// From the software distribution accompanying the textbook
// "A Practical Introduction to Data Structures and Algorithm Analysis,
// Third Edition (C++)" by Clifford A. Shaffer.
// Source code Copyright (C) 2007-2011 by Clifford A. Shaffer.

// 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;
};