Re: Dangerous deallocation/circular reference...

[Brady Duga <duga at ljug dot com>]

On Thursday, July 11, 2002, at 11:42  AM, Joseph J. Strout wrote:

> At 11:01 AM -0700 7/11/02, Brady Duga wrote:
> >  I am worried because the reference count of the Tickler becomes 0 
> > from a call the Tickler is making.
> No, it doesn't.  You said that you nil the reference TO the Tickler. 
> The Tickled object still has a reference to it -- it must, since it 
> invoked the Tickle method somehow.
Right - I nil the reference to the Tickler - the reference that the 
Tickled has to it. So the Tickled object no longer has a reference to 
the tickler (the Tickle method was invoked by the Tickler, not the 
Tickled).
> And in fact, as you've described it, the Tickled object won't go away 
> at all.  The Tickler still has a reference to it, so until the Tickler 
> object dies (which presumably won't be until the app exits), anything 
> it has a reference to will stay alive too.
I assume the Tickler would go away when its ref count dropped to 0.

> So, what you need to do is tell the Tickler to release its reference to 
> the object that no longer needs tickles.  Then, the object will be 
> destroyed.  And yes, this is safe.
I'm not sure I explained this correctly. Let me try again - in some 
object, we have this code:
sub StartMeUp()
        dim lv as SomeClassWhichImplementsTickled

        lv = new SomeClassWhichImplementsTickled
        lv.DoSomethingforTenSeconds()
        return
end sub

In SomeClassWhichImplementsTickled:

sub SomeClassWhichImplementsTickled()
        mTickler = new Tickler(me)
end sub

sub Tickle()
        If IAmDone() then       
                mTickler = nil
        else
                KeepDoingWhatINeedTo()
        end if
end sub

In Tickler:

sub Tickler(aTickeled as Tickeled)
        mTickeled = aTickeled
end sub


During StartMeUp, we have the following references:

      1                            2                1
mTickler <----> mTickeled <--- lv

When we exit StartMeUp(), lv goes out of scope, leaving:

      1                            1
mTickler <----> mTickeled

Then, in the action of mTickler, we say:

mTickeled.tickle()

In that call, it sets the mTickler reference to nil if IAmDone() returns 
true, leaving:
       0                          1
mTickler  ----> mTickeled

Doesn't this cause mTickler to vanish? And when it does, won't the 
remaining reference to mTickeled vanish as well? This seemed dangerous, 
as mTickler is no longer referenced, even though it is currently 
involved with the call chain.

--Brady
The La Jolla Underground