[#unordered_flat_set]
== Class Template unordered_flat_set

:idprefix: unordered_flat_set_

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

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_.

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`: 

  - `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.

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

=== Synopsis

[listing,subs="+macros,+quotes"]
-----
// #include <boost/unordered/unordered_flat_set.hpp>

namespace boost {
  template<class Key,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<Key>>
  class unordered_flat_set {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    using init_type            = Key;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename std::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename std::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = std::size_t;
    using difference_type      = std::ptrdiff_t;

    using iterator             = _implementation-defined_;
    using const_iterator       = _implementation-defined_;

    // construct/copy/destroy
    xref:#unordered_flat_set_default_constructor[unordered_flat_set]();
    explicit xref:#unordered_flat_set_bucket_count_constructor[unordered_flat_set](size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      xref:#unordered_flat_set_iterator_range_constructor[unordered_flat_set](InputIterator f, InputIterator l,
                         size_type n = _implementation-defined_,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    xref:#unordered_flat_set_copy_constructor[unordered_flat_set](const unordered_flat_set& other);
    xref:#unordered_flat_set_move_constructor[unordered_flat_set](unordered_flat_set&& other);
    template<class InputIterator>
      xref:#unordered_flat_set_iterator_range_constructor_with_allocator[unordered_flat_set](InputIterator f, InputIterator l, const allocator_type& a);
    explicit xref:#unordered_flat_set_allocator_constructor[unordered_flat_set](const Allocator& a);
    xref:#unordered_flat_set_copy_constructor_with_allocator[unordered_flat_set](const unordered_flat_set& other, const Allocator& a);
    xref:#unordered_flat_set_move_constructor_from_concurrent_flat_set[unordered_flat_set](concurrent_flat_set<Key, Hash, Pred, Allocator>&& other);
    xref:#unordered_flat_set_initializer_list_constructor[unordered_flat_set](std::initializer_list<value_type> il,
                       size_type n = _implementation-defined_
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    xref:#unordered_flat_set_bucket_count_constructor_with_allocator[unordered_flat_set](size_type n, const allocator_type& a);
    xref:#unordered_flat_set_bucket_count_constructor_with_hasher_and_allocator[unordered_flat_set](size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      xref:#unordered_flat_set_iterator_range_constructor_with_bucket_count_and_allocator[unordered_flat_set](InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      xref:#unordered_flat_set_iterator_range_constructor_with_bucket_count_and_hasher[unordered_flat_set](InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
    xref:#unordered_flat_set_initializer_list_constructor_with_allocator[unordered_flat_set](std::initializer_list<value_type> il, const allocator_type& a);
    xref:#unordered_flat_set_initializer_list_constructor_with_bucket_count_and_allocator[unordered_flat_set](std::initializer_list<value_type> il, size_type n,
                       const allocator_type& a);
    xref:#unordered_flat_set_initializer_list_constructor_with_bucket_count_and_hasher_and_allocator[unordered_flat_set](std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);
    xref:#unordered_flat_set_destructor[~unordered_flat_set]();
    unordered_flat_set& xref:#unordered_flat_set_copy_assignment[operator++=++](const unordered_flat_set& other);
    unordered_flat_set& xref:#unordered_flat_set_move_assignment[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);++
    unordered_flat_set& xref:#unordered_flat_set_initializer_list_assignment[operator++=++](std::initializer_list<value_type>);
    allocator_type xref:#unordered_flat_set_get_allocator[get_allocator]() const noexcept;

    // iterators
    iterator       xref:#unordered_flat_set_begin[begin]() noexcept;
    const_iterator xref:#unordered_flat_set_begin[begin]() const noexcept;
    iterator       xref:#unordered_flat_set_end[end]() noexcept;
    const_iterator xref:#unordered_flat_set_end[end]() const noexcept;
    const_iterator xref:#unordered_flat_set_cbegin[cbegin]() const noexcept;
    const_iterator xref:#unordered_flat_set_cend[cend]() const noexcept;

    // capacity
    ++[[nodiscard]]++ bool xref:#unordered_flat_set_empty[empty]() const noexcept;
    size_type xref:#unordered_flat_set_size[size]() const noexcept;
    size_type xref:#unordered_flat_set_max_size[max_size]() const noexcept;

