Use of abstract tagged types without access types

Use of abstract tagged types without access types

[Prof. Dr. Carl Weierstrass]

Hi,

I have a question to all Ada gurus:

I want to do something like:

package Test is

   type Parent_Type is new Ada.Finalization.Controlled with private;
   type Child_Type is abstract new Ada.Finalization.Controlled with
private;
   type Child_Type_1 is new Child_Type with private;
   type Child_Type_N is new Child_Type with private;

private

   type Parent_Type is new Ada.Finalization.Controlled with record
         Child : Child_Type'Class;
   end record;
   type Child_Type is abstract new Ada.Finalization.Controlled with
null record;
   type Child_Type_1 is new Child_Type with null record;
   type Child_Type_N is new Child_Type with null record;

end Test;

This doesn't work, because:
class-wide subtype with unknown discriminants in component declaration

Of course I could

Re: Use of abstract tagged types without access types

[AdaMagica]

On 9 Jun., 19:44, "Prof. Dr. Carl Weierstrass"
<carl.weierstr...@googlemail.com> wrote:
>    type Parent_Type is new Ada.Finalization.Controlled with record
        Child : access Child_Type'Class;  -- You definitely need an
access here...
>    end record;
>
> Of course I could
... as (I'm sure) you wanted to say here. Tags are some kind of
(hidden) discriminant.

Alternatively:

  type Parent_Type (<>) --(Child: access Child_Type'Class)
    is new Ada.Finalization.Controlled with private;

private

  type Parent_Type (Child: access Child_Type'Class)
    is new Ada.Finalization.Controlled with null record;

Re: Use of abstract tagged types without access types

[Simon Wright]

AdaMagica <christ-usch.grein@t-online.de> writes:

>   type Parent_Type (Child: access Child_Type'Class)
>     is new Ada.Finalization.Controlled with null record;

Does that have to be (Child : not null access Child_Type'Class)
nowadays?

Re: Use of abstract tagged types without access types

[AdaMagica]

On 10 Jun., 09:17, Simon Wright <si...@pushface.org> wrote:
> AdaMagica <christ-usch.gr...@t-online.de> writes:
> >   type Parent_Type (Child: access Child_Type'Class)
> >     is new Ada.Finalization.Controlled with null record;
>
> Does that have to be (Child : not null access Child_Type'Class)
> nowadays?

Not necessarily, only if you would prevent null values.

Re: Use of abstract tagged types without access types

[Yannick Duchêne (Hibou57)]

Le Thu, 09 Jun 2011 19:44:05 +0200, Prof. Dr. Carl Weierstrass  
<carl.weierstrass@googlemail.com> a écrit:
>    type Parent_Type is new Ada.Finalization.Controlled with record
>          Child : Child_Type'Class;
>    end record;
That looks a strange design to my eyes, just like if you were to reinvent  
the concept of class-wide type.

I would enjoy enough to know the finality of this design.

-- 
“Syntactic sugar causes cancer of the semi-colons.”  [Epigrams on  
Programming — Alan J. — P. Yale University]
“Structured Programming supports the law of the excluded muddle.” [Idem]
“c++; /* this makes c bigger but returns the old value */” [Anonymous]