.NET/Java PDF, Tiff, Barcode SDK Library

where there is no change. The result is that a is unaltered, while b and c get the text set to "test." This is illustrated in Figure 1-4, in which you can see how the text "test" propagates through the objects. Now try to trace the following call. Can you tell what the outcome will be a->setText( "Qt" );

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We already saw how our public API is effectively a contract with our clients. With virtual functions, though, we are defining not only a contract for the caller, as usual, but also a contract for anyone who might choose to override that method. That requires more documentation, and a greater degree of control over how you implement the method.

By declaring a method as virtual, the base class gives derived classes permission to replace whole pieces of its own innards. That s a very powerful but very dangerous technique, rather like organ transplant surgery on an animal you ve never seen before. Even a trained surgeon might balk at replacing the kidneys of a dromedary armed with nothing more than developer-quality documentation about the process.

For example, some method in your base class calls its MethodA, then its MethodB, to do some work. You then (perhaps unknowingly) rely on that ordering when you provide overrides for MethodA and MethodB. If a future version of the base class changes that ordering, you will break. Let s go back to our example to look at that in more detail, because it is really important. First, let s change the implementation of Firefighter.ExtinguishFire so that it makes use of a couple of helper methods: TurnOnHose and TrainHoseOnFire (see Example 4-8).

class Firefighter { // This calls TrainHoseOnFire and TurnOnHose as part of the // process for extinguishing the fire public virtual void ExtinguishFire() { Console.WriteLine("{0} is putting out the fire!", Name); TrainHoseOnFire(); TurnOnHose(); } private void TurnOnHose() { Console.WriteLine("The fire is going out."); } private void TrainHoseOnFire() { Console.WriteLine("Training the hose on the fire."); } } // ...

By providing a signal for each slot (for example, textChanged corresponds to setText), you make Tip it possible to tie two objects together. In the previous example, the objects b and c always have the same value because a change in one triggers a change in the other. This is a very useful feature when one object is a part of a graphical user interface, as you will see later.

Let s also simplify our Main function so that we can see what is going on, as shown in Example 4-9.

Visual Studio 2005 provides a designer for the UpdatePanel control, including a Tasks pane that helps you set up the control. To use an UpdatePanel control, you simply drag and drop it onto the design surface of your web form (see Figure 6-15).

static void Main(string[] args) { // A reference to Joe, Harry's number one Firefighter joe = new Firefighter { Name = "Joe" }; joe.ExtinguishFire(); } Console.ReadKey();

If we compile and run, we ll see the following output:

The last time building Qt applications was mentioned, the reason for using the QMake tool was platform independence. Another big reason is that QMake handles the generation of meta-objects and includes them in the final application. Figure 1-5 shows how a standard C++ project is built.

All is well so far, but what happens if we add a trainee firefighter into the mix The trainee is extremely fastidious and follows his instructor s guidelines to the letter. We re going to make a class for him and override the TurnOnHose and TrainHoseOnFire methods so that the work is done in the trainee s own particular idiom. Hang on a moment, though! Our helper methods are private members. We can t get at them, except from other members of our Firefighter class itself. Before we can do anything, we need to make them accessible to derived classes.

In the preceding chapter, we mentioned that there were two additional accessibility modifiers that we would deal with later: protected and protected internal. Well, this is where they come into their own. They make members accessible to derived classes. If you want a member to be available either to derived classes or to other classes in your own assembly, you mark that member protected internal. It will be visible to other classes in the library, or to clients that derive classes from your base, but inaccessible to other clients who just reference your assembly. If, on the other hand, you want your class to make certain methods available only to derived classes, you just mark those methods protected. In terms of code out there in the wild, this is the most common usage, but it is not necessarily the best one!

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