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## Class template hashtable

boost::intrusive::hashtable

## Synopsis

    // In header: <boost/intrusive/hashtable.hpp>

    template<typename T, class... Options>
    class hashtable {
    public:
      // types
      typedef Config::value_traits                                  value_traits;
      typedef real_value_traits::pointer                            pointer;
      typedef real_value_traits::const_pointer                      const_pointer;
      typedef real_value_traits::value_type                         value_type;
      typedef std::iterator_traits< pointer >::reference            reference;
      typedef std::iterator_traits< const_pointer >::reference      const_reference;
      typedef std::iterator_traits< pointer >::difference_type      difference_type;
      typedef Config::size_type                                     size_type;
      typedef value_type                                            key_type;
      typedef Config::equal                                         key_equal;
      typedef Config::hash                                          hasher;
      typedef unspecified                                           bucket_type;
      typedef boost::pointer_to_other< pointer, bucket_type >::type bucket_ptr;
      typedef slist::iterator                                       siterator;
      typedef slist::const_iterator                                 const_siterator;
      typedef unspecified                                           iterator;
      typedef unspecified                                           const_iterator;
      typedef real_value_traits::node_traits                        node_traits;
      typedef node_traits::node                                     node;
      typedef boost::pointer_to_other< pointer, node >::type        node_ptr;
      typedef boost::pointer_to_other< node_ptr, const node >::type const_node_ptr;
      typedef slist::node_algorithms                                node_algorithms;
      typedef unspecified                                           insert_commit_data;
      typedef unspecified                                           local_iterator;
      typedef unspecified                                           const_local_iterator;

      // construct/copy/destruct
      hashtable(const bucket_traits &, const hasher & = hasher(),
                const key_equal & = key_equal(),
                const value_traits & = value_traits());
      ~hashtable();

      // public member functions
      const real_value_traits & get_real_value_traits() const;
      real_value_traits & get_real_value_traits();
      iterator begin();
      const_iterator begin() const;
      const_iterator cbegin() const;
      iterator end();
      const_iterator end() const;
      const_iterator cend() const;
      hasher hash_function() const;
      key_equal key_eq() const;
      bool empty() const;
      size_type size() const;
      void swap(hashtable &);
      template<typename Cloner, typename Disposer>
        void clone_from(const hashtable &, Cloner, Disposer);
      iterator insert_equal(reference);
      template<typename Iterator> void insert_equal(Iterator, Iterator);
      std::pair< iterator, bool > insert_unique(reference);
      template<typename Iterator> void insert_unique(Iterator, Iterator);
      template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
        std::pair< iterator, bool >
        insert_unique_check(const KeyType &, KeyHasher, KeyValueEqual,
                            insert_commit_data &);
      iterator insert_unique_commit(reference, const insert_commit_data &);
      void erase(const_iterator);
      void erase(const_iterator, const_iterator);
      size_type erase(const_reference);
      template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
        size_type erase(const KeyType &, KeyHasher, KeyValueEqual);
      template<typename Disposer> void erase_and_dispose(const_iterator, Disposer);
      template<typename Disposer>
        void erase_and_dispose(const_iterator, const_iterator, Disposer);
      template<typename Disposer>
        size_type erase_and_dispose(const_reference, Disposer);
      template<typename KeyType, typename KeyHasher, typename KeyValueEqual,
               typename Disposer>
        size_type erase_and_dispose(const KeyType &, KeyHasher, KeyValueEqual,
                                    Disposer);
      void clear();
      template<typename Disposer> void clear_and_dispose(Disposer);
      size_type count(const_reference) const;
      template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
        size_type count(const KeyType &, const KeyHasher &, const KeyValueEqual &) const;
      iterator find(const_reference);
      template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
        iterator find(const KeyType &, KeyHasher, KeyValueEqual);
      const_iterator find(const_reference) const;
      template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
        const_iterator find(const KeyType &, KeyHasher, KeyValueEqual) const;
      std::pair< iterator, iterator > equal_range(const_reference);
      template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
        std::pair< iterator, iterator >
        equal_range(const KeyType &, KeyHasher, KeyValueEqual);
      std::pair< const_iterator, const_iterator >
      equal_range(const_reference) const;
      template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
        std::pair< const_iterator, const_iterator >
        equal_range(const KeyType &, KeyHasher, KeyValueEqual) const;
      iterator iterator_to(reference);
      const_iterator iterator_to(const_reference) const;
      local_iterator local_iterator_to(reference);
      const_local_iterator local_iterator_to(const_reference) const;
      size_type bucket_count() const;
      size_type bucket_size(size_type) const;
      size_type bucket(const key_type &) const;
      template<typename KeyType, typename KeyHasher>
        size_type bucket(const KeyType &, const KeyHasher &) const;
      bucket_ptr bucket_pointer() const;
      local_iterator begin(size_type);
      const_local_iterator begin(size_type) const;
      const_local_iterator cbegin(size_type) const;
      local_iterator end(size_type);
      const_local_iterator end(size_type) const;
      const_local_iterator cend(size_type) const;
      void rehash(const bucket_traits &);
      bool incremental_rehash(bool = true);
      bool incremental_rehash(const bucket_traits &);
      size_type split_count() const;

