A Switch Statement State Machine

This is one of the easiest state machines to implement, and is one of the first that programmers will encounter, most often by intuitive happenstance.

The code below demonstrates the switch() state machine with a basic motor-based example:

enum class MotorStates
{
    NONE,
    INIT,
    STOPPED,
    RUNNING
};

int main()
{
    MotorStates currState = MotorStates::INIT;

    // Loop indefinetly
    while(true)
    {
        switch(currState)
        {
            case MotorStates::NONE :
                break;
            case MotorStates::INIT :
                // Init complete, goto STOPPED state
                currState = MotorStates::STOPPED;
                break;
            case MotorStates::STOPPED :
                break;
            case MotorStates::RUNNING :
                break;
        }
    }
}

You can add code which runs only on the entry to each state by keeping track of the previous state. You will now see why I added the MotorStates::NONE state.

int main()
{
    MotorStates currState = MotorStates::INIT;
    MotorStates prevState = MotorStates::NONE;

    // Loop indefinitely
    while(true)
    {
        switch(currState)
        {
            case MotorStates::NONE :
                break;
            case MotorStates::INIT :
                if(prevState != currState)
                {
                    // This code will run once on entry to state
                    prevState = currState;
                }
                // Init complete, goto STOPPED state
                currState = MotorStates::STOPPED;
                break;
            case MotorStates::STOPPED :
                if(prevState != currState)
                {
                    // This code will run once on entry to state
                    prevState = currState;
                }
                break;
            case MotorStates::RUNNING :
                if(prevState != currState)
                {
                    // This code will run once on entry to state
                    prevState = currState;
                }
                break;
        }
    }
}

One of the drawbacks is that you have to remember to break out of every case, otherwise you will get fall through to the next state.