Re: Self-referential types

[Ted Dennison <dennison@telepath.com>]

In article <3802597B.9205AEE8@averstar.com>,
  Tucker Taft <stt@averstar.com> wrote:
> Ted Dennison wrote:
> >
> > Below is a code sample that is taken directly from Cohen's AAASL,
> > section 11.8.3:
> >
> > procedure Test is
> >
> >    type Cell_Type;
> >
> >    type List_Type is access all Cell_Type;
> >
> >    type Cell_Type is
> >      limited record
> >         Contents_Part      : String (1..20);
> >         Next_Cell_Part     : List_Type := Cell_Type'Access;
> >         Previous_Cell_Part : List_Type := Cell_Type'Access;
> >      end record;

> invokes the various accessibility rules.  In particular, when
> inside the definition of a type, <type_name>'Access is presumed
> to have an accessibility level deeper than that of the enclosing type,
> meaning it is also deeper than List_Type.  Note that there is
> nothing preventing the values of next and previous being copied
> into global variables, whereas you might have a local object of type
> Cell_Type.
OK. After masticating on this parargraph for a while, I think what you
are saying is that I theoreticly could do this:

   Global : List_Type;
   procedure Local is
      Cell : Cell_Type;
   begin
      Global := Cell.Next_Cell_Part;
   end Local;

..and then after a call to Local, I now have in Global a pointer into
unallocated stack space.

I think what was confusing me was that I'm used to thinking about
accessability levels in record *objects*, not types. But I can't speak
for what was confusing Cohen. :-)

> To avoid the use of "'Unchecked_Access" you could simply default
> initialize to "null,"  You could define a separate function to do the

..or use a second null record with an access discriminant, as I have
seen done elsewhere.

--
T.E.D.


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