      // public static functions
      static local_iterator s_local_iterator_to(reference);
      static const_local_iterator s_local_iterator_to(const_reference);
      static size_type suggested_upper_bucket_count(size_type);
      static size_type suggested_lower_bucket_count(size_type);
      static const bool stateful_value_traits;
      static const bool store_hash;
      static const bool unique_keys;
      static const bool constant_time_size;
      static const bool cache_begin;
      static const bool compare_hash;
      static const bool incremental;
      static const bool power_2_buckets;
      static const bool optimize_multikey;
    };

## Description

The class template hashtable is an intrusive hash table container, that is used to construct intrusive unordered_set and unordered_multiset containers. The no-throw guarantee holds only, if the Equal object and Hasher don't throw.

hashtable is a semi-intrusive container: each object to be stored in the container must contain a proper hook, but the container also needs additional auxiliary memory to work: hashtable needs a pointer to an array of type `bucket_type` to be passed in the constructor. This bucket array must have at least the same lifetime as the container. This makes the use of hashtable more complicated than purely intrusive containers. `bucket_type` is default-constructible, copyable and assignable

The template parameter `T` is the type to be managed by the container. The user can specify additional options and if no options are provided default options are used.

The container supports the following options: `base_hook<>/member_hook<>/value_traits<>`, `constant_time_size<>`, `size_type<>`, `hash<>` and `equal<>` `bucket_traits<>`, power_2_buckets<>, cache_begin<> and incremental<>.

hashtable only provides forward iterators but it provides 4 iterator types: iterator and const_iterator to navigate through the whole container and local_iterator and const_local_iterator to navigate through the values stored in a single bucket. Local iterators are faster and smaller.

It's not recommended to use non constant-time size hashtables because several key functions, like "empty()", become non-constant time functions. Non constant_time size hashtables are mainly provided to support auto-unlink hooks.

hashtables, does not make automatic rehashings nor offers functions related to a load factor. Rehashing can be explicitly requested and the user must provide a new bucket array that will be used from that moment.

Since no automatic rehashing is done, iterators are never invalidated when inserting or erasing elements. Iterators are only invalidated when rehashing.

### `hashtable` public construct/copy/destruct

1.     hashtable(const bucket_traits & b_traits, const hasher & hash_func = hasher(),
                 const key_equal & equal_func = key_equal(),
                 const value_traits & v_traits = value_traits());

   Requires: buckets must not be being used by any other resource.

   Effects: Constructs an empty unordered_set, storing a reference to the bucket array and copies of the key_hasher and equal_func functors.

   Complexity: Constant.

   Throws: If value_traits::node_traits::node constructor throws (this does not happen with predefined Boost.Intrusive hooks) or the copy constructor or invocation of hash_func or equal_func throws.

   Notes: buckets array must be disposed only after this is disposed.

