String COW 写时复制

String COW 写时复制"/>

String COW 写时复制

String 类Copy_On_Write 技术实现： 申请堆空间时额外申请sizeof(cahr)大小空间用于保存引用计数值
Proxy 模式区别operator[]运算符的write(引用计数自减) , read情况 —Effective C++

自定义一个String 类

/*String 类Copy_On_Write 技术实现： 申请堆空间时而外申请sizeof(cahr)大小空间用于保存引用计数值Proxy 模式区别operator[]运算符的write(引用计数自减) , read情况  ————Effective C++
*/#include <stdio.h>
#include <string.h>
#include <iostream>
using std::cout;
using std::endl;class String{
public:class CharProxy;         //代理类 可以设置为友元String();~String();String(const char* pstr);String(const String &rhs);            //2种复制语义String & operator = (const String & rhs);CharProxy operator[] (size_t idx);    //重载[]//char & operator[](size_t idx);    //原重载的方式  返回char 类型的引用friend std::ostream & operator<< (std::ostream &os ,const String &rhs);const char* c_str() const{return _pstr;}size_t size() const{return strlen(_pstr);}size_t refcount() const{         //获取引用计数值return _pstr[size()+1];}private:void initRefcount(){             //引用计数的相关函数_pstr[(size()+1)] = 1;}void increaseRefcount(){++ _pstr[size()+1];}void decreaseRefcount(){-- _pstr[size()+1];}char * _pstr;
};String::String()
:_pstr(new char [2]){//cout << "string()" << endl;initRefcount();
}String::String(const char * pstr)
:_pstr(new char[strlen(pstr) + 2]){//cout << "String(const char *)" << endl;strcpy(_pstr,pstr);initRefcount();               //strcopy,初始化refcount=1
}String::String(const String & rhs)
:_pstr(rhs._pstr){      //复制语义->浅拷贝->rhs指针赋值increaseRefcount();
}String & String::operator = (const String &rhs){if(this != &rhs){decreaseRefcount();if(refcount() == 0){    //只有当this->refcount变为0时，才去回收左操作数的空间delete [] _pstr;cout<< "delete left _pstr" << endl;}_pstr = rhs._pstr;   //复制语义->浅拷贝->rhs指针赋值increaseRefcount();}return *this;       //注意返回值
}String::~String(){decreaseRefcount();if(refcount()==0){delete [] _pstr;cout<< "~String" << endl;}
}std::ostream & operator<< (std::ostream & os,const String &rhs){os << rhs._pstr;return os;
}//*************************************CharProxy定义
class String::CharProxy{
public:CharProxy(String &, int );CharProxy & operator=(const char &);friend std::ostream& operator<<(std::ostream &os, const CharProxy &rhs){return os << rhs._mystr.c_str()[rhs._idx];    //避免 _mystr[idx]递归;}
private:String & _mystr;     //********传递一个引用!!!! 代理类不需要析构函数size_t _idx;
};String::CharProxy::CharProxy(String & mystr, int idx)
:_mystr(mystr)
,_idx(idx){cout<<"String::CharProxy::CharProxy(String & mystr, int idx)() ok"<<endl;
}String::CharProxy & String::CharProxy::operator=(const char & c){if(_idx < _mystr.size()){if(_mystr.refcount()>1){          //待更新的str多个引用时_mystr.decreaseRefcount();char * ptmp = new char[_mystr.size()+2];strcpy(ptmp,_mystr._pstr);_mystr._pstr=ptmp;          //浅拷贝,同一块堆区_mystr.initRefcount();}_mystr._pstr[_idx]=c;  //写更新}return (*this);
}//保证CharProxy定义complete  string operator需要在最后
String::CharProxy String::operator[] (size_t idx){return CharProxy(*this , idx);
}int main(int argc,char** argv){//*****************************************************测试复制语义String str1;String str2("hello,world");String str3(str2);cout << "str1 = " << str2 << endl;cout << "str2 = " << str2 << endl;cout << "str3 = " << str3 << endl;cout << "str1's refcount = " << str1.refcount() << endl;cout << "str2's refcount = " << str2.refcount() << endl;cout << "str3's refcount = " << str3.refcount() << endl;printf("&str1 = %p\n", str3.c_str());printf("&str2 = %p\n", str2.c_str());printf("&str3 = %p\n", str3.c_str());cout<<"----------------------------------"<<endl;str1 = str2;                       //str2 str3指向的字符串 引用计数+1cout << "str1 = " << str2 << endl;cout << "str2 = " << str2 << endl;cout << "str3 = " << str3 << endl;cout << "str1's refcount = " << str2.refcount() << endl;cout << "str2's refcount = " << str2.refcount() << endl;cout << "str3's refcount = " << str3.refcount() << endl;printf("&str1 = %p\n", str3.c_str());printf("&str2 = %p\n", str2.c_str());printf("&str3 = %p\n", str3.c_str());cout<<"----------------------------------"<<endl;String str4 = "wangdao";String str5(str4);         //此时str4引用计数=2str4=str2;                  //此时str2 的引用计数=4cout << "str2 = " << str2 << endl;cout << "str4 = " << str4 << endl;cout << "str5 = " << str5 << endl;cout << "str2's refcount = " << str2.refcount() << endl;cout << "str4's refcount = " << str4.refcount() << endl;cout << "str5's refcount = " << str5.refcount() << endl;printf("&str2 = %p\n", str2.c_str());printf("&str4 = %p\n", str4.c_str());printf("&str5 = %p\n", str5.c_str());//*************************************************测试operator [] 写操作cout<<"----------------------------------"<<endl;str1[0] = 'X';cout << "str1 = " << str1 << endl;cout << "str2 = " << str2 << endl;cout << "str3 = " << str3 << endl;cout << "str4 = " << str4 << endl;cout << "str1's refcount = " << str1.refcount() << endl;cout << "str2's refcount = " << str2.refcount() << endl;cout << "str3's refcount = " << str3.refcount() << endl;cout << "str4's refcount = " << str4.refcount() << endl;printf("&str1 = %p\n", str1.c_str());printf("&str2 = %p\n", str2.c_str());printf("&str3 = %p\n", str3.c_str());printf("&str4 = %p\n", str4.c_str());//*************************************************测试operator [] 读操作cout<<"----------------------------------"<<endl;cout<<"read str2[0]"<<str2[0]<<endl;       //read str[2] 不会发生COW情况 refcout 不改变cout << "str1 = " << str1 << endl;cout << "str2 = " << str2 << endl;cout << "str3 = " << str3 << endl;cout << "str4 = " << str4 << endl;cout << "str1's refcount = " << str1.refcount() << endl;cout << "str2's refcount = " << str2.refcount() << endl;cout << "str3's refcount = " << str3.refcount() << endl;cout << "str4's refcount = " << str4.refcount() << endl;printf("&str1 = %p\n", str1.c_str());printf("&str2 = %p\n", str2.c_str());printf("&str3 = %p\n", str3.c_str());printf("&str4 = %p\n", str4.c_str());return 0;}/*1.思考 private static 来保存引用计数值的错误设计2.String 重载[]但是无法进行左值、右值判断。=》只返回代理类对象，将左值右值判断放入代理类内部,再次重载代理类的 operator<<（右值情况）与 operator=（左值情况）
*/