Guide An Introduction to Design Patterns in C++ with Qt 4

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Every chapter of this edition has been improved with new content, better organization, or both. It has been. The authors introduce several new design patterns, add many quiz questions and labs, and present more efficient solutions relying on new Qt features and best practices. Alan Ezust received his M.

An Introduction to Design Patterns in C++ with Qt 4

Sc in computer science from McGill University in Montreal. He lives in Victoria, BC, Canada. He has done extensive outside consulting, contract programming, and research in computational mathematics. Alan Ezust , Paul Ezust. As you must have observed in the above example, Client creates objects of either TwoWheeler or FourWheeler based on some input during constructing its object. Say, library introduces a new class ThreeWheeler to incorporate three wheeler vehicles also.

What would happen? Client will end up chaining a new else if in the conditional ladder to create objects of ThreeWheeler.

An introduction to design patterns in C++ with QT 4 - CERN Document Server

Which in turn will need Client to be recompiled. So, each time a new change is made at the library side, Client would need to make some corresponding changes at its end and recompile the code.

PART I: Introduction to C++ and Qt 4 2

Sounds bad? For example, should the copy have the same parent as the original? Should the copy have in some sense the children of the original? A shallow copy of the child list would not work because then each of the children would have two parents. Furthermore, if the copy gets destroyed e. Even with resource sharing methods, this approach would introduce some serious difficulties. A deep copy of the child list could be a costly operation if the number of children were large and the objects pointed to were large.

Since each child could also have arbitrarily many children, this questionable approach would also generate serious difficulties. The complete implementation is shown in Example 9. All heap objects were implicitly destroyed. The output of this function, after all objects have been created, should look like this:.

According to [Gamma95], the Composite pattern is intended to facilitate building complex composite objects from simpler component parts by representing the part-whole hierarchies as tree-like structures.

An Introduction to Design Patterns in C++ with Qt™, Second Edition

This must be done in such a way that clients do not need to distinguish between simple parts and more complex parts that are made up of i. In Figure 9. We can express the whole-part relationship as a parent-child relationship between QObject s. The highest level i. The simplest QObject s i. Client code can recursively deal with each node of the tree. For an example of how this pattern might be used, let's look at Suffolk University.

In the founder, Gleason Archer, decided to start teaching the principles of law to a small group of tradesmen who wanted to become lawyers. He was assisted by one secretary and, after a while, a few instructors. The organizational chart for this new school was quite simple: a single office consisting of several employees with various tasks. As the enterprise grew, the chart gradually became more complex with the addition of new offices and departments. Today, years later, the Law School has been joined with a College of Arts and Sciences, a School of Management, a School of Art and Design, campuses abroad, and many specialized offices so that the organizational chart has become quite complex and promises to become more so.

Figure 9.

Factory Design Pattern in C++

Each box in the chart is a component. It may be composite and have sub-components which, in turn, may be composite or simple components.

See a Problem?

For example, the PresidentOffice has individual employees e. The leaves of this tree are the individual employees of the organization. We can use the Composite pattern to model this structure. Each node of the tree can be represented by an object of.

The QObject public interface allows us to build up a tree-like representation of the organization with code that instantiates an OrgUnit and then calls setParent to add it to the appropriate child list. A call to getSalary from any particular node returns the total salary for the part of the university represented by the subtree whose root is that node.

QObject provides convenient and powerful functions named findChildren for finding children in the child list.


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The signature of one of its overloaded forms looks like this:. If name is an empty string, findChildren works as a class filter by returning a QList holding pointers to all children, which can be typecast to type T. To call the function, you must supply a template parameter after the function name, as shown in Example 9. Interactive Qt applications with GUI have a different control flow from console applications and filter applications 2 because they are event-based, and often multi-threaded.


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Objects are frequently sending messages to each other, making a linear hand-trace through the code rather difficult. Observer Pattern - When writing event-driven programs, GUI views need to respond to changes in the state of data model objects, so that they can display the most recent information possible. When a particular subject object changes state, it needs an indirect way to alert and perhaps send additional information to all the other objects that are listening to state-change events, known as observers.

A design pattern that enables such a message-passing mechanism is called the Observer pattern , sometimes also known as the Publish-Subscribe pattern. There are many different implementations of this pattern. Some common characteristics that tie them together are. The mechanism used to send information from subjects to observers is completely specified in the subject's base class.


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Qt's approach is very different from Java's approach, because signals and slots rely on generated code, while Java just renames observer to listener. The Qt class QEvent encapsulates the notion of an event. QEvent objects can be created by the window system in response to actions of the user e. The type member function returns an enum that has nearly a hundred specific values that can identify the particular kind of event. A typical Qt program creates objects, connects them, and then tells the application to exec. At that point, the objects can send information to each other in a variety of ways.

QWidget s send QEvent s to other QObject s in response to user actions such as mouse clicks and keyboard events. A widget can also respond to events from the window manager such as repaints, resizes, or close events.