Class Acts::FiniteStateMachine

template<typename Derived, typename ...States>
class FiniteStateMachine

Implementation of a finite state machine engine.

Allows setting up a system of states and transitions between them. States are definedd as empty structs (footprint: 1 byte). Tranitions call functions using overload resolution. This works by subclassing this class, providing the deriving type as the first template argument (CRTP) and providing methods like

event_return on_event(const S&, const E&);

The arguments are the state S and the triggered event E. Their values can be discarded (you can attach values to events of course, if you like) The return type of these functions is effectively std::optional<State>, so you can either return std::nullopt to remain in the same state, or an instance of another state. That state will then become active.

You can also define a method template, which will serve as a catch-all handler (due to the fact that it will match any state/event combination):

template <typename State, typename Event>
  event_return on_event(const State&, const Event&) const {
  return Terminated{};

If for a given state and event no suitable overload of on_event (and you also haven’t defined a catch-all as described above), a transition to Terminated will be triggered. This is essentially equivalent to the method template above.

If this triggers, it will switch to the Terminated state (which is always included in the FSM).

Additionally, the FSM will attempt to call functions like

void on_enter(const State&);
void on_exit(const State&);
when entering/exiting a state. This can be used to perform actions regardless of the source or destination state in a transition to a given state. This is also fired in case a transition to Terminated occurs.

The base class also calls

void on_process(const Event&);
void on_process(const State&, const Event&);
void on_process(const State1& const Event&, const State2&);
during event processing, and allow for things like event and transition logging.

The on_event, on_enter, on_exit and on_process methods need to be implemented exhaustively, i.e. for all state/event combinations. This might require you to add catch-all no-op functions like

template <typename...Args>
event_return on_event(Args&&...args) {} // noop
and so on.

The public interface for the user of the FSM are the

template <typename... Args>
void setState(StateVariant state, Args&&... args);

template <typename Event, typename... Args>
void dispatch(Event&& event, Args&&... args) {

setState triggers a transition to a given state, dispatch triggers processing on an event from the given state. Both will call the appropriate on_exit and on_enter overloads. Both also accept an arbitrary number of additional arguments that are passed to the on_event, on_exit and on_enter overloads.

Template Parameters
  • Derived – Class deriving from the FSM

  • States – Argument pack with the state types that the FSM can be handled.

Public Types

using StateVariant = std::variant<Terminated, States...>

Variant type allowing tagged type erased storage of the current state of the FSM.

Public Functions

inline FiniteStateMachine()

Default constructor.

The default state is taken to be the first in the States template arguments

inline FiniteStateMachine(StateVariant state)

Constructor from an explicit state.

The FSM is initialized to this state.


state – Initial state for the FSM.

template<typename Event, typename ...Args>
inline void dispatch(Event &&event, Args&&... args)

Public interface to handle an event.

Will call the appropriate event handlers and perform any required transitions.

Template Parameters
  • Event – Type of the event being triggered

  • Args – Additional arguments being passed to overload handlers.

  • event – Instance of the event being triggere

  • args – Additional arguments

inline const StateVariant &getState() const noexcept

Get the current state of the FSM (as a variant).


StateVariant The current state of the FSM.

template<typename S>
inline bool is(const S &state) const noexcept

Returns whether the FSM is in the specified state.

Template Parameters

State – type to check against


state – State instance to check against


Whether the FSM is in the given state.

template<typename S>
inline bool is() const noexcept

Returns whether the FSM is in the specified state.

Alternative version directly taking only the template argument.

Template Parameters

State – type to check against


Whether the FSM is in the given state.

template<typename State, typename ...Args>
inline void setState(State state, Args&&... args)

Sets the state to a given one.

Triggers on_exit and on_enter for the given states.

Template Parameters
  • State – Type of the target state

  • Args – Additional arguments passed through callback overloads.

  • state – Instance of the target state

  • args – The additional arguments

inline bool terminated() const noexcept

Returns whether the FSM is in the terminated state.


Whether the FSM is in the terminated state.

struct Terminated

Contractual termination state.

Is transitioned to if State+Event do not have a transition defined.

Public Static Attributes

static constexpr std::string_view name = "Terminated"

Name of this state (useful for logging)