C++ comparing string elements to an int

I have number lying in a string variable. I wanna check if every one of its elements is equal to some value, so I use the for loop to loop over every element and use if:

        int zera = 0, jedynki = 0;
        for (int i = 0; i < liczba.length(); i++) {
            if (liczba[i] == 0) zera ++;
            else if (liczba[i] == 1) jedynki ++;
        }

liczba is a string.

I know now that I can't do that. I tried to convert this int into char but still, nothing happened. What's wrong here? What should I do?