Andreas Huber <ah2003@gmx.net> writes:
If anything, I'd want to prohibit exceptions in entry actions, since those can lead to the FSM being in a state from which it's impossible to continue.
Interesting, it seems "failure-proof" entry actions would be an inevitable consequence when you allow failing exit actions.
Not AFAICT. Even if you don't allow exit actions to fail, how will you recover from a failed entry action? I am assuming here that when going A->B, you exit A before you enter B. If you're doing it in the other order, of course, my arguments don't apply.
At least with exit actions, you can stay where you are and keep all your FSM's abstract invariants intact.
As I have already pointed out: Only if your machines are flat and non- orthogonal, right?
I don't understand what you mean. Certainly w.r.t. "flat", only one exit action in a stack of nested states can be allowed to fail.
My approach is almost symmetrical to yours, isn't it? Since I allow failing entry actions I am forced to disallow failing exit actions.
I don't understand that. You can't neccessarily re-enter a valid state if you've already exited and entry throws an exception.
With your approach exit actions can fail what forces you to disallow failing entry actions. In both approaches allowing the failure-proof action to suddenly fail can result in an FSM having an invalid state from where it is impossible to recover.
However, I consider my approach superior because I can guarantee that for every successfully completed entry action there will always be a matching call to the exit action. You cannot give such a guarantee.
If this is needed only rarely I'd rather have users work around the problem by setting a boolean and posting an error event in a failing exit action and then testing that boolean in all following exit actions. Eeeeew, that's gross. Sorry, but that just smells like an awful hack.
I very much agree. But don't we have to do the same when we want to signal a destructor failure?
Circular argument again ;-) -- Dave Abrahams Boost Consulting http://www.boost-consulting.com