## multimap

    Container

- Summary
- Data Type and Member Function Indexes
- Synopsis
- Description
- Interface
- Constructors
- Destructors
- Assignment Operators
- Allocators
- Iterators
- Member Functions
- Non-member Operators
- Specialized Algorithms
- Example
- Warnings
- See Also

### Summary

An associative container that gives access to non-key values using keys. multimap keys are not required to be unique. A multimap supports bidirectional iterators.

### Data Type and Member Function Indexes
(exclusive of constructors and destructors)

Member Functions
begin() clear() count() empty() end() equal_range() | erase() find() get_allocator() insert() key_comp() lower_bound() | max_size() operator!=() operator>() operator>=() operator<() operator<=() | operator=() operator==() rbegin() rend() size() swap() upper_bound() value_comp()

### Synopsis

    #include <map>
    template <class Key, class T, class Compare = less<Key>,
              class Allocator = allocator<pair<const Key, T>> >
    class multimap;

### Description

multimap <Key ,T, Compare, Allocator> gives fast access to stored values of type `T` that are indexed by keys of type `Key`. The default operation for key comparison is the `<` operator. Unlike map, multimap allows insertion of duplicate keys.

multimap uses bidirectional iterators that point to an instance of `pair<const Key x, T y>` where `x` is the key and `y` is the stored value associated with that key. The definition of multimap includes a `typedef` to this pair called `value_type`.

The types used for both the template parameters `Key` and `T` must include the following (where `T` is the `type`, `t` is a value of `T` and `u` is a `const` `value` of `T`):

Copy constructors | `T(t)` and `T(u)`
Destructor | `t.~T()`
Address of | `&t` and `&u` yielding `T*` and `const T*` respectively
Assignment | `t = a` where `a` is a (possibly `const`) value of `T`

The type used for the `Compare` template parameter must satisfy the requirements for binary functions.

### Interface

    template <class Key, class T, class Compare = less<Key>,
              class Allocator = allocator<pair<const Key, T>> >
     class multimap {

    public:

    // types

       typedef Key key_type;
       typedef T mapped_type;
       typedef pair<const Key, T> value_type;
       typedef Compare key_compare;
       typedef Allocator allocator_type;

       typedef typename
               Allocator::reference        reference;
       typedef typename
               Allocator::const_reference  const_reference;

       class iterator;
       class const_iterator;

       typedef typename
               Allocator::size_type        size_type;
       typedef typename
               Allocator::difference_type  difference_type;

       typedef typename std::reverse_iterator<iterator>
                             reverse_iterator;
       typedef typename std::reverse_iterator<const_iterator>
                             const_reverse_iterator;

     class value_compare
        : public binary_function<value_type, value_type, bool>

        {
         friend class multimap<Key, T, Compare, Allocator>;

         protected :
           Compare comp;
           value_compare (Compare C) : comp(c) {}
         public :
           bool operator() (const value_type&,
                            const value_type&) const;
        };

    // Construct/Copy/Destroy

       explicit multimap (const Compare& = Compare(),
                          const Allocator& =
                          Allocator());
       template <class InputIterator>
        multimap (InputIterator, InputIterator,
                  const Compare& = Compare(),
                  const Allocator& = Allocator());
       multimap (const multimap<Key, T, Compare, Allocator>&);
       ~multimap ();
       multimap<Key, T, Compare, Allocator>& operator=
           (const multimap<Key, T, Compare, Allocator>&);
       allocator_type get_allocator () const;

    // Iterators

       iterator begin ();
       const_iterator begin () const;
       iterator end ();
       const_iterator end () const;
       reverse_iterator rbegin ();
       const_reverse_iterator rbegin () const;
       reverse_iterator rend ();
       const_reverse_iterator rend () const;

    // Capacity

       bool empty () const;
       size_type size () const;
       size_type max_size () const;

    // Modifiers

       iterator insert (const value_type&);
       iterator insert (iterator, const value_type&);
       template <class InputIterator>
        void insert (InputIterator, InputIterator);

       void erase (iterator);
       size_type erase (const key_type&);
       void erase (iterator, iterator);
       void swap (multimap<Key, T, Compare, Allocator>&);
       void clear ();

