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,
|`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.
`std::allocator_traits<Allocator>::pointer` and `std::allocator_traits<Allocator>::const_pointer`
must be convertible to/from `value_type*` and `const value_type*`, respectively.
|===
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^].
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^].
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^].
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`.
---
==== 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 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
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^]
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`.
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.
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.
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.
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.
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 <typename 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 <typename 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 <typename 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.
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 <typename 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.
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.
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.
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>
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).
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>
