--- title: "reference_flat_map" --- {{< callout type="info">}} Header: `reference_flat_map.h` Similar to: `std::reference_flat_map` {{< /callout >}} A fixed capacity reference map 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. Uses `etl::less` as the default key comparison method. ```cpp etl::reference_flat_map ``` Inherits from `etl::ireference_flat_map`. `etl::ireference_flat_map` may be used as a size independent pointer or reference type for any `etl::reference_flat_map` instance. ## Template deduction guides C++17 and above ```cpp template etl::reference_flat_map(TPairs...) ``` ### Example ```cpp etl::reference_flat_map data{ etl::pair{0, 1}, etl::pair{2, 3}, etl::pair{4, 5}, etl::pair{6, 7} }; ``` Defines data as an reference_flat_map of int, of length 4, containing the supplied data. ## Make template C++11 and above ```cpp template , typename... TPairs> constexpr auto make_reference_flat_map(TValues&&... values) ``` ### Example auto data = etl::make_reference_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 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 const_reverse_iterator ETL_OR_STD::reverse_iterator ``` ## Constructor ```cpp etl::reference_flat_map() ``` **Description** Default constructor. --- ```cpp etl::reference_flat_map(const flat_map& other) ``` **Description** Copy constructor. --- ```cpp template etl::reference_flat_map(TIterator begin, TIterator end) ``` **Description** If the map is full then raises an `etl::reference_flat_map_full`. If asserts or exceptions are not enabled then undefined behaviour occurs. ## Element access ### at ```cpp TMapped& at(const key_type& key) const TMapped& at(const key_type& key) const ``` **Description** Returns a reference or const reference to the indexed element. Raises 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 template mapped_type& at(const K& key) const mapped_type& at(const K& key) const ``` **Description** Returns a reference or const reference to the indexed element. Raises 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. C++11 or above. For comparators that define `is_transparent`. Since: `20.21.0` ### find ```cpp template iterator find(const key_type& key) const_iterator find(const key_type& key) const ``` **Description** Returns a reference or const reference to the indexed element. Raises 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 template iterator find(const K& key) const_iterator find(const K& key) const ``` **Description** Returns a reference or const reference to the indexed element. Raises 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. C++11 or above. For comparators that define `is_transparent`. Since: `20.21.0` ### lower_bound ```cpp iterator lower_bound(const key_type& key) const_iterator lower_bound(const key_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). --- ```cpp template iterator lower_bound(const K& key) const_iterator lower_bound(const K& key) const ``` 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 ```cpp iterator upper_bound(const key_type& key) const_iterator upper_bound(const key_type& key) const ``` **Description** Returns an iterator pointing to the first element in the container whose key is considered to go after `key`. --- ```cpp template 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 ```cpp pair equal_range(const key_type& key) pair equal_range(const key_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`. C++11 or above. For comparators that define `is_transparent`. Since: `20.21.0` --- ```cpp template pair equal_range(const K& key) pair 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 ```cpp bool contains(const value_type& key) const ``` **Description** Check if the container contains the key. Since: `20.21.0` --- ```cpp template bool contains(const K& k) const ``` **Description** Check if the container contains the key. C++11 or above. For comparators that define `is_transparent`. Since: `20.21.0` ## 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 reference_flat_map& operator = (const reference_flat_map& rhs) ``` **Description** Copies the data from another flat map. --- ```cpp pair insert(const value_type& value) ``` **Description** Inserts `value` into the map. --- ```cpp iterator insert(iterator position, const value_type& value) ``` **Description** Inserts `value` into the map at th esuggested `position`. --- ```cpp template void insert(TIterator first, TIterator last) ``` **Description** Inserts values in to the map. If the map is full then raises an `etl::flat_map_full`. If asserts or exceptions are not enabled then undefined behaviour occurs. --- ```cpp size_t erase(const key_type& key) ``` **Description** Erase the element with `key`. --- ```cpp void erase(iterator i_element) ``` **Description** Erase the element at `i_element`. --- ```cpp void erase(iterator first, iterator last) ``` **Description** Erase elements in the range [`first`, `last`). --- ```cpp template size_t erase(K&& key) ``` **Description** Erases values in the map. Since: `20.21.0` --- ```cpp void clear() ``` **Description** Clears the lookup to a size of zero. Iterator are not checked for validity. ## Non-member functions Lexicographically comparisons ```cpp operator == ``` **Description** `true` if the contents of the maps are equal, otherwise `false`. --- ```cpp operator != ``` **Description** `true` if the contents of the maps are not equal, otherwise `false`. ## Technical stuff Reference flat maps 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 map that contains it. Unlike most other reference containers, the map has a finite capacity. 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 reference 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.