Re: Accessibility levels, continued

[Robert A Duff <bobduff@world.std.com>]

adam@irvine.com writes:

> Should this program raise an exception?

It's illegal (ie compile-time error).  ;-)

(OK, sorry for being a smart-aleck.  If you fix the compile-time error,
it should not raise an exception.)

>     procedure Test is
> 
>         type T is record
>             Field : Integer;
>         end record;
> 
>         Var : T;
> 
>         type Access_T is access T;
>         Acc_Var : Access_T;
> 
>         procedure Outer (Outer_Param : access T) is
> 
>             procedure Inner (Inner_Param : access T) is
>             begin
>                 Acc_Var := Inner_Param.all'access; -- [1]
>             end Inner;
> 
>         begin
>             Inner (Outer_Param);
>         end Outer;
> 
>     begin
>         Outer (Var'access);                        -- [2]
>     end Test;
> 
> It doesn't look like it should, if you follow the intent of the
> accessibility rules in 3.10.2.  Acc_Var will be assigned to an access
> to Var, and since Acc_Var's type does not have a deeper accessibility
> level than Var, everything should be fine.

Right.

>...However, I believe that
> the way the RM is written, the program should raise Program_Error at
> line [1].  Here's what seems to be going on:
> 
> 3.10.2(29): 'ACCESS will succeed as long as the accessibility level of
> Inner_Param.all is not deeper than that of Access_T.
> 
> 3.10.2(15): The accessibility level of Inner_Param.all equals that of
> Inner_Param's type, which is an anonymous access type.
> 
> 3.10.2(13): The accessibility level of Inner_Param's anonymous access
> type is the same as that of the view designated by the actual, in this
> case Outer_Param.

So far, so good.

> I don't know the exact meaning of the "view designated by the actual"
> when the actual is another parameter.  I haven't found any reason to
> believe that the last clause of the above paragraph is different from,
> more simply, "the accessibility level of Outer_Param".

"Designated" means the thing it points to.  It's not the same thing as
"denoted".  The actual is Outer_Param; the designated object is
Outer_Param.all.  So you then invoke the same paragraphs you mentioned
above, again.  So you end up with the accessibility level of Var, which
is the *same* as Access_T, so all is well.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.