Finally understand why C + + istream, such as the return of objects can directly carry out bool judgment

It's been confusing before. C + + with >>, getline and other functions to read data, such as:

        while (CIN>>C) or while (Getline (CIN, str))

Why is it possible to read directly the end of the data or read the data error, which is equivalent to returning a bool type

Because I see the function its prototype is like this, return is a 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);

The original is its parent class Basic_ios implemented the following two functions, haha, finally understand:

@{
      /**
       *  @brief the  quick-and-easy status check.
       *
       * This allows your 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 ();}
      //@}