2.     ~hashtable();

   Effects: Detaches all elements from this. The objects in the unordered_set are not deleted (i.e. no destructors are called).

   Complexity: Linear to the number of elements in the unordered_set, if it's a safe-mode or auto-unlink value. Otherwise constant.

   Throws: Nothing.

### `hashtable` public member functions

1.     const real_value_traits & get_real_value_traits() const;

2.     real_value_traits & get_real_value_traits();

3.     iterator begin();

   Effects: Returns an iterator pointing to the beginning of the unordered_set.

   Complexity: Amortized constant time. Worst case (empty unordered_set): O(this->bucket_count())

   Throws: Nothing.

4.     const_iterator begin() const;

   Effects: Returns a const_iterator pointing to the beginning of the unordered_set.

   Complexity: Amortized constant time. Worst case (empty unordered_set): O(this->bucket_count())

   Throws: Nothing.

5.     const_iterator cbegin() const;

   Effects: Returns a const_iterator pointing to the beginning of the unordered_set.

   Complexity: Amortized constant time. Worst case (empty unordered_set): O(this->bucket_count())

   Throws: Nothing.

6.     iterator end();

   Effects: Returns an iterator pointing to the end of the unordered_set.

   Complexity: Constant.

   Throws: Nothing.

7.     const_iterator end() const;

   Effects: Returns a const_iterator pointing to the end of the unordered_set.

   Complexity: Constant.

   Throws: Nothing.

8.     const_iterator cend() const;

   Effects: Returns a const_iterator pointing to the end of the unordered_set.

   Complexity: Constant.

   Throws: Nothing.

9.     hasher hash_function() const;

   Effects: Returns the hasher object used by the unordered_set.

   Complexity: Constant.

   Throws: If hasher copy-constructor throws.

10.     key_equal key_eq() const;

    Effects: Returns the key_equal object used by the unordered_set.

    Complexity: Constant.

    Throws: If key_equal copy-constructor throws.

11.     bool empty() const;

    Effects: Returns true if the container is empty.

    Complexity: if constant-time size and cache_begin options are disabled, average constant time (worst case, with empty() == true: O(this->bucket_count()). Otherwise constant.

    Throws: Nothing.

12.     size_type size() const;

    Effects: Returns the number of elements stored in the unordered_set.

    Complexity: Linear to elements contained in *this if constant_time_size is false. Constant-time otherwise.

    Throws: Nothing.

13.     void swap(hashtable & other);

    Requires: the hasher and the equality function unqualified swap call should not throw.

    Effects: Swaps the contents of two unordered_sets. Swaps also the contained bucket array and equality and hasher functors.

    Complexity: Constant.

    Throws: If the swap() call for the comparison or hash functors found using ADL throw. Basic guarantee.

14.     template<typename Cloner, typename Disposer>
          void clone_from(const hashtable & src, Cloner cloner, Disposer disposer);

    Requires: Disposer::operator()(pointer) shouldn't throw Cloner should yield to nodes that compare equal and produce the same hash than the original node.

    Effects: Erases all the elements from *this calling Disposer::operator()(pointer), clones all the elements from src calling Cloner::operator()(const_reference ) and inserts them on *this. The hash function and the equality predicate are copied from the source.

    If store_hash option is true, this method does not use the hash function.

    If any operation throws, all cloned elements are unlinked and disposed calling Disposer::operator()(pointer).

    Complexity: Linear to erased plus inserted elements.

    Throws: If cloner or hasher throw or hash or equality predicate copying throws. Basic guarantee.

15.     iterator insert_equal(reference value);

    Requires: value must be an lvalue

    Effects: Inserts the value into the unordered_set.

    Returns: An iterator to the inserted value.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws. Strong guarantee.

    Note: Does not affect the validity of iterators and references. No copy-constructors are called.

16.     template<typename Iterator> void insert_equal(Iterator b, Iterator e);

    Requires: Dereferencing iterator must yield an lvalue of type value_type.

    Effects: Equivalent to this->insert_equal(t) for each element in [b, e).

