String类的写时拷贝实例分享

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实例如下：

  #include<iostream>  using namespace std;     class String;  ostream& operator<<(ostream &out, const String&s);  //引用计数器类  class String_rep   {    friend class String;    friend ostream& operator<<(ostream &out, const String&s);    public:        String_rep(const char *str )        :use_count(0)      {        if (str == NULL)        {          data = new char[1];          data[0] = '';        }        else        {          data = new char[strlen(str) + 1];          strcpy(data, str);        }      }          String_rep(const String_rep &rep) :use_count(0)      {        data = new char[strlen(rep.data) + 1];        strcpy(data, rep.data);      }      String_rep& operator=(const String_rep &rep)      {        if (this != &rep)        {          delete[]data;          data = new char[strlen(rep.data) + 1];          strcpy(data, rep.data);        }        return *this;      }      ~String_rep()      {          delete[]data;          data = NULL;      }    public:      void increase()    {      ++use_count;    }        void decrease()    {      if (use_count == 0)      {        delete this; //自杀行为  释放this所指的空间，在释放之前调动这个类的析构函数       }    }    private:        char *data;      int use_count;  };  ////////////////////////////////////////////////////////////////////////////////////////  class String  {     friend ostream& operator<<(ostream &out, const String&s);  public:    String(const char* str = " ")    {      rep = new String_rep(str);      rep->increase();    }    String(const String &s)    {      rep = s.rep;   //浅拷贝      rep->increase();    }    String& operator=(const String &s)    {      if (this != &s)      {        rep->decrease();  //模拟delete        rep = s.rep;      //模拟new        rep->increase();   //模拟strcpy        /*rep = s.rep;  //这会更改引用计数器指针 ，造成s内存泄漏        rep->increase();*/      }      return *this;    }      ~String()      {        rep->decrease();      }    public:      void to_upper()    {      if (rep->use_count > 1)      {        String_rep* new_rep = new String_rep(rep->data);        rep->decrease();        rep = new_rep;        rep->increase();      }      char* ch = rep->data;      while (*ch != '')      {        *ch -= 32;        ++ch;      }    }    private:      String_rep *rep; //引用计数器  };    ostream& operator<<(ostream &out, const String&s)  {    out << s.rep->data;    return out;  }  void main()  {    String s1("hello");    String s2(s1);    String s3;    s3 = s2;    cout << "s1=" << s1 << endl;    cout << "s2=" << s2 << endl;    cout << "s3=" << s3 << endl;       s2.to_upper();      cout << "-----------------------------------------------" << endl;        cout << "s1=" << s1 << endl;    cout << "s2=" << s2 << endl;    cout << "s3=" << s3 << endl;  }

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