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etl/docs/containers/sets/multiset.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: "multiset"
---
{{< callout type="info">}}
Header: `multiset.h`
Similar to: `std::multiset`
{{< /callout >}}
A fixed capacity multiset.
Uses etl or `std::less` as the default key comparison method.
```cpp
etl::multiset<typename T, const size_t SIZE, TKeyCompare = etl::less>
```
Inherits from `etl::iset<T, TKeyCompare>`.
`etl::iset` may be used as a size independent pointer or reference type for any `etl::multiset` instance.
## Template deduction guides
C++17 and above
```cpp
template <typename... TPairs>
etl::multiset(TPairs...)
```
### Example
```cpp
etl::multiset data{ 0, 1, 2, 3, 4, 5, 6, 7 };
```
Defines data as an multiset of `int`, of length 8, containing the supplied data.
## Make template
**C++11 and above**
```cpp
template <typename TKey,
typename TKeyCompare = etl::less<TKey>,
typename... T>
constexpr auto make_multiset(T&&... keys) -> etl::multiset<TKey, sizeof...(T), TKeyCompare>
```
### Example
```cpp
auto data = etl::make_multiset<int>(0, 1, 2, 3, 4, 5, 6, 7);
```
## Member types
```cpp
value_type T
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<iterator>
const_reverse_iterator ETL_OR_STD::reverse_iterator<const_iterator>
```
## Constructors
```cpp
etl::multiset<T, SIZE, TKeyCompare>()
```
**Description**
---
```cpp
template <typename TIterator>
etl::multiset<T, SIZE, TKeyCompare>(TIterator begin, TIterator end)
```
**Description**
If the map is full then emits an `etl::reference_flat_set_full`.
If asserts or exceptions are not enabled then undefined behaviour occurs.
## Element access
```cpp
iterator find(const value_type& v)
const_iterator find(const value_type& v) const
```
**Description**
Searches the container for an element with a value of `v` and returns an iterator to it if found, otherwise it returns an iterator to `etl::multiset::end()`.
---
```cpp
size_type count(const value_type& v) const
```
**Description**
Count elements with a specific key.
Searches the container for elements with a value of `v` and returns the number of matches
---
```cpp
iterator lower_bound(const value_type& v)
const_iterator lower_bound(const value_type& v) const
```
**Description**
Searches for an element with the key not less than `v`.
Returns an iterator to the element, or `end()` if not found.
---
```cpp
iterator upper_bound(const value_type& v)
const_iterator upper_bound (const value_type& v) const
```
**Description**
Searches for an element with the key greater than `v`.
Returns an iterator to the element, or `end()` if not found.
---
```cpp
pair<const_iterator, const_iterator> equal_range(const value_type& v) const
pair<iterator, iterator> equal_range(const value_type& v)
```
**Description**
Returns the range of elements with a key equal to `v`.
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**
Since: `20.21.0`
C++11 or above.
---
```cpp
template <typename K>
const_iterator find(const K& key) const
```
**Description**
Since: `20.21.0`
C++11 or above.
---
```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.
Since: `20.21.0`
C++11 or above.
---
```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 a const iterator to the element, or `end()` if not found.
Since: `20.21.0`
C++11 or above.
---
```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.
Since: `20.21.0`
C++11 or above.
---
```cpp
template <typename K>
const_iterator upper_bound(const K& key) const
```
**Description**
Searches for an element with the key greater than `key`.
Returns a const iterator to the element, or `end()` if not found.
Since: `20.21.0`
C++11 or above.
---
```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`)
Since: `20.21.0`
C++11 or above.
---
```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`)
Since: `20.21.0`
C++11 or above.
---
```cpp
bool contains(key_value_parameter_t key) const
```
**Description**
Check if the container contains the key.
Since: `20.21.0`
---
For comparators that define is_transparent.
```cpp
template <typename K>
```
**Description**
Since: `20.21.0`
C++11 or above.
---
```cpp
bool contains(const K& k) const
```
Check if the container contains the key.
Since: `20.21.0`
C++11 or above.
## 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 rbegin()
const_iterator rbegin() const
const_iterator crbegin() const
```
**Description**
Returns a reverse iterator to the beginning 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 map is SIZE, otherwise false.
---
```cpp
size_t size() const
```
**Description**
Returns the size of the map.
---
```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
<=20.19.0
```cpp
template <typename TIterator>
void insert(TIterator begin, TIterator end)
```
**Description**
---
>=20.20.0
```cpp
template <typename TIterator>
iterator insert(TIterator begin, TIterator end)
```
**Description**
---
```cpp
iterator insert(parameter_t value)
```
**Description**
Inserts values in to the map.
If the multiset is full then emits an `etl::multiset_full`.
If asserts or exceptions are not enabled then undefined behaviour occurs.
---
```cpp
template <typename TIterator>
iterator erase(TIterator begin, TIterator end)
```
**Description**
---
```cpp
iterator erase(TIterator element)
```
**Description**
Erases values in the multiset.
Iterators are not checked for validity.
---
```cpp
size_t erase(const key_type& key)
```
**Description**
----
20.21.0
```cpp
template <typename K>
size_t erase(K&& key)
```
**Description**
---
```cpp
void clear()
```
**Description**
Clears the map to a size of zero.
## Non-member functions
```cpp
operator ==
```
**Description**
`true` if the contents of the sets are equal, otherwise `false`.
---
```cpp
operator !=
```
**Description**
`true` if the contents of the sets are not equal, otherwise `false`.
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