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## Class template flat_multimap

boost::container::flat_multimap

## Synopsis

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

    template<typename Key, typename T, typename Compare = std::less<Key>,
             typename Allocator = new_allocator< std::pair< Key, T> > >
    class flat_multimap {
    public:
      // types
      typedef Key                                                              key_type;
      typedef T                                                                mapped_type;
      typedef std::pair< Key, T >                                              value_type;
      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                                           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 implementation_defined                                           movable_value_type;

      // construct/copy/destruct
      flat_multimap();
      explicit flat_multimap(const Compare &,
                             const allocator_type & = allocator_type());
      explicit flat_multimap(const allocator_type &);
      template<typename InputIterator>
        flat_multimap(InputIterator, InputIterator, const Compare & = Compare(),
                      const allocator_type & = allocator_type());
      template<typename InputIterator>
        flat_multimap(InputIterator, InputIterator, const allocator_type &);
      template<typename InputIterator>
        flat_multimap(ordered_range_t, InputIterator, InputIterator,
                      const Compare & = Compare(),
                      const allocator_type & = allocator_type());
      flat_multimap(std::initializer_list< value_type >,
                    const Compare & = Compare(),
                    const allocator_type & = allocator_type());
      flat_multimap(std::initializer_list< value_type >, const allocator_type &);
      flat_multimap(ordered_range_t, std::initializer_list< value_type >,
                    const Compare & = Compare(),
                    const allocator_type & = allocator_type());
      flat_multimap(const flat_multimap &);
      flat_multimap(flat_multimap &&);
      flat_multimap(const flat_multimap &, const allocator_type &);
      flat_multimap(flat_multimap &&, const allocator_type &);
      flat_multimap & operator=(const flat_multimap &);
      flat_multimap &
      operator=(flat_multimap &&) noexcept(allocator_traits_type::is_always_equal::value &&boost::container::container_detail::is_nothrow_move_assignable< Compare >::value));
      flat_multimap & operator=(std::initializer_list< value_type >);

      // public member functions
      allocator_type get_allocator() const noexcept;
      stored_allocator_type & get_stored_allocator() noexcept;
      const stored_allocator_type & get_stored_allocator() const noexcept;
      iterator begin() noexcept;
      const_iterator begin() const noexcept;
      iterator end() noexcept;
      const_iterator end() const noexcept;
      reverse_iterator rbegin() noexcept;
      const_reverse_iterator rbegin() const noexcept;
      reverse_iterator rend() noexcept;
      const_reverse_iterator rend() const noexcept;
      const_iterator cbegin() const noexcept;
      const_iterator cend() const noexcept;
      const_reverse_iterator crbegin() const noexcept;
      const_reverse_iterator crend() const noexcept;
      bool empty() const noexcept;
      size_type size() const noexcept;
      size_type max_size() const noexcept;
      size_type capacity() const noexcept;
      void reserve(size_type);
      void shrink_to_fit();
      iterator nth(size_type) noexcept;
      const_iterator nth(size_type) const noexcept;
      size_type index_of(iterator) noexcept;
      size_type index_of(const_iterator) const noexcept;
      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(impl_value_type &&);
      iterator insert(const_iterator, const value_type &);
      iterator insert(const_iterator, value_type &&);
      iterator insert(const_iterator, impl_value_type &&);
      template<typename InputIterator> void insert(InputIterator, InputIterator);
      template<typename InputIterator>
        void insert(ordered_range_t, InputIterator, InputIterator);
      void insert(std::initializer_list< value_type >);
      void insert(ordered_range_t, std::initializer_list< value_type >);
      iterator erase(const_iterator);
      size_type erase(const key_type &);
      iterator erase(const_iterator, const_iterator);
      void swap(flat_multimap &) noexcept(allocator_traits_type::is_always_equal::value &&boost::container::container_detail::is_nothrow_swappable< Compare >::value));
      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;

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

## Description

A flat_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 flat_multimap class supports random-access iterators.

A flat_multimap satisfies all of the requirements of a container and of a reversible container and of an associative container. For a flat_multimap<Key,T> the key_type is Key and the value_type is std::pair<Key,T> (unlike std::multimap<Key, T> which value_type is std::pair<const Key, T>).

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

Allocator is the allocator to allocate the value_types (e.g. allocator< std::pair<Key, T> >).

flat_multimap is similar to std::multimap but it's implemented like an ordered vector. This means that inserting a new element into a flat_map invalidates previous iterators and references

Erasing an element invalidates iterators and references pointing to elements that come after (their keys are bigger) the erased element.