    // Observers

       key_compare key_comp () const;
       value_compare value_comp () const;

    // Multimap operations

       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;
       pair<iterator, iterator> equal_range (const key_type&);
       pair<const_iterator, const_iterator>
         equal_range (const key_type&) const;
    };
    // Non-member Operators

    template <class Key, class T, class Compare,
              class Allocator>
     bool operator== (const multimap<Key, T, Compare,
                      Allocator>&,
                      const multimap<Key, T, Compare,
                      Allocator>&);

    template <class Key, class T, class Compare,
              class Allocator>
     bool operator!= (const multimap<Key, T, Compare,
                      Allocator>&,
                      const multimap<Key, T, Compare,
                      Allocator>&);

    template <class Key, class T, class Compare,
              class Allocator>
     bool operator< (const multimap<Key, T, Compare,
                     Allocator>&,
                     const multimap<Key, T, Compare,
                     Allocator>&);

    template <class Key, class T, class Compare,
              class Allocator>
     bool operator> (const multimap<Key, T, Compare,
                     Allocator>&,
                     const multimap<Key, T, Compare,
                     Allocator>&);

    template <class Key, class T, class Compare,
              class Allocator>
     bool operator<= (const multimap<Key, T, Compare,
                      Allocator>&,
                      const multimap<Key, T, Compare,
                      Allocator>&);

    template <class Key, class T, class Compare,
              class Allocator>
     bool operator>= (const multimap<Key, T, Compare,
                      Allocator>&,
                      const multimap<Key, T, Compare,
                      Allocator>&);

    // Specialized Algorithms

    template <class Key, class T, class Compare,
              class Allocator>
     void swap (multimap<Key, T, Compare, Allocator>&,
                multimap<Key, T, Compare, Allocator>&;

### Constructors

    explicit multimap(const Compare& comp = Compare(),
                      const Allocator& alloc = Allocator());

Constructs an empty multimap that uses the optional relation `comp` to order keys and the allocator `alloc` for all storage management.

    template <class InputIterator>
    multimap(InputIterator first,
              InputIterator last,
              const Compare& comp = Compare()
              const Allocator& alloc = Allocator());

Constructs a multimap containing values in the range `[first, last)`. Creation of the new multimap is only guaranteed to succeed if the iterators `first` and `last` return values of type `pair<class Key, class T>.`

    multimap(const multimap<Key, T, Compare, Allocator>& x);

Creates a new multimap by copying all pairs of `key` and `value` from `x`.

### Destructors

    ~multimap();

Releases any allocated memory for this multimap.

### Assignment Operators

    multimap<Key, T, Compare, Allocator>&
    operator=(const multimap<Key, T, Compare, Allocator>& x);

Replaces the contents of `*this` with a copy of the multimap `x`.

### Allocators

    allocator_type
    get_allocator() const;

Returns a copy of the allocator used by self for storage management.

### Iterators

    iterator
    begin();

Returns a bidirectional `iterator` pointing to the first element stored in the multimap. "First" is defined by the multimap's comparison operator, `Compare`.

    const_iterator
    begin() const;

Returns a `const_iterator` pointing to the first element stored in the multimap. "First" is defined by the multimap's comparison operator, `Compare`.

    iterator
    end();

Returns a bidirectional `iterator` pointing to the last element stored in the multimap (in other words, the off-the-end value).

    const_iterator
    end() const;

Returns a `const_iterator` pointing to the last element stored in the multimap.

    reverse_iterator
    rbegin();

Returns a `reverse_iterator` pointing to the first element stored in the multimap. "First" is defined by the multimap's comparison operator, `Compare`.

    const_reverse_iterator
    rbegin() const;

Returns a `const_reverse_iterator` pointing to the first element stored in the multimap.

    reverse_iterator
    rend();

Returns a `reverse_iterator` pointing to the last element stored in the multimap (in other words, the off-the-end value).

    const_reverse_iterator
    rend() const;

Returns a `const_reverse_iterator` pointing to the last element stored in the multimap.

