[vtk-developers] vtk-developers Digest, Vol 158, Issue 21

Sat Jun 24 19:18:16 EDT 2017

Hi Andras,

One thing that may be relevant is this link:
http://marc.info/?l=vtk-developers&m=126471789304821
Note: I think
vtkSmartPointer<vtkPolyData> output;
should be:
vtkSmartPointer<vtkPolyData> readerOutput;

I think the example there nicely demonstrates the difference between vtkNew
and vtkSmartPointer.

Maybe something like this could be incorporated into the testing in
https://gitlab.kitware.com/vtk/vtk/merge_requests/2961

Regards
   Andrew

On Sun, Jun 25, 2017 at 6:07 AM, Andras Lasso <lasso at queensu.ca> wrote:

> It is great that you've brought this up! There is a very important,
> fundamental difference. Unlike std::shared_ptr or QSharedPointer, VTK smart
> pointer does NOT need clear distinction between the pointer object and the
> raw pointer it holds, because reference counting is implemented in the
> managed object itself.
>
> See in std::shared_ptr documentation: "Constructing a new shared_ptr using
> the raw underlying pointer owned by another shared_ptr leads to undefined
> behavior." (http://en.cppreference.com/w/cpp/memory/shared_ptr)
>
> In VTK, if you assign a vtkSmartPointer to another, internally the raw
> pointer is retrieved and used (Common/Core/vtkSmartPointer.h):
>
>   vtkSmartPointer& operator=(const vtkSmartPointer<U>& r)
>   {
>     this->vtkSmartPointerBase::operator=(CheckType(r.GetPointer()));
>     return *this;
>   }
>
> VTK smart pointers work differently from STL and Qt, so it is reasonable
> to use a different API that reflects this difference. As VTK allows safe
> use of a simpler API, we should take advantage of that and use it.
>
> Andras
>
> -----Original Message-----
> From: Elvis Stansvik [mailto:elvis.stansvik at orexplore.com]
> Sent: Saturday, June 24, 2017 12:36 PM
> To: Andras Lasso <lasso at queensu.ca>
> Cc: David Lonie <david.lonie at kitware.com>; Shawn Waldon <
> shawn.waldon at kitware.com>; VTK Developers <vtk-developers at vtk.org>;
> Andrew Maclean <andrew.amaclean at gmail.com>
> Subject: Re: [vtk-developers] vtk-developers Digest, Vol 158, Issue 21
>
> 2017-06-24 15:53 GMT+02:00 Andras Lasso <lasso at queensu.ca>:
> > Based on the feedbacks and the poll results (11 out of 13 votes for
> > changing current behavior), it seems there is a strong consensus for
> adding implicit conversion.
>
> I was away and missed the vote, but frankly I'm not sure what I would have
> voted.
>
> Looking at what others do, I guess vtkNew is comparable to std::unique_ptr
> or Qt's QScopedPointer, none of which do implicit conversion to the
> underlying pointer (they have .get() and .data() respectively). So VTK
> would be rather unique in doing that for a scoped smart pointer I think..
>
> Elvis
>
> >
> > Here is a pull request with the change:
> > https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitla
> > b.kitware.com%2Fvtk%2Fvtk%2Fmerge_requests%2F2961&data=02%7C01%7Classo
> > %40queensu.ca%7C14c5314b70e443f5be4908d4bb1f0bf4%7Cd61ecb3b38b142d582c
> > 4efb2838b925c%7C1%7C0%7C636339189405346369&sdata=2Jv0QS1qlRUgSJvN%2BO8
> > 0pXOt89kKC0NP7iXXRr2mg30%3D&reserved=0
> >
> > Andras
> >
> > -----Original Message-----
> > From: David Lonie [mailto:david.lonie at kitware.com]
> > Sent: Friday, June 23, 2017 1:51 PM
> > To: Shawn Waldon <shawn.waldon at kitware.com>
> > Cc: Andras Lasso <lasso at queensu.ca>; VTK Developers
> > <vtk-developers at vtk.org>; Andrew Maclean <andrew.amaclean at gmail.com>
> > Subject: Re: [vtk-developers] vtk-developers Digest, Vol 158, Issue 21
> >
> > I'm so glad someone brought this up =)
> >
> > On Fri, Jun 23, 2017 at 10:52 AM, Shawn Waldon <shawn.waldon at kitware.com>
> wrote:
> >>  But realistically what it does is
> >> discourage use of vtkNew.  Most people I talk to say "I would use
> >> vtkNew but then I have to clutter up my code with GetPointer calls
> >> everywhere" and so they still use vtkSmartPointer.  Honestly I only
> >> used it to clean up declarations (the problem discussed earlier in
> >> this thread).  Now that we can use auto in VTK, I'll probably go back
> >> to vtkSmartPointer since it is easier to use.
> >
> > I agree with Shawn. It's an unnecessary hassle to require the Get() to
> be used.
> >
> > I suppose I'm just a fan of the design philosophy for C++ about "don't
> prevent developers from shooting themselves in the foot." If a dev doesn't
> understand VTK's reference counting well enough to understand that you
> can't simply return a vtkNew'd pointer without increfing it first, they're
> going to have far greater problems using the toolkit than this.
> >
> > Besides, you could easily (and safely) return a vtkNew'd pointer from a
> function:
> >
> > vtkObject* function()
> > {
> >   vtkNew<vtkObject> obj;
> >   obj->Register();
> >   return obj; // If we had implicit conversions, anyway }
> >
> > Making the class unwieldly simply so that people don't need to learn how
> reference counts work is a poor justification IMO.
> >
> > Just my 2c,
> > Dave
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-- 
___________________________________________
Andrew J. P. Maclean

___________________________________________
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