Re: Abstract Completion Question

[Tucker Taft <stt@averstar.com>]

Nick Roberts wrote:
> 
> -- Consider the following code:
> 
> with Ada.Text_IO; use Ada.Text_IO;
> 
> procedure Test_1 is
> 
>    package P1 is
>       type T1 is abstract tagged null record;
>       procedure A (X: in T1) is abstract;
>    end;
> 
>    type T2 is new P1.T1 with
>       record
>          Data: String(1..10) := "***DATA***";
>       end record;
> 
>    procedure A (X: in T2) is
>    begin
>       Put_Line(X.Data);
>    end;
> 
>    type T1CA is access all P1.T1'Class;
> 
>    Y: aliased T2;
>    Z: T1CA := Y'Access;
> 
> begin
> 
>    P1.A(Z.all); -- dispatching call
> 
> end;
> 
> -- The operation "A" of type "T2" is overridden, so fulfilling the promise
> -- of type "T1" (as far as Ada is concerned).  This program compiles
> -- and runs (GNAT 3.11p Win32), but nevertheless "A" is not a primitive
> -- operation of "T2" (because it is not declared in a package spec), so
> -- "A" cannot be dispatched to. 

A *is* a primitive operation of T2, because it overrides an
inherited subprogram (RM95 3.2.3(7)).

> ... Upon executing this program under
> -- GNAT, Constraint_Error is raised.  This behaviour could be a
> -- 'surprise' for users (a nasty one).  Comments?

Perhaps you have run into a GNAT bug, or perhaps the Constraint_Error
is due to some other error in your program.  From a language
point of view, the dispatching call should be going to the
body of A containing the Put_Line.
 
> -------------------------------------
> -- Nick Roberts
> -------------------------------------

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA