NAME¶

std::flat_set - std::flat_set

Synopsis¶

Defined in header <flat_set>
template<

class Key,
class Compare = std::less<Key>, (since C++23)
class KeyContainer = std::vector<Key>

> class flat_set;

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.

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.
The type of the underlying SequenceContainer to store the elements.
The iterators of such container should satisfy
KeyContainer - LegacyRandomAccessIterator or model random_access_iterator.

The standard containers std::vector and std::deque satisfy these
requirements.

Member types¶

Member type Definition
container_type KeyContainer
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 name Definition
c (private) the underlying container of container_type
(exposition-only member object*)
compare (private) the comparison function object of type key_compare
(exposition-only member object*)

Member functions¶

constructor constructs the flat_set
(public member function)
destructor destroys every element of the container adaptor
(implicitly declared) (public member function)
operator= assigns values to the container adaptor
(public member function)

Iterators¶

begin returns an iterator to the beginning
cbegin (public member function)
end returns an iterator to the end
cend (public member function)
rbegin returns a reverse iterator to the beginning
crbegin (public member function)
rend returns a reverse iterator to the end
crend (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)
returns an iterator to the first element not less than the
lower_bound given key
(public member function)
returns an iterator to the first element greater than the
upper_bound 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)
returns the function that compares keys in objects of type
value_comp value_type
(public member function)

Non-member functions¶

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

Helper classes¶

std::uses_allocator<std::flat_set> specializes the std::uses_allocator type trait
(C++23) (class template specialization)

Tags

sorted_unique indicates that elements of a range are sorted and unique
sorted_unique_t (tag)
(C++23)

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

Categories:¶

* Todo without reason
* Todo no example