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Class Factories and Factory Templates

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Microsoft DirectShow 9.0

Class Factories and Factory Templates

Before a client creates an instance of a COM object, it creates an instance of the object's class factory, using a call to the CoGetClassObject function. The client then calls the class factory's IClassFactory::CreateInstance method. It is the class factory that actually creates the component and returns a pointer to the requested interface. (The CoCreateInstance function combines these steps, inside the function call.)

The following illustration shows the sequence of method calls.

Method calls to create a class factory

CoGetClassObject calls the DllGetClassObject function, which is defined in the DLL. This function creates the class factory and returns a pointer to an interface on the class factory. DirectShow implements DllGetClassObject for you, but the function relies on your code in a specific way. To understand how it works, you must understand how DirectShow implements class factories.

A class factory is a COM object dedicated to creating another COM object. Each class factory has one type of object that it creates. In DirectShow, every class factory is an instance of the same C++ class, CClassFactory. Class factories are specialized by means of another class, CFactoryTemplate, also called the factory template. Each class factory holds a pointer to a factory template. The factory template contains information about a specific component, such as the component's class identifier (CLSID), and a pointer to a function that creates the component.

The DLL declares a global array of factory templates, one for each component in the DLL. When DllGetClassObject makes a new class factory, it searches the array for a template with a matching CLSID. Assuming it finds one, it creates a class factory that holds a pointer to the matching template. When the client calls IClassFactory::CreateInstance, the class factory calls the instantiation function defined in the template.

The following illustration shows the sequence of method calls.

Class factory templates in a DLL

The benefit of this architecture is that you can define just a few things that are specific to your component, such as the instantiation function, without implementing the entire class factory.