Visual C++/MFC Tutorial - Lesson 6: SDI and MDI Applications

Lesson 6: SDI and MDI Applications

We are getting to some advanced stuff now. In this lesson I am not going to go in depth at all. I will just give you a flavor of the structure of a SDI (single document interface) and a MDI (multiple document interface) application. In the last lesson we will build a SDI application and you can see the nitty gritty there.

The SDI application is typically used when you intend to work with only one data set at a time. For instance, the program notepad.exe is a SDI application. Netscape is also an SDI application. At any one time, there is only one document open at a time. Word for Windows and the VC++ developer studio are MDI applications. In these you can have several documents opened at once. This is particularly useful when you want to cut and paste between documents. Another use for MDI applications is to have one document, but several different views open that view the data differently.  A graphing application comes to mind where in one window you have a spreadsheet-like data list, and in another window you have a plot of the data. For small applications, a SDI application will usually be all you need. After you master it, the jump to MDI is a snap. Let's go over the structure of a SDI app.

Remember that in a dialog app, we had just two main classes. CWinApp and CDialog. Here again we have a CWinApp which serves the same purpose as it did in lesson 5. The CDialog class however is replaced by 3 other classes: CMainFrame, CDocument, and CView.

CDocument is a class that has no display, and typically doesn't react much with the messaging system of windows. It is used as a class to manage your data. MFC will create the code automatically which handles the event of File->Save, File->SaveAs, File->Close, and File->Open. All you need to do is to fill in the blank functions in the CDocument class.

Next is the CView. Most likely you will spend more time writing code to display and interact with the document's data then you will writing any other code. This is where the CView comes in. The CView is a class derived from CWnd, which is used for displaying your CDocument in some way. It is also one of the places where you can handle events like mouse clicks and what not. The heart of the CView is usually a call to get a pointer to your document followed by some drawing routines to display the data in your document.

The CMainFrame acts as a way to bridge the gap between your document/view classes and the rest of the application. Do you see that frame which goes all around applications boarders? That is the Main Frame window of the application. The title bar, the menu, the scroll bars, the status bar, and the tool bars are all part of the main frame window in an SDI application.  You typically put the code to handle these objects in the CMainFrame class. The CMainFrame class is the main window of the application. The CView class is typically a child window of the CMainFrame. (For the most part, the child/parent window relations just tell windows what windows are 'stuck' to what other windows. If you move a parent window, all of the children will move also. If you destroy a parent window, all of the children will be destroyed. Etc.)

You should have a pretty good idea now of how SDI applications are constructed. MDI applications are similar, but there can be several CDocument classes in existence at the same time and there is an added class called CChildFrame which acts as a connection between the CView and the CMainFrame.