Re: Memory Access

[Adam Beneschan <adam@irvine.com>]

On Nov 8, 9:46 am, awdorrin <awdor...@gmail.com> wrote:
>
> Now I just have to try to understand what is actually going on with
> those commands.
>
> I'm assuming that the subtype is setting the bounds on the array. so
> that the Dev_Table_Ptr_Type is now able to see the elements.

Something like that.  The subtype declaration says that every object
of that subtype has those bounds.  This also means that for any object
X of type Dev_Table_Ptr_Type, X.all has those bounds too.  The reason
that's important is because the bounds don't have to be stored
somewhere else.  If you have an access-to-an-unconstrained array, then
for any object X with that access type, the bounds have to be stored
somewhere along with X.  The problem is that it's left up to the
implementation just how to store those bounds.  It looks like AdaMulti
makes X a three-word object containing a data address, the lower
bound, and the upper bound; while GNAT stores them differently.


> The Dev_Table_Pointer.To_Pointer is mapping the address to an access
> type.
> I'm not quite sure I understand why the package definition for
> Dev_Table_Pointer is needed - and why
> System.Address_To_Access_Conversions cannot be used directly... i'm
> guessing its due to Ada type checking?

System.Address_To_Access_Conversions is a generic package.  You can't
use a generic package directly; you have to instantiate it.  In this
case, you have to instantiate it with a type.  And yes, I think that
this needs to be a generic because of type checking; Ada doesn't have
the equivalent of a (void *) that you can typecast willy-nilly between
other pointer types and screw yourself up royally.

                        -- Adam