Need help with C++ templates

Question

I'm fairly sure this is a template question, since I can't seem to solve it any other way - but non-template solutions are also welcome.

A Finite State Machine has a number of program States and each state can react to a number of Events.

So, I want to define classes for Event, State and FSM. FSM has a collection (probably vector, might be linked list if STL gives problems in an embedded system) of States, and State has a collection of Events.

Each state and event have a unique Id and a name string for debugging porpoises.

To be awkward, I don't want the Ids to be integers, but elements of an enum. Each FSM has different enums for its states & events.

How best to code this? Can you give an example with two simple FSMs, or one FSm with two states, each with two events?

For example, if I have

enum myEvents {a, b, c};
enum hisEvents {d, e, f, g};

I want to be able to declare an Event class which accpts constructors params

(myEvents a,char *"event_a")

and

(hisEvents g,char* "event_g")

Note that I don't want to just overload the constructor, since that is restrictive - what if new event enums are added?

And similarly with states, then have my FSMs each have a list of states.

Or am I just being anel, insisting on enums for eventId, when it would be much simpler to pass an int?

thanks.

---

Btw, I'd rather avoid Boost as it is itself undecided on how well it works in embedded systems. I prefer in-house developed, for complete control.

Answer 1

I'm not shure if I'm understanding things correctly but I'll take a stab at it:

I'm assuming you want to define a state machine by defining the transitions; e.g. "when in state 'myEvents' and you see 'a' do 'event_a'"

class State {};
template<T> RealState : State
{
    static void Add(T event, char*) { /* save stuff */ }
};

class Event {};
template<T> RealEvent : Event    {
    RealEvent(T event, char* name) {RealState<T>(event, name); }
};

Some how you would need to tack in actions and whatnot and you will want to muck it up a bit to get more than one state machine, but I hope that gets you started.

Answer 2

While i understand why you want to avoid code redundancies by implementing the state machine basics generically, i don't understand why you want to implement this yourself.

Take a look at existing implementations like Boost.Statechart or whatever suits your usage scenario best.