std::unordered_multimap

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Containers library
 
std::unordered_multimap
Member types
Member functions
Iterators
Capacity
Modifiers
(C++23)
(C++17)
(C++17)
Lookup
(C++20)
Bucket interface
Hash policy
Observers
Non-member functions
(until C++20)
(C++20)
Deduction guides(C++17)
 
Defined in header <unordered_map>
template<

    class Key,
    class T,
    class Hash = std::hash<Key>,
    class KeyEqual = std::equal_to<Key>,
    class Allocator = std::allocator< std::pair<const Key, T> >

> class unordered_multimap;
 (1) (since C++11)
namespace pmr {

template<
    class Key, class T,
    class Hash = std::hash<Key>,
    class Pred = std::equal_to<Key>
> using unordered_multimap = std::unordered_multimap<Key, T, Hash, Pred,
                                 std::pmr::polymorphic_allocator<std::pair<const Key,T>>>;

}
 (2) (since C++17)

std::unordered_multimap is an unordered associative container that supports equivalent keys (an unordered_multimap may contain multiple copies of each key value) and that associates values of another type with the keys. The unordered_multimap class supports forward iterators. 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 key. This allows fast access to individual elements, since once the 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_multimaps), 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_multimap meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

Template parameters

This section is incomplete
Reason: Add descriptions of the template parameters.

Member types

Member type Definition
key_type Key
mapped_type T
value_type std::pair<const Key, T>
size_type Unsigned integer type (usually std::size_t)
difference_type Signed integer type (usually std::ptrdiff_t)
hasher Hash
key_equal KeyEqual
allocator_type Allocator
reference value_type&
const_reference const value_type&
pointer std::allocator_traits<Allocator>::pointer
const_pointer std::allocator_traits<Allocator>::const_pointer
iterator LegacyForwardIterator to value_type
const_iterator LegacyForwardIterator to const value_type
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
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
node_type (since C++17) a specialization of node handle representing a container node

Member functions

(C++11)
 constructs the unordered_multimap
(public member function)
(C++11)
 destructs the unordered_multimap
(public member function)
(C++11)
 assigns values to the container
(public member function)
(C++11)
 returns the associated allocator
(public member function)
Iterators
(C++11)
 returns an iterator to the beginning
(public member function)
(C++11)
 returns an iterator to the end
(public member function)
Capacity
(C++11)
 checks whether the container is empty
(public member function)
(C++11)
 returns the number of elements
(public member function)
(C++11)
 returns the maximum possible number of elements
(public member function)
Modifiers
(C++11)
 clears the contents
(public member function)
(C++11)
 inserts elements or nodes (since C++17)
(public member function)
(C++23)
 inserts a range of elements
(public member function)
(C++11)
 constructs element in-place
(public member function)
(C++11)
 constructs elements in-place using a hint
(public member function)
(C++11)
 erases elements
(public member function)
(C++11)
 swaps the contents
(public member function)
(C++17)
 extracts nodes from the container
(public member function)
(C++17)
 splices nodes from another container
(public member function)
Lookup
(C++11)
 returns the number of elements matching specific key
(public member function)
(C++11)
 finds element with specific key
(public member function)
(C++20)
 checks if the container contains element with specific key
(public member function)
(C++11)
 returns range of elements matching a specific key
(public member function)
Bucket interface
(C++11)
 returns an iterator to the beginning of the specified bucket
(public member function)
(C++11)
 returns an iterator to the end of the specified bucket
(public member function)
(C++11)
 returns the number of buckets
(public member function)
(C++11)
 returns the maximum number of buckets
(public member function)
(C++11)
 returns the number of elements in specific bucket
(public member function)
(C++11)
 returns the bucket for specific key
(public member function)
Hash policy
(C++11)
 returns average number of elements per bucket
(public member function)
(C++11)
 manages maximum average number of elements per bucket
(public member function)
(C++11)
 reserves at least the specified number of buckets and regenerates the hash table
(public member function)
(C++11)
 reserves space for at least the specified number of elements and regenerates the hash table
(public member function)
Observers
(C++11)
 returns function used to hash the keys
(public member function)
(C++11)
 returns the function used to compare keys for equality
(public member function)

Non-member functions

(removed in C++20)
 compares the values in the unordered_multimap
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++20)
 erases all elements satisfying specific criteria
(function template)

Deduction guides

 (since C++17)

Notes

Feature-test macro Value Std Comment
__cpp_lib_containers_ranges 202202L (C++23) Ranges construction and insertion for containers

Example

This section is incomplete
Reason: no example
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