State Design Pattern: Error Handling

Question

I was just playing around with the "state design pattern" and had a couple of questions on how exactly errors are handled in a state machine. Let us take the case below

class state_machine
{
  private:
   state state1;
   state state2;

  public:

}

class state
{
   private: 
       state_machine*  m_state_machine;    /** Will pass the pointer to states **/
   public: 
     void perform_state1_action();
     void perform_state2_action();
}

class state1: public state
{
   public:
     void perform_state1_action()
      {
         /**
            Functionality
         **/
      }

     void perform_state2_action();   // Have nothing to do for this function


}


class state2: public state
{
   public:
     void perform_state2_action()
      {
         /**
            Functionality
         **/
      }

     void perform_state1_action();   // Have nothing to do for this function


}

My question is how do I gracefully handle the case where we call perform_state2_action when its in state1. Do I write a base function implementation with nothing or maybe error logging functionality?

Answer 1

This design pattern requires you to provide public methods that are available for every state. If you come across a situation that you feel an urge to add an action that is valid only for one of them, it could mean one of the following:

• You should make it private and call it from more general, public method(s), which can be implemented for all of your states
• This method should be moved outside of the state machine because it is not related to the state
• This is a specific case where empty implementation is a correct behaviour (so no error log needed)
• You have chosen wrong design pattern

Answer 2

I decided to use the state design pattern with minor changes:

Use a generic name for a function like "do_task" and use this to call the needed private functions.

This provided the benefits of the state design pattern at the same time preventing creation of surplus absolute virtual functions