Difference between revisions of "cpp/container/unordered multiset"

Latest revision as of 15:45, 1 November 2024

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_multiset;	(1)	(since C++11)
namespace pmr { template< class Key, class Hash = std::hash<Key>, class Pred = std::equal_to<Key> > using unordered_multiset = std::unordered_multiset<Key, Hash, Pred, std::pmr::polymorphic_allocator<Key>>; }	(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 two std::unordered_multisets), except that every group of elements whose keys compare equivalent (compare equal with key_eq() as the comparator) forms a contiguous subrange in the iteration order, also accessible with equal_range().

std::unordered_multiset meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

Type	Definition
`key_type`	`Key`[edit]
`value_type`	`Key`[edit]
`size_type`	Unsigned integer type (usually std::size_t)[edit]
`difference_type`	Signed integer type (usually std::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`	Constant LegacyForwardIterator to `value_type`[edit]
`const_iterator`	LegacyForwardIterator to const value_type[edit]
`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[edit]
`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[edit]
`node_type` (since C++17)	a specialization of node handle representing a container node[edit]

[edit] Member functions

(constructor)	constructs the `unordered_multiset` (public member function) [edit]
(destructor)	destructs the `unordered_multiset` (public member function) [edit]
operator=	assigns values to the container (public member function) [edit]
get_allocator	returns the associated allocator (public member function) [edit]
Iterators
begincbegin	returns an iterator to the beginning (public member function) [edit]
endcend	returns an iterator to the end (public member function) [edit]
Capacity
empty	checks whether the container is empty (public member function) [edit]
size	returns the number of elements (public member function) [edit]
max_size	returns the maximum possible number of elements (public member function) [edit]
Modifiers
clear	clears the contents (public member function) [edit]
insert	inserts elements or nodes(since C++17) (public member function) [edit]
insert_range (C++23)	inserts a range of elements (public member function) [edit]
emplace	constructs element in-place (public member function) [edit]
emplace_hint	constructs elements in-place using a hint (public member function) [edit]
erase	erases elements (public member function) [edit]
swap	swaps the contents (public member function) [edit]
extract (C++17)	extracts nodes from the container (public member function) [edit]
merge (C++17)	splices nodes from another container (public member function) [edit]
Lookup
count	returns the number of elements matching specific key (public member function) [edit]
find	finds element with specific key (public member function) [edit]
contains (C++20)	checks if the container contains element with specific key (public member function) [edit]
equal_range	returns range of elements matching a specific key (public member function) [edit]
Bucket interface
begin(size_type)cbegin(size_type)	returns an iterator to the beginning of the specified bucket (public member function) [edit]
end(size_type)cend(size_type)	returns an iterator to the end of the specified bucket (public member function) [edit]
bucket_count	returns the number of buckets (public member function) [edit]
max_bucket_count	returns the maximum number of buckets (public member function) [edit]
bucket_size	returns the number of elements in specific bucket (public member function) [edit]
bucket	returns the bucket for specific key (public member function) [edit]
Hash policy
load_factor	returns average number of elements per bucket (public member function) [edit]
max_load_factor	manages maximum average number of elements per bucket (public member function) [edit]
rehash	reserves at least the specified number of buckets and regenerates the hash table (public member function) [edit]
reserve	reserves space for at least the specified number of elements and regenerates the hash table (public member function) [edit]
Observers
hash_function	returns function used to hash the keys (public member function) [edit]
key_eq	returns the function used to compare keys for equality (public member function) [edit]

[edit] Non-member functions

operator==operator!= (C++11)(C++11)(removed in C++20)	compares the values in the unordered_multiset (function template) [edit]
std::swap(std::unordered_multiset) (C++11)	specializes the std::swap algorithm (function template) [edit]
erase_if(std::unordered_multiset) (C++20)	erases all elements satisfying specific criteria (function template) [edit]

Deduction guides

(since C++17)

