Re: Q: Endless loop by dispatching

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On Fri, 11 Jul 2003 10:22:54 +0200, Michael Erdmann
<michael.erdmann@snafu.de> wrote:

>i got a problem with the following code fragment below. The Idea is that 
>the procedure Serialize defines the strategy how to display object which
>are derived from A.Object. Searialize simply dispatches into the class
>by calling a procedure Write, which does the job for the class.

[...]

>In the following fragment i am defining a package B with an
>derived objec B.Object as below. When an instance of B.Object
>shall be serialized, the element of the super class (A.Object)
>shall be serialzed as well. The code looks nice and compiles
>but does not work, since the Serialize dispatches again with
>Object.B even ther was a a case given  A.Object( This ).

[...]

>Does any body know, what this loop causes?! I am not sure
>if this is a bug or simply i missed the point.

Serialize is class-wide and a view conversion does not change the
tag..I.e. you have a re-dispatching case. Re-dispatching is a
dangerous thing. [I would like it removed from Ada]

So Serialize re-dispatches to B.Write which again calls Serialize.
From design point of view, one should never call class-wide routines
from specific ones, this warranties that dispatch happens only once.

I suppose that you want sort of procedure expansion instead of
complete overriding, if so, then you should explicitly call parent's
Write, similarly as one does it for Initialize/Finalize:

package body B is
...
procedure Write (This : in Object) is
begin
--       A.Serialize( A.Object( This ) );
   A.Write (A.Object (This)); -- Write parent's things
   Put_Line( "Q =" & Natural'Image(This.Q));
end Write;

BTW, why do not you use Ada.Streams? It is for things like this.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de