Re: Construction initialization problem

["Randy Brukardt" <randy@rrsoftware.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:oscmgxrpod50$.g7h7snlssha0$.dlg@40tude.net...
> Consider this:
>
>   type A is tagged limited null record;
>   type B (X : not null access A'Class) is tagged limited null record;
>
> Now we want to specialize B so that its instances would already contain A:

You never explained why you would want the discriminant in the first place. 
I would think that it would be better to do what you've said about C 
(include the instance directly in it) from the beginning, and the problem 
doesn't come up.

Access discriminants have a fairly high overhead in Ada (because of the 
possibility of coextensions), so unless you *need* coextensions, they ought 
to be avoided.

One could use a named access type instead to declare the discriminant if you 
really need a discriminant. But the only time you must use a discriminant is 
if you have a discriminant-dependent component, and there is no interesting 
way to make a component dependent on an access discriminant. So I'd prefer 
to just avoid the discriminant.

If you're using discriminants to stand in for a proper constructor, I say 
"don't!!" Use a constructor function as the language intends (and force it 
with a (<>) if you need it).

                               Randy.






>   type C is new B with record
>      Y : aliased A;
>   end record;
>
> Now there seems no way to either create an object of C so that C.X would
> point to C.Y or else to derive from C a new type without discriminant:
>
>   type D is new C (X => C.Y'Access) with null record; -- Illegal
>   type D is new C (X => D.Y'Access) with null record; -- Illegal
>
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de