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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Controlled types and exception safety
Date: 02 Dec 2005 14:15:20 -0500
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Date: 2005-12-02T14:15:20-05:00
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tmoran@acm.org writes:

> > it's *my* responsibility to judge the tradeoffs for each case separately.
>   If you want "X := Y;" to leave X unchanged if there are problems during
> the assignment, consider how you might handle a similar problem if you were
> copying an important file on disk.  You would probably do something like
>   rename X -> X.bak
>   copy Y -> X
>   delete X.bak
> so that if their was a failure during copy you could still retrieve the
> file's data.  How about doing something similar for the stack object?
>   type stack is ...
>     backup_copy : access stack;
> Finalize then makes a backup copy (perhaps just by swapping pointers
> around), which Adjust then deletes when it successfuly finishes, or which
> Adjust's exception handler uses to restore X if there's a problem.
> Finalize also puts the backup_copy, with a "delete after time T" time
> marker, on a queue for later garbage collection if this is a "terminal"
> Finalize - analogous to a nightly "delete *.bak".

But X := Y overwrites X before calling Adjust on it, so you can't store
the backup copy, or any way of accessing the backup copy, in X.

- Bob