Continuing previous post, I explore Object-Oriented Programming with VBScript.

Now I want to take a closer look at inheritance and practice its workaround, called delegation.

Delegator Class example

Class ScreenPoint
  '' Properties
 
  'Ancestor Point2D
  Private P2D
  'Point color
  Private Color
  '----------------------
 
  '' Methods
  'Constructor - called automatically
  Private Sub Class_Initialize()
    Set P2D = new Point2D
  End Sub
  '----------------------
  'Destructor - called automatically
  Private Sub Class_Terminate()
    Set P2D = Nothing
  End Sub
  '----------------------
  'A pair of methods to access private property X
  Property Get X
    X = P2D.X
  End Property

  Property Let X(ByVal in_X)
    P2D.X = in_X
  End Property
  '----------------------
  'A pair of methods to access private property Y
  Property Get Y
    Y = P2D.Y
  End Property

  Property Let Y(ByVal in_Y)
    P2D.Y = in_Y
  End Property
  '----------------------
End Class

As you can see, ScreenPoint class has Point2D property. When initialized, it refers to Point2D object. Note, that ScreenPoint class does not have X or Y properties, but Property Let / Property Get methods don’t have to be in sync with property declarations.

Here’s the example of using the new class.

Public Sub Test1()
  Dim SP
  Dim V1
 
  Set SP = new ScreenPoint
  V1 = SP.X
  SP.Y = 10
 
  Set SP = Nothing
 
End Sub

As you see, with Delegation we can effectively re-use data and code. The only inconvenience I found, is a boring amount of typing declaring Property Let / Property Get code blocks. Read next posts to see how I addressed it.