--- title: "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. ```cpp etl::flat_map ``` Inherits from `etl::iflat_map`. `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 ```cpp template etl::flat_map(TPairs...) ``` ### Example ```cpp 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 ```cpp template , typename... TPairs> constexpr auto make_flat_map(TValues&&... values) ``` ### Example ```cpp auto data = etl::make_flat_map(etl::pair{0, 1}, etl::pair{2, 3}, etl::pair{4, 5}, etl::pair{6, 7}); ``` ## Member types ```cpp key_type TKey mapped_type TMapped value_type pair 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 const_reverse_iterator reverse_iterator ``` ## Static Constants `MAX_SIZE` The maximum size of the flat map. ## Constructors ```cpp etl::flat_map() ``` **Description** Default constructor. --- ```cpp etl::flat_map(const flat_map& other) ``` **Description** Copy constructor. --- ```cpp etl::flat_map(flat_map&& other) ``` **Description** Move constructor. --- ```cpp template etl::flat_map(TIterator begin, TIterator end); ``` **Description** Construct from the range [`begin`, `end`). ## Element access ```cpp 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. --- ```cpp 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 ```cpp iterator begin() const_iterator begin() const const_iterator cbegin() const ``` **Description** Returns an iterator to the beginning of the map. --- ```cpp iterator end() const_iterator end() const const_iterator cend() const ``` **Description** Returns an iterator to the end of the map. --- ```cpp iterator rbegin() const_iterator rbegin() const const_iterator crbegin() const ``` **Description** Returns a reverse iterator to the beginning of the map. --- ```cpp iterator rend() const_iterator rend() const const_iterator crend() const ``` **Description** Returns a reverse iterator to the end of the map. ## Capacity ```cpp bool empty() const ``` **Description** Returns `true` if the size of the map is zero, otherwise `false`. --- ```cpp bool full() const ``` **Description** Returns `true` if the size of the lookup is `SIZE`, otherwise `false`. --- ```cpp size_t size() const ``` **Description** Returns the size of the lookup. --- ```cpp size_t max_size() const ``` **Description** Returns the maximum possible size of the map. --- ```cpp size_t available() const ``` **Description** Returns the remaining available capacity in the map. ## Modifiers ```cpp flat_map& operator = (const flat_map& rhs) flat_map& operator = (flat_map&& rhs) ``` **Description** Copies or moves the data from another flat map. --- ```cpp pair insert(const value_type& value) pair 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. ```cpp template 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`) --- ```cpp pair emplace((const value_type& value)) pair 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`) ```cpp template pair emplace(const key_type& key, const T1& value1) ``` **Description** Emplaces a value constructed from `key` and 1 argument into the map. ```cpp template pair emplace(const key_type& key, const T1& value1, const T2& value2) ``` **Description** Emplaces a value constructed from `key` and 2 arguments into the map. ```cpp template pair 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. ```cpp template pair 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** ```cpp template pair emplace(const key_type& key, Args&& ... args) ``` **Description** Emplaces a value constructed from the `key` and arguments into the map. --- ```cpp 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. --- ```cpp iterator erase(const_iterator i_element) iterator erase(const_iteratorfirst, const_iteratorlast) ``` **Description** Erase elements from the map. From: `20.20.0` --- ```cpp template 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` --- ```cpp void clear(); ``` **Description** Clears the map to a size of zero. ## Search ```cpp 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. ```cpp 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. ```cpp 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. ```cpp pair equal_range(key_value_parameter_t key) pair 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.** ```cpp template 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 ```cpp template 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 --- ```cpp template 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 ```cpp template 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 --- ```cpp template 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 ```cpp template 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 --- ```cpp template pair 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 ```cpp template pair 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 --- ```cpp bool contains(key_value_parameter_t key) const ``` Check if the container contains the key. From: `20.21.0` --- ```cpp template 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** ```cpp operator == ``` `true` if the contents of the maps are equal, otherwise `false`. --- ```cpp 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.