COM Objects: Passing Data Between Classes Using INHERIT

[Kev Peel]

'______________________________________________________________________________
'
' Inheritance: Shared Data Demo
' -----------------------------
'
' Demo showing how to share data between the base class and derived interfaces.
'
'
' By Kev Peel, August 2008. [URL]http://www.kgpsoftware.com[/URL]
'______________________________________________________________________________
#Compile Exe
#Dim All
#Include "win32api.inc"                         ' For the DebugPrint function.

'------------------------------------------------------------------------------
' For debugging purposes. Creates and displays text in a console.
'------------------------------------------------------------------------------
Function DebugPrint(ByVal sText As String) As Long
  Static hCons As Dword
  hCons = GetStdHandle(%STD_ERROR_HANDLE)
  If (hCons = 0) Or (hCons = %INVALID_HANDLE_VALUE) Then
     AllocConsole
     hCons = GetStdHandle(%STD_ERROR_HANDLE)
     If (hCons = %INVALID_HANDLE_VALUE) Then Exit Function
  End If
  ' Write to the screen...
  Local lCount As Long
  sText = sText + $CrLf
  If WriteConsole(hCons, ByVal StrPtr(sText), Len(sText), lCount, %NULL) Then Function = %TRUE
End Function
 
'------------------------------------------------------------------------------
' The base class. One simple interface.
'------------------------------------------------------------------------------
Class CBase
  Instance bInit As Byte
  Instance saved_data As Long
  Class Method Create
     ' Initialize...
     DebugPrint "CBase.Create"
  End Method
  Class Method Destroy
     ' Uninitialize...
     DebugPrint "CBase.Destroy"
  End Method
  Interface IBase
     Inherit IDispatch
     
     '------------------------------------------------------------------------------
     ' Internal: Saves data in nValue (could be a pointer), for future base access.
     '------------------------------------------------------------------------------
     Method PassData(ByVal nValue As Long)
        If (bInit = %FALSE) Then
           ' Save data ONCE only, since this is public...
           bInit = %TRUE
           saved_data = nValue
           DebugPrint "IBase.PassData(" + Format$(saved_data) + ")"
        End If
     End Method
     ' Put your base METHOD/PROPERTY code here
     '------------------------------------------------------------------------------
     ' Displays the saved data that was passed from PassData.
     '------------------------------------------------------------------------------
     Method ShowData
        DebugPrint "IBase.ShowData: saved_data is " + Format$(saved_data)
     End Method
  End Interface
End Class

'------------------------------------------------------------------------------
' Simple class #1, uses an interface derived from IBase.
'------------------------------------------------------------------------------
Class CDerive1
  Class Method Create
     ' Initialize...
     DebugPrint "CDerive1.Create"
     ' Access our secret METHOD...
     ' Note: it passes a number here, but you can pass anything, even a pointer ;)
     Local oSecret As IDerive1
     Let oSecret = Me
     oSecret.PassData(111)
  End Method
  Class Method Destroy
     ' Uninitialize...
     DebugPrint "CDerive1.Destroy"
  End Method
  Interface IDerive1
     Inherit CBase, IBase
     ' Put your derived METHOD/PROPERTY code here
  End Interface
End Class
 
'------------------------------------------------------------------------------
' Simple class #2, uses an interface derived from IBase.
'------------------------------------------------------------------------------
Class CDerive2
  Class Method Create
     ' Initialize...
     DebugPrint "CDerive2.Create"
     ' Access our secret METHOD...
     ' Note: it passes a number here, but you can pass anything, even a pointer ;)
     Local oSecret As IDerive2
     Let oSecret = Me
     oSecret.PassData(222)
  End Method
  Class Method Destroy
     ' Uninitialize...
     DebugPrint "CDerive2.Destroy"
  End Method
  Interface IDerive2
     Inherit CBase, IBase
     ' Put your derived METHOD/PROPERTY code here
  End Interface
End Class

'------------------------------------------------------------------------------
' The test code, creates some objects.
'------------------------------------------------------------------------------
Function PBMain
  Local Obj1 As IDerive1
  Local Obj2 As IDerive2
  Obj1 = Class "CDerive1"
  Obj2 = Class "CDerive2"
  Obj1.ShowData
  Obj2.ShowData
  Obj1 = Nothing
  Obj2 = Nothing
  ? "END"
End Function