Re: limited/non-limited in Ada95

[mheaney@ni.net (Matthew Heaney)]

In article <ufhgaghueo.fsf@synquiry.com>, Jon S Anthony <jsa@synquiry.com>
wrote:

>mheaney@ni.net (Matthew Heaney) writes:
>
>> What "important capabilities" do limited types have?  That they're passed
>> by reference?  You get that for free anyway for tagged types, even if the
>> tagged type is non-limited.
>
>Well, they are _not_ necessarily passed by reference.  That is part of
>what Henry Baker is (correctly) whinging about.
>
>The most important part is that they can't be implicitly aliased -
>especially via assignment.

I don't understand Henry's problem.  If I implement a bank account as

package Bank_Accounts is

   type Bank_Account is limited private;
...
private

   type Bank_Account is
      limited record
         Money : Dollars := 0.0;
      end record;

end;

Then what's the problem?  Always implement the full view as a limited
record.  (My only complaint is that you can't apply the limited qualifier
to other types, not even arrays.  Bummer.  So you have to wrap the type in
a limited record.  Oh well.)

I agree that aliasing can comprimise the safely of a limited type (everyone
ought to read Baker's Limited Robbery paper), but if the programmer always
implements a limited private type by using a full view that is itself
limited - which one can do in Ada 95 - then there is no problem.

<ftp://ftp.netcom.com/pub/hb/hbaker/LimitedRobbery.ps.Z>
<ftp://ftp.netcom.com/pub/hb/hbaker/LimitedRobbery.html>

If the programmer doesn't use a by-reference type, then it's a case of
programmer indolence, not a language problem.

All the same, it would be really swell if compilers would tell you when you
implement a limited private type using a type that isn't passed by
reference.  It would be cool too if that were added as an argument to
pragma Restrictions, something like
Full_View_Of_Limited_Private_Must_Be_Limited.

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