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From: Georg Bauhaus <sb463ba@d2-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: automatic tag conversion?
Date: Mon, 26 May 2003 18:16:05 +0000 (UTC)
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Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
:> The system would always dispatch the most special case, am I right?
: 
: No, Element is of NodeClassAccess which is different from anonymous "access 
: Node" (that will dispatch). Thus LookProcedure.all is not dispatching at 
: all. However it will be sort of "class-wide" when NodeClassAccess is 
: class-wide, as it is. So you practically have everything to make it 
: working. Just define a wrapper in the same package, where Node is defined:
: 
: procedure Print_Any (Ptr : NodeClassAccess) is
: begin
:   Print (Ptr); -- Dispatches
: end Print_Any;

Yes. I've made something similar, using a slightly modified set with
a generic "look" function,

    generic
       with procedure accessor(e: in Element);
    procedure look(s: in Set);

(and also deriving Node from a Printable abstract tagged type,

   type Printable is abstract tagged private;
   procedure print(p: access Printable) is abstract;
)
Maybe both the Booch components and the Charles library
might provide some ideas (in addition to code :)
of how iteration can be done.

The slight change results in

with Sets;
with nodes; use nodes;

procedure run is  -- testing

   package Node_Sets is new Sets(Element => Node_ptr);

   net: Node_sets.Set;

   procedure print_any(n: Node_ptr) is
   begin
      print(n.all'access);
   end print_any;

   procedure print_all is
      new node_sets.look(accessor => print_any);

begin
   node_sets.insert(new Transition, net);
   node_sets.insert(new Place, net);
   node_sets.insert(new Transition, net);
   node_sets.insert(new Transition, net);
   print_all(net);
end run;

$ ./run
a transition
a transition
a place
a transition
$


Georg