[vtkusers] About Pitfalls section in smart pointers tutorial

[Elvis Stansvik]

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.

Elvis


>
> I am not sure the pitfalls section has any real value as the only
> remaining point is quite generically true, don't create objects you
> are not going to use (on the stack, pointers, or smart pointers).
>
> Anyone see something I am missing there?
>
> Thanks,
>
> Marcus
>
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