Рассмотрим следующий пример:

int main()
{
    string x = "hello";
    //copy constructor has been called here.
    string y(x);
    //c_str return const char*, but this usage is quite popular.
    char* temp = (char*)y.c_str();

    temp[0] = 'p';

    cout < "x = " < x < endl;
    cout < "y = " < y < endl;

    cin >> x;
    return 0;
}