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From: Philippe Tarroux <philippe.tarroux@limsi.fr>
Newsgroups: comp.lang.ada
Subject: Re: Interfaces and private types
Date: Tue, 29 Jan 2008 10:35:40 +0100
Organization: University Paris-Sud, France.
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Date: 2008-01-29T10:35:40+01:00
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Randy Brukardt wrote:
> Specifically, it violates 7.3(7.3/2): "the partial view shall be a
> descendant of an interface type (see 3.9.4) if and only if the full type is
> a descendant of the interface type."
>   
> As written, this is illegal because type T does not have the interface 
> Int.
As it is written the compiler i use doesn't mention any error and i 
interpreted this construct as legal because :

1/ the interface is synchronized thus allowing to derive concurrent or 
non concurrent types
2/ The partial view is a descendant of the interface type
3/ The full view precises that the partial view correspond to a 
concurrent type but hides this detail to the user

Writing, as you propose

task type T is new Int with private;

tells the user that T is a concurrent type, what precisely i wanted to 
avoid.
> Assuming that this is just a mistake in your interface and you meant
>
>    task type T is new Int with
>
> then your program is legal, and you should complain your your compiler
> vendor about the bug. In no case should there be an error on the declaration
> of the object, so that makes it pretty clear that there is a compiler bug.
>
> You could work around the bug with something like:
>
> package Test_Interfaces is
>
>    type Int is synchronized interface;
>    procedure Init (I : in out Int) is abstract;
>
>    type T is new Int with private;
>    overriding procedure Init (I : in out T);
>
> private
>
>    task type T is new Int with
>       entry Init;
>    end T;
> end Test_Interfaces;
>
> and then have
>     procedure Init (I : in out T) is
>     begin
>           I.Init;
>     end Init;
> in the body. (But note that this runs into a known bug in the Ada standard,
> so it isn't clear that the code will work right.)
>   
But the overriding procedure Init needs to be redefined both as an entry 
of T and as a procedure. I assume that when i will be trying to call 
O.Init with O a type T task there will be a mistake due to the double 
definition.

But, anyway, thanks a lot for the comments. I am waiting for comments 
from the guys who are in charge of the gnat compiler.

Philippe Tarroux