[Insight-developers] MakeOutput : Raw pointer... Smart pointer ?

[Luis Ibanez]

Hi Antonin,


1) MakeOutput returns a SmartPointer,
    the function signature is:

   DataObjectPointer MakeOutput(unsigned int idx)



3) Smart Pointers do not support casting from
    derived classes.

    For example

  typedef itk::ImageBase< 3 > ImageBaseType;
  typedef itk::Image< unsigned short, 3 > ImageType;
  ImageType::Pointer image = ImageType::New();
  ImageBaseType::Pointer baseImage = image;


Will fail compilation with the message:

conversion from ‘itk::SmartPointer<itk::Image<short unsigned int, 3u> >’ to
 non-scalar type ‘itk::SmartPointer<itk::ImageBase<3u> >’ requested


While the code:

  typedef itk::ImageBase< 3 > ImageBaseType;
  typedef itk::Image< unsigned short, 3 > ImageType;
  ImageType::Pointer image = ImageType::New();
  ImageBaseType::Pointer baseImage = image.GetPointer();


will compile and work fine.

-----

With SmartPointers you can't assign the smartpointer of a
derived class to the smartpointer of a base class.

With SmartPointers you can assigne the RAW pointer of a
derived class to the smartpointer of a base class.

That's the reason why GetPointer() is used in MakeOutput().


3) That said, the particular example that you are
   looking at, has some unnecessary GetPointer() calls.
   Particularly, the one in the return statement is not
   needed.

   The code:

> ::MakeOutput(unsigned int idx)
> {
>   DataObject::Pointer output;
>
>   switch ( idx )
>     {
>     case 0:
>       output = ( EigenValueImageType::New() ).GetPointer();
>       break;
>     case 1:
>       output = ( EigenValueImageType::New() ).GetPointer();
>       break;
>     case 2:
>       output = ( EigenVectorImageType::New() ).GetPointer();
>       break;
>     }
>   return output.GetPointer();
> }
>

    Should just be:

> ::MakeOutput(unsigned int idx)
> {
>   DataObject::Pointer output;
>
>   switch ( idx )
>     {
>     case 0:
>       output = ( EigenValueImageType::New() ).GetPointer();
>       break;
>     case 1:
>       output = ( EigenValueImageType::New() ).GetPointer();
>       break;
>     case 2:
>       output = ( EigenVectorImageType::New() ).GetPointer();
>       break;
>     }
>   return output;
> }
>

   Given that the "output" variable already
   has the same type as the expected return
   type of the function.

   The calls

     output = ( EigenValueImageType::New() ).GetPointer();

   Need the GetPointer() method, because "output"
   is a smart pointer to DataObject, and the
   EigenValueImageType is a derived class from
   DataObject.




    Luis



----------------------------------------------
On Fri, Apr 1, 2011 at 11:57 AM, Antonin Perrot-Audet
<antonin07130 at gmail.com> wrote:
> Hello,
>
> I still have trouble understanding the function of
> ProcessObject::MakeOutput
> method.
>
> I have a filter with multiple outputs, and I must admit that I have trouble
> understanding the principle of the example provided in
> itkEigenAnalysis2DImageFilter
> MakeOutput seems to *return a RAW pointer instead of a SMART POINTER* ?
>
> template< class TInputImage, class TEigenValueImage, class TEigenVectorImage
>>
> DataObject::Pointer
> EigenAnalysis2DImageFilter< TInputImage, TEigenValueImage, TEigenVectorImage
>>
> ::MakeOutput(unsigned int idx)
> {
>   DataObject::Pointer output;
>
>   switch ( idx )
>     {
>     case 0:
>       output = ( EigenValueImageType::New() ).GetPointer();
>       break;
>     case 1:
>       output = ( EigenValueImageType::New() ).GetPointer();
>       break;
>     case 2:
>       output = ( EigenVectorImageType::New() ).GetPointer();
>       break;
>     }
>   return output.GetPointer();
> }
>
> Can anyone give me more insights about *when to return smart pointers and
> raw pointers* ?
>
> Thanks,
> --
> Antonin
>
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