VBA inheritance, analog of super

This is the way I have used it a long time to simulate an abstract class through the interface.

'class module: IBase
'We define a base interface
'
Sub go(): End Sub

Sub gogo(): End Sub

Now let's define the other classes, beginning with the abstract class 'B'.

'
'class module: B
'
Implements IBase

'Daughter classes should implement 'go'
'Note that the method is 'Public'
Public Sub go(): End Sub

'Let's agree that every daughter class implements the method
'abstract 'super' that returns the IBase interface
Public Property Get super() As IBase: End Property

'
'The signature of other abstract methods can be stated here
'
'Public Sub goGoChild(): End Sub
'Public Function goGoGoChild2(): End Function
'

'
'Note that the methods are accessible through the IBase interface
'
Private Sub IBase_go()
    Debug.Print "B: super.go()"
End Sub

Private Sub IBase_gogo()
    Debug.Print "B: super.gogo()"
End Sub

Let's create class 'A' which implements the abstract class 'B'

'
'class module: 'A'
'

'We started implementing the abstract class B
Implements B

'we define a private type 'myType'
Private Type myType

    'variable that references an instance of 'B'
    objB As B

    'variable that references the interface of 'B'
    objIB As IBase

End Type

'VBA version of 'this'
Private this As myType

'
'Every class that implements 'B' (abstract class)
'you must initialize in your constructor some variables
'of instance.
'
Private Sub Class_Initialize()

    With this

        'we create an instance of object B
        Set .objB = New B

        'the variable 'objIB' refers to the IBase interface, implemented by class B
        Set .objIB = .objB

    End With

End Sub


'Visible only for those who reference interface B
Private Property Get B_super() As IBase

    'returns the methods implemented by 'B', through the interface IBase
    Set B_super = this.objIB

End Property

Private Sub B_go()
    Debug.Print "A: go()"
End Sub
'==================================================

'Class 'A' local method
Sub localMethod1()
    Debug.Print "A: Local method 1"
End Sub

And finally, let's create the 'main' module.

Sub testA()

    'reference to class 'A'
    Dim objA As A

    'reference to interface 'B'
    Dim objIA As B

    'we create an instance of 'A'
    Set objA = New A

    'we access the local methods of instance 'A'
    objA.localMethod1

    'we obtain the reference to interface B (abstract class) implemented by 'A'
    Set objIA = objA

    'we access the 'go' method, implemented by interface 'B'
    objIA.go

    'we go to the 'go' method of the super class
    objIA.super.go

    'we access the 'gogo' method of the super class
    objIA.super.gogo

End Sub

And the output, in the verification window, will be:

A: Local method 1
A: go()
B: super.go()
B: super.gogo()

One can pull a trick to mimic inheritance. It works by using the default member property.

If you give your derived class a property called Super whose type is the superclass then make that the default member (by exporting and editing file to include Attribute Item.VB_UserMemId = 0, re-importing) then you can reach the superclass by just a pair of round brackets (which resolves to the default member).

This blog post gives the full details but author (disclosure, me) there uses 'Base' instead of 'Super'

Hopefully that syntax is tight enough for you.

I also point out that this does not expose all the base class's internal guts like C# does. This means my method does not suffer from the fragile base class problem. I.e. my method retains encapsulation making it better IMHO.

The usual way to do this in VBA is to have A contain an instance of B as well as having A implement B's interface, and then delegate calls to the B interface of A to the internal B.

This is old stuff, but see the Visual Studio 6.0 Programmer's Guide:

http://msdn.microsoft.com/en-us/library/aa716285(VS.60).aspx

There is a chapter on "The Many (Inter)Faces of Code Reuse" that describes this convention:

http://msdn.microsoft.com/en-us/library/aa240846(v=VS.60).aspx

The way MS describes it is:

In addition to implementing abstract interfaces, you can reuse your code by implementing the interface of an ordinary class, and then selectively delegating to a hidden instance of the class.

This means that implementation inheritance requires lots of explicit delegation methods. There's even a chapter subheading: "Doesn't This Get Tedious?". Yet another reason why OOP in VBA is a PITA (TM)...

EDIT THAT WON'T FIT IN A COMMENT:

To answer the question you posed in your comment, well, an A is a B. When you make A implement B's interface, you are essentially saying that you can treat an instance of A as if it is actually of type B. In VBA, the way you do that is by declaring a variable of type B, and then setting it to an instance of A. VBA will know what to do when you call it like a B:

Dim usedAsB as B
Dim anA as A

Set anA = New A
Set usedAsB = anA    'fine since A implements B

usedAsB.something()    'will call B_something() defined in class A

As far as what you see in the debug window, I don't why it appears that way. And as far as forced delegation, I'm not sure what you mean. VBA automatically dispatches calls to the B interface to the right methods in the A class. If you mean automatically generating the code to inherit B's implementation in the manner described above, there's nothing like that I know of for VBA. I think the various "professional" versions of VB6 could do that, but I've never used VB6 so I don't know.