Begin defining a Walker_iterator class which isn't a member of Octree

Octree/include/CGAL/Octree.h

@@ -24,6 +24,7 @@
 
 #include <CGAL/Octree/Octree_node.h>
 #include <CGAL/Octree/Split_criterion.h>
+#include <CGAL/Octree/Walker_iterator.h>
 
 #include <CGAL/bounding_box.h>
 #include <boost/iterator/transform_iterator.hpp>

Octree/include/CGAL/Octree/Walker_iterator.h

@@ -0,0 +1,41 @@
+
+#ifndef OCTREE_WALKER_ITERATOR_H
+#define OCTREE_WALKER_ITERATOR_H
+
+#include <boost/iterator/iterator_facade.hpp>
+
+namespace CGAL {
+
+  template<class Value>
+  class Walker_iterator :
+          public boost::iterator_facade<Walker_iterator<Value>, Value, boost::forward_traversal_tag> {
+
+  public:
+
+    Walker_iterator() : m_value(nullptr) {}
+
+    Walker_iterator(Value *value) : m_value(value) {}
+
+  private:
+    friend class boost::iterator_core_access;
+
+    bool equal(Walker_iterator<Value> const &other) const {
+      return m_value == other.m_value;
+    }
+
+    void increment() {
+      // TODO
+    }
+
+    Value &dereference() const {
+      return m_value;
+    }
+
+  private:
+
+    Value *m_value;
+  };
+
+}
+
+#endif //OCTREE_WALKER_ITERATOR_H