终于明白c++中为什么返回的istream等对象能直接进行bool判断

      之前一直很困惑c++中用>>, getline等函数读进数据，如：    

        while(cin>>c) 或者 while(getline(cin, str))

 为什么能直接判读是否读到数据尾或者读取数据出错，相当于返回的是个bool类型

     因为我看函数其原型是这样的，返回的是个istream&

    istream& operator>> (bool& val );    istream& operator>> (short& val );    istream& operator>> (unsigned short& val );    istream& operator>> (int& val );    istream& operator>> (unsigned int& val );    istream& operator>> (long& val );     

    istream& getline ( istream& is, string& str, char delim );    istream& getline ( istream& is, string& str );

       原来是其父类basic_ios 中实现了以下两个函数，哈哈，终于弄明白了：

 //@{      /**       *  @brief  The quick-and-easy status check.       *       *  This allows you to write constructs such as       *  "if (!a_stream) ..." and "while (a_stream) ..."      */      operator void*() const      { return this->fail() ? 0 : const_cast<basic_ios*>(this); }      bool      operator!() const      { return this->fail(); }      //@}