caution

Note, this article is not finished! You can help by editing this doc page.

Overview

Simplified (since C++98)

template< class Key, /* ... */ >
class unorderd_multiset;

Detailed

Regular (since C++98)

template<
    class Key,
    class Hash = std::hash<Key>,
    class KeyEqual = std::equal_to<Key>,
    class Allocator = std::allocator<Key>
> 
class unordered_multiset;

Polymorphic (since C++17)

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>>
}

The std::unordered_multiset is an unordered associative container for storing unique objects.

Technical details

Technical definition of an unordered_multiset

std::unordered_multiset is an associative container that contains a set of 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 a hash is computed, it refers to the exact bucket the element is placed into.

Container elements may not be modified (even by non const iterators) since modification could change an element's hash and corrupt the container.

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

std::unordered_multiset

Defined in

unordered_set

Template parameters

pub	Key	The type of the stored keys.
pub	Hash	A function object that perfoms hashing on two objects of type Key.
pub	Compare	A comparator type satisfying Compare.
pub	Allocator	An allocator type responsible for allocating and deallocating memory. Must satisfy Allocator.

Type names

pub	key_type	Key
pub	value_type	Key
pub	size_type	Unsigned integer type (usually std::size_t).
pub	difference_type	Signed integer type (usually std::ptrdiff_t).
pub	hasher	Hash
pub	key_equal	KeyEqual
pub	allocator_type	Allocator
pub	reference	value_type&
pub	const_reference	const value_type&
pub	pointer	std::allocator_traits<Allocator>::pointer
pub	const_pointer	std::allocator_traits<Allocator>::const_pointer
pub	iterator ❗	Constant LegacyBidirectionalIterator to value_type
pub	const_iterator ❗	LegacyBidirectionalIterator to const value_type
pub	local_iterator	Like iterator but only to iterate across a single bucket.
pub	const_local_iterator	Like const_iterator but only to iterate across a single bucket.
pub	node_type (since C++17)	A specialization of node handle representing a container node.

Unordered multiset's iterators

iterator is said to be constant because it is impossible to modify the values it's pointing to. This is because modyfing a value the iterator is pointing to could possibly invalidate the internal structure of the unordered multiset, which could then lead to all kinds of incorrect and unexpected behaviours.

The member type aliases iterator and const_iterator may be the same. This is left entirely to the implementation. This means that any overloaded functions/specializations differing only in the iterator types may be incorrect and may lead to ODR violation.

Member functions

pub	(constructor)	Constructs a new unordered set.
pub	(destructor)	Destructs an unordered set.
pub	operator=	Assigns one unordered set to another.
pub	get_allocator	Returns an associated allocator.

Iterators

pub	begin cbegin	Returns an iterator to the beginning.
pub	end cend	Returns an iterator to the end.

Capacity

pub	empty	Returns true if a set is empty, otherwise false.
pub	size	Returns the number of elements in an unordered set.
pub	max_size	Returns the maximum possible number of elements.

Modifiers

pub	clear	Clears the contents of an unordered set.
pub	insert	Inserts elements (or nodes (extracted with .extract()) (since C++17)).
pub	emplace	Constructs a new element in place.
pub	emplace_hint	Constructs elements in-place using a hint (iterator).
pub	erase	Erases elements.
pub	swap	Swaps two unordered sets.
pub	extract (since C++17)	Extracts nodes from an unordered set (can be later inserted somewhere else).
pub	merge (since C++17)	Merges two unordered sets together.

Lookup

pub	count	Returns the number of elements matching a specific key.
pub	find	Searches for an element and returns an iterator to it, or an end iterator if not found.
pub	contains (since C++20)	Returns true if an element is inside a set, otherwise false.
pub	equal_range	Returns a range of elements matching a specific key.

Bucket interface

pub	begin (size_type) cbegin (size_type)	Returns an iterator to the beginning of a specified bucket.
pub	end (size_type) cend (size_type)	Returns an iterator to the end of a specified bucket.
pub	bucket_count	Returns a number of buckets.
pub	max_bucket_count	Returns a maximum number of buckets.
pub	bucket_size	Returns a number of elements in a specific bucket.
pub	bucket	Returns a bucket for a specific key.

Hash policy

pub	load_factor	Returns an average number of elements per bucket.
pub	max_load_factor	Manages a maximum average number of elements per bucket.
pub	rehash	Reserves at least the specified number of buckets and regenerates the hash table.
pub	reserve	Reserves space for at least a specified number of elements and regenerates the hash table.

