David Abrahams wrote:
What is "class state?" Can you describe its domain? What are the principle abstractions in that domain? Ok, you catched me. In most cases, these members are shared across states. So, they have to be members of Fsm. Class states might hold a "baton" to pass on to a next state but I don't see any important use case.
If I try to map what you're describing onto the state machine designs that we did for "C++ Template Metaprogramming" (and that I posted here), I could achieve something similar by simply putting storage for auxilliary data (like speed) in a data member of the derived FSM class.
The man who invented examples is the greatest inventor of all time! After looking at mpl example I understand what do you mean. My code can be rewriten to support your style. You could typedef Running as state<0>, Stopped as state<1> and so on. The framework could find out that all state<N> are stateless (what a mess! :) and left out storage generation for them. I wonder when and why you need these numbers? Not for passing to switch statement, I hope? I suppose, you use them to check for a particular state of set of states like whether FSM is in final state(s) or not. If you do so, I do so as well.
While I have to admit that your particular example seems to naturally call for associating that auxilliary data directly with the state class, that model doesn't generalize well AFAICT. There could be auxilliary data that's needed -- persistently -- for more than one state.
Agree.