C++中重载赋值运算符应该注意的地方

string& string::operator=(const string& str){
      if( this==&str )
            return *this;
      delete []data;
      m_data = null;
      m_data = new char[ strlen(str.data)+1] ;
       strcpy(data, str.data);
       return *this;
}

string& string::operator=(const string& str){
     if( this != &str){
          string temp(str);
          char *p = temp.data;
          temp.data = data;
          data = p;
      }
      return *this;
}