Re: Rep Specing Variant Records

["Randy Brukardt" <randy@rrsoftware.com>]

I don't like responding to ancient messages, but...

"Adam Beneschan" <adam@irvine.com> wrote in message 
news:e70b8d9a-3868-4d09-b745-794674ea610a@m3g2000pri.googlegroups.com...
...
>Finally, although it's theoretically possible to have tags in
>different places for two different (non-derived) tagged types, it
>makes implementing the language a lot more difficult.  This is even
>more true now that interface types have been introduced in Ada 2005.
>(If you declare a root type Root_Type with a tag in a nonstandard
>place, and then later "type Child is new Root_Type and Interface_Type
>with...", then how is an operation on Interface_Type'Class going to
>know where to find the tag if you give it a Child?)  So it is entirely
>reasonable for a compiler to insist on the tag being in a set place
>for every tagged type.

This is wildly incorrect and I want to set the record straight. We were very 
concerned about this, since at least two existing Ada 95 compilers (IBM 
Rational and Janus/Ada) supported non-standard tag locations, and we didn't 
want to force those compilers to remove that capability (it seems likely 
that they have customers depending on it). So there are rules specifically 
added to eliminate the extra "difficulty implementing the language".

In particular, the case postulated above is allowed to be rejected by a 
compiler, on the basis of 13.1(13.1/2):
"A type_declaration is illegal if it has one or more progenitors, and a 
representation item applies to an ancestor, and this representation item 
conflicts with the representation of some other ancestor. The cases that 
cause conflicts are implementation defined." Note that in this case it is 
the type declaration of Child that can be illegal, not the representation 
clause on Root_Type. The AARM notes on 13.1(13.1/2) explain this further.

The result is that using non-default locations for tags is no more difficult 
to implement in Ada 2005 as it is in Ada 95.

                               Randy.