    Complexity: Average case O(N), where N is std::distance(b, e). Worst case O(N*this->size()).

    Throws: If the internal hasher or the equality functor throws. Basic guarantee.

    Note: Does not affect the validity of iterators and references. No copy-constructors are called.

17.     std::pair< iterator, bool > insert_unique(reference value);

    Requires: value must be an lvalue

    Effects: Tries to inserts value into the unordered_set.

    Returns: If the value is not already present inserts it and returns a pair containing the iterator to the new value and true. If there is an equivalent value returns a pair containing an iterator to the already present value and false.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws. Strong guarantee.

    Note: Does not affect the validity of iterators and references. No copy-constructors are called.

18.     template<typename Iterator> void insert_unique(Iterator b, Iterator e);

    Requires: Dereferencing iterator must yield an lvalue of type value_type.

    Effects: Equivalent to this->insert_unique(t) for each element in [b, e).

    Complexity: Average case O(N), where N is std::distance(b, e). Worst case O(N*this->size()).

    Throws: If the internal hasher or the equality functor throws. Basic guarantee.

    Note: Does not affect the validity of iterators and references. No copy-constructors are called.

19.     template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
          std::pair< iterator, bool >
          insert_unique_check(const KeyType & key, KeyHasher hash_func,
                              KeyValueEqual equal_func,
                              insert_commit_data & commit_data);

    Requires: "hash_func" must be a hash function that induces the same hash values as the stored hasher. The difference is that "hash_func" hashes the given key instead of the value_type.

    "equal_func" must be a equality function that induces the same equality as key_equal. The difference is that "equal_func" compares an arbitrary key with the contained values.

    Effects: Checks if a value can be inserted in the unordered_set, using a user provided key instead of the value itself.

    Returns: If there is an equivalent value returns a pair containing an iterator to the already present value and false. If the value can be inserted returns true in the returned pair boolean and fills "commit_data" that is meant to be used with the "insert_commit" function.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If hash_func or equal_func throw. Strong guarantee.

    Notes: This function is used to improve performance when constructing a value_type is expensive: if there is an equivalent value the constructed object must be discarded. Many times, the part of the node that is used to impose the hash or the equality is much cheaper to construct than the value_type and this function offers the possibility to use that the part to check if the insertion will be successful.

    If the check is successful, the user can construct the value_type and use "insert_commit" to insert the object in constant-time.

    "commit_data" remains valid for a subsequent "insert_commit" only if no more objects are inserted or erased from the unordered_set.

    After a successful rehashing insert_commit_data remains valid.

20.     iterator insert_unique_commit(reference value,
                                      const insert_commit_data & commit_data);

    Requires: value must be an lvalue of type value_type. commit_data must have been obtained from a previous call to "insert_check". No objects should have been inserted or erased from the unordered_set between the "insert_check" that filled "commit_data" and the call to "insert_commit".

    Effects: Inserts the value in the unordered_set using the information obtained from the "commit_data" that a previous "insert_check" filled.

    Returns: An iterator to the newly inserted object.

    Complexity: Constant time.

    Throws: Nothing.

    Notes: This function has only sense if a "insert_check" has been previously executed to fill "commit_data". No value should be inserted or erased between the "insert_check" and "insert_commit" calls.

    After a successful rehashing insert_commit_data remains valid.

21.     void erase(const_iterator i);

    Effects: Erases the element pointed to by i.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: Nothing.

    Note: Invalidates the iterators (but not the references) to the erased element. No destructors are called.

22.     void erase(const_iterator b, const_iterator e);

    Effects: Erases the range pointed to by b end e.

    Complexity: Average case O(std::distance(b, e)), worst case O(this->size()).

    Throws: Nothing.

    Note: Invalidates the iterators (but not the references) to the erased elements. No destructors are called.

23.     size_type erase(const_reference value);

    Effects: Erases all the elements with the given value.

    Returns: The number of erased elements.

    Complexity: Average case O(this->count(value)). Worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws. Basic guarantee.

