Introduction to MFC
Programming with Visual C++ Version 6.x
by Marshall Brain
A Simple
MFC Program
In this tutorial
we will examine a simple MFC program piece by piece to gain an
understanding of its structure and conceptual framework. We will
start by looking at MFC itself and then examine how MFC is used
to create applications.
MFC is a large and extensive C++ class hierarchy that makes Windows application development significantly easier. MFC is compatible across the entire Windows family. As each new version of Windows comes out, MFC gets modified so that old code compiles and works under the new system. MFC also gets extended, adding new capabilities to the hierarchy and making it easier to create complete applications.
The advantage of using MFC and C++ - as opposed to directly accessing the Windows API from a C program-is that MFC already contains and encapsulates all the normal "boilerplate" code that all Windows programs written in C must contain. Programs written in MFC are therefore much smaller than equivalent C programs. On the other hand, MFC is a fairly thin covering over the C functions, so there is little or no performance penalty imposed by its use. It is also easy to customize things using the standard C calls when necessary since MFC does not modify or hide the basic structure of a Windows program.
The best part about using MFC is that it does all of the hard work for you. The hierarchy contains thousands and thousands of lines of correct, optimized and robust Windows code. Many of the member functions that you call invoke code that would have taken you weeks to write yourself. In this way MFC tremendously accelerates your project development cycle.
MFC is fairly large. For example, Version 4.0 of the hierarchy contains something like 200 different classes. Fortunately, you don't need to use all of them in a typical program. In fact, it is possible to create some fairly spectacular software using only ten or so of the different classes available in MFC. The hierarchy is broken into several different class categories which include (but is not limited to):
| Visualizing MFC |
|---|
| One of the most frusterating things when you are first learning MFC is the "Where am I?" feeling you get. MFC has hundreds of classes. A good way to get around this feeling is to use a class hierarchy visualization tool like CodeVizor. With CodeVizor you can drag the source code for MFC into the CodeVizor tool and in about 30 seconds have a beautiful, clickable (and printable!) class hierarchy chart. Get CodeVizor and see how much easier undestanding MFC becomes! |
We will concentrate on visual objects in these tutorials. The list below shows the portion of the class hierarchy that deals with application support and windows support.
There are several things to notice in this list. First, most classes in MFC derive from a base class called CObject. This class contains data members and member functions that are common to most MFC classes. The second thing to notice is the simplicity of the list. The CWinApp class is used whenever you create an application and it is used only once in any program. The CWnd class collects all the common features found in windows, dialog boxes, and controls. The CFrameWnd class is derived from CWnd and implements a normal framed application window. CDialog handles the two normal flavors of dialogs: modeless and modal respectively. CView is used to give a user access to a document through a window. Finally, Windows supports six native control types: static text, editable text, push buttons, scroll bars, lists, and combo boxes (an extended form of list). Once you understand this fairly small number of pieces, you are well on your way to a complete understanding of MFC. The other classes in the MFC hierarchy implement other features such as memory management, document control, data base support, and so on.
To create a program in MFC, you either use its classes directly or, more commonly, you derive new classes from the existing classes. In the derived classes you create new member functions that allow instances of the class to behave properly in your application. You can see this derivation process in the simple program we used in Tutorial 1, which is described in greater detail below. Both CHelloApp and CHelloWindow are derived from existing MFC classes.
Before discussing the code itself, it is worthwhile to briefly discuss the program design process under MFC. As an example, imagine that you want to create a program that displays the message "Hello World" to the user. This is obviously a very simple application but it still requires some thought.
A "hello world" application first needs to create a window on the screen that holds the words "hello world". It then needs to get the actual "hello world" words into that window. Three objects are required to accomplish this task:
Every program that you create in MFC will contain the first two objects. The third object is unique to this particular application. Each application will define its own set of user interface objects that display the application's output as well as gather input from the user.
Once you have completed the user interface design and decided on the controls necessary to implement the interface, you write the code to create the controls on the screen. You also write the code that handles the messages generated by these controls as they are manipulated by the user. In the case of a "hello world" application, only one user interface control is necessary. It holds the words "hello world". More realistic applications may have hundreds of controls arranged in the main window and dialog boxes.
