STYLE: Add in-class {} initializers to classes with virtual functions

Added in-class `{}` default member initializers to data members of classes
that have one or more virtual member functions.

For an object that has one or more virtual member functions, the performance
cost of adding a `{}` default member initializer to each of its non-static data
members should be acceptable, relative to cost of the initialization of its
virtual table.

Follow-up to pull request #3851
commit 5e2c49f
"STYLE: Add in-class `{}` member initializers to objects created by New()"
(which only looked at classes that have a `itkNewMacro`).

Author: N-Dekker

Modules/Bridge/VTK/include/itkVTKImageExportBase.h

@@ -231,7 +231,7 @@ class ITKVTK_EXPORT VTKImageExportBase : public ProcessObject
 
 private:
   /** PipelineMTime from the last call to PipelineModifiedCallback. */
-  ModifiedTimeType m_LastPipelineMTime;
+  ModifiedTimeType m_LastPipelineMTime{};
 };
 } // end namespace itk
 

Modules/Compatibility/Deprecated/include/itkChildTreeIterator.h

@@ -81,8 +81,8 @@ class ITK_TEMPLATE_EXPORT ChildTreeIterator : public TreeIteratorBase<TTreeType>
   HasNext() const override;
 
 private:
-  mutable ChildIdentifier m_ListPosition;
-  TreeNodeType *          m_ParentNode;
+  mutable ChildIdentifier m_ListPosition{};
+  TreeNodeType *          m_ParentNode{};
 };
 
 } // end namespace itk

Modules/Compatibility/Deprecated/include/itkLevelOrderTreeIterator.h

@@ -106,9 +106,9 @@ class ITK_TEMPLATE_EXPORT LevelOrderTreeIterator : public TreeIteratorBase<TTree
   int
   GetLevel(const TreeNodeType * node) const;
 
-  int                                      m_StartLevel;
-  int                                      m_EndLevel;
-  mutable std::queue<const TreeNodeType *> m_Queue;
+  int                                      m_StartLevel{};
+  int                                      m_EndLevel{};
+  mutable std::queue<const TreeNodeType *> m_Queue{};
 };
 
 } // end namespace itk

Modules/Compatibility/Deprecated/include/itkMutexLock.h

@@ -102,7 +102,7 @@ class ITKDeprecated_EXPORT SimpleMutexLock
   }
 
 protected:
-  MutexType m_MutexLock;
+  MutexType m_MutexLock{};
 };
 
 /**

Modules/Compatibility/Deprecated/include/itkTreeChangeEvent.h

@@ -85,7 +85,7 @@ class ITK_TEMPLATE_EXPORT TreeChangeEvent : public ModifiedEvent
   {}
 
 protected:
-  const TreeIteratorBase<TTreeType> * m_ChangePosition;
+  const TreeIteratorBase<TTreeType> * m_ChangePosition{};
 };
 
 /**  \class TreeNodeChangeEvent

Modules/Compatibility/Deprecated/include/itkTreeIteratorBase.h

@@ -274,9 +274,9 @@ class ITK_TEMPLATE_EXPORT TreeIteratorBase
   TreeIteratorBase(const TTreeType * tree, const TreeNodeType * start);
 
   mutable TreeNodeType * m_Position; // Current position of the iterator
-  mutable TreeNodeType * m_Begin;
-  const TreeNodeType *   m_Root;
-  TTreeType *            m_Tree;
+  mutable TreeNodeType * m_Begin{};
+  const TreeNodeType *   m_Root{};
+  TTreeType *            m_Tree{};
 
   virtual bool
   HasNext() const = 0;

Modules/Core/Common/include/itkCellInterface.h

@@ -490,7 +490,7 @@ class ITK_TEMPLATE_EXPORT CellInterface
 
 protected:
   /** Store the set of cells using this boundary. */
-  UsingCellsContainer m_UsingCells;
+  UsingCellsContainer m_UsingCells{};
 };
 
 /** \class CellTraitsInfo

Modules/Core/Common/include/itkConditionalConstIterator.h

@@ -112,10 +112,10 @@ class ITK_TEMPLATE_EXPORT ConditionalConstIterator
 protected: // made protected so other iterators can access
   /** Smart pointer to the source image. */
   // SmartPointer<const ImageType> m_Image;
-  typename ImageType::ConstWeakPointer m_Image;
+  typename ImageType::ConstWeakPointer m_Image{};
 
