en	Class Template unordered_flat_set

en	:idprefix: unordered_flat_set_

en	`boost::unordered_flat_set` — An open-addressing unordered associative container that stores unique values.

en	The performance of `boost::unordered_flat_set` is much better than that of `boost::unordered_set` or other implementations of `std::unordered_set`. Unlike standard unordered associative containers, which are node-based, the elements of a `boost::unordered_flat_set` are held directly in the bucket array, and insertions into an already occupied bucket are diverted to available buckets in the vicinity of the original position. This type of data layout is known as _open addressing_.

en	As a result of its using open addressing, the interface of `boost::unordered_flat_set` deviates in a number of aspects from that of `boost::unordered_flat_set`/`std::unordered_flat_set`:

en	- `value_type` must be move-constructible. - Pointer stability is not kept under rehashing. - `begin()` is not constant-time. - There is no API for bucket handling (except `bucket_count`) or node extraction/insertion. - The maximum load factor of the container is managed internally and can't be set by the user.

en	Other than this, `boost::unordered_flat_set` is mostly a drop-in replacement of node-based standard unordered associative containers.

en	Synopsis

en	---

en	Description

en	*Template Parameters*

en	_Key_

en	`Key` must be https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^] into the container

en	and https://en.cppreference.com/w/cpp/named_req/Erasable[Erasable^] from the container.

en	_Hash_

en	A unary function object type that acts a hash function for a `Key`. It takes a single argument of type `Key` and returns a value of type `std::size_t`.

en	_Pred_

en	A binary function object that induces an equivalence relation on values of type `Key`. It takes two arguments of type `Key` and returns a value of type `bool`.

en	_Allocator_

en	An allocator whose value type is the same as the container's value type.

en	Allocators using https://en.cppreference.com/w/cpp/named_req/Allocator#Fancy_pointers[fancy pointers] are supported.

en	The elements of the container are held into an internal _bucket array_. An element is inserted into a bucket determined by its hash code, but if the bucket is already occupied (a _collision_), an available one in the vicinity of the original position is used.

en	The size of the bucket array can be automatically increased by a call to `insert`/`emplace`, or as a result of calling `rehash`/`reserve`. The _load factor_ of the container (number of elements divided by number of buckets) is never greater than `max_load_factor()`, except possibly for small sizes where the implementation may decide to allow for higher loads.

en	If `link:../../../../../container_hash/doc/html/hash.html#ref_hash_is_avalanchinghash[hash_is_avalanching]<Hash>::value` is `true`, the hash function is used as-is; otherwise, a bit-mixing post-processing stage is added to increase the quality of hashing at the expense of extra computational cost.

en	---

en	Configuration Macros

en	`BOOST_UNORDERED_ENABLE_STATS`

en	Globally define this macro to enable xref:reference/stats.adoc#stats[statistics calculation] for the container. Note that this option decreases the overall performance of many operations.

en	---

en	Typedefs

en	A constant iterator whose value type is `value_type`.

en	The iterator category is at least a forward iterator.

en	Convertible to `const_iterator`.

en	---

en	A constant iterator whose value type is `value_type`.

en	The iterator category is at least a forward iterator.

en	Constructors

en	Default Constructor

en	```c++ unordered_flat_set(); ```

en	Constructs an empty container using `hasher()` as the hash function, `key_equal()` as the key equality predicate and `allocator_type()` as the allocator.

en	Postconditions:;; `size() == 0` Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Bucket Count Constructor

en	```c++ explicit unordered_flat_set(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); ```

en	Constructs an empty container with at least `n` buckets, using `hf` as the hash function, `eql` as the key equality predicate, and `a` as the allocator.

en	Postconditions:;; `size() == 0` Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Iterator Range Constructor

en	Constructs an empty container with at least `n` buckets, using `hf` as the hash function, `eql` as the key equality predicate and `a` as the allocator, and inserts the elements from `[f, l)` into it.

en	Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Copy Constructor

en	```c++ unordered_flat_set(unordered_flat_set const& other); ```

en	The copy constructor. Copies the contained elements, hash function, predicate and allocator.

en	If `Allocator::select_on_container_copy_construction` exists and has the right signature, the allocator will be constructed from its result.

