class String{    //类型的定义    struct Srep;        //表示能被几个同样值的String共享    Srep *rep;public：    class Cref;         //实现下标运算，区别对待读操作和写操作    class Range();   //范围错误时抛出的异常    //构造、赋值和析构函数    String();                                         //x = "";    String(const char*);                       //x ="abc";    String(const String&);                    //x = other_string;    String& operator=(const char*);    String& operator=(const String&);    ~String();        //带检查的访问运算符    void check(int i) const{if(i < 0 || rep->sz < i) throw Range();}    char read(int i) const{return rep->s[i];}    void write(int i, char c){        rep = rep->get_own_copy();         rep->s[i] =c;    }    Cref operator[](int i){        check(i);        return Cref(*this, i);    }    char operator[](int i) const{        check(i);        return rep->s[i];    }    int size() const{return rep->sz;}    //其他String操作    String& operator+=(const String&);    String& operator+=(const char*);    friend ostream& operator<<(ostream&, const String&);    friend istream& operator>>(ostream&, String&);    friend bool operator==(const String& x, const char *s){        return strcmp(x.rep->s, s)==0;    }    friend bool operator==(const String& x, const String& y){        return strcmp(x.rep->s, y.rep->s)==0;    }};String operator+(const String&, const String&);String operator+(const String&, const char*);String operator+(const char*, const String&);    

struct String::Srep{    char *s;        //到元素的指针    int sz;           //字符个数    int n;             //引用计数    Srep(int nsz, const char *p){        n = 1;        sz = nsz;        s = new char[sz + 1];        strcpy(s, p);    }    ~Srep(){        delete s;    }    Srep* get_own_copy(){        //实现写时复制。        if(n == 1) return this;        n--;        return new Srep(sz, s);    }    void assign(int nsz, const char* p){        if(sz != nsz){            delete []s;            sz = nsz;            s = new char[sz + 1];        }           strcpy(s, p);    }private:    Srep(const Srep&);    Srep& operator=(const Srep&);};

String::String(){                                 //空串为默认值    rep = new Srep(0, "");}String::String(const String& x){         //拷贝构造函数    x.rep->n++;    rep = x.rep;}String::~String(){                              //析构函数    if(--n == 0) delete rep;}String& operator=(const String& x){   //复制赋值     x.rep->n++;     if(--n == 0) delete rep;     rep = x.rep;     return *this;}String::String(const char *s){    rep = new Srep(strlen(s), s);}String& operator=(const char *s){   //复制赋值     if(rep->n == 1)         rep->assign(strlen(s), s);     else{        rep->n--;        rep = new Srep(strlen(s), s);    }     return *this;}

class String::Cref{    friend class String;    String &s;    int i;    Cref(String &ss, int ii):s(ss), i(ii){}public:    operator char() const{ return s.read(i);}    void operator=(char c){s.write(i, c);}};