Using ActiveX

Microsoft's ActiveX technology allows applications to incorporate user interface components provided by other applications or libraries. It is built on Microsoft COM and defines one set of interfaces for applications that use components and another set of interfaces for applications and libraries that provide components.

Qt/Windows Enterprise Edition provides the ActiveQt framework to seamlessly combine ActiveX and Qt. ActiveQt consists of two modules:

  • The QAxContainer module allows us to use COM objects and to embed ActiveX controls in Qt applications.
  • The QAxServer module allows us to export custom COM objects and ActiveX controls written using Qt.

Our first example will embed the Windows Media Player in a Qt application using QAxContainer. The Qt application adds an Open button, a Play/Pause button, a Stop button, and a slider to the Windows Media Player ActiveX control.

Figure 18.2. The Media Player application


The application's main window is of type PlayerWindow:

class PlayerWindow : public QWidget
 enum PlayStateConstants { Stopped = 0, Paused = 1, Playing = 2 };
 enum ReadyStateConstants { Uninitialized = 0, Loading = 1,
 Interactive = 3, Complete = 4 };

 PlayerWindow(QWidget *parent = 0, const char *name = 0);

 void timerEvent(QTimerEvent *event);
private slots:
 void onPlayStateChange(int oldState, int newState);
 void onReadyStateChange(ReadyStateConstants readyState);
 void onPositionChange(double oldPos, double newPos);
 void sliderValueChanged(int newValue);
 void openFile();

The PlayerWindow class inherits from QWidget. The Q_ENUMS() macro is necessary to tell moc that the ReadyStateConstants type used in the onReadyStateChange() slot is an enum type.

 QAxWidget *wmp;
 QToolButton *openButton;
 QToolButton *playPauseButton;
 QToolButton *stopButton;
 QSlider *seekSlider;
 QString fileFilters;
 int updateTimer;

In the private section, we declare a QAxWidget * data member.

PlayerWindow::PlayerWindow(QWidget *parent, const char *name)
 : QWidget(parent, name)
 wmp = new QAxWidget(this);

In the constructor, we create a QAxWidget object to encapsulate the Windows Media Player ActiveX control. The QAxContainer module consists of three classes: QAxObject encapsulates a COM object, QAxWidget encapsulates an ActiveX control, and QAxBase implements the core COM functionality for QAxObject and QAxWidget.

Figure 18.3. Inheritance tree for the QAxContainer module


We call setControl() on the QAxWidget with the class ID of the Windows Media Player 6.4 control. This will create an instance of the required component. From then on, all the properties, events, and methods of the ActiveX control are available as Qt properties, signals, and slots through the QAxWidget object.

The COM data types are automatically converted into the corresponding Qt types, as summarized in Figure 18.4. For example, an in-parameter of type VARIANT_BOOL becomes a bool, and an out-parameter of type VARIANT_BOOL becomes a bool &. If the resulting type is a Qt class (QString, QDateTime, etc.), the in-parameter is a const reference (for example, const QString &).

Figure 18.4. Relationship between COM types and Qt types


To obtain the list of all the properties, signals, and slots available in a QAxObject or QAxWidget with their Qt data types, call generateDocumentation() or use Qt's dumpdoc command-line tool, located in Qt's extensionsactiveqtexample directory.

wmp->setProperty("ShowControls", QVariant(false, 0));
connect(wmp, SIGNAL(PlayStateChange(int, int)),
 this, SLOT(onPlayStateChange(int, int))),
connect(wmp, SIGNAL(ReadyStateChange(ReadyStateConstants)),
 this, SLOT(onReadyStateChange(ReadyStateConstants)));
connect(wmp, SIGNAL(PositionChange(double, double)),
 this, SLOT(onPositionChange(double, double)));

After calling setControl() in the PlayerWindow constructor, we call setProperty() to set the ShowControls property of the Windows Media Player to false, since we provide our own buttons to manipulate the component. The setProperty() function is defined in QObject and can be used both for COM properties and for normal Qt properties. Its second parameter is of type QVariant. Because some C++ compilers don't support the bool type properly yet, the QVariant constructor that takes a bool also has a dummy int parameter. For types other than bool, the conversion to QVariant is automatic.

