OLE applications make use of callback functions. Callbacks are functions that must be written in a client or server application by the developer. The callback functions are then called directly by the OLE DLLs. Implementing OLE client or server support requires adding callbacks to the application code so that the OLE DLLs can provide data to the application and cause the application to perform specific operations.
To implement callbacks, OLE client and server applications use a mechanism called a VTBL. A VTBL is a data structure that contains pointers to functions (also known as "methods" or "member functions"). The term VTBL comes from the object-oriented concept of a Virtual Table in which methods are associated with instances of an object; each method is a member function of the VTBL associated with the data that forms that particular object. In OLE, VTBLs are the name of the specific data structures used to associate callback functions with the data forming a specific object.
Note For more information on VTBLs and callbacks, see Chapter 7, "Callback Functions and Data Structures."