How to implement entry actions in the finite state machine? - c++

Question

Let's say I have been implementing a state machine in the C++ based on the switch statement i.e.
I have following piece of code
class StateMachine {
public:
enum class State {
State_A,
State_B,
State_C
};
enum class Event {
Event_Default,
Event_A,
Event_B,
Event_C
};
StateMachine();
void handleEvent(Event e);
private:
State state;
void doStateA();
void doStateB();
void doStateC();
};
StateMachine::StateMachine()
{
state = State::State_A;
}
void StateMachine::handleEvent(Event e)
{
switch (state)
{
case State::State_A:
doStateA(e);
break;
case State::State_B:
doStateB(e);
break;
case State::State_C:
doStateC(e);
break;
}
}
void StateMachine::doStateA(Event e)
{
// once upon entry into the state do some actions here
// the actions have to be done only once after state entry
switch(e) {
case Event::Event_Default:
break;
case Event::Event_A:
state = State::State_B;
break;
case Event::Event_B:
state = State::State_C;
break;
case StateMachine::Event::Event_C:
state = State::State_A;
break;
}
}
void StateMachine::doStateB(Event e)
{
// ...
}
void StateMachine::doStateC(Event e)
{
// ...
}
I have found that I need to have the ability to do some actions once upon entry into the states. Of cource I could do those entry actions before the state transitions in the doStateX() methods. But I see following disadvantage of this approach. The entry action of a given state isn't done in that state but in the state (or states) from which the transition occurs. This approach seems to me to be error prone. So I have been looking for a solution where the entry action of a given state is localized directly in that state. Does anybody have any idea?