Next, we call setSizePolicy() to make the ActiveX control take all the available space in the layout, and we connect three ActiveX events from the COM component to three slots.

The rest of the PlayerWindow constructor follows the usual pattern, except that we connect some Qt signals to slots provided by the COM object (Play(), Pause(), and Stop()).

Let's leave the constructor and look at the timerEvent() function:

void PlayerWindow::timerEvent(QTimerEvent *event)
 if (event->timerId() == updateTimer) {
 double curPos = wmp->property("CurrentPosition").toDouble();
 onPositionChange(-1, curPos);
 } else {

The timerEvent() function is called at regular intervals while a media clip is playing. We use it to advance the slider. This is done by calling property() on the ActiveX control to obtain the value of the CurrentPosition property as a QVariant and calling toDouble() to convert it to a double. We then call onPositionChange() to perform the update.

We will not review the rest of the code because most of it isn't directly relevant to ActiveX and doesn't show anything that we haven't covered already. The code is included on the CD.

In the .pro file, we need this entry to link with the QAxContainer module:

LIBS += -lqaxcontainer

One frequent need when dealing with COM objects is to be able to call a COM method directly (as opposed to connecting it to a Qt signal). The easiest way to do this is to call dynamicCall() with the name and signature of the method as first parameter and the arguments to the method as additional parameters. For example:

wmp->dynamicCall("TitlePlay(uint)", 6);

The dynamicCall() function takes up to eight parameters of type QVariant and returns a QVariant. If we need to pass an IDispatch * or an IUnknown * this way, we can encapsulate the component in a QAxObject and call asVariant() on it to convert it to a QVariant. If we need to call a COM method that returns an IDispatch * or an IUnknown *, or if we need to access a COM property of one of those types, we must use querySubObject() instead:

QAxObject *session = outlook.querySubObject("Session");
QAxObject *defaultContacts =

If we want to call functions that have unsupported data types in their parameter list, we can use QAxBase::queryInterface() to retrieve the COM interface and call the function directly. We must call Release() when we have finished using the interface.

If we often need to call such functions, we can subclass QAxObject or QAxWidget and provide member functions that encapsulate the COM interface calls. However, be aware that QAxObject and QAxWidget subclasses cannot define their own properties, signals, and slots.

We will now review the QAxServer module. This module enables us to turn a standard Qt program into an ActiveX server. The server can either be a shared library or a stand-alone application. Servers built as shared libraries are often called in-process servers, and stand-alone applications are called out-of-process servers.

Our first QAxServer example is an in-process server that provides a widget that shows a ball bouncing left and right. We will also see how to embed the widget in Internet Explorer.

Figure 18.5. The AxBouncer widget in Internet Explorer


Here's the beginning of the class definition of the AxBouncer widget:

class AxBouncer : public QWidget, public QAxBindable
 Q_PROPERTY(QColor color READ color WRITE setColor)
 Q_PROPERTY(Speed speed READ speed WRITE setSpeed)
 Q_PROPERTY(int radius READ radius WRITE setRadius)
 Q_PROPERTY(bool running READ isRunning)

AxBouncer inherits from both QWidget and QAxBindable. The QAxBindable class provides an interface between the widget and an ActiveX client. Any QWidget can be exported as an ActiveX control, but by subclassing QAxBindable we can notify the client when a property's value changes, and we can implement COM interfaces to supplement those already implemented by QAxServer.

When doing multiple inheritance involving a QObject-derived class, we must always put the QObject-derived class first so that moc can pick it up.

