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certainly not bad enough to stop you from using the feature However, if you make an addition or modification to the implemented interface and you've used the interface in multiple classes, you have a legitimate code-maintenance problem that can be bad enough to prevent you from ever using Implements again By adding a level of indirection to the calling and implementation code, you can avoid this code maintenance problem entirely and substantially reduce the delegation code you need to write Indirecting an interface involves moving the implementation of a commonly used interface into a separate class The main class now implements a simple interface with a single property used to retrieve an implementation of the interface 'clsExposed Implements GetIStuff Private m_Stuff As IStuffImpl Private Sub Class_Initialize() Set m_Stuff = New IStuffImpl 'See multiple behaviors section m_StuffType = ForClsExposed End Sub Private Property Get GetIStuff_GetStuff() As IStuff Set GetIStuff_GetStuff = m_Stuff End Property 'Calling code Dim clsX As clsExposed 'Call helper DoStuff clsX 'Helpers Sub DoStuff(ByVal Indirect As GetIStuff) Dim Stuff As IStuff Set Stuff = IndirectGetStuff 'IStuff code here End Sub The extra level of indirection allows you to make modifications to the IStuff interface without affecting the clsExposed implementation at all For example, if you add a method to IStuff to support a new behavior in class clsOverExposed, OBJECT DESIGN CONSTRUCTS
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only IStuffImpI and the code that actually calls the new method need to be changed The change has no effect on clsExposed or the hypothetical myriad other classes that also use IStuff indirectly Without the additional level of indirection, you would need to change every one of these classes Sharing Data with Base Classes When you call a procedure on an object, the procedure's behavior is generally determined by three interacting sets of data: The code in the procedure, the passed parameters, and the current value of the member variables all act together to determine the action taken A fourth set of data (which I'll ignore in this discussion) is the global state of the project (You don't need an advanced book to tell you that you should keep instance interaction with globals to a minimum) Let's apply this observation to the IStuffImpI class in the previous discussion IStuffImpI Implements IStuff, so its external behavior is defined by the definition of the IStuff interface clsExposed must make IStuffImpI behave as if it had been implemented directly, not indirectly This means that clsExposed must set all the member variables of the IStuffImpI class If clsExposed has full control over the member variable state in IStuffImpI, it essentially has full control of its runtime behavior as well The easiest way to share all of a class's member variables is to have only one member variable The single member variable can have a user-defined type Class modules don't support a public variable with a UDT type, so you have to place the Type definition in a standard module Then clsExposed can fill the structure and assign it to IStuffImpI through a Friend function 'In a module Public Type StuffImplData Name As String End Type 'In IStuffImpl Private m_Data As StuffImplData Friend Property Get Data() As StuffImplData Data = m_Data End Property Friend Property Let Data(RHS As StuffImplData)
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m_Data = RHS End Property 'In clsExposed Private m_Stuff As IStuffImpl Private Sub Class_Initialize() Dim Data As StuffImplData DataName = "clsExposed" Set m_Stuff = New IStuffImpl m_StuffData = Data End Sub Although this code works well for accessing all the data at once, it makes it necessary to make a copy of the data to initialize it and another deep copy whenever you set it A deep data copy duplicates all the data in the structure, whereas a shallow copy duplicates only pointer values to the original data If there are no variable-length strings or variable-size arrays in the UDT, the copy operation works reasonably well for small structures However, if the structure includes these pointer types, the structure copy becomes much more expensive If you want to avoid the copy or let clsExposed have more direct access to the data in IStuffImpl, you'll have to use the array descriptor techniques, described in "Writing to Array Variables" in 2, to give the clsExposed class direct access to the m_Data structure in the IStuffImpl instance that it owns By simply pointing a variable in each class to the same data, we eliminate the notification and copy code required to synchronize two copies of the structure 'IStuffImpl class Implements IStuff Private m_Data As StuffImplData Friend Property Get DataPtr() As Long DataPtr = VarPtr(m_Data) End PropertyPublic Property Get IStuff_Name() As String IStuff_Name = m_DataName End Property 'clsExposed class Implements GetIStuff
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