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TEMPLATES AND ITERATORS
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List<string> List() ~List() insert() remove() isEmpty() print() newNode() ListNode<string> ListNode() friends first
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Bowen, Van
ListNode<string>
ListNode<string>
ListNode<string>
ListNode<string>
White, Ann
string
This shows the situation at the moment that the insert() function has invoked the newNode() function which has invoked the ListNode() constructor to create a new node for "White, Ann".
13.8 ITERATOR CLASSES A common activity performed on a container object is the traversal of the object. For example, to traverse a List object means to travel through the list, visiting each element. This was done by means of a for loop in both the destructor and the print() function in our List class template. (See Example 13.7 on page 309.) An iterator is an object that has the ability to traverse through a container object. It acts like a pointer, locating one item in the container at a time. All iterators have the same basic functionality, regardless of the type of container to which they are attached. The five fundamental operations are:
initialize the iterator at some initial position in the container; return the data value stored at the current position; change the data value stored at the current position; determine whether there actually is an item at the iterator s current position; advance to the next position in the container.
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TEMPLATES AND ITERATORS
Since these five operations should be implemented by every iterator, it makes sense to declare an abstract base class with these functions. We actually need an abstract base class template because the container classes will be template instances:
template<class T> class Iterator { public: virtual void reset() =0; virtual T operator()() =0; virtual void operator=(T t) =0; virtual int operator!() =0; virtual int operator++() =0; };
// initialize the iterator // read current value // write current value // determine whether item exists // advance to next item
Recall that every pure virtual function prototype begins with the keyword virtual and ends with the code () =0 . The parentheses are required because it is a function, and the initializer =0 makes it a pure virtual function. Also recall that an abstract base class is any class that contains at least one pure virtual function. (See Section 12.9 on page 286.) Now we can use this abstract base class template to derive iterator templates for various container classes. The List class template in Example 13.7 on page 309 had an obvious shortcoming: it allowed insertions and deletions only at the front of the list. A list iterator will solve this problem, as shown in the next example. EXAMPLE 13.8 An Iterator Class Template for the List Class Template
#include "List.h" #include "Iterator.h" template<class T> class ListIter : public Iterator<T> { public: ListIter(List<T>& l) : list(l) { reset(); } virtual void reset() { previous = NULL; current = list.first; } virtual T operator()() { return current->data; } virtual void operator=(T t) { current->data = t; } virtual int operator!(); // determine whether current exists virtual int operator++(); // advance iterator to next item void insert(T t); // insert t after current item void preInsert(T t); // insert t before current item void remove(); // remove current item protected: ListNode<T>* current; // points to current node ListNode<T>* previous; // points to previous node List<T>& list; // this is the list being traversed }; In addition to a constructor and the five fundamental operations, we have added three other functions that will make lists much more useful. They allow the insertion and deletion of items anywhere in the list. The operator!() function serves two purposes. First it resets the current pointer if necessary, and then it reports back whether that pointer is NULL. The first purpose is to clean up after a call to the remove() function which deletes the node to which current points.
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