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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Idiom for a class and an object in Ada
Date: 20 Oct 2004 20:37:42 +0100
Organization: Pushface
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Date: 2004-10-20T20:37:42+01:00
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"Matthew Heaney" <mheaney@on2.com> writes:

> "Simon Wright" <simon@pushface.org> wrote in message 
> news:x7v4qkqrlr1.fsf@smaug.pushface.org...
> >
> > package body ADC is
> >
> >   task type T is ...
> >   type T_P is access T;
> >   Converter_1 : T_P;
> >
> > begin
> >
> >   Converter_1 := new T (...);
> >
> > end ADC;
> 
> This doesn't buy you anything.  The rule is that the "task object" 
> elaborates in linear order (the same as for any other declaration), but then 
> the "task" itself "activates" when the begin statement is reached.
> 
> We can write your example as:
> 
> package body ADC is
>    task type T;
> 
>    O : T;  -- this is the "task object"
> 
>    task body T is ... end;
> 
> end;
> 
> No explicit allocation is necessary.  Task object O elaborates in the normal 
> way, and its associated task activates at the completion of elaboration of 
> the package body.

And this is exactly why you end up with elaboration order problems!

package A is
   procedure P;
end A;
with B;
package body A is
   procedure P is
   begin
      null;
   end P;
end A;
package B is
   procedure Q;
end B;
with A;
package body B is
   task type T is
      entry Start;
   end T;
   O : T;
   task body T is
   begin
      accept Start;
      A.P;
   end T;
   procedure Q is
   begin
      null;
   end Q;
end B;
with A;
procedure M is
begin
   null;
end M;

smaug.pushface.org[7]$ gnatmake m
gcc -c m.adb
gcc -c a.adb
gcc -c b.adb
gnatbind -x m.ali
error: elaboration circularity detected
info:    "b (body)" must be elaborated before "b (body)"
info:       reason: implicit Elaborate_All in unit "b (body)"
info:       recompile "b (body)" with -gnatwl for full details
info:          "b (body)"
info:             must be elaborated along with its spec:
info:          "b (spec)"
info:             which is withed by:
info:          "a (body)"
info:             which must be elaborated along with its spec:
info:          "a (spec)"
info:             which is withed by:
info:          "b (body)"

gnatmake: *** bind failed.

-- 
Simon Wright                               100% Ada, no bugs.