[vtkusers] Difference between vtkSmartPointer assignment operator and ShallowCopy

[Berti Krüger]

Thanks Oleksandr for the great answer.

OK, i see, so when i use ShallowCopy, i copy the underlying structure of the vtk object e.g. attributes like the pointers the vtk object consists of without copying the content which the pointers point to.

So for example by using a ShallowCopy on a vtkPolydata object i copy all the underlying pointers to things like vtkPointSet, vtkDataSet, Cells, Lines, Vertices, Points etc. which a vtkPolydata object is composed of but unlike using a DeepCopy not the actual content (vertices and stuff arrays) which these pointers point to.

By using the vtkSmartPointer i get a reference counted pointer to the vtkObject without transfering the ownership of this object. It only increments the lease count by one and if the lease count becomes zero the destructor of the object is called.



Just one question:

Does every vtk filter copies the data it is given to?

e.g.

When using


    vtkNew<vtkTransformPolyDataFilter> transformFilter;

    transformFilter->SetInputData(polyData);

    transformFilter->SetTransform(transformation);

    transformFilter->Update();



does the polyData exists twice in memory (the original polyData and the filter keeps its own transformed copy of the polyData)?



Thanks and regards,


Berti


Am Do, 25. Okt, 2018 um 2:15 VORMITTAGS schrieb Oleksandr Malyushytsky <omalyushytskyvtkuser at gmail.com>:
vtkSmartPointer has nothing to do with neither deep or shallow copy.

if you assign vtk object to smartpointer the reference count of vtk object is incremented,
there is only one vtkObject in the memory. Once smartpointer is released (going out of scope  for example) vtk object reference count  it was pointing is decremented.
In another words vtkSmartPointer is like a normal pointer, which adds does reference couning.

Operations with smartpointer DO NOT copy any data related to the object they point to.


While deep and shalow copy are always performed with 2 different objects.





On Wed, Oct 24, 2018 at 3:19 PM Berti Krüger <berti_krueger at hotmail.com<mailto:berti_krueger at hotmail.com>> wrote:
Hello.

I (hopefully) already understand the difference between the ShallowCopy and the DeepCopy method (e.g. copying only the pointer vs. copying the pointer and the content pointed to).

But what i don't understand yet is, what is the difference between using the overloaded assignment operator of vtkSmartPointer and the ShallowCopy method, e.g. what is the difference between


vtkSmartPointer<vtkPolydata> myStrangePolydata = vtkSmartPointer<vtkPolydata>::New();


myStrangePolydata = vtkTransmogrifyFilter->GetOutput();


and


myStangePolydata->ShallowCopy(vtkTransmogrifyFilter->GetOutput();



Does it make a difference to the reference count of the vtkSmartPointer?


Thank you very much in advance.


Cheers,
Berti
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