COMP: Move inline = default destructors to .cxx for 30 exported classes

[hjmjohnson]

Summary

This PR fixes an ABI issue in shared library builds (investigated and documented in issue #6000):

• Problem: Non-template exported classes with ~ClassName() override = default; inline in the header have their destructor thunks (D1Ev/D0Ev) hidden under -fvisibility-inlines-hidden (an ITK global build flag), even when the class vtable and typeinfo remain exported via itkOverrideGetNameOfClassMacro. This causes dynamic_cast failures across DSO boundaries in shared library consumers.

• Fix: Move the destructor definition to the corresponding .cxx file as ClassName::~ClassName() = default;. This compiles it in one TU with default (exported) visibility, correctly anchoring all destructor thunks.

• Scope: 30 concrete non-template exported classes across 7 ITK modules (ITKCommon, ITKIOImageBase, ITKFEM, ITKOptimizers, ITKStatistics, ITKWatersheds, ITKVideoCore). Template classes were intentionally excluded — their destructors cannot be defined in .cxx files without explicit template instantiation boilerplate.

• Verification: Full ITK build passes cleanly; all affected module tests pass.

Affected Classes (30)

Module	Classes
ITKCommon	EquivalencyTable, LoggerManager, LoggerOutput
ITKIOImageBase	ArchetypeSeriesFileNames, NumericSeriesFileNames, RegularExpressionSeriesFileNames
ITKFEM	FEMLightObject
ITKOptimizers	ExhaustiveOptimizer, GradientDescentOptimizer, InitializationBiasedParticleSwarmOptimizer, MultipleValuedNonLinearOptimizer, OnePlusOneEvolutionaryOptimizer, Optimizer, QuaternionRigidTransformGradientDescentOptimizer, RegularStepGradientDescentBaseOptimizer, RegularStepGradientDescentOptimizer, SingleValuedNonLinearOptimizer, SPSAOptimizer, VersorTransformOptimizer
ITKStatistics	ChiSquareDistribution, DenseFrequencyContainer2, GaussianDistribution, MaximumDecisionRule, MaximumRatioDecisionRule, MinimumDecisionRule, SparseFrequencyContainer2, TDistribution
ITKWatersheds	OneWayEquivalencyTable, WatershedMiniPipelineProgressCommand
ITKVideoCore	TemporalProcessObject

Related

• Closes part of PERF/STYLE: Investigate code bloat from inline destructors vs. out-of-line definitions #6000
• Companion to COMP: Use std::unique_ptr in FEMLinearSystemWrapperDenseVNL #5995 (which fixes LinearSystemWrapperDenseVNL)

[greptile-apps]

Greptile Summary

This PR fixes a shared-library ABI visibility bug (issue #6000) across 30 exported ITK classes: under -fvisibility-inlines-hidden (an ITK-wide build flag), inline = default destructors in headers have their thunks (D1Ev/D0Ev) hidden, causing dynamic_cast failures across DSO boundaries even when the class vtable and typeinfo are exported. The fix — moving each destructor definition to its corresponding .cxx file — is the canonical solution for this class of problem and is applied uniformly and correctly across all 60 changed files.

• All 30 class headers have the destructor changed from ~ClassName() override = default; (inline) to a plain declaration ~ClassName() override;.
• All 30 corresponding .cxx files add ClassName::~ClassName() = default; at the top of their namespace block, anchoring the thunk in a single TU with default (exported) visibility.
• Namespace handling is consistent: namespace itk for most classes, namespace itk::fem for FEMLightObject, and namespace itk::Statistics for the Statistics module classes.
• Template classes are intentionally left out, as is appropriate — they require explicit instantiation boilerplate to be defined in a .cxx.
• Three .cxx files (itkExhaustiveOptimizer.cxx, itkSPSAOptimizer.cxx, itkInitializationBiasedParticleSwarmOptimizer.cxx) introduce a cosmetic double blank line after the new destructor definition, because the original code already had one blank line before the first definition and the insertion adds a second.

