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