Il Class Definition: class Queue ( public standard Queue methods Queue) bool empty) const Error _code append(const Queue entry &item) Error code serve) Error_code retrieve(Queue entry &item) const I/ safety features for linked structures Queue( ) Queue(const Queue &original): void operator (const Queue &original): protected Node front, *rear; Error_code Queue :: append(const Queue entry &item /* Post: Add item to the rear of the Queue and return a code of success or return a code of overflow if dynamic memory is exhausted. "/ Node *newrear = new Node(item); if (newrear NULL) return overflow; Il fill the missing lines that complete the function: if (rear== NULL) else f return success