Confidence Score: 5/5

• Safe to merge — the fix is mechanically correct and addresses a real ABI defect; only cosmetic style nits remain.
• All 30 classes are handled identically and correctly. No functional issues were found. The single finding (double blank line in three files) is purely cosmetic and does not affect compilation, runtime behaviour, or ABI correctness.
• No files require special attention beyond the optional cleanup of the double blank lines in itkExhaustiveOptimizer.cxx, itkSPSAOptimizer.cxx, and itkInitializationBiasedParticleSwarmOptimizer.cxx.

Important Files Changed

Filename	Overview
Modules/Core/Common/include/itkEquivalencyTable.h	Destructor changed from = default inline to a declaration; definition moved to .cxx to fix ABI visibility.
Modules/Core/Common/src/itkEquivalencyTable.cxx	Adds EquivalencyTable::~EquivalencyTable() = default; at the top of the namespace block to anchor destructor thunks with exported visibility.
Modules/Numerics/Optimizers/src/itkExhaustiveOptimizer.cxx	Destructor definition added correctly, but leaves a double blank line before the constructor definition (minor style issue).
Modules/Numerics/Optimizers/src/itkSPSAOptimizer.cxx	Destructor definition added correctly; introduces a double blank line before the constructor definition (same minor style issue as ExhaustiveOptimizer).
Modules/Numerics/Optimizers/src/itkInitializationBiasedParticleSwarmOptimizer.cxx	Destructor definition added correctly; introduces a double blank line before the constructor definition (same minor style issue).
Modules/Numerics/FEM/src/itkFEMLightObject.cxx	Correctly adds FEMLightObject::~FEMLightObject() = default; inside namespace itk::fem, consistent with the file's existing namespace style.
Modules/Numerics/Statistics/src/itkChiSquareDistribution.cxx	Correctly adds destructor definition inside namespace itk::Statistics; single blank line spacing is consistent.
Modules/Video/Core/src/itkTemporalProcessObject.cxx	Correctly adds TemporalProcessObject::~TemporalProcessObject() = default; to anchor destructor thunks.
Modules/Segmentation/Watersheds/src/itkOneWayEquivalencyTable.cxx	Correctly moves destructor definition to translation unit; consistent spacing.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Exported class header (.h)\n~ClassName() override = default"] -->|"Before fix\n(-fvisibility-inlines-hidden)"| B["Destructor thunk hidden\n(not exported from DSO)"]
    B --> C["dynamic_cast fails\nacross DSO boundary"]

    D["Exported class header (.h)\n~ClassName() override (declaration only)"] -->|"After fix"| E["Destructor defined in .cxx\nClassName::~ClassName() = default"]
    E --> F["Compiled in one TU\nwith default visibility"]
    F --> G["Destructor thunks exported\nalongside vtable & typeinfo"]
    G --> H["dynamic_cast works correctly\nacross DSO boundary"]

    style C fill:#f88,stroke:#c00
    style H fill:#8f8,stroke:#080

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_{Reviews (1): Last reviewed commit: "COMP: Move inline = default destructors ..." | Re-trigger Greptile}

[hjmjohnson]

Fixed in the follow-up commit (STYLE: Fix double blank lines in three Optimizers .cxx files). All three files — itkExhaustiveOptimizer.cxx, itkSPSAOptimizer.cxx, and itkInitializationBiasedParticleSwarmOptimizer.cxx — now have a single blank line between the destructor and constructor definitions.

[dzenanz]

Looks good on a glance.

[N-Dekker]

Thanks Hans!

[hjmjohnson]

Thanks Hans!

It was your helpful commit message that made this possible! Thank you for noticing the issue. I used AI to "Create an issue to encapsulate problem that @N-Dekker thinks exists. Investigate the problem with metrics, fix based on findings." ... and then baby-sat and cajoled the solution. :)