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etl/docs/containers/maps/flat-map.md
rleigh-electradx c8bc1bd896
Correct the documented make_xxx factory signatures and examples (#1530) 
* Corrected the documented make_xxx factory signatures and examples

The Make template sections across the container documentation had
drifted from the implementations, largely through copy-paste:

- map.md, multimap.md and flat-multimap.md all documented
  make_flat_map; multiset.md documented make_set; flat-multiset.md
  documented make_flat_set; unordered-multimap.md documented
  make_unordered_map. Names and examples now match each container.
- Every signature omitted the C++11-required trailing return type
  despite the sections being labelled C++11 and above; several also
  used a parameter name (TValues) that did not match any template
  parameter, and the unordered map headers were missing a comma.
- forward-list.md documented a make_array-style signature with an
  explicit element type; the real function deduces the element type as
  the common type of the arguments, as documented for make_list.
- The flat_set/flat_multiset template headers were missing the
  variadic parameter pack, and their examples passed a braced list to
  a variadic function, which does not compile; set.md used TKeyCompare
  where the implementation uses TCompare.
- deque.md and the reference container docs were missing code fences.
- Updated the four reference_flat_* function.  However these functions
  cannot be instantiated at all (the reference containers have no
  initializer_list constructor, since one would dangle by design), so
  the documented examples have never compiled.

Every remaining documented example verified to compile with clang 22
(C++17) against the current headers.

* Fixed C++17-only syntax in factory examples and flagged the non-compilable ones

Review feedback on the make_xxx documentation examples:

- All the map-family factory examples are labelled C++11 and above but
  constructed their pairs with class template argument deduction
  (etl::pair{0, 1}), which requires C++17. The pairs in the make_flat_map,
  make_flat_multimap, make_map, make_multimap, make_unordered_map,
  make_unordered_multimap, make_reference_flat_map and
  make_reference_flat_multimap examples now name their template arguments
  explicitly. The template deduction guide examples are unchanged, as
  those sections are labelled C++17 and above. Verified with clang 22 in
  C++14 mode, which, like C++11, has no deduction guides.

- The four make_reference_flat_* examples are now preceded by a warning
  callout stating that they do not currently compile: the functions
  cannot be instantiated because the reference containers have no
  initializer_list constructor. The examples themselves are retained,
  rewritten to pass named lvalues rather than temporaries, since the
  reference containers store references to external objects and the
  temporaries would dangle even if the functions compiled. The defect
  is being reported separately.

---------

Co-authored-by: John Wellbelove <jwellbelove@users.noreply.github.com>
2026-08-04 11:45:49 +01:00

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---
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<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
```cpp
template <typename... TPairs>
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 TKey,
typename TMapped,
typename TKeyCompare = etl::less<TKey>,
typename... TPairs>
constexpr auto make_flat_map(TPairs&&... pairs) -> etl::flat_map<TKey, TMapped, sizeof...(TPairs), TKeyCompare>
```
### Example
```cpp
auto data = etl::make_flat_map<int, int>(etl::pair<int, int>{0, 1}, etl::pair<int, int>{2, 3},
etl::pair<int, int>{4, 5}, etl::pair<int, int>{6, 7});
```
## Member types
```cpp
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
```cpp
etl::flat_map<Tkey, TMapped, SIZE, TKeyCompare>()
```
**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 <typename TIterator>
etl::flat_map<Tkey, TMapped, SIZE, TKeyCompare>(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<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.
```cpp
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`)
---
```cpp
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`)
```cpp
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.
```cpp
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.
```cpp
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.
```cpp
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**
```cpp
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.
---
```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 <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`
---
```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<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.**
```cpp
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
```cpp
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
---
```cpp
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
```cpp
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
---
```cpp
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
```cpp
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
---
```cpp
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
```cpp
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
---
```cpp
bool contains(key_value_parameter_t key) const
```
Check if the container contains the key.
From: `20.21.0`
---
```cpp
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**
```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.
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