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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"><HTML><HEAD> <TITLE>Teach Yourself Visual C++® 5 in 24 Hours -- Hour 9 -- The Document/View Architecture</TITLE></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF"><CENTER><H1><IMG SRC="../button/sams.gif" WIDTH="171" HEIGHT="66" ALIGN="BOTTOM" BORDER="0"><BR><FONT COLOR="#000077">Teach Yourself Visual C++® 5 in 24 Hours</FONT></H1></CENTER><CENTER><P><A HREF="../ch08/ch08.htm"><IMG SRC="../button/previous.gif" WIDTH="128" HEIGHT="28"ALIGN="BOTTOM" ALT="Previous chapter" BORDER="0"></A><A HREF="../ch10/ch10.htm"><IMGSRC="../button/next.gif" WIDTH="128" HEIGHT="28" ALIGN="BOTTOM" ALT="Next chapter"BORDER="0"></A><A HREF="../index.htm"><IMG SRC="../button/contents.gif" WIDTH="128"HEIGHT="28" ALIGN="BOTTOM" ALT="Contents" BORDER="0"></A> <HR></CENTER><CENTER><H1><FONT COLOR="#000077">- Hour 9 -<BR>The Document/View Architecture</FONT></H1></CENTER><P>The main topic for this hour is Document/View, the architecture used by programswritten using AppWizard and the MFC class library. In this hour, you will learn<UL> <LI>The support offered for Document/View by the MFC class library and tools such as AppWizard and ClassWizard<BR> <BR> <LI>The MFC classes used to implement Document/View<BR> <BR> <LI>Using pointers and references and the role they play in Document/View</UL><P>Also in this hour you will build DVTest, a sample program that will help illustratehow documents and views interact with each other in an MFC program.<H2><FONT COLOR="#000077"><B>Visual C++ Support for Document/View</B></FONT></H2><P>MFC and AppWizard use the Document/View architecture to organize programs writtenfor Windows. Document/View separates the program into four main classes:<UL> <LI>A document class derived from <TT>CDocument</TT><BR> <BR> <LI>A view class derived from <TT>CView</TT><BR> <BR> <LI>A frame class derived from <TT>CFrameWnd</TT><BR> <BR> <LI>An application class derived from <TT>CWinApp</TT></UL><P>Each of these classes has a specific role to play in an MFC Document/View application.The document class is responsible for the program's data. The view class handlesinteraction between the document and the user. The frame class contains the viewand other user interface elements, such as the menu and toolbars. The applicationclass is responsible for actually starting the program and handling some general-purposeinteraction with Windows. Figure 9.1 shows the four main parts of a Document/Viewprogram.</P><P><A NAME="01"></A><A HREF="01.htm"><B>Figure 9.1.</B></A> <I><BR>The Document/View architecture.</I></P><P>Although the name "Document/View" might seem to limit you to only word-processingapplications, the architecture can be used in a wide variety of program types. Thereis no limitation as to the data managed by <TT>CDocument</TT>; it can be a word processingfile, a spreadsheet, or a server at the other end of a network connection providinginformation to your program. Likewise, there are many types of views. A view canbe a simple window, as used in the simple SDI applications presented so far, or itcan be derived from <TT>CFormView</TT>, with all the capabilities of a dialog box.You will learn about form views in Hour 23, "Advanced Views."<H3><FONT COLOR="#000077"><B>SDI and MDI Applications</B></FONT></H3><P>There are two basic types of Document/View programs:<UL> <LI>SDI, or Single Document Interface<BR> <BR> <LI>MDI, or Multiple Document Interface</UL><P>An SDI program supports a single type of document and almost always supports onlya single view. Only one document can be open at a time. An SDI application focuseson a particular task and usually is fairly straightforward.</P><P>Several different types of documents can be used in an MDI program, with eachdocument having one or more views. Several documents can be open at a time, and theopen document often uses a customized toolbar and menus that fit the needs of thatparticular document.<H3><FONT COLOR="#000077"><B>Why Use Document/View?