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## Class template multiset

boost::container::multiset

## Synopsis

    // In header: <boost/container/set.hpp>

    template<typename Key, typename Compare = std::less<Key>,
             typename Allocator = std::allocator<Key>,
             typename MultiSetOptions = tree_assoc_defaults>
    class multiset {
    public:
      // types
      typedef Key                                                                key_type;
      typedef Key                                                                value_type;
      typedef Compare                                                            key_compare;
      typedef Compare                                                            value_compare;
      typedef ::boost::container::allocator_traits< Allocator >                  allocator_traits_type;
      typedef ::boost::container::allocator_traits< Allocator >::pointer         pointer;
      typedef ::boost::container::allocator_traits< Allocator >::const_pointer   const_pointer;
      typedef ::boost::container::allocator_traits< Allocator >::reference       reference;
      typedef ::boost::container::allocator_traits< Allocator >::const_reference const_reference;
      typedef ::boost::container::allocator_traits< Allocator >::size_type       size_type;
      typedef ::boost::container::allocator_traits< Allocator >::difference_type difference_type;
      typedef Allocator                                                          allocator_type;
      typedef implementation_defined                                             stored_allocator_type;
      typedef implementation_defined                                             iterator;
      typedef implementation_defined                                             const_iterator;
      typedef implementation_defined                                             reverse_iterator;
      typedef implementation_defined                                             const_reverse_iterator;

      // construct/copy/destruct
      multiset();
      explicit multiset(const Compare &,
                        const allocator_type & = allocator_type());
      explicit multiset(const allocator_type &);
      template<typename InputIterator>
        multiset(InputIterator, InputIterator, const Compare & = Compare(),
                 const allocator_type & = allocator_type());
      template<typename InputIterator>
        multiset(ordered_range_t, InputIterator, InputIterator,
                 const Compare & = Compare(),
                 const allocator_type & = allocator_type());
      multiset(std::initializer_list< value_type >, const Compare & = Compare(),
               const allocator_type & = allocator_type());
      multiset(ordered_unique_range_t, std::initializer_list< value_type >,
               const Compare & = Compare(),
               const allocator_type & = allocator_type());
      multiset(const multiset &);
      multiset(multiset &&);
      multiset(const multiset &, const allocator_type &);
      multiset(multiset &&, const allocator_type &);
      multiset & operator=(const multiset &);
      multiset & operator=(multiset &&);
      multiset & operator=(std::initializer_list< value_type >);

      // public member functions
      allocator_type get_allocator() const;
      stored_allocator_type & get_stored_allocator();
      const stored_allocator_type & get_stored_allocator() const;
      iterator begin();
      const_iterator begin() const;
      const_iterator cbegin() const;
      iterator end() noexcept;
      const_iterator end() const noexcept;
      const_iterator cend() const noexcept;
      reverse_iterator rbegin() noexcept;
      const_reverse_iterator rbegin() const noexcept;
      const_reverse_iterator crbegin() const noexcept;
      reverse_iterator rend() noexcept;
      const_reverse_iterator rend() const noexcept;
      const_reverse_iterator crend() const noexcept;
      bool empty() const;
      size_type size() const;
      size_type max_size() const;
      template<class... Args> iterator emplace(Args &&...);
      template<class... Args> iterator emplace_hint(const_iterator, Args &&...);
      iterator insert(const value_type &);
      iterator insert(value_type &&);
      iterator insert(const_iterator, const value_type &);
      iterator insert(const_iterator, value_type &&);
      template<typename InputIterator> void insert(InputIterator, InputIterator);
      void insert(std::initializer_list< value_type >);
      iterator erase(const_iterator);
      size_type erase(const key_type &);
      iterator erase(const_iterator, const_iterator);
      void swap(flat_multiset &);
      void clear() noexcept;
      key_compare key_comp() const;
      value_compare value_comp() const;
      iterator find(const key_type &);
      const_iterator find(const key_type &) const;
      size_type count(const key_type &) const;
      iterator lower_bound(const key_type &);
      const_iterator lower_bound(const key_type &) const;
      iterator upper_bound(const key_type &);
      const_iterator upper_bound(const key_type &) const;
      std::pair< const_iterator, const_iterator >
      equal_range(const key_type &) const;
      std::pair< iterator, iterator > equal_range(const key_type &);
      void rebalance();

