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| title |
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| flat_map |
{{< callout type="info">}}
Header: flat_map.h
Similar to: std::map
{{< /callout >}}
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.
Uses etl::less as the default key comparison method.
etl::flat_map<typename TKey, typename TMapped, size_t SIZE, TKeyCompare = etl::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.
Template deduction guides
C++17 and above
template <typename... TPairs>
etl::flat_map(TPairs...)
Example
etl::flat_map data{ etl::pair{0, 1}, etl::pair{2, 3}, etl::pair{4, 5}, etl::pair{6, 7} };
Defines data as an flat_map of int/int pairs, of length 4, containing the supplied data.
make_flat_map
C++11 and above
template <typename TKey,
typename TMapped,
typename TKeyCompare = etl::less<TKey>,
typename... TPairs>
constexpr auto make_flat_map(TValues&&... values)
Example
auto data = etl::make_flat_map<int, int>(etl::pair{0, 1}, etl::pair{2, 3},
etl::pair{4, 5}, etl::pair{6, 7});
Member types
key_type TKey
mapped_type TMapped
value_type pair<const key_type, mapped_type>
The type is either std::pair (default) or etl::pair (ETL_NO_STL)
size_type std::size_t
difference_type std::ptrdiff_t
reference T&
const_reference const T&
rvalue_reference T&&
pointer T*
const_pointer const T*
iterator Random access iterator
const_iterator Constant random access iterator
reverse_iterator reverse_iterator<iterator>
const_reverse_iterator reverse_iterator<const_iterator>
Static Constants
MAX_SIZE The maximum size of the flat map.
Constructors
etl::flat_map<Tkey, TMapped, SIZE, TKeyCompare>()
Description
Default constructor.
etl::flat_map(const flat_map& other)
Description
Copy constructor.
etl::flat_map(flat_map&& other)
Description
Move constructor.
template <typename TIterator>
etl::flat_map<Tkey, TMapped, SIZE, TKeyCompare>(TIterator begin, TIterator end);
Description
Construct from the range [begin, end).
Element access
TMapped& at(const_key_reference key)
const TMapped& at(const_key_reference key) const
Description
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.
If asserts or exceptions are not enabled then undefined behaviour occurs.
TMapped& operator[](const_key_reference key)
const TMapped& operator[](const_key_reference key) const
Description
Returns a reference or const reference to the indexed element.
If the key is not in the table then a new entry is created.
Iterators
iterator begin()
const_iterator begin() const
const_iterator cbegin() const
Description
Returns an iterator to the beginning of the map.
iterator end()
const_iterator end() const
const_iterator cend() const
Description
Returns an iterator to the end of the map.
iterator rbegin()
const_iterator rbegin() const
const_iterator crbegin() const
Description
Returns a reverse iterator to the beginning of the map.
iterator rend()
const_iterator rend() const
const_iterator crend() const
Description
Returns a reverse iterator to the end of the map.
Capacity
bool empty() const
Description
Returns true if the size of the map 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 map.
size_t available() const
Description
Returns the remaining available capacity in the map.
Modifiers
flat_map& operator = (const flat_map& rhs)
flat_map& operator = (flat_map&& rhs)
Description
Copies or moves the data from another flat map.
pair<iterator, bool> insert(const value_type& value)
pair<iterator, bool> insert(value_type&& value)
iterator insert(iterator position, const value_type& value)
iterator insert(iterator position, value_type&& value)
Description
Inserts a value into the map.
template <typename TIterator>
void insert(TIterator first, TIterator last)
Description
Inserts values in to the map.
If the map is full then emits an etl::flat_map_full. If asserts or exceptions are not enabled then undefined behaviour occurs.
The return type is either std::pair (default) or etl::pair (ETL_NO_STL)
pair<iterator, bool> emplace((const value_type& value))
pair<iterator, bool> emplace(const key_type& key, const mapped_type& value)
Description
Inserts key/value pairs into the map by constructing directly into storage.
The return type is either std::pair (default) or etl::pair (ETL_NO_STL)
C++03
The emplace functions differ from that of std::map in that, due to C++03 not supporting 'perfect forwarding', the values for constructing mapped types must be listed as parameters and not nested in a 'mapped' value parameter.
