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Showing content from https://cplusplus.com/unordered_multiset below:

class template

<unordered_set>

std::unordered_multiset

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

Unordered Multiset

Unordered multisets are containers that store elements in no particular order, allowing fast retrieval of individual elements based on their value, much like unordered_set containers, but allowing different elements to have equivalent values.

In an unordered_multiset, the value of an element is at the same time its key, used to identify it. Keys are immutable, therefore, the elements in an unordered_multiset cannot be modified once in the container - they can be inserted and removed, though.

Internally, the elements in the unordered_multiset are not sorted in any particular, 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).

Elements with equivalent values are grouped together in the same bucket and in such a way that an iterator (see equal_range) can iterate through all of them.

Iterators in the container are at least forward iterators.

Notice that this container is not defined in its own header, but shares header <unordered_set> with unordered_set.

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.
Multiple equivalent keys: The container can hold multiple elements with 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_multiset is also identified by this value.
Aliased as member types unordered_multiset::key_type and unordered_multiset::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_multiset 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_multiset::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_multiset object uses this expression to determine whether two element keys are equivalent. This container supports multiple elements with equivalent keys.
Aliased as member type unordered_multiset::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_multiset::allocator_type.

In the reference for the unordered_multiset 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_multiset. 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 a unordered_multiset 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_multiset (public member function)

(destructor): Destroy unordered multiset (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 type)

cend: Return const_iterator to end (public member type)

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 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_multiset): Relational operators for unordered_multiset (function template)

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

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