      // friend functions
      friend bool operator==(const multiset &, const multiset &);
      friend bool operator!=(const multiset &, const multiset &);
      friend bool operator<(const multiset &, const multiset &);
      friend bool operator>(const multiset &, const multiset &);
      friend bool operator<=(const multiset &, const multiset &);
      friend bool operator>=(const multiset &, const multiset &);
      friend void swap(multiset &, multiset &);
    };

## Description

A multiset is a kind of associative container that supports equivalent keys (possibly contains multiple copies of the same key value) and provides for fast retrieval of the keys themselves. Class multiset supports bidirectional iterators.

A multiset satisfies all of the requirements of a container and of a reversible container, and of an associative container). multiset also provides most operations described for duplicate keys.

### Template Parameters

1.     typename Key

   is the type to be inserted in the set, which is also the key_type

2.     typename Compare = std::less<Key>

   is the comparison functor used to order keys

3.     typename Allocator = std::allocator<Key>

   is the allocator to be used to allocate memory for this container

4.     typename MultiSetOptions = tree_assoc_defaults

   is an packed option type generated using using `boost::container::tree_assoc_options`.

### `multiset` public construct/copy/destruct

1.     multiset();

   Effects: Default constructs an empty set.

   Complexity: Constant.

2.     explicit multiset(const Compare & comp,
                         const allocator_type & a = allocator_type());

   Effects: Constructs an empty set using the specified comparison object and allocator.

   Complexity: Constant.

3.     explicit multiset(const allocator_type & a);

   Effects: Constructs an empty set using the specified allocator object.

   Complexity: Constant.

4.     template<typename InputIterator>
         multiset(InputIterator first, InputIterator last,
                  const Compare & comp = Compare(),
                  const allocator_type & a = allocator_type());

   Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range [first ,last ).

   Complexity: Linear in N if the range [first ,last ) is already sorted using comp and otherwise N logN, where N is last - first.

5.     template<typename InputIterator>
         multiset(ordered_range_t, InputIterator first, InputIterator last,
                  const Compare & comp = Compare(),
                  const allocator_type & a = allocator_type());

   Effects: Constructs an empty multiset using the specified comparison object and allocator, and inserts elements from the ordered range [first ,last ). This function is more efficient than the normal range creation for ordered ranges.

   Requires: [first ,last) must be ordered according to the predicate.

   Complexity: Linear in N.

   Note: Non-standard extension.

6.     multiset(std::initializer_list< value_type > il,
                const Compare & comp = Compare(),
                const allocator_type & a = allocator_type());

   Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range [il.begin(), il.end()).

   Complexity: Linear in N if the range [il.begin(), il.end()) is already sorted using comp and otherwise N logN, where N is il.begin() - il.end().

7.     multiset(ordered_unique_range_t, std::initializer_list< value_type > il,
                const Compare & comp = Compare(),
                const allocator_type & a = allocator_type());

   Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the ordered unique range [il.begin(), il.end()). This function is more efficient than the normal range creation for ordered ranges.

   Requires: [il.begin(), il.end()) must be ordered according to the predicate and must be unique values.

   Complexity: Linear in N.

   Note: Non-standard extension.

8.     multiset(const multiset & x);

   Effects: Copy constructs a set.

   Complexity: Linear in x.size().

9.     multiset(multiset && x);

10.     multiset(const multiset & x, const allocator_type & a);

11.     multiset(multiset && x, const allocator_type & a);

12.     multiset & operator=(const multiset & x);

    Effects: Makes *this a copy of x.

    Complexity: Linear in x.size().

13.     multiset & operator=(multiset && x);

    Effects: this->swap(x.get()).

    Throws: If allocator_traits_type::propagate_on_container_move_assignment is false and (allocation throws or value_type's move constructor throws)

    Complexity: Constant if allocator_traits_type:: propagate_on_container_move_assignment is true or this->get>allocator() == x.get_allocator(). Linear otherwise.

14.     multiset & operator=(std::initializer_list< value_type > il);

### `multiset` public member functions

1.     allocator_type get_allocator() const;

   Effects: Returns a copy of the Allocator that was passed to the object's constructor.

