LISTSERV mailing list manager LISTSERV 16.0

Help for TEAM-ADA Archives


TEAM-ADA Archives

TEAM-ADA Archives


TEAM-ADA@LISTSERV.ACM.ORG


View:

Message:

[

First

|

Previous

|

Next

|

Last

]

By Topic:

[

First

|

Previous

|

Next

|

Last

]

By Author:

[

First

|

Previous

|

Next

|

Last

]

Font:

Proportional Font

LISTSERV Archives

LISTSERV Archives

TEAM-ADA Home

TEAM-ADA Home

TEAM-ADA  March 2000

TEAM-ADA March 2000

Subject:

Re: Access to procedures

From:

Mark Lundquist <[log in to unmask]>

Reply-To:

Mark Lundquist <[log in to unmask]>

Date:

Thu, 2 Mar 2000 12:55:03 -0800

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (246 lines)

From:  Fortier Andres <[log in to unmask]>
Subject:       Access to procedures

> HI,
>    First of all I'm completly new so if I'm doing somethig wrong please =
> tell me.

OK... you're doing something wrong :-) :-)  See below...

For your future reference... [log in to unmask] isn't really the place to
ask technical questions about Ada itself.  It's more of a place to talk
about issues relating to how to get people to use more Ada!  The place
to ask about how to program in Ada is comp.lang.ada.  If you don't have
news access, there are ways to get news via the web or email (I'll
forward you the info separately -- from a post in this very mailing list
:-).

But I'll still reply regarding your problem and how to solve it... :-)

> I'm working in something we may call a "framework".
> The idea is to made a platform which can be used in many different =
> applications. This is based in the idea of passing procedures (or access =
> to procedures) as parameters.
>
> Now here are the problems, let's suppose this package:
>
> ADS:
>
> Package ProcTry is
>
>     type PtrProc is access procedure;
>
>     Procedure TeEjecuto (Proc : PtrProc);
>
> end ProcTry;
>
> ADB:
>
> Package body ProcTry is
>
> Procedure TeEjecuto (Proc : PtrProc) is
> begin
>   Proc.all;
> end;
>
> end ProcTry;
>
> MAIN.ADB
>
> With Text_IO; Use Text_IO;
> With ProcTry; Use ProcTry;
>
> procedure main is
>
> procedure KBHandler is
> begin
>   Put_Line("HI");
> end KBHandler;
>
> C : Character;
>
> begin
>    TeEjecuto(KBHandler'Access);
>    Get_Immediate(C);
>    While (C/='A') loop
>      TeEjecuto(KBHandler'Access);
>      Get_Immediate(C);
>    end loop;
> end main;
>
> When I compile this, I get this:
>
> TeEjecuto(KBHandler'Access);
>                 |
>                 |
>  Subprogram must not be deeper than access type
>
> What does this means??

OK, access-to-subprogram types and parameters have a rule in Ada that
makes them unsuitable for general "callback"-style use.  This rule is
called the accessibility rule, and you can read about it in RM 3.10.2.
The problem that Ada has with your code is that there is no way for the
declaration of TeEjecuto to guarantee that the body of TeEjecuto is not
going to take the value of its Proc parameter and save it away (in some
more global variable) for later use.  What could be wrong with that?
Well, suppose you had this:

    procedure Fortune (Message: String)
    is

        procedure Print is
        begin
            Text_Io.Put_Line ("Your cookie says: " & Message);
        end;

    begin
        Proc_Try.TeEjecuto (Print'Access);
    end Fortune;

Now we have trouble if TeEjecuto does something to make the value of
parameter Proc outlive Fortune itself (like add it to a list, or
something).  Then if something (like somewhere else in ProcTry) tries
later to make a call through this access value, it doesn't make sense,
because procedure Print's code to access Fortune's Message paramter is
not going to be valid when Fortune is no longer active.

There are at least three ways to get around this:

1) 'Unrestricted_Access

    If you're using GNAT, you can use 'Unrestricted_Access instead of
    'Access.  This is how you tell the compiler "Trust me, I really am
    not going to do anything bad".  But be aware that (a) you had really
    better not do anything bad, because you'll get what you asked for,
    and (b) 'Unrestricted_Access is a GNAT-specific attribute, so in
    general it isn't portable to other compilers.

2) Use a generic

    You already figured this one out.  It works because it's the
    instantiation, not the generic, that has accessibility level.
    So you instantiate it at the same level as your access type, and
    everything is happy.

3) Use dispatching instead of access-to-subprogram

    You can do something like this:

        package ProcTry is

            type Effector is abstract tagged private;

            -- A primitive operation (overridable, and must be
            -- overridden since Effector is abstract)...
            --
            procedure Action (Using : Effector) is abstract;

            -- A classwide operation
            --
            procedure TeEjecuto (This: access Effector'Class);

        private
            type Effector is abstract tagged null record;
        end ProcTry;


        package body ProcTry is

            procedure TeEjecuto (This: access Effector'Class) is
            begin
                Action (Using => This);  -- A dispatching call
            end TeEjecuto;

        end ProcTry;

    Now, you can do:

        With Text_IO; Use Text_IO;
        With ProcTry; Use ProcTry;

        procedure main is

            -- Derive a new Effector type.  (We could also add a record
            -- extension if we wanted...)
            --
            type KBHandler is new ProcTry.Effector;

            -- Override Action...
            --
            procedure Action (Using: KBHandler) is
            begin
              Put_Line("HI");
            end Action;

            C : Character;

        begin
           TeEjecuto(KBHandler'Access);
           Get_Immediate(C);
           While (C /= 'A') loop
             TeEjecuto(KBHandler'Access);
             Get_Immediate(C);
           end loop;
        end main;

    This way has hardly any more code to write, and the body of your
    Main is identical to what you had before.

