[Insight-developers] Advice needed from C++ Template wizards.

[Bradley Lowekamp]

Hello Kent,

This is a lot easier now that we can use partial template specialization. This reminds me of the common IsSameType construct (which you tried something very similar with a function):

 /// generic programming to test if T is the same type as U 
    template <typename T, typename U> 
    struct IsSameType {
      /// true if T is the same type as U 
      static const bool result = false;
    };
    
    
    /// generic programming to test if T is the same type as U 
    template <typename T>
    struct IsSameType<T,T> {
      /// true if T is the same type as U 
      static const bool result = true;
    };


One way to address this is to define a class in the member function or as a member of class. I believe you can do the needed partial specialization in both of these scopes. Then just add a member function to the above construct that is specific to that specialization. 

This is what I would try... not 100% it'll work though.

Good luck,
Brad

On Oct 26, 2010, at 12:36 PM, kent williams wrote:

> I'm trying to fix this bug:
> http://public.kitware.com/Bug/view.php?id=3610
> 
> the problem is this code:
> 
>   typename TFeatureImageType::Pointer tempFeature;
>   if ( typeid(TImageType) == typeid(TFeatureImageType))
>     {
>     m_Canny->SetInput(tempFeature);
>     }
>   else
>     {
>     m_Caster->SetInput(tempFeature);
>     m_Canny->SetInput(m_Caster->GetOutput());
>     }
> 
> This code breaks if typeid(TImageType) != typeid(TFeatureImageType), since the compiler still compiles the top branch of the if statement, and the m_Canny->SetInput() call will be in error because of incorrect parameter type.
> 
> I thought I could fix this with specialization, like this:
>> // IN CLASS DEFINITION
>> template <class _TImageType, class _TFeatureImageType>
>> void AssignCannyInput(typename _TFeatureImageType::Pointer &input)
>> {
>>   m_Caster->SetInput(input);
>>   m_Canny->SetInput(m_Caster->GetOutput());
>> }
>> template <>
>> void AssignCannyInput<TImageType,TImageType>(typename TImageType::Pointer &input)
>> {
>>   m_Canny->SetInput(input);
>> }
> 
> But the compiler objects to the specialization inside the class definition, and moving the specialization out of the class like this:
> 
>> // Removed from class definition, moved to TXX file
>> template< class TImageType, class TFeatureImageType >
>> template <>
>> void
>> CannySegmentationLevelSetFunction< TImageType, TFeatureImageType >
>> ::AssignCannyInput<TImageType,TImageType>
>> (typename TImageType::Pointer &input)
>> {
>>   m_Canny->SetInput(input);
>> }
> 
> Is illegal as well.
> 
> Does anyone know of a good Template MetaProgramming solution to this issue?
> <ATT00001..txt>

========================================================
Bradley Lowekamp  
Lockheed Martin Contractor for
Office of High Performance Computing and Communications
National Library of Medicine 
blowekamp at mail.nih.gov


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