4.2. Iterators

4.2. Iterators
	Chapter 4. Lists

[ fromfile: iterators.xml id: iterators ]

Any time you have a container of Things, sooner or later, you need to loop through the container and do something with (or to) each individual Thing. An iterator is an object that provides indirect access to each element in a container. It is specifically designed to be used in a loop.

Qt supports the following styles of iteration:

Qt style foreach loops, similar to Perl and Python
Java 1.2 style Iterator, which always points between elements
Standard Library style ContainerType::iterator
Hand-made while or for loops that use the getters of the container
QDirIterator, for iterating through entries in a directory structure

STL-style iterators behave more like pointers than do Java-style iterators. One important difference between iterators and pointers is that there is no iterator value that corresponds to the null value for pointers. For example, a function that uses pointers to search for something in a collection of items can return a null pointer if the search fails. There is no corresponding, generally recognizable iterator value to return that would indicate an unsuccessful iterator-based search.

The Iterator Pattern

A class, function, or programming element that provides a generic way to access elements of a collection sequentially without being type-restricted is an implementation of the Iterator design pattern. ^[36] C++ iterators, Java-style Iterators, and foreach loops are all examples of the Iterator pattern.

There are many classes in Qt that provide specialized iteration for various types of data, such as QDirIterator, QSortFilterProxyModel, QTreeWidgetItemIterator, and QDomNodeList. When an iterator is introduced, its definition must contain enough information to enable it to move from one item to the next in the container.

Section 4.2.1 demonstrates the various styles of iteration available in C++ with Qt.

^[36]We discuss design patterns in more detail in Section 7.4.


4.1. Introduction to Containers		4.2.1. `QStringList` and iteration