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## Class template multimap

boost::container::multimap

## Synopsis

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

    template<typename Key, typename T, typename Compare = std::less<Key>,
             typename Allocator = std::allocator< std::pair< const Key, T> >,
             typename MultiMapOptions = tree_assoc_defaults>
    class multimap {
    public:
      // types
      typedef Key                                                              key_type;
      typedef T                                                                mapped_type;
      typedef std::pair< const Key, T >                                        value_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                                           value_compare;
      typedef Compare                                                          key_compare;
      typedef implementation_defined                                           iterator;
      typedef implementation_defined                                           const_iterator;
      typedef implementation_defined                                           reverse_iterator;
      typedef implementation_defined                                           const_reverse_iterator;
      typedef std::pair< key_type, mapped_type >                               nonconst_value_type;
      typedef implementation_defined                                           movable_value_type;

      // construct/copy/destruct
      multimap();
      explicit multimap(const Compare &,
                        const allocator_type & = allocator_type());
      explicit multimap(const allocator_type &);
      template<typename InputIterator>
        multimap(InputIterator, InputIterator, const Compare & = Compare(),
                 const allocator_type & = allocator_type());
      template<typename InputIterator>
        multimap(ordered_range_t, InputIterator, InputIterator,
                 const Compare & = Compare(),
                 const allocator_type & = allocator_type());
      multimap(std::initializer_list< value_type >, const Compare & = Compare(),
               const allocator_type & = allocator_type());
      multimap(ordered_range_t, std::initializer_list< value_type >,
               const Compare & = Compare(),
               const allocator_type & = allocator_type());
      multimap(const multimap &);
      multimap(multimap &&);
      multimap(const multimap &, const allocator_type &);
      multimap(multimap &&, const allocator_type &);
      multimap & operator=(const multimap &);
      multimap & operator=(multimap &&);
      multimap & 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(const nonconst_value_type &);
      iterator insert(nonconst_value_type &&);
      iterator insert(movable_value_type &&);
      iterator insert(const_iterator, const value_type &);
      iterator insert(const_iterator, const nonconst_value_type &);
      iterator insert(const_iterator, nonconst_value_type &&);
      iterator insert(const_iterator, movable_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< iterator, iterator > equal_range(const key_type &);
      std::pair< const_iterator, const_iterator >
      equal_range(const key_type &) const;
      void rebalance();

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

## Description

A multimap is a kind of associative container that supports equivalent keys (possibly containing multiple copies of the same key value) and provides for fast retrieval of values of another type T based on the keys. The multimap class supports bidirectional iterators.

A multimap satisfies all of the requirements of a container and of a reversible container and of an associative container. The `value_type` stored by this container is the value_type is std::pair<const Key, T>.

### Template Parameters

1.     typename Key

   is the key_type of the map

2.     typename T

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

   is the ordering function for Keys (e.g. std::less<Key>).

4.     typename Allocator = std::allocator< std::pair< const Key, T> >

   is the allocator to allocate the `value_type`s (e.g. allocator< std::pair<const Key, T> > ).

5.     typename MultiMapOptions = tree_assoc_defaults

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

### `multimap` public construct/copy/destruct

1.     multimap();

   Effects: Default constructs an empty multimap.

   Complexity: Constant.

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

   Effects: Constructs an empty multimap using the specified allocator.

   Complexity: Constant.

3.     explicit multimap(const allocator_type & a);

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

   Complexity: Constant.

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

   Effects: Constructs an empty multimap 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>
         multimap(ordered_range_t, InputIterator first, InputIterator last,
                  const Compare & comp = Compare(),
                  const allocator_type & a = allocator_type());

   Effects: Constructs an empty multimap 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.     multimap(std::initializer_list< value_type > il,
                const Compare & comp = Compare(),
                const allocator_type & a = allocator_type());

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

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

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

8.     multimap(const multimap & x);

   Effects: Copy constructs a multimap.

   Complexity: Linear in x.size().

9.     multimap(multimap && x);

   Effects: Move constructs a multimap. Constructs *this using x's resources.

   Complexity: Constant.

   Postcondition: x is emptied.

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

    Effects: Copy constructs a multimap.

    Complexity: Linear in x.size().

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

    Effects: Move constructs a multimap using the specified allocator. Constructs *this using x's resources. Complexity: Constant if a == x.get_allocator(), linear otherwise.

    Postcondition: x is emptied.

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

    Effects: Makes *this a copy of x.

    Complexity: Linear in x.size().

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

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

    Complexity: Constant.

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

    Effects: Assign content of il to *this.

### `multimap` 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 T constructed with std::forward<Args>(args)... 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.

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

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

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(const nonconst_value_type & x);

    Effects: Inserts a new value constructed from x and returns the iterator pointing to the newly inserted element.

    Complexity: Logarithmic.

23.     iterator insert(nonconst_value_type && x);

    Effects: Inserts a new value move-constructed from x and returns the iterator pointing to the newly inserted element.

    Complexity: Logarithmic.

24.     iterator insert(movable_value_type && x);

    Effects: Inserts a new value move-constructed from x and returns the iterator pointing to the newly inserted element.

    Complexity: Logarithmic.

25.     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.

26.     iterator insert(const_iterator p, const nonconst_value_type & x);

    Effects: Inserts a new value 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.

27.     iterator insert(const_iterator p, nonconst_value_type && x);

    Effects: Inserts a new 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.

28.     iterator insert(const_iterator p, movable_value_type && x);

    Effects: Inserts a new 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.

29.     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)

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

    Effects: inserts each element from the range [il.begin(), il.end().

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

31.     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

32.     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)

33.     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.

34.     void swap(flat_multiset & x);

    Effects: Swaps the contents of *this and x.

    Throws: Nothing.

    Complexity: Constant.

35.     void clear() noexcept;

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

    Postcondition: size() == 0.

    Complexity: linear in size().

36.     key_compare key_comp() const;

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

    Complexity: Constant.

37.     value_compare value_comp() const;

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

    Complexity: Constant.

38.     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.

39.     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.

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

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

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

41.     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

42.     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

43.     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

44.     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

45.     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

46.     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

47.     void rebalance();

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

    Complexity: Linear

### `multimap` friend functions

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

   Effects: x.swap(y)

   Complexity: Constant.