// Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies). // SPDX-License-Identifier: BSD-3-Clause /*! \page winmigrate-walkthrough.html \title MFC to Qt Migration - Walkthrough \tableofcontents \section1 Introduction This walkthrough covers the migration of an MFC based program, generated by Microsoft Visual Studio's MFC Application Wizard, to the Qt toolkit using the \e{Windows Migration Framework}. \section2 Goals for this Walkthrough The original application, located in the \c step1 directory, is a trivial SDI program with a main window including a menu bar, a single child view and an about dialog. The walkthrough will demonstrate how easy it is to gradually replace the MFC code in an existing application with multiplatform Qt code, while being able to extend the program with new functionality based on Qt. The final step will be a complete replacement of the Windows-only MFC code with a single source that can be compiled for any platform supported by Qt, including Windows, Mac OS X, Linux/Unix and embedded Linux. \section2 A note about MFC's memory leak detection MFC contains a checkpoint-based memory leak detection mechanism. This mechanism does not handle well Qt's system of allocating global static objects. The result is that when running applications that combine Qt and MFC (like the examples below) from within Visual Studio, one will get a report about leaked objects upon application exit. These warnings can safely be ignored. \section1 Getting Started Load the project file \c qtmfc1.dsp into a Workspace in Visual Studio and make sure that everything is set up correctly by building and running the project. The MFC application has an interface to use dialogs provided by an external DLL that will be explicitly loaded. The interface is fairly simple: the DLL must export a C function called \c showDialog that can take a window handle \e parent. The DLL must show its dialog modally, and when the function returns the DLL is unloaded again. The code that does this in the MFC application is in the OnAppAbout command handler. \quotefromfile step1/qtmfc.cpp \skipto WindowsApp::OnAppAbout \printto ////// If the DLL can be loaded and exports the \c showDialog symbol the exported function is called, otherwise a default MFC about dialog is displayed. \section2 Plugin extension The project in the \c qtdll example directory implements the plugin interface using the QMessageBox class. To use this class a QApplication object must exist in the current process, and the Qt events must be processed in addition to the standard event dispatching performed by the running MFC application. The DLL also has to make sure that it can be loaded together with other Qt based DLLs in the same process (in which case a QApplication object will probably exist already), and that the DLL that creates the QApplication object remains loaded in memory to avoid other DLLs using memory that is no longer available to the process. All these issues are handled by the QMfcApp::pluginInstance() function. This function creates a QApplication object and installs a message hook that merges the Qt event loop with the existing standard Win32 message pump of MFC. If an instance to the DLL is passed as a parameter the function will also increase the DLL's reference count so that it is not unloaded before the process exits. This function can be used in an implementation of the \c DllMain entry point function when the DLL is loaded. A static bool variable is used to remember whether this DLL is responsible for the QApplication object, and when the DLL is unloaded the QApplication object, accessible through the global \c qApp pointer, can be deleted. To use the function and the other Qt classes involved we also need to include a few header files. \quotefromfile qtdll/main.cpp \skipto #include \printuntil } The DLL interface is then implemented using an exported C function called \c showDialog. The QWinWidget class is used to provide the proper placement and stacking of the Qt dialog. \printuntil } \section2 Linking against Qt To use Qt classes directly in the MFC application we must link the application against the Qt libraries, and add the location of the Qt header files to the compiler's include directories. The \c step2 directory includes a \c .pro file that generates a proper \c .dsp file that has all the required settings. Run \ c{qmake -tp vc} in the directory to generate that \c .dsp file, and check the settings necessary in case you want to modify the Visual Studio project manually. \section2 Replacing the MFC event loop To be able to use Qt, we need to create a QApplication object. The QApplication class controls the event delivery and display management for all Qt objects and widgets. In the original MFC project the wizard generated \c WindowsApp class, a subclass of \c CWinApp, runs the event loop in the default implementation of \c Run(). The MFC event loop is a standard Win32 event loop, but uses the CWinApp API \c PreTranslateMessage() to activate accelerators. In order to keep MFC accelerators working we must use the \c QApplication subclass \c QMfcApp that is provided by the Windows Migration framework and which merges the Qt and the MFC event loops. The first step of the Qt migration is to reimplement the \c Run() function in the \c WindowsApp class. We can either use the wizard to add a reimplementation of Run(), or add the reimplementation ourselves to the class declaration in the \c qtmfc.h header file: \quotefromfile step2/qtmfc.h \skipto class WindowsApp \printuntil }; The implementation of this function is in the \c qtmfc.cpp source file. To use the QMfcApp API we need to \c #include the \c qmfcapp.h header file. \quotefromfile step2/qtmfc.cpp \skipto #include \printto #ifdef _DEBUG \skipto BOOL WindowsApp::Run() \printuntil } The implementation uses the static run() function of the QMfcApp class to implicitly instantiate