[vtkusers] Questions About Python Wrapping

[David Gobbi]

The vtkSmartPointer isn't very safe in this regard. It is easy to
accidentally do the following, since VTK smart pointers implicitly convert
to raw pointers:

  vtkSmartPointer<vtkObject> MyMethod();
  vtkObject *obj = MyMethod(); // ownership is not caught, object is
autodeleted

The preferred way of dealing with transfer of ownership for returned values
in VTK is to add the VTK_NEWINSTANCE hint to the return value.  Then the
wrappers will automatically handle the reference count.  As for people who
use these methods from C++, they still have to worry about memory leaks,
but at least they don't have to worry about memory corruption caused by
mistakes like the one in the example above.

 - David


On Thu, May 17, 2018 at 4:29 PM, Andras Lasso <lasso at queensu.ca> wrote:

> The main advantage of the ability to return smart pointers would be that
> ownership of VTK objects could be transferred safely to the caller, without
> requiring calling UnRegister(None). C++ developers are aware of pointers
> but for Python users it is a mistery why sometimes they need to call
> UnRegister to avoid memory leaks.
>
> Andras
> ------------------------------
> *From:* David Gobbi <david.gobbi at gmail.com>
> *Sent:* Thursday, May 17, 2018 4:55 PM
> *To:* Kyle Sunderland
> *Cc:* vtkusers at vtk.org
> *Subject:* Re: [vtkusers] Questions About Python Wrapping
>
> Hi Kyle,
>
> Thanks for sending the files.  I see that the parameters for those methods
> are actually smart pointers, and as you have already noted, smart pointers
> aren't wrapped.
>
> This has never been an issue in VTK proper or for most VTK projects, since
> VTK methods generally use raw pointers.  The header file you sent is closer
> to ITK style than to VTK style.
>
> I definitely think that it would be worthwhile to add better smart pointer
> handling to the wrappers.
>
>  - David
>
>
> On Thu, May 17, 2018 at 12:13 PM, David Gobbi <david.gobbi at gmail.com>
> wrote:
>
>> On Thu, May 17, 2018 at 10:10 AM, Kyle Sunderland <1krs1 at queensu.ca>
>> wrote:
>> > It (mostly) works, however I noticed that no functions are wrapped
>> which have a definition that contains any of the wrapped classes. Does
>> anyone have insight into what I might have missed?
>>
>> Is a vtk..Hierarchy.txt file generated for the wrapped module when you
>> build it?  If so, send it to me along with the header files you're
>> wrapping, and I can take a look.
>>
>> > Also, as an additional question, is it still not possible to wrap
>> functions that return a smart pointer?
>>
>> If this is a feature that you need, then add it to the bug tracker.
>>
>> Cheers,
>>  - David
>>
>
>
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