VBA: is there something like Abstract Class?

Question

I\'m using an interface to ensure some similar classes implements some mandatory methods (subs/functions).

Example:

• interface I1 declares M1 and M2 me

Answer 1

You could implement semi-decorator :-) pattern in VBA. Lets say we have a base class and two child classes. If you use Implements keyword in child classes you will garatee that child classes has the same interface as the base class and at the same time declare a private instance of base class in each child class and redirect calls from children to base class.

Note: base class here is normal class, you can still create instances of it (so it is not real abstract class).

Example:

' Standard module

Sub main()
    Dim a As New ChildA
    Dim b As New ChildB

    a.State = 2
    b.State = 5

    Debug.Print TypeOf a Is Base
    Debug.Print TypeOf b Is Base

    TestPolymorphic a
    TestPolymorphic b
End Sub

Private Sub TestPolymorphic(ByRef obj As Base)
    obj.Polymorphic
End Sub

' -----------------------------------------------

' Base class module

Private m_state As Byte

Public Event StateChanged(oldState As Byte, newState As Byte)

Public Property Get State() As Byte
    State = m_state
End Property

Public Property Let State(ByVal newState As Byte)
    Dim oldState As Byte
    oldState = m_state
    m_state = newState
    RaiseEvent StateChanged(oldState, newState)
End Property

Sub Polymorphic()
    Err.Raise 123, , "Implement in child class"
End Sub

Private Sub Class_Initialize()
    m_state = 1
End Sub

' -----------------------------------------------

' ChildA class module

Implements Base
Private WithEvents m_base As Base

Private Sub Class_Initialize()
    Set m_base = New Base
End Sub

Public Property Get State() As Byte
    State = Base_State
End Property

Public Property Let State(ByVal newState As Byte)
    Base_State = newState
End Property

Public Sub Polymorphic()
    Base_Polymorphic
End Sub

Private Property Get Base_State() As Byte
    Base_State = m_base.State
End Property

Private Property Let Base_State(ByVal newState As Byte)
    m_base.State = newState
End Property

Private Sub Base_Polymorphic()
    Debug.Print "In Child A ..."
End Sub

Private Sub m_base_StateChanged(oldState As Byte, newState As Byte)
    Debug.Print "State of 'Child A' instance has changed from " & oldState & " to " & newState
End Sub

Output:
' State of 'Child A' instance has changed from 1 to 2
' State of 'Child B' instance has changed from 1 to 5
' True
' True
' In Child A ...
' In Child B ...