Re: Gem 39 - compiler specific?

[Adam Beneschan <adam@irvine.com>]

On Sep 3, 9:38 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Thu, 3 Sep 2009 08:26:50 -0700 (PDT), Adam Beneschan wrote:
> > On Sep 3, 12:26 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> > wrote:
> >> 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))
>
> But Acc_T is a pool-specific access, while Address_To_Access_Conversions
> creates a general access type.

Sigh... I missed that---you can't convert a general access type to a
pool-specific one.  4.6 can be hard to follow, the way it's organized,
but that's not a good excuse.  I've read it enough times that I should
know better.

However, one could make that case that if you declare a pool-specific
access type, then by definition it should point to something that's in
a pool; and that if you want an access that could point to an
arbitrary address (as Access_To_Address_Conversions would let you do),
you should use a general access type.

                                    -- Adam