Quick Tutorial

1. Include the C/C++ headers

#include <assert.h>

2. Configure optional HFSM2 functionality using #defines (in this case we're using Plans to make transition cycle more straightforward):


3. Include HFSM2 header:

#include <hfsm2/machine.hpp>

4. Define interface class between the state machine and its host (also ok to use the host object itself):

struct Context {
bool powerOn;

5. (Optional) Define type config:

using Config = hfsm2::Config::ContextT<Context>;

6. (Optional, recommended) Definehfsm2::Machine for convenience:

using M = hfsm2::MachineT<Config>;

7. Declare state machine structure. States need to be forward declared, e.g. with a magic macro:

#define S(s) struct s
using FSM = M::PeerRoot<
S(Off), // initial top-level state
M::Composite<S(On), // sub-machine region with a head state (On) and and 3 sub-states
S(Red), // initial sub-state of the region
#undef S

8. (Optional) While HFSM2 transitions aren't event-based, events can be used to have FSM react to external stimuli:

struct Event {};

9. Define states and override required state methods:

struct Off
: FSM::State
void entryGuard(FullControl& control) { // called before state activation, use to re-route transitions
if (control.context().powerOn) // access shared data
control.changeTo<On>(); // initiate a transition into 'On' region
struct On
: FSM::State
void enter(PlanControl& control) { // called on state activation
auto plan = control.plan(); // access the plan for the region
plan.change<Red, Yellow>(); // sequence plan steps, executed when the previous state succeeds
plan.change<Yellow, Green>();
plan.change<Green, Yellow>();
plan.change<Yellow, Red>();
void exit(PlanControl& /*control*/) {} // called on state deactivation
void planSucceeded(FullControl& control) { // called on the successful completion of all plan steps
void planFailed(FullControl& /*control*/) {} // called if any of the plan steps fails
struct Red
: FSM::State
void update(FullControl& control) { // called on periodic state machine updates
control.succeed(); // notify successful completion of the plan step
} // plan will advance to the 'Yellow' state
struct Yellow
: FSM::State
void update(FullControl& control) {
control.succeed(); // plan will advance to the 'Green' state on the first entry
// and 'Red' state on the second one
struct Green
: FSM::State
void react(const Event&, FullControl& control) { // called on external events
control.succeed(); // advance to the next plan step
struct Done
: FSM::State

10. Write the client code to use your new state machine:

int main() {

11. Create context and state machine instances:

Context context;
context.powerOn = true;
FSM::Instance fsm{context};
assert(fsm.isActive<On>()); // activated by Off::entryGuard()
assert(fsm.isActive<Red>()); // On's initial sub-state

12. Call FSM::update() for the FSM to process transitions:

assert(fsm.isActive<Yellow>()); // 1st setp of On's plan
assert(fsm.isActive<Green>()); // 2nd setp of On's plan

13. (Optional) Event reactions also cause transitions to be processed:

assert(fsm.isActive<Yellow>()); // 3rd setp of On's plan

14. Keep updating the FSM for as long as necessary:

assert(fsm.isActive<Red>()); // 4th setp of On's plan
assert(fsm.isActive<Done>()); // activated by On::planSucceeded()
return 0;

See Also