[edit] 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

[edit] Example

[edit] 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

[edit] See also

unordered_set (C++11)	collection of unique keys, hashed by keys (class template) [edit]
multiset	collection of keys, sorted by keys (class template) [edit]
flat_multiset (C++23)	adapts a container to provide a collection of keys, sorted by keys (class template) [edit]

@@ Line 1: / Line 1: @@
 {{cpp/title|unordered_multiset}}
-{{cpp/container/unordered_multiset/sidebar}}
+{{cpp/container/unordered_multiset/navbar}}
-{{ddcl | header=unordered_set | notes={{mark c++0x feature}} |
+{{dcl begin}}
+{{dcl header|unordered_set}}
+{{dcl|num=1|since=c++11|1=
 template<
      class Key,
-    class T,
+     class Hash = std::hash<Key>,
-     class Hash {{=}} std::hash<Key>,
+     class KeyEqual = std::equal_to<Key>,
-     class KeyEqual {{=}} std::equal_to<Key>,
+     class Allocator = std::allocator<Key>
-     class Allocator {{=}} std::allocator<Key>
 > class unordered_multiset;
 }}
+{{dcl|num=2|since=c++17|1=
+namespace pmr {
+    template<
+        class Key,
+        class Hash = std::hash<Key>,
+        class Pred = std::equal_to<Key>
+    > using unordered_multiset = std::unordered_multiset<Key, Hash, Pred,
+                                     std::pmr::polymorphic_allocator<Key>>;
+}
+}}
+{{dcl end}}
-Unordered multiset is an associative container that contains set of possibly non-unique objects of type Key.
+{{tt|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.
-{{tdcl list begin}}
+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.
-{{tdcl list h1 | Member types}}
-{{tdcl list hitem | Member type | Definition}}
-{{tdcl list template | cpp/container/tdcl list key_type | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list value_type | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list size_type | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list difference_type | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list hasher | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list key_equal | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list allocator_type | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list reference | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list const_reference | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list pointer | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list const_pointer | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list iterator | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list const_iterator | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list local_iterator | unordered_multiset}}
-{{tdcl list template | cpp/container/tdcl list const_local_iterator | unordered_multiset}}
-{{tdcl list end}}
+The iteration order of this container is not required to be stable (so, for example, {{lc|std::equal}} cannot be used to compare two {{tt|std::unordered_multiset}}s), except that every group of elements whose keys compare ''equivalent'' (compare equal with {{lc|key_eq()}} as the comparator) forms a contiguous subrange in the iteration order, also accessible with {{lc|equal_range()}}.
+{{tt|std::unordered_multiset}} meets the requirements of {{named req|Container}}, {{named req|AllocatorAwareContainer}}, {{named req|UnorderedAssociativeContainer}}.
-{{dcl list begin}}
+===Template parameters===
-{{dcl list h1 | Member functions}}
+{{todo|Add descriptions of the template parameters.}}
-{{dcl list template | cpp/container/dcl list constructor | unordered_multiset}}
-{{dcl list template | cpp/container/dcl list destructor | unordered_multiset}}
-{{dcl list template | cpp/container/dcl list operator{{=}} | unordered_multiset}}
