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

[kent williams]

The problem ‹ even allowing for partial specialization, runs into problems
of specializing template member functions in template classes ‹ according to
what I¹ve been able to read, you have to fully qualify the class before you
can specialize the template member function.

I¹m just going to go caveman on the problem and use reinterpret_cast.  It¹s
one case where we know for certain that it won¹t be a problem since the
typeid is tested before SetInput is called.  The reinterpret_cast will have
bogus results only in the case where the call is unreachable at runtime.


On 10/26/10 12:16 PM, "Bradley Lowekamp" <blowekamp at mail.nih.gov> wrote:

> 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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