ShapedNeighborhoodIterator silently breaks const-correctness on image argument

[N-Dekker]

Description

ShapedNeighborhoodIterator has a constructor that accepts a pointer to a const Image:

ShapedNeighborhoodIterator(const SizeType & radius, const ImageType * ptr, const RegionType & region)

At

ITK/Modules/Core/Common/include/itkShapedNeighborhoodIterator.h

Line 222 in 056f5eb

ShapedNeighborhoodIterator(const SizeType & radius, const ImageType * ptr, const RegionType & region)

This offers a "backdoor" for users to modify an image via a pointer-to-const, thereby breaking logical const-correctness. It is also unusual within ITK. Constructors of other non-const ITK iterators do not have such a const ImageType * parameter.

Steps to Reproduce

The following code shows how to modify an image via a const ImageType, by exploiting this ShapedNeighborhoodIterator bug. It does so by calling shapedNeighborhoodIterator.SetCenterPixel(1), but it might as well do so iteratively, by setting the pixel values from shapedNeighborhoodIterator.Begin() to shapedNeighborhoodIterator.End()

#include "itkShapedNeighborhoodIterator.h"
#include "itkImageBufferRange.h"

void
TestModifyingConstImage()
{
  static constexpr unsigned int dimension{ 2 };
  using ImageType = itk::Image<int, dimension>;

  constexpr auto modifyConstImage = [](const ImageType * const constImage) {
    constexpr auto radius = itk::Size<dimension>::Filled(1);
    const auto     region = constImage->GetBufferedRegion();

    itk::ShapedNeighborhoodIterator<ImageType> shapedNeighborhoodIterator(radius, constImage, region);

    shapedNeighborhoodIterator.SetLocation(itk::Index<dimension>::Filled(1));
    shapedNeighborhoodIterator.SetCenterPixel(1);
  };

  const auto image = ImageType::New();
  image->SetRegions(itk::Size<dimension>::Filled(3));
  image->AllocateInitialized();

  const auto printImage = [image] {
    for (const int pixelValue : itk::ImageBufferRange<ImageType>(*image))
    {
      std::cout << pixelValue;
    }
    std::cout << '\n';
  };

  printImage(); // Prints "000000000".
  modifyConstImage(image);
  printImage(); // Prints "000010000"!
}

Expected behavior

The following line should not compile. ShapedNeighborhoodIterator should not accept a "const" image as argument.

itk::ShapedNeighborhoodIterator<ImageType> shapedNeighborhoodIterator(radius, constImage, region);

Actual behavior

It prints the following lines, indicating the center pixel value of the 3x3 test image has changed.

000000000
000010000

Reproducibility

100%

Versions

Today's main revision: commit 056f5eb

Environment

Any supported ITK platform

Additional Information

I'm considering to submit a pull request to fix this issue.

---

For the record, it appears that this mistake (having a const ImageType * constructor parameter) was there already when ShapedNeighborhoodIterator was introduced, with commit fc6d1b2, Jan 8, 2003:

ITK/Code/Common/itkShapedNeighborhoodIterator.h

Lines 181 to 183 in fc6d1b2

	ShapedNeighborhoodIterator(const SizeType &radius,
	const ImageType * ptr,
	const RegionType &region

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