(C++17) | ||||
| Sequence | ||||
(C++11) | ||||
(C++26) | ||||
(C++26) | ||||
(C++11) | ||||
| Associative | ||||
| Unordered associative | ||||
(C++11) | ||||
unordered_multiset (C++11) | ||||
(C++11) | ||||
(C++11) | ||||
| Adaptors | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
| Views | ||||
(C++20) | ||||
(C++23) | ||||
| Tables | ||||
| Iterator invalidation | ||||
| Member function table | ||||
| Non-member function table |
std::unordered_multiset| Member types | ||||||
| Member functions | ||||||
| Non-member functions | ||||||
| ||||||
| Deduction guides(C++17) | ||||||
Defined in header <unordered_set> | ||
template< class Key, | (1) | (since C++11) |
namespace pmr{ template< | (2) | (since C++17) |
std::unordered_multiset is an associative container that contains set of possibly non-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 hash is computed, it refers to the exact bucket the element is placed into.
The iteration order of this container is not required to be stable (so, for example,std::equal cannot be used to compare twostd::unordered_multisets), except that every group of elements whose keys compareequivalent (compare equal withkey_eq() as the comparator) forms a contiguous subrange in the iteration order, also accessible withequal_range().
std::unordered_multiset meets the requirements ofContainer,AllocatorAwareContainer,UnorderedAssociativeContainer.
All member functions ofstd::unordered_multiset areconstexpr: it is possible to create and usestd::unordered_multiset objects in the evaluation of a constant expression.However, | (since C++26) |
Contents |
| This section is incomplete Reason: Add descriptions of the template parameters. |
| Type | Definition |
key_type | Key[edit] |
value_type | Key[edit] |
size_type | Unsigned integer type (usuallystd::size_t)[edit] |
difference_type | Signed integer type (usuallystd::ptrdiff_t)[edit] |
hasher | Hash[edit] |
key_equal | KeyEqual[edit] |
allocator_type | Allocator[edit] |
reference | value_type&[edit] |
const_reference | const value_type&[edit] |
pointer | std::allocator_traits<Allocator>::pointer[edit] |
const_pointer | std::allocator_traits<Allocator>::const_pointer[edit] |
iterator | ConstantLegacyForwardIterator andConstexprIterator(since C++26) tovalue_type[edit] |
const_iterator | LegacyForwardIterator andConstexprIterator(since C++26) toconst value_type[edit] |
local_iterator | An iterator type whose category, value, difference, pointer and reference types are the same as iterator. This iteratorcan be used to iterate through a single bucket but not across buckets[edit] |
const_local_iterator | An iterator type whose category, value, difference, pointer and reference types are the same as const_iterator. This iteratorcan be used to iterate through a single bucket but not across buckets[edit] |
node_type(since C++17) | a specialization ofnode handle representing a container node[edit] |
constructs theunordered_multiset(public member function)[edit] | |
destructs theunordered_multiset(public member function)[edit] | |
| assigns values to the container (public member function)[edit] | |
| returns the associated allocator (public member function)[edit] | |
Iterators | |
| returns an iterator to the beginning (public member function)[edit] | |
| returns an iterator to the end (public member function)[edit] | |
Capacity | |
| checks whether the container is empty (public member function)[edit] | |
| returns the number of elements (public member function)[edit] | |
| returns the maximum possible number of elements (public member function)[edit] | |
Modifiers | |
| clears the contents (public member function)[edit] | |
| inserts elementsor nodes(since C++17) (public member function)[edit] | |
(C++23) | inserts a range of elements (public member function)[edit] |
| constructs element in-place (public member function)[edit] | |
| constructs elements in-place using a hint (public member function)[edit] | |
| erases elements (public member function)[edit] | |
| swaps the contents (public member function)[edit] | |
(C++17) | extracts nodes from the container (public member function)[edit] |
(C++17) | splices nodes from another container (public member function)[edit] |
Lookup | |
| returns the number of elements matching specific key (public member function)[edit] | |
| finds element with specific key (public member function)[edit] | |
(C++20) | checks if the container contains element with specific key (public member function)[edit] |
| returns range of elements matching a specific key (public member function)[edit] | |
Bucket interface | |
| returns an iterator to the beginning of the specified bucket (public member function)[edit] | |
| returns an iterator to the end of the specified bucket (public member function)[edit] | |
| returns the number of buckets (public member function)[edit] | |
| returns the maximum number of buckets (public member function)[edit] | |
| returns the number of elements in specific bucket (public member function)[edit] | |
| returns the bucket for specific key (public member function)[edit] | |
Hash policy | |
| returns average number of elements per bucket (public member function)[edit] | |
| manages maximum average number of elements per bucket (public member function)[edit] | |
| reserves at least the specified number of buckets and regenerates the hash table (public member function)[edit] | |
| reserves space for at least the specified number of elements and regenerates the hash table (public member function)[edit] | |
Observers | |
| returns function used to hash the keys (public member function)[edit] | |
| returns the function used to compare keys for equality (public member function)[edit] | |
(C++11)(C++11)(removed in C++20) | compares the values in the unordered_multiset (function template)[edit] |
| specializes thestd::swap algorithm (function template)[edit] | |
| erases all elements satisfying specific criteria (function template)[edit] |
Deduction guides | (since C++17) |
The member typesiterator andconst_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 theOne Definition Rule. Sinceiterator is convertible toconst_iterator, a single function with aconst_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 |
__cpp_lib_constexpr_unordered_set | 202502L | (C++26) | constexprstd::unordered_muliset |
| This section is incomplete Reason: no example |
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 ofreference,const_reference,pointerand const_pointer were based onallocator_type | based onvalue_type andstd::allocator_traits |
(C++11) | collection of unique keys, hashed by keys (class template)[edit] |
| collection of keys, sorted by keys (class template)[edit] | |
(C++23) | adapts a container to provide a collection of keys, sorted by keys (class template)[edit] |
