Variadic State Machine

A C++17 header-only state machine implementation, leveraging variadic templates for compile-time checking of transitions and event-handling.

struct State {
  void OnEnter();
  void OnEnter(const ButtonEvent& event); // optional specialization for transition from event
  auto Process() -> Maybe<TransitionTo<OtherState>>; // might transition to 'OtherState'
  void OnExit();

  auto Handle(const Event& event) -> TransitionTo<AnotherState>; // transitions to 'AnotherState'
};

int main() {
  StateMachine sm(State{}, OtherState{}, AnotherState{});
  sm.Process(); // process currently active state
  sm.Handle(Event{}); // currently active state will receive Event
}

States can be very lean, they only have to handle all events.

struct SwitchPressed{};

struct LightOff {
  auto Handle(const SwitchPressed &) -> vsm::TransitionTo<LightOn> {
    std::cout << "Light is now on\n";
    return {};
  }
};

struct LightOn {
  auto Handle(const SwitchPressed &) -> vsm::TransitionTo<LightOff> {
    std::cout << "Light is now off\n";
    return {};
  }
};
// ...
sm.Handle(SwitchPressed{});

Checkout the examples.

Build instructions

meson setup build
meson test --print-errorlogs -C build

meson setup build
meson compile -C build
./build/examples/traffic_lights

Based on this article by Michael Adamczyk.