Access™ 2007 VBA Programmer's Reference

13.9. Creating a Clone Method

Let's say you want to create a separate copy of a class object. It might be tempting to write something like this:

Dim objTeacher1 As clsTeacher
Dim objTeacher2 As clsTeacher

Set objTeacher 1 = New clsTeacher
Set objTeacher 2 = objTeacher1

On the surface, this looks as if it would create a new object named objTeacher2 that has the same property values as objTeacher2, and that is true. However, these objects actually point to the same data. If you change a property for objTeacher2, objTeacher1 also reflects the same value. The following code demonstrates:

Sub TestCopy()
    Dim objTeacher1 As clsTeacher
    Dim objTeacher2 As clsTeacher

    ' create the first instance
    Set objTeacher1 = New clsTeacher
    objTeacher1.Name = "Steven Buchanan"
    Debug.Print "Before: " & objTeacher1.Name

    ' create the second instance
    Set objTeacher2 = objTeacher1

    ' set the second name, then print the first
    objTeacher2.Name = "Nancy Davolio"
    Debug.Print "After: " & objTeacher1.Name

    ' Verify that the objects point to different locations
    Debug.Assert ObjPtr(objTeacher1) <> ObjPtr(objTeacher2)

    ' cleanup

Set objTeacher1 = Nothing
    Set objTeacher2 = Nothing
End Sub

The first time the Name is printed, the code prints Steven Buchanan. Then after changing the name of objTeacher2, objTeacher1 has been changed to Nancy Davolio.

Note the use of the ObjPtr function. This is a hidden function in VBA that returns the memory address of an object variable. You use this function in a Debug.Assert statement ...