    Note: Invalidates the iterators (but not the references) to the erased elements. No destructors are called.

24.     template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
          size_type erase(const KeyType & key, KeyHasher hash_func,
                          KeyValueEqual equal_func);

    Requires: "hash_func" must be a hash function that induces the same hash values as the stored hasher. The difference is that "hash_func" hashes the given key instead of the value_type.

    "equal_func" must be a equality function that induces the same equality as key_equal. The difference is that "equal_func" compares an arbitrary key with the contained values.

    Effects: Erases all the elements that have the same hash and compare equal with the given key.

    Returns: The number of erased elements.

    Complexity: Average case O(this->count(value)). Worst case O(this->size()).

    Throws: If hash_func or equal_func throw. Basic guarantee.

    Note: Invalidates the iterators (but not the references) to the erased elements. No destructors are called.

25.     template<typename Disposer>
          void erase_and_dispose(const_iterator i, Disposer disposer);

    Requires: Disposer::operator()(pointer) shouldn't throw.

    Effects: Erases the element pointed to by i. Disposer::operator()(pointer) is called for the removed element.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: Nothing.

    Note: Invalidates the iterators to the erased elements.

26.     template<typename Disposer>
          void erase_and_dispose(const_iterator b, const_iterator e,
                                 Disposer disposer);

    Requires: Disposer::operator()(pointer) shouldn't throw.

    Effects: Erases the range pointed to by b end e. Disposer::operator()(pointer) is called for the removed elements.

    Complexity: Average case O(std::distance(b, e)), worst case O(this->size()).

    Throws: Nothing.

    Note: Invalidates the iterators to the erased elements.

27.     template<typename Disposer>
          size_type erase_and_dispose(const_reference value, Disposer disposer);

    Requires: Disposer::operator()(pointer) shouldn't throw.

    Effects: Erases all the elements with the given value. Disposer::operator()(pointer) is called for the removed elements.

    Returns: The number of erased elements.

    Complexity: Average case O(this->count(value)). Worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws. Basic guarantee.

    Note: Invalidates the iterators (but not the references) to the erased elements. No destructors are called.

28.     template<typename KeyType, typename KeyHasher, typename KeyValueEqual,
                 typename Disposer>
          size_type erase_and_dispose(const KeyType & key, KeyHasher hash_func,
                                      KeyValueEqual equal_func, Disposer disposer);

    Requires: Disposer::operator()(pointer) shouldn't throw.

    Effects: Erases all the elements with the given key. according to the comparison functor "equal_func". Disposer::operator()(pointer) is called for the removed elements.

    Returns: The number of erased elements.

    Complexity: Average case O(this->count(value)). Worst case O(this->size()).

    Throws: If hash_func or equal_func throw. Basic guarantee.

    Note: Invalidates the iterators to the erased elements.

29.     void clear();

    Effects: Erases all of the elements.

    Complexity: Linear to the number of elements on the container. if it's a safe-mode or auto-unlink value_type. Constant time otherwise.

    Throws: Nothing.

    Note: Invalidates the iterators (but not the references) to the erased elements. No destructors are called.

30.     template<typename Disposer> void clear_and_dispose(Disposer disposer);

    Requires: Disposer::operator()(pointer) shouldn't throw.

    Effects: Erases all of the elements.

    Complexity: Linear to the number of elements on the container. Disposer::operator()(pointer) is called for the removed elements.

    Throws: Nothing.

    Note: Invalidates the iterators (but not the references) to the erased elements. No destructors are called.

31.     size_type count(const_reference value) const;

    Effects: Returns the number of contained elements with the given value

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws.

32.     template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
          size_type count(const KeyType & key, const KeyHasher & hash_func,
                          const KeyValueEqual & equal_func) const;

    Requires: "hash_func" must be a hash function that induces the same hash values as the stored hasher. The difference is that "hash_func" hashes the given key instead of the value_type.

    "equal_func" must be a equality function that induces the same equality as key_equal. The difference is that "equal_func" compares an arbitrary key with the contained values.

