Visual C++ and MFC Programming 2nd Edition

Chapter 5: The Document/View Architecture Visual C++ and MFC Fundamentals

4.1 Introduction to Messages............................................................................

4.1.1 Overview...............................................................................................

Some of your applications will be made of various objects. Most of the time, more than

one application is running on the computer. These two scenarios mean that the operating

system is constantly asked to perform some assignments. Because there can be so many

requests presented unpredictably, the operating system leaves it up to the objects to

specify what they want, when they want it, and what behavior or result they expect.

The Microsoft Windows operating system cannot predict what kinds of requests one

object would need to be taken care of and what type of assignment another object would

need. To manage all these assignments and requests, the objects send messages, one

message at a time, to the operating system. For this reason, Microsoft Windows is said to

be a message-driven operating system.

The messages are divided in various categories but as mentioned already, each object has

the responsibility to decided what message to send and when. Therefore, most of the

messages we will review here are part of a window frame. Others will be addressed when

necessary.

Once a control has composed a message, it must send it to the right target which could be

the operating system. In order to send a message, a control must create an event. It is also

said to fire an event. To make a distinction between the two, a message's name usually

starts with WM_ which stands for Window Message. The name of an event usually starts

with On which indicates an action. Remember, the message is what needs to be sent. The

event is the action of sending the message.

4.1.2 A Map of Messages.............................................................................

For the compiler to manage messages, they should be included in the class definition. The

list of messages starts on a section driven by, but that ends with, the

DECLARE_MESSAGE_MAP macro. The section can be created as follows:

#include <afxwin.h>

class CSimpleFrame : public CFrameWnd

{

public:

CSimpleFrame();

DECLARE_MESSAGE_MAP()

};

The DECLARE_MESSAGE_MAP macro should be provided at the end of the class

definition. The actual messages (as we will review them shortly) should be listed just

above the DECLARE_MESSAGE_MAP line. This is just a rule. In some

circumstances, and for any reason, you may want, or have, to provide one message or a

few messages under the DECLARE_MESSAGE_MAP line.