Re: Gem 39 - compiler specific?

[Adam Beneschan <adam@irvine.com>]

On Sep 3, 12:26 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Wed, 2 Sep 2009 18:20:36 -0500, Randy Brukardt wrote:
> > "Maciej Sobczak" <see.my.homep...@gmail.com> wrote in message
> >news:9e0bbbcd-260f-48ed-8043-d6280c633e85@h3g2000yqa.googlegroups.com...
> >> Consider:
>
> >>http://www.adacore.com/2008/06/09/gem-39/
>
> >> The example code performs Unchecked_Conversion between addresses of
> >> two different array types.
> >> As far as I understand, there is no standard provision for arrays to
> >> occupy contiguous memory space (and not even for the alignment of
> >> their components) and such a conversion relies heavily on the
> >> assumption that arrays can be "overlaid" by plain address
> >> reinterpretation.
>
> >> Still, this technique is quite attractive. What is the chance (in
> >> practice) to hit the compiler that does not get it "right"?
>
> > This is horrible code; it should never be written in Ada 95 or newer! The
> > problem is that System.Address is not necessarily the same as a general
> > access type, so this Unchecked_Conversion may not work (or may not even
> > compile) on another implementation. (It won't on at least some versions of
> > Janus/Ada, for instance.)
>
> > Ada 95 provides System.Address_to_Access_Conversions for this purpose, so at
> > the very least the writer of the Rock (it surely isn't a "Gem"!!!) should
> > have used it:
>
> > procedure Write_Buffer
> >       (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
> >        Item   : in Buffer)  is
> >       Item_Size : constant Stream_Element_Offset :=
> >                      Buffer'Object_Size / Stream_Element'Size;
>
> >      subtype SEA is Stream_Element_Array (1..Item_Size);
>
> >      package A2A is new System.Address_to_Access_Conversions (SEA);
>
> > begin
> >       Ada.Streams.Write (Stream.all, A2A.To_Pointer (Item'Address).all);
> > end Write_Buffer;
>
> > (As an aside, this technique could not work in Janus/Ada if Buffer was an
> > unconstrained array: 'Address of an unconstrained array points at the array
> > descriptor, not the data.
>
> It must point to the first array element 13.3(14). BTW, it is interesting
> where it should point when the array is empty.
>
> > (That's in fact true for all objects in Janus/Ada:
> > there is a static part and an optional dynamic part, and all operations like
> > 'Address and 'Size point at the static part. That was decided long before
> > Ada 95 came around and tried to change these definitions; it's not practical
> > to change in Janus/Ada as address clauses could not be made to work - among
> > other things - if the static part is ignored.)
>
> Skewed address is not a problem here. The problem is that
> System.Address_To_Access_Conversions should take an access type as a
> parameter rather than declare it new. This makes its use quite limited.

I don't see any limitation, at least where To_Pointer is concerned
(which is what the above code needed to use).  If you declare an
instance of this package for any object type T:

  type Convert_T_Access is new System.Address_To_Access_Conversions
(T);

then an object of type Convert_T_Access.Object_Pointer can be
converted to any access type whose designated type is T.  I checked
through the legality rules in 4.6, and I couldn't find a way to
declare an access-to-T type that you couldn't convert an
Object_Pointer to.  Thus, if you have any access type

  type Acc_T is access T;  [or access all T, or access constant T]

you can convert an address Addr to an Acc_T by saying:

  Acc_T (Convert_T_Access.To_Pointer (Addr))

There do seem to be some limitations on using To_Address, though; you
can't convert an "access constant T" to an Object_Pointer, and you
can't convert an access-to-T type that is nested inside a subprogram.
So you couldn't use To_Address on those types.  Offhand, though, I'd
guess that there is much less need for this than for To_Pointer, so
this may not be a severe enough limitation to require fixing.

                              -- Adam