std::flat_multimap

Defined in header `<flat_map>`
template< class Key, class T, class Compare = std::less<Key>, class KeyContainer = std::vector<Key>, class MappedContainer = std::vector<T> > class flat_multimap;		(since C++23)

The flat multimap is a container adaptor that gives the functionality of an associative container that contains key-value pairs, while permitting multiple entries with the same key value. Keys are sorted by using the comparison function Compare.

The class template flat_multimap acts as a wrapper to the two underlying containers, passed as objects of type KeyContainer and MappedContainer respectively. The first container is sorted, and for each key its corresponding value is in the second container at the same index (offset). The number of elements in both containers is the same.

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

std::flat_multimap meets the requirements of Container, ReversibleContainer, optional container requirements, and all requirements of AssociativeContainer (including logarithmic search complexity), except that:

requirements related to nodes are not applicable,
iterator invalidation requirements differ,
the complexity of insertion and erasure operations is linear.

A flat multimap supports most of AssociativeContainer's operations that use equal keys.

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

However, defining a constexpr std::flat_multimap variable is generally an error, because constant evaluation requires any dynamically allocated storage to be released in the same evaluation, which is usually not the case with std::flat_multimap's initializer.

(since C++26)

Iterator invalidation

Template parameters

Key	-	The type of the keys. The program is ill-formed if `Key` is not the same type as `KeyContainer::value_type`.
T	-	The type of mapped values. The program is ill-formed if `T` is not the same type as `MappedContainer::value_type`.
Compare	-	A Compare type providing a strict weak ordering.
KeyContainer MappedContainer	-	The types of the underlying SequenceContainer to store keys and mapped values correspondingly. The iterators of such containers should satisfy LegacyRandomAccessIterator or model `random_access_iterator`. Invocations of their member functions `size` and `max_size` should not exit via an exception. The standard containers `std::vector` and `std::deque` satisfy these requirements.

Member types

Type	Definition
`key_container_type`	`KeyContainer`
`mapped_container_type`	`MappedContainer`
`key_type`	`Key`
`mapped_type`	`T`
`value_type`	std::pair<key_type,mapped_type>
`key_compare`	`Compare`
`reference`	std::pair<constkey_type&,mapped_type&>
`const_reference`	std::pair<constkey_type&,constmapped_type&>
`size_type`	`std::size_t`
`difference_type`	`std::ptrdiff_t`
`iterator`	implementation-defined LegacyInputIterator, ConstexprIterator(since C++26) and `random_access_iterator` to `value_type`
`const_iterator`	implementation-defined LegacyInputIterator, ConstexprIterator(since C++26) and `random_access_iterator` to constvalue_type
`reverse_iterator`	std::reverse_iterator<iterator>
`const_reverse_iterator`	std::reverse_iterator<const_iterator>
`containers`	type describing the underlying containers `struct containers { key_container_type keys; mapped_container_type values; };`

Member classes

value_compare

compares objects of type value_type
(class)

Member objects

Member	Description
`containers` `c` (private)	the adapted containers (exposition-only member object*)
`key_compare` `compare` (private)	the comparison function object (exposition-only member object*)

Member functions

(constructor)	constructs the `flat_multimap` (public member function)
(destructor) (implicitly declared)	destroys every element of the container adaptor (public member function)
operator=	assigns values to the container adaptor (public member function)
Iterators
begincbegin	returns an iterator to the beginning (public member function)
endcend	returns an iterator to the end (public member function)
rbegincrbegin	returns a reverse iterator to the beginning (public member function)
rendcrend	returns a reverse iterator to the end (public member function)
Capacity
empty	checks whether the container adaptor 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
emplace	constructs element in-place (public member function)
emplace_hint	constructs elements in-place using a hint (public member function)
insert	inserts elements (public member function)
insert_range	inserts a range of elements (public member function)
extract	extracts the underlying containers (public member function)
replace	replaces the underlying containers (public member function)
erase	erases elements (public member function)
swap	swaps the contents (public member function)
clear	clears the contents (public member function)
Lookup
find	finds element with specific key (public member function)
count	returns the number of elements matching specific key (public member function)
contains	checks if the container contains element with 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)
equal_range	returns range of elements matching a specific 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)
keys	direct access to the underlying keys container (public member function)
values	direct access to the underlying values container (public member function)

Non-member functions

operator==operator<=> (C++23)	lexicographically compares the values of two `flat_multimap`s (function template)
std::swap(std::flat_multimap) (C++23)	specializes the `std::swap` algorithm (function template)
erase_if(std::flat_multimap) (C++23)	erases all elements satisfying specific criteria (function template)

Helper classes

std::uses_allocator<std::flat_multimap>

(C++23)

specializes the std::uses_allocator type trait
(class template specialization)

Deduction guides

Notes

The member types iterator and const_iterator may be aliases to the same type. This means defining a pair of function overloads using the two types as parameter types may violate the One Definition Rule. Since iterator is convertible to const_iterator, a single function with a const_iterator as parameter type will work instead.

Feature-test macro	Value	Std	Feature
`__cpp_lib_flat_map`	`202207L`	(C++23)	`std::flat_map` and `std::flat_multimap`
`__cpp_lib_constexpr_flat_map`	`202502L`	(C++26)	Constexpr `std::flat_multimap`

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

std::flat_multimap

Iterator invalidation

Template parameters

Member types

Member classes

Member objects

Member functions

Iterators

Capacity

Modifiers

Lookup

Observers

Non-member functions

Helper classes

Tags

Deduction guides

Notes

Example

See also