Functions with one parameter problem

[MMac1218]

Hey guys, I think I am very close on this problem but I dont know how to end my program after the zero is entered because it just continues to promt the question.

Also we are not allowed to use global variables.. Is this a global variable?

/*	Write a program with a function called mySquareRoot. 
	In main, prompt the user for a number (a double), then
	 by calling the mySquareRoot function  calculate and 
	display the square root of the number. Use the standard 
	cmath function to calculate the square root. Use the 
	follow screen shot as a guide.
*/


#include <iostream>
#include <string>
#include <cmath>

using namespace std; 

double mySquareRoot (double num=0) 
{  
    return sqrt(num); 
} 
void main () 
{ 

    double NewNum; 
    double num=0; 

    
    while(num != '0') 
    {cout << "Enter a number (a double): "; cin >> num; 

    NewNum = mySquareRoot(num); 
    cout << "The square root is " << NewNum << endl; 

    } 
    if (num =='0') 
        return;  
}