[vtk-developers] Shared ownership of an array of pointers

[David Gobbi]

Hi Phillippe,

Actually I was wrong about vtkVariant not having any size overhead.  It needs
to store the type along with the value.  So vtkVariantArray is no good for this.

A helper class might be the best option.  You could make a vtkObjectArray
class which would be just like vtkVoidArray except that it would objects.

 - David


On Tue, Nov 20, 2012 at 9:54 AM, Philippe Pébay
<philippe.pebay at kitware.com> wrote:
> Hello David
>
> Thanks for the summary about the absence of performance hit when using SPs
> as opposed to RPs. I will remember it and use them more often as a result.
>
> Regarding the variant array, yes, the conversion to vtkObject is a concern.
> I think that the most efficient option would be to create a small helper
> class that would internally store a raw array of pointers. That helper would
> derive from vtkObjectBase. That should avoid any perfomance hit.
>
> Philippe
>
>
> On Tue, Nov 20, 2012 at 5:49 PM, David Gobbi <david.gobbi at gmail.com> wrote:
>>
>> Hi Phillippe,
>>
>> A vtkVariant is 64 bits, it should be no larger than a pointer.  There
>> is a function call overhead in ToVTKObject(), which might be a concern to
>> you.
>>
>> The vtkSmartPointer is exactly the same size as a pointer, so there is no
>> size
>> overhead.  It has no computational overhead, either, except for the
>> "operator=",
>> the copy constructor, and the destructor (I wouldn't even count the
>> overhead in
>> the destructor, because you have to call ->Delete() eventually anyway).
>> So do
>> not avoid smart pointers because you are worried about efficiency.  The
>> VTK
>> smart pointers are very efficient because vtkObjectBase already has a
>> built-in
>> reference count.
>>
>>  - David
>>
>>
>> On Tue, Nov 20, 2012 at 9:35 AM, Philippe Pébay
>> <philippe.pebay at kitware.com> wrote:
>> > Hello David
>> >
>> > Thanks for the suggestion: I am concerned by the overhead that comes
>> > with
>> > variants. Wouldn't it be more lightweight to use a vtkVoidArray instead?
>> > The
>> > array can potentially contain O(10⁶) pointers. By the way this is for
>> > the
>> > same reason that I have, so far, refrained from using vtkSmartPointers
>> > as
>> > opposed to raw pointers, by fear of the overhead involved. Is this extra
>> > caution excessive?
>> >
>> > Thanks
>> > Philippe
>> >
>> > On Tue, Nov 20, 2012 at 5:31 PM, David Gobbi <david.gobbi at gmail.com>
>> > wrote:
>> >>
>> >> Hi Philippe,
>> >>
>> >> If the pointers are pointers to VTK objects, then you can use
>> >> vtkVariantArray.
>> >>
>> >>  - David
>> >>
>> >>
>> >> On Tue, Nov 20, 2012 at 9:29 AM, Philippe Pébay
>> >> <philippe.pebay at kitware.com> wrote:
>> >> > Just an idea: couldn't a vtkVoidArray be used (or abused) as
>> >> > container
>> >> > of
>> >> > raw pointers, that would be appropriately downcasted internally to
>> >> > the
>> >> > concrete type to which they are supposed to point? For instance,
>> >> > instead
>> >> > of
>> >> > having a
>> >> >  vtkMyInternalObject** obj
>> >> > instance variable, couldn't I use a
>> >> >  vtkVoidArray* obj
>> >> > instead, where each entry in obj would be the raw pointer to a
>> >> > vtkSmartPointer<vtkMyInternalObject>?
>> >> >
>> >> > Thanks
>> >> > Philippe
>> >> >
>> >> >
>> >> > On Tue, Nov 20, 2012 at 5:14 PM, Philippe Pébay
>> >> > <philippe.pebay at kitware.com>
>> >> > wrote:
>> >> >>
>> >> >> Hello Kyle
>> >> >>
>> >> >> Yes, I would want to avoid a dependency on Boost. But this is
>> >> >> basically
>> >> >> what I am tried to reproduce (the Boost shared array of pointers).
>> >> >>
>> >> >> Thanks
>> >> >> P
>> >> >>
>> >> >>
>> >> >> On Tue, Nov 20, 2012 at 5:13 PM, Kyle Lutz <kyle.lutz at kitware.com>
>> >> >> wrote:
>> >> >>>
>> >> >>> Would using boost suffice? Or do you want a solution using just
>> >> >>> VTK?
>> >> >>>
>> >> >>> -kyle
>> >> >>>
>> >> >>> On Tue, Nov 20, 2012 at 7:58 AM, Philippe Pébay
>> >> >>> <philippe.pebay at kitware.com> wrote:
>> >> >>> > Hello all,
>> >> >>> >
>> >> >>> > I need to keep track of shared ownership of an array of pointers.
>> >> >>> > Can
>> >> >>> > someone point me towards an example of code that already does
>> >> >>> > this?
>> >> >>> > This
>> >> >>> > should amount to a small helper class that contains the array of
>> >> >>> > pointers
>> >> >>> > along with a reference count.
>> >> >>> >
>> >> >>> > Thank you!
>> >> >>> > Philippe
>> >> >>> >
>> >> >>> >
>> >> >>> > --
>> >> >>> > Philippe Pébay, PhD
>> >> >>> > Director of Visualization and High Performance Computing /
>> >> >>> > Directeur de la Visualisation et du Calcul Haute Performance
>> >> >>> > Kitware SAS
>> >> >>> > 26 rue Louis Guérin, 69100 Villeurbanne, France
>> >> >>> > +33 (0) 6.83.61.55.70 / 4.37.45.04.15
>> >> >>> > http://www.kitware.fr
>
>
>
>
> --
> Philippe Pébay, PhD
> Director of Visualization and High Performance Computing /
> Directeur de la Visualisation et du Calcul Haute Performance
> Kitware SAS
> 26 rue Louis Guérin, 69100 Villeurbanne, France
> +33 (0) 6.83.61.55.70 / 4.37.45.04.15
> http://www.kitware.fr