en	Requires:;; `value_type` is copy constructible

en	---

en	Move Constructor

en	```c++ unordered_flat_set(unordered_flat_set&& other); ```

en	The move constructor. The internal bucket array of `other` is transferred directly to the new container. The hash function, predicate and allocator are moved-constructed from `other`. If statistics are xref:unordered_flat_set_boost_unordered_enable_stats[enabled], transfers the internal statistical information from `other` and calls `other.reset_stats()`.

en	---

en	Iterator Range Constructor with Allocator

en	```c++ template<class InputIterator> unordered_flat_set(InputIterator f, InputIterator l, const allocator_type& a); ```

en	Constructs an empty container using `a` as the allocator, with the default hash function and key equality predicate and inserts the elements from `[f, l)` into it.

en	Requires:;; `hasher`, `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Allocator Constructor

en	```c++ explicit unordered_flat_set(Allocator const& a); ```

en	Constructs an empty container, using allocator `a`.

en	---

en	Copy Constructor with Allocator

en	```c++ unordered_flat_set(unordered_flat_set const& other, Allocator const& a); ```

en	Constructs a container, copying ``other``'s contained elements, hash function, and predicate, but using allocator `a`.

en	---

en	Move Constructor with Allocator

en	```c++ unordered_flat_set(unordered_flat_set&& other, Allocator const& a); ```

en	If `a == other.get_allocator()`, the elements of `other` are transferred directly to the new container; otherwise, elements are moved-constructed from those of `other`. The hash function and predicate are moved-constructed from `other`, and the allocator is copy-constructed from `a`. If statistics are xref:unordered_flat_set_boost_unordered_enable_stats[enabled], transfers the internal statistical information from `other` iff `a == other.get_allocator()`, and always calls `other.reset_stats()`.

en	---

en	Move Constructor from concurrent_flat_set

en	```c++ unordered_flat_set(concurrent_flat_set<Key, Hash, Pred, Allocator>&& other); ```

en	Move construction from a xref:#concurrent_flat_set[`concurrent_flat_set`]. The internal bucket array of `other` is transferred directly to the new container. The hash function, predicate and allocator are moved-constructed from `other`. If statistics are xref:unordered_flat_set_boost_unordered_enable_stats[enabled], transfers the internal statistical information from `other` and calls `other.reset_stats()`.

en	Complexity:;; Constant time. Concurrency:;; Blocking on `other`.

en	---

en	Initializer List Constructor

en	Constructs an empty container with at least `n` buckets, using `hf` as the hash function, `eql` as the key equality predicate and `a`, and inserts the elements from `il` into it.

en	Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Bucket Count Constructor with Allocator

en	```c++ unordered_flat_set(size_type n, allocator_type const& a); ```

en	Constructs an empty container with at least `n` buckets, using `hf` as the hash function, the default hash function and key equality predicate and `a` as the allocator.

en	Postconditions:;; `size() == 0` Requires:;; `hasher` and `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Bucket Count Constructor with Hasher and Allocator

en	```c++ unordered_flat_set(size_type n, hasher const& hf, allocator_type const& a); ```

en	Constructs an empty container with at least `n` buckets, using `hf` as the hash function, the default key equality predicate and `a` as the allocator.

en	Postconditions:;; `size() == 0` Requires:;; `key_equal` needs to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Iterator Range Constructor with Bucket Count and Allocator

en	Constructs an empty container with at least `n` buckets, using `a` as the allocator and default hash function and key equality predicate, and inserts the elements from `[f, l)` into it.

en	Requires:;; `hasher`, `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Iterator Range Constructor with Bucket Count and Hasher

en	Constructs an empty container with at least `n` buckets, using `hf` as the hash function, `a` as the allocator, with the default key equality predicate, and inserts the elements from `[f, l)` into it.

en	Requires:;; `key_equal` needs to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	initializer_list Constructor with Allocator

en	```c++ unordered_flat_set(std::initializer_list<value_type> il, const allocator_type& a); ```

en	Constructs an empty container using `a` and default hash function and key equality predicate, and inserts the elements from `il` into it.

en	Requires:;; `hasher` and `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	initializer_list Constructor with Bucket Count and Allocator

en	```c++ unordered_flat_set(std::initializer_list<value_type> il, size_type n, const allocator_type& a); ```

en	Constructs an empty container with at least `n` buckets, using `a` and default hash function and key equality predicate, and inserts the elements from `il` into it.

