std:: flat_map

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_map ;		(since C++23)

The flat map is a container adaptor that gives the functionality of an associative container that contains key-value pairs with unique keys. Keys are sorted by using the comparison function Compare .

The class template flat_map 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_map 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 map supports most AssociativeContainer 's operations that use unique keys.

Iterator invalidation

This section is incomplete

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 < const key_type & , mapped_type & >
const_reference	std:: pair < const key_type & , const mapped_type & >
size_type	std::size_t
difference_type	std::ptrdiff_t
iterator	implementation-defined LegacyInputIterator and random_access_iterator to value_type
const_iterator	implementation-defined LegacyInputIterator and random_access_iterator to const value_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_map (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)
Element access
at	access specified element with bounds checking (public member function)
operator[]	access or insert specified element (public member function)
Iterators
begin cbegin	returns an iterator to the beginning (public member function)
end cend	returns an iterator to the end (public member function)
rbegin crbegin	returns a reverse iterator to the beginning (public member function)
rend crend	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)
try_emplace	inserts in-place if the key does not exist, does nothing if the key exists (public member function)
insert	inserts elements (public member function)
insert_range	inserts a range of elements (public member function)
insert_or_assign	inserts an element or assigns to the current element if the key already exists (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_map s (function template)
std::swap (std::flat_map) (C++23)	specializes the std::swap algorithm (function template)
erase_if (std::flat_map) (C++23)	erases all elements satisfying specific criteria (function template)

Helper classes

std::uses_allocator <std::flat_map> (C++23)

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

Tags

sorted_unique sorted_unique_t (C++23)

indicates that elements of a range are sorted and unique (tag)

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

Example

This section is incomplete Reason: no example

See also

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