[Insight-developers] Need for decoupling Traits from Vector-like containers

[Luis Ibanez]

Gaetan, Bill,


The change
http://review.source.kitware.com/#change,224


Is intended to decouple the Traits from
the Vector like containers.



There are two main reasons for this change:


A) The Generic Programming justifications:

  Traits take two flavors:

  a) They may be types defined inside a class X

  b) They may be types defined in a helper class
       that is templated (and potentially specialized)
       over class X

  When we opt for the case (b) is because the class
  X can't or shouldn't not be aware of the traits, or
  because we have no access to modify class X
  as to allow the introduction of the traits (that case(a)
  will require).  Think, of the std::vector for example,
  or the basic types (int,double...) over which we have
  no control nor access to modifications.

Once we opt for (b), as we did in NumericTraits,
in order to avoid circular dependencies, class X
shouldn't  know about the Traits helper class,
because that helper class  in some cases needs
to know about Class X.

This specific circular dependency is the reason
that lead us to decouple the Specialization of
the Numerics traits from the actual Vector-like
classes.


I agree with you in that, in itself the change,
doesn't seem to justify the pain that it will
inflict in users, who will have to reintroduce
the traits.  However, this changes is a stepping
stone in a much larger modification that is required.

and that modification is our reason (B) below:


B) The Statistics framework to be of large
      general use, should be able to instantiate
      itk::Sample types over a large variety of
      vector-like classes (that we usually refer to,
     as the "MeasurementVectorType".

     This includes typically:

     * itk::Array<t>
     * itk::FixedArray<t,n>
     * itk::Vector<t,n>
     * itk::Point<t,n>
     * itk::CovariantVector<t,n>
     * vnl_vector<t>
     * vnl_fixed_vector<t,n>
     * std::vector<t>


  To make the itk::Sample< MV > be
  instantiatable with MV= any of the
  types above, a certain number of
  operations and types must be standardized.

  In particular:

       int  GetLength()
       void SetLength(int)
       Set to Zero
       The Real type of the equivalent MV

  Up to now, those traits were encoded in
  a new family of Traits that were introduced
  ad-hoc in the Statistics refactoring, and
  were generally called

        MeasurementVectorTraits<>

  As part of recent cleanup in the Statistics
  Framework, Brad L. pointed out (and I agree)
  that the role of the MeasurementVectorTraits
  should have been fused with the pre-existing
  NumericTraits that are available to existing
  classes.

   Additionally, in order to use the Statistics
   framework with scalar images, we typically
   instantiate vector-like types of size = 1.

   We discussed the options with Brad and
   Gabe and converged to the idea that
   a VectorType trait should be added to the
   basic types NumericTraits in order to
  generalize access to this concept.

   So,

    NumericTraits<double>::VectorType

    should be defined as

           itk::FixedArray<double>



------------------------

To summarize,

This change that Gabe is introducing
is a atomic step that prepares the code
base to much larger changes that are
required to improve consistency in the
Statistics Framework.

Regarding backward compatibility and
the need to assist users during the
migration, Gabe is actually using this
changes as the role model for how
migration information should be collected
and presented to future users.

Gabe have prepared a very detailed
description of the draft proposal for
migration support at:

http://itk.org/Wiki/ITK_Release_4/Users_Migration_Guide/Developer_Migration_Guide_Workflow

(looking at the PDF is quite helpful since
it looks closer to an animation of the process).

Gabe's commit includes an XML file

http://review.source.kitware.com/#patch,sidebyside,224,2,Migration/DecoupleNumericTraitsFromContainers.xml

(that we have discussed with Julien Jomier
and Hans Johnson) based on what Qt did
for migration from Qt3 to Qt4:

http://doc.qt.nokia.com/4.0/qt3to4.html


This XML file contains all the documentation
that will be then converted into a web page
for the Migration Guide web site.

At this time, we are experimenting with two
systems:

1) phpbb:

http://www.insight-journal.org/itkforum/viewforum.php?f=2&sid=fd5100d0689c9299fed0ca72a5f5b955

see for example:
http://www.insight-journal.org/itkforum/viewtopic.php?f=2&t=5

and

2) phpFAQ

   http://ij.itk.org/itkfaq/



---


     Luis
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