[Insight-users] destruction of smart pointers

[Miller, James V (Research)]

There are very few places in ITK where we call UnRegister() on 
a SmartPointer.  Essentially, this is the beauty of the SmartPointer
implementation is that you do not need to remember to unregister
your hold on an object.  When the SmartPointer goes out of scope,
UnRegister() is called automatically.

What is happening in your example, is that the call to UnRegister()
is deleting the underlying object and then when the SmartPointer goes
out of scope (at the end of the program) it tries to delete it a second
time.

I imagine if you change your code to never call UnRegister(), you will be
fine.

Example #1:  Scope releasing memory

{  
   ImageImportType::Pointer itkImporter
   itkImporter = ImageImportType::New();
   itkImporter->DebugOn();
} // itkImporter goes out of scope here, so it is deleted


Example #2:  Pointer reassign releasing memory

   ImageImportType::Pointer itkImporter
   itkImporter = ImageImportType::New();
   itkImporter = 0;  // Pointer assigned to itkImporter is overwritten, 
                     // so underlying object is deleted if reference count
was 1

	
For your ITK example 

   vtkPtr = 0;
   vtkPtr = vtkSomeClass::New();
   vtkPtr->Destroy();
   vtkPtr = 0;

the equivalent ITK code would be

   itkPtr = itk::SomeClass::New();
   itkPtr = 0; // pointer overwrite automatically causes UnRegister to be
called on the 
               // original object.



> -----Original Message-----
> From: dean.inglis@on.aibn.com [mailto:dean.inglis@on.aibn.com]
> Sent: Saturday, February 15, 2003 11:09 PM
> To: insight-users@public.kitware.com
> Subject: [Insight-users] destruction of smart pointers
> 
> 
> still trying to get Borland GUI working here.
> I did a simple test: in my Form class definition 
> I have in the public section:
> 
> typedef itk::Image<float, 2> ImageType;
> typedef itk::VTKImageImport<ImageType>
> ImageImportType;
> ImageImportType::Pointer itkImporter;
> 
> in the Form's constructor, I try to create and
> then immediately destroy the pointer:
> 
>   itkImporter = ImageImportType::New();
>   itkImporter->DebugOn();
>   itkImporter->UnRegister();
> 
> the Win32 OuputWindow shows the Reference count goes
> to zero and the pointer destructs.  However, after
> running and then exiting the app, the debugger shows
> the line 
> 
>   void UnRegister()
>     {
> ***->    if(m_Pointer) { m_Pointer->UnRegister(); }
>     }
> 
> in itkSmartPointer.h as being the cause of an access
> violation.  Is there some other itk object that
> need to be destroyed that I am missing? I am stumped ... but 
> am determined to  get GUI development working with BCB5 and ITK!
> Can someone explain to me the difference(s) in 
> terms of object persistence/lifetime between
> typical class pointers and SmartPointers. i.e.,
> if it were a VTK class object:
>  vtkSomeClass* vtkPtr;
> 
> one can do this: 
>   vtkPtr = 0;
> and
>   vtkPtr = vtkSomeClass::New();
> and 
>   vtkPtr->Destroy();
> and maybe again
>   vtkPtr = 0;
> 
> One always 'knows' what one is getting in terms of
> pointer validity/existence.  How does this compare
> with itk smart pointers?  Perhaps its obvious, but
> I need to be hit on the head here ...
> 
> Dean
> 
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