Python Programming On Win32

Interfaces and Objects

COM makes a clear distinction between interfaces and objects . An interface describes certain functionality, while an object implements that functionality (that is, implements the interface). An interface describes how an object is to behave, while the object itself implements the behavior. For example, COM defines an IStream interface, a generic interface for reading and writing, in a manner similar to a file. Although COM defines the IStream interface, it’s the responsibility of objects to implement the interface; thus, you may have an object that implements the IStream interface writing to and from files or an object implementing the IStream interface using sockets, and so forth. This is a huge advantage to users of these interfaces, because you can code to the IStream interface, and your code works regardless of whether your data goes to a file or out over a socket. Each COM interface has a unique 128-bit GUID known as an interface ID (IID).

An interface defines a series of methods: interfaces can’t have properties. An interface is defined in terms of a C++ vtable . Highly experienced C++ programmers will know that a vtable implements virtual methods in C++.

Just as with C++, COM allows one interface to derive from, or extend, another interface; in fact, COM explicitly requires it. COM defines an interface known as IUnknown , which is the root (or base) of all COM interfaces; that is, all COM interfaces explicitly support the IUnknown interface. IUnknown ...