How to test if two object variables reference the same object?

  • Thread starter Thread starter joeu2004
  • Start date Start date
J

joeu2004

How can I test if two object variables reference the same object?

For example, suppose I construct a circular linked list of objects.
Then I want to walk the list until I get to the last object --
actually, until I wrap around to the first object again.

I might declare the following class module myTest1:

Private objInst As Long
Public nextObj As myTest1

Private Sub Class_Initialize()
objCnt = objCnt + 1
objInst = objCnt
End Sub

Property Get instNum() 'read-only
instNum = objInst
End Property

I would like to write the following in a standard module:

Public objcnt As Long

Private Sub testit()
Dim first As myTest1, last As myTest1
Dim i As Long, s As String, p As myTest1
'***** build circular linked list
Set first = New myTest1
Set last = first
Set first.nextObj = first
For i = 2 To 10
Set last.nextObj = New myTest1
Set last = last.nextObj
Set last.nextObj = first
Next
'***** walk circular list
s = first.instNum
Set p = first.nextObj
Do Until p = first '<--- does not work
s = s & Chr(10) & p.instNum
Set p = p.nextObj
Loop
MsgBox s
End Sub

Work-around: I must write `Do Until p.instNum = first.instNum`. I
would like to avoid this.
 
PS....

Do Until p = first        '<--- does not work
Click to expand...

My intent is test if p and first reference the same object, not if the
content of the both objects are the same.
 
Do Until p = first '<--- does not work
My intent is test if p and first reference the same
object, not if the content of the both objects are
the same.
Click to expand...

Just a guess here, does using the "Is" operator do what you want...

Do Until p Is first

Rick Rothstein (MVP - Excel)
 
Do Until p = first '<--- does not work
Click to expand...

Do Until p.instNum = first.instNum
My intent is test if p and first reference the same object, not if the
content of the both objects are the same.
Click to expand...

In that case more generally as Rick suggests
Do Until p Is first

Or you could also do
Do Until ObjPtr(p) = ObjPtr(first)

All three above will give same results in your test. And a very nasty little
test it is too, even if deliberately designed to leak memory :-)

Regards,
Peter T
 
joeu2004 said:
Peter and Rick (et al),

I would really appreciate your response to my other questions:

"Diff btwn Dim x and Private x in class module?"
http://groups.google.com/group/micr...ogramming/browse_frm/thread/ac893f9e4f0e8e40#
Click to expand...

That's n easy one, no difference. Dim/Private at module level in any type of
module are equivalent. As are Global/Public. But suggest use Private &
Public at module level for convention, even though the others are retained
for backwards compatibility.
"What kernel function returns process stats"
http://groups.google.com/group/micr...ogramming/browse_frm/thread/94e23f46bdaa3865#
Click to expand...

Don't know, I've never looked into that

Regards,
Peter T
 
That's n easy one, no difference. Dim/Private at module
level in any type of module are equivalent. As are
Global/Public. But suggest use Private & Public at module
level for convention
Click to expand...

That's what I determined empirically, too. And I agree with your
convention. It was just that Walkenbach used Dim in one (and only
one) example in the class module chapter; he used Private in the other
examples, but without comment about the difference. Since
Walkenbach's book are highly regarded by many (not I), I thought he
might have been trying to convey a subtle difference without
explanation :-(.

Good to hear that's not the case. And perhaps he fixed the Dim
declaration in later revisions of the book. Mine is for XL2003, 2nd
printing.
 
Hi all,

If you would like it to be a two way linked list you can use:

For the classe:
-----
Option Explicit

Private objInst As Long
Public nextObj As myTest1
Public prevObj As myTest1

Private Sub Class_Initialize()
objcnt = objcnt + 1
objInst = objcnt
End Sub

Property Get instNum() 'read-only
instNum = objInst
End Property
-----

And for the module:
-----
Public objcnt As Long

Private Sub testit()
Dim first As myTest1, last As myTest1
Dim i As Long, s As String, p As myTest1
'***** build circular linked list
Set first = New myTest1
Set last = first
Set first.nextObj = first
Set last.prevObj = first
For i = 2 To 10
Set last.nextObj = New myTest1
Set last.nextObj.prevObj = last
Set last = last.nextObj
Set last.nextObj = first
Set first.prevObj = last
Next
'***** walk circular list
s = first.instNum
Set p = first.nextObj
Do Until p Is first '<--- does not work
s = s & Chr(10) & p.instNum
Set p = p.nextObj
Loop
MsgBox s
End Sub
 
Hi,

I have inserted some code to show how to remove a instance from the
list:

Public objcnt As Long

Private Sub testit()
Dim first As myTest1, last As myTest1
Dim i As Long, s As String, p As myTest1
Dim r As Long, j As Long
'***** build circular linked list
Set first = New myTest1
Set last = first
Set first.nextObj = first
Set last.prevObj = first
For i = 2 To 10
Set last.nextObj = New myTest1
Set last.nextObj.prevObj = last
Set last = last.nextObj
Set last.nextObj = first
Set first.prevObj = last
Next
'***** walk circular list
s = first.instNum
Set p = first.nextObj
Do Until p Is first '<--- does not work
s = s & Chr(10) & p.instNum
Set p = p.nextObj
Loop
MsgBox s, vbOKOnly, "All"

For j = 1 To 5
' select a random number to
' remove a object from the list
Randomize (Timer)
Do
r = first.instNum + (Fix(Rnd() * _
first.prevObj.instNum - first.instNum))
Loop Until r >= first.instNum And _
r <= first.prevObj.instNum
Set p = first
Do Until p.instNum = r Or p Is last
Set p = p.nextObj
Loop

If p.instNum = r Then
' ajust next and prev
Set p.nextObj.prevObj = p.prevObj
Set p.prevObj.nextObj = p.nextObj
' remove selected
If first.instNum = p.instNum Then
Set first = first.nextObj
End If
If last.instNum = p.instNum Then
Set last = last.prevObj
End If

Set p = Nothing

'***** walk circular list
s = last.instNum
Set p = last.prevObj
Do Until p Is last
s = s & Chr(10) & p.instNum
Set p = p.prevObj
Loop
MsgBox s, vbOKOnly, "loop " & j & ", " & r & " removed"
Else
MsgBox r & " not found", vbOKOnly, "loop " & j
End If
Next

End Sub
 
That's one way but to release all the objects you'd need to increase For j =
1 To 5
to a lot more. But even with that you'll end up with p.nextObj & p.prevObj
both referring to 'self', ie the same object as p, so finally need to do

Set p.nextObj = Nothing
Set p.prevObj = Nothing

then the object still assigned to p will self destruct as p falls out of
scope.

In real life of course never construct something like this!

Regards,
Peter T
 
HI Peter,

I was only a way to remove an object and maintai the connection from
the previous en next.

I was not my intention to remove all.

Regards,

Wouter
 
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