String类的写时拷贝

#include
    
     #include
     
      using namespace std;class String{public:    friend ostream& operator <<(ostream &os, String &str);    String(char *str = "") :_str(new char[strlen(str) + 1 + 4])    {        _str = _str + 4;        getCount() = 1;        strcpy(_str, str);    }    String(String &str) :_str(str._str)    {        ++getCount();    }    String &operator=(String &str)    {        if (this != &str)        {            release();            _str = str._str;            ++getCount();        }        return *this;    }    char &operator[](int index)    {        assert(index >= 0);        assert(index<(int)strlen(_str));        if (getCount()>1)        {            --getCount();            char *temp = new char[strlen(_str) + 4 + 1];            strcpy(temp + 4, _str);            _str = temp + 4;            getCount() = 1;        }        return _str[index];    }    ~String()    {        release();    }private:    void release()    {        if (_str&&(--getCount() == 0))        {            delete[](_str-4);            _str = NULL;        }    }    int &getCount()    {        return *(int*)(_str - 4);    }    char *_str;};ostream &operator<<(ostream &os, String &str){    os << str._str;    return os;}void test1(){    String str1("hello ");    String str2(str1);    String str3("world");    String str4(str3);    str1[0] = 'c';    str1[1] = 'h';    str1[2] = 'a';    str1[3] = 'n';    str1[4] = 'g';    str1[5] = 'e';    str2 = str3;    cout << str1 << endl;    cout << str3 << endl;}int main(){    test1();    system("pause");    return 0;}