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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Task entries and access to subprograms.
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Date: Wed, 6 Apr 2005 14:06:23 +0200
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Date: 2005-04-06T14:02:00+02:00
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On Wed, 06 Apr 2005 12:25:27 +0200, Alex R. Mosteo wrote:

> this is a question about the validity of some approach I'm trying. I 
> know that task entries can't have access parameters. My gnat gap 1.1.0 
> linux behaves like that:
> 
> type AInt is access all Integer;
> type Code is access procedure;
> 
> task type Blah is
>     entry One (I : access Integer); -- Illegal and detected.
> 
>     entry Two (I : AInt); -- No complaint. Legal?
> 
>     entry Three (C : not null access procedure); -- Illegal and detected.
>     --  I have the Ada0Y features enabled.
> 
>     entry Four (C : Code); -- No complaint.
> end Blah;
> 
> I want to execute some arbitrary code inside a task. This is because 
> GtkAda in win32 requires all Gtk calls to be made from the same thread, 
> and I want to have code outside of that thread able to make Gtk calls 
> (for abstraction and future extension purposes I don't want to hardcode 
> all my Gtk calls in that task).
> 
> Apparently the fourth approach works under linux, but I'm worried it may 
> be a bad trick. If this isn't legal I could use a tagged type with a 
> defined  Execute method, so I'd pass instances of derived types of this 
> base class to the task, and execute the dispatching call.
> 
> I'm interested in your oppinion about the second and fourth entries 
> above,

They are fully legal.

> and in other alternatives to the tagged type workaround.

Tagged is also OK:

type Code is abstract tagged ...;
procedure Execute (This : in out Code) is abstract;

task type Blah is
   entry Do_It (X : in out Code'Class);
end Blah;

Note also that tagged object has one big advantage over an
access-to-procedure object. You can pass the parameters for the action to
be executed within the object using additional object's members for that
purpose.

---------
If tasks are permanently assigned to the same code you can use either
generics or access attribute:

task type Blah (What_To_Do : access Code['Class]) is
   entry Do_It;
end Blah;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de