    Effects: Returns the number of contained elements with the given key

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If hash_func or equal throw.

33.     iterator find(const_reference value);

    Effects: Finds an iterator to the first element is equal to "value" or end() if that element does not exist.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws.

34.     template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
          iterator find(const KeyType & key, KeyHasher hash_func,
                        KeyValueEqual equal_func);

    Requires: "hash_func" must be a hash function that induces the same hash values as the stored hasher. The difference is that "hash_func" hashes the given key instead of the value_type.

    "equal_func" must be a equality function that induces the same equality as key_equal. The difference is that "equal_func" compares an arbitrary key with the contained values.

    Effects: Finds an iterator to the first element whose key is "key" according to the given hash and equality functor or end() if that element does not exist.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If hash_func or equal_func throw.

    Note: This function is used when constructing a value_type is expensive and the value_type can be compared with a cheaper key type. Usually this key is part of the value_type.

35.     const_iterator find(const_reference value) const;

    Effects: Finds a const_iterator to the first element whose key is "key" or end() if that element does not exist.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws.

36.     template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
          const_iterator
          find(const KeyType & key, KeyHasher hash_func, KeyValueEqual equal_func) const;

    Requires: "hash_func" must be a hash function that induces the same hash values as the stored hasher. The difference is that "hash_func" hashes the given key instead of the value_type.

    "equal_func" must be a equality function that induces the same equality as key_equal. The difference is that "equal_func" compares an arbitrary key with the contained values.

    Effects: Finds an iterator to the first element whose key is "key" according to the given hasher and equality functor or end() if that element does not exist.

    Complexity: Average case O(1), worst case O(this->size()).

    Throws: If hash_func or equal_func throw.

    Note: This function is used when constructing a value_type is expensive and the value_type can be compared with a cheaper key type. Usually this key is part of the value_type.

37.     std::pair< iterator, iterator > equal_range(const_reference value);

    Effects: Returns a range containing all elements with values equivalent to value. Returns std::make_pair(this->end(), this->end()) if no such elements exist.

    Complexity: Average case O(this->count(value)). Worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws.

38.     template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
          std::pair< iterator, iterator >
          equal_range(const KeyType & key, KeyHasher hash_func,
                      KeyValueEqual equal_func);

    Requires: "hash_func" must be a hash function that induces the same hash values as the stored hasher. The difference is that "hash_func" hashes the given key instead of the value_type.

    "equal_func" must be a equality function that induces the same equality as key_equal. The difference is that "equal_func" compares an arbitrary key with the contained values.

    Effects: Returns a range containing all elements with equivalent keys. Returns std::make_pair(this->end(), this->end()) if no such elements exist.

    Complexity: Average case O(this->count(key, hash_func, equal_func)). Worst case O(this->size()).

    Throws: If hash_func or the equal_func throw.

    Note: This function is used when constructing a value_type is expensive and the value_type can be compared with a cheaper key type. Usually this key is part of the value_type.

39.     std::pair< const_iterator, const_iterator >
        equal_range(const_reference value) const;

    Effects: Returns a range containing all elements with values equivalent to value. Returns std::make_pair(this->end(), this->end()) if no such elements exist.

    Complexity: Average case O(this->count(value)). Worst case O(this->size()).

    Throws: If the internal hasher or the equality functor throws.

40.     template<typename KeyType, typename KeyHasher, typename KeyValueEqual>
          std::pair< const_iterator, const_iterator >
          equal_range(const KeyType & key, KeyHasher hash_func,
                      KeyValueEqual equal_func) const;

    Requires: "hash_func" must be a hash function that induces the same hash values as the stored hasher. The difference is that "hash_func" hashes the given key instead of the value_type.

    "equal_func" must be a equality function that induces the same equality as key_equal. The difference is that "equal_func" compares an arbitrary key with the contained values.

    Effects: Returns a range containing all elements with equivalent keys. Returns std::make_pair(this->end(), this->end()) if no such elements exist.

