class String
{
private:char* _str;static int count;
};

class String
{
public:String(char* str = "")    //不能strlen（NULL）{_str = new char[strlen( str) + 1];strcpy(_str, str);count++;}String(const String &s){_str = s._str;count++;}String& operator=( String& s)  {_str = s._str;count++;return *this;}~String(){if (--count == 0){delete[] _str;_str = NULL;cout << "~String " << endl;}}friend ostream& operator<<( ostream& output, const String &s);friend istream& operator>>( istream& input, const String &s);
private:char* _str;static int count;
};int String::count = 0;      //初始化count

void Test()    //用例测试
{String s1("abcdefg");String s2(s1);String s3;s3 = s2;cout << s1 << endl;cout << s2 << endl;cout << s3 << endl;String s4("opqrst");String s5(s4);String s6 (s5);s6 = s4;cout << s4 << endl;cout << s5 << endl;cout << s6 << endl;
}

class String
{private:char* _str;int* _refCount;
};

 //错误代码String& operator=(String& s)  {if (_str!= s._str){delete[] _str;delete _refCount;          _str = s._str;_refCount = s._refCount;(*_refCount)++;}      return *this;}　

//案例三class String
{
public:String(char* str = "")    //不能strlen（NULL）{_refCount = new int(1);     //给_refCount开辟空间，并赋初值1_size = strlen(str);_capacity = _size + 1;_str = new char[strlen(str) + 1];strcpy(_str, str);}String(const String &s){_refCount = s._refCount;_str = s._str;_size = strlen(s._str);_capacity = _size + 1;(*_refCount)++;      //拷贝一次_refCount都要加1}//要考虑是s1=s2时，s1原先不为空的情况，要先释放原内存//如果要释放原内存时，要考虑它的_refCount减1后是否为0，为零再释放，否则其它对象指针无法再访问这片空间String& operator=(String& s)  {if (_str!= s._str){_size = strlen(s._str);_capacity = _size + 1;if (--(*_refCount) == 0){delete[] _str;delete _refCount;}_str = s._str;_refCount = s._refCount;(*_refCount)++;}      return *this;}//如果修改了字符串的内容，那所有指向这块内存的对象指针的内容间接被改变//如果还有其它指针指向这块内存，我们可以从堆上重新开辟一块内存空间，//把原字符串拷贝过来//再去改变它的内容，就不会产生链式反应//  1.减引用计数  2.拷贝   3.创建新的引用计数char& String::operator[](const size_t index) //参考深拷贝      {if (*_refCount==1){return *(_str + index);}else{--(*_refCount);char* tmp = new char[strlen(_str)+1];strcpy(tmp, _str);_str = tmp;_refCount = new int(1);return *(_str+index);}}~String(){if (--(*_refCount)== 0)  //当_refCount=0的时候就释放内存{delete[] _str;delete _refCount;_str = NULL;cout << "~String " << endl;}_size = 0;_capacity = 0;}friend ostream& operator<<(ostream& output, const String &s);friend istream& operator>>(istream& input, const String &s);
private:char* _str;      //指向字符串的指针size_t  _size;      //字符串大小size_t  _capacity;   //容量int* _refCount;    //计数指针
};ostream& operator<<(ostream& output, const String &s)
{output << s._str;return output;
}
istream& operator>>(istream& input, const String &s)
{input >> s._str;return input;
}void Test()    //用例测试
{String s1("abcdefg");String s2(s1);String s3;s3 = s2;cout << s1 << endl;cout << s2 << endl;cout << s3 << endl;s2[3] = '0';cout << s1 << endl;cout << s2 << endl;cout << s3 << endl;
}

方案四

class String
{private:char* _str;
};

方案四与方案三类似。方案四把用来计数的整型指针变量放在所开辟的内存空间的首部。

用*((int*)_str)就能取得计数值

class String
{
public:String(char * str = "" )    //不能strlen（NULL）{_str = new char[strlen( str) + 5];_str += 4;strcpy(_str, str);GetRefCount(_str) = 1;}String(const String &s){_str = s._str;++GetRefCount(_str);}//要考虑是s1=s2时，s1原先不为空的情况，要先释放原内存//如果要释放原内存时，要考虑它的_refCount减1后是否为0，//为零再释放，否则其它对象指针无法再访问这片空间String& operator=(String& s){if (this != &s ){if (GetRefCount(_str ) == 1){delete (_str-4);_str = s._str;++GetRefCount(_str );}else{--GetRefCount(_str );_str = s._str;++GetRefCount(_str );}}return *this ;}//如果修改了字符串的内容，那所有指向这块内存的对象指针的内容间接被改变//如果还有其它指针指向这块内存，我们可以从堆上重新开辟一块内存空间，//把原字符串拷贝过来.//再去改变它的内容，就不会产生链式反应char& String ::operator[](const size_t index ) //深拷贝     {if (GetRefCount(_str) == 1){return _str[index ];}else{//  1.减引用计数--GetRefCount(_str );//  2.拷贝     3.创建新的引用计数char* tmp = new char [strlen(_str) + 5];   *((int *)tmp) = 1;tmp += 4;strcpy(tmp, _str);_str = tmp;return _str[index ];}}int& GetRefCount(char* ptr)    //获取引用计数(隐式内联函数){return *((int *)(ptr -4));}~String(){if (--GetRefCount(_str) == 0){cout << "~String" << endl;delete[] (_str-4);             }}friend ostream& operator<<( ostream& output, const String &s);friend istream& operator>>( istream& input, const String &s);
private:char* _str;};ostream& operator<<(ostream& output, const String &s)
{output << s._str;return output;
}
istream& operator>>(istream& input, const String &s)
{input >> s._str;return input;
}void Test()  //用例测试
{String s1("abcdefg" );String s2(s1);String s3;s3 = s2;cout << s1 << endl;cout << s2 << endl;cout << s3 << endl;s2[3] = '0';cout << s1 << endl;cout << s2 << endl;cout << s3 << endl;
}