Difference between Pointer & Reference.

What are the differences between Pointers & References? 

1. Both can be used to refer to any object of its type. But there is a difference in systax.

Eg:
  int *p=&a;
  int &r=a;

2. A pointer can be NULL, whereas a reference has to be always pointed to some object.

Eg: 

     For pointers: 

            int *p; // This is valid. 

            int *p=NULL; // This is valid. 

            int *p=&a; // This is valid. 

    For references: 

        int &r; // This is In-valid. 

        int &r=NULL; // This is In-valid. 

        int &r=a; // This is valid. 

3. A reference once it is binded to an object, it cannot be rebinded to a different object as, reference is inherently const. 

    Eg: int a, b; 

        int *p=&a; 

        p=&b; // This is valid as it is a pointer. 

        int a,b; 

        int &r=a; 

        r=b; // This is invalid as a reference cannot be referenced to a different object. 

Note: A reference can be considered as constant pointers, except that, even a const pointer can be pointed to NULL.