Re: Extending discriminant types

["Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org>]

Stefan Bellon wrote:
> 
>    type Kind is (A, B);
> 
>    type Base (K : Kind) is abstract tagged record
>       case K is
>          when A => null;
>          when B => Dummy : Natural;
>       end case;
>    end record;
> 
>    type Base_Access is access all Base'Class;
> 
>    type Child is new Base (B) with null record;
> 
> In this case I would have thought that type Child is now constrained to
> Kind B and no object of type Child with Kind A can be created.
> 
> However, the following compiles (with GNAT) and does not throw an
> exception at runtime:
> 
>    Var : Base_Access := new Child'(K => A);
> 
> Am I misunderstanding the Ada semantics? If so, is there a way to
> achieve what I'm trying, i.e. that I can determine the Kind while
> extending and not change it when creating objects?

The following does not compile:

Var : Child (K => A);

You probably don't need the access type that is causing you this confusion.

-- 
Jeff Carter
"In the frozen land of Nador they were forced to
eat Robin's minstrels, and there was much rejoicing."
Monty Python & the Holy Grail
70