++20) (public member function)
equal_range returns range of elements matching a specific key
(public member function)

Bucket interface¶

begin(size_type) returns an iterator to the beginning of the specified bucket
cbegin(size_type) (public member function)
end(size_type) returns an iterator to the end of the specified bucket
cend(size_type) (public member function)
bucket_count returns the number of buckets
(public member function)
max_bucket_count returns the maximum number of buckets
(public member function)
bucket_size returns the number of elements in specific bucket
(public member function)
bucket returns the bucket for specific key
(public member function)

Hash policy¶

load_factor returns average number of elements per bucket
(public member function)
max_load_factor manages maximum average number of elements per bucket
(public member function)
reserves at least the specified number of buckets and regenerates
rehash the hash table
(public member function)
reserves space for at least the specified number of elements and
reserve regenerates the hash table
(public member function)

Observers¶

hash_function returns function used to hash the keys
(public member function)
key_eq returns the function used to compare keys for equality
(public member function)

Non-member functions¶

operator==
operator!= compares the values in the unordered_set
(C++11) (function template)
(C++11)(removed in C++20)
std::swap(std::unordered_set) specializes the std::swap algorithm
(C++11) (function template)
erase_if(std::unordered_set) erases all elements satisfying specific criteria
(C++20) (function template)

Deduction guides (since C++17)

Notes¶

The member types iterator and const_iterator may be aliases to the same type. This
means defining a pair of function overloads using the two types as parameter types
may violate the One Definition Rule. Since iterator is convertible to
const_iterator, a single function with a const_iterator as parameter type will work
instead.

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges construction and insertion for
containers

Example¶

// Run this code

#include <iostream>
#include <unordered_set>

void print(const auto& set)
{
for (const auto& elem : set)
std::cout << elem << ' ';
std::cout << '\n';
}

int main()
{
std::unordered_set<int> mySet{2, 7, 1, 8, 2, 8}; // creates a set of ints
print(mySet);

mySet.insert(5); // puts an element 5 in the set
print(mySet);

if (auto iter = mySet.find(5); iter != mySet.end())
mySet.erase(iter); // removes an element pointed to by iter
print(mySet);

mySet.erase(7); // removes an element 7
print(mySet);
}

Possible output:¶

8 1 7 2
5 8 1 7 2
8 1 7 2
8 1 2

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
the definitions of reference,
LWG 2050 C++11 const_reference, pointer based on value_type and
and const_pointer were based on std::allocator_traits
allocator_type

See also¶

set collection of unique keys, sorted by keys
(class template)

Category:¶

* Todo with reason