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From: mjv@objects.mv.com (Michael J. Vilot)
Newsgroups: comp.lang.c++
Subject: Re: Copy constructors
Message-ID: <1110@zinn.MV.COM>
Date: 12 Jan 91 20:48:11 GMT
Sender: nuucp@zinn.MV.COM
Lines: 31

David Masterson asks about the effect of casting bits to create an object:
> Assuming that type of the object is known after transmission, it
> should be possible to cast the transmission buffer to that type and copy
> construct a true object of that type using the buffer as input.

It may indeed be possible, but as Joe Buck pointed out it may be neither
portable nor guaranteed to work by any of the language's rules.  The mechanics
of virtual functions are clearly implementation-dependent (as they should be).

Assuming you really want to pursue a design based on assembling bits in a buffer
and not on using constructors to build up the objects, then you may be able to
rely on the semantics of operator new.  Specifically, the use of a ``placement
expression.''

The number and type of arguments are up to you -- #include <new.h>, declare the
operator new() that you need, and you may be able to develop the kind of 
solution you are after.  Rather than a cast, you would ``allocate'' a new object
and provide arguments to operator new() and/or the class' constructor.  You can
then dereference the pointer thus obtained and treat the resulting object in
the usual way.

Obviously, this design approach is a bit tricky, and requires careful 
cooperation between operator new, the constructor(s), and the destructor for
the class.  But it's more portable than relying on the result of a cast.
More importantly, it stays within the type system.

Hope this helps,

--
Mike Vilot,  ObjectWare Inc, Nashua NH
mjv@objects.mv.com  (UUCP:  ...!decvax!zinn!objects!mjv)