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From: "Matthew Heaney" <matthew_heaney@acm.org>
Subject: Re: Newbie Access Types
Date: 1999/08/27
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In article <wcc4shlgtdw.fsf@world.std.com> , Robert A Duff 
<bobduff@world.std.com>  wrote:

> Also, in Ada 95, we have anonymous access types (for parameters and
> discriminants), and they have no null value, so it's not possible to
> initialize such parameters to null.

One thing I would have liked for inclusion is subtypes for named access
types.  That way you could declare a subtype that excludes null:

  type T_Access_Base is access all T;

  subtype T_Access is T_Access_Base range not null;
  -- or some other syntax

It's occasionally necessary to have a parameter of a named access type, and
at the same type require that a non-null value be passed in.  It would be
nice to have the compiler do the check, that we now have to do manually.

--
Matt

It is impossible to feel great confidence in a negative theory which has
always rested its main support on the weak points of its opponent.

Joseph Needham, "A Mechanistic Criticism of Vitalism"