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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Legit Warnings or not
Date: Thu, 21 Jul 2011 11:13:01 -0400
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Date: 2011-07-21T11:13:01-04:00
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Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> There are 2 ways to handle such unconstrained objects. The easy way,
> which GNAT uses, is to allocate enough space for the largest
> variant. The harder way only allocates enough space for the current
> value, and actually changes the space of the object if the size is
> changed through an assignment.

Right.  I know of two Ada compilers that used this
"deallocate-and-reallocate" method to change the size.
One of them got it wrong.  I don't remember the details,
but I think it had to do with renaming a subcomponent,
and then an assignment to the whole record caused the
renaming to become a dangling pointer.  The other compiler,
as far as I know, got it right.

> Some who have gone to the trouble to implement the 2nd way say they
> think it was the intention of the language designers.

Could be -- I don't know.  I think for a low-level language like Ada,
it's probably not a good idea to introduce implicit heap allocation.
I'd at least want a way to turn it off.

Implicit heap allocation could be a good feature in a different
language, but then I'd want that language to allow:

    type Tree is tagged
        record
            Left, Right : Tree'Class; -- doubly illegal!
        end record;

- Bob