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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Subject: Re: Exchanging objects between programs with different inheritance
 hierarchies
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"Steven Deller" <deller@smsail.com> writes:

>    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?

No, to convert to an access type, it has to be a general access type
("access all", usually).  (I'm ignoring the uninteresting case of
derived access types.)

> 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 can't think of any reason why a compiler would have to generate any
code other than the Tag check.  An access value is usually represented
as a machine address, and converting a machine address to a machine
address is easy.  ;-)

> 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 :-) ).

I don't understand what you're getting at here.  In most compilers,
"all" has no effect on the representation of access types.

Anyway, even on compilers that do funny things with *some* access types,
I suspect that a whole bunch of "access all" types pointing to various
places in a tagged type hierarchy will all be represented the same
(whether class-wide or not).

- Bob