class template


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

Elements in associative containers are referenced by their key and not by their absolute position in the container.
Unordered containers organize their elements using hash tables that allow for fast access to elements by their key.
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.
The container uses an allocator object to dynamically handle its storage needs.

Template parameters

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.
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.
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.
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 typedefinitionnotes
key_typethe first template parameter (Key)
value_typethe first template parameter (Key)The same as key_type
hasherthe second template parameter (Hash)defaults to: hash<key_type>
key_equalthe third template parameter (Pred)defaults to: equal_to<key_type>
allocator_typethe fourth template parameter (Alloc)defaults to: allocator<value_type>
const_referenceconst value_type&
pointerallocator_traits<Alloc>::pointerfor the default allocator: value_type*
const_pointerallocator_traits<Alloc>::const_pointerfor the default allocator: const value_type*
iteratora forward iterator to const value_type* convertible to const_iterator
const_iteratora forward iterator to const value_type*
local_iteratora forward iterator to const value_type* convertible to const_local_iterator
const_local_iteratora forward iterator to const value_type*
size_typean unsigned integral typeusually the same as size_t
difference_typea signed integral typeusually 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



Element lookup



Hash policy


Non-member function overloads