d_lqueue.h

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#ifndef LINKEDQUEUE_CLASS
#define LINKEDQUEUE_CLASS

#ifndef NULL
#include <cstddef>
#endif  // NULL

#include "d_node.h"		// node class
#include "d_except.h"	// for underflowError exception

// implements a linked queue
template <typename T>
class linkedQueue
{
    public:
		linkedQueue();
			// Constructor: creates an empty queue.
			// Postcondition: qsize = 0, qfront = qback = NULL

		linkedQueue(const linkedQueue<T>& obj);
			// copy constructor. builds a copy of obj

		~linkedQueue();
			// destructor. clear the linked list

		linkedQueue<T>& operator= (const linkedQueue<T>& rhs);

		void push(const T& item);
			// insert an element at the rear of the queue
			// Postcondition: the queue has one more element

		void pop();
			// remove an element from the front of the queue.
			// Precondition: the queue is not empty. if the queue
			// is empty, throw an underflowError exception
			// Postcondition: the queue has one less element

		T& front();
			// return a reference to the front of the queue.
			// Precondition: the queue is not empty. if the queue
			// is empty, throw an underflowError exception
		const T& front() const;
			// constant version

      int size() const;
			// return the current size of the queue

		bool empty() const;
			// return true if the queue is empty; otherwise return false

    private:
		node<T> *qfront, *qback;
			// the linked list with qfront stores the elements.
			// qback points to the rear of the queue
		int qsize;
			// number of elements in the queue

		void copy(const linkedQueue<T>& q);
			// copy q so the current list is identical to q

		void clear();
			// clear the list. used by the destructor and assignment

		node<T> *getNode(const T& item);
			// allocate a node with value item and pointer value NULL
			// and return a pointer to it. if memory allocation fails,
			// throw the memoryAllocationError exception
};

template <typename T>
void linkedQueue<T>::copy(const linkedQueue<T>& q)
{
	// qback moves through the list we are building and
	// winds up at the rear of our new list. p
	// moves through the list we are copying
	node<T> *newNode, *p = q.qfront;

	// initially, the list is empty
	qfront = qback = NULL;

	// nothing to do if p is NULL
	if (p != NULL)
	{
		// create the first node in the queue and assign
		// its addres to qback
		qfront = qback = getNode(p->nodeValue);

		// move forward in the list we are copying
		p = p->next;

		// copy remaining items
		while(p != NULL)
		{
			// insert new node at the back
			newNode = getNode(p->nodeValue);
			qback->next = newNode;

			// qback is the new node
			qback = newNode;

			// move to the next node of the list we are copying
			p = p->next;
		}
	}

	// the size of the new list is the size of q
	qsize = q.qsize;
}

template <typename T>
void linkedQueue<T>::clear()
{
	node<T> *curr = qfront, *p;

	// delete nodes until we arrive at the end of the list
	while (curr != NULL)
	{
		// save curr in p and move curr forward
		// to the next list node
		p = curr;
		curr = curr->next;

		// delete node p
		delete p;
	}

	// specify an emtpy list
	qfront = qback = NULL;
	qsize = 0;
}

template <typename T>
node<T> *linkedQueue<T>::getNode(const T& item)
{
	// allocate a new node
	node<T> *newNode = new node<T> (item);

	// check for failure
	if (newNode == NULL)
		throw memoryAllocationError(
			"linkedQueue: memory allocation failure");

	return newNode;
}

// create the empty list qfront = qback = NULL and qsize = 0
template <typename T>
linkedQueue<T>::linkedQueue(): qfront(NULL), qback(NULL), qsize(0)
{}

template <typename T>
linkedQueue<T>::linkedQueue(const linkedQueue<T>& obj)
{
	// call copy() and pass the pointer to the front of
	// the linked list in obj
	copy(obj);
}

template <typename T>
linkedQueue<T>::~linkedQueue()
{
	// call clear()
	clear();
}

template <typename T>
linkedQueue<T>& linkedQueue<T>::operator= (const linkedQueue<T>& rhs)
{
	// delete the current list
	clear();

	// make the current object a copy of rhs
	copy(rhs);

	return *this;
}

// insert item into the queue by inserting it at
// the back of the list
template <typename T>
void linkedQueue<T>::push(const T& item)
{
	// allocate space for the new node
	node<T> *newNode = getNode(item);

	// if the queue is empty, insert the new element at the front of
	// the list and update both qfront and qback
	if (qfront == NULL)
	{
		qfront = newNode;
		qback = newNode;
	}
	// in a non-empty list, insert new element at back and update qback
	else
	{
		qback->next = newNode;
		qback = newNode;
	}

	// increment the queue size
	qsize++;
}

// remove the item at the front of the queue by erasing
// the first element in the linked list
template <typename T>
void linkedQueue<T>::pop()
{
	// save the location of the front of the queue
	node<T> *p = qfront;

	// if the queue is empty, throw underflowError
	if (qsize == 0)
		throw underflowError("queue pop(): empty queue");

	// move the front forward one node
	qfront= qfront->next;

	// if the queue is now empty, set qback to NULL
	if (qfront == NULL)
		qback = NULL;

	// delete the node
	delete p;

	// decrement the queue size
	qsize--;
}

template <typename T>
T& linkedQueue<T>::front()
{
	// if the queue is empty, throw underflowError
	if (qsize == 0)
		throw underflowError("queue front(): empty queue");

	// return the value at the front
	return qfront->nodeValue;
}

template <typename T>
const T& linkedQueue<T>::front() const
{
	// if the queue is empty, throw underflowError
	if (qsize == 0)
		throw underflowError("queue front(): empty queue");

	// return the value at the front
	return qfront->nodeValue;
}

template <typename T>
int linkedQueue<T>::size() const
{
	return qsize;
}

template <typename T>
bool linkedQueue<T>::empty() const
{
	return qsize == 0;
}

#endif	// LINKEDQUEUE_CLASS