RE: Exchanging objects between programs with different inheritance hierarchies

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

> > Steven Deller wrote:
> >...  Using unchecked_conversion depends
> > > on the tags being integral to the
> > > record structures.  On GNAT they are not, so the code fails.
> 
> Is this really true?  I thought all Ada compilers store the 
> tag field as a field in the record (although not always at 
> the same offset)?  What is GNAT doing that messes up your 
> unchecked conversions?
> 
> Maybe I'm missing your point.
> 
> - Bob

I looked at the generated code.  For the tag comparison it was
generating 3 dereferences for an access type and 2 dereferences for an
object.  It appears the single tag word in the record is actually a
pointer to structure with a pointer to the actual value.  In different
programs, those pointers are to different locations, so just uchecked
converting to the unchecked, saved 32-bit value in the original record
does not work.

For Apex, the tag comparison was a simple value comparison and, since it
seemed to work across programs, apparently that value is not a pointer.

Regards,
Steve