Multiset Class and its Iterator The multiset class is a container class in the Standard Template Library (STL) Include a feature called the iterator, which permits a programmer to easily step through all the elements of an STL container class. Data Structures and Other Objects Using C++

Multiset and Set classes Multiset is similar to bag Set has the same interface as the multiset, except that it stores elements without repetition

Multiset example Template instantiation multiset<int> first; first.insert(8); fisrt.insert(4); first.insert(4); Template instantiation

Multiset restriction The type of item in a multiset must be possible to compare two items using “<” operator Satisfy the rules of a strict weak ordering

Multiset members value_type size_type size_type count(const value_type& target) const; size_type erase(const value_type& target); size_type size( ) const; iterator insert(const value_type& entry);

Iterators An iterator is an object that permits a programmer to easily step through all the items in a container, examining the items changing them. Member functions: begin end * ++

begin member function Its return value is an iterator that provides access to the first item in the container multiset<string> actors; multiset<string>::iterator role; role = actors.begin();

end member function Its return value is an iterator that provides access to the last item in the container multiset<string> actors; multiset<string>::iterator role; role = actors.end();

* operator The * operator can be used to access (cannot change) the current element of the iterator multiset<string> actors; multiset<string>::iterator role; role = actors.begin(); cout<< *role <<endl;

++ operator ++ operator can be used to move an iterator forward to the item in its collection multiset<string> actors; multiset<string>::iterator role; role = actors.begin(); ++role;

The [ … )Pattern

Pitfall Do not access an iterator’s item after reaching end() Remember end() is one location past the last item of the container

Other multiset operations != == It is an error to compare two iterators from different containers find erase multiset<int> m; multiset<int>::iterator position; position = m.find(42); if(position != m.end()) m.erase(position);