en	Requires:;; `hasher` and `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	initializer_list Constructor with Bucket Count and Hasher and Allocator

en	```c++ unordered_flat_set(std::initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a); ```

en	Constructs an empty container with at least `n` buckets, using `hf` as the hash function, `a` as the allocator and default key equality predicate,and inserts the elements from `il` into it.

en	Requires:;; `key_equal` needs to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].

en	---

en	Destructor

en	```c++ ~unordered_flat_set(); ```

en	Note:;; The destructor is applied to every element, and all memory is deallocated

en	---

en	Assignment

en	Copy Assignment

en	```c++ unordered_flat_set& operator=(unordered_flat_set const& other); ```

en	The assignment operator. Destroys previously existing elements, copy-assigns the hash function and predicate from `other`, copy-assigns the allocator from `other` if `Alloc::propagate_on_container_copy_assignment` exists and `Alloc::propagate_on_container_copy_assignment::value` is `true`, and finally inserts copies of the elements of `other`.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^]

en	---

en	Move Assignment

en	```c++ unordered_flat_set& operator=(unordered_flat_set&& other) noexcept((boost::allocator_traits<Allocator>::is_always_equal::value || boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value) && std::is_same<pointer, value_type*>::value); ``` The move assignment operator. Destroys previously existing elements, swaps the hash function and predicate from `other`, and move-assigns the allocator from `other` if `Alloc::propagate_on_container_move_assignment` exists and `Alloc::propagate_on_container_move_assignment::value` is `true`. If at this point the allocator is equal to `other.get_allocator()`, the internal bucket array of `other` is transferred directly to the new container; otherwise, inserts move-constructed copies of the elements of `other`. If statistics are xref:unordered_flat_set_boost_unordered_enable_stats[enabled], transfers the internal statistical information from `other` iff the final allocator is equal to `other.get_allocator()`, and always calls `other.reset_stats()`.

en	---

en	Initializer List Assignment

en	```c++ unordered_flat_set& operator=(std::initializer_list<value_type> il); ```

en	Assign from values in initializer list. All previously existing elements are destroyed.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^]

en	Iterators

en	begin

en	```c++ iterator begin() noexcept; const_iterator begin() const noexcept; ```

en	Returns:;; An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container. Complexity:;; O(`bucket_count()`)

en	---

en	end

en	```c++ iterator end() noexcept; const_iterator end() const noexcept; ```

en	Returns:;; An iterator which refers to the past-the-end value for the container.

en	---

en	cbegin

en	```c++ const_iterator cbegin() const noexcept; ```

en	Returns:;; A `const_iterator` referring to the first element of the container, or if the container is empty the past-the-end value for the container. Complexity:;; O(`bucket_count()`)

en	---

en	cend

en	```c++ const_iterator cend() const noexcept; ```

en	Returns:;; A `const_iterator` which refers to the past-the-end value for the container.

en	---

en	Size and Capacity

en	empty

en	```c++ [[nodiscard]] bool empty() const noexcept; ```

en	Returns:;; `size() == 0`

en	---

en	size

en	```c++ size_type size() const noexcept; ```

en	Returns:;; `std::distance(begin(), end())`

en	---

en	max_size

en	```c++ size_type max_size() const noexcept; ```

en	Returns:;; `size()` of the largest possible container.

en	---

en	Modifiers

en	emplace

en	```c++ template<class... Args> std::pair<iterator, bool> emplace(Args&&... args); ```

en	Inserts an object, constructed with the arguments `args`, in the container if and only if there is no element in the container with an equivalent key.

en	Requires:;; `value_type` is constructible from `args`. Returns:;; The `bool` component of the return type is `true` if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. +

en	---

en	emplace_hint

en	```c++ template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); ```

en	Inserts an object, constructed with the arguments `args`, in the container if and only if there is no element in the container with an equivalent key.

en	`position` is a suggestion to where the element should be inserted. This implementation ignores it.