### Member Functions

    void
    clear();

Erases all elements from the self.

    size_type
    count(const key_type& x) const;

Returns the number of elements in the multimap with the key value `x`.

    bool
    empty() const;

Returns `true` if the multimap is empty, `false` otherwise.

    pair<iterator,iterator>
    equal_range(const key_type& x);
    pair<const_iterator,const_iterator>
    equal_range(const key_type& x) const;

Returns the pair `(lower_bound(x), upper_bound(x))`.

    void
    erase(iterator first, iterator last);

If the iterators `first` and `last` point to the same multimap and last is reachable from first, all elements in the range (`first, last`) are deleted from the multimap. Returns an `iterator` pointing to the element following the last deleted element or `end(),` if there were no elements after the deleted range.

    void
    erase(iterator position);

Deletes the multimap element pointed to by the iterator `position`. Returns an `iterator` pointing to the element following the deleted element, or `end(),` if the deleted item was the last one in this list.

    size_type
    erase(const key_type& x);

Deletes the elements with the key value `x` from the map, if any exist. Returns the number of deleted elements, or `0` otherwise.

    iterator
    find(const key_type& x);

Searches the multimap for a pair with the key value `x` and returns an `iterator` to that pair if it is found. If such a pair is not found the value `end()` is returned.

    const_iterator
    find(const key_type& x) const;

Same as find above but returns a `const_iterator`.

    iterator
    insert(const value_type& x);
    iterator
    insert(iterator position, const value_type& x);

`x` is inserted into the multimap. A position may be supplied as a hint regarding where to do the insertion. If the insertion is done right after `position`, then it takes amortized constant time. Otherwise it takes `O(log N)` time.

    template <class InputIterator>
    void
    insert(InputIterator first, InputIterator last);

Copies of each element in the range `[first, last)` are inserted into the multimap. The iterators `first` and `last` must return values of type `pair<T1,T2>`. This operation takes approximately `O(N*log(size()+N))` time.

    key_compare
    key_comp() const;

Returns a function object capable of comparing key values using the comparison operation, `Compare`, of the current multimap.

    iterator
    lower_bound(const key_type& x);

Returns an `iterator` to the first multimap element whose key is greater than or equal to `x`. If no such element exists, then `end()` is returned.

    const_iterator
    lower_bound(const key_type& x) const;

Same as `lower_bound` above but returns a `const_iterator.`

    size_type
    max_size() const;

Returns the maximum possible size of the multimap.

    size_type
    size() const;

Returns the number of elements in the multimap.

    void
    swap(multimap<Key, T, Compare, Allocator>& x);

Swaps the contents of the multimap `x` with the current multimap, `*this`.

    iterator
    upper_bound(const key_type& x);

Returns an `iterator` to the first element whose key is less than or equal to `x`. If no such element exists, then `end()` is returned.

    const_iterator
    upper_bound(const key_type& x) const;

Same as `upper_bound` above but returns a `const_iterator`.

    value_compare
    value_comp() const;

Returns a function object capable of comparing `value_types` (`key,value` pairs) using the comparison operation, `Compare`, of the current multimap.

### Non-member Operators

    bool
    operator==(const multimap<Key, T, Compare, Allocator>& x,
               const multimap<Key, T, Compare, Allocator>& y);

Returns `true` if all elements in `x` are element-wise equal to all elements in `y`, using `(T::operator==).` Otherwise it returns `false`.

    bool
    operator!=(const multimap<Key, T, Compare, Allocator>& x,
               const multimap<Key, T, Compare, Allocator>& y);

Returns `!(x==y)`.

    bool
    operator<(const multimap<Key, T, Compare, Allocator>& x,
               const multimap<Key, T, Compare, Allocator>& y);

Returns `true` if `x` is lexicographically less than `y`. Otherwise, it returns `false`.

    bool
    operator>(const multimap<Key, T, Compare, Allocator>& x,
               const multimap<Key, T, Compare, Allocator>& y);

Returns `y < x`.

    bool
    operator<=(const multimap<Key, T, Compare, Allocator>& x,
               const multimap<Key, T, Compare, Allocator>& y);

Returns `!(y < x)`.

    bool
    operator>=(const multimap<Key, T, Compare, Allocator>& x,
               const multimap<Key, T, Compare, Allocator>& y);

Returns `!(x < y)`.