    // modifiers
    template<class... Args> std::pair<iterator, bool> xref:#unordered_flat_set_emplace[emplace](Args&&... args);
    template<class... Args> iterator xref:#unordered_flat_set_emplace_hint[emplace_hint](const_iterator position, Args&&... args);
    std::pair<iterator, bool> xref:#unordered_flat_set_copy_insert[insert](const value_type& obj);
    std::pair<iterator, bool> xref:#unordered_flat_set_move_insert[insert](value_type&& obj);
    template<class K> std::pair<iterator, bool> xref:#unordered_flat_set_transparent_insert[insert](K&& k);
    iterator xref:#unordered_flat_set_copy_insert_with_hint[insert](const_iterator hint, const value_type& obj);
    iterator xref:#unordered_flat_set_move_insert_with_hint[insert](const_iterator hint, value_type&& obj);
    template<class K> iterator xref:#unordered_flat_set_transparent_insert_with_hint[insert](const_iterator hint, K&& k);
    template<class InputIterator> void xref:#unordered_flat_set_insert_iterator_range[insert](InputIterator first, InputIterator last);
    void xref:#unordered_flat_set_insert_initializer_list[insert](std::initializer_list<value_type>);

    _convertible-to-iterator_     xref:#unordered_flat_set_erase_by_position[erase](iterator position);
    _convertible-to-iterator_     xref:#unordered_flat_set_erase_by_position[erase](const_iterator position);
    size_type                   xref:#unordered_flat_set_erase_by_key[erase](const key_type& k);
    template<class K> size_type xref:#unordered_flat_set_erase_by_key[erase](K&& k);
    iterator  xref:#unordered_flat_set_erase_range[erase](const_iterator first, const_iterator last);
    void      xref:#unordered_flat_set_swap[swap](unordered_flat_set& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
               boost::allocator_traits<Allocator>::propagate_on_container_swap::value);
    void      xref:#unordered_flat_set_clear[clear]() noexcept;

    template<class H2, class P2>
      void xref:#unordered_flat_set_merge[merge](unordered_flat_set<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void xref:#unordered_flat_set_merge[merge](unordered_flat_set<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher xref:#unordered_flat_set_hash_function[hash_function]() const;
    key_equal xref:#unordered_flat_set_key_eq[key_eq]() const;

    // set operations
    iterator         xref:#unordered_flat_set_find[find](const key_type& k);
    const_iterator   xref:#unordered_flat_set_find[find](const key_type& k) const;
    template<class K>
      iterator       xref:#unordered_flat_set_find[find](const K& k);
    template<class K>
      const_iterator xref:#unordered_flat_set_find[find](const K& k) const;
    size_type        xref:#unordered_flat_set_count[count](const key_type& k) const;
    template<class K>
      size_type      xref:#unordered_flat_set_count[count](const K& k) const;
    bool             xref:#unordered_flat_set_contains[contains](const key_type& k) const;
    template<class K>
      bool           xref:#unordered_flat_set_contains[contains](const K& k) const;
    std::pair<iterator, iterator>               xref:#unordered_flat_set_equal_range[equal_range](const key_type& k);
    std::pair<const_iterator, const_iterator>   xref:#unordered_flat_set_equal_range[equal_range](const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             xref:#unordered_flat_set_equal_range[equal_range](const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> xref:#unordered_flat_set_equal_range[equal_range](const K& k) const;

    // bucket interface
    size_type xref:#unordered_flat_set_bucket_count[bucket_count]() const noexcept;

    // hash policy
    float xref:#unordered_flat_set_load_factor[load_factor]() const noexcept;
    float xref:#unordered_flat_set_max_load_factor[max_load_factor]() const noexcept;
    void xref:#unordered_flat_set_set_max_load_factor[max_load_factor](float z);
    size_type xref:#unordered_flat_set_max_load[max_load]() const noexcept;
    void xref:#unordered_flat_set_rehash[rehash](size_type n);
    void xref:#unordered_flat_set_reserve[reserve](size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>>,
           class Pred = std::equal_to<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>>,
           class Allocator = std::allocator<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>>>
    unordered_flat_set(InputIterator, InputIterator, typename xref:#unordered_flat_set_deduction_guides[__see below__]::size_type = xref:#unordered_flat_set_deduction_guides[__see below__],
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_flat_set<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>, Hash, Pred, Allocator>;

  template<class T, class Hash = boost::hash<T>, class Pred = std::equal_to<T>,
           class Allocator = std::allocator<T>>
    unordered_flat_set(std::initializer_list<T>, typename xref:#unordered_flat_set_deduction_guides[__see below__]::size_type = xref:#unordered_flat_set_deduction_guides[__see below__],
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_flat_set<T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_flat_set(InputIterator, InputIterator, typename xref:#unordered_flat_set_deduction_guides[__see below__]::size_type, Allocator)
      -> unordered_flat_set<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>,
                            boost::hash<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>>,
                            std::equal_to<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_flat_set(InputIterator, InputIterator, Allocator)
      -> unordered_flat_set<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>,
                            boost::hash<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>>,
                            std::equal_to<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_flat_set(InputIterator, InputIterator, typename xref:#unordered_flat_set_deduction_guides[__see below__]::size_type, Hash,
                       Allocator)
      -> unordered_flat_set<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>, Hash,
                            std::equal_to<xref:#unordered_flat_set_iter_value_type[__iter-value-type__]<InputIterator>>, Allocator>;