en	Requires:;; `value_type` is constructible from `args`. Returns:;; The `bool` component of the return type is `true` if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. +

en	---

en	Copy Insert

en	```c++ std::pair<iterator, bool> insert(const value_type& obj); ```

en	Inserts `obj` in the container if and only if there is no element in the container with an equivalent key.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^]. Returns:;; The `bool` component of the return type is `true` if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

en	---

en	Move Insert

en	```c++ std::pair<iterator, bool> insert(value_type&& obj); ```

en	Inserts `obj` in the container if and only if there is no element in the container with an equivalent key.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^]. Returns:;; The `bool` component of the return type is `true` if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

en	---

en	Transparent Insert

en	```c++ template<class K> std::pair<iterator, bool> insert(K&& k); ```

en	Inserts an element constructed from `std::forward<K>(k)` in the container if and only if there is no element in the container with an equivalent key.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] from `k`. Returns:;; The bool component of the return type is true if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. + + This overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs and neither `iterator` nor `const_iterator` are implicitly convertible from `K`. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type.

en	---

en	Copy Insert with Hint

en	```c++ iterator insert(const_iterator hint, const value_type& obj); ``` Inserts `obj` in the container if and only if there is no element in the container with an equivalent key.

en	`hint` is a suggestion to where the element should be inserted. This implementation ignores it.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^]. Returns:;; The `bool` component of the return type is `true` if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

en	---

en	Move Insert with Hint

en	```c++ iterator insert(const_iterator hint, value_type&& obj); ```

en	Inserts `obj` in the container if and only if there is no element in the container with an equivalent key.

en	`hint` is a suggestion to where the element should be inserted. This implementation ignores it.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^]. Returns:;; The `bool` component of the return type is `true` if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

en	---

en	Transparent Insert with Hint

en	```c++ template<class K> std::pair<iterator, bool> insert(const_iterator hint, K&& k); ```

en	Inserts an element constructed from `std::forward<K>(k)` in the container if and only if there is no element in the container with an equivalent key.

en	`hint` is a suggestion to where the element should be inserted. This implementation ignores it.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] from `k`. Returns:;; The bool component of the return type is true if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. + + This overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs and neither `iterator` nor `const_iterator` are implicitly convertible from `K`. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type.

en	---

en	Insert Iterator Range

en	```c++ template<class InputIterator> void insert(InputIterator first, InputIterator last); ```

en	Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] into the container from `*first`. Throws:;; When inserting a single element, if an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

en	---

en	Insert Initializer List

en	```c++ void insert(std::initializer_list<value_type>); ```

en	Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

en	Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^] into the container. Throws:;; When inserting a single element, if an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

en	---

en	Erase by Position

en	Erase the element pointed to by `position`.

en	Returns:;; An opaque object implicitly convertible to the `iterator` or `const_iterator` immediately following `position` prior to the erasure. Throws:;; Nothing. Notes:;; The opaque object returned must only be discarded or immediately converted to `iterator` or `const_iterator`.

en	---

en	Erase by Key

en	```c++ size_type erase(const key_type& k); template<class K> size_type erase(K&& k); ```

en	Erase all elements with key equivalent to `k`.

en	Returns:;; The number of elements erased. Throws:;; Only throws an exception if it is thrown by `hasher` or `key_equal`. Notes:;; The `template<class K>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs and neither `iterator` nor `const_iterator` are implicitly convertible from `K`. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type.

en	---

en	Erase Range

en	```c++ iterator erase(const_iterator first, const_iterator last); ```

en	Erases the elements in the range from `first` to `last`.

en	Returns:;; The iterator following the erased elements - i.e. `last`. Throws:;; Nothing in this implementation (neither the `hasher` nor the `key_equal` objects are called).

en	---

en	swap

en	```c++ void swap(unordered_flat_set& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value || boost::allocator_traits<Allocator>::propagate_on_container_swap::value); ```

en	Swaps the contents of the container with the parameter.

en	If `Allocator::propagate_on_container_swap` is declared and `Allocator::propagate_on_container_swap::value` is `true` then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

en	Throws:;; Nothing unless `key_equal` or `hasher` throw on swapping.

en	---

en	pull

en	```c++ init_type pull(const_iterator position); ```

en	Move-constructs an `init_value` `x` from the element pointed to by `position`, erases the element and returns `x`.

en	---

en	clear

en	```c++ void clear() noexcept; ```

en	Erases all elements in the container.

