[Insight-developers] Casting constructors for DTI and Sym Mat pixeltypes

Luke Bloy luke.bloy at gmail.com
Tue Mar 16 08:58:04 EDT 2010 

My main concern is the ability to use existing filters on images of 
these pixeltypes.

Many filters depend on the ability to cast to the 
NumericTraits<PixelType>::RealType via static_cast. which I don't think 
will work without a templated constructor.

I haven't noticed alot of code using a templated assignment such as

floatTensor2 = intTensor;


So perhaps making the constructor explicit might not have any 
detrimental effects.

My vote would be to have these classes behave like their parent class 
(FixedArray), which has implicit templated constructors and templated 
assignment operators.

Regardless I can implement either depending on the decision of the group.

-Luke

Karthik Krishnan wrote:
> For FixedArray, Vector, RGBPixel classes, we've already templated the
> operator= and constructor classes to provide implicit casting
> operations and they seem to work well.
>
> Thanks
> --
> karthik
>
> On Mon, Mar 15, 2010 at 4:43 PM, Luis Ibanez <luis.ibanez at kitware.com> wrote:
>   
>> Hi Brad,
>>
>> I can't think of a drawback for the explicit constructors
>> right now,...  (but I may just be low in coffee).     :-/
>>
>>
>> I worry about the "invisibility" of these methods, and how
>> easy it is to add them, and how hard it will be to remove
>> them if something goes wrong with them.
>>
>>
>> I still would vote for the "CastFrom()" methods, but if
>> enough developers are happy with explicit constructors
>> and are willing to help with the maintenance of the code
>> for the next decade, then I'm happy to follow the crowd.   :-)
>>
>>
>>     Luis
>>
>>
>>
>> -----------------------------------------------------------------------------
>> On Mon, Mar 15, 2010 at 4:03 PM, Bradley Lowekamp
>> <blowekamp at mail.nih.gov> wrote:
>>     
>>> Hello Luis,
>>>
>>> Thanks for the info regarding the itk::Array, I forgot the usefulness of all the inherited methods!
>>>
>>>
>>> In regards to this, what is the down side to using an explicit constructor? For example:
>>>
>>> explicit
>>> template<TOtherValueType>
>>> SymmetricSecondRankTensor( const SymmetricSecondRankTensor<TOtherValueType> &);
>>>
>>> This uses standard conventions along with requiring the contractor/conversion to be explicitly defined?
>>>
>>> Making the following valid code:
>>>
>>> Float3DTensorType floatTensor2(intTensor);
>>> floatTensor2 = Float3DTensorType(intTensor);
>>>
>>> But not:
>>> floatTensor2 = intTensor;
>>>
>>> Just a thought on an alternative approach....
>>>
>>> Thanks,
>>> Brad
>>>
>>> p.s.
>>> This is my understanding of C++, compilers and standards may differ :)
>>>
>>>
>>> On Mar 15, 2010, at 3:45 PM, Luis Ibanez wrote:
>>>
>>>       
>>>> Hi Luke,
>>>>
>>>> Historically we have been cautious when adding implicit
>>>> conversions, since once that syntactic sugar is introduced
>>>> in the code, it is very easy to get unintended consequences
>>>> particularly when using templated code.
>>>>
>>>>
>>>> For the case of the itkPoint we were a lot more explicit, and
>>>> added a method:
>>>>
>>>>
>>>> itk::Point<T>::CastFrom( const itk::Point< R > & otherPoint );
>>>>
>>>>
>>>> See the definition in:
>>>>
>>>>                   Insight/Code/Common/itkPoint.h
>>>>
>>>> in lines 212-221:
>>>>
>>>>  /** Copy from another Point with a different representation type.
>>>>   *  Casting is done with C-Like rules  */
>>>>  template < typename TCoordRepB >
>>>>  void CastFrom( const Point<TCoordRepB,NPointDimension> & pa )
>>>>    {
>>>>    for(unsigned int i=0; i<NPointDimension; i++ )
>>>>      {
>>>>      (*this)[i] = static_cast<TCoordRep>( pa[i] );
>>>>      }
>>>>    }
>>>>
>>>>
>>>>
>>>> I would suggest that we follow this Explicit approach for
>>>> the Tensor classes.
>>>>
>>>>
>>>> Would you like to give it a try at implementing and testing
>>>> the CastFrom() methods for these classes ?
>>>>
>>>>
>>>>  Thanks
>>>>
>>>>
>>>>         Luis
>>>>
>>>>
>>>> ----------------------------------------------------------------------------------
>>>> On Fri, Mar 5, 2010 at 2:00 PM, Luke Bloy <luke.bloy at gmail.com> wrote:
>>>>         
>>>>> The classes itkDiffusionTensor3D and itkSymmetricSecondRankTensor don't have
>>>>> constructors that enable casting. ( http://www.itk.org/Bug/view.php?id=10323
>>>>> ) so code such as the following fails.
>>>>>
>>>>>  typedef itk::SymmetricSecondRankTensor<int,3>     Int3DTensorType;
>>>>>  typedef itk::SymmetricSecondRankTensor<float,3>   Float3DTensorType;
>>>>>  typedef itk::SymmetricSecondRankTensor<double,3>  Double3DTensorType;
>>>>>  Int3DTensorType intTensor(1);
>>>>>
>>>>>  //Test constructors
>>>>>  Float3DTensorType floatTensor(intTensor);
>>>>>  Double3DTensorType doubleTensor(floatTensor);
>>>>>
>>>>>  //test Assignment
>>>>>  Float3DTensorType floatTensor2 = intTensor;
>>>>>
>>>>>
>>>>> adding templated constructors and assignment operators will allow this type
>>>>> of behavior.
>>>>>
>>>>> Is there a reason these constructors have been not been defined?
>>>>>
>>>>> -Luke
>>>>>
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>>>> _______________________________________________
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>>>       
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