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"Team Ada: Ada Advocacy Issues (83 & 95)" <[log in to unmask]>
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Fortier Andres <[log in to unmask]>
Date:
Thu, 2 Mar 2000 12:55:03 -0800
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Mark Lundquist <[log in to unmask]>
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Mark Lundquist <[log in to unmask]>
In-Reply-To:
Message from Fortier Andres <[log in to unmask]> of "Wed, 01 Mar 2000 12:29:04 -0300." <002a01bf8393$7545c0c0$ddc5f4d8@andrs>
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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

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