std:: flat_set

Defined in header <flat_set>
template < class Key, class Compare = std:: less < Key > , class KeyContainer = std:: vector < Key > > class flat_set ;		(since C++23)

The flat set is a container adaptor that gives the functionality of an associative container that stores a sorted set of unique objects of type Key . Sorting is done using the key comparison function Compare .

The class template flat_set acts as a wrapper to the underlying sorted container passed as object of type KeyContainer .

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_set 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 set supports most AssociativeContainer 's operations that use unique keys.

Iterator invalidation

This section is incomplete

Template parameters

Key	-	The type of the stored elements. The program is ill-formed if Key is not the same type as KeyContainer::value_type .
Compare	-	A Compare type providing a strict weak ordering.
KeyContainer	-	The type of the underlying SequenceContainer to store the elements. The iterators of such container should satisfy LegacyRandomAccessIterator or model random_access_iterator . The standard containers std::vector and std::deque satisfy these requirements.

Member types

Type	Definition
container_type	Key Container
key_type	Key
value_type	Key
key_compare	Compare
value_compare	Compare
reference	value_type &
const_reference	const value_type &
size_type	typename KeyContainer :: size_type
difference_type	typename KeyContainer :: difference_type
iterator	implementation-defined LegacyRandomAccessIterator and random_access_iterator to value_type
const_iterator	implementation-defined LegacyRandomAccessIterator and random_access_iterator to const value_type
reverse_iterator	std:: reverse_iterator < iterator >
const_reverse_iterator	std:: reverse_iterator < const_iterator >

Member objects

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

Member functions

(constructor)	constructs the flat_set (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
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)
insert	inserts elements (public member function)
insert_range	inserts a range of elements (public member function)
extract	extracts the underlying container (public member function)
replace	replaces the underlying container (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)

Non-member functions

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

Helper classes

std::uses_allocator <std::flat_set> (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.

Some advantages of flat set over other standard container adaptors are:

• Potentially faster lookup (even though search operations have logarithmic complexity).
• Much faster iteration: random access iterators instead of bidirectional iterators .
• Less memory consumption for small objects (and for big objects if KeyContainer :: shrink_to_fit ( ) is available).
• Better cache performance (depending on KeyContainer , keys are stored in a contiguous block(s) of memory).

Some disadvantages of flat set are:

• Non-stable iterators (iterators are invalidated when inserting and erasing elements).
• Non-copyable and non-movable type values can not be stored.
• Weaker exception safety (copy/move constructors can throw when shifting values in erasures and insertions).
• Slower (i.e. linear) insertion and erasure, especially for non-movable types.

Feature-test macro	Value	Std	Feature
__cpp_lib_flat_set	202207L	(C++23)	std::flat_set and std::flat_multiset

Example

This section is incomplete Reason: no example

See also

flat_multiset (C++23)	adapts a container to provide a collection of keys, sorted by keys (class template)
set	collection of unique keys, sorted by keys (class template)
unordered_set (C++11)	collection of unique keys, hashed by keys (class template)