Re: limited/non-limited in Ada95

[mheaney@ni.net (Matthew Heaney)]

In article <344a4329.812635@SantaClara01.news.InterNex.Net>, tmoran@bix.com
(Tom Moran) wrote:

>>type DT is ...;
>>type DT_Access is access all DT;
>
>>type T (D : DT_Access) is private;
>
>>You can then use Unchecked_Access:
>
>>declare
>>   D : aliased DT;
>>   O : T (D'Unchecked_Access);
>>begin
>Could you expand on that, I'm confused.  I'd like 
>   -- existing type T is limited, tagged
>  type Newer(X:in some_access_type) is new T with ...

I thought the problem was that, because T was non-limited, you couldn't add
an access discriminant.  But you can add a discriminant of a named access
type as a discriminant, even if the type is non-limited.  (Note: "access
discriminant" and "discriminant of a named access type" are two different
things.  The former is only allowed for types that are limited.)

If T is limited, then it's probably simpler just to add an access
discrimant to the derived type:

type T is tagged limited private;

type NT (D : access DT) is new T with private;

Although it's perfectly legal to make the discriminant of NT have a named type:

type NT (D : DT_Access) is new T with private;


>Another problem is that once a tagged type is non-limited, no
>extension can be, or have components that are,llimited.

Yes, that's true.  But you could always add a discriminant of a named
access type to the derived type, to point to objects that are limited.

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