   /** Region type to iterate over. */
-  RegionType m_Region;
+  RegionType m_Region{};
 
   /** Is the iterator at the end of its walk? */
   bool m_IsAtEnd{ false };

Modules/Core/Common/include/itkConstNeighborhoodIterator.h

@@ -590,7 +590,7 @@ class ITK_TEMPLATE_EXPORT ConstNeighborhoodIterator
   const InternalPixelType * m_Begin{ nullptr };
 
   /** The image on which iteration is defined. */
-  typename ImageType::ConstWeakPointer m_ConstImage;
+  typename ImageType::ConstWeakPointer m_ConstImage{};
 
   /** A pointer to one past the last pixel in the iteration region. */
   const InternalPixelType * m_End{ nullptr };
@@ -603,7 +603,7 @@ class ITK_TEMPLATE_EXPORT ConstNeighborhoodIterator
   IndexType m_Loop{ { 0 } };
 
   /** The region over which iteration is defined. */
-  RegionType m_Region;
+  RegionType m_Region{};
 
   /** The internal array of offsets that provide support for regions of
    *  interest.
@@ -616,7 +616,7 @@ class ITK_TEMPLATE_EXPORT ConstNeighborhoodIterator
    * By default this points to m_BoundaryCondition, but
    * OverrideBoundaryCondition allows a user to point this variable an external
    * boundary condition.  */
-  ImageBoundaryConditionPointerType m_BoundaryCondition;
+  ImageBoundaryConditionPointerType m_BoundaryCondition{};
 
   /** Denotes which of the iterators dimensional sides spill outside
    * region of interest boundaries. By default `false` for each dimension. */
@@ -631,19 +631,19 @@ class ITK_TEMPLATE_EXPORT ConstNeighborhoodIterator
   mutable bool m_IsInBoundsValid{ false };
 
   /** Lower threshold of in-bounds loop counter values. */
-  IndexType m_InnerBoundsLow;
+  IndexType m_InnerBoundsLow{};
 
   /** Upper threshold of in-bounds loop counter values. */
-  IndexType m_InnerBoundsHigh;
+  IndexType m_InnerBoundsHigh{};
 
   /** Default boundary condition. */
-  TBoundaryCondition m_InternalBoundaryCondition;
+  TBoundaryCondition m_InternalBoundaryCondition{};
 
   /** Does the specified region need to worry about boundary conditions? */
   bool m_NeedToUseBoundaryCondition{ false };
 
   /** Functor type used to access neighborhoods of pixel pointers */
-  NeighborhoodAccessorFunctorType m_NeighborhoodAccessorFunctor;
+  NeighborhoodAccessorFunctorType m_NeighborhoodAccessorFunctor{};
 };
 
 template <typename TImage>

Modules/Core/Common/include/itkConstNeighborhoodIteratorWithOnlyIndex.h

@@ -375,7 +375,7 @@ class ITK_TEMPLATE_EXPORT ConstNeighborhoodIteratorWithOnlyIndex : public Neighb
   IndexType m_Bound{ { 0 } };
 
   /** The image on which iteration is defined. */
-  typename ImageType::ConstPointer m_ConstImage;
+  typename ImageType::ConstPointer m_ConstImage{};
 
   /** The end index for iteration within the itk::Image region
    * on which this ConstNeighborhoodIteratorWithOnlyIndex is defined. */
@@ -385,7 +385,7 @@ class ITK_TEMPLATE_EXPORT ConstNeighborhoodIteratorWithOnlyIndex : public Neighb
   IndexType m_Loop{ { 0 } };
 
   /** The region over which iteration is defined. */
-  RegionType m_Region;
+  RegionType m_Region{};
 
   /** Denotes which of the iterators dimensional sides spill outside
    * region of interest boundaries. By default `false` for each dimension. */
@@ -400,10 +400,10 @@ class ITK_TEMPLATE_EXPORT ConstNeighborhoodIteratorWithOnlyIndex : public Neighb
   mutable bool m_IsInBoundsValid{ false };
 
   /** Lower threshold of in-bounds loop counter values. */
-  IndexType m_InnerBoundsLow;
+  IndexType m_InnerBoundsLow{};
 
   /** Upper threshold of in-bounds loop counter values. */
-  IndexType m_InnerBoundsHigh;
+  IndexType m_InnerBoundsHigh{};
 
   /** Does the specified region need to worry about boundary conditions? */
   bool m_NeedToUseBoundaryCondition{ false };