[vtkusers] Reference counting in 6.3

[Weiguang guan]

Hi David,

Thank you very much for helping me understand reference counting in VTK. 
Most of my questions are answered except this one:

A->SetInputConnection(B->GetOutputPort());

Why A's reference count is incremented after the above statement, not 
B's? I think B is used by A, so that B's reference count should be 
incremented. Am I right?

Best,
Weiguang

On 23/10/2015 5:07 PM, David Cole wrote:
> There is nothing that guarantees the reference count of an object
> derived from vtkObject will be any certain value immediately after
> construction.
>
> It totally depends on what happens to the object during its
> constructor. It might set up a whole network of other objects, and
> then give references to itself to some of them. Some of them might
> increase the reference count of the object, some of them might not.
>
> If you call Register, the reference count goes up. If you call
> Unregister, it goes down. If it goes down far enough, the object's
> destructor is called and the memory is released.
>
> The mechanism itself has not changed significantly lately (that I know
> of...), but individual objects may have changed their behavior with
> respect to "what's the reference count immediately after construction"
> or " what's the count after a single or repeated calls to
> SetInputConnection..."
>
> You should not rely on the reference count value for anything other
> than automatic cleanup when no more references exist.
>
>
> HTH,
> David C.
>
>
>
> On Fri, Oct 23, 2015 at 4:24 PM, Weiguang guan <guanw at rhpcs.mcmaster.ca> wrote:
>> Hi Dan,
>>
>> Thank you very much for the reply.
>>
>> I still use the old way (vtkABC *v= vtkABC::New()) rather than using VTK
>> smart pointer. The reference counting mechanism of VTK must have changed
>> although I don't know when (which version) that happened.
>>
>> The reference count of a VTK object used to be 1 right after its creation.
>> But now it is not always the case. I find source objects (like
>> vtkCubeSource, vtkImageEllipsoidSource, etc) have reference count = 2 right
>> after being created while filter object have reference count = 1. I haven't
>> checked every class to see if there is an exception to the above finding.
>>
>> Here is a three-line code section together with running results:
>> vtkCubeSource* cube = vtkCubeSource::New();                    //
>> ReferenceCount of cube = 2
>> vtkShrinkPolyData* shrink = vtkShrinkPolyData::New();     // ReferenceCount
>> of cube = 2, ReferenceCount of shrink= 1
>> shrink->SetInputConnection(cube->GetOutputPort());         // ReferenceCount
>> of cube = 2, ReferenceCount of shrink= 2
>> It can not be explained by the old VTK reference counting mechanism.
>>
>> Best,
>> Weiguang
>>
>> On 21/10/2015 3:11 PM, Dan Lipsa wrote:
>>
>> Hi Weiguang,
>> Take a look at
>> http://www.vtk.org/Wiki/VTK/Tutorials/SmartPointers
>> This might clarify some of your questions. You have to remember that the
>> reference count is stored in the objects themselves unlike the c++
>> shared_ptr where the reference count is stored in the shared pointer object.
>>
>> For the first question: do you assign the object to a vtk smart pointer?
>> That would explain why the reference count is 2.
>> For the rest of the questions: when you set up the pipeline there are other
>> objects involved besides the filter themselves, so it is harder to
>> understand of the reference count.
>>
>> Also you can set VTK_DEBUG_LEAKS when you build VTK if you are concerned
>> about leaks.
>>
>> Dan
>>
>>
>> On Thu, Oct 15, 2015 at 4:13 PM, Weiguang guan <guanw at rhpcs.mcmaster.ca>
>> wrote:
>>> Hello,
>>>
>>> I find, in debug mode, that the ReferenceCount of a
>>> vtkImageEllipsoidSource object becomes 2 right after being initiated
>>> (vtkImageEllipsoidSourc::New()). It should be 1 not 2, right? How could this
>>> happen?
>>>
>>> My second question is --- when you make a connection
>>> A->SetInputConnection(B->GetOutputPort()); Whose reference count will be
>>> increased by 1, A or B? I think B's reference count should be increased
>>> because it is used by A. But what I find is just the opposite.
>>>
>>> My third question is about setting up more than 1 connection for a filter.
>>> Taking vtkImageMathematics as an example, I do
>>> vtkImageMathematics* A = vtkImageMathematics::New();
>>> A->SetInputConnection(0, B1->GetOutputPort();
>>> A->SetInputConnection(0, B2->GetOutputPort();
>>>
>>> I find that the reference count of A gets increased by the first
>>> SetInputConnection() call only.
>>>
>>> Can someone explain how reference counting works in VTK? Thanks.
>>>
>>> Best,
>>> Weiguang
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>>
>>
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