"Andreas Huber" <ah2003@[...].net>  writes:

>  I don't think I have missed your point and I know that I'm talking to an
>  exception handling expert. Have you read that whole paragraph? 

Yep

>  Agreed, after
>  posting I noticed that the first half isn't exactly the best explanation of
>  why exit actions must not fail. However, I believe that the second half is a
>  good argument why throwing exit actions are a bad idea:
>  <quote>
>  ... However, even if exit actions are called in the
>  course of a normal transition what are you going to do if B's exit action
>  throws an exception? 

Stay at B, of course (?)

>  Technically, the state machine still resides in B, so
>  you have pretty few options for continuing. You cannot exit A as that would
>  violate the state machine invariant that inner states are always exited
>  before outer states. You cannot make a transition to another state, as that
>  would also require successful exit from B. So, the only sensible thing is to
>  try to handle the error inside B's exit action. 

I don't understand why.  The user of the FSM could handle the
exception on the outside if its state doesn't change.

>  If the error cannot be handled there it must either be ignored or,
>  in the case of a really serious error, be handled with a different
>  mechanism than exceptions.  </quote>
> 
>  A maybe more convincing argument:
>  In the course of a transition a state entry action throws an exception. Note
>  that the state machine is in an invalid state (unstable) when this happens.
>  If the state machine does not successfully handle the exception, the
>  exception must be propagated to the state machine client. Because the state
>  machine is unstable, it *must* be terminated before propagating to the
>  client. 

OK so far...

>  Termination calls the exit actions of all currently active states.

that sounds like it could be the wrong design decision, to me.

>  What are you going to do with exceptions thrown by these exit actions?

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com

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