Observers

pub	hash_function	Returns an internal function object that hashes keys.
pub	key_eq	Returns an internal function object that compares keys.

Non-member functions

pub	operator== operator!= (removed in C++20)	Compares values in an unordered_map.
pub	std::swap (std::unordered_multiset)	An overload for a std::swap algorithm.
pub	std::erase_if (std::unordered_multiset) (since C++20)	Overload for a std::erase_if algorithm.

Deduction guides (since C++17)

Click to expand

Deduction guides

// (1)
template< class InputIt, class Alloc >
unordered_multiset( InputIt, InputIt, Alloc )
  -> unordered_multiset<iter_key_t<InputIt>, iter_val_t<InputIt>,
            std::hash<iter_key_t<InputIt>>,
            std::equal_to<iter_key_t<InputIt>>, Alloc>;

// (2)
template< class InputIt, class Hash, class Alloc >
unordered_multiset( InputIt, InputIt, typename /*see below*/::size_type, Hash, Alloc )
  -> unordered_multiset<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash,
              std::equal_to<iter_key_t<InputIt>>, Alloc>;

// (3)
template< class InputIt, class Alloc >
unordered_multiset( InputIt, InputIt, typename /*see below*/::size_type, Alloc )
  -> unordered_multiset<iter_key_t<InputIt>, iter_val_t<InputIt>,
           std::hash<iter_key_t<InputIt>>,
           std::equal_to<iter_key_t<InputIt>>, Alloc>;

// (4)
template< class InputIt,
          class Hash = std::hash<iter_key_t<InputIt>>,
          class Pred = std::equal_to<iter_key_t<InputIt>>,
          class Alloc = std::allocator<iter_to_alloc_t<InputIt>>>
unordered_multiset( InputIt, InputIt,
         typename /*see below*/::size_type = /*see below*/,
         Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
  -> unordered_multiset<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash, Pred, Alloc>;

(1 - 4) allow deduction from an iterator range

// (5)
template< class Key, class T, typename Alloc >
unordered_multiset( std::initializer_list<std::pair<Key, T>>, Alloc )
  -> unordered_multiset<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;

// (6)
template< class Key, class T, class Hash, class Alloc >
unordered_multiset( std::initializer_list<std::pair<Key, T>>,
       typename /*see below*/::size_type, Hash, Alloc )
  -> unordered_multiset<Key, T, Hash, std::equal_to<Key>, Alloc>;

// (7)
template< class Key, class T, typename Alloc >
unordered_multiset( std::initializer_list<std::pair<Key, T>>,
     typename /*see below*/::size_type, Alloc )
  -> unordered_multiset<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;

// (8)
template< class Key, class T, class Hash = std::hash<Key>,
          class Pred = std::equal_to<Key>,
          class Alloc = std::allocator<std::pair<const Key, T>> >
unordered_multiset( std::initializer_list<std::pair<Key, T>>,
         typename /*see below*/::size_type = /*see below*/,
         Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
-> unordered_multiset<Key, T, Hash, Pred, Alloc>;

(5 - 8) allow deduction from a std::initializer_list

Aliases iter_key_t, iter_val_t and iter_to_alloc_t are defined as if follows:

template< class InputIt >
using iter_key_t = std::remove_const_t<
                     typename std::iterator_traits<InputIt>::value_type::first_type>;

template< class InputIt >
using iter_val_t = typename std::iterator_traits<InputIt>::value_type::second_type;

template< class InputIt >
using iter_to_alloc_t = std::pair<
  std::add_const_t<typename std::iterator_traits<InputIt>::value_type::first_type>,
  typename std::iterator_traits<InputIt>::value_type::second_type
>

note

Note that these aliases aren't guaranteed to be found anywhere in the standard library. They are defined solely for presentation purposes of these deduction guides and weren't present in the standard library at the point of writing of this document.

Overload resolution

In order for any of the deduction guides to participate in overload resolution, the folllowing requirements must be met:

• InputIt satisfies LegacyInputIterator
• Alloc satisfies Allocator
• Hash doesn't deduce to an integral type or doesn't satisfy Allocator
• Pred doesn't satisfy Allocator

note

The extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum:

• Integral types do not qualify as input iterators.

Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum:

• The member type Alloc::value_type must exist.
• The expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.