class String {
public:
        String():rep(new StringRep("")) { }
        String(const String &s): rep(s.rep) { rep->count++; }
        String &operator=(const String &s){
            String(s).swap(*this); // copy-and-swap idiom
            return *this;
        }
        ~String() { // StringRep deleted only when the last handle goes out of scope.
                if(rep && --rep->count <= 0) delete rep;
        }
        String(const char *s): rep(new StringRep(s)) { }
        void swap (String & s) throw ()  {
          std::swap(this->rep, s.rep);
        }
private:
        StringRep *rep;
};