STYLE: Use (const) unique_ptr for SparseFieldLayer::m_HeadNode

Following C++ Core Guidelines, February 15, 2024:
"Use unique_ptr or shared_ptr to avoid forgetting to delete objects created using new"
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rh-smart

Also defaulted the destructor of `SparseFieldLayer`.

Modules/Core/Common/include/itkSparseFieldLayer.h

@@ -21,6 +21,7 @@
 #include "itkObjectFactory.h"
 #include "itkObject.h"
 #include <vector>
+#include <memory> // For unique_ptr.
 
 namespace itk
 {
@@ -217,7 +218,7 @@ class ITK_TEMPLATE_EXPORT SparseFieldLayer : public Object
   PopFront()
   {
     m_HeadNode->Next = m_HeadNode->Next->Next;
-    m_HeadNode->Next->Previous = m_HeadNode;
+    m_HeadNode->Next->Previous = m_HeadNode.get();
     m_Size -= 1;
   }
 
@@ -226,7 +227,7 @@ class ITK_TEMPLATE_EXPORT SparseFieldLayer : public Object
   PushFront(NodeType * n)
   {
     n->Next = m_HeadNode->Next;
-    n->Previous = m_HeadNode;
+    n->Previous = m_HeadNode.get();
     m_HeadNode->Next->Previous = n;
     m_HeadNode->Next = n;
     m_Size += 1;
@@ -260,22 +261,22 @@ class ITK_TEMPLATE_EXPORT SparseFieldLayer : public Object
   Iterator
   End()
   {
-    return Iterator(m_HeadNode);
+    return Iterator(m_HeadNode.get());
   }
 
   /** Returns a const iterator pointing one node past the end of the list. */
   ConstIterator
   End() const
   {
-    return ConstIterator(m_HeadNode);
+    return ConstIterator(m_HeadNode.get());
   }
 
   /** Returns TRUE if the list is empty, FALSE otherwise. Executes in constant
    *  time. */
   bool
   Empty() const
   {
-    if (m_HeadNode->Next == m_HeadNode)
+    if (m_HeadNode->Next == m_HeadNode.get())
     {
       return true;
     }
@@ -297,15 +298,15 @@ class ITK_TEMPLATE_EXPORT SparseFieldLayer : public Object
 
 protected:
   SparseFieldLayer();
-  ~SparseFieldLayer() override;
+  ~SparseFieldLayer() override = default;
   void
   PrintSelf(std::ostream & os, Indent indent) const override;
 
 private:
   /** The anchor node of the list.  m_HeadNode->Next is the first node in the
    *  list. If m_HeadNode->Next == m_HeadNode, then the list is empty. */
-  NodeType *   m_HeadNode{};
-  unsigned int m_Size{};
+  const std::unique_ptr<NodeType> m_HeadNode{ std::make_unique<NodeType>() };
+  unsigned int                    m_Size{};
 };
 } // end namespace itk
 

Modules/Core/Common/include/itkSparseFieldLayer.hxx

@@ -24,25 +24,18 @@ namespace itk
 template <typename TNodeType>
 SparseFieldLayer<TNodeType>::SparseFieldLayer()
 {
-  m_HeadNode = new NodeType;
-  m_HeadNode->Next = m_HeadNode;
-  m_HeadNode->Previous = m_HeadNode;
+  m_HeadNode->Next = m_HeadNode.get();
+  m_HeadNode->Previous = m_HeadNode.get();
   m_Size = 0;
 }
 
-template <typename TNodeType>
-SparseFieldLayer<TNodeType>::~SparseFieldLayer()
-{
-  delete m_HeadNode;
-}
-
 template <typename TNodeType>
 void
 SparseFieldLayer<TNodeType>::PrintSelf(std::ostream & os, Indent indent) const
 {
   Superclass::PrintSelf(os, indent);
 
-  os << indent << "HeadNode: " << m_HeadNode << std::endl;
+  os << indent << "HeadNode: " << m_HeadNode.get() << std::endl;
   os << indent << "Size: " << m_Size << std::endl;
 }