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Subject: Re: Some help for a C++ guy
Date: 8 Apr 2005 16:03:22 -0700
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tmo...@acm.org wrote:
> >That's a problem because that callback subroutine needs to be passed
to
> >a function expecting some of the type BC_Completion_Handler_T,
defined
> >as:
> >
> >   type BC_Completion_Handler_T is access
> >      procedure (Status : in IO_Status_T);
> >
> >Changing the that type would then break a bunch of other code.
>
> So you need to specify for a callback the particular Message_* that
> it pertains to?

Correct. Right now the very simple callback just increments a counter.
That way I can see how times a particlar message ran.

> Just make a set of wrapper routines:
>     procedure Message_0_Completion_Handler(Status : in IO_Status_T)
is
>     begin
>       Completion_Handler(Message(0), Status);
>     end Message_0_Completion_Handler;
> etc.
>
> and register the specific Message_*_Completion_Handler procedures.
> I presume this registration is done once so even if you add a new
> Message_237_Completion_Handler it's a minor addition to a single
package.

Plus an additional blurp of code to register that callback. What I
wanted to be able to do was have all the registration stuff in a loop.

So rather than a whole bunch of

       Stat := M1553.Register_Message_Callback(msg_id,
                Message_0_Handler.Message_Callback'access);

       if Stat /= M1553.No_Errors
          then
            GNAT.IO.Put_Line("Error Register_Message_Callback");
          end if;

Blocks, one for each message (and similar ones for the two buffers that
have to be registered), I'd have:

   for i in Handlers'range loop
       Stat := M1553.Register_Message_Callback(msg_id,
                Handlers(i).Message_Callback'access);

       if Stat /= M1553.No_Errors
          then
            GNAT.IO.Put_Line("Error Register_Message_Callback");
          end if;
   end loop;

I'd have been perfectly happy with a way to create an array of those
generic package instantiations.

> I'm presuming you want to make small changes to the existing code.
> If you want to use tagged types you are contemplating larger
architectural
> changes.

I don't want to break existing code.

> In your current code, "registering a callback" means essentially
> "note a particular routine, and a particular Message_n's data for
future
> use".

It means pass these to already written registration routines through an
established API.

> Passing an explicit tagged Handler parameter accomplishes the
> "note a particular Message_n's data" part.  If the particular routine
> is not in fact dynamic, but always BC_Completion_Handler, then
instead
> of registering the procedure callback you want to register the
Message_n
> data and always call BC_Completion_Handler(Registered_Message,
Status);

Well, there will be a specific callback function dedicated the each
message. For right now, as I said above, it just tracks how many times
the message ran. Later, they'd be expanded to do more things.

So each message needs to have a distinct data buffer, status buffer,
and callback subroutine.

This is tricky in C++ as well. Usually you go for callback objects or
functors rather than trying to pass a class member function.


Brian