This container provides random-access iterators.

### 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 = new_allocator< std::pair< Key, T> >

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

### `flat_multimap` public construct/copy/destruct

1.     flat_multimap();

   Effects: Default constructs an empty `flat_map`.

   Complexity: Constant.

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

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

   Complexity: Constant.

3.     explicit flat_multimap(const allocator_type & a);

   Effects: Constructs an empty `flat_multimap` using the specified allocator.

   Complexity: Constant.

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

   Effects: Constructs an empty `flat_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>
         flat_multimap(InputIterator first, InputIterator last,
                       const allocator_type & a);

   Effects: Constructs an empty `flat_multimap` using the specified 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.

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

   Effects: Constructs an empty `flat_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.

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

   Effects: Constructs an empty `flat_map` 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 last - first.

8.     flat_multimap(std::initializer_list< value_type > il,
                     const allocator_type & a);

   Effects: Constructs an empty `flat_map` using the specified 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 last - first.

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

   Effects: Constructs an empty `flat_multimap` using the specified comparison object and allocator, and inserts elements from the ordered 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.

   Complexity: Linear in N.

   Note: Non-standard extension.

10.     flat_multimap(const flat_multimap & x);

    Effects: Copy constructs a `flat_multimap`.

    Complexity: Linear in x.size().

11.     flat_multimap(flat_multimap && x);

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

    Complexity: Constant.

    Postcondition: x is emptied.

12.     flat_multimap(const flat_multimap & x, const allocator_type & a);

    Effects: Copy constructs a `flat_multimap` using the specified allocator.

    Complexity: Linear in x.size().

13.     flat_multimap(flat_multimap && x, const allocator_type & a);

    Effects: Move constructs a `flat_multimap` using the specified allocator. Constructs *this using x's resources.

    Complexity: Constant if a == x.get_allocator(), linear otherwise.

14.     flat_multimap & operator=(const flat_multimap & x);

    Effects: Makes *this a copy of x.

    Complexity: Linear in x.size().

15.     flat_multimap &
        operator=(flat_multimap && x) noexcept(allocator_traits_type::is_always_equal::value &&boost::container::container_detail::is_nothrow_move_assignable< Compare >::value));

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

    Complexity: Constant.

16.     flat_multimap & operator=(std::initializer_list< value_type > il);

    Effects: Assign content of il to *this

    Complexity: Linear in il.size().

### `flat_multimap` public member functions

1.     allocator_type get_allocator() const noexcept;

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

   Complexity: Constant.

2.     stored_allocator_type & get_stored_allocator() noexcept;

   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 noexcept;

   Effects: Returns a reference to the internal allocator.

   Throws: Nothing

   Complexity: Constant.

   Note: Non-standard extension.

4.     iterator begin() noexcept;

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

   Throws: Nothing.

   Complexity: Constant.

5.     const_iterator begin() const noexcept;

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

   Throws: Nothing.

   Complexity: Constant.

6.     iterator end() noexcept;

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

   Throws: Nothing.

   Complexity: Constant.

7.     const_iterator end() const noexcept;

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

   Throws: Nothing.

   Complexity: Constant.

8.     reverse_iterator rbegin() noexcept;

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

   Throws: Nothing.

   Complexity: Constant.

9.     const_reverse_iterator rbegin() const noexcept;

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

   Throws: Nothing.

   Complexity: Constant.

10.     reverse_iterator rend() noexcept;

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

    Throws: Nothing.

    Complexity: Constant.

11.     const_reverse_iterator rend() const noexcept;

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

    Throws: Nothing.

    Complexity: Constant.

12.     const_iterator cbegin() const noexcept;

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

    Throws: Nothing.

    Complexity: Constant.

13.     const_iterator cend() const noexcept;

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

    Throws: Nothing.

    Complexity: Constant.

14.     const_reverse_iterator crbegin() const noexcept;

    Effects: Returns a const_reverse_iterator pointing to the beginning 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 noexcept;

    Effects: Returns true if the container contains no elements.

    Throws: Nothing.

    Complexity: Constant.

17.     size_type size() const noexcept;

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

    Throws: Nothing.

    Complexity: Constant.

18.     size_type max_size() const noexcept;

    Effects: Returns the largest possible size of the container.

    Throws: Nothing.

    Complexity: Constant.

19.     size_type capacity() const noexcept;

    Effects: Number of elements for which memory has been allocated. capacity() is always greater than or equal to size().

