Regarding nested class in stack implementation using Linked List

Question

In the stack using Linked List example impl, as below, why do we need to create Element as a nested class in the Stack class? Also, why do we need to make it protected? I am looking for the reason behind this design?

class Stack{
  public:
  Stack();
  ~Stack();
  void push(void *data);
  void *pop();
  protected:
   class Element{
   public:
     Element();
     Element *getNext() const {return next; }
     void *value() const {return data;}
   private:
     Element *next;
     void *data;
   };
   Element *head;
 };

Answer 1

The general rule is that any item should have the most restricted visibility possible. If Element were a top level class, anyone could refer to it. If it were public, anyone could refer to Stack::Element. This is "showing your privates" -- revealing implementation details that are used only internally to the world.

As to why it's protected rather than private, that's a bit odd. The Stack class is not obviously designed to be subclassed (its destructor is not virtual, for example), so private would almost certainly be more appropriate.