[vtkusers] About Pitfalls section in smart pointers tutorial

[Elvis Stansvik]

2016-06-30 17:33 GMT+02:00 Marcus D. Hanwell <marcus.hanwell at kitware.com>:

> On Thu, Jun 30, 2016 at 3:02 AM, Elvis Stansvik
> <elvis.stansvik at orexplore.com> wrote:
> > 2016-06-29 21:42 GMT+02:00 Marcus D. Hanwell <marcus.hanwell at kitware.com
> >:
> >>
> >> On Wed, Jun 29, 2016 at 3:28 PM, Elvis Stansvik
> >> <elvis.stansvik at orexplore.com> wrote:
> >> > At
> >> >
> >> >    http://www.vtk.org/Wiki/VTK/Tutorials/SmartPointers#Pitfalls
> >> >
> >> > is this warning:
> >> >
> >> > "If you create an object and then change where it is pointing, the
> >> > reference
> >> > count will be incorrect. e.g.
> >> >
> >> > vtkSmartPointer<vtkPolyData> Polydata =
> >> > vtkSmartPointer<vtkPolyData>::New();
> >> > Polydata = Reader->GetOutput();
> >> >
> >> > In this case, memory is allocated for Polydata, but then we change
> >> > Polydata
> >> > to point to the output of Reader rather than the memory we just
> >> > allocated.
> >> > Instead, we should have done simply:
> >> >
> >> > vtkPolyData* Polydata = Reader->GetOutput();
> >> >
> >> > It was not necessary to use a smart pointer because we did not
> actually
> >> > create any new objects"
> >> >
> >> > I fail to see how the refcount would be incorrect in the first
> approach.
> >> > First a new object is created and its refcount increased to 1, then
> when
> >> > Reader->GetOutput() (a raw pointer) is assigned to Polydata, wouldn't
> >> > the
> >> > smart pointer take care of decreasing the refcount for the object it
> >> > already
> >> > points to (so it would go to 0), before it starts managing the new
> >> > pointer?
> >> > Or is that not how it will work?
> >> >
> >> I have the same interpretation, it looks like it was added by David
> >> Doria in 2009. It is silly to allocate a new object, and then
> >> immediately point to something else without ever using that object,
> >> but the reference count for that object should be correctly
> >> decremented and so deleted as far as I can see.
> >
> >
> > Yes, I forgot to say: I of course also think it's silly, I was just
> curious
> > about the wording of that warning since it didn't make sense to me.
> >
> I removed the section from the wiki article, it would be better to
> balance it with the other smart pointer types too possibly. All
> vtkObject derived classes are implicitly reference counted for better
> or worse, so the behavior is different to the STL smart pointers.
>
> They interact with the objects reference count, I am a little biased
> as I wrote it, but
> https://blog.kitware.com/a-tour-of-vtk-pointer-classes/ (linked at the
> top of that wiki page) provides a broad overview of the different
> smart pointers and when/where they might be used.
>

Yep, I read that before I came across the tutorial page in the wiki
actually. It was very good.

Elvis
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://public.kitware.com/pipermail/vtkusers/attachments/20160630/7cf0db90/attachment.html>