Using ActiveX controls and COM in Qt

The QAxContainer module is part of the ActiveQt framework. It provides a library implementing a QWidget subclass, QAxWidget, that acts as a container for ActiveX controls, and a QObject subclass, QAxObject, that can be used to easily access non-visual COM objects. Scripting COM objects embedded using these classes is possible through the QAxScript, QAxScriptManager and QAxScriptEngine classes, and a set of tools makes it easy to access COM objects programmatically.

The module consists of six classes

1. QAxBase is an abstract class that provides an API to initialize and access a COM object or ActiveX control.
2. QAxObject provides a QObject that wraps a COM object.
3. QAxWidget is a QWidget that wraps an ActiveX control.
4. QAxScriptManager, QAxScript and QAxScriptEngine provide an interface to the Windows Script Host.

Some example applications that use standard ActiveX controls to provide high-level user interface functionality are provided.

Topics:

Using the Library

To build Qt applications that can host COM objects and ActiveX controls link the application against the QAxContainer module by adding

QT += axcontainer

to your application's .pro file.

Distributing QAxContainer Applications

The QAxContainer library is static, so there is no need to redistribute any additional files when using this module. Note however that the ActiveX server binaries you are using might not be installed on the target system, so you have to ship them with your package and register them during the installation process of your application.

Instantiating COM Objects

To instantiate a COM object use the QAxBase::setControl() API, or pass the name of the object directly into the constructor of the QAxBase subclass you are using.

The control can be specified in a variety of formats, but the fastest and most powerful format is to use the class ID (CLSID) of the object directly. The class ID can be prepended with information about a remote machine that the object should run on, and can include a license key for licensed controls.

Typical Error Messages

ActiveQt prints error messages to the debug output when it encounters error situations at runtime. Usually you must run your program in the debugger to see these messages (e.g. in Visual Studio's Debug output).

Requested control could not be instantiated

The control requested in QAxBase::setControl() is not installed on this system, or is not accessible for the current user.

The control might require administrator rights, or a license key. If the control is licensed, pass the license key to QAxBase::setControl as documented.

Accessing the Object API

ActiveQt provides a Qt API to the COM object, and replaces COM datatypes with Qt equivalents.

There are four ways to call APIs on the COM object:

• Generating a C++ namespace
• Call-by-name
• Through a script engine
• Using the native COM interfaces

Generating a C++ Namespace

To generate a C++ namespace for the type library you want to access, use the dumpcpp tool. Run this tool manually on the type library you want to use, or integrate it into the build system by adding the type libraries to the TYPELIBS variable in your application's .pro file:

TYPELIBS = file.tlb

Note that dumpcpp might not be able to expose all APIs in the type library.

Include the resulting header file in your code to access the object APIs through the generated C++ classes. See the Qutlook example for more information.

Call-by-Name

Use QAxBase::dynamicCall() and QAxBase::querySubObject() as well as the QObject::setProperty() and QObject::property() APIs to call the methods and properties of the COM object through their name. Use the dumpdoc tool to get the documentation of the Qt API for any COM object and its subobjects; note that not all of the COM object's APIs might be available.

See the Media Player example for more information.

Calling Function Through a Script Engine

A Qt application can host any ActiveScript engine installed on the system. The script engine can then run script code that accesses the COM objects.

To instantiate a script engine, use QAxScriptManager::addObject() to register the COM objects you want to access from script, and QAxScriptManager::load() to load the script code into the engine. Then call the script functions using QAxScriptManager::call() or QAxScript::call().

Which APIs of the COM object are available through scripting depends on the script language used.

The ActiveX Test Container demonstrates loading of script files.

Calling a Function Using the Native COM Interfaces

To call functions of the COM object that can not be accessed via any of the above methods it is possible to request the COM interface directly using QAxBase::queryInterface(). To get a C++ definition of the respective interface classes use the #import directive with the type library provided with the control; see your compiler manual for details.

Typical Error Messages

ActiveQt prints error messages to the debug output when it encounters error situations at runtime. Usually you must run your program in the debugger to see these messages (e.g. in Visual Studio's Debug output).

QAxBase::internalInvoke: No such method

A QAxBase::dynamicCall() failed - the function prototype did not match any function available in the object's API.

Error calling IDispatch member: Non-optional parameter missing

A QAxBase::dynamicCall() failed - the function prototype was correct, but too few parameters were provided.

Error calling IDispatch member: Type mismatch in parameter n

A QAxBase::dynamicCall() failed - the function prototype was correct, but the parameter at index n was of the wrong type and could not be coerced to the correct type.

QAxScriptManager::call(): No script provides this function

You try to call a function that is provided through an engine that doesn't provide introspection (ie. ActivePython or ActivePerl). You need to call the function directly on the respective QAxScript object.

See also ActiveQt Framework.