Reverse Polish Calculator help!

[Mchubbs]

Hello!
So I have my Reverse Polish calculator compiling and everything but for the assignment I need to handle a few exceptions that I can't seem to get.
First off I'm trying to make the program exit if the user enters only "0"
but since the input i'm using is string, I cant figure out how to code
"If the first node is 0 and the next node = NULL, return true"

Here is my code:

#include<iomanip>
#include<iostream>
#include<stdlib.h>
#include<stdio.h>
#include<string>
#include<sstream>
using namespace std;

class Stack 
{
	private:
		double data;
		Stack *top;
		Stack *next;
		int counter;

	public:
		Stack()
		{
			top = NULL;
			next = NULL;
		}

		bool isEmpty()
		{
			return (top == NULL);
		}

		bool full()
		{
			return 0;
		}

		void push(double input)
		{   
			Stack *q = new Stack;
			q->data=input;
			q->next=top;
			top=q;
			counter++;
		}

		double pop()
		{
			if(isEmpty())
			{
				cout << "Too many operators!" << endl;
			}
			else
			{
			counter--;
			Stack *q = top->next;
			double value = top->data;
			delete top;
			top = q;
			return value;
			}
		}

		int count()
		{
			cout << "There are " << counter << "elements on the stack";
		}

		void print()
		{
			cout << top->data << endl;
		}

		bool isOperator(const string& input)
		  {
			 string ops[] = {"-", "+", "*", "/"};
			 for(int i = 0; i < 4; i++)
			  {
				 if(input == ops[i])
				  {
					 return true;
				  }
			  }
			 return false;
		   }

		void performOp(const string& input, Stack& stack)
		 {
			 double fVal, sVal, result;
			 int error = 0;

			 sVal = stack.pop();
			 fVal = stack.pop();

			  if(input == "-")
				 {
				  stack.push(fVal - sVal);
				 }
			  else if (input == "+")
				 {
				   stack.push(fVal + sVal);
				  }
				else if (input == "*")
				  {
				   stack.push(fVal * sVal);
				  }
				else if(input == "/" && sVal!=0)
			    	{
					stack.push(fVal / sVal);
					}
				if(input == "/" && sVal == 0)
				{
					cout << "Division by zero error" << endl;
					error = 1;
				}

				if(error == 0)
				{
					stack.print();
				}
			}
		bool quitter(const string& input, Stack *top, Stack *next)
		{ //This is the function I cant seem to get working

			if(top == 0 && next == NULL)
			{
				return true;
			}

		}

};

int main()
{
	Stack a;
	string input;

	cout << "Reverse Polish Calculator! Enter Calculation or Enter 0 to quit.\n";
	cout << "Enter a Reverse Polish Expression: ";

	while(true)
	{
		cin >> input;
		double num;

		if(a.quitter(input))//check for quit
			{
			cout << "QUITTER!";
			return 0;
			}
		else if(istringstream(input) >> num)
			{
			a.push(num);
			}
		else if (a.isOperator(input))
			{
			a.performOp(input, a);
			}
		else 
			{
			cout << "Invalid input" << endl;
			}


	}

	system("PAUSE");
	return 0;
}

[Mchubbs]

I got it to accept this but when i try calculations I get this error

Unhandled exception at 0x00E18019 in RPN.exe: 0xC0000005: Access violation reading location 0x00000000.

bool quitter(const string& input)
		{ 
			if(top->data == 0 && top->next == NULL)
			{
				return true;
			}

		}

I also swapped:

if(a.quitter(input))//check for quit
			{
			cout << "QUITTER!";
			return 0;
			}

to under else if (a.isOperator(input)) function in main