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From: Anton Gibbs <agibbs@dera.gov.uk>
Subject: Re: Subverting 'Access for Sub-programs
Date: 1999/08/04
Message-ID: <37A7FDE8.4F5@dera.gov.uk>#1/1
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Newsgroups: comp.lang.ada
Date: 1999-08-04T00:00:00+00:00
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Dear Ada Community,

Thank you, everyone, for all the very helpful responses to my question
on 'Access.

Unfortunately, in my eagerness to provide a simplified statement of my
problem, I had omitted an important detail: in fact, the procedure which
I called Main is really not the main program and, more importantly, it
has a parameter which I need to access.

What I mean is:-

package Database is
   type Action_Type is access procedure( I : in Integer );
   -- Perform calls Action for every entry in the database
   procedure Perform( Action : in Action_Type );
end Database;

with Database;
with Text_IO;
package body Something is
   procedure Not_Main( F : in Integer ) is
      procedure Print_If_Multiple( I : in Integer ) is
      begin
         if I rem F = 0 then
            Text_IO.Put_Line( Integer'Image( I ) );
         end if;
      end Print_If_Multiple;
   begin
      Database.Perform( Action => Print_If_Multiple'Access );
   end Not_Main;
end Something;

Here, the problem is retaining visibility of the parameter F within the
procedure whose 'Access I am trying to pass to the Perform operation. If
I move Print_If_Multiple out to its own package (ie. out of the scope of
Not_Main) then the only way I can see F is by using global data. I know
this will work but I am reluctant to adopt this approach because I have
often found that non-re-entrant code bites back in years to come when it
gets re-used (usually by me) in a way the author never originally
intended.

I am attracted by Robert Dewar's 'Unrestricted_Access: it is obviously
there for a reason. Thank you too for the candid explanation of why
'Unchecked_Access is prohibited on sub-programs. As someone who is often
baffled by some of the language restrictions it is nice to get an
explanation one can understand. I just wish I could understand the Ada95
LRM as well as I could the Ada83 one.

Many thanks everyone.

Regards

Anton Gibbs.
Software Engineer

-- 
Civil Air Traffic Management Group
Defence Evaluation and Research Agency
Bedford, UK

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