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std::unordered_multimap

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C++
 
Containers library
 
std::unordered_multimap
Member types
Member functions
Non-member functions
(until 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.

All member functions of std::unordered_multimap are constexpr: it is possible to create and use std::unordered_multimap objects in the evaluation of a constant expression.

However, std::unordered_multimap objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.

 (since C++26)

Contents

[edit] Template parameters

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

[edit] Member types

Type Definition
key_type Key[edit]
mapped_type T[edit]
value_type std::pair<const Key, T>[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 LegacyForwardIterator and ConstexprIterator(since C++26) to value_type[edit]
const_iterator LegacyForwardIterator and ConstexprIterator(since C++26) 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

 constructs the unordered_multimap
(public member function) [edit]
destructs the unordered_multimap
(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]