A C++ template library for embedded applications
Designed and maintained by
Aster Consulting Ltd

flat_map / flat_multmap

A fixed capacity map based on a sorted vector.
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::map.
Uses std::less as the default key comparison method.

etl::flat_map<typename TKey, typename TMapped, const size_t SIZE, TKeyCompare = std::less>

Inherits from etl::iflat_map<TKey, TMapped, TKeyCompare>
etl::iflat_map may be used as a size independent pointer or reference type for any etl::flat_map instance.

Member types

key_type                TKey
mapped_type             TMapped
value_type              std::pair<const key_type, mapped_type>
size_type               std::size_t
difference_type         std::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        std::reverse_iterator<iterator>
const_reverse_iterator  std::reverse_iterator<const_iterator>


etl::flat_map<Tkey, TMapped, SIZE, TKeyCompare>();

etl::flat_map(const flat_map& other)

template <typename TIterator>
etl::flat_map<Tkey, TMapped, SIZE, TKeyCompare>(TIterator begin, TIterator end);

Element access

TMapped& at(size_t i)
const TMapped& at(size_t i) const
Returns a reference or const reference to the indexed element. Emits an etl::flat_map_out_of_range if the key is not
in the table.

TMapped& operator[](size_t i)
const TMapped& operator[](size_t i) const
Returns a reference or const reference to the indexed element.


iterator begin()
const_iterator begin() const
const_iterator cbegin() const
Returns an iterator to the beginning of the map.

iterator end()
const_iterator end() const
const_iterator cend() const
Returns an iterator to the end of the map.

iterator rbegin()
const_iterator rbegin() const
const_iterator crbegin() const
Returns a reverse iterator to the beginning of the map.

iterator rend()
const_iterator rend() const
const_iterator crend() const
Returns a reverse iterator to the end of the map.


bool empty() const
Returns true if the size of the map is zero, otherwise false.

bool full() const
Returns true if the size of the lookup is SIZE, otherwise false.

size_t size() const
Returns the size of the lookup.

size_t max_size() const
Returns the maximum possible size of the map.

size_t available() const
Returns the remaining available capacity in the map.


flat_map& operator = (const flat_map& rhs)
Copies the data from another flat map.

std::pair<iterator, bool> insert(const value_type& value)
iterator insert(iterator position, const value_type& value)
template <class TIterator>
void insert(TIterator first, TIterator last)
Inserts values in to the map. If the map is full then emits an etl::flat_map_full.

size_t erase(key_value_parameter_t key)
void erase(iterator i_element)
void erase(iterator first, iterator last)
Erases values in the map.

void clear();
Clears the lookup to a size of zero.


iterator find(key_value_parameter_t key)
const_iterator find(key_value_parameter_t key) const

iterator lower_bound(key_value_parameter_t key)
const_iterator lower_bound(key_value_parameter_t key) const

iterator upper_bound(key_value_parameter_t key)
const_iterator upper_bound(key_value_parameter_t key) const

std::pair<iterator, iterator> equal_range(key_value_parameter_t key)
std::pair<const_iterator, const_iterator> equal_range(key_value_parameter_t key) const

Non-member functions

Lexicographically comparisons

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

Technical stuff

Flat maps are usually implemented internally as a sorted vector of key/value pairs. Whilst this makes searching fast, it can
have a detrimental effect when items are inserted into a container that stores complex, non-trivial keys or 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 flat maps are implemented as vectors of pointers to key/value pairs, sorted by key
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 map implementation does not have this overhead.
flat_map.h / flat_multimap.h