It is important to note that there are actually two different ways to create user controls in a program. The method described here uses straight C++ code to create the controls. In a large application, however, this method becomes painful. Creating the controls for an application containing 50 or 100 dialogs using C++ code to do it would take an eon. Therefore, a second method uses resource files to create the controls with a graphical dialog editor. This method is much faster and works well on most dialogs.
The listing below shows the code for the simple "hello world" program that you entered, compiled and executed in Tutorial 1. Line numbers have been added to allow discussion of the code in the sections that follow. By walking through this program line by line, you can gain a good understanding of the way MFC is used to create simple applications.
If you have not done so already, please compile and execute the code below by following the instructions given in Tutorial 1.
1 //hello.cpp
2 #include <afxwin.h>
3 // Declare the application class
4 class CHelloApp : public CWinApp
5 {
6 public:
7 virtual BOOL InitInstance();
8 };
9 // Create an instance of the application class
10 CHelloApp HelloApp;
11 // Declare the main window class
12 class CHelloWindow : public CFrameWnd
13 {
14 CStatic* cs;
15 public:
16 CHelloWindow();
17 };
18 // The InitInstance function is called each
19 // time the application first executes.
20 BOOL CHelloApp::InitInstance()
21 {
22 m_pMainWnd = new CHelloWindow();
23 m_pMainWnd->ShowWindow(m_nCmdShow);
24 m_pMainWnd->UpdateWindow();
25 return TRUE;
26 }
27 // The constructor for the window class
28 CHelloWindow::CHelloWindow()
29 {
30 // Create the window itself
31 Create(NULL,
32 "Hello World!",
33 WS_OVERLAPPEDWINDOW,
34 CRect(0,0,200,200));
35 // Create a static label
36 cs = new CStatic();
37 cs->Create("hello world",
38 WS_CHILD|WS_VISIBLE|SS_CENTER,
39 CRect(50,80,150,150),
40 this);
41 }
Take a moment and look through this program. Get a feeling for the "lay of the land." The program consists of six small parts, each of which does something important.
The program first includes afxwin.h (line 2). This header file contains all the types, classes, functions, and variables used in MFC. It also includes other header files for such things as the Windows API libraries.
Lines 3 through 8 derive a new application class named CHelloApp from the standard CWinApp application class declared in MFC. The new class is created so the InitInstance member function in the CWinApp class can be overridden. InitInstance is a virtual function that is called as the application begins execution.
In Line 10, the code declares an instance of the application object as a global variable. This instance is important because it causes the program to execute. When the application is loaded into memory and begins running, the creation of that global variable causes the default constructor for the CWinApp class to execute. This constructor automatically calls the InitInstance function defined in lines 18 though 26.
In lines 11 through 17, the CHelloWindow class is derived from the CFrameWnd class declared in MFC. CHelloWindow acts as the application's window on the screen. A new class is created so that a new constructor, destructor, and data member can be implemented.
Lines 18 through 26 implement the InitInstance function. This function creates an instance of the CHelloWindow class, thereby causing the constructor for the class in Lines 27 through 41 to execute. It also gets the new window onto the screen.
Lines 27 through 41 implement the window's constructor. The constructor actually creates the window and then creates a static control inside it.
An interesting thing to notice in this program is that there is no main or WinMain function, and no apparent event loop. Yet we know from executing it in Tutorial 1 that it processed events. The window could be minimized and maximized, moved around, and so on. All this activity is hidden in the main application class CWinApp and we therefore don't have to worry about it-event handling is totally automatic and invisible in MFC.
The following sections describe the different pieces of this program in more detail. It is unlikely that all of this information will make complete sense to you right now: It's best to read through it to get your first exposure to the concepts. In Tutorial 3, where a number of specific examples are discussed, the different pieces will come together and begin to clarify themselves.
Every program that you create in MFC will contain a single application object that you derive from the CWinApp class. This object must be declared globally (line 10) and can exist only once in any given program.
An object derived from the CWinApp class handles initialization of the application, as well as the main event loop for the program. The CWinApp class has several data members, and a number of member functions. For now, almost all are unimportant. If you would like to browse through some of these functions however, search for CWinApp in the MFC help file by choosing the Search option in the Help menu and typing in "CWinApp". In the program above, we have overridden only one virtual function in CWinApp, that being the InitInstance function.
