C++ string class

If you have a C++ string and a char pointer

string str = "mudkipz";
char * ptr;

how would you iterate through each char in the string using only the pointer and only pointer notation? In other words, you can't assign the address with ptr = &str[0];

I don't see how it's possible, since a string isn't an array. If it were, I'd just do this

string str = "mudkipz";
    char * ptr = str;

    for(int i = 0; i < str.length(); ++i)
    {
        cout << *(ptr + i);
    }

Any ideas?

>>8
Okay, nevermind. I guess my professor is an idiot.