en	Postconditions:;; `size() == 0`, `max_load() >= max_load_factor() * bucket_count()`

en	---

en	merge

en	```c++ template<class H2, class P2> void merge(unordered_flat_set<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_flat_set<Key, T, H2, P2, Allocator>&& source); ```

en	Move-inserts all the elements from `source` whose key is not already present in `*this`, and erases them from `source`.

en	---

en	Observers

en	get_allocator

en	``` allocator_type get_allocator() const noexcept; ```

en	Returns:;; The container's allocator.

en	---

en	hash_function

en	``` hasher hash_function() const; ```

en	Returns:;; The container's hash function.

en	---

en	key_eq

en	``` key_equal key_eq() const; ```

en	Returns:;; The container's key equality predicate

en	---

en	Lookup

en	find

en	```c++ iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k);

en	```

en	Returns:;; An iterator pointing to an element with key equivalent to `k`, or `end()` if no such element exists. Notes:;; The `template<class K>` overloads only participate in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type.

en	---

en	count

en	```c++ size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; ```

en	Returns:;; The number of elements with key equivalent to `k`. Notes:;; The `template<class K>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type.

en	---

en	contains

en	```c++ bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; ```

en	Returns:;; A boolean indicating whether or not there is an element with key equal to `key` in the container Notes:;; The `template<class K>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type.

en	---

en	equal_range

en	```c++ std::pair<iterator, iterator> equal_range(const key_type& k); std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> std::pair<iterator, iterator> equal_range(const K& k); template<class K> std::pair<const_iterator, const_iterator> equal_range(const K& k) const; ```

en	Returns:;; A range containing all elements with key equivalent to `k`. If the container doesn't contain any such elements, returns `std::make_pair(b.end(), b.end())`. Notes:;; The `template<class K>` overloads only participate in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type.

en	---

en	Bucket Interface

en	bucket_count

en	```c++ size_type bucket_count() const noexcept; ```

en	Returns:;; The size of the bucket array.

en	---

en	Hash Policy

en	load_factor

en	```c++ float load_factor() const noexcept; ```

en	Returns:;; `static_cast<float>(size())/static_cast<float>(bucket_count())`, or `0` if `bucket_count() == 0`.

en	---

en	max_load_factor

en	```c++ float max_load_factor() const noexcept; ```

en	Returns:;; Returns the container's maximum load factor.

en	---

en	Set max_load_factor

en	```c++ void max_load_factor(float z); ```

en	Effects:;; Does nothing, as the user is not allowed to change this parameter. Kept for compatibility with `boost::unordered_set`.

en	---

en	max_load

en	```c++ size_type max_load() const noexcept; ```

en	Returns:;; The maximum number of elements the container can hold without rehashing, assuming that no further elements will be erased. Note:;; After construction, rehash or clearance, the container's maximum load is at least `max_load_factor() * bucket_count()`. This number may decrease on erasure under high-load conditions.

en	---

en	rehash

en	```c++ void rehash(size_type n); ```

en	Changes if necessary the size of the bucket array so that there are at least `n` buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the `bucket_count()` associated with the container.

en	When `size() == 0`, `rehash(0)` will deallocate the underlying buckets array. If the provided Allocator uses fancy pointers, a default allocation is subsequently performed.

en	Invalidates iterators, pointers and references, and changes the order of elements.

en	Throws:;; The function has no effect if an exception is thrown, unless it is thrown by the container's hash function or comparison function.

en	---

en	reserve

en	```c++ void reserve(size_type n); ```

en	Equivalent to `a.rehash(ceil(n / a.max_load_factor()))`.

en	Similar to `rehash`, this function can be used to grow or shrink the number of buckets in the container.

en	Invalidates iterators, pointers and references, and changes the order of elements.

en	Throws:;; The function has no effect if an exception is thrown, unless it is thrown by the container's hash function or comparison function.

en	---

en	Statistics

en	get_stats

en	```c++ stats get_stats() const; ```

en	Returns:;; A statistical description of the insertion and lookup operations performed by the container so far. Notes:;; Only available if xref:reference/stats.adoc#stats[statistics calculation] is xref:unordered_flat_set_boost_unordered_enable_stats[enabled].

en	---

en	reset_stats

en	```c++ void reset_stats() noexcept; ```

en	Effects:;; Sets to zero the internal statistics kept by the container. Notes:;; Only available if xref:reference/stats.adoc#stats[statistics calculation] is xref:unordered_flat_set_boost_unordered_enable_stats[enabled].

