VB Shell Programming

Classes

One of the details VB hides from the programmer is the fact that an interface and a class are two distinct entities. This is easier to visualize once you know what a class really looks like; Figure 2.1 attempts to depict the Cow class graphically. A class is nothing more than a pointer to an array of function pointers (member functions), followed by public and private data. This array of function pointers is called a virtual function table, or vtable. This arrangement allows multiple instances of a COM component to share the same vtable, which is very efficient in terms of memory. Of course, member variables are not shared. Every instance of a component has its own copy of any public or private member variables. Also, if a class such as Cow had any methods of its own or implemented any additional interfaces, these would be added to the vtable. The order of a vtable is very important, because for all practical purposes the vtable is the physical representation of the interfaces an object has implemented.

Figure 2-1. The Cow class

Interface Identifiers

The Animal and Cow interfaces are unique. Behind the scenes, VB has assigned a globally unique identifier (GUID) to both of these interfaces. A GUID that names an interface is called an IID. The IID for Animal is {101E95AB-018E-11D3-BB7C-444553540000}. Well, actually this is a string representation of an IID. An IID is a unique ...