</B></FONT></H3><P>The first reason to use Document/View is that it provides a large amount of applicationcode for free. You should always try to write as little new source code as possible,and that means using MFC classes and letting AppWizard and ClassWizard do a lot ofthe work for you. A large amount of the code that is written for you in the formof MFC classes and AppWizard code uses the Document/View architecture.</P><P>The Document/View architecture defines several main categories for classes usedin a Windows program. Document/View provides a flexible framework that you can useto create almost any type of Windows program. One of the big advantages of the Document/Viewarchitecture is that it divides the work in a Windows program into well-defined categories.Most classes fall into one of the four main class categories:<UL> <LI>Controls and other user-interface elements related to a specific view<BR> <BR> <LI>Data and data-handling classes, which belong to a document<BR> <BR> <LI>Work that involves handling the toolbar, status bar, and menus, usually belonging to the frame class<BR> <BR> <LI>Interaction between the application and Windows occurring in the class derived from <TT>CWinApp</TT></UL><P>Dividing work done by your program helps you manage the design of your programmore effectively. Extending programs that use the Document/View architecture is fairlysimple because the four main Document/View classes communicate with each other throughwell-defined interfaces. For example, to change an SDI program to an MDI program,you must write little new code. Changing the user interface for a Document/View programimpacts only the view class or classes; no changes are needed for the document, frame,or application classes.<H3><FONT COLOR="#000077"><B>Using AppWizard</B></FONT></H3><P>Use AppWizard to create SDI and MDI applications. In earlier chapters, you useAppWizard to create the SDI programs used as examples. Although doing so is morecomplicated, you can use AppWizard to create an MDI application almost as easilyas an SDI.</P><P>The basic difference between an SDI application and an MDI application is thatan MDI application must manage multiple documents and, usually, multiple views. TheSDI application uses only a single document, and normally only a single view.</P><P>Both SDI and MDI applications use an object called a document template to createa relationship between a view, a document, and a frame class, as well as an identifierused for the program's menu, icon, and other resources. You use the <TT>CSingleDocTemplate</TT>class for SDI applications and the <TT>CMultiDocTemplate</TT> class for MDI applications.These two classes share a common base class, <TT>CDocTemplate</TT>. Listing 9.1 isan example of a document template used for an SDI program.<H4><FONT COLOR="#000077">TYPE: Listing 9.1. How AppWizard uses a document templatein an SDI application.</FONT></H4><PRE><FONT COLOR="#0066FF"><TT>CSingleDocTemplate* pDocTemplate;</TT><TT>pDocTemplate = new CSingleDocTemplate( IDR_MAINFRAME,</TT><TT> RUNTIME_CLASS(CTestDoc),</TT><TT> RUNTIME_CLASS(CMainFrame),</TT><TT> RUNTIME_CLASS(CTestView));</TT><TT>AddDocTemplate(pDocTemplate);</TT></FONT></PRE><P>Two types of frame windows exist in an MDI program: the main frame, which encompassesthe entire client area, and the child frame, which contains each MDI child window.The different windows used in an MDI program are shown in Figure 9.2.</P><P><A NAME="02"></A><A HREF="02.htm"><B>Figure 9.2.</B> </A><BR><I>The windows used in a typical MDI program.</I></P><P>The C++ source code generated by Developer Studio for an MDI program is slightlydifferent than the code it generates for an SDI program. Examine this code, shownin Listing 9.2, to see some of the differences between MDI objects and SDI objects.<H4><FONT COLOR="#000077">TYPE: Listing 9.2. AppWizard code that uses a documenttemplate in an MDI application.