We declare three read-write properties and one read-only property. The Q_ENUMS() macro is necessary to tell moc that the Speed type is an enum type. The Speed enum is declared in the public section of the class.

 enum Speed { Slow, Normal, Fast };

 AxBouncer(QWidget *parent = 0, const char *name = 0);

 void setSpeed(Speed newSpeed);
 Speed speed() const { return ballSpeed; }
 void setRadius(int newRadius);
 int radius() const { return ballRadius; }
 void setColor(const QColor &newColor);
 QColor color() const { return ballColor; }
 bool isRunning() const { return myTimerId != 0; }
 QSize sizeHint() const;
 QAxAggregated *createAggregate();

public slots:
 void start();
 void stop();

 void bouncing();

The AxBouncer constructor is a standard constructor for a widget, with a parent and a name parameter. The QAXFACTORY_DEFAULT() macro, which we will use to export the component, expects a constructor with this signature.

The createAggregate() function is reimplemented from QAxBindable. We will explain it in a moment.

 void paintEvent(QPaintEvent *event);
 void timerEvent(QTimerEvent *event);

 int intervalInMilliseconds() const;

 QColor ballColor;
 Speed ballSpeed;
 int ballRadius;
 int myTimerId;
 int x;
 int delta;

The protected and private sections of the class are the same as what we would have in a standard Qt widget.

AxBouncer::AxBouncer(QWidget *parent, const char *name)
 : QWidget(parent, name, WNoAutoErase)
 ballColor = blue;
 ballSpeed = Normal;
 ballRadius = 15;
 myTimerId = 0;
 x = 20;
 delta = 2;

The AxBouncer constructor initializes the class's private variables.

void AxBouncer::setColor(const QColor &newColor)
 if (newColor != ballColor && requestPropertyChange("color")) {
 ballColor = newColor;

The setColor() function sets the value of the color property. It calls update() to repaint the widget.

The unusual part is the requestPropertyChange() and propertyChanged() calls. These functions are inherited from QAxBindable and should ideally be called whenever we change a property. The requestPropertyChange() asks the client's permission to change a property, and returns true if the client allows the change. The propertyChanged() function notifies the client that the property has been changed.

The setSpeed() and setRadius() property setters also follow this pattern, and so do the start() and stop() slots, since they change the value of the running property.

There is one interesting AxBouncer member function left:

QAxAggregated *AxBouncer::createAggregate()
 return new ObjectSafetyImpl;

The createAggregate() function is reimplemented from QAxBindable. It allows us to implement COM interfaces that the QAxServer module doesn't already implement or to bypass QAxServer's default COM interfaces. Here, we do it to provide the IObjectSafety interface, which is used by Internet Explorer to access a component's safety options. This is the standard trick to get rid of Internet Explorer's infamous "Object not safe for scripting" error message.

Here's the definition of the class that implements the IObjectSafety interface:

class ObjectSafetyImpl : public QAxAggregated, public IObjectSafety
 long queryInterface(const QUuid &iid, void **iface);


 HRESULT WINAPI GetInterfaceSafetyOptions(REFIID riid,
 DWORD *pdwSupportedOptions, DWORD *pdwEnabledOptions);
 HRESULT WINAPI SetInterfaceSafetyOptions(REFIID riid,
 DWORD pdwSupportedOptions, DWORD pdwEnabledOptions);

The ObjectSafetyImpl class inherits both QAxAggregated and IObjectSafety. The QAxAggregated class is an abstract base class for implementations of additional COM interfaces. The COM object that the QAxAggregated extends is accessible through controllingUnknown(). This COM object is created behind the scenes by the QAxServer module.

The QAXAGG_IUNKNOWN macro provides standard implementations of QueryInterface(), AddRef(), and Release(). These implementations simply call the same functions on the controlling COM object.

long ObjectSafetyImpl::queryInterface(const QUuid &iid, void **iface)
 *iface = 0;
 if (iid == IID_IObjectSafety)
 *iface = (IObjectSafety *)this;

 return S_OK;

The queryInterface() function is a pure virtual function of QAxAggregated. It is called by the controlling COM object to give access to the interfaces provided by the QAxAggregated subclass. We must return E_NOINTERFACE for interfaces that we don't implement and for IUnknown.

