Question

I was wondering if there’s a way I could code some kind of “generic” FSM for a game with C++?.

My game has a component oriented design, so I use a FSM Component.

my Finite State Machine (FSM) Component, looks more or less this way.

class gecFSM : public gecBehaviour 
{

public:
    //Constructors
    gecFSM() { state = kEntityState_walk; }
    gecFSM(kEntityState s) { state = s; }

    //Interface
    void setRule(kEntityState initialState, int inputAction, kEntityState resultingState);
    void performAction(int action);

private:
    kEntityState fsmTable[MAX_STATES][MAX_ACTIONS];

};

I would love to hear your opinions/ideas or suggestions about how to make this FSM component, generic. With generic I mean:

1) Creating the fsmTable from an xml file is really easy, I mean it’s just a bunch of integers that can be loaded to create an MAX_STATESxMAX_ACTION matrix.

void gecFSM::setRule(kEntityState initialState, int inputAction, kEntityState resultingState)
{
    fsmTable[initialState][inputAction] = resultingState;
}

2) But what about the “perform Action” method ?

void gecFSM::performAction(int action)
{
    switch( smTable[ ownerEntity->getState() ][ action ] )
    {
        case WALK:
            /*Walk action*/
        break;
        case STAND:
            /*Stand action*/
        break;
        case JUMP:
            /*Jump action*/
        break;
        case RUN:
            /*Run action*/
        break;
    }
}

What if I wanted to make the “actions” and the switch generic?

This in order to avoid creating a different FSM component class for each GameEntity? (gecFSMZombie, gecFSMDryad, gecFSMBoss etc ?). That would allow me to go “Data Driven” and construct my generic FSM’s from files for example.

What do you people suggest?

Answer 1

Have a look at the Boost Statechart Library. (formerly known as boost::fsm)
It comes with a very nice photo camera example.