NAME¶

std::multimap - std::multimap

Synopsis¶

Defined in header <map>
template<

class Key,
class T, (1)
class Compare = std::less<Key>,
class Allocator = std::allocator<std::pair<const Key, T>>

> class multimap;
namespace pmr {

template<
class Key,
class T, (2) (since C++17)
class Compare = std::less<Key>
> using multimap = std::multimap<Key, T, Compare,
std::pmr::polymorphic_allocator<std::pair<const Key, T>>>;

}

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 (since C++11)
order of insertion and does not change.

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¶

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)
key_compare Compare
allocator_type Allocator
reference value_type&
const_reference const value_type&
Allocator::pointer (until C++11)
pointer std::allocator_traits<Allocator>::pointer (since C++11)

Allocator::const_pointer (until
C++11)
const_pointer 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¶

begin returns an iterator to the beginning
cbegin (public member function)
(C++11)
end returns an iterator to the end
cend (public member function)
(C++11)
rbegin returns a reverse iterator to the beginning
crbegin (public member function)
(C++11)
rend returns a reverse iterator to the end
crend (public member function)
(C++11)

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)
inserts elements
insert or nodes
(since C++17)
(public member function)
insert_range inserts a range of elements
(C++23) (public member function)
emplace constructs element in-place
(C++11) (public member function)
emplace_hint constructs elements in-place using a hint
(C++11) (public member function)
erase erases elements
(public member function)
swap swaps the contents
(public member function)
extract extracts nodes from the container
(C++17) (public member function)
merge splices nodes from another container
(C++17) (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 checks if the container contains element with specific key
(C++20) (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>= lexicographically compares the values of two multimaps
operator<=> (function template)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(C++20)
std::swap(std::multimap) specializes the std::swap algorithm
(function template)
erase_if(std::multimap) erases all elements satisfying specific criteria
(C++20) (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
Key was not required to be
LWG 230 C++98 CopyConstructible Key is also required to
(a key of type Key might not be able to be CopyConstructible
be constructed)

Categories:¶

* Todo with reason
* Todo no example