[Insight-developers] correct form of dimension specific code.

[Karthik Krishnan]

In addition,

inlined member template functions are also allowed,

class foo
{

template< unsigned int ImageDimension >
virtual PixelType ComputeThreshold( const NeighborhoodType &neighborhood )
{
   // You can have appropriate typedefs inside this function as in
   typedef itk::Image< PixelType, ImageDimension > 
ImageOfAppropriateDimension;
  ...
}

};


The catch is that member template functions must appear in the header 
and not in the txx file

-karthik

Miller, James V (Research) wrote:

>Richard, 
>
>It can be tricky to put in dimension specific code.  All the code has 
>to be compilable regardless of dimension.  So if you use an if block
>on dimension, the code inside that block will need to compile even if the
>image is not of that dimension.  One bad example is 
>
>Index<InputImageDimension> index;
>
>if (InputImageDimension == 3)
>  {
>  index[2] = 100;
>  }
>
>If InputImageDimension is 2, then the code inside the if block will access past 
>the end of the array.  This will probably cause a compiler warning.
>
>
>It is tempting to provide separate functions for each dimension, like:
>
>  switch (InputImageDimension)
>    {
>    case 2: this->ComputeThreshold2D();  break;
>    case 3: this->ComputeThreshold3D();  break;
>    default: this->ComputeThreshold(); break;
>    }
>
>However, this requires that ComputeThreshold2D(), ComputeThreshold3D() and 
>ComputeThreshold() can all be compiled regardless of image dimension.  So this
>technique has not addressed the issue.
>
>
>One pattern that does work frequently is to use function overloading. 
>
>  struct DispatchBase {};
>  template<signed int VDimension>
>  struct Dispatch : DispatchBase {};
>  
>  /** This method computes the threshold by averaging the intensity
>   *  in direction perpendicular to the image gradient. */
>  virtual PixelType ComputeThreshold( const Dispatch<2> &,
>                                      const NeighborhoodType & neighborhood ) const;
>  virtual PixelType ComputeThreshold( const Dispatch<3> &,
>                                      const NeighborhoodType & neighborhood ) const;
>  virtual PixelType ComputeThreshold( const DispatchBase &,
>                                      const NeighborhoodType & neighborhood ) const;
>
>
>Here we use three functions called ComputeThreshold() that differ in the type
>of the first parameter.  The version whose first parameter is Dispatch<2> is used
>for 2D images. The code inside that function can use optimizations for 2D images.
>The version whose first parameter is Dispatch<3> is used for 3D images.  The version 
>whose first parameter is DispatchBase is used for any other image dimension.
>
>In your code, when you want to call the correct ComputeThreshold() method, you simply
>call ComputeThreshold() with a Dispatch templated over image dimension, 
>
>   this->ComputeThreshold(Dispatch<InputImageDimension>, neighborhood);
>
>If your image is 2D, it will end up compiling and calling 
>ComputeThreshold(Dispatch<2>, ...).  If your image is 3D, it will end up calling
>ComputeThreshold(Dispatch<3>, ...).  If your image is any other dimension, it
>will call the more general ComputeThreshold(DispatchBase, ...).
>
>This is best pattern we can use until we drop support for compilers that do not
>allow partial specialization.
>
>For the specific optimizations that you want to perform, there could be other 
>techniques that may be appropriate.  The Dispatch pattern is a general solution.
>
>Jim
>
>
>-----Original Message-----
>From: insight-developers-bounces+millerjv=crd.ge.com at itk.org
>[mailto:insight-developers-bounces+millerjv=crd.ge.com at itk.org]On Behalf
>Of Richard Beare
>Sent: Thursday, October 06, 2005 9:52 PM
>To: insight-developers at itk.org
>Subject: [Insight-developers] correct form of dimension specific code.
>
>
>Hi,
>I realise that the need to write dimension specific code has been much
>reduced by iterators, but there are some optimizations that I can't
>see how to achieve without exploiting dimension information. I was
>wondering what the correct way of doing this in itk's c++ style? Does
>an if block dependent on a template parameter have the correct
>behaviour, or is something else more appropriate? If nothing else this
>capability will provide a mechanism to compare the optimization
>approaches.
>
>Thanks
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>