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.