[Insight-developers] Templates and preprocessor macros

[Williams, Norman K]

I think what Benoit wants is to execute different processing steps based on ImageType::ImageDimension.

What seems like you'd want -- specialization of template member function, isn't legal C++.  And if partial specialization of non-class helper functions works, I haven't figured out the syntax recipe for it.

Here, though is a mockup of how you could do it with member function overloads:

/*
 * Demonstrate using template + overload pattern to
 * handle different ImageDimension cases differently
 */
#include <iostream>
#include <itkImage.h>

template <typename TImage>
class TestClass
{
public:
  typedef TImage ImageType;
  typedef typename ImageType::Pointer ImagePointer;
  void Execute();
private:
  /** fallback case -- doesn't do anything */
  template <typename TImage2>
  ImagePointer _Execute(TImage2 *input);
  /** specifically handle 2D */
  ImagePointer _Execute(typename itk::Image<typename TImage::PixelType,2> *input);
  /** specifically handle 3D */
  ImagePointer _Execute(typename itk::Image<typename TImage::PixelType,3> *input);
};


/** ND case */
template <class TImage>
template <class TImage2>
typename TestClass<TImage>::ImagePointer
TestClass<TImage>
::_Execute(TImage2 *input)
{
  std::cerr << "Unsupported Dimension" << std::endl;
  return input;
}

/** 2D Case */
template <class TImage>
typename TestClass<TImage>::ImagePointer
TestClass<TImage>
::_Execute(typename itk::Image<typename TImage::PixelType,2> *input)
{
  std::cerr << "2D" << std::endl;
  ImagePointer output;
  // do 2D processing
  return output;
}

/** 3D Case */
template <class TImage>
typename TImage::Pointer
TestClass<TImage>
::_Execute(typename itk::Image<typename TImage::PixelType,3> *input)
{
  std::cerr << "3D" << std::endl;
  ImagePointer output;
  // do 3D processing
  return output;
}

template <class TImage>
void
TestClass<TImage>
::Execute()
{
  ImagePointer p;
  this->_Execute(p.GetPointer());
}

int main(int argc, char *argv[])
{
  typedef itk::Image<unsigned char,4> Image4DType;
  typedef itk::Image<unsigned char,3> Image3DType;
  typedef itk::Image<unsigned char,2> Image2DType;

  TestClass<Image4DType> test4D;
  TestClass<Image3DType> test3D;
  TestClass<Image2DType> test2D;
  test4D.Execute();
  test3D.Execute();
  test2D.Execute();
}

On 7/12/11 8:48 PM, "Bradley Lowekamp" <blowekamp at mail.nih.gov<mailto:blowekamp at mail.nih.gov>> wrote:

With C++0x there is the static_assert:

http://en.wikipedia.org/wiki/C%2B%2B0x#Static_assertions

Which is by far the easiest was to check what you are looking for, as it is just :

static_assert( VImageDimension == 3 );

*poof* compile time assertion. However, this is not supported by all compilers that ITK needs to support so we can't do that for the main code. But if it's just your own project you could use it.

I would be  in favor for adding a itkStaticAssert as a macro to ITKv4. But I am also a meta-programing wacko, so I may be alone. We could use the C++0x implementation if available or fall back to something else, or just do nothing.

Brad

On Jul 11, 2011, at 1:28 PM, Williams, Norman K wrote:

The only way to solve this is with template metaprogramming.  This makes
for some extra work getting the syntax right, but you can write the code
so that it only uses a filter on 3D.
On the other hand, as one of the people who worked on OrientImageFilter
back in the day, I don't think anyone should be using OrientImageFilter
and it should be deprecated, because it destroys information. It only does
the right thing if all of the direction cosines are aligned with one of
the world coordinate axes.
On 7/11/11 11:10 AM, "Benoit Scherrer" <benoitscherrer at gmail.com<mailto:benoitscherrer at gmail.com>> wrote:
Hi,
Is there a way to use the C++ preprocessor macros that test the value of
a template parameter?
For example i want to do a generic image filter ( using itk::Image<type,
VImageDimension> ) that
will use itk::OrientImageFilter only for VImageDimension==3
However, it seems that the preprocessor tool does not 'expand' the
templates and
just don't know when VImageDimension==3 or not.
Is there any trick to do that, or is it a lost cause ?
Thanks a lot!
Benoit
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