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Lesson -10 : ActiveX Concepts
10.6 - The Component Object Model
The heart of ActiveX is the Component Object Model (COM). This is an incredibly complex topic that deserves a book of its own. Luckily, the Microsoft Foundation Classes and the Visual C++ AppWizard do much of the behind-the-scenes work for you. The discussion in these units is just what you need to know to use COM as a developer. COM is a binary standard for Windows objects. That means that the executable code (in a DLL or EXE) that describes an object can be executed by other objects. Even if two objects were written in different languages, they are able to interact using the COM standard. How do they interact? Through an interface. An ActiveX interface is a collection of functions, or really just function names. It’s a C++ class with no data, only pure virtual functions. Your objects inherit from this class and provide code for the functions. (Remember, as discussed in Appendix A, "C++ Review and Object-Oriented Concepts," a class that inherits a pure virtual function doesn’t inherit code for that function.) Other programs get to your code by calling these functions. All ActiveX objects must have an interface named IUnknown (and most have many more, all with names that start with I, the prefix for interfaces). The IUnknown interface has only one purpose: finding other interfaces. It has a function called QueryInterface() that takes an interface ID and returns a pointer to that interface for this object. All the other interfaces inherit from IUnknown, so they have a QueryInterface() too, and you have to write the code—or you would if there was no MFC. MFC implements a number of macros that simplify the job of writing interfaces and their functions, as you will shortly see. The full declaration of IUnknown is in Listing 101. The macros take care of some of the work of declaring an interface and won’t be discussed here. There are three functions declared: QueryInterface(), AddRef(), and Release(). These latter two functions are used to keep track of which applications are using an interface. All three functions are inherited by all interfaces and must be implemented by the developer of the interface. Listing 10.1 IUnknown, Defined in \Program Files\Microsoft Visual Studio\VC98\Include\unknwn.h MIDL_INTERFACE("00000000-0000-0000-C000-000000000046") IUnknown { public: BEGIN_INTERFACE virtual HRESULT STDMETHODCALLTYPE QueryInterface( /* [in] / REFIID riid, / [iid_is][out] / void __RPC_FAR __RPC_FAR ppvObject) = 0; virtual ULONG STDMETHODCALLTYPE AddRef( void) = 0; virtual ULONG STDMETHODCALLTYPE Release( void) = 0; #if (_MSC_VER >= 1200) // VC6 or greater template <class Q> HRESULT STDMETHODCALLTYPE QueryInterface(Q* pp) { return QueryInterface(__uuidof(Q), (void**)pp); } #endif END_INTERFACE };

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