Message maps may not be simple to read, but they are simple to create if you use ClassWizard. There are two ways to add an entry to a message map in Visual C++ 6.0: with the main ClassWizard dialog box or with one of the new dialog boxes that add message handlers or virtual functions. This section shows you these dialog boxes for ShowString, rather than work you through creating a sample application.

3.4.1 The ClassWizard Tabbed Dialog Box

The main ClassWizard dialog box is displayed by choosing View, ClassWizard or by pressing Ctrl+W. ClassWizard is a tabbed dialog box, and Figure 3.1 shows the Message Maps tab. At the top of the dialog box are two drop-down list boxes, one that reminds you which project you are working on (ShowString in this case) and the other that reminds you which class owns the message map you are editing. In this case, it is the CShowStringApp class, whose message map you have already seen.

FIG. 3.1 ClassWizard makes catching messages simple.

Below those single-line boxes is a pair of multiline boxes. The one on the left lists the class itself and all the commands that the user interface can generate. Commands are discussed in the "Commands" section later in this unit. With the classname highlighted, the box on the right lists all the Windows messages this class might catch. It also lists a number of virtual functions that catch common messages.

To the right of those boxes are buttons where you can add a new class to the project, add a function to the class to catch the highlighted message, remove a function that was catching a message, or open the source code for the function that catches the highlighted message. Typically, you select a class, select a message, and click Add Function to catch the message. Here’s what the Add Function button sets in motion:

• Adds a skeleton function to the bottom of the source file for the application

• Adds an entry to the message map in the source file

• Adds an entry to the message map in the include file

• Updates the list of messages and member functions in the dialog box

After you add a function, clicking Edit Code makes it simple to start filling in the behavior of that function. If you prefer, double-click the function name in the Member Functions list box.

Below the Object IDs and Messages boxes is a list of the member functions of this class that are related to messages. This class has two such functions:

The InitInstance function is called whenever an application first starts. You don’t need to understand this function to see that ClassWizard reminds you the function has been over-ridden.

Finally, under the Member Functions box is a reminder of the meaning of the highlighted message. called to implement wait cursors is a description of the DoWaitCursor virtual function.

In release 5.0 of Visual C++, a new way of catching messages was added. Rather than opening ClassWizard and then remembering to set the right classname in a drop-down list box, you right-click on the classname in ClassView and then choose Add Windows Message Handler from the shortcut menu that appears. Figure 3.2 shows the dialog box that appears when you make this choice.

FIG. 3.2 The New Windows Message and Event Handlers dialog box is another way to catch messages.

This dialog box doesn’t show any virtual functions that were listed in the main ClassView dialog box. It is easy to see that this class catches the command ID_APP_ABOUT but doesn’t catch the command update. (Commands and command updating are discussed in more detail later in this unit.) To add a new virtual function, you right-click on the class in ClassView and choose Add New Virtual Function from the shortcut menu.

CShowStringApp already overrides the InitInstance() virtual function, and you can see what other functions are available to be overridden. As in the tabbed dialog box, a message area at the bottom of the dialog box reminds you of the purpose of each function: In fact, the text—Called to implement wait cursors—is identical to that in Figure 3.1.

The only tricky part of message maps and message handling is deciding which class should catch the message. That’s a decision you can’t make until you understand all the different message and command targets that make up a typical application. The choice is usually one of the following:

• The active view

• The document associated with the active view

• The frame window that holds the active view

• The application object

Views, documents, and frames are discussed in Unit4, "Documents and Views."