The return type is either std::pair (default) or etl::pair (ETL_NO_STL)
template <typename T1>
pair<iterator, bool> emplace(const key_type& key, const T1& value1)
Description
Emplaces a value constructed from key and 1 argument into the map.
template <typename T1, typename T2>
pair<iterator, bool> emplace(const key_type& key, const T1& value1, const T2& value2)
Description
Emplaces a value constructed from key and 2 arguments into the map.
template <typename T1, typename T2, typename T3>
pair<iterator, bool> emplace(const key_type& key, const T1& value1, const T2& value2, const T3& value3)
Description
Emplaces a value constructed from key and 3 arguments into the map.
template <typename T1, typename T2, typename T3, typename T4>
pair<iterator, bool> emplace(const key_type& key, const T1& value1, const T2& value2, const T3& value3, const T4& value4)
Description
Emplaces a value constructed from key and 4 arguments into the map.
C++11
template <typename ... Args>
pair<iterator, bool> emplace(const key_type& key, Args&& ... args)
Description
Emplaces a value constructed from the key and arguments into the map.
size_t erase(key_value_parameter_t key)
void erase(iterator i_element)
void erase(iterator first, iterator last)
Description
Erase elements from the map.
iterator erase(const_iterator i_element)
iterator erase(const_iteratorfirst, const_iteratorlast)
Description
Erase elements from the map.
From: 20.20.0
template <typename K>
size_t erase(K&& key)
Description
Erases values in the map.
Returns an iterator to the next element in the map.
Iterator parameters are not checked for validity.
From: 20.21.0
void clear();
Description
Clears the map to a size of zero.
Search
iterator find(key_value_parameter_t key)
const_iterator find(key_value_parameter_t key) const
Description
Searches for an element with the key key.
Returns an iterator to the element, or end() if not found.
iterator lower_bound(key_value_parameter_t key)
const_iterator lower_bound(key_value_parameter_t key) const
Description
Searches for an element with the key not less than key.
Returns an iterator to the element, or end() if not found.
iterator upper_bound(key_value_parameter_t key)
const_iterator upper_bound(key_value_parameter_t key) const
Description
Searches for an element with the key greater than key.
Returns an iterator to the element, or end() if not found.
pair<iterator, iterator> equal_range(key_value_parameter_t key)
pair<const_iterator, const_iterator> equal_range(key_value_parameter_t key) const
Description
Returns the range of elements with a key equal to key.
The return type is either std::pair (default) or etl::pair (ETL_NO_STL)
For comparators that define is_transparent.
template <typename K>
iterator find(const K& key)
Description
Searches for an element with the key key.
Returns an iterator to the element, or end() if not found.
From: 20.21.0
Since: C++11
template <typename K>
const_iterator find(const K& key) const
Description
Searches for an element with the key key.
Returns an iterator to the element, or end() if not found.
From: 20.21.0
Since: C++11
template <typename K>
iterator lower_bound(const K& key)
Description
Searches for an element with the key not less than key.
Returns an iterator to the element, or end() if not found.
From: 20.21.0
Since: C++11
template <typename K>
const_iterator lower_bound(const K& key) const
Description
Searches for an element with the key not less than key.
Returns an iterator to the element, or end() if not found.
From: 20.21.0
Since: C++11
template <typename K>
iterator upper_bound(const K& key)
Description
Searches for an element with the key greater than key.
Returns an iterator to the element, or end() if not found.
From: 20.21.0
Since: C++11
template <typename K>
const_iterator upper_bound(const K& key) const
Description
Searches for an element with the key greater than key.
Returns an iterator to the element, or end() if not found.
From: 20.21.0
Since: C++11
template <typename K>
pair<iterator, iterator> equal_range(const K& key)
Description
Returns the range of elements with a key equal to key.
The return type is either std::pair (default) or etl::pair (ETL_NO_STL)
From: 20.21.0
Since: C++11
template <typename K>
pair<const_iterator, const_iterator> equal_range(const K& key) const
Description
Returns the range of elements with a key equal to key.
The return type is either std::pair (default) or etl::pair (ETL_NO_STL)
From: 20.21.0
Since: C++11
bool contains(key_value_parameter_t key) const
Check if the container contains the key.
From: 20.21.0
template <typename K>
bool contains(const K& k) const
Check if the container contains the key.
For comparators that define is_transparent.
From: 20.21.0
Since: C++11
Non-member functions
Lexicographically comparisons
operator ==
true if the contents of the maps are equal, otherwise false.
operator !=
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 to the right of 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 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.