en	---

en	Deduction Guides

en	A deduction guide will not participate in overload resolution if any of the following are true:

en	- It has an `InputIterator` template parameter and a type that does not qualify as an input iterator is deduced for that parameter. - It has an `Allocator` template parameter and a type that does not qualify as an allocator is deduced for that parameter. - It has a `Hash` template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter. - It has a `Pred` template parameter and a type that qualifies as an allocator is deduced for that parameter.

en	A `size_­type` parameter type in a deduction guide refers to the `size_­type` member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

en	__iter-value-type__

en	Equality Comparisons

en	operator

en	```c++ template<class Key, class T, class Hash, class Pred, class Alloc> bool operator==(const unordered_flat_set<Key, T, Hash, Pred, Alloc>& x, const unordered_flat_set<Key, T, Hash, Pred, Alloc>& y); ```

en	Return `true` if `x.size() == y.size()` and for every element in `x`, there is an element in `y` with the same key, with an equal value (using `operator==` to compare the value types).

en	Notes:;; Behavior is undefined if the two containers don't have equivalent equality predicates.

en	---

en	operator!

en	```c++ template<class Key, class T, class Hash, class Pred, class Alloc> bool operator!=(const unordered_flat_set<Key, T, Hash, Pred, Alloc>& x, const unordered_flat_set<Key, T, Hash, Pred, Alloc>& y); ```

en	Return `false` if `x.size() == y.size()` and for every element in `x`, there is an element in `y` with the same key, with an equal value (using `operator==` to compare the value types).

en	Notes:;; Behavior is undefined if the two containers don't have equivalent equality predicates.

en	Swap

en	```c++ template<class Key, class T, class Hash, class Pred, class Alloc> void swap(unordered_flat_set<Key, T, Hash, Pred, Alloc>& x, unordered_flat_set<Key, T, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); ```

en	Swaps the contents of `x` and `y`.

en	If `Allocator::propagate_on_container_swap` is declared and `Allocator::propagate_on_container_swap::value` is `true` then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

en	Effects:;; `x.swap(y)` Throws:;; Nothing unless `key_equal` or `hasher` throw on swapping.

en	---

en	erase_if

en	```c++ template<class K, class T, class H, class P, class A, class Predicate> typename unordered_flat_set<K, T, H, P, A>::size_type erase_if(unordered_flat_set<K, T, H, P, A>& c, Predicate pred); ```

en	Traverses the container `c` and removes all elements for which the supplied predicate returns `true`.

en	Returns:;; The number of erased elements. Notes:;; Equivalent to: + + ```c++ auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size(); ```

en	Serialization

en	``unordered_flat_set``s can be archived/retrieved by means of link:../../../../../serialization/index.html[Boost.Serialization^] using the API provided by this library. Both regular and XML archives are supported.

en	Saving an unordered_flat_set to an archive

en	Saves all the elements of an `unordered_flat_set` `x` to an archive (XML archive) `ar`.

en	Requires:;; `value_type` is serializable (XML serializable), and it supports Boost.Serialization `save_construct_data`/`load_construct_data` protocol (automatically suported by https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^] types).

en	---

en	Loading an unordered_flat_set from an archive

en	Deletes all preexisting elements of an `unordered_flat_set` `x` and inserts from an archive (XML archive) `ar` restored copies of the elements of the original `unordered_flat_set` `other` saved to the storage read by `ar`.

en	Requires:;; `x.key_equal()` is functionally equivalent to `other.key_equal()`.

en	---

en	Saving an iterator/const_iterator to an archive

en	Saves the positional information of an `iterator` (`const_iterator`) `it` to an archive (XML archive) `ar`. `it` can be and `end()` iterator.

en	Requires:;; The `unordered_flat_set` `x` pointed to by `it` has been previously saved to `ar`, and no modifying operations have been issued on `x` between saving of `x` and saving of `it`.

en	---

en	Loading an iterator/const_iterator from an archive

en	Makes an `iterator` (`const_iterator`) `it` point to the restored position of the original `iterator` (`const_iterator`) saved to the storage read by an archive (XML archive) `ar`.

en	Requires:;; If `x` is the `unordered_flat_set` `it` points to, no modifying operations have been issued on `x` between loading of `x` and loading of `it`.