BUG: ImageRandomIteratorWithIndex should not assign data in constructor

[N-Dekker]

The constructors of ImageRandomConstIteratorWithIndex and ImageRandomConstIteratorWithOnlyIndex that support two arguments (image and region) accidentally still assigned their data members, while they were already initialized by in-class default member initialization. This in-class default member initialization was added as part of pull request #3929 commit 4e46cb6 "STYLE: Default default-constructor of ImageRandom ConstIterator classes", merged on February 24, 2023 and included with tag ITK v5.4rc01.

This caused extra MersenneTwisterRandomVariateGenerator::New() calls, which changed the seeds of random number generation. These changes can possibly cause regression failures in unit tests of client applications, including elastix.

This commit removes all data member assignments from the bodies of these constructors.

---

The bug was my fault, actually 😧, introduced with commit 4e46cb6

Some background information: Each MersenneTwisterRandomVariateGenerator::New() call changes the random seed. Now it would not be a problem to have the very same MersenneTwisterRandomVariateGenerator::New() call both inside the class definition (in-class default member initialization) and in the constructor member initializer list:

ImageRandomConstIteratorWithIndex<TImage>::ImageRandomConstIteratorWithIndex(const ImageType *  ptr,
                                                                             const RegionType & region)
  : ImageConstIteratorWithIndex<TImage>(ptr, region),
  m_Generator(MersenneTwisterRandomVariateGenerator::New()) // member initializer list
{
}

An initialization in the constructor member initializer list would overrule (supersede) the in-class default member initialization of the same member variable.

However, an assignment to a member variable in the constructor body does not supersede the in-class default member initialization.

{
  // Constructor body. Produces an extra MersenneTwisterRandomVariateGenerator::New() call.
  m_Generator = Statistics::MersenneTwisterRandomVariateGenerator::New();
}

So that's why MersenneTwisterRandomVariateGenerator::New() was called twice, instead of once, when the in-class default member initialization was added.

[tbirdso]

Thanks for tracking this down @N-Dekker . Can you confirm that Elastix regression tests pass again once this fix is applied?

[N-Dekker]

Can you confirm that Elastix regression tests pass again once this fix is applied?

Yes I can 😃 Thanks for asking, @tbirdso

Regression failures in elastix + v5.4rc01 branch: https://github.com/SuperElastix/elastix/tree/Upgrade-ITK-to-v5.4rc01

Fixed with this PR branch: https://github.com/SuperElastix/elastix/tree/ImageRandomIteratorWithIndex-should-not-assign-it-constructor

[tbirdso]

@N-Dekker thanks for confirming this fixes application behavior 👍