Review my "draw triangle out of signs" code (absolute beginner)

[Fresh Grass]

I've just started learning C++ and I decided to make a program that asks user for a length of a triangle's side and then draws the triangle with asterisks. This is what program looks like when executed:

Define number of signs per side of a triangle. To exit the program, enter 0.

Number of signs: 4

    *
   * *
  * * *
 * * * *

Number of signs: 2

  *
 * *

etc...

I want to know if there's more convenient way of doing this, using other loop types, a break... I'm here to learn, so any suggestions and corrections are more than welcome!

Here is my code:

#include <iostream>
using namespace std;
int main() {
	int num=1, line, sign;
	cout << "Define number of signs per side of a triangle. To exit the program, enter 0.\n";
	while (num != 0) {		//This loop will repeat until user enters 0.
		cout << "\nNumber of signs: "; cin >> num; cout << "\n";
	//Line's number will decrease until 0, because number of empty spaces decreases.
		for (line = num; line != 0; line--) {
			sign = 1;
		//The following loop will output empty spaces until line's number is reached.
			for (;sign < line; sign++) cout << " ";
			sign = num;
		//The following loop will output asterisks until sign number decreases to line's number.
			for (;sign >= line; sign--) {
				if (sign > line) cout << " *";
			//Outputs last asterisk and goes to new line when number of signs equals line number.
				else cout << " *\n"; 
			}
		}
	}
	return 0;
}

Thanks in advance!