a QApplication object, and run the event loops for both Qt and MFC: The code in the plugin DLL does not need to be modified: Since we have a QApplication object created in the application itself the pluginInstance() function will do nothing, and the DLL will open the message box in the exported function just like before. \section2 Replacing a Dialog Instead of using the Qt plugin DLL we will now replace the MFC About Dialog directly in the application source code. Using the Visual Studio integration toolbar we could quickly create a new QDialog with \e{Qt Designer} from scratch. Or convert the existing dialogs from Microsoft resource files into \e{Qt Designer} \c .ui files by using a conversion tool such as \l {http://www.kdab.net/?page=products&sub=knut} {KDAB's KNUT} tool. But for a basic dialog as the one in this example, it is even easier to use the QMessageBox::about() function as we did in the plugin code shown earlier. \quotefromfile step3/qtmfc.cpp \skipto #include \printto #ifdef _DEBUG To use the QMessageBox API we must include the appropriate header. Since we need to create the QMessageBox as a child of the MFC based main window we also need to use the \c QWinWidget class again and include that header as well. \skipto WindowsApp message handlers \printuntil } We can remove the class declaration and implementation of the CAboutDlg class from the source file, and use the QWinWidget and QMessageBox API in the implementation of the \c WindowsApp::OnAppAbout() command handler. A QWinWidget object is created on the stack, using the MFC application's main window as the parent window. The showCentered() API is used to make sure that the Qt message box, which uses the QWinWidget object as its parent, will open centered over the main window. \section1 New Functionality with Qt We can now add new functionality to the MFC application using Qt. We will add a Qt based user interface to the MFC child view, and add an additional modeless options dialog created with \e{Qt Designer}. \section2 Creating Qt widgets To be able to create Qt widgets in the initialization of the MFC application we must first create an instance of QApplication. The current use of the static \c QMfcApp::run() API creates the QApplication object in the \c Run() reimplementation of the CWinApp subclass, while the GUI is already being created in the \c InitInstance reimplementation. \quotefromfile step4/qtmfc.cpp \skipto ::InitInstance \printuntil MainFrame* pFrame = \skipto return true; \printuntil } To create the QApplication object in the \c InitInstance implementation we must use the static function \c QMfcApp::instance(). \printuntil } \c QMfcApp:run() will then use that instance, which must then be deleted explicitly using the global \c qApp pointer. MFC's window creation infrastructure is rather complicated, and we must add a message handler for the \c WM_CREATE and \c WM_DESTROY messages to be able to add children only when the creation of the MFC window is complete, and to delete the children before the MFC window is destroyed. \quotefromfile step4/childview.h \skipto ChildView window \printuntil ChildView(); \code ... \endcode \skipto // Generated message map \printuntil }; We can again use Visual Studio's wizards, or simply add the code ourselves. We also add a forward declaration of the QWinWidget class, and add a pointer to that class as a member of the ChildView. \quotefromfile step4/childview.cpp \skipto #include \printto #ifdef _DEBUG We include the headers for the Qt widget we want to use, as well as the header for the QWinWidget class. \skipto ChildView::ChildView \printuntil } We initialize the pointer to the QWinWidget member to zero. We cannot create it yet, since the MFC window has not yet been created. This happens only when the \c MainWindow::OnCreate() message handler calls the \c Create function, which then calls our \c ChildView::OnCreate implementation. \skipto BEGIN_MESSAGE_MAP \printuntil END_MESSAGE_MAP The message handlers are added to the message map. \skipto ChildView::OnCreate \printuntil return -1; The implementation of the \c OnCreate message handler calls and verifies the parent class function and returns an error code if an error occurred. \printuntil } Now we can create the QWinWidget instance with \c this CWnd instance as a parent window, and use that instance as a parent to the QWidgets we want to use to create the user interface. Since QWinWidget is a proper QWidget it can be laid out, and we move the Qt GUI to the upper left corner of the MFC child and show() the user interface immediately. \printuntil } Since the \c ChildView class of MFC was not supposed to be a container for other windows we now see some bad flickering whenever we resize the main window. The \c QLabel widget is obviously painted over by the \c ChildView window on every resize before it has a chance to fill its own background. To prevent this we must change the style of the window to include the \c CS_CLIPCHILDREN flag. \printuntil } In the \c OnDestroy message handler we delete the QWinWidget instance, which deletes all other QWidgets we have created as children. \section2 A new Qt Dialog To add a new dialog we use the Visual Studio integration toolbar's "New Qt Dialog" button. We add a \e{Qt Designer} \c{.ui} file "optionsdialog.ui" to the current project, and add the required build steps to generate a C++ class from that file. \footnote If you don't have the integration toolbar, add the following commands to the custom build step of the \c .ui file: \code %qtdir%\bin\uic.exe $(InputPath) -o $(InputDir)\$(InputName).h %qtdir%\bin\uic.exe $(InputPath) -i $(InputName).h -o $(InputDir)\$(InputName).cpp %qtdir%\bin\moc.exe $(InputDir)\$(InputName).h -o $(InputDir)\moc_$(InputName).cpp \endcode And the following Output files: \code $(InputDir)\$(InputName).h $(InputDir)\$(InputName).cpp $(InputDir)\moc_$(InputName).cpp \endcode \endfootnote MFC projects have the precompiled header option turned on by default, and since Qt or \e{Qt Designer} cannot rely on the compiler used supporting precompiled headers the respective preprocessor directives are missing from the generated \c{.cpp} files. We must turn the precompiled headers option off for those files, but we can just as well turn them off for the complete project. To be able to invoke the dialog we add a new entry to the MFC menu using the Visual Studio resource editor. The menu entry is called "Options", and has the ID \c ID_EDIT_OPTIONS. \quotefromfile step4/qtmfc.h \skipto class WindowsApp \printuntil public: \code ... \endcode \skipto // Implementation \printuntil }; We add a command handler for that option to the WindowsApp class and add the mapping to the message map. We also include the generated header file. \quotefromfile step4/qtmfc.cpp \skipto #include \printto #ifdef _DEBUG \skipto BEGIN_MESSAGE_MAP \printuntil END_MESSAGE_MAP The implementation of the command handler once again uses the QWinWidget class to make sure that the dialog is properly positioned and stacked. Since we want the dialog to be modeless we cannot create the QWinWidget on the stack, since it would be deleted when it leaves the scope, and all its children, including the dialog, would be deleted as well. \skipto WindowsApp::OnAppOptions \printuntil } Instead we create QWinWidget on the heap, using operator \c new, and use the \c WDestructiveClose widget flag when creating the dialog as a child, also with operator \c new. Both the QWinWidget object and the modeless dialog will be destroyed when the dialog is closed, e.g. when clicking the OK button. \section1 Removing MFC We will now turn the complete MFC/Qt hybrid application into a genuine, multiplatform Qt application. To make our application compilable with different compilers for different platforms we need to remove the dependency on Visual C++ and Visual Studio, and replace the Visual Studio project file with a qmake project. \section2 Using the Qt build system The project file lists all the sources involved. \quotefromfile step4/step4.pro \printuntil qtwinmigrate Until we have completed the transition we must still link against the Windows Migration Framework, compile the Visual C++ resources, set the preprocessor define to pull in the MFC DLL, and turn off UNICODE to avoid library conflicts with the non-UNICODE MFC version. We must also remove the \c qtmain library which implements the \c WinMain entry point function to call the multiplatform \c main entry point. Running \c{qmake -tp vc} on the \c .pro file will generate a new \c .dsp file that we can use in Visual Studio to compile and link the application. \section2 Replacing the ChildView The first MFC class we will move over to Qt is the ChildView class. We replace the old class declaration with the declaration of a QWidget subclass. \quotefromfile step5/childview.h \skipto #ifndef \printuntil #endif We don't need any creation and destruction message handlers anymore, and the QWinWidget member is obsolete as well. However, we will keep an event handler for paint events as in the original class. \quotefromfile step5/childview.cpp \skipto ChildView::ChildView \printuntil QPainter \printuntil } The implementation of the class creates the user interface elements directly in the constructor and sets the erase color property to white. The paintEvent does nothing, at least for now. \section2 Replacing the MainFrame The next MFC class we will move over to Qt is the MainFrame class. We could use \e{Qt Designer} to generate a main window in a visual environment, but it's just as easy to add the few elements manually. \quotefromfile step5/mainframe.h \skipto #ifndef \printuntil #endif The class implements the constructor, and keeps a reference to the ChildView object. The \c Q_OBJECT macro is used to allow this class to declare signals and slots. We add two slots, \c editOptions and \c helpAbout. \quotefromfile step5/mainframe.cpp \skipto MainFrame:: \printuntil } The implementation of the class creates the menu, instantiates the ChildView as the central widget, and adds a status bar. \printuntil } \printuntil } The slot implementations are identical to the application command handlers in the Qt/MFC hybrid, but of course don't need to use the QWinWidget class anymore. \section2 Replacing the MFC application The final step is to remove the WindowsApp class completely, and handle the application startup and initialization in a multiplatform \c main entry point function. \quotefromfile step5/step5.pro \printuntil RC_FILE We can delete the \c qtmfc.h header file and remove it from the \c HEADERS section in the qmake \c .pro file. We can also remove the linking against the Migration Framework library, and the modifications of the preprocessor symbols. Then we rerun qmake to regenerate the \c .dsp file. Since we added the \c Q_OBJECT macro to the \c MainFrame class declaration we have to make sure that the meta object compiler, \c{moc}, is added to the build step for the \c MainFrame class, and this is also done when running qmake. The \c qtmfc.cpp file is completely replaced. \quotefromfile step5/qtmfc.cpp \skipto #include \printuntil } All it does now is to include the required headers, and to implement a standard main entry point function. \section2 Cleaning up Finally we edit the resource script (with a plain text editor, Visual Studio is too powerful for this) and remove the obsolete entries. The only entry we keep is the application icon. \quotefromfile step5/qtmfc.rc \printuntil IDR_MAINFRAME Now we can delete the files that are no longer needed: \code StdAfx.h, StdAfx.cpp, qtmfc.h, res/qtmfc.rc2, resource.h \endcode The code we have now does the same job as the MFC based code, is much smaller, has less files to maintain and will compile with different compilers for different platforms. */