class template

<unordered_set>

std::unordered_set

template < class Key,                        // unordered_set::key_type/value_type           class Hash = hash<Key>,           // unordered_set::hasher           class Pred = equal_to<Key>,       // unordered_set::key_equal           class Alloc = allocator<Key>      // unordered_set::allocator_type           > class unordered_set;

Unordered Set

Unordered sets are containers that store unique elements in no particular order, and which allow for fast retrieval of individual elements based on their value.

In an

unordered_set, the value of an element is at the same time its key, that identifies it uniquely. Keys are immutable, therefore, the elements in an unordered_set cannot be modified once in the container - they can be inserted and removed, though.

Internally, the elements in the

unordered_set are not sorted in any particular order, but organized into buckets depending on their hash values to allow for fast access to individual elements directly by their values (with a constant average time complexity on average).unordered_set containers are faster than set containers to access individual elements by their key, although they are generally less efficient for range iteration through a subset of their elements.

Iterators in the container are at least forward iterators.

Container properties

Associative: Elements in associative containers are referenced by their key and not by their absolute position in the container.
Unordered: Unordered containers organize their elements using hash tables that allow for fast access to elements by their key.
Set: The value of an element is also the key used to identify it.
Unique keys: No two elements in the container can have equivalent keys.
Allocator-aware: The container uses an allocator object to dynamically handle its storage needs.

Template parameters

Key: Type of the elements. Each element in an unordered_set is also uniquely identified by this value.
Aliased as member types unordered_set::key_type and unordered_set::value_type.
Hash: A unary function object type that takes an object of the same type as the elements as argument and returns a unique value of type size_t based on it. This can either be a class implementing a function call operator or a pointer to a function (see constructor for an example). This defaults to hash<Key>, which returns a hash value with a probability of collision approaching 1.0/std::numeric_limits<size_t>::max().
The unordered_set object uses the hash values returned by this function to organize its elements internally, speeding up the process of locating individual elements.
Aliased as member type unordered_set::hasher.
Pred: A binary predicate that takes two arguments of the same type as the elements and returns a bool. The expression pred(a,b), where pred is an object of this type and a and b are key values, shall return true if a is to be considered equivalent to b. This can either be a class implementing a function call operator or a pointer to a function (see constructor for an example). This defaults to equal_to<Key>, which returns the same as applying the equal-to operator (a==b).
The unordered_set object uses this expression to determine whether two element keys are equivalent. No two elements in an unordered_set container can have keys that yield true using this predicate.
Aliased as member type unordered_set::key_equal.
Alloc: Type of the allocator object used to define the storage allocation model. By default, the allocator class template is used, which defines the simplest memory allocation model and is value-independent.
Aliased as member type unordered_set::allocator_type.

In the reference for the unordered_set member functions, these same names (Key, Hash, Pred and Alloc) are assumed for the template parameters.

Member types

The following aliases are member types of unordered_set. They are widely used as parameter and return types by member functions:

member type	definition	notes
`key_type`	the first template parameter (`Key`)
`value_type`	the first template parameter (`Key`)	The same as `key_type`
`hasher`	the second template parameter (`Hash`)	defaults to: `hash<key_type>`
`key_equal`	the third template parameter (`Pred`)	defaults to: `equal_to<key_type>`
`allocator_type`	the fourth template parameter (`Alloc`)	defaults to: `allocator<value_type>`
`reference`	`Alloc::reference`
`const_reference`	`Alloc::const_reference`
`pointer`	`Alloc::pointer`	for the default allocator: `value_type*`
`const_pointer`	`Alloc::const_pointer`	for the default allocator: `const value_type*`
`iterator`	a forward iterator to `const value_type`	* convertible to `const_iterator`
`const_iterator`	a forward iterator to `const value_type`	*
`local_iterator`	a forward iterator to `const value_type`	* convertible to `const_local_iterator`
`const_local_iterator`	a forward iterator to `const value_type`	*
`size_type`	an unsigned integral type	usually the same as size_t
`difference_type`	a signed integral type	usually the same as ptrdiff_t

member type	definition	notes
`key_type`	the first template parameter (`Key`)
`value_type`	the first template parameter (`Key`)	The same as `key_type`
`hasher`	the second template parameter (`Hash`)	defaults to: `hash<key_type>`
`key_equal`	the third template parameter (`Pred`)	defaults to: `equal_to<key_type>`
`allocator_type`	the fourth template parameter (`Alloc`)	defaults to: `allocator<value_type>`
`reference`	`value_type&`
`const_reference`	`const value_type&`
`pointer`	`allocator_traits<Alloc>::pointer`	for the default allocator: `value_type*`
`const_pointer`	`allocator_traits<Alloc>::const_pointer`	for the default allocator: `const value_type*`
`iterator`	a forward iterator to `const value_type`	* convertible to `const_iterator`
`const_iterator`	a forward iterator to `const value_type`	*
`local_iterator`	a forward iterator to `const value_type`	* convertible to `const_local_iterator`
`const_local_iterator`	a forward iterator to `const value_type`	*
`size_type`	an unsigned integral type	usually the same as size_t
`difference_type`	a signed integral type	usually the same as ptrdiff_t

*Note: All iterators in an unordered_set point to const elements. Whether the const_ member type is the same type as its non-const_ counterpart depends on the particular library implementation, but programs should not rely on them being different to overload functions: const_iterator is more generic, since iterator is always convertible to it.
The same applies to local_ and non-local_ iterator types: they may either be the same type or not, but a program should not rely on them being different.

Member functions

(constructor): Construct unordered_set (public member function)

(destructor): Destroy unordered set (public member function)

operator=: Assign content (public member function)

Capacity

empty: Test whether container is empty (public member function)

size: Return container size (public member function)

max_size: Return maximum size (public member function)

Iterators

begin: Return iterator to beginning (public member type)

end: Return iterator to end (public member type)

cbegin: Return const_iterator to beginning (public member function)

cend: Return const_iterator to end (public member function)

Element lookup

find: Get iterator to element (public member function)

count: Count elements with a specific key (public member function)

equal_range: Get range of elements with a specific key (public member function)

Modifiers

emplace: Construct and insert element (public member function)

emplace_hint: Construct and insert element with hint (public member function)

insert: Insert elements (public member function)

erase: Erase elements (public member function)

clear: Clear content (public member function)

swap: Swap content (public member function)

Buckets

bucket_count: Return number of buckets (public member function)

max_bucket_count: Return maximum number of buckets (public member function)

bucket_size: Return bucket size (public member type)

bucket: Locate element's bucket (public member function)

Hash policy

load_factor: Return load factor (public member function)

max_load_factor: Get or set maximum load factor (public member function)

rehash: Set number of buckets (public member function)

reserve: Request a capacity change (public member function)

Observers

hash_function: Get hash function (public member type)

key_eq: Get key equivalence predicate (public member type)

get_allocator: Get allocator (public member function)

Non-member function overloads

operators (unordered_set): Relational operators for unordered_set (function template)

swap (unordered_set): Exchanges contents of two unordered_set containers (function template)