C++

Reference

<map>

multimap

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:

1
 
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 typedefinitionnotes
key_typeThe first template parameter (Key)
mapped_typeThe second template parameter (T)
value_typepair<const key_type,mapped_type>
key_compareThe third template parameter (Compare)defaults to: less<key_type>
value_compareNested function class to compare elementssee value_comp
allocator_typeThe fourth template parameter (Alloc)defaults to: allocator<value_type>
referenceallocator_type::referencefor the default allocator: value_type&
const_referenceallocator_type::const_referencefor the default allocator: const value_type&
pointerallocator_type::pointerfor the default allocator: value_type*
const_pointerallocator_type::const_pointerfor the default allocator: const value_type*
iteratora bidirectional iterator to value_typeconvertible to const_iterator
const_iteratora bidirectional iterator to const value_type
reverse_iteratorreverse_iterator<iterator>
const_reverse_iteratorreverse_iterator<const_iterator>
difference_typea signed integral type, identical to: iterator_traits<iterator>::difference_typeusually the same as ptrdiff_t
size_typean unsigned integral type that can represent any non-negative value of difference_typeusually the same as size_t
member typedefinitionnotes
key_typeThe first template parameter (Key)
mapped_typeThe second template parameter (T)
value_typepair<const key_type,mapped_type>
key_compareThe third template parameter (Compare)defaults to: less<key_type>
value_compareNested function class to compare elementssee value_comp
allocator_typeThe fourth template parameter (Alloc)defaults to: allocator<value_type>
referencevalue_type&
const_referenceconst value_type&
pointerallocator_traits<allocator_type>::pointerfor the default allocator: value_type*
const_pointerallocator_traits<allocator_type>::const_pointerfor the default allocator: const value_type*
iteratora bidirectional iterator to value_typeconvertible to const_iterator
const_iteratora bidirectional iterator to const value_type
reverse_iteratorreverse_iterator<iterator>
const_reverse_iteratorreverse_iterator<const_iterator>
difference_typea signed integral type, identical to:
iterator_traits<iterator>::difference_type		usually the same as ptrdiff_t
size_typean unsigned integral type that can represent any non-negative value of difference_typeusually 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)