    Throws: Nothing.

    Complexity: Constant.

20.     void reserve(size_type cnt);

    Effects: If n is less than or equal to capacity(), this call has no effect. Otherwise, it is a request for allocation of additional memory. If the request is successful, then capacity() is greater than or equal to n; otherwise, capacity() is unchanged. In either case, size() is unchanged.

    Throws: If memory allocation allocation throws or T's copy constructor throws.

    Note: If capacity() is less than "cnt", iterators and references to to values might be invalidated.

21.     void shrink_to_fit();

    Effects: Tries to deallocate the excess of memory created

    Throws: If memory allocation throws, or T's copy constructor throws.

    Complexity: Linear to size().

22.     iterator nth(size_type n) noexcept;

    Requires: size() >= n.

    Effects: Returns an iterator to the nth element from the beginning of the container. Returns end() if n == size().

    Throws: Nothing.

    Complexity: Constant.

    Note: Non-standard extension

23.     const_iterator nth(size_type n) const noexcept;

    Requires: size() >= n.

    Effects: Returns a const_iterator to the nth element from the beginning of the container. Returns end() if n == size().

    Throws: Nothing.

    Complexity: Constant.

    Note: Non-standard extension

24.     size_type index_of(iterator p) noexcept;

    Requires: size() >= n.

    Effects: Returns an iterator to the nth element from the beginning of the container. Returns end() if n == size().

    Throws: Nothing.

    Complexity: Constant.

    Note: Non-standard extension

25.     size_type index_of(const_iterator p) const noexcept;

    Requires: begin() <= p <= end().

    Effects: Returns the index of the element pointed by p and size() if p == end().

    Throws: Nothing.

    Complexity: Constant.

    Note: Non-standard extension

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

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

    Complexity: Logarithmic search time plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

27.     template<class... Args>
          iterator emplace_hint(const_iterator hint, 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 search time (constant time if the value is to be inserted before p) plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

28.     iterator insert(const value_type & x);

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

    Complexity: Logarithmic search time plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

29.     iterator insert(value_type && x);

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

    Complexity: Logarithmic search time plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

30.     iterator insert(impl_value_type && x);

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

    Complexity: Logarithmic search time plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

31.     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 search time (constant time if the value is to be inserted before p) plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

32.     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 search time (constant time if the value is to be inserted before p) plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

33.     iterator insert(const_iterator p, impl_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 search time (constant time if the value is to be inserted before p) plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

34.     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) search time plus N*size() insertion time.

    Note: If an element is inserted it might invalidate elements.

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

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

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

    Effects: inserts each element from the range [first,last) if and only if there is no element with key equivalent to the key of that element. This function is more efficient than the normal range creation for ordered ranges.

    Complexity: At most N log(size()+N) (N is the distance from first to last) search time plus N*size() insertion time.

    Note: If an element is inserted it might invalidate elements.

    Note: Non-standard extension.

36.     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 first to last) search time plus N*size() insertion time.

    Note: If an element is inserted it might invalidate elements.

37.     void insert(ordered_range_t, std::initializer_list< value_type > il);

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

    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. This function is more efficient than the normal range creation for ordered ranges.

    Complexity: At most N log(size()+N) (N is the distance from first to last) search time plus N*size() insertion time.

    Note: If an element is inserted it might invalidate elements.

    Note: Non-standard extension.

38.     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: Linear to the elements with keys bigger than p

    Note: Invalidates elements with keys not less than the erased element.

39.     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: Logarithmic search time plus erasure time linear to the elements with bigger keys.

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

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

    Returns: Returns last.

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

    Complexity: Logarithmic search time plus erasure time linear to the elements with bigger keys.

41.     void swap(flat_multimap & x) noexcept(allocator_traits_type::is_always_equal::value &&boost::container::container_detail::is_nothrow_swappable< Compare >::value));

    Effects: Swaps the contents of *this and x.

    Throws: Nothing.

    Complexity: Constant.

42.     void clear() noexcept;

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

    Postcondition: size() == 0.

    Complexity: linear in size().

43.     key_compare key_comp() const;

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

    Complexity: Constant.

44.     value_compare value_comp() const;

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

    Complexity: Constant.

45.     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.

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

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

    Complexity: Logarithmic.

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

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

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

48.     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

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

    Returns: A 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

50.     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

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

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

    Complexity: Logarithmic

52.     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

53.     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

### `flat_multimap` friend functions

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

   Effects: x.swap(y)

   Complexity: Constant.