[vtkusers] Breaking GC loops

[Michael Jackson]

And just to add to the fine examples that Ken wrote Parent-Child  
relationships where each holds a reference to another is another prime  
example of where SmartPointer/WeakPointer can come into play.

   Say you have some hierarchical data structure. If they both the  
parent and child need to hold references to each other then one of  
those references really needs to be a weak reference otherwise  
destroying objects can be troublesome. Just adding to the examples.
___________________________________________________________
Mike Jackson                      www.bluequartz.net
Principal Software Engineer       mike.jackson at bluequartz.net
BlueQuartz Software               Dayton, Ohio


On Feb 19, 2010, at 4:15 PM, Moreland, Kenneth wrote:

> Reference counting loops can be automatically detected and cleaned  
> up by the VTK garbage collector if all the references in the loop  
> are properly reported.  Details on reporting references to the  
> garbage collector are documented in the vtkGarbageCollector class:
>
> http://www.vtk.org/doc/nightly/html/classvtkGarbageCollector.html#_details
>
> Keep in mind that in order for this to work, you have to be sure  
> that all the references in the loop are reported.  If any are  
> missing, the references can never be collected.
>
> As an alternative, you might want to consider using a vtkWeakPointer  
> in place of one of the vtkSmartPointers.  vtkWeakPointer is used in  
> much the same way as vtkSmartPointer.  However, vtkWeakPointer does  
> not actually hold a reference.  The object pointed to by the  
> vtkWeakPointer can at any time be deleted if all the other  
> references to it are deleted; however vtkWeakPointer will detect  
> when the object is deleted and set itself to NULL.  Weak pointers  
> are useful in situations where you don’t care if the object gets  
> deleted or if you can be reasonably sure that the pointed to object  
> will not be deleted until the end of its normal use.  These are  
> clearly limited uses, but if you fall into one of these it can be  
> more convenient and efficient than invoking the garbage collection.
>
> As far as the render window/interactor goes, I would not look at  
> that as an example.  That is an exceptional case that exists only  
> because the order in which the two objects get destroyed matters.   
> They were also written before the garbage collector existed.  If  
> they were to be rewritten today, I would expect that specialty code  
> would not exist.
>
> -Ken
>
>
> On 2/18/10 8:50 PM, "Kenneth Porter" <shiva at sewingwitch.com> wrote:
>
> I added some code to an example using smart pointers to explicitly  
> break a
> GC loop, but it's a hack because it's not exception-safe. What's the
> correct way to break this kind of loop? I see a similar loop between  
> the
> render window and the interactor and there seems to be explicit code  
> in
> vtkRenderWindowInteractor::UnRegister to break this loop. (Why is  
> the break
> condition equal to 3 there? I guess each holds a reference to the  
> other,
> and one holds a reference to itself, and the other has had its  
> reference
> count go to zero pending deletion. But if another object also holds  
> such a
> loop, this won't go to 3.) I think something similar is needed in a  
> custom
> UnRegister method in the example.
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>
>    ****      Kenneth Moreland
>     ***      Sandia National Laboratories
> ***********
> *** *** ***  email: kmorel at sandia.gov
> **  ***  **  phone: (505) 844-8919
>     ***      web:   http://www.cs.unm.edu/~kmorel
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