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From: Chris Knight (cknite_at_[hidden])
Date: 2008-08-05 18:02:02

On Tuesday 05 August 2008 4:34:30 pm David Abrahams wrote:

> I don't know if you've seen the FSM examples from "C++ Template
> Metaprogramming" or not. It would be interesting for this library's
> documentation to explain its advantages over that approach (I can see
> some from here already).
> One other thing I'd like to see is an implementation of nested states.
> I once promised some people who needed that functionality that I'd
> extend our framework to support them, but was never able to figure out
> what their semantics are ;-)

Nested states would be nice. I looked through the "C++ Template
Metaprogramming" book as another reference when developing our internal
implementation. I didn't get too fancy in our own but some protocol's like
FIX for instance has many, many dark corners. I would see nested states as
simply a further restriction on the matching of the from state. This could
provide further optimizations as a well designed transition table could
reduce the set of states to match against. However, I'm not quite sure how
that plays out in a design with the enter/exit state handlers, transition
handlers, et al.

The finite state machine I came up with for the FIX protocol was well over 50
transitions. I implemented ours so that the from state could be boost::any
which acts acts like '.' in regexps and I suppose means our implementation
should be renamed kmt::nfsm. It was a zero-cost implentation as it simply
adds one last transition to each from state's action linked-list.I was able
to get the transition list down to 22 with that one feature. Mostly all the
different error conditions that you want to handle gracefully in a text
protocol originally designed in the 1980's and still used by finance
exchanges today. <sigh>

The other feature that I'm not sure whether FSM supports or not is static
registering of posted event types so as to skip around run-time double
dispatching. I suspect any implementation that seeks to provide a
simplified/speedier interface over StateChart should support that as well.


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