The purpose of the application object is to initialize and control your application. Because Windows allows multiple instances of the same application to run simultaneously, MFC breaks the initialization process into two parts and uses two functions-InitApplication and InitInstance-to handle it. Here we have used only the InitInstance function because of the simplicity of the application. It is called each time a new instance of the application is invoked. The code in Lines 3 through 8 creates a class called CHelloApp derived from CWinApp. It contains a new InitInstance function that overrides the existing function in CWinApp (which does nothing):
3 // Declare the application class
4 class CHelloApp : public CWinApp
5 {
6 public:
7 virtual BOOL InitInstance();
8 };
Inside the overridden InitInstance function at lines 18 through 26, the program creates and displays the window using CHelloApp's data member named m_pMainWnd:
18 // The InitInstance function is called each
19 // time the application first executes.
20 BOOL CHelloApp::InitInstance()
21 {
22 m_pMainWnd = new CHelloWindow();
23 m_pMainWnd->ShowWindow(m_nCmdShow);
24 m_pMainWnd->UpdateWindow();
25 return TRUE;
26 }
The InitInstance function returns a TRUE value to indicate that initialization completed successfully. Had the function returned a FALSE value, the application would terminate immediately. We will see more details of the window initialization process in the next section.
When the application object is created at line 10, its data members (inherited from CWinApp) are automatically initialized. For example, m_pszAppName, m_lpCmdLine, and m_nCmdShow all contain appropriate values. See the MFC help file for more information. We'll see a use for one of these variables in a moment.
MFC defines two types of windows: 1) frame windows, which are fully functional windows that can be re-sized, minimized, and so on, and 2) dialog windows, which are not re-sizable. A frame window is typically used for the main application window of a program.
In the code shown in listing 2.1, a new class named CHelloWindow is derived from the CFrameWnd class in lines 11 through 17:
11 // Declare the main window class
12 class CHelloWindow : public CFrameWnd
13 {
14 CStatic* cs;
15 public:
16 CHelloWindow();
17 };
The derivation contains a new constructor, along with a data member that will point to the single user interface control used in the program. Each application that you create will have a unique set of controls residing in the main application window. Therefore, the derived class will have an overridden constructor that creates all the controls required in the main window. Typically this class will also have an overridden destructor to delete them when the window closes, but the destructor is not used here. In Tutorial 4, we will see that the derived window class will also declare a message handler to handle messages that these controls produce in response to user events.
Typically, any application you create will have a single main application window. The CHelloApp application class therefore defines a data member named m_pMainWnd that can point to this main window. To create the main window for this application, the InitInstance function (lines 18 through 26) creates an instance of CHelloWindow and uses m_pMainWnd to point to the new window. Our CHelloWindow object is created at line 22:
18 // The InitInstance function is called each
19 // time the application first executes.
20 BOOL CHelloApp::InitInstance()
21 {
22 m_pMainWnd = new CHelloWindow();
23 m_pMainWnd->ShowWindow(m_nCmdShow);
24 m_pMainWnd->UpdateWindow();
25 return TRUE;
26 }
Simply creating a frame window is not enough, however. Two other steps are required to make sure that the new window appears on screen correctly. First, the code must call the window's ShowWindow function to make the window appear on screen (line 23). Second, the program must call the UpdateWindow function to make sure that each control, and any drawing done in the interior of the window, is painted correctly onto the screen (line 24).
You may wonder where the ShowWindow and UpdateWindow functions are defined. For example, if you wanted to look them up to learn more about them, you might look in the MFC help file (use the Search option in the Help menu) at the CFrameWnd class description. CFrameWnd does not contain either of these member functions, however. It turns out that CFrameWnd inherits its behavior-as do all controls and windows in MFC-from the CWnd class (see figure 2.1). If you refer to CWnd in the MFC documentation, you will find that it is a huge class containing over 200 different functions. Obviously, you are not going to master this particular class in a couple of minutes, but among the many useful functions are ShowWindow and UpdateWindow.
