etl/docs/containers/sets/reference-flat-multiset.md
John Wellbelove 4a88884b39
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---------

Co-authored-by: Roland Reichwein <Roland.Reichwein@bmw.de>
Co-authored-by: John Wellbelove <john.wellbelove@etlcpp.com>
Co-authored-by: John Wellbelove <john.wellbelove@etlcpp.co.uk>
2026-06-06 13:12:44 +01:00

8.6 KiB

title
reference_flat_multiset

{{< callout type="info">}} Header: reference_flat_multiset.h
Similar to: std::multiset {{< /callout >}}

A fixed capacity set based on a sorted vector.
The container stores references to objects, rather than the objects themselves.
The container is an associative lookup table with O(N) insertion and erase, and O(log N) search.
This container is best used for tables that are occasionally updated and spend most of their time being searched.
The interface is most similar to std::set.
Uses etl::less as the default key comparison method.

etl::reference_flat_set<typename T, const size_t SIZE, TCompare = etl::less>

Inherits from etl::ireference_flat_set<T, TCompare>.
etl::ireference_flat_set may be used as a size independent pointer or reference type for any etl::reference_flat_set instance.

Template deduction guides

C++17 and above

template <typename... T> etl::reference_flat_set(T...)

Example

etl::reference_flat_setdata{ 0, 1, 2, 3, 4, 5, 6, 7 }; Defines data as a set of int/int pairs, of length 8, containing the supplied data.

Make template

C++11 and above template constexpr auto make_reference_flat_set(T&&... values)

Example

auto data = etl::make_reference_flat_set(0, 1, 2, 3, 4, 5, 6, 7);

Member types

key_type                T
value_type              T
size_type               size_t
difference_type         ptrdiff_t
reference               value_type&
const_reference         const value_type&
pointer                 value_type*
const_pointer           const value_type*
iterator                Random access iterator
const_iterator          Constant random access iterator
reverse_iterator        ETL_OR_STD:reverse_iterator<iterator>
const_reverse_iterator  ETL_OR_STD::reverse_iterator<const_iterator>

Constructor

etl::flat_set<T, SIZE, TCompare>();

Description


etl::flat_set(const flat_set& other)

Description


template <typename TIterator>
etl::flat_set<T, SIZE, TCompare>(TIterator begin, TIterator end);

Description
If the set is full then raises an etl::reference_flat_set_full.
If asserts or exceptions are not enabled then undefined behaviour occurs.

Iterators

iterator begin()
const_iterator begin() const
const_iterator cbegin() const

Description
Returns an iterator to the beginning of the set.


iterator end()
const_iterator end() const
const_iterator cend() const

Description
Returns an iterator to the end of the set.


iterator rbegin()
const_iterator rbegin() const
const_iterator crbegin() const

Description
Returns a reverse iterator to the beginning of the set.


iterator rend()
const_iterator rend() const
const_iterator crend() const

Description
Returns a reverse iterator to the end of the set.

Capacity

bool empty() const

Description
Returns true if the size of the set is zero, otherwise false.


bool full() const

Description
Returns true if the size of the lookup is SIZE, otherwise false.


size_t size() const

Description
Returns the size of the lookup.


size_t max_size() const

Description
Returns the maximum possible size of the set.


size_t available() const

Description
Returns the remaining available capacity in the set.

Modifiers

flat_set& operator = (const flat_set& rhs)

Description
Copies the data from another flat set.


ETL_OR_STD::pair<iterator, bool> insert(const value_type& value)
iterator insert(iterator position, const value_type& value)

Description


template <typename TIterator>
void insert(TIterator first, TIterator last)

Description
Inserts values in to the set.
If the set is full then raises an etl::reference_flat_set_full.
If asserts or exceptions are not enabled then undefined behaviour occurs.


size_t erase(const value_type& key)
void erase(iterator i_element)
void erase(iterator first, iterator last)

Description
Erases values in the set.
Iterators are not checked for validity.

Since: 20.21.0

template <typename K>
size_t erase(K&& key)

Description
Erases values in the map.


void clear()

Description
Clears the lookup to a size of zero.

find

iterator find(const value_type& key)
const_iterator find(const value_type& key) const

Description
Finds the element that contains key, or end() if not found,

template <typename K>
iterator find(const K& key)
const_iterator find(const K& key) const

Description
Finds the element that contains key, or end() if not found,

C++11 or above.
For comparators that define is_transparent.
Since: 20.21.0

lower_bound

iterator lower_bound(const value_type& key)
const_iterator lower_bound(const value_type& key) const

Description
Returns an iterator pointing to the first element in the container whose key is not considered to go before key (i.e., either it is equivalent or goes after).


Since: 20.21.0 C++11 or above

template <typename K>
iterator lower_bound(const K& key)
const_iterator lower_bound(const K& key) const

Description
Returns an iterator pointing to the first element in the container whose key is not considered to go before key (i.e., either it is equivalent or goes after).

C++11 or above.
For comparators that define is_transparent.
Since: 20.21.0

upper_bound

iterator upper_bound(const value_type& key)
const_iterator upper_bound(const value_type& key) const

Description
Returns an iterator pointing to the first element in the container whose key is considered to go after key.


template <typename K>
iterator upper_bound(const K& key)
const_iterator upper_bound(const K& key) const

Description
Returns an iterator pointing to the first element in the container whose key is considered to go after key.

C++11 or above.
For comparators that define is_transparent.
Since: 20.21.0

equal_range

ETL_OR_STD::pair<iterator, iterator> equal_range(const value_type& key)
ETL_OR_STD::pair<const_iterator, const_iterator> equal_range(const value_type& key) const

Description
Returns the bounds of a range that includes all the elements in the container which have a key equivalent to key.


template <typename K>
pair<iterator, iterator> equal_range(const K& key)
pair<const_iterator, const_iterator> equal_range(const K& key) const

Description
Returns the bounds of a range that includes all the elements in the container which have a key equivalent to key.

C++11 or above.
For comparators that define is_transparent.
Since: 20.21.0

contains

bool contains(const value_type&  key) const

Description
Check if the container contains the key.

Since: 20.21.0 C++11 or above For comparators that define is_transparent.

template <typename K>
bool contains(const K& k) const

Description
Check if the container contains the key.

Since: 20.21.0 C++11 or above For comparators that define is_transparent.

Non-member functions

Lexicographically comparisons

operator ==

Description
true if the contents of the maps are equal, otherwise false.


operator !=

Description
true if the contents of the maps are not equal, otherwise false.

How it works

Reference flat sets are different from the normal version in that the elements are not copied, but linked directly.
This means that the lifetime of the element inserted must be as great as that of the set that contains it.
Unlike most other reference containers, the set has a finite capacity.

Flat sets are usually implemented internally as a sorted vector of values. Whilst this makes searching fast, it can have a detrimental effect when items are inserted into a container that stores complex, non-trivial values.
As Inserting requires that all of the items above the insert position must be shifted, this can become an expensive operation for larger containers.

To improve insertion performance ETL reference flat sets are implemented as vectors of pointers to values, sorted by value. An insertion will involve a copy of a range of pointers; an operation that can be made very fast.

The downside is that access to an item via an iterator will involve one indirection and the overhead of the container will be one pointer per item. A normal flat set implementation does not have this overhead.