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From: David Abrahams (dave_at_[hidden])
Date: 2004-05-30 18:31:03

"Andreas Huber" <ah2003_at_[hidden]> writes:

>> I'm also concerned that the current fsm destruction/exit action
>> implementation model results in an environment where I should avoid
>> using exit actions that have any significant external effects because
>> I don't want/expect them to be run when/if the fsm is destructed.
> Concern noted. This is actually the central disagreement between me and Dave
> Abrahams. If you have any real-world examples, I'd be *very* interested to
> see them.

Here's one I'm pulling out of the air, since I don't have any
real-world experience *with FSM exit actions*. I hope my extensive
real-world experience with programs in general, state and invariant
management, and error handling counts for something. I'm certain I
could come up with five more of these given an hour, but if you demand
an example that I know exists in some real FSM code, I can't help you.

A state X has a local buffer of data to be written to disk upon making
a successful transition from X. The presence of that data on disk
indicates something important about what's happened in the state
machine, and you *don't* want the data on disk unless you have made
the transition from X successfully.

It would be a bad idea to write that data out if the FSM's controlling
process decided to simply discard the FSM when it was in state X.
Also, putting the instructions to write the data in a state destructor
could require the otherwise-unneccessary quashing of exceptions from
the function used to write the data to disk.

I said long ago that I was going to stop poking at this one, and
didn't, because I had the sense that issues in this area, and the way
you were reacting to my poking, might indicate that there were other
problems. I don't feel as though much progress has been made, and you
seem to feel that trying to convince me of anything by example is
futile. So really, now I'm going to stop worrying this issue. May
other peoples' posts be more helpful than mine have been!

Dave Abrahams
Boost Consulting

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