> Now if I change de definition of the access to private:
>
> Package ProcTry is
>
>     type PtrProc is Private;
>     Procedure TeEjecuto (Proc : PtrProc);
> Private
>     type PtrProc is access procedure;
> end ProcTry;
>
> This is what I get:
>
> TeEjecuto(KBHandler'Access);
>                 |
>                 |
> Expected private type "PtrProc" defined at proctry:4
> Found type access to "KBHandler" defined at line 14
>
> How do I "tell the compiler" that KBHandler is actually of PtrProc type =
> ???

That question doesn't make sense.  But as you discovered, when you make
PtrProc private, you can't pass KBHandler'Access as a value of PtrProc,
any more than you can pass the number 5 or the string "Foobar"! :-)
Anyway, the publicness of PtrProc isn't the problem, it's the
accessibility level.

Have fun!
Mark

P.S. You have this code...

       TeEjecuto(KBHandler'Access);
       Get_Immediate(C);
       While (C /= 'A') loop
         TeEjecuto(KBHandler'Access);
         Get_Immediate(C);
       end loop;

     Here's a nicer way to write it:

       loop
         TeEjecuto(KBHandler'Access);
         Get_Immediate(C);
         exit when C /= 'A';
       end loop;


--

Mark Lundquist
Senior Software Engineer
Rational Software
Development Solutions Business Unit
UNIX Suites Group
Aloha, OR, USA

Top of Message | Previous Page | Permalink

Advanced Options


Options

Log In

Log In

Get Password

Get Password


Search Archives

Search Archives


Subscribe or Unsubscribe

Subscribe or Unsubscribe


Archives

December 2017
November 2017
October 2017
September 2017
June 2017
May 2017
April 2017
January 2017
December 2016
November 2016
October 2016
August 2016
July 2016
June 2016
May 2016
April 2016
March 2016
February 2016
January 2016
December 2015
November 2015
October 2015
September 2015
August 2015
June 2015
May 2015
April 2015
March 2015
February 2015
January 2015
December 2014
November 2014
September 2014
August 2014
July 2014
June 2014
May 2014
April 2014
March 2014
February 2014
January 2014
December 2013
November 2013
October 2013
September 2013
June 2013
May 2013
April 2013
March 2013
February 2013
January 2013
December 2012
November 2012
October 2012
September 2012
August 2012
July 2012
June 2012
May 2012
April 2012
February 2012
January 2012
December 2011
November 2011
October 2011
September 2011
August 2011
July 2011
June 2011
May 2011
April 2011
December 2010
November 2010
October 2010
September 2010
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
December 2009
November 2009
October 2009
September 2009
August 2009
July 2009
June 2009
May 2009
April 2009
February 2009
January 2009
December 2008
November 2008
October 2008
September 2008
August 2008
June 2008
May 2008
April 2008
March 2008
February 2008
January 2008
December 2007
November 2007
October 2007
September 2007
June 2007
May 2007
March 2007
February 2007
December 2006
November 2006
October 2006
September 2006
August 2006
July 2006
June 2006
May 2006
April 2006
March 2006
February 2006
January 2006
December 2005
November 2005
October 2005
August 2005
July 2005
June 2005
May 2005
April 2005
March 2005
February 2005
January 2005
December 2004
November 2004
October 2004
September 2004
August 2004
July 2004
June 2004
May 2004
March 2004
February 2004
January 2004
December 2003
November 2003
October 2003
September 2003
August 2003
July 2003
June 2003
May 2003
April 2003
March 2003
February 2003
January 2003
December 2002
November 2002
October 2002
September 2002
August 2002
July 2002
June 2002
May 2002
April 2002
March 2002
February 2002
December 2001
November 2001
October 2001
September 2001
August 2001
July 2001
June 2001
May 2001
April 2001
March 2001
February 2001
January 2001
December 2000
November 2000
October 2000
September 2000
August 2000
July 2000
June 2000
May 2000
April 2000
March 2000
February 2000
January 2000
December 1999
November 1999
October 1999
September 1999
August 1999
July 1999
June 1999
May 1999
April 1999
March 1999
February 1999
January 1999
December 1998
November 1998
October 1998
September 1998
August 1998
July 1998
June 1998
May 1998
April 1998
March 1998
February 1998
January 1998
December 1997
November 1997
October 1997
September 1997
August 1997
July 1997
June 1997
May 1997
April 1997
March 1997
February 1997
January 1997
December 1996
November 1996
October 1996

ATOM RSS1 RSS2



LISTSERV.ACM.ORG

Secured by F-Secure Anti-Virus CataList Email List Search Powered by the LISTSERV Email List Manager