[vtkusers] Re: is VTKDelete() intended for public use?

[David Gobbi]

Steve M. Robbins wrote:

>On Fri, Oct 07, 2005 at 07:23:45AM -0400, Jeff Lee wrote:
>  
>
>>Hi Steve,
>>One way would be to make sure you remove all references to the vtk 
>>object, then call System.gc() which will do the finalize, and actually 
>>delete the pointer immediately.  
>>    
>>
>
>But explicit garbage collection is only a hint, and it is said to be a
>bad idea:
>
>    A third category where developers often mistakenly think they are
>    helping the garbage collector is the use of System.gc(), which
>    triggers a garbage collection (actually, it merely suggests that
>    this might be a good time for a garbage
>    collection). Unfortunately, System.gc() triggers a full
>    collection, which includes tracing all live objects in the heap
>    and sweeping and compacting the old generation. This can be a lot
>    of work. In general, it is better to let the system decide when it
>    needs to collect the heap, and whether or not to do a full
>    collection. Most of the time, a minor collection will do the
>    job.
>
>    http://www-128.ibm.com/developerworks/java/library/j-jtp01274.html
>
>
>  
>
>>I am not opposed to your patch, but 
>>VTKDelete has reference counting in it, and if someone else has a 
>>reference somewhere, the object won't be deleted anyway.
>>    
>>
>
>The java object won't be deleted, no.  But I only care about removing
>a reference from the C++ object that holds the resources.
>
>Yes, it is dangerous in that someone may now be holding an invalid
>java object.  I can live with that.  It would be even better if the
>JNI code would detect and throw IllegalStateException in this
>situation.  I bet we could do that without too much trouble.
>
>  
>
>>For me, explicitly calling VTKDelete is philosophically not the
>>right thing to do.  In java, you should make sure the reference tree
>>is clean for the object, and then it will be finalized when garbage
>>collection occurs.  If the vm is memory bound, then the garbage
>>collection will be invoked more often, ensuring that the
>>vtkImageData will be deleted shortly after all references are
>>removed.
>>    
>>
>
>Our software is an image viewer with a mixture of small, short-lived
>things (typically generated by mouse events), and very large,
>long-lived things (like image buffers).  Hard-won experience taught us
>that we should not rely on GC for the latter, for several reasons.  In
>practice, this is not an impediment because we know precisely when we
>can dispose of the buffer.  Hence our search for a way to dispose of
>the VTK C++ objects.
>
>If VTKDelete is not the right method, can we design a better one?
>  
>
Hi Steve,

I think that you are taking the wrong approach.  You don't actually need 
to delete
any VTK objects, you just need to free the memory that they are using to 
store
the image array.

Each data object has a "ReleaseData()" method that will allegedly do the 
trick.

 - David