6. The Node class constructor

Each node keeps user data and a pointer to the next node in the list.

It is a good idea to initialize the pointer with NULL, indicating that no further linkage is present.

NULL becomes the end marker of the singly-linked list.

The definition of NULL comes from a number of the standard library headers, such as <iostream> and <cstddef>.

NULL can be assigned to a pointer of any type.

class Node {
    Node* pnext;
public:
    int data;

    // Constructor taking initial value:
    Node( int value = 0 )
    : pnext( NULL ), data( value )
    {
    }
};