-{{dcl list template | cpp/container/dcl list get_allocator | unordered_multiset}}
-{{dcl list h2 | Iterators}}
+===Member types===
-{{dcl list template | cpp/container/dcl list begin | unordered_multiset}}
+{{dsc begin}}
-{{dcl list template | cpp/container/dcl list end | unordered_multiset}}
+{{dsc hitem|Type|Definition}}
+{{dsc inc|cpp/container/dsc key_type|unordered_multiset}}
+{{dsc inc|cpp/container/dsc value_type|unordered_multiset}}
+{{dsc inc|cpp/container/dsc size_type|unordered_multiset}}
+{{dsc inc|cpp/container/dsc difference_type|unordered_multiset}}
+{{dsc inc|cpp/container/dsc hasher|unordered_multiset}}
+{{dsc inc|cpp/container/dsc key_equal|unordered_multiset}}
+{{dsc inc|cpp/container/dsc allocator_type|unordered_multiset}}
+{{dsc inc|cpp/container/dsc reference|unordered_multiset}}
+{{dsc inc|cpp/container/dsc const_reference|unordered_multiset}}
+{{dsc inc|cpp/container/dsc pointer|unordered_multiset}}
+{{dsc inc|cpp/container/dsc const_pointer|unordered_multiset}}
+{{dsc inc|cpp/container/dsc iterator|unordered_multiset}}
+{{dsc inc|cpp/container/dsc const_iterator|unordered_multiset}}
+{{dsc inc|cpp/container/dsc local_iterator|unordered_multiset}}
+{{dsc inc|cpp/container/dsc const_local_iterator|unordered_multiset}}
+{{dsc inc|cpp/container/dsc node_type|unordered_multiset}}
+{{dsc end}}
-{{dcl list h2 | Capacity}}
+===Member functions===
-{{dcl list template | cpp/container/dcl list empty | unordered_multiset}}
+{{dsc begin}}
-{{dcl list template | cpp/container/dcl list size | unordered_multiset}}
+{{dsc inc|cpp/container/dsc constructor|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list max_size | unordered_multiset}}
+{{dsc inc|cpp/container/dsc destructor|unordered_multiset}}
+{{dsc inc|cpp/container/dsc operator{{=}}|unordered_multiset}}
+{{dsc inc|cpp/container/dsc get_allocator|unordered_multiset}}
-{{dcl list h2 | Modifiers}}
+{{dsc h2|Iterators}}
-{{dcl list template | cpp/container/dcl list clear | unordered_multiset}}
+{{dsc inc|cpp/container/dsc begin|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list insert | unordered_multiset}}
+{{dsc inc|cpp/container/dsc end|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list emplace | unordered_multiset}}
-{{dcl list template | cpp/container/dcl list emplace_hint | unordered_multiset}}
-{{dcl list template | cpp/container/dcl list erase | unordered_multiset}}
-{{dcl list template | cpp/container/dcl list swap | unordered_multiset}}
-{{dcl list h2 | Lookup}}
+{{dsc h2|Capacity}}
-{{dcl list template | cpp/container/dcl list count | unordered_multiset}}
+{{dsc inc|cpp/container/dsc empty|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list find | unordered_multiset}}
+{{dsc inc|cpp/container/dsc size|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list equal_range | unordered_multiset}}
+{{dsc inc|cpp/container/dsc max_size|unordered_multiset}}
-{{dcl list h2 | Bucket interface}}
+{{dsc h2|Modifiers}}
-{{dcl list template | cpp/container/dcl list begin(int) | unordered_multiset}}
+{{dsc inc|cpp/container/dsc clear|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list end(int) | unordered_multiset}}
+{{dsc inc|cpp/container/dsc insert|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list bucket_count | unordered_multiset}}
+{{dsc inc|cpp/container/dsc insert_range|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list max_bucket_count | unordered_multiset}}
+{{dsc inc|cpp/container/dsc emplace|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list bucket_size | unordered_multiset}}
+{{dsc inc|cpp/container/dsc emplace_hint|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list bucket | unordered_multiset}}
+{{dsc inc|cpp/container/dsc erase|unordered_multiset}}
