STYLE: Default constructors that only initialized one member in hxx file

Removed user-defined constructors of the form:

    template <typename TParameter>
    ClassTemplate<TParameter>::ClassTemplate()
      : m_DataMember(initialValue)
    {}

Defaulted each of those constructors in its header file. Added the specified
initial value of the data member to its default member initializer.

Cases found by Notepad++, Find in Files, doing:

  Find what: `^template <.+>\r\n(\w+)<.+>::\1\(\)$[\r\n]+  : m_\w+\(\w+\)[\r\n]+{}\r\n\r\n`
  Filters: itk*.hxx
  [v] Match case
  Search Mode: Regular expression

Following C++ Core Guidelines, Jul 8, 2025, "Don’t define a default constructor
that only initializes data members; use default member initializers instead",
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c45-dont-define-a-default-constructor-that-only-initializes-data-members-use-default-member-initializers-instead

Author: N-Dekker

Modules/Core/FiniteDifference/include/itkFiniteDifferenceSparseImageFilter.h

@@ -114,7 +114,7 @@ class ITK_TEMPLATE_EXPORT FiniteDifferenceSparseImageFilter
   itkGetConstMacro(PrecomputeFlag, bool);
 
 protected:
-  FiniteDifferenceSparseImageFilter();
+  FiniteDifferenceSparseImageFilter() = default;
   ~FiniteDifferenceSparseImageFilter() override = default;
   void
   PrintSelf(std::ostream & os, Indent indent) const override;
@@ -206,7 +206,7 @@ class ITK_TEMPLATE_EXPORT FiniteDifferenceSparseImageFilter
   bool m_PrecomputeFlag{};
 
   /** The Sparse function type. */
-  SparseFunctionType * m_SparseFunction{};
+  SparseFunctionType * m_SparseFunction{ nullptr };
 
   /** A list of subregions of the active set of pixels in the sparse image
       which are passed to each thread for parallel processing. */

Modules/Core/FiniteDifference/include/itkFiniteDifferenceSparseImageFilter.hxx

@@ -21,11 +21,6 @@
 
 namespace itk
 {
-template <typename TInputImageType, typename TSparseOutputImageType>
-FiniteDifferenceSparseImageFilter<TInputImageType, TSparseOutputImageType>::FiniteDifferenceSparseImageFilter()
-  : m_SparseFunction(nullptr)
-{}
-
 template <typename TInputImageType, typename TSparseOutputImageType>
 void
 FiniteDifferenceSparseImageFilter<TInputImageType, TSparseOutputImageType>::PrintSelf(std::ostream & os,

Modules/Core/ImageFunction/include/itkScatterMatrixImageFunction.h

@@ -117,13 +117,13 @@ class ITK_TEMPLATE_EXPORT ScatterMatrixImageFunction
   itkGetConstReferenceMacro(NeighborhoodRadius, unsigned int);
   /** @ITKEndGrouping */
 protected:
-  ScatterMatrixImageFunction();
+  ScatterMatrixImageFunction() = default;
   ~ScatterMatrixImageFunction() override = default;
   void
   PrintSelf(std::ostream & os, Indent indent) const override;
 
 private:
-  unsigned int m_NeighborhoodRadius{};
+  unsigned int m_NeighborhoodRadius{ 1 };
 };
 } // end namespace itk
 

Modules/Core/ImageFunction/include/itkScatterMatrixImageFunction.hxx

@@ -23,11 +23,6 @@
 namespace itk
 {
 
-template <typename TInputImage, typename TCoordinate>
-ScatterMatrixImageFunction<TInputImage, TCoordinate>::ScatterMatrixImageFunction()
-  : m_NeighborhoodRadius(1)
-{}
-
 template <typename TInputImage, typename TCoordinate>
 void
 ScatterMatrixImageFunction<TInputImage, TCoordinate>::PrintSelf(std::ostream & os, Indent indent) const

Modules/Core/ImageFunction/include/itkVarianceImageFunction.h

@@ -111,13 +111,13 @@ class ITK_TEMPLATE_EXPORT VarianceImageFunction
   itkGetConstReferenceMacro(NeighborhoodRadius, unsigned int);
   /** @ITKEndGrouping */
 protected:
-  VarianceImageFunction();
+  VarianceImageFunction() = default;
   ~VarianceImageFunction() override = default;
   void
   PrintSelf(std::ostream & os, Indent indent) const override;
 
 private:
-  unsigned int m_NeighborhoodRadius{};
+  unsigned int m_NeighborhoodRadius{ 1 };
 };
 } // end namespace itk
 

Modules/Core/ImageFunction/include/itkVarianceImageFunction.hxx

@@ -24,11 +24,6 @@
 namespace itk
 {
 
-template <typename TInputImage, typename TCoordinate>
-VarianceImageFunction<TInputImage, TCoordinate>::VarianceImageFunction()
-  : m_NeighborhoodRadius(1)
-{}
-
 template <typename TInputImage, typename TCoordinate>
 void
 VarianceImageFunction<TInputImage, TCoordinate>::PrintSelf(std::ostream & os, Indent indent) const

Modules/Core/ImageFunction/include/itkVectorMeanImageFunction.h

@@ -113,13 +113,13 @@ class ITK_TEMPLATE_EXPORT VectorMeanImageFunction
   itkGetConstReferenceMacro(NeighborhoodRadius, unsigned int);
   /** @ITKEndGrouping */
 protected:
-  VectorMeanImageFunction();
+  VectorMeanImageFunction() = default;
   ~VectorMeanImageFunction() override = default;
   void
   PrintSelf(std::ostream & os, Indent indent) const override;
 
 private:
-  unsigned int m_NeighborhoodRadius{};
+  unsigned int m_NeighborhoodRadius{ 1 };
 };
 } // end namespace itk
 

Modules/Core/ImageFunction/include/itkVectorMeanImageFunction.hxx

@@ -24,11 +24,6 @@
 namespace itk
 {
 
-template <typename TInputImage, typename TCoordinate>
-VectorMeanImageFunction<TInputImage, TCoordinate>::VectorMeanImageFunction()
-  : m_NeighborhoodRadius(1)
-{}
-
 template <typename TInputImage, typename TCoordinate>
 void
 VectorMeanImageFunction<TInputImage, TCoordinate>::PrintSelf(std::ostream & os, Indent indent) const

Modules/Core/SpatialObjects/include/itkCastSpatialObjectFilter.h

@@ -221,15 +221,15 @@ class ITK_TEMPLATE_EXPORT CastSpatialObjectFilter : public Object
   }
 
 protected:
-  CastSpatialObjectFilter();
+  CastSpatialObjectFilter() = default;
   ~CastSpatialObjectFilter() override = default;
 
   void
   PrintSelf(std::ostream & os, Indent indent) const override;
 
 
 private:
-  typename InputSpatialObjectType::Pointer m_Input{};
+  typename InputSpatialObjectType::Pointer m_Input{ nullptr };
 
 }; // End class CastSpatialObjectFilter
 

Modules/Core/SpatialObjects/include/itkCastSpatialObjectFilter.hxx

@@ -25,14 +25,6 @@
 namespace itk
 {
 
-/**
- *    Constructor
- */
-template <unsigned int ObjectDimension>
-CastSpatialObjectFilter<ObjectDimension>::CastSpatialObjectFilter()
-  : m_Input(nullptr)
-{}
-
 /**
  *  Print Self
  */