std::unordered_set

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C++
 
Containers library
 
std::unordered_set
Member types
Member functions
Non-member functions
(until C++20)
Deduction guides (C++17)
 
Defined in header <unordered_set>
template<

    class Key,
    class Hash = std::hash<Key>,
    class KeyEqual = std::equal_to<Key>,
    class Allocator = std::allocator<Key>

> class unordered_set;
 (1) (since C++11)
namespace pmr {

    template<
        class Key,
        class Hash = std::hash<Key>,
        class Pred = std::equal_to<Key>
    > using unordered_set = std::unordered_set<Key, Hash, Pred,
                                std::pmr::polymorphic_allocator<Key>>;

}
 (2) (since C++17)

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.

Iterator invalidation

Operations Invalidated
All read only operations, swap, std::swap Never
clear, rehash, reserve, operator= Always
insert, emplace, emplace_hint Only if causes rehash
erase Only to the element erased

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

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
local_iterator An iterator type whose category, value, difference, pointer and
reference types are the same as iterator. This iterator
can be used to iterate through a single bucket but not across buckets
const_local_iterator An iterator type whose category, value, difference, pointer and
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
insert_return_type (since C++17) type describing the result of inserting a node_type, a specialization of

template<class Iter, class NodeType>
struct /*unspecified*/
{
    Iter     position;
    bool     inserted;
    NodeType node;
};
instantiated with template arguments iterator and node_type.

Member functions

 constructs the unordered_set
(public member function)
destructs the unordered_set
(public member function)
assigns values to the container
(public member function)
returns the associated allocator
(public member function)
Iterators
returns an iterator to the beginning
(public member function)
returns an iterator to the end
(public member function)
Capacity
checks whether the container is empty
(public member function)
returns the number of elements
(public member function)
returns the maximum possible number of elements
(public member function)
Modifiers
clears the contents
(public member function)
inserts elements or nodes(since C++17)
(public member function)
(C++23)
 inserts a range of elements
(public member function)
constructs element in-place
(public member function)
constructs elements in-place using a hint
(public member function)
erases elements
(public member function)
swaps the contents
(public member function)
(C++17)
 extracts nodes from the container
(public member function)
(C++17)
 splices nodes from another container
(public member function)
Lookup
returns the number of elements matching specific key
(public member function)
finds element with specific key
(public member function)
(C++20)
 checks if the container contains element with specific key
(public member function)
returns range of elements matching a specific key
(public member function)
Bucket interface
returns an iterator to the beginning of the specified bucket
(public member function)
returns an iterator to the end of the specified bucket
(public member function)
returns the number of buckets
(public member function)
returns the maximum number of buckets
(public member function)
returns the number of elements in specific bucket
(public member function)
returns the bucket for specific key
(public member function)
Hash policy
returns average number of elements per bucket
(public member function)
manages maximum average number of elements per bucket
(public member function)
reserves at least the specified number of buckets and regenerates the hash table
(public member function)
reserves space for at least the specified number of elements and regenerates the hash table
(public member function)
Observers
returns function used to hash the keys
(public member function)
returns the function used to compare keys for equality
(public member function)

Non-member functions

(C++11)(C++11)(removed in C++20)
 compares the values in the unordered_set
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++20)
 erases all elements satisfying specific criteria
(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
LWG 2050 C++11 the definitions of reference, const_reference, pointer
and const_pointer were based on allocator_type 		based on value_type and
std::allocator_traits

See also

(C++11)
 collection of keys, hashed by keys
(class template)
collection of unique keys, sorted by keys
(class template)
(C++23)
 adapts a container to provide a collection of unique keys, sorted by keys
(class template)
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