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From: "Randy Brukardt" <randy@rrsoftware.com>
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 <wccirtygr41.fsf@shell01.TheWorld.com>
Subject: Re: Dynamically tagged expression required
Date: Fri, 9 Dec 2005 14:41:30 -0600
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Date: 2005-12-09T14:41:30-06:00
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"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
news:wccirtygr41.fsf@shell01.TheWorld.com...
...
> If you want to change the Tag of an object, you can't -- you have to use
> access types, and allocate a new object with the new Tag you want.
> In my experience, class-wide types nearly always require use
> of access types and heap allocation.

That's half right. Class-wide types nealry always require the use of access
types somewhere. But you don't necessarily have to use heap allocation; the
objects can be allocated on the stack, and 'Unchecked_Access used to create
the access values.

With proper use of controlled types, this can be completely safe, and much
of the time, it is better than heap allocation for all of the normal
reasons.

For this to work, the access types have to have a fairly controlled usage
(so that it is practical for Finalize to update them); it works best in a
library. Claw works this way, for instance.

                       Randy.