</FONT></H4><PRE><FONT COLOR="#0066FF"><TT>CMultiDocTemplate* pDocTemplate;</TT><TT>pDocTemplate = new CMultiDocTemplate( IDR_TESTTYPE,</TT><TT> RUNTIME_CLASS(CTestDoc),</TT><TT> RUNTIME_CLASS(CChildFrame),</TT><TT> RUNTIME_CLASS(CTestView));</TT><TT>AddDocTemplate(pDocTemplate);</TT></FONT></PRE><P><TT>CChildFrame</TT> is a class included in every MDI project created by AppWizard,and is derived from <TT>CMDIChildFrame</TT>. This class is provided to make customizingthe frame to suit your needs easy. Every MDI child window has a frame that owns theminimize, maximize, and close buttons and the frame around the view. Any customizationyou want to do to the frame is done in the <TT>CChildFrame</TT> class.<H3><FONT COLOR="#000077"><B>Using ClassWizard</B></FONT></H3><P>You have used ClassWizard in previous hours to add member variables to dialogbox classes, add new classes to a project, and handle messages sent to view windowsand dialog boxes. You also use ClassWizard to add interfaces defined as part of theDocument/View architecture. In most cases, default behavior provided by the MFC frameworkis enough for simple programs.</P><P>You will learn about the interfaces used by the document and view classes in thenext section. However, you add almost all these interfaces using ClassWizard. Let'slook at one of these interfaces, <TT>GetFirstViewPosition</TT>. A document can obtaina pointer to the first view associated with the document using this function. Normally,the framework will maintain a list of the views associated with a document, but youcan keep this list yourself by overriding this function. Because the <TT>GetFirstViewPosition</TT>function is virtual, your implementation of it is always called if available.</P><P>To add an implementation for one of the Document/View interface functions, followthese steps, which are similar to the steps used to add message-handling functions:<DL> <DD>1. Open ClassWizard.<BR> <BR> 2. Select the name of the class that supplies the interface to be added; in this case, a class derived from <TT>CDocument</TT>.<BR> <BR> 3. Select the Message Maps tab.<BR> <BR> 4. Select the <TT>CDocument</TT>-derived class as the object ID.<BR> <BR> 5. Select the interface function to be added from the list box.<BR> <BR> 6. Click the Add Function button.<BR> <BR> 7. Close ClassWizard.</DL><P>You can use ClassWizard to override all the interfaces defined for programs usingthe Document/View architecture. Interfaces such as <TT>GetFirstViewPosition</TT>are rarely overridden, except when debugging. If you provide a new version of <TT>GetFirstViewPosition</TT>,you probably should override the related function <TT>GetNextView</TT> as well.</P><P>For the remaining examples in this hour, you will create an MDI project namedDVTest. To create the DVTest example, use AppWizard to create a default MDI program.Name the program DVTest. Feel free to accept or change any of the default parametersoffered by AppWizard because they have no impact on these examples. When finished,DVTest displays a collection of names stored by the document class.<H2><FONT COLOR="#000077"><B>Pointers and References</B></FONT></H2><P>Pointers are important topics in C++ programming. A good understanding of theways in which pointers are used will help you write programs that are more flexibleand reliable. C++, and MFC in particular, relies very heavily on proper understandingand use of pointers.</P><P><FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>A <I>pointer</I> is simplya numeric variable. This numeric variable is an <I>address</I>, or location in memorywhere the actual data resides.<I> </I>Pointers must also follow the same rules thatare applied to other variables. They must have unique names, and they must be declaredbefore they can be used.</P><P>Every object or variable that is used in an application takes up a location ormultiple locations in memory. This memory location is accessed via an address (seeFigure 9.3).</P><P><A NAME="03"></A><A HREF="03.htm"><B>Figure 9.3.</B> </A><BR><I>The text <TT>Hello</TT> stored beginning at address 1000.