如何将ifstream读入对象成员？

Question

The file opened in binary mode, the first variation gives exception, the second one is not. How can I read directly to my mhead object with ifstream? Please help me. Here is my code:

class mhead {   

public:

  long length;  
  void readlong(std::ifstream *fp);
}  

void mhead::readlong(std::ifstream *fp)    
{
    //this one is not work
    fp->read((char*)this->length,sizeof(this->length));     

    //this is working
    long other;
    fp->read((char*)other,sizeof(other));
}
}

Answer 1

Try this:

fp->read(&this->length,sizeof(this->length));

Writing to (char *)this->length means:

• Get some number you just made up
• Write to that memory location
• Hope for the best

Answer 2

If read was successful readlong return true.

class mhead
{   
  public:
    long length;  

    bool readlong(std::istream &is)    
    {
      is.read(reinterpret_cast<char *>( &this->length ), sizeof(long) );
      return ( is.gcount() == sizeof(long) )
    };
}

Or (I suggest this one):

istream & operator >> ( istream &is, mhead &_arg )
{
  long temp = 0;
  is.read(reinterpret_cast<char *>( &temp ), sizeof(long) );
  if ( is.gcount() == sizeof(long) )
    _arg.length = temp;
  return is;
}