[vtkusers] Reference counting in 6.3

[David Cole]

It depends on what A and B are, and what happens when you connect them ...

With this line of code:

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

I would expect that the object returned by B's GetOutputPort method is
the one whose ref count is increased. (But it also may not be,
depending on what connections already exist prior to the call...)

Neither, or one only, or both ref counts may go up depending on
whether they are already ref-counted by other objects in the pipeline.
In general, you cannot depend on the ref-counts changing when you
connect or disconnect objects.

You can count on this: if something needs to be kept alive by
something else, it will either already have an existing ref count on
it, or increase the ref count to keep it alive.

If something is being detached or disconnected, it may or may not see
its ref count decremented. It's totally context dependent: you have to
dig in and learn about all the objects involved and understand all
their interconnections to figure out what's going on.


Hope this helps,
David C.



On Mon, Oct 26, 2015 at 10:31 AM, Weiguang guan <guanw at rhpcs.mcmaster.ca> wrote:
> 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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