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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Q: access to subprogram
Date: 1996/07/03
Message-ID: <Dtzro3.CKn@world.std.com>#1/1
X-Deja-AN: 163632547
references: <Dtxu2p.97F@world.std.com> <4rd5lu$q4@mulga.cs.mu.OZ.AU>
 <DtytvM.Mw2@world.std.com> <4re8bj$qfk@krusty.irvine.com>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-07-03T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <4re8bj$qfk@krusty.irvine.com>,
Adam Beneschan <adam@irvine.com> wrote:
>It should be noted that the new object-oriented features of Ada 95
>should make subprogram accesses less necessary than before.  Abstract
>types inherently possess subprogram pointers, so it should be possible
>to take advantage of this feature as an alternative to explicit
>subprogram accesses.

Quite right.  The only problem is that it's a bit verbose.

>    procedure P1 is
>        
>        type My_Error_Handler is new Error_Handler with record
>            Error_File : Text_IO.File_Type;
>        end record;

You need to put:

        procedure Handle_Error (Handler : in My_Error_Handler;
                                Message : in string);

here, to avoid freezing rules rearing their ugly head.

>        EH : My_Error_Handler;
>                -- variable name chosen to avoid being fired  :)
>        
>        procedure Handle_Error (Handler : in My_Error_Handler;
>                                Message : in string) is 
>        begin
>            Text_IO.Put_Line (Handler.Error_File, Message);
>        end Handle_Error;
>
>    begin
>        Text_IO.Create (EH.Error_File, ...
>        ...
>        Pack1.Do_Some_Big_Procedure_That_Can_Report_Errors (EH);
>
>This mechanism doesn't let you *directly* store an access to a
>subprogram in a global variable, the way subprogram accesses do; but
>using accesses to Error_Handler objects should let you accomplish the
>same thing.

Right.  Quite often, if you think you want a pointer to a procedure, a
class-wide object is better.

>Once again, my apologies if my Ada 95 syntax is incorrect.  I haven't
>yet learned all the nuances of Ada 95 programming.  Mainly, I wanted
>to discuss the concepts involved; someone else may need to fill in the
>programming details.

You've got the concepts quite right.  No need to apologize for violating
a freezing rule.  ;-)

My only objection to this technique is that it's a lot of verbiage for a
pretty trivial thing.

- Bob