How does a sentinel node offer benefits over NULL?

Question

On the Sentinel Node wikipedia page it says that the benefits of a sentinel node over NULL are :

• Increased speed of operations
• Reduced algorithmic cod

Answer 1

I will try to answer in the context of the standard template library:

1) In a call to "next()", NULL does not necessarily indicate end-of-list. What if a memory error occurred? Returning a sentinel node, is a definitive way to indicate that end-of-list had occurred, and not some other outcome. In other words, NULL could indicate a variety of things, not just end-of-list.

2) This is just one possible method: when you create your list, create a private node that is not shared outside of the class (called "lastNode" for instance). Upon detecting that you have iterated to the end of the list, have "next()" return a reference to "lastNode". Also have a method called "end()" return a reference to "lastNode". Lastly, depending on how you implement your class, you might need to override the comparison operator for this to work properly.

Example:

class MyNode{

};

class MyList{

public:
    MyList () : lastNode();

    MyNode * next(){

       if (isLastNode) return &lastNode;
       else return //whatever comes next
    }

    MyNode * end() {

       return &lastNode;
    }

    //comparison operator
    friend bool operator == (MyNode &n1, MyNode &n2){

        return (&n1 == &n2); //check that both operands point to same memory
    }


private:
    MyNode lastNode;
};


int main(){

  MyList list;
  MyNode * node = list.next();

  while ( node != list.end() ){ 

     //do stuff!

     node = list.next();
  }

  return 0; 
}