(newbie) converting string to int..

I'm trying to convert string to int with atoi, but I always get this error when I try to compile:
test.cpp: In function ‘int main()’:
test.cpp:11: error: cannot convert ‘std::string’ to ‘const char*’ for argument ‘1’ to ‘int atoi(const char*)’

Code:

---

int main() {
        cout<<"insert string\n";
        string a;
        cin>>a;
        int b=0;
        while(a!="s") {
                b=atoi(a);
                cout<<b<<"\n";
                cin>>a;
        }
}

---

This is prototype, no checking if string is not number or smth.. How to make this prototype to work? :(

b=atoi( a.c_str() );
But consider using a string stream.

Thank you, now you're my new superhero :)