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Showing content from https://cplusplus.com/multimap below:

class template

<map>

std::multimap

template < class Key,                                     // multimap::key_type           class T,                                       // multimap::mapped_type           class Compare = less<Key>,                     // multimap::key_compare           class Alloc = allocator<pair<const Key,T> >    // multimap::allocator_type           > class multimap;

Multiple-key map

Multimaps are associative containers that store elements formed by a combination of a key value and a mapped value, following a specific order, and where multiple elements can have equivalent keys.

In a multimap, the key values are generally used to sort and uniquely identify the elements, while the mapped values store the content associated to this key. The types of key and mapped value may differ, and are grouped together in member type value_type, which is a pair type combining both:

typedef pair<const Key, T> value_type;

Internally, the elements in a multimap are always sorted by its key following a specific strict weak ordering criterion indicated by its internal comparison object (of type Compare).

multimap containers are generally slower than unordered_multimap containers to access individual elements by their key, but they allow the direct iteration on subsets based on their order.

Multimaps are typically implemented as binary search trees.

Container properties

Associative: Elements in associative containers are referenced by their key and not by their absolute position in the container.
Ordered: The elements in the container follow a strict order at all times. All inserted elements are given a position in this order.
Map: Each element associates a key to a mapped value: Keys are meant to identify the elements whose main content is the mapped value.
Multiple equivalent keys: Multiple elements in the container can have equivalent keys.
Allocator-aware: The container uses an allocator object to dynamically handle its storage needs.

Template parameters

Key: Type of the keys. Each element in a map is identified by its key value.
Aliased as member type multimap::key_type.
T: Type of the mapped value. Each element in a multimap stores some data as its mapped value.
Aliased as member type multimap::mapped_type.
Compare: A binary predicate that takes two element keys as arguments and returns a bool. The expression comp(a,b), where comp is an object of this type and a and b are element keys, shall return true if a is considered to go before b in the strict weak ordering the function defines.
The multimap object uses this expression to determine both the order the elements follow in the container and whether two element keys are equivalent (by comparing them reflexively: they are equivalent if !comp(a,b) && !comp(b,a)).
This can be a function pointer or a function object (see constructor for an example). This defaults to less<T>, which returns the same as applying the less-than operator (a<b).
Aliased as member type multimap::key_compare.
Alloc: Type of the allocator object used to define the storage allocation model. By default, the allocator class template is used, which defines the simplest memory allocation model and is value-independent.
Aliased as member type multimap::allocator_type.

Member types member type definition notes key_type The first template parameter (Key) mapped_type The second template parameter (T) value_type pair<const key_type,mapped_type> key_compare The third template parameter (Compare) defaults to: less<key_type> value_compare Nested function class to compare elements see value_comp allocator_type The fourth template parameter (Alloc) defaults to: allocator<value_type> reference allocator_type::reference for the default allocator: value_type& const_reference allocator_type::const_reference for the default allocator: const value_type& pointer allocator_type::pointer for the default allocator: value_type* const_pointer allocator_type::const_pointer for the default allocator: const value_type* iterator a bidirectional iterator to value_type convertible to const_iterator const_iterator a bidirectional iterator to const value_type reverse_iterator reverse_iterator<iterator> const_reverse_iterator reverse_iterator<const_iterator> difference_type a signed integral type, identical to: iterator_traits<iterator>::difference_type usually the same as ptrdiff_t size_type an unsigned integral type that can represent any non-negative value of difference_type usually the same as size_t member type definition notes key_type The first template parameter (Key) mapped_type The second template parameter (T) value_type pair<const key_type,mapped_type> key_compare The third template parameter (Compare) defaults to: less<key_type> value_compare Nested function class to compare elements see value_comp allocator_type The fourth template parameter (Alloc) defaults to: allocator<value_type> reference value_type& const_reference const value_type& pointer allocator_traits<allocator_type>::pointer for the default allocator: value_type* const_pointer allocator_traits<allocator_type>::const_pointer for the default allocator: const value_type* iterator a bidirectional iterator to value_type convertible to const_iterator const_iterator a bidirectional iterator to const value_type reverse_iterator reverse_iterator<iterator> const_reverse_iterator reverse_iterator<const_iterator> difference_type a signed integral type, identical to:
iterator_traits<iterator>::difference_type usually the same as ptrdiff_t size_type an unsigned integral type that can represent any non-negative value of difference_type usually the same as size_t
Member functions

(constructor): Construct multimap (public member function)

(destructor): Multimap destructor (public member function)

operator=: Copy container content (public member function)

Iterators:

begin: Return iterator to beginning (public member function)

end: Return iterator to end (public member function)

rbegin: Return reverse iterator to reverse beginning (public member function)

rend: Return reverse iterator to reverse end (public member function)

cbegin: Return const_iterator to beginning (public member function)

cend: Return const_iterator to end (public member function)

crbegin: Return const_reverse_iterator to reverse beginning (public member function)

crend: Return const_reverse_iterator to reverse end (public member function)

Capacity:

empty: Test whether container is empty (public member function)

size: Return container size (public member function)

max_size: Return maximum size (public member function)

Modifiers:

insert: Insert element (public member function)

erase: Erase elements (public member function)

swap: Swap content (public member function)

clear: Clear content (public member function)

emplace: Construct and insert element (public member function)

emplace_hint: Construct and insert element with hint (public member function)

Observers:

key_comp: Return key comparison object (public member function)

value_comp: Return value comparison object (public member function)

Operations:

find: Get iterator to element (public member function)

count: Count elements with a specific key (public member function)

lower_bound: Return iterator to lower bound (public member function)

upper_bound: Return iterator to upper bound (public member function)

equal_range: Get range of equal elements (public member function)

Allocator:

get_allocator: Get allocator (public member function)

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