Re: controlled type in generic package?

[Ted Dennison <dennison@telepath.com>]

In article <95q1fa$9e$1@usenet.rational.com>,
  "Mark Lundquist" <mark@rational.com> wrote:
>
> Ted Dennison <dennison@telepath.com> wrote in message
> > Also, any instantiations of this generic will have to be done at the
> > library level, since Limited_Controlled is declared at the library
> > level, and types can't be declared at a lower level of scope than
> > their parent types.
...
> > (As a user, I hate that rule.)
>
> Do you hate it as much as what might happen if it weren't for the
> rule? :-)

I have to admit I'm a little unclear as to what nasty things could
happen to me if I were allowed to declare a derived tagged type at a
lower scope level than their parent type. The worst I can see that some
objects that are pointed to in a classwide aggregated data structure
might go invalid if I have things badly programmed. Since none of the
Ada compilers I use have garbage collection (and I can't use controlled
types as a general solution), I have to worry about deallocation issues
anyway, so its not like this is really saving me from anything.

On the other hand, I loose the ability to make use of the safer
controlled types in all but the most restrictive of circumstances. This
is not a theoretical issue; Its prevented me from using controlled types
in quite a few situations. In a very real sense, the baby got thrown out
with the bathwater. So perhaps this is just my ignorance speaking, but
yes, I do hate it more.

Of course there's nothing saying that there couldn't have been a
solution that keept the rule, but allowed non-library-level controlled
types. This whole situation is just an unfortunate side-effect of the
implementation method chosen for controlled types, and has nothing to do
with controlled types themselves. As such, I think its clear its a *bug*
in the Ada spec, which hopefully will be fixed in the next serious
revision. Making Ada.Controlled a generic would seem an obvious solution
(if a bit of a hack), but I'll leave the actual fix up to those who are
good at that sort of thing.

--
T.E.D.

http://www.telepath.com/~dennison/Ted/TED.html


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