fsm.cpp

template <typename State, typename ...Trans> struct FSMRunner;

template <typename State, typename T, typename ...Ts> struct FSMRunner<State, T, Ts...> {
    inline static void step(State &state) {
        if (T::from == state && T::precond()) {
            T::action();
            state = T::to;
        } else {
            FSMRunner<State, Ts...>::step(state);
        }
    }
};
template <typename State> struct FSMRunner<State> {
    inline static void step(State &state) {}
};

template <typename State, typename ...Trans> struct FSM {
    State state; // initial sstate

    inline void step() {
        FSMRunner<State, Trans...>::step(state);
    }
};


enum class State {
    A,
    B,
    C
};

struct TransitionX {
    static const State from = State::A;
    static const State to = State::B;

    static bool precond() {
        return true;
    }

    static void action() {
    }
};


int main()
{
    FSM<State, TransitionX, TransitionX, TransitionX> fsm;
    fsm.state = State::A;
    fsm.step();
    cout << (int)fsm.state << endl;
    return 0;
}