【C++】String类拷贝构造函数——浅拷贝优化的三种方式（引用计数）

这里提供三种优化方式：

1.使用静态变量计数

2.单独开辟计数空间

3.对象和计数变量使用同一连续空间

代码示例：

第一种

class String{public:String(char* pStr = ""):_pStr(new char[strlen(pStr)+1]){strcpy(_pStr, pStr);_pCount++;}String(String& s):_pStr(s._pStr){_pCount++;}//赋值运算符重载只有面对同一个对象，无需重写~String(){if (--_pCount == 0){delete[] _pStr;}}private:char* _pStr;static int _pCount;};

int String::_pCount = 0;

第二种：

使用单独内存的引用计数class String{public:String(char* pStr = ""):_pStr(new char[strlen(pStr) + 1]),_pCount(new int(1)){strcpy(_pStr, pStr);}String(String& s):_pStr(s._pStr){_pCount++;}String& operator=(const String& s){if (_pStr != s._pStr){Delete();_pStr = s._pStr;_pCount = s._pCount;_pCount++;}return *this;}~String(){Delete();}private:void Delete(){if (0 == (--(*_pCount))){delete[] _pStr;delete _pCount;}}private:char* _pStr;int* _pCount;};

第三种：

//在同一空间中使用引用计数—— _pCount ====(int*)(_pStr-4)class String{public:String(char* pStr = ""):_pStr(new char[strlen(pStr) + 5]){*_pStr = 1;_pStr -= 4;strcpy(_pStr, pStr);}String(String& s):_pStr(s._pStr){//++(*((int*)(_pStr - 4)));++GetRefCount();}String& operator=(const String& s){if (_pStr != s._pStr){Delete();_pStr = s._pStr;//_pCount = s._pCount;//_pCount++;++GetRefCount();}return *this;}~String(){Delete();}private:void Delete(){if (0 == (--(GetRefCount()))){delete[] (_pStr-4);//delete _pCount;}}int& GetRefCount(){return *((int*)(_pStr - 4));}private:char* _pStr;};void fun1(){String s1("hello");String s2(s1);s2 = s1;}void fun2(){String s1("hello");String s2(s1);String s3("world");String s4(s3);s3 = s2;}void fun3(){String s1("hello");String s2("world");s2 = s1;}int main(){fun1();fun2();fun3();return 0;}