String类的写时拷贝实例

#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] = '/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 != '/0')    {      *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;}