   Complexity: Constant.

2.     stored_allocator_type & get_stored_allocator();

   Effects: Returns a reference to the internal allocator.

   Throws: Nothing

   Complexity: Constant.

   Note: Non-standard extension.

3.     const stored_allocator_type & get_stored_allocator() const;

   Effects: Returns a reference to the internal allocator.

   Throws: Nothing

   Complexity: Constant.

   Note: Non-standard extension.

4.     iterator begin();

   Effects: Returns an iterator to the first element contained in the container.

   Throws: Nothing.

   Complexity: Constant

5.     const_iterator begin() const;

   Effects: Returns a const_iterator to the first element contained in the container.

   Throws: Nothing.

   Complexity: Constant.

6.     const_iterator cbegin() const;

   Effects: Returns a const_iterator to the first element contained in the container.

   Throws: Nothing.

   Complexity: Constant.

7.     iterator end() noexcept;

   Effects: Returns an iterator to the end of the container.

   Throws: Nothing.

   Complexity: Constant.

8.     const_iterator end() const noexcept;

   Effects: Returns a const_iterator to the end of the container.

   Throws: Nothing.

   Complexity: Constant.

9.     const_iterator cend() const noexcept;

   Effects: Returns a const_iterator to the end of the container.

   Throws: Nothing.

   Complexity: Constant.

10.     reverse_iterator rbegin() noexcept;

    Effects: Returns a reverse_iterator pointing to the beginning of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

11.     const_reverse_iterator rbegin() const noexcept;

    Effects: Returns a const_reverse_iterator pointing to the beginning of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

12.     const_reverse_iterator crbegin() const noexcept;

    Effects: Returns a const_reverse_iterator pointing to the beginning of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

13.     reverse_iterator rend() noexcept;

    Effects: Returns a reverse_iterator pointing to the end of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

14.     const_reverse_iterator rend() const noexcept;

    Effects: Returns a const_reverse_iterator pointing to the end of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

15.     const_reverse_iterator crend() const noexcept;

    Effects: Returns a const_reverse_iterator pointing to the end of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

16.     bool empty() const;

    Effects: Returns true if the container contains no elements.

    Throws: Nothing.

    Complexity: Constant.

17.     size_type size() const;

    Effects: Returns the number of the elements contained in the container.

    Throws: Nothing.

    Complexity: Constant.

18.     size_type max_size() const;

    Effects: Returns the largest possible size of the container.

    Throws: Nothing.

    Complexity: Constant.

19.     template<class... Args> iterator emplace(Args &&... args);

    Effects: Inserts an object of type Key constructed with std::forward<Args>(args)... and returns the iterator pointing to the newly inserted element.

    Complexity: Logarithmic.

20.     template<class... Args>
          iterator emplace_hint(const_iterator p, Args &&... args);

    Effects: Inserts an object of type Key constructed with std::forward<Args>(args)...

    Returns: An iterator pointing to the element with key equivalent to the key of x.

    Complexity: Logarithmic in general, but amortized constant if t is inserted right before p.

21.     iterator insert(const value_type & x);

    Effects: Inserts x and returns the iterator pointing to the newly inserted element.

    Complexity: Logarithmic.

22.     iterator insert(value_type && x);

    Effects: Inserts a copy of x in the container.

    Returns: An iterator pointing to the element with key equivalent to the key of x.

    Complexity: Logarithmic in general, but amortized constant if t is inserted right before p.

23.     iterator insert(const_iterator p, const value_type & x);

    Effects: Inserts a copy of x in the container. p is a hint pointing to where the insert should start to search.

    Returns: An iterator pointing to the element with key equivalent to the key of x.

    Complexity: Logarithmic in general, but amortized constant if t is inserted right before p.

24.     iterator insert(const_iterator p, value_type && x);

    Effects: Inserts a value move constructed from x in the container. p is a hint pointing to where the insert should start to search.

    Returns: An iterator pointing to the element with key equivalent to the key of x.

    Complexity: Logarithmic in general, but amortized constant if t is inserted right before p.

25.     template<typename InputIterator>
          void insert(InputIterator first, InputIterator last);

    Requires: first, last are not iterators into *this.

    Effects: inserts each element from the range [first,last) .

