Rogue Wave banner
Previous fileTop of DocumentContentsIndex pageNext file
Essential Tools Module User's Guide

6.13 Iterators in Class Templates

The Essential Tools Module provides several distinct methods for iterating over a collection class. Most collections offer an apply member function, which applies your supplied function to every element of a collection class before returning. Another form of iteration is provided by separate collaborating iterator classes associated with many of the collections. For example, an RWTPtrDlistIterator<T,A> can be used to visit each element of an RWTPtrDlist<T,A> in turn. Iterators are described in Section 6.6.

6.13.1 Standard C++ Library Iterators

Al Essential Tools Module standard library-based collection class templates provide standard iterators. These iterators are fully compliant with the Standard C++ Library requirements for iterators, making them a powerful tool for using the classes in conjunction with the Standard C++ Library -- especially the algorithms. Although full treatment of iterators is beyond the scope of this guide, your Standard C++ Library reference and tutorials will provide ample information.

The standard library-based collection class templates provide three types of iterators: forward, bi-directional, and random-access. Forward iterators allow unidirectional traversal from beginning to end. As suggested by the name, bidirectional iterators allow traversal in both directions -- front to back, and back to front. Random-access iterators are bidirectional as well, and further distinguished by their ability to advance over an arbitrary number of elements in constant time. All of these iterators allow access to the item at the current position via the dereference operator *.

Given iterator iter and an integral value n, the following basic operations are just some of those supported:

Expression

Meaning

Supported by:

++iter;

advance to next item and return

Forw, Bidir, Random

iter++;

advance to next item, return original value

Forw, Bidir, Random

*iter;

return reference to item at current position

Forw, Bidir, Random

--iter;

retreat to previous item and return

Bidir, Random

iter--;

retreat to previous item, return original value

Bidir, Random

iter+=n;

advance n items and return

Random

iter-=n;

retreat n items and return

Random

Again, your standard library documentation will describe all the operators and functions available for each type of iterator.

In addition to the iterators just described, the standard library-based collection class templates also provide two typedefs used to iterate over the items in a collection class: iterator, and const_iterator. You can use the iterator typedef to traverse a collection class and modify the elements within. You can use instances of const_iterator to traverse, but not modify, the collection class and access elements. For the associative container-based and sorted sequence-based collections, which do not allow modification of elements once they are in the collection class, the iterator and const_iterator types are the same.

Finally, the templates also provide two member functions that return actual iterators you can use for traversing their respective collection classes. These member functions are begin() and end(). Each of these member functions is overloaded by a const receiver so that the non-const version returns an instance of type iterator, and the const version returns an instance of type const_iterator.

Member function begin() always returns an iterator already positioned at the first item in the collection class. Member function end() returns an iterator which has a past-the-end value, the way a pointer to the NULL character of a null-terminated character string has a value that points "past the end." An iterator of past-the-end value can be used to compare with another iterator to see if you've finished visiting all the elements in the collection class. It can also be used as a starting point for moving backwards through collection classes that provide either bidirectional or random-access iterators. The one thing you cannot do with an end() iterator is deference it. Here's an example using iterators to move through a list and search for a match:

6.13.2 Map-Based Iteration and Pairs

In the case of a map-based collection class, like RWTValMap<K,T,C,A>, iterators refer to instances of the Standard C++ Library structure pair<const K, T>. As you iterate over a map-based collection, you have access to both the key and its associated data at each step along the traversal. The pair structure provides members first and second, which allow you to individually access the key and its data, respectively. For example:

Note that given a non-const reference to such a pair, you can still modify only the second element -- the data associated with the key. This is because the first element is declared to be of type const K. Because the placement of objects within the collection class is maintained internally, based on the value of the key, declaring it as const protects the integrity of the collection class. In order to change a key within a map, you have to remove the key and its data entirely, and replace them with a new entry.

6.13.3 Iterators as Generalized Pointers

It may not be obvious at first, but you can think of an iterator as a generalized pointer. Imagine a pointer to an array of int s. The array itself is a collection class, and a pointer to an element of that array is a random-access iterator. To advance to the next element, you simply use the unary operator ++. To move back to a previous element, you use --. To access the element at the current position of the iterator, you use the unary operator *. Finally, it is important to know when you have visited all the elements. C++ guarantees that you can always point to the first address past the end of an allocated array. For example:

If you compare this code fragment to the one using standard iterators in Section 6.13.1, you can see the similarities. If you need a bit of help imagining how the standard iterators work, you can always picture them as generalized pointers.



Previous fileTop of DocumentContentsIndex pageNext file

©2004 Copyright Quovadx, Inc. All Rights Reserved.
Rogue Wave and SourcePro are registered trademarks of Quovadx, Inc. in the United States and other countries. All other trademarks are the property of their respective owners.
Contact Rogue Wave about documentation or support issues.