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From: "Mark Lundquist" <mark@rational.com>
Newsgroups: comp.lang.ada
Subject: Re: controlled type in generic package?
Date: Wed, 7 Feb 2001 13:57:17 -0800
Organization: Rational Software
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Date: 2001-02-07T13:57:17-08:00
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Lutz Donnerhacke <lutz@iks-jena.de> wrote in message
news:slrn9827l8.ml.lutz@taranis.iks-jena.de...
> * Mark Lundquist wrote:
> >The compilation error that was reported would have been due to the
> >instantiation.  The instantiation is a copy of the generic template, so
you
> >can't instantiate a generic that does something that would be illegal to
do
> >in the same place if it weren't in a generic :-).  In this case, that
> >something is to declare a type extension that's statically deeper than
the
> >parent.
>
> I don't understand the reason, but will keep it in mind, until I came
across
> this situation.

OK, here's the reason:

Suppose you have

    type T is tagged record
        .
        .

and

    procedure P (X : access T'Class);        -- (or a parameter of a named
access type)

Now suppose in a nested scope we say

    type D is new T with record
        .
        .

If we have an object of type D, we can pass a pointer to it in a call to P.
But maybe P does something squirrely with this pointer, like saving it on a
list.
Once the scope of D goes away, so have all objects of type D, so that
pointer is now dangling.  The result: The Kind Of Error That Is Not Supposed
To Happen To An Ada Program :-) :-)

I think Bob Duff's "limited access" proposal would fix this.  I really like
that proposal.

-- mark