Fatal Error: One Unresolved External

[firstlast]

I'm trying to implement this queue class, and I don't know why it won't compile. I wrote a main function in the main.cpp file and even commented out the calls to functions add and removed and still gave me the fatal error.

queue.h

#ifndef QUEUE_H
#define QUEUE_H
namespace queuesavitch
{
	struct QueueNode
	{
	    char data;
		char next;
		QueueNode *link;
	};
	typedef QueueNode* QueueNodePtr;

	class Queue
	{
	public:
		Queue();
		Queue(const Queue& aQueue);
		~Queue();

		void add(char item);
		char remove();
		bool empty() const;
		void displayqueue();

	private:
		QueueNodePtr front;
		//QueueNodePtr next;
		QueueNodePtr back;
		int count;
	};
}
#endif 

main.cpp

#include <iostream>
#include "queue.h"
using namespace queuesavitch;
using namespace std;

Queue::Queue()
{
	front = NULL;
	back = NULL;
}

bool Queue::empty() const
{
return count == 0 ? true : false;
}

void Queue::add(char item)
{
   
       // Create a new node
       QueueNode* tmp = new QueueNode;
       tmp->data = item;
       tmp->link = NULL;

       if ( empty() ) 
	   {
           // Add the first item
           front = back; 
		   back = tmp;
       }
       else
	   {
           // Append to the list
           back->link = back;
           back = tmp;
       }
} 


char Queue::remove()
 { 
       if ( empty() )
	   {
           cout << "Queue is empty";
	   }

       char ret = front->data;
       QueueNode* tmp = front;

       // Move the front pointer to next node
       front = front->link;

       count--;
       delete tmp;
       return ret;   
}

void Queue::displayqueue()
{
  cout << "You entered: \n";
  while (! empty())
	cout << remove();
  cout << endl; 
}

int main()
{
	Queue q;
    char next, ans;

    do
    {
        cout << "Enter a name:\n";
        cin.get(next);
        while (next != '\n')
        {
			cin.get(next);
            q.add(next);
        }  

		//q.add(item);
		q.remove();
		q.displayqueue();

        cout << "Again?(y/n): ";
        cin >> ans;
        cin.ignore(10000, '\n');
    }while (ans != 'n' && ans != 'N');  

	system ("pause");
    return 0;
}

[Peter87]

You have not implemented the destructor.

[firstlast]

Okay, I've implemented the instructor, but now I have an "Access violation at writing location 0x00000004."

{
         // Append to the list
           back->link = back;
           back = tmp;
}

[Peter87]

0x00000004 is very close to 0 (null) so my guess is that you are dereferencing a null pointer somewhere.

[cire]

void Queue::add(char item)
{
   
       // Create a new node
       QueueNode* tmp = new QueueNode;
       tmp->data = item;
       tmp->link = NULL;

       if ( empty() ) 
	   {
           // Add the first item
           front = back; 
		   back = tmp;
       }
       else
	   {
           // Append to the list
           back->link = back;
           back = tmp;
       }
} 

When you create a queue, it is empty and front and back are set to null. Here, when you encounter an empty list and add a character, you set front equal to back or null, then set back to point to tmp.

Any attempt to use front past that point will result in dereferencing a null pointer.