  template<class T, class Allocator>
    unordered_flat_set(std::initializer_list<T>, typename xref:#unordered_flat_set_deduction_guides[__see below__]::size_type, Allocator)
      -> unordered_flat_set<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Allocator>
    unordered_flat_set(std::initializer_list<T>, Allocator)
      -> unordered_flat_set<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Hash, class Allocator>
    unordered_flat_set(std::initializer_list<T>, typename xref:#unordered_flat_set_deduction_guides[__see below__]::size_type, Hash, Allocator)
      -> unordered_flat_set<T, Hash, std::equal_to<T>, Allocator>;

  // Equality Comparisons
  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool xref:#unordered_flat_set_operator_2[operator==](const unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_flat_set<Key, T, Hash, Pred, Alloc>& y);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool xref:#unordered_flat_set_operator_3[operator!=](const unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_flat_set<Key, T, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void xref:#unordered_flat_set_swap_2[swap](unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
              unordered_flat_set<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  // Erasure
  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_flat_set<K, T, H, P, A>::size_type
       xref:#unordered_flat_set_erase_if[erase_if](unordered_flat_set<K, T, H, P, A>& c, Predicate pred);
}
-----

---

=== Description

*Template Parameters*

[cols="1,1"]
|===

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

|_Hash_
|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`.

|_Pred_
|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`.

|_Allocator_
|An allocator whose value type is the same as the container's value type.
Allocators using https://en.cppreference.com/w/cpp/named_req/Allocator#Fancy_pointers[fancy pointers] are supported.

|===

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.

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.

If `xref:hash_traits_hash_is_avalanching[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.

---

=== Typedefs

[source,c++,subs=+quotes]
----
typedef _implementation-defined_ iterator;
----

A constant iterator whose value type is `value_type`.

The iterator category is at least a forward iterator.

Convertible to `const_iterator`.

---

[source,c++,subs=+quotes]
----
typedef _implementation-defined_ const_iterator;
----

A constant iterator whose value type is `value_type`.

The iterator category is at least a forward iterator.

=== Constructors

==== Default Constructor
```c++
unordered_flat_set();
```

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

[horizontal]
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^].

---

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

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.

[horizontal]
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^].

---

==== Iterator Range Constructor
[source,c++,subs="+quotes"]
----
template<class InputIterator>
  unordered_flat_set(InputIterator f, InputIterator l,
                     size_type n = _implementation-defined_,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());
----

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.

[horizontal]
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^].

---

==== Copy Constructor
```c++
unordered_flat_set(unordered_flat_set const& other);
```

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

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

[horizontal]
Requires:;; `value_type` is copy constructible

---

==== Move Constructor
```c++
unordered_flat_set(unordered_flat_set&& other);
```

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`.

---

==== Iterator Range Constructor with Allocator
```c++
template<class InputIterator>
  unordered_flat_set(InputIterator f, InputIterator l, const allocator_type& a);
```

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.

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

---

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

Constructs an empty container, using allocator `a`.

---

==== Copy Constructor with Allocator
```c++
unordered_flat_set(unordered_flat_set const& other, Allocator const& a);
```

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

---

==== Move Constructor with Allocator
```c++
unordered_flat_set(unordered_flat_set&& other, Allocator const& a);
```

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`.

---

==== Move Constructor from concurrent_flat_set

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

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`.

[horizontal]
Complexity:;; Constant time.
Concurrency:;; Blocking on `other`.

---

==== Initializer List Constructor
[source,c++,subs="+quotes"]
----
unordered_flat_set(std::initializer_list<value_type> il,
              size_type n = _implementation-defined_
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());
----

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.

[horizontal]
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^].

---

==== Bucket Count Constructor with Allocator
```c++
unordered_flat_set(size_type n, allocator_type const& a);
```

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.

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

---

==== Bucket Count Constructor with Hasher and Allocator
```c++
unordered_flat_set(size_type n, hasher const& hf, allocator_type const& a);
```

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.

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

---

==== Iterator Range Constructor with Bucket Count and Allocator
[source,c++,subs="+quotes"]
----
template<class InputIterator>
  unordered_flat_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
----

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.

