# std::multimap

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Defined in header `<map>` |  |
template< class Key, class T, class Compare = std::less <Key>, class Allocator = std::allocator < std::pair <const Key, T>> > class multimap; | (1) |
namespace pmr { template< class Key, class T, class Compare = std::less <Key> > using multimap = std::multimap<Key, T, Compare, std::pmr::polymorphic_allocator < std::pair <const Key, T>>>; } | (2) | (since C++17)

`std::multimap` is an associative container that contains a sorted list of key-value pairs, while permitting multiple entries with the same key. Sorting is done according to the comparison function `Compare`, applied to the keys. Search, insertion, and removal operations have logarithmic complexity.

Iterators of `std::multimap` iterate in non-descending order of keys, where non-descending is defined by the comparison that was used for construction. That is, given

- m, a `std::multimap`
- it_l and it_r, dereferenceable iterators to m, with it_l < it_r.

m.value_comp()(*it_r, *it_l) == false (least to greatest if using the default comparison).

The order of the key-value pairs whose keys compare equivalent is the order of insertion and does not change. | (since C++11)

Everywhere the standard library uses the Compare requirements, equivalence is determined by using the equivalence relation as described on Compare. In imprecise terms, two objects a and b are considered equivalent if neither compares less than the other: !comp(a, b) && !comp(b, a).

`std::multimap` meets the requirements of Container, AllocatorAwareContainer, AssociativeContainer and ReversibleContainer.

### Template parameters

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

### Member types

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)
`key_compare` | `Compare`
`allocator_type` | `Allocator`
`reference` | value_type&
`const_reference` | const value_type&

`pointer`

`Allocator::pointer` | (until C++11)
std::allocator_traits <Allocator>::pointer | (since C++11)

`const_pointer`

`Allocator::const_pointer` | (until C++11)
std::allocator_traits <Allocator>::const_pointer | (since C++11)

`iterator` | LegacyBidirectionalIterator to `value_type`
`const_iterator` | LegacyBidirectionalIterator to const value_type
`reverse_iterator` | std::reverse_iterator <iterator>
`const_reverse_iterator` | std::reverse_iterator <const_iterator>
`node_type` (since C++17) | a specialization of node handle representing a container node

### Member classes

value_compare | compares objects of type `value_type` (class)

### Member functions

(constructor) | constructs the `multimap` (public member function)
(destructor) | destructs the `multimap` (public member function)
operator= | assigns values to the container (public member function)
get_allocator | returns the associated allocator (public member function)

##### Iterators

begincbegin (C++11) | returns an iterator to the beginning (public member function)
endcend (C++11) | returns an iterator to the end (public member function)
rbegincrbegin (C++11) | returns a reverse iterator to the beginning (public member function)
rendcrend (C++11) | returns a reverse iterator to the end (public member function)

##### Capacity

empty | checks whether the container is empty (public member function)
size | returns the number of elements (public member function)
max_size | returns the maximum possible number of elements (public member function)

##### Modifiers

clear | clears the contents (public member function)
insert | inserts elements or nodes(since C++17) (public member function)
insert_range (C++23) | inserts a range of elements (public member function)
emplace (C++11) | constructs element in-place (public member function)
emplace_hint (C++11) | constructs elements in-place using a hint (public member function)
erase | erases elements (public member function)
swap | swaps the contents (public member function)
extract (C++17) | extracts nodes from the container (public member function)
merge (C++17) | splices nodes from another container (public member function)

##### Lookup

count | returns the number of elements matching specific key (public member function)
find | finds element with specific key (public member function)
contains (C++20) | checks if the container contains element with specific key (public member function)
equal_range | returns range of elements matching a specific key (public member function)
lower_bound | returns an iterator to the first element not less than the given key (public member function)
upper_bound | returns an iterator to the first element greater than the given key (public member function)

##### Observers

key_comp | returns the function that compares keys (public member function)
value_comp | returns the function that compares keys in objects of type `value_type` (public member function)

### Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20) | lexicographically compares the values of two `multimap`s (function template)
std::swap(std::multimap) | specializes the std::swap algorithm (function template)
erase_if(std::multimap) (C++20) | erases all elements satisfying specific criteria (function template)

### Deduction guides

(since C++17)

### Notes

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

### Example

| This section is incomplete Reason: no example

### 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 230 | C++98 | `Key` was not required to be CopyConstructible (a key of type `Key` might not be able to be constructed) | `Key` is also required to be CopyConstructible

### See also

map | collection of key-value pairs, sorted by keys, keys are unique (class template)
unordered_multimap (C++11) | collection of key-value pairs, hashed by keys (class template)
flat_multimap (C++23) | adapts two containers to provide a collection of key-value pairs, sorted by keys (class template)

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