// Abstract queue class
template <typename E> class Queue {
private:
void operator =(const Queue&) {} // Protect assignment
Queue(const Queue&) {} // Protect copy constructor
public:
Queue() {} // Default
virtual ˜Queue() {} // Base 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;
};