//abstract queue class

template <typename E> class Queue

{

private:

void operator = (const Queue&) {} //protect assignment

Queue(const Queue&) {} //protect copy constructor

public:

Queue() {} //default constructor

virtual ~Queue() {} //destructor

//reinitialize the queue. The user is responsible for

//reclaiming the storage used by the queue elements

virtual void clear() =0;

//place an element at the rear of the queue

//it: the element being enqueued

virtual void enqueue(const E&) =0;

//remove and return element at the front of the queue

//return: the element at the front of the queue

virtual E dequeue() =0;

//return: a copy of the front element

virtual const E& frontValue() const =0;

//return: the number of elements in the queue

virtual int length() const =0;

};