finite state machine : transitions in absence of external events

srp113 <>
Wed, 16 Sep 2009 06:49:01 -0700 (PDT)
 I am desigining a FSM (plan to use C++ to implement it). One of the
issues I am encountering is dealing with transient states ex:
                       | |
                           | |
                               | |
The transition from A-B happens because of some external event (like
timer expiration).
The transition from B-C is always immediate, basically state B exists
to perform some actions before entering state C. Its also possible
that FSM starts off with initial state as B instead of A which is
state B is needed. The transition from C-D is again immediate, C
executes some actions before entering state D. The reason we have
state C is because we need to reexecute these actions when we
transition from E to C(loopback)
Transition from D to E and from E to C are based on external events
(timer events).
(Transition from C to E not possible)

So in above FSM A,D,E are states that transition to next state only
on receipt of some external event while states B,C are transient/
temporary states that exist only to execute some actions before
transitioning to next state. One way I could think of achieving this
is to provide entry/exit routines for each of the state. Essentially
B::onEntry() {
  do B's actions


B::onExit() {
  //Do nothing or if there are any exit actions for B


C::onEntry() {
  do C's actions


C::onExit() {
  //Do nothing or if there are any exit actions for C


//Some class that has pointer to currentState
moveToState(newState) {
   currentState = newState;


This was only solution I could think of. What I dont like about what
happens when we move from say A-B, we finally end in D (A-B,B-C,C-D)
and then at runtime call graph looks like: procesTimeoutInA()-


()->B's actions->B()::exit()- (B'S::exit called from with B::s entry
()!!)->C::entry()->C's actions->C's::exit() (again C's exit is called
from C's entry)->D's::entry() and unwind in the opposite order. Is
there any better way to handle this?
Thanks Much,

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