Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!uunet!pilchuck!dataio!bright
From: bright@Data-IO.COM (Walter Bright)
Newsgroups: comp.lang.c++
Subject: Re: Objectifying contents read in from disk
Message-ID: <2069@dataio.Data-IO.COM>
Date: 24 Jul 89 18:40:46 GMT
References: <7316@athertn.Atherton.COM>
Reply-To: bright@dataio.Data-IO.COM (Walter Bright)
Distribution: usa
Organization: Data I/O Corporation; Redmond, WA
Lines: 21

In article <7316@athertn.Atherton.COM> thanh@athertn.Atherton.COM (Thanh Diec) writes:
<    After objects are written out to disk, and then read back in again,
<possibly by a different program, what does the reader need to do to set the
<content read in to behave correctly as objects?  For example, the runtime
<_vptr's in the writer program are going to be different from the reader's,
<even though both programs include the same ".h".
<Writer:
<  write(fd, &object_array[0], sizeof(class1) * 1024);
<Reader:
<  read(fd, buf, sizeof(class1) * 1024);

This will not work unless class1 has no virtual functions and no
virtual base classes. It also violates the C++ paradigm that only
a constructor can create an instance of a class (read() is creating
the instances).

The solution is to define a member function to write out the object,
and a constructor to read it in. Unfortunately, this can be significantly
less efficient than the write()/read() above. If you know what you're
doing and are willing to modify your code for new compilers, you can
use the write()/read() above.