I don't know if you posted your entire code, but if you did, than you only declare the constructors, you never actually define them. Try changing the class definition to this:
class Mystring {
char *s;
int size;
public:
Mystring(const char *str)
{
}
Mystring()
{
}
operator const char * () { return s; }
};
You may also define the constructors outside the class - but my suggestion is to do some reading about c++ first. Good luck!!