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From: William FRANCK <william.franck@free.fr>
Newsgroups: comp.lang.ada
Date: Mon, 14 Oct 2019 22:58:42 +0200
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Subject: Re: How to transfer Class-Wide object to a Task ?
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Xref: reader01.eternal-september.org comp.lang.ada:57283
Date: 2019-10-14T22:58:42+02:00
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On 2019-10-14 19:58:34 +0000, Dmitry A. Kazakov said:

> On 2019-10-14 21:41, William FRANCK wrote:
> 
>> 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.
> 
> Strange design, why not to pipe streams using a FIFO?
> 
>> 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 ...)
> 
> Yes, if using FIFO, a protected object can be used to signal 
> non-empty/not-full events at the FIFO ends.
> 
> Otherwise, use a reference-counted handle or a plain access type to the 
> target object. The reader task allocates the object in the pool and 
> passes a handle or access to it to the writer. The writer task writes 
> the object and then disposes it.

Thanks you Dimitry for your follow-up.

Here is the multitasking part (simplified) (working, no issue)
for reading the datafile, and writing it back (after some data-process)

My first intention was : while Writing.Bloc is busy writing on the 
output file, Reading.Bloc can take 1 record in advance

Now I have to insert the class-wide object passing in the Bloc.

As You mentionned, should I use a protected type (FIFO)  instead of 2 
// tasks ?

--================
with Ada.Text_io;

procedure main_tasks is

   task reading is
      entry Open;
      entry Bloc;
      entry Stop;
   end reading;

   task  writing is
      entry Create;
      entry Bloc;
      entry Stop;
   end writing;

   task body reading is
   begin
      loop
         select
            accept Open;
               Ada.Text_IO.put_line("Opening file ...");
         or
            accept Bloc do
               Ada.Text_IO.put_line("Reading ...");
            end Bloc;
         or
            accept Stop;
            Ada.Text_IO.put_line("Reading Stopped !");
            exit;
         end select;
      end loop;
   end reading;

   task body writing is
   begin
      loop
      select
         accept Create;
         Ada.Text_IO.put_line("Creating file ...");
         or
            accept Bloc do
               Ada.Text_IO.put_line("Got bloc !");
            end Bloc;
            Ada.Text_IO.put_line("Writing bloc ...");
         or
            accept Stop;
            Ada.Text_IO.put_line("Writing Stopped !");
            exit;
         end select;
      end loop;
   end writing;

begin

   Writing.Create;
   Reading.Open;
   for i in 1..10 loop -- While not end_of_file()
      Reading.Bloc;
      Writing.Bloc;
   end loop;

   Reading.Stop;
   Writing.Stop;

end main_tasks;