// Source: "Software Design ...", John A Robinson, Newnes, 2004, page 212.
class queue
	{
	struct item
		{
		itemtype *value;
		item *pnext;
		} *head, *tail;										
	int queue_length;		// To allow the caller to set max length
public:
	queue(int size)				
		{
		queue_length = size;	// Unused in this implementation
		head = 0;
		tail = 0;
		};
	~queue()									
		{
		item *p;
		while(head)		// Step through, deleting items one by one
			{
			p = head;
			head = head->pnext;
			delete p;
			}
		}
	int enqueue(itemtype *);
	int dequeue(itemtype **);
	};

int queue::enqueue(itemtype *value)		
		{
		item *p = new item;	// Make new item
		if (tail)		// Already something on queue
			tail = tail->pnext = p;		// so tack new item on end
		else
			head = tail = p;	// Otherwise this is single item on queue so far
		tail->pnext = 0;			// New end of queue
		tail->value = value;
		return(0);
		}
int queue::dequeue(itemtype **value)
		{
		if (head == 0)
			return(-1);	// Queue already empty
		*value = head->value;	// Get value in head item
		item *p;
		p = head;
		head = head->pnext;	// Move head pointer along one
		delete p;		// and delete former head item
		if (head == 0)		// Last item just removed
			tail = 0;
		return(0);
		}