Whay '<'signed /unsigned mismatch error??

Question

stack <char>stack1;

string my_string="hello";
for (int i =0; i< my_string.length(); i++)
{
stack1.push(my_string[i]);

}

Answer 1

I compiled and ran this that worked perfectly:
#include <iostream>
#include <stack>
using namespace std;
int main() {
stack <char>stack1;
string my_string = "hello";
for (int i = 0; i < my_string.length(); i++) {
stack1.push(my_string[i]);
}
return 0;
}

Answer 2

Change the int to size_t?

Answer 3

@John Wells
may be my compiler it dosn't work

Answer 4

Interface says size_type, which is usually size_t.