Remove local type aliases of Image Iterators in *.hxx, use CTAD

[N-Dekker]

Using Notepad++, Replace in Files, doing:

Find what: ^[ ]+ using (\w+) = (Image\w+Iterator<\w+>);\r\n(.*?[^\w])\1
Filters: itk*.hxx
[v] Match case
(*) Regular expression
[v] . matches newline

Followed by manual replacements of uses of those type aliases.

• Follow-up to pull request Remove local type aliases of ImageRegionIterator and ImageRegionConstIterator in *.hxx files #5981

[greptile-apps]

Greptile Summary

This is a pure C++17 modernization PR that removes redundant local type aliases (e.g., using IteratorType = ImageRegionIterator<OutputImageType>) across 29 .hxx implementation files and replaces their uses with either the full explicit type or CTAD (Class Template Argument Deduction) where the template argument can be deduced from the constructor's image-pointer argument. No logic or algorithmic changes are made.

Key patterns applied:

• using IteratorType = SomeIterator<T>; ... IteratorType var(img, region) → SomeIterator var(img, region) (CTAD)
• In contexts where CTAD cannot be used (C-array declarations, make_unique_for_overwrite<T[]>, std::vector<T*>, typename std::vector<T*>::iterator), the full explicit type SomeIterator<T> is kept — handled correctly throughout.
• Assignment to a pre-declared variable with an explicit type (e.g., ImageRegionIterator<TOutputImage> it = ImageRegionIterator(...)) is valid because the CTAD-deduced type on the RHS matches the declared type on the LHS.

Notable files:

• itkSTAPLEImageFilter.hxx: Both IteratorType and FuzzyIteratorType aliases removed; make_unique_for_overwrite call correctly expands to the full ImageScanlineConstIterator<TInputImage>[] since CTAD is not applicable to template arguments.
• itkBSplineDeformableTransform.hxx / itkBSplineTransform.hxx: C-array declaration retains the explicit <ImageType> (necessary because there is no initializer from which to deduce), while the loop assignment correctly uses CTAD.
• itkNaryFunctorImageFilter.hxx: Vector-of-pointer and new expressions retain explicit template arguments — correct, as CTAD cannot appear in template-argument position.

Confidence Score: 5/5

Safe to merge — purely mechanical modernization with no functional changes and no new logic introduced.

All changes are cosmetic C++17 CTAD clean-ups; every CTAD use deduces the same type that was previously spelled out explicitly, and every location where CTAD cannot apply correctly retains the explicit template argument. No bugs, no regressions, no logic alterations found across all 29 files.

No files require special attention.

Important Files Changed

Filename	Overview
Modules/Core/Common/include/itkImageAlgorithm.hxx	Removes explicit template arguments from ImageScanlineConstIterator and ImageScanlineIterator using CTAD; straightforward cleanup.
Modules/Core/Transform/include/itkBSplineDeformableTransform.hxx	Removes using IteratorType alias; array declaration keeps explicit (required for C-arrays), CTAD used only in the assignment inside the loop.
Modules/Core/Transform/include/itkBSplineTransform.hxx	Identical pattern to itkBSplineDeformableTransform.hxx; alias removed, C-array retains explicit type, loop assignment uses CTAD.
Modules/Filtering/ImageCompare/include/itkSTAPLEImageFilter.hxx	Removes IteratorType/FuzzyIteratorType aliases; make_unique_for_overwrite call correctly expands to the full explicit type ImageScanlineConstIterator[] since CTAD cannot apply there.
Modules/Filtering/ImageIntensity/include/itkNaryFunctorImageFilter.hxx	Removes ImageScanlineConstIteratorType alias; explicit template argument retained in vector-of-pointer and new expressions where CTAD cannot be used; memory ownership via manual delete is pre-existing.
Modules/Filtering/ImageFeature/include/itkZeroCrossingImageFilter.hxx	Right-hand side switches to CTAD while the declared type keeps explicit ; the deduced type matches, so this compiles correctly and is functionally equivalent.
Modules/Filtering/Smoothing/include/itkBinomialBlurImageFilter.hxx	Removes TempReverseIterator alias; ImageRegionReverseIterator variable declaration uses CTAD directly, which is cleaner.
Modules/Registration/PDEDeformable/include/itkCurvatureRegistrationFilter.hxx	CTAD applied to ImageRegionConstIterator and ImageRegionIterator assignments; ImageRegionConstIteratorWithIndex retains explicit type (not changed in this PR).

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Local alias present\n(using IteratorType = SomeIterator<T>)"]
    A -->|"alias used only as constructor\n(IteratorType it(img, region))"| B["Remove alias\nUse CTAD: SomeIterator it(img, region)"]
    A -->|"alias used in assignment RHS\n(it = IteratorType(img, region))"| C["Remove alias\nUse CTAD on RHS: it = SomeIterator(img, region)"]
    A -->|"alias used in C-array declaration\n(IteratorType arr[N])"| D["Remove alias\nKeep explicit type: SomeIterator<T> arr[N]"]
    A -->|"alias used as template arg\n(vector<IteratorType*>, make_unique<IteratorType[]>)"| E["Remove alias\nKeep explicit type: SomeIterator<T> in template arg"]
    B --> F["CTAD deduces T from image ptr"]
    C --> F
    D --> G["Explicit type required\n(no initializer for deduction)"]
    E --> G

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_{Reviews (1): Last reviewed commit: "STYLE: Use CTAD when assigning an Image ..." | Re-trigger Greptile}

[hjmjohnson]

Thanks.

[dzenanz]

Do all iterator types have CTAD guides? Code is both less verbose and more readable when using them. We should have them and apply them for all the iterator types. Perhaps something to consider in follow-up PR(s)?

[N-Dekker]

Do all iterator types have CTAD guides? Code is both less verbose and more readable when using them. We should have them and apply them for all the iterator types. Perhaps something to consider in follow-up PR(s)?

The Image Iterators all have a CTAD guide for ImageIteratorType(image, region). As tested here:

ITK/Modules/Core/Common/test/itkImageIteratorsGTest.cxx

Lines 187 to 213 in 8fcd691

	CheckIteratorsSupportClassTemplateArgumentDeduction<itk::ImageConstIterator,
	itk::ImageConstIteratorWithIndex,
	itk::ImageConstIteratorWithOnlyIndex,
	itk::ImageIterator,
	itk::ImageIteratorWithIndex,
	itk::ImageLinearConstIteratorWithIndex,
	itk::ImageLinearIteratorWithIndex,
	itk::ImageRandomConstIteratorWithIndex,
	itk::ImageRandomConstIteratorWithOnlyIndex,
	itk::ImageRandomIteratorWithIndex,
	itk::ImageRandomNonRepeatingConstIteratorWithIndex,
	itk::ImageRandomNonRepeatingIteratorWithIndex,
	itk::ImageRegionConstIterator,
	itk::ImageRegionConstIteratorWithIndex,
	itk::ImageRegionConstIteratorWithOnlyIndex,
	itk::ImageRegionExclusionConstIteratorWithIndex,
	itk::ImageRegionExclusionIteratorWithIndex,
	itk::ImageRegionIterator,
	itk::ImageRegionIteratorWithIndex,
	itk::ImageRegionReverseConstIterator,
	itk::ImageRegionReverseIterator,
	itk::ImageReverseConstIterator,
	itk::ImageReverseIterator,
	itk::ImageScanlineConstIterator,
	itk::ImageScanlineIterator,
	itk::ImageSliceConstIteratorWithIndex,
	itk::ImageSliceIteratorWithIndex>();