+{{dsc inc|cpp/container/dsc swap|unordered_multiset}}
+{{dsc inc|cpp/container/dsc extract|unordered_multiset}}
+{{dsc inc|cpp/container/dsc merge|unordered_multiset}}
-{{dcl list h2 | Hash policy}}
+{{dsc h2|Lookup}}
-{{dcl list template | cpp/container/dcl list load_factor | unordered_multiset}}
+{{dsc inc|cpp/container/dsc count|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list max_load_factor | unordered_multiset}}
+{{dsc inc|cpp/container/dsc find|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list rehash | unordered_multiset}}
+{{dsc inc|cpp/container/dsc contains|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list reserve | unordered_multiset}}
+{{dsc inc|cpp/container/dsc equal_range|unordered_multiset}}
-{{dcl list h2 | Observers}}
+{{dsc h2|Bucket interface}}
-{{dcl list template | cpp/container/dcl list hash_function | unordered_multiset}}
+{{dsc inc|cpp/container/dsc begin(int)|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list key_eq | unordered_multiset}}
+{{dsc inc|cpp/container/dsc end(int)|unordered_multiset}}
+{{dsc inc|cpp/container/dsc bucket_count|unordered_multiset}}
+{{dsc inc|cpp/container/dsc max_bucket_count|unordered_multiset}}
+{{dsc inc|cpp/container/dsc bucket_size|unordered_multiset}}
+{{dsc inc|cpp/container/dsc bucket|unordered_multiset}}
-{{dcl list h1 | Non-member functions}}
+{{dsc h2|Hash policy}}
-{{dcl list template | cpp/container/dcl list operator_comp_unord | unordered_multiset}}
+{{dsc inc|cpp/container/dsc load_factor|unordered_multiset}}
-{{dcl list template | cpp/container/dcl list std_swap | unordered_multiset}}
+{{dsc inc|cpp/container/dsc max_load_factor|unordered_multiset}}
-{{dcl list end}}
+{{dsc inc|cpp/container/dsc rehash|unordered_multiset}}
+{{dsc inc|cpp/container/dsc reserve|unordered_multiset}}
+{{dsc h2|Observers}}
+{{dsc inc|cpp/container/dsc hash_function|unordered_multiset}}
+{{dsc inc|cpp/container/dsc key_eq|unordered_multiset}}
+{{dsc end}}
+===Non-member functions===
+{{dsc begin}}
+{{dsc inc|cpp/container/dsc operator_cmp_unord|unordered_multiset}}
+{{dsc inc|cpp/container/dsc swap2|unordered_multiset}}
+{{dsc inc|cpp/container/dsc erase_if|unordered_multiset}}
+{{dsc end}}
+{{rrev|since=c++17|
+==={{rl|deduction guides|Deduction guides}}===
+}}
+===Notes===
+{{cpp/container/assoc note}}
+{{ftm begin|std=1|comment=1}}
+{{ftm|__cpp_lib_containers_ranges|value=202202L|std=C++23|Ranges construction and insertion for containers}}
+{{ftm end}}
+===Example===
+{{example}}
+===Defect reports===
+{{dr list begin}}
+{{dr list item|wg=lwg|dr=2050|std=C++11|before=the definitions of {{tt|reference}}, {{tt|const_reference}}, {{tt|pointer}}<br>and {{tt|const_pointer}} were based on {{tt|allocator_type}}|after=based on {{tt|value_type}} and<br>{{lc|std::allocator_traits}}}}
+{{dr list end}}
+===See also===
+{{dsc begin}}
+{{dsc inc|cpp/container/dsc unordered_set}}
+{{dsc inc|cpp/container/dsc multiset}}
+{{dsc inc|cpp/container/dsc flat_multiset}}
+{{dsc end}}
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Sequence
array (C++11)
vector
vector<bool>
inplace_vector (C++26)
deque
forward_list (C++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
Adaptors
stack
queue
priority_queue
flat_set (C++23)
flat_multiset (C++23)
flat_map (C++23)
flat_multimap (C++23)
Views
span (C++20)
mdspan (C++23)
Tables
Iterator invalidation
Member function table
Non-member function table

cppreference.com

Namespaces

Variants

Views

Actions

Difference between revisions of "cpp/container/unordered multiset"

Latest revision as of 15:45, 1 November 2024

Contents

[edit] Template parameters

[edit] Member types

[edit] Member functions

Iterators

Capacity

Modifiers

Lookup

Bucket interface

Hash policy

Observers