    Complexity: At most N log(size()+N) (N is the distance from first to last)

26.     void insert(std::initializer_list< value_type > il);

    Effects: inserts each element from the range [il.begin(),il.end()) if and only if there is no element with key equivalent to the key of that element.

    Complexity: At most N log(size()+N) (N is the distance from il.begin() to il.end())

27.     iterator erase(const_iterator p);

    Effects: Erases the element pointed to by p.

    Returns: Returns an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, returns end().

    Complexity: Amortized constant time

28.     size_type erase(const key_type & x);

    Effects: Erases all elements in the container with key equivalent to x.

    Returns: Returns the number of erased elements.

    Complexity: log(size()) + count(k)

29.     iterator erase(const_iterator first, const_iterator last);

    Effects: Erases all the elements in the range [first, last).

    Returns: Returns last.

    Complexity: log(size())+N where N is the distance from first to last.

30.     void swap(flat_multiset & x);

    Effects: Swaps the contents of *this and x.

    Throws: Nothing.

    Complexity: Constant.

31.     void clear() noexcept;

    Effects: erase(a.begin(),a.end()).

    Postcondition: size() == 0.

    Complexity: linear in size().

32.     key_compare key_comp() const;

    Effects: Returns the comparison object out of which a was constructed.

    Complexity: Constant.

33.     value_compare value_comp() const;

    Effects: Returns an object of value_compare constructed out of the comparison object.

    Complexity: Constant.

34.     iterator find(const key_type & x);

    Returns: An iterator pointing to an element with the key equivalent to x, or end() if such an element is not found.

    Complexity: Logarithmic.

35.     const_iterator find(const key_type & x) const;

    Returns: Allocator const_iterator pointing to an element with the key equivalent to x, or end() if such an element is not found.

    Complexity: Logarithmic.

36.     size_type count(const key_type & x) const;

    Returns: The number of elements with key equivalent to x.

    Complexity: log(size())+count(k)

37.     iterator lower_bound(const key_type & x);

    Returns: An iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.

    Complexity: Logarithmic

38.     const_iterator lower_bound(const key_type & x) const;

    Returns: Allocator const iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.

    Complexity: Logarithmic

39.     iterator upper_bound(const key_type & x);

    Returns: An iterator pointing to the first element with key not less than x, or end() if such an element is not found.

    Complexity: Logarithmic

40.     const_iterator upper_bound(const key_type & x) const;

    Returns: Allocator const iterator pointing to the first element with key not less than x, or end() if such an element is not found.

    Complexity: Logarithmic

41.     std::pair< const_iterator, const_iterator >
        equal_range(const key_type & x) const;

    Effects: Equivalent to std::make_pair(this->lower_bound(k), this->upper_bound(k)).

    Complexity: Logarithmic

42.     std::pair< iterator, iterator > equal_range(const key_type & x);

    Effects: Equivalent to std::make_pair(this->lower_bound(k), this->upper_bound(k)).

    Complexity: Logarithmic

43.     void rebalance();

    Effects: Rebalances the tree. It's a no-op for Red-Black and AVL trees.

    Complexity: Linear

### `multiset` friend functions

1.     friend bool operator==(const multiset & x, const multiset & y);

   Effects: Returns true if x and y are equal

   Complexity: Linear to the number of elements in the container.

2.     friend bool operator!=(const multiset & x, const multiset & y);

   Effects: Returns true if x and y are unequal

   Complexity: Linear to the number of elements in the container.

3.     friend bool operator<(const multiset & x, const multiset & y);

   Effects: Returns true if x is less than y

   Complexity: Linear to the number of elements in the container.

4.     friend bool operator>(const multiset & x, const multiset & y);

   Effects: Returns true if x is greater than y

   Complexity: Linear to the number of elements in the container.

5.     friend bool operator<=(const multiset & x, const multiset & y);

   Effects: Returns true if x is equal or less than y

   Complexity: Linear to the number of elements in the container.

6.     friend bool operator>=(const multiset & x, const multiset & y);

   Effects: Returns true if x is equal or greater than y

   Complexity: Linear to the number of elements in the container.

7.     friend void swap(multiset & x, multiset & y);

   Effects: x.swap(y)

   Complexity: Constant.