HRESULT WINAPI ObjectSafetyImpl::GetInterfaceSafetyOptions(
 REFIID riid, DWORD *pdwSupportedOptions,
 DWORD *pdwEnabledOptions)
 *pdwEnabledOptions = *pdwSupportedOptions;
 return S_OK;

HRESULT WINAPI ObjectSafetyImpl::SetInterfaceSafetyOptions(REFIID,
 return S_OK;

The GetInterfaceSafetyOptions() and SetInterfaceSafetyOptions() functions are declared in IObjectSafety. We implement them to tell the world that our object is safe for scripting.

Let's now review main.cpp:


#include "axbouncer.h"

int main()
 return 0;

The QAXFACTORY_DEFAULT() macro exports an ActiveX control. We can use it for ActiveX servers that export only one control. Otherwise, we must subclass QAxFactory and use a macro called QAXFACTORY_EXPORT(). The next example in this section shows how to do it.

The first argument to QAXFACTORY_DEFAULT() is the name of the Qt class to export. This is also the name under which the control is exported. The other five arguments are the class ID, the interface ID, the event interface ID, the type library ID, and the application ID. We can use standard tools like guidgen or uuidgen to generate these identifiers.

Because the server is a library, we don't need a real main() function. We must still provide a fake implementation to pacify the linker.

Here's the .pro file for our in-process ActiveX server:

CONFIG += activeqt dll
HEADERS = axbouncer.h 
SOURCES = axbouncer.cpp 
RC_FILE = qaxserver.rc
DEF_FILE = qaxserver.def

The qaxserver.rc and qaxserver.def files referred to in the .pro file are standard files that can be copied from Qt's extensionsactiveqtcontrol directory.

The makefile or Visual C++ project file generated by qmake contains rules to register the server in the Windows registry. To register the server on end user machines, we can use the regsvr32 tool available on all Windows systems.

We can then include the Bouncer component in an HTML page using the tag:

The ActiveX control is not available. Make sure you have built and
registered the component server.

We can create buttons that invoke slots:


And we can manipulate the widget using JavaScript or VBScript just like any other ActiveX control. See the demo.html file on the CD for a rudimentary page that uses the ActiveX server.

Our last example is a scriptable Address Book application. The application can serve as a standard Qt/Windows application or an out-of-process ActiveX server. The latter possibility allows us to script the application using, say, Visual Basic.

class AddressBook : public QMainWindow
 Q_PROPERTY(int count READ count)
 AddressBook(QWidget *parent = 0, const char *name = 0);
 ~AddressBook() ;

 int count() const;

public slots:
 ABItem *createEntry(const QString &contact) ;
 ABItem *findEntry(const QString &contact) const;
 ABItem *entryAt(int index) const;

The AddressBook widget is the application's main window. The property and the slots it provides will be available for scripting.

class ABItem : public QObject, public QListViewItem
 Q_PROPERTY(QString contact READ contact WRITE setContact)
 Q_PROPERTY(QString address READ address WRITE setAddress)
 Q_PROPERTY(QString phoneNumber READ phoneNumber
 WRITE setPhoneNumber)
 ABItem(QListView *listView);

 void setContact(const QString &contact);
 QString contact() const { return text (0); }
 void setAddress(const QString &address);
 QString address() const { return text(1); }
 void setPhoneNumber(const QString &number);
 QString phoneNumber() const { return text(2); }
public slots:
 void remove();

The ABItem class represents one entry in the address book. It inherits from QListViewItem so that it can be shown in a QListView and from QObject so that it can be exported as a COM object.

int main(int argc, char *argv[])
 QApplication app(argc, argv);
 if (!QAxFactory::isServer()) {
 AddressBook addressBook;
 return app.exec();
 return app.exec();

In main(), we check whether the application is being run stand-alone or as a server. The -activex command-line option makes it run as a server. If the application isn't run as a server, we create the main widget and show it as we would normally do in any stand-alone Qt application.

In addition to -activex, ActiveX servers understand the following commandline options:

  • -regserver registers the server in the system registry.
  • -unregserver unregisters the server from the system registry.
  • -dumpidl file writes the server's IDL to the specified file.

For the case where the application is run as a server, we need to export the AddressBook and ABItem classes as COM components:


The QAXFACTORY_EXPORT() macro exports a factory for creating COM objects. Since we want to export two types of COM objects, we cannot simply use QAXFACTORY_DEFAULT() as we did in the previous example.

