[Insight-developers] Template function specialization

[Williams, Norman K]

The problem you're running into is that C++ is very touchy about templates
and specialization. It's rather a confusing topic, about which books are
written.  The solution involves some form of Template Metaprogramming.

Below is one solution to your problem, that I came up with off the top of
my head. It uses member function overloading, and doesn't require partial
template specialization, which is what I think you were trying to do.
There are other ways to do this, but this is the one that came immediately
to mind:

#include <iostream>
#include <itkImage.h>

template <class ImageType>
class TestOverLoad
{
public:
  void GenerateData();
private:
  // dummy type to discriminate dimension at compile time
  template <unsigned dim> struct DimType {};
  // the general ND case -- template member function
  template <unsigned dim>  void GenerateData(DimType<dim> *);
  // the 2D special case -- non-template member function
  void GenerateData(DimType<2> *);
};

// The 2D special case
template <class ImageType>
void
TestOverLoad<ImageType>::
GenerateData(DimType<2> *)
{
  std::cout << "TwoD case" << std::endl;
}

// General Case ND
template <class ImageType>
template <unsigned dim>
void
TestOverLoad<ImageType>::
GenerateData(DimType <dim> *)
{
  std::cout << "General Case, Dim = "
            << ImageType::ImageDimension
            << std::endl;
}

template <class ImageType>
void
TestOverLoad<ImageType>::
GenerateData()
{
  // depending on dimension of ImageType, only one version of
  // GenerateData will actually be instantiated
  // The DimType<dim> * parameter never used, but it's a low-cost way
  // to force compile-time type discrimination
  this->GenerateData(static_cast< DimType<ImageType::ImageDimension>
*>(0));
}

int
main(int argc, char **argv)
{
  TestOverLoad<itk::Image<unsigned char,2> > x2;
  x2.GenerateData();

  TestOverLoad<itk::Image<unsigned char,3> > x3;
  x3.GenerateData();
}