RE: Exchanging objects between programs with different inheritance hierarchies

["Steven Deller" <deller@smsail.com>]

> -----Original Message-----
> From: comp.lang.ada-admin@ada.eu.org
> [mailto:comp.lang.ada-admin@ada.eu.org] On Behalf Of Grein, Christoph
> You should have a look at the attribute 'External_Tag. ...

Christoph,
Thanks.  As I thought about the problem last night without an RM handy,
I wondered if one could rep spec the external tag.  Lo and behold, one
can!!!  Just rep spec the derived types, use 'Class'Output/Input on the
"prime" type and we are cooking :-).  Don't even have to override the
default type outputs as long as the same compiler is used on machines
with the same endian-ness.

Love that Ada.

One other question.  Suppose I have
   type Top is tagged ...  -- our "prime" type
   type X <derived from Top>
   type A_Top is access Top'Class ; -- Only allocated objects are used
   type A_X is access X ;

   Obj : A_Top ;

   X : A_X ;

   Obj := function_returning_A_top ;

   X := A_X ( Obj) ;  -- This is allowed, right?

Does the conversion of an A_Top class access type to a specific A_X
access type have any code associated with it, besides a tag check?
(That is, in general does an Ada compiler have to do anything except
check the tag and copy the "pointer"?)  A specific answer for GNAT would
suffice.

I'm guessing there is nothing except the tag check, since "all" was not
used for defining the access types, but I feel a little like I'm
tiptoeing through land mines here (I'd prefer tulips :-) ).

Oh -- does C++ have anything similar for easy exchange of different
class objects across a single stream (without knowing ahead of time
which object is to be read)?  If not, this is another big win for Ada
:-).

Regards,
Steve