The first argument to QAXFACTORY_EXPORT() is the name of the QAxFactory class that provides the application's COM objects. The other two arguments are the type library ID and the application ID.

class ABFactory : public QAxFactory
 ABFactory(const QUuid &lib, const QUuid &app);
 QStringList featureList() const;
 QWidget *create(const QString &key, QWidget *parent,
 const char *name);
 QUuid classID(const QString &key) const;
 QUuid interfaceID(const QString &key) const;
 QUuid eventsID(const QString &key) const;
 QString exposeToSuperClass(const QString &key) const;

The ABFactory class inherits QAxFactory and reimplements virtual functions to export the AddressBook class as an ActiveX control and the ABItem class as a COM component.

ABFactory::ABFactory(const QUuid &lib, const QUuid &app)
 : QAxFactory(lib, app)

The ABFactory constructor simply forwards its two parameters to the base class constructor.

QStringList ABFactory::featureList() const
 return QStringList() << "AddressBook" << "ABItem";

The featureList() function returns a list of the COM components provided by the factory.

QWidget *ABFactory::create(const QString &key, QWidget *parent,
 const char *name)
 if (key == "AddressBook")
 return new AddressBook(parent, name);
 return 0;

The create() function creates an instance of an ActiveX control. We return a null pointer for ABItem because we don't want users to create ABItem objects. Furthermore, the return type of create() is QWidget *, which prevents it from returning COM objects that aren't ActiveX controls.

QUuid ABFactory::classID(const QString &key) const
 if (key == "AddressBook")
 return QUuid("{588141ef-110d-4beb-95ab-ee6a478b576d}");
 else if (key == "ABItem")
 return QUuid("{bc82730e-5f39-4e5c-96be-461c2cd0d282}");
 return QUuid();

The classId() function returns the class ID for all the classes exported by the factory.

QUuid ABFactory::interfaceID(const QString &key) const
 if (key == "AddressBook")
 return QUuid("{718780ec-b30c-4d88-83b3-79b3d9e78502}");
 else if (key == "ABItem")
 return QUuid("{c8bc1656-870e-48a9-9937-fbe1ceff8b2e}");
 return QUuid();

The interfaceID() function returns the interface ID for the classes exported by the factory.

QUuid ABFactory::eventsID(const QString &key) const
 if (key == "AddressBook")
 return QUuid("{0a06546f-9f02-4f14-a269-d6d56ffeb861}");
 else if (key == "ABItem")
 return QUuid("{105c6b0a-3fc7-460b-ae59-746d9d4b1724}");
 return QUuid();

The eventsId() function returns the event interface ID for the classes exported by the factory.

QString ABFactory::exposeToSuperClass(const QString &key) const
 return key;

By default, ActiveX controls expose not only their own properties, signals, and slots to clients, but also those of their superclasses up to QWidget. We can reimplement the exposeToSuperClass() function to return the highest superclass (in the inheritance tree) that we want to expose.

Here, we return the class name of the component("AddressBook" or"ABItem") as the highest superclass to export, meaning that properties, signals, and slots defined in AddressBook's and ABItem's superclasses will not be exported.

This is the .pro file for our out-of-process ActiveX server:

CONFIG += activeqt
HEADERS = abfactory.h 
SOURCES = abfactory.cpp
RC_FILE = qaxserver.rc

The qaxserver.rc file referred to in the .pro file is a standard file that can be copied from Qt's extensionsactiveqtcontrol directory.

Look in the example's vb directory for a Visual Basic project that uses the Address Book server.

This completes our overview of the ActiveQt framework. The Qt distribution includes additional examples, and the documentation contains information about how to build the QAxContainer and QAxServer modules and how to solve common interoperability issues.

C++ GUI Programming with Qt 3
C++ GUI Programming with Qt 3
ISBN: 0131240722
EAN: 2147483647
Year: 2006
Pages: 140
Simiral book on Amazon © 2008-2017.
If you may any questions please contact us: