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From: William FRANCK <william.franck@free.fr>
Newsgroups: comp.lang.ada
Date: Mon, 14 Oct 2019 22:21:48 +0200
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Subject: Re: How to transfer Class-Wide object to a Task ?
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Date: 2019-10-14T22:21:48+02:00
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On 2019-10-14 19:41:53 +0000, William FRANCK said:

> Hey all !
> Here is a nice issue I have with Ada (GNAT 2012) when trying to do OO 
> dispatching with streams in different tasks ...
> 
> Here it is :
> I'd like to get from a task RdV (Entry-Access) an object which could be 
> any subclass of a root'Class, and pass it to another task
> 
> Context : read (Root'Class'Input() ) tagged records from an input 
> stream, and send them to anther task which will write 
> (Root'Class'Output() ) the given records to another output stream.
> 
> I'm stuck with task memory isolation with does NOT allow to pass any 
> access object to a Root'Class.
> 
> Should I try to use a protected object ?
> (not shore this solves the passing of a Class-wide object ...)
> 
> Thanks for your feed-back,
> William

Here is some code to illustrate
(code needs soem fixings)

with
   Ada.Text_io;

procedure main_ClassWide_to_2Task is

   type Geo2D is tagged record
      Y, X : integer;
      Name : String(1..6) := "Object";
   end record;

   type Circle is new Geo2D with record
      Radius : integer;
   end record;

   myObject : access Geo2D'Class;

   -- ==============================

   task reading is
      entry Object(This : in out Geo2D'Class);
      entry Stop;
   end reading;

   task writing is
      entry Object(This : in Geo2D'Class);
      entry Stop;
   end writing;

   -- ==============================
   task body reading is
      O2D : access Geo2D'Class;
   begin
      loop
         select
            accept Object(This : in out Geo2D'Class);
            declare
               -- (shortcut to simulate real case) ::  Object : 
constant Geo2D'Class := Dispatching_Input (Ada.Tags.Internal_Tag 
(External_Tag), Stream);
            begin
               O2D := new Circle'(0,0,"Cercle",10);
               Ada.Text_io.Put_line("Reading 2D Object: " & O2D.Name); 
-- debug trace
  --             This.all := O2D.all; --Compiler error : "This" is undefined
            end;
         or
            accept Stop;
            exit;
         end select;
      end loop;
   end reading;

   -- ==============================
   task body writing is
      myObject : access Geo2D'Class;
   begin
      loop
         select
            accept Object(This : in Geo2D'Class) do
               myObject := This'Access;
            end Object;
            Ada.Text_io.Put_line("Writing 2D Object: " & This.Name); -- 
debug trace
         or
            accept Stop;
            exit;
         end select;
      end loop;
   end writing;

 -- ==============================
begin
   reading.Object(myObject.all);
   writing.Object(myObject.all);

   reading.Stop;
   writing.Stop;


end main_ClassWide_to_2Task;