    Complexity: Average case O(this->count(key, hash_func, equal_func)). Worst case O(this->size()).

    Throws: If the hasher or equal_func throw.

    Note: This function is used when constructing a value_type is expensive and the value_type can be compared with a cheaper key type. Usually this key is part of the value_type.

41.     iterator iterator_to(reference value);

    Requires: value must be an lvalue and shall be in a unordered_set of appropriate type. Otherwise the behavior is undefined.

    Effects: Returns: a valid iterator belonging to the unordered_set that points to the value

    Complexity: Constant.

    Throws: If the internal hash function throws.

42.     const_iterator iterator_to(const_reference value) const;

    Requires: value must be an lvalue and shall be in a unordered_set of appropriate type. Otherwise the behavior is undefined.

    Effects: Returns: a valid const_iterator belonging to the unordered_set that points to the value

    Complexity: Constant.

    Throws: If the internal hash function throws.

43.     local_iterator local_iterator_to(reference value);

    Requires: value must be an lvalue and shall be in a unordered_set of appropriate type. Otherwise the behavior is undefined.

    Effects: Returns: a valid local_iterator belonging to the unordered_set that points to the value

    Complexity: Constant.

    Throws: Nothing.

44.     const_local_iterator local_iterator_to(const_reference value) const;

    Requires: value must be an lvalue and shall be in a unordered_set of appropriate type. Otherwise the behavior is undefined.

    Effects: Returns: a valid const_local_iterator belonging to the unordered_set that points to the value

    Complexity: Constant.

    Throws: Nothing.

45.     size_type bucket_count() const;

    Effects: Returns the number of buckets passed in the constructor or the last rehash function.

    Complexity: Constant.

    Throws: Nothing.

46.     size_type bucket_size(size_type n) const;

    Requires: n is in the range [0, this->bucket_count()).

    Effects: Returns the number of elements in the nth bucket.

    Complexity: Constant.

    Throws: Nothing.

47.     size_type bucket(const key_type & k) const;

    Effects: Returns the index of the bucket in which elements with keys equivalent to k would be found, if any such element existed.

    Complexity: Constant.

    Throws: If the hash functor throws.

    Note: the return value is in the range [0, this->bucket_count()).

48.     template<typename KeyType, typename KeyHasher>
          size_type bucket(const KeyType & k, const KeyHasher & hash_func) const;

    Requires: "hash_func" must be a hash function that induces the same hash values as the stored hasher. The difference is that "hash_func" hashes the given key instead of the value_type.

    Effects: Returns the index of the bucket in which elements with keys equivalent to k would be found, if any such element existed.

    Complexity: Constant.

    Throws: If hash_func throws.

    Note: the return value is in the range [0, this->bucket_count()).

49.     bucket_ptr bucket_pointer() const;

    Effects: Returns the bucket array pointer passed in the constructor or the last rehash function.

    Complexity: Constant.

    Throws: Nothing.

50.     local_iterator begin(size_type n);

    Requires: n is in the range [0, this->bucket_count()).

    Effects: Returns a local_iterator pointing to the beginning of the sequence stored in the bucket n.

    Complexity: Constant.

    Throws: Nothing.

    Note: [this->begin(n), this->end(n)) is a valid range containing all of the elements in the nth bucket.

51.     const_local_iterator begin(size_type n) const;

    Requires: n is in the range [0, this->bucket_count()).

    Effects: Returns a const_local_iterator pointing to the beginning of the sequence stored in the bucket n.

    Complexity: Constant.

    Throws: Nothing.

    Note: [this->begin(n), this->end(n)) is a valid range containing all of the elements in the nth bucket.

52.     const_local_iterator cbegin(size_type n) const;

    Requires: n is in the range [0, this->bucket_count()).

    Effects: Returns a const_local_iterator pointing to the beginning of the sequence stored in the bucket n.

    Complexity: Constant.

    Throws: Nothing.

    Note: [this->begin(n), this->end(n)) is a valid range containing all of the elements in the nth bucket.

53.     local_iterator end(size_type n);

    Requires: n is in the range [0, this->bucket_count()).

