Interfaces isolate the client from the server One of the cardinal rules of COM is that you can only access a COM object through an interface. The client program is completely isolated from the server's implementation through interfaces. This is an extremely important point.
The client program knows nothing about the COM object or C++ class that implements the COM object. All it can see is the interface. The interface is like window into the COM object. The interface designer allows the client to see only those parts of the object that he or she wishes to expose. Figure 2-1 illustrates how all client access to the COM object is funneled through the interface.
The notation used here, a small circle connected by a stick, is the conventional way to draw a COM interface. There are many important rules associated with interfaces. While critical for understanding the details how COM works, we can leave them until later. For now, we'll concentrate on the broad concepts of interfaces.