</I></P><P>In this figure, the text <I><TT>Hello</TT></I> is stored in memory beginning ataddress 1000. Each character takes up a unique address space in memory. Pointersprovide a method for holding and getting to these addresses in memory. Pointers makemanipulating the data easier because they hold the address of another variable ordata location.<BLOCKQUOTE> <P><HR><B> </B><FONT COLOR="#000077"><B>Just a Minute:</B></FONT><B> </B>Pointers give flexibility to C++ programs and enable the programs to grow dynamically. By using a pointer to a block of memory that is allocated at runtime, a program can be much more flexible than one that allocates all its memory at once. <HR></BLOCKQUOTE><P>A pointer is also easier to store than a large structure or class object. Becausea pointer just stores an address, it can easily be passed to a function. However,if an object is passed to a function, the object must be constructed, copied, anddestroyed, which can be costly for large objects.<H3><FONT COLOR="#000077"><B>How Are Pointers Used?</B></FONT></H3><P>When using pointers and memory addresses, it is often useful to know the amountof memory required for each object pointed to. If you must know the amount of storagerequired for a particular object or variable, you can use the <TT>sizeof</TT> operator.You can also use <TT>sizeof</TT> to determine the amount of storage required foryour own classes and structures, as shown in Listing 9.3.<H4><FONT COLOR="#000077">TYPE: Listing 9.3. Using sizeof with class types.</FONT></H4><PRE><FONT COLOR="#0066FF"><TT>void DisplayBtnSize()</TT><TT>{</TT><TT> int nSize = sizeof(CButton);</TT><TT> CString strMsg;</TT><TT> strMsg.Format("The size of CButton is %d bytes", nSize);</TT><TT> AfxMessageBox(strMsg);</TT><TT>}</TT></FONT></PRE><H3><FONT COLOR="#000077"><B>The Indirection and Address Operators</B></FONT></H3><P>Two operators are used when working with addresses in a C++ program: the <I>address-ofoperator</I> (<TT>&</TT>) and the <I>indirection operator</I> (<TT>*</TT>). Theseoperators are different from operators seen previously because they are <I>unary</I>,meaning that they work with only one operand.</P><P>The address-of operator, <TT>&</TT>, returns the address of a variable orobject. This operator is associated with the object to its right, like this:</P><PRE><FONT COLOR="#0066FF"><TT>&myAge;</TT></FONT></PRE><P>This line returns the address of the <TT>myAge</TT> variable.</P><P>The indirection operator, <TT>*</TT>, works like the address-of operator in reverse.It also is associated with the object to its right, and it takes an address and returnsthe object contained at that address. For example, the following line determinesthe address of the <TT>myAge</TT> variable; then it uses the indirection operatorto access the variable and give it a value of 42:</P><PRE><FONT COLOR="#0066FF"><TT>*(&myAge) = 42;</TT></FONT></PRE><H3><FONT COLOR="#000077"><B>Using the Indirection Operator</B></FONT></H3><P>You can use a pointer with the indirection operator to change the value of theother variable, as shown in the console-mode program in Listing 9.4.<H4><FONT COLOR="#000077">TYPE: Listing 9.4. Using a pointer variable with the indirectionoperator.</FONT></H4><PRE><FONT COLOR="#0066FF"><TT>#include <iostream></TT><TT>using namespace std;</TT><TT>int main()</TT><TT>{</TT><TT> int nVar;</TT><TT> int* pVar;</TT><TT> // Store a value in nVar, and display it. Also</TT><TT> // display nVar's address.</TT><TT> nVar = 5;</TT><TT> cout << "nVar's value is " << nVar << "." << endl;</TT><TT> cout << "nVar's address is " << &nVar << "." << endl;</TT><TT> // Store the address of nVar in pointer pVar. Display</TT><TT> // information about pVar and the address it points to.</TT><TT> pVar = &nVar;</TT><TT> cout << "pVar's value is " << pVar << "." << endl;</TT><TT> cout << "*pVar's value is " << *pVar << "." << endl;</TT><TT> // Change the value of the variable pointed to by pVar.</TT><TT> *pVar = 7;</TT><TT> cout << "nVar's value is " << nVar << "." << endl;</TT>?? 快捷鍵說明
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