STYLE: Default default-constructor of RecursiveGaussianImageFilter

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/Filtering/Smoothing/include/itkRecursiveGaussianImageFilter.h

@@ -200,7 +200,7 @@ class ITK_TEMPLATE_EXPORT RecursiveGaussianImageFilter : public RecursiveSeparab
   SetSecondOrder();
 
 protected:
-  RecursiveGaussianImageFilter();
+  RecursiveGaussianImageFilter() = default;
   ~RecursiveGaussianImageFilter() override = default;
   void
   PrintSelf(std::ostream & os, Indent indent) const override;
@@ -254,12 +254,12 @@ class ITK_TEMPLATE_EXPORT RecursiveGaussianImageFilter : public RecursiveSeparab
   ComputeRemainingCoefficients(bool symmetric);
 
   /** Sigma of the gaussian kernel. */
-  ScalarRealType m_Sigma{};
+  ScalarRealType m_Sigma{ 1.0 };
 
   /** Normalize the image across scale space */
   bool m_NormalizeAcrossScale{};
 
-  GaussianOrderEnum m_Order{};
+  GaussianOrderEnum m_Order{ GaussianOrderEnum::ZeroOrder };
 };
 } // end namespace itk
 

Modules/Filtering/Smoothing/include/itkRecursiveGaussianImageFilter.hxx

@@ -24,12 +24,6 @@
 
 namespace itk
 {
-template <typename TInputImage, typename TOutputImage>
-RecursiveGaussianImageFilter<TInputImage, TOutputImage>::RecursiveGaussianImageFilter()
-  : m_Sigma(1.0)
-  , m_Order(GaussianOrderEnum::ZeroOrder)
-{}
-
 /**
  *   Explicitly set a zeroth order derivative.
  */