    Effects: Returns a local_iterator pointing to the end of the sequence stored in the bucket n.

    Complexity: Constant.

    Throws: Nothing.

    Note: [this->begin(n), this->end(n)) is a valid range containing all of the elements in the nth bucket.

54.     const_local_iterator end(size_type n) const;

    Requires: n is in the range [0, this->bucket_count()).

    Effects: Returns a const_local_iterator pointing to the end of the sequence stored in the bucket n.

    Complexity: Constant.

    Throws: Nothing.

    Note: [this->begin(n), this->end(n)) is a valid range containing all of the elements in the nth bucket.

55.     const_local_iterator cend(size_type n) const;

    Requires: n is in the range [0, this->bucket_count()).

    Effects: Returns a const_local_iterator pointing to the end of the sequence stored in the bucket n.

    Complexity: Constant.

    Throws: Nothing.

    Note: [this->begin(n), this->end(n)) is a valid range containing all of the elements in the nth bucket.

56.     void rehash(const bucket_traits & new_bucket_traits);

    Requires: new_buckets must be a pointer to a new bucket array or the same as the old bucket array. new_size is the length of the the array pointed by new_buckets. If new_buckets == this->bucket_pointer() n can be bigger or smaller than this->bucket_count(). 'new_bucket_traits' copy constructor should not throw.

    Effects: Updates the internal reference with the new bucket erases the values from the old bucket and inserts then in the new one. Bucket traits hold by *this is assigned from new_bucket_traits. If the container is configured as incremental<>, the split bucket is set to the new bucket_len().

    If store_hash option is true, this method does not use the hash function.

    Complexity: Average case linear in this->size(), worst case quadratic.

    Throws: If the hasher functor throws. Basic guarantee.

57.     bool incremental_rehash(bool grow = true);

    Requires:

    Effects:

    Complexity:

    Throws:

    Note: this method is only available if incremental<true> option is activated.

58.     bool incremental_rehash(const bucket_traits & new_bucket_traits);

    Effects: If new_bucket_traits.bucket_count() is not this->bucket_count()/2 or this->bucket_count()*2, or this->split_bucket() != new_bucket_traits.bucket_count() returns false and does nothing.

    Otherwise, copy assigns new_bucket_traits to the internal bucket_traits and transfers all the objects from old buckets to the new ones.

    Complexity: Linear to size().

    Throws: Nothing

    Note: this method is only available if incremental<true> option is activated.

59.     size_type split_count() const;

    Requires:

    Effects:

    Complexity:

    Throws:

### `hashtable` public static functions

1.     static local_iterator s_local_iterator_to(reference value);

   Requires: value must be an lvalue and shall be in a unordered_set of appropriate type. Otherwise the behavior is undefined.

   Effects: Returns: a valid local_iterator belonging to the unordered_set that points to the value

   Complexity: Constant.

   Throws: Nothing.

   Note: This static function is available only if the value traits is stateless.

2.     static const_local_iterator s_local_iterator_to(const_reference value);

   Requires: value must be an lvalue and shall be in a unordered_set of appropriate type. Otherwise the behavior is undefined.

   Effects: Returns: a valid const_local_iterator belonging to the unordered_set that points to the value

   Complexity: Constant.

   Throws: Nothing.

   Note: This static function is available only if the value traits is stateless.

3.     static size_type suggested_upper_bucket_count(size_type n);

   Effects: Returns the nearest new bucket count optimized for the container that is bigger or equal than n. This suggestion can be used to create bucket arrays with a size that will usually improve container's performance. If such value does not exist, the higher possible value is returned.

   Complexity: Amortized constant time.

   Throws: Nothing.

4.     static size_type suggested_lower_bucket_count(size_type n);

   Effects: Returns the nearest new bucket count optimized for the container that is smaller or equal than n. This suggestion can be used to create bucket arrays with a size that will usually improve container's performance. If such value does not exist, the lowest possible value is returned.

   Complexity: Amortized constant time.

   Throws: Nothing.