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

---

==== Iterator Range Constructor with Bucket Count and Hasher
[source,c++,subs="+quotes"]
----
    template<class InputIterator>
      unordered_flat_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
----

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.

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

---

==== initializer_list Constructor with Allocator

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

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

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

---

==== initializer_list Constructor with Bucket Count and Allocator

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

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.

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

---

==== initializer_list Constructor with Bucket Count and Hasher and Allocator

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

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.

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

---

=== Destructor

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

[horizontal]
Note:;; The destructor is applied to every element, and all memory is deallocated

---

=== Assignment

==== Copy Assignment

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

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`.

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

---

==== Move Assignment
```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`.

---

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

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

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

=== Iterators

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

[horizontal]
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()`)

---

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

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

---

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

[horizontal]
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()`)

---

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

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

---

=== Size and Capacity

==== empty

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

[horizontal]
Returns:;; `size() == 0`

---

==== size

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

[horizontal]
Returns:;; `std::distance(begin(), end())`

---

==== max_size

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

[horizontal]
Returns:;; `size()` of the largest possible container.

---

=== Modifiers

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

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.

[horizontal]
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. +

---

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

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.

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

[horizontal]
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. +

---

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

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

[horizontal]
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.

---

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

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

[horizontal]
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.

---

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

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.

[horizontal]
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.

---

==== Copy Insert with Hint
```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.

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

[horizontal]
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.

---

==== Move Insert with Hint
```c++
iterator insert(const_iterator hint, value_type&& obj);
```

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

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

[horizontal]
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.

---

==== Transparent Insert with Hint
```c++
template<class K> std::pair<iterator, bool> insert(const_iterator hint, K&& k);
```

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.

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

[horizontal]
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.

---

==== Insert Iterator Range
```c++
template<class InputIterator> void insert(InputIterator first, InputIterator last);
```

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.

[horizontal]
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.

---

==== Insert Initializer List
```c++
void insert(std::initializer_list<value_type>);
```

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.

[horizontal]
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.

---

==== Erase by Position

[source,c++,subs=+quotes]
----
_convertible-to-iterator_ erase(iterator position);
_convertible-to-iterator_ erase(const_iterator position);
----

Erase the element pointed to by `position`.

[horizontal]
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`.

---

==== Erase by Key
```c++
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);
```

Erase all elements with key equivalent to `k`.

[horizontal]
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.

---

==== Erase Range

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

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

[horizontal]
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).

---

==== swap
```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);
```

Swaps the contents of the container with the parameter.

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.

[horizontal]
Throws:;; Nothing unless `key_equal` or `hasher` throw on swapping.

---

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

Erases all elements in the container.

[horizontal]
Postconditions:;; `size() == 0`, `max_load() >= max_load_factor() * bucket_count()`

---

==== merge
```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);
```

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

---

=== Observers

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

[horizontal]
Returns:;; The container's allocator.

---

==== hash_function
```
hasher hash_function() const;
```

[horizontal]
Returns:;; The container's hash function.

---

==== key_eq
```
key_equal key_eq() const;
```

[horizontal]
Returns:;; The container's key equality predicate

---

=== Lookup

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

```

[horizontal]
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.

---

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

[horizontal]
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.

---

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

[horizontal]
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.

---

==== equal_range
```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;
```

[horizontal]
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.

---

=== Bucket Interface

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

[horizontal]
Returns:;; The size of the bucket array.

---

=== Hash Policy

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

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

---

==== max_load_factor

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

[horizontal]
Returns:;; Returns the container's maximum load factor.

---

==== Set max_load_factor
```c++
void max_load_factor(float z);
```

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

---


==== max_load

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

[horizontal]
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.

---

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

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.

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

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

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

---

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

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

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

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

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

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

  - 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.

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.

==== __iter-value-type__
[listings,subs="+macros,+quotes"]
-----
template<class InputIterator>
  using __iter-value-type__ =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only
-----

=== Equality Comparisons

==== operator==
```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);
```

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).

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

---

==== operator!=
```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);
```

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).

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

=== Swap
```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)));
```

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

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.

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

---

=== erase_if
```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);
```

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

[horizontal]
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();
```

=== Serialization

``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. 

==== Saving an unordered_flat_set to an archive

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

[horizontal]
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).  

---

==== Loading an unordered_flat_set from an archive

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`.

[horizontal]
Requires:;; `x.key_equal()` is functionally equivalent to `other.key_equal()`.

---

==== Saving an iterator/const_iterator to an archive

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

[horizontal]
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`.

---

==== Loading an iterator/const_iterator from an archive

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`.

[horizontal]
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`.
