[Insight-developers] itk::Index, itk::Offset, ... as subclass of itk::FixedArray?

Tue Nov 16 16:25:42 EST 2010

The backward compatibility issue is not good. We need a better solution.

2010/11/16 Gaëtan Lehmann <gaetan.lehmann at jouy.inra.fr>:
>
> Le 16 nov. 10 à 16:56, Tom Vercauteren a écrit :
>
>> Hi all,
>>
>> On Tue, Nov 16, 2010 at 16:35, Luis Ibanez <luis.ibanez at kitware.com>
>> wrote:
>>>
>>> Hi Gaetan,
>>>
>>> Probably only historical reasons...
>>>
>>> I don't see why we couldn't make the itk::Index and
>>> the itk::Offset be derived classes of the itk::FixedArray.
>>>
>>> Note however that Dave Cole is working hard on
>>> fixing the Windows 64 bits issues, so you may want
>>> to coordinate those changes (since the itk::Index and
>>> the itk::Offset are at the heart of the 64bits problem).
>>>
>>> --
>>>
>>> Regarding the intentional lack of a default constructor,
>>> we may have to run these experiments again.  I would
>>> suspect that it is still a valid issue, given that this relates
>>> to making the itkFixedArray appear as plain-old-data,
>>> instead of as a class. (there was also an issue of padding
>>> if I remember correctly...)
>>
>> Indeed, currently it is still easy to access the memory help by an
>>  itk::Image< itk::Vector<float,2>, 2 >
>> by a simple
>>  float * floatptr = image->GetBufferPointer()[0].GetDataPointer()
>>
>> Not sure if it is standard conformant since itk::Vector seems to be
>> only POD like and not strictly POD but at least it works on the
>> systems I have tested so far. This is important to avoid copying
>> memory between different libraries.
>>
>> This works because itk::Vector inherits from itk::FixedArray and is
>> also POD-like. The less POD it becomes the more fragile my hypothesis
>> becomes. Remember that we had to remove an empty destructor to make
>> itk::FixedArray closer to being POD. This led to a better memory
>> alignment and improved the performance of some of our code by a factor
>> 2:
>>
>> http://www.itk.org/mailman/private/insight-developers/2008-June/010444.html
>>
>> Note that I am not the only one caring about this:
>> http://www.itk.org/pipermail/insight-users/2010-October/038525.html
>>
>> Anyhow, we may want to improve the documentation of itk::FixedArray
>> with respect to that point.
>>
>
> ok, so itk::FixedArray is the class tweaked for efficiency and which act
> like POD.
> And itk::FixedArray, unlike itk::Index, itk::Offset or itk::Size, also
> provide the constructor I'm looking for.
>
>  http://www.itk.org/Doxygen318/html/classitk_1_1FixedArray.html
>
> So I guess it should be possible to implement those 3 classes as subclasses
> of FixedArray, or at least to implement the same kind of constructor for
> those classes.
>
>> My two cents,
>> Tom
>>
>>>
>>> What would be the argument in favor of adding the
>>> default constructor ?
>>>
>>> (initialization ?,
>>> note that we don't know what T is, in advance...
>>> I'm not sure that we can provide a reasonable
>>> initialization anyways.  T could be a std::string,
>>> or a highly complex class...)
>
>  Backward compatibility.
>
> This is related to this change:
>
>  http://review.source.kitware.com/#change,374
>
> I had to modify a lot of tests to make it build. For example:
>
>  http://review.source.kitware.com/#patch,sidebyside,374,1,Testing/Code/Algorithms/itkNormalizedCorrelationImageMetricTest.cxx
>
> In that test, only the size parameter break, because itk::Size doesn't
> provide the right constructor. The other parameters I have changed like the
> spacing or the origin are not breaking anything, I think because their type
> is derived from itk::FixedArray
>
> Regards,
>
> Gaëtan
>
>
>>>
>>>
>>>     Luis
>>>
>>>
>>> ------------------------------------------------------------------------
>>> 2010/11/16 Gaëtan Lehmann <gaetan.lehmann at jouy.inra.fr>
>>>>
>>>> Hi,
>>>>
>>>> Is there a reason to have the basic array types itk::Index and
>>>> itk::Offset, and maybe others, not implemented as a subclass of
>>>> itk::FixedArray?
>>>> Also, I see in the doc that no constructor are provided for performance
>>>> reasons. Is it still true with the compilers supported by itk 4?
>>>>
>>>> Thanks,
>>>>
>>>> Gaëtan
>>>>
>>>>
>>>> --
>>>> Gaëtan Lehmann
>>>> Biologie du Développement et de la Reproduction
>>>> INRA de Jouy-en-Josas (France)
>>>> tel: +33 1 34 65 29 66    fax: 01 34 65 29 09
>>>> http://voxel.jouy.inra.fr  http://www.itk.org
>>>> http://www.mandriva.org  http://www.bepo.fr
>>>>
>>>>
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>>>
>>>
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>
> --
> Gaëtan Lehmann
> Biologie du Développement et de la Reproduction
> INRA de Jouy-en-Josas (France)
> tel: +33 1 34 65 29 66    fax: 01 34 65 29 09
> http://voxel.jouy.inra.fr  http://www.itk.org
> http://www.mandriva.org  http://www.bepo.fr
>
>
> _______________________________________________
> Powered by www.kitware.com
>
> Visit other Kitware open-source projects at
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>
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> http://kitware.com/products/protraining.html
>
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>