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In practice, there is rarely more than a handful of variations on the base class in a particular project By moving all the implementations into one class and setting a type property on the class, you can share a lot of code and jump quickly to the correct code, based on the class's type The result is predictable code with a centralized implementation If the behavioral variations become extreme, or if the data required for the implementations is substantially different, just use Implements and code another version of the base class with the same interface
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The term aggregation, as generally used in COM programming, refers to an identity trick initiated when an object is created The identity of a COM object is determined by its controlling IUnknown pointer (as described in 3) When an object is created as an aggregate, it is handed a controlling IUnknown pointer, and it defers all external IUnknown operations to that controlling pointer The outer object is given a second IUnknown implementation used to communicate with the inner object In this fashion, a COM object can expose an interface by creating a second, inner object as an aggregate on the controlling IUnknown of the outer object The underlying concept of aggregation is much less stringent than the aggregation described by the COM specification In general, an aggregate refers to the combination of two or more objects into a single object The composite object is then used as if it were a single object, with some parts of the object coming from each of the aggregates The concept behind aggregation is that it enables objects to expose complete interface implementations with very little work You can use a single implementation of an interface to provide equivalent capabilities to several classes VB objects cannot be created as COM aggregates, and there is no built-in mechanism that allows a VB object to create another object as an aggregate This lack of support shuts VB programmers out of an entire mechanism of code reuse: The implementation of any interface must be done function by function in the class module rather than using COM aggregation to defer control of an entire interface to a previously implemented object Because VB can't aggregate at object-creation time, and gaining control over an object before it is created is difficult (if not impossible), I won't try to show
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you how to support classic aggregation However, I can show you how to combine one or more previously created objects and aggregate them into an existing object Making two objects appear the same requires control of the IUnknown calls that come into the object By hooking the IUnknown functions of an existing object and wrapping a COM object with a super-lightweight delegation layer (known as a blind vtable delegator) you can create composite objects that provide all the code-reuse advantages of aggregation If you're interested only in creating aggregates and don't care how it's done, jump to the "Aggregating Existing Objects" section of this chapter and refer to the "Aggregation Functions" section in the Appendix Blind VTable Delegation VTable delegation is a very low-level technique (implemented in assembly code) for wrapping objects so that the wrapped object behaves exactly like the unwrapped object VTable delegation is useful for modifying the behavior of certain functions in a vtable while leaving the rest of the functions alone In the vtable delegator used for aggregation, I provide an implementation for the three IUnknown functions and let the rest pass through In order to understand how vtable delegation works, you first have to know how a vtable works The COM binary standard specifies that every COM object has a pointer to an IUnknown derived virtual function table (vtable) as its first element So if pObj is a pointer to a COM object, *pObj (the value at the memory location pObj) is the vtable The vtable is an ordered array of function pointers in which the functions' order corresponds to the order defined in the interface that describes the COM object Therefore, (*pObj)[0] is the first function in the function pointer array and it is an implementation of the IUnknownQueryInterface function All outside calls to a COM object come through the vtable, so the external behavior of an object is the same if all functions in its vtable have the same behavior To see how a blind delegator works, let's consider the case of a fully blind delegator; that is, one in which all functions simply defer to the real implementation of the function I do this for illustration only: There is no point in creating a fully blind delegator A blind delegator object has two required elements: a vtable pointer (m_pVTable) and a pointer to the real object (m_punklnner) The m_pVTable
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