Since we are on the subject, take a minute now to look up the CWnd::ShowWindow function in the MFC help file. You do this by clicking the help file's Search button and entering "ShowWindow". As an alternative, find the section describing the CWnd class using the Search button, and then find the ShowWindow function under the Update/Painting Functions in the class member list. Notice that ShowWindow accepts a single parameter, and that the parameter can be set to one of ten different values. We have set it to a data member held by CHelloApp in our program, m_nCmdShow (line 23). The m_nCmdShow variable is initialized based on conditions set by the user at application start-up. For example, the user may have started the application from the Program Manager and told the Program Manager to start the application in the minimized state by setting the check box in the application's properties dialog. The m_nCmdShow variable will be set to SW_SHOWMINIMIZED, and the application will start in an iconic state. The m_nCmdShow variable is a way for the outside world to communicate with the new application at start-up. If you would like to experiment, you can try replacing m_nCmdShow in the call to ShowWindow with the different constant values defined for ShowWindow . Recompile the program and see what they do.
Line 22 initializes the window. It allocates memory for it by calling the new function. At this point in the program's execution the constructor for the CHelloWindow is called. The constructor is called whenever an instance of the class is allocated. Inside the window's constructor, the window must create itself. It does this by calling the Create member function for the CFrameWnd class at line 31:
27 // The constructor for the window class
28 CHelloWindow::CHelloWindow()
29 {
30 // Create the window itself
31 Create(NULL,
32 "Hello World!",
33 WS_OVERLAPPEDWINDOW,
34 CRect(0,0,200,200));
Four parameters are passed to the create function. By looking in the MFC documentation you can see the different types. The initial NULL parameter indicates that a default class name be used. The second parameter is the title of the window that will appear in the title bar. The third parameter is the style attribute for the window. This example indicates that a normal, overlappable window should be created. Style attributes are covered in detail in Tutorial 3. The fourth parameter specifies that the window should be placed onto the screen with its upper left corner at point 0,0, and that the initial size of the window should be 200 by 200 pixels. If the value rectDefault is used as the fourth parameter instead, Windows will place and size the window automatically for you.
Since this is an extremely simple program, it creates a single static text control inside the window. In this particular example, the program uses a single static text label as its only control, and it is created at lines 35 through 40. More on this step in the next section.
The program derives the CHelloWindow class from the CFrameWnd class (lines 11 through 17). In doing so it declares a private data member of type CStatic*, as well as a constructor.
As seen in the previous section, the CHelloWindow constructor does two things. First it creates the application's window by calling the Create function (line 31), and then it allocates and creates the control that belongs inside the window. In this case a single static label is used as the only control. Object creation is always a two-step process in MFC. First, the memory for the instance of the class is allocated, thereby calling the constructor to initialize any variables. Next, an explicit Create function is called to actually create the object on screen. The code allocates, constructs, and creates a single static text object using this two-step process at lines 36 through 40:
27 // The constructor for the window class
28 CHelloWindow::CHelloWindow()
29 {
30 // Create the window itself
31 Create(NULL,
32 "Hello World!",
33 WS_OVERLAPPEDWINDOW,
34 CRect(0,0,200,200));
35 // Create a static label
36 cs = new CStatic();
37 cs->Create("hello world",
38 WS_CHILD|WS_VISIBLE|SS_CENTER,
39 CRect(50,80,150,150),
40 this);
41 }
The constructor for the CStatic item is called when the memory for it is allocated, and then an explicit Create function is called to create the CStatic control's window. The parameters used in the Create function here are similar to those used for window creation at Line 31. The first parameter specifies the text to be displayed by the control. The second parameter specifies the style attributes. The style attributes are discussed in detail in the next tutorial but here we requested that the control be a child window (and therefore displayed within another window), that it should be visible, and that the text within the control should be centered. The third parameter determines the size and position of the static control. The fourth indicates the parent window for which this control is the child. Having created the static control, it will appear in the application's window and display the text specified.
In looking at this code for the first time,
it will be unfamiliar and therefore potentially annoying. Don't
worry about it. The only part in the entire program that matters
from an application programmer's perspective is the CStatic
creation code at lines 36 through 40. The rest you will type in
once and then ignore. In the next tutorial you will come to a
full understanding of what lines 36 through 40 do, and see a number
of options that you have in customizing a CStatic control.