### Specialized Algorithms

    template<class Key, class T, class Compare, class Allocator>
    void swap(multimap<Key, T, Compare, Allocator>& a,
               multimap<Key, T, Compare, Allocator>& b);

Swaps the contents of `a` and `b`.

### Example

    //
    // multimap.cpp
    //
     #include <string>
     #include <map>
     #include <iostream>
     using namespace std;

     typedef multimap<int, string, less<int> > months_type;

     // Print out a pair
     template <class First, class Second>
     ostream& operator<<(ostream& out,
                         const pair<First,Second>& p)
     {
       cout << p.second << " has " << p.first << " days";
       return out;
     }

     // Print out a multimap
     ostream& operator<<(ostream& out, months_type l)
     {
       copy(l.begin(),l.end(), ostream_iterator
                 <months_type::value_type,char>(cout,"\n"));
       return out;
     }

     int main(void)
     {
       // create a multimap of months and the number of
       // days in the month
       months_type months;

       typedef months_type::value_type value_type;

       // Put the months in the multimap
       months.insert(value_type(31, string("January")));
       months.insert(value_type(28, string("February")));
       months.insert(value_type(31, string("March")));
       months.insert(value_type(30, string("April")));
       months.insert(value_type(31, string("May")));
       months.insert(value_type(30, string("June")));
       months.insert(value_type(31, string("July")));
       months.insert(value_type(31, string("August")));
       months.insert(value_type(30, string("September")));
       months.insert(value_type(31, string("October")));
       months.insert(value_type(30, string("November")));
       months.insert(value_type(31, string("December")));

       // print out the months
       cout << "All months of the year" << endl << months
            << endl;

       // Find the Months with 30 days
       pair<months_type::iterator,months_type::iterator> p =
              months.equal_range(30);

       // print out the 30 day months
       cout << endl << "Months with 30 days" << endl;
       copy(p.first,p.second,
            ostream_iterator<months_type::value_type,char>
           (cout,"\n"));

       return 0;
     }

Program Output

    All months of the year
    February has 28 days
    April has 30 days
    June has 30 days
    September has 30 days
    November has 30 days
    January has 31 days
    March has 31 days
    May has 31 days
    July has 31 days
    August has 31 days
    October has 31 days
    December has 31 days

    Months with 30 days
    April has 30 days
    June has 30 days
    September has 30 days
    November has 30 days

### Warnings

Member function templates are used in all containers included in the Standard Template Library. An example of this feature is the constructor for multimap<Key,T,Compare,Allocator> that takes two templatized iterators:

    template <class InputIterator>
     multimap (InputIterator, InputIterator,
               const Compare& = Compare(),
               const Allocator& = Allocator());

multimap also has an `insert` function of this type. These functions, when not restricted by compiler limitations, allow you to use any type of input iterator as arguments. For compilers that do not support this feature, substitute functions allow you to use an iterator obtained from the same type of container as the one you are constructing (or calling a member function on), or you can use a pointer to the type of element you have in the container.

For example, if your compiler does not support member function templates, you can construct a multimap in the following two ways:

    multimap<int,int>::value_type intarray[10];
    multimap<int,int> first_map(intarry, intarray + 10);
    multimap<int,int> second_multimap(first_multimap.begin(),
                      first_multimap.end());

but not this way:

    multimap<long,long>
     long_multimap(first_multimap.begin(),first_multimap.end());

since the `long_multimap` and `first_multimap` are not the same type.

Also, many compilers do not support default template arguments. If your compiler is one of these you always need to supply the `Compare` template argument and the `Allocator` template argument. For instance, you have to write:

`multimap<int, int, less<int>, allocator<int> >`

instead of:

`multimap<int, int>`

If your compiler does not support namespaces, then you do not need the using declaration for `std`.

### See Also

allocator, Containers, Iterators, map

Copyright (c) 1998, Rogue Wave Software, Inc.
Send mail to report errors or comment on the documentation.
OEM Release, June 1998