VBA: is there something like Abstract Class?

There is no inheritance in VBA.

You can define an interface and you can implement it in a class by using the Implements keyword. But if you want shared functionality pre-implemented by a base class, you have to use the copy-paste approach.

Related reading:
How to use the Implements in Excel VBA
How to use comparison methods between class object modules in VBA in a similar manner as VB.NET?

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 moduleSub 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 bEnd SubPrivate Sub TestPolymorphic(ByRef obj As Base)    obj.PolymorphicEnd Sub' -----------------------------------------------' Base class modulePrivate m_state As BytePublic Event StateChanged(oldState As Byte, newState As Byte)Public Property Get State() As Byte    State = m_stateEnd PropertyPublic Property Let State(ByVal newState As Byte)    Dim oldState As Byte    oldState = m_state    m_state = newState    RaiseEvent StateChanged(oldState, newState)End PropertySub Polymorphic()    Err.Raise 123, , "Implement in child class"End SubPrivate Sub Class_Initialize()    m_state = 1End Sub' -----------------------------------------------' ChildA class moduleImplements BasePrivate WithEvents m_base As BasePrivate Sub Class_Initialize()    Set m_base = New BaseEnd SubPublic Property Get State() As Byte    State = Base_StateEnd PropertyPublic Property Let State(ByVal newState As Byte)    Base_State = newStateEnd PropertyPublic Sub Polymorphic()    Base_PolymorphicEnd SubPrivate Property Get Base_State() As Byte    Base_State = m_base.StateEnd PropertyPrivate Property Let Base_State(ByVal newState As Byte)    m_base.State = newStateEnd PropertyPrivate Sub Base_Polymorphic()    Debug.Print "In Child A ..."End SubPrivate Sub m_base_StateChanged(oldState As Byte, newState As Byte)    Debug.Print "State of 'Child A' instance has changed from " & oldState & " to " & newStateEnd SubOutput:' 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 ...

My solution for abstract class in vba:View

'------------------------- ' Standard moduleSub Main()    Dim objC1 As C1    Dim objC2 As C2    Dim objCollection As New Collection    Set objC1 = New C1    Set objC2 = New C2    With objC1        .getInterface.M1 "Hello C1!"        temp1 = .getInterface.M2(objCollection)        .getSuperClass.SM1 "Hi C1!!!"        temp3 = .getSuperClass.SM2(objCollection)    End With    Debug.Print vbCrLf    With objC2        .getInterface.M1 "Hello C2!"        temp1 = .getInterface.M2(objCollection)        .getSuperClass.SM1 "Hi C2!!!"        temp3 = .getSuperClass.SM2(objCollection)    End WithEnd Sub' -----------------------------------------------' IAbstracat class moduleSub SM1(strString As String): End SubFunction SM2(varItem As Variant) As String: End Function' -----------------------------------------------' Abstracat class moduleImplements IAbstract'Each class must implement these methods, in a particular waySub M1(strString As String): End SubFunction M2(varItem As Variant) As String: End Function'The sub classes will extend SM1 and SM2Private Sub IAbstract_SM1(strString As String)    Debug.Print "Sub IAbstract_SM1: " & strStringEnd Sub'The sub classes will extend SM1 and SM2 Private Function IAbstract_SM2(varItem As Variant) As String    Dim strMyString As String    strMyString = "Function IAbstract_SM2 => ObjPtr(varItem): " & ObjPtr(varItem)    Debug.Print strMyString    IAbstract_SM2 = strMyStringEnd Function' -----------------------------------------------' C1 class moduleImplements AbstractPrivate Type TC1    objConcretSuperClass As Abstract    objInterfaceSuperClass As IAbstract    objInterfaceSubClass As AbstractEnd TypePrivate this As TC1'if you do not need to initialize anything, then this is it:Private Sub Class_Initialize()    With this        'creating an instance of class' Abstract'        Set .objConcretSuperClass = New Abstract        'Referencing the' Abstract 'interface, where are the extended methods        Set .objInterfaceSuperClass = .objConcretSuperClass        'Creating a refence for the C1 interface, which is the class' Abstract'        'Here we have the particular implementations of M1 and M2        Set .objInterfaceSubClass = Me    End WithEnd Sub'With this we can do:'   set objC1 = New C1'   objC1.getInterface.Abstract_M1'   objC1.getInterface.Abstract_M2Property Get getInterface() As Abstract    Set getInterface = this.objInterfaceSubClassEnd Property'With this we can call the methods defined in' Abstract ': SM1 and SM2'   set objC1 = New C1'   objC1.getSuperClass.SM1 "hello!"'   temp = objC1.getSuperClass.SM2 (New Collection)Property Get getSuperClass() As IAbstract    Set getSuperClass = this.objInterfaceSuperClassEnd Property'Here we have the particular implementations of M1 and M2Private Sub Abstract_M1(strString As String)    Debug.Print "Class C1 => Sub Abstract_M1: " & strStringEnd SubPrivate Function Abstract_M2(varItem As Variant) As String    Debug.Print "Class C1 => Function Abstract_M2: " & ObjPtr(varItem)End Function' -----------------------------------------------' C2 class moduleImplements AbstractPrivate Type TC2    objConcretSuperClass As Abstract    objInterfaceSuperClass As IAbstract    objInterfaceSubClass As AbstractEnd TypePrivate this As TC2'if you do not need to initialize anything, then this is it:Private Sub Class_Initialize()    With this        'creating an instance of class' Abstract'        Set .objConcretSuperClass = New Abstract        'Referencing the' Abstract 'interface, where are the extended methods        Set .objInterfaceSuperClass = .objConcretSuperClass        'Creating a refence for the C1 interface, which is the class' Abstract'        'Here we have the particular implementations of M1 and M2        Set .objInterfaceSubClass = Me    End WithEnd Sub'With this we can do:'   set objC2 = New C2'   objC2.getInterface.Abstract_M1'   objC2.getInterface.Abstract_M2Property Get getInterface() As Abstract    Set getInterface = this.objInterfaceSubClassEnd Property'With this we can call the methods defined in' Abstract ': SM1 and SM2'   set objC1 = New C1'   objC1.getSuperClass.SM1 "hello!"'   temp = objC1.getSuperClass.SM2 (New Collection)Property Get getSuperClass() As IAbstract    Set getSuperClass = this.objInterfaceSuperClassEnd Property'Here we have the particular implementations of M1 and M2Private Sub Abstract_M1(strString As String)    Debug.Print "Class C2 => Sub Abstract_M1: " & strStringEnd SubPrivate Function Abstract_M2(varItem As Variant) As String    Debug.Print "Class C2 => Function Abstract_M2: " & ObjPtr(varItem)End Function

Immediate Check Window (CTRL + G):

Class C1 => Sub Abstract_M1: Hello C1!Class C1 => Function Abstract_M2: 550324728Sub IAbstract_SM1: Hi C1!!!Function IAbstract_SM2 => ObjPtr(varItem): 550324728Class C2 => Sub Abstract_M1: Hello C2!Class C2 => Function Abstract_M2: 550324728Sub IAbstract_SM1: Hi C2!!!Function IAbstract_SM2 => ObjPtr(varItem): 550324728