Designing and implementing an interface in C++: I'm damned if I do and I'm damned if I don't

I need some help with inheritance and coding to interfaces in C++. I'm creating a state machine from which I'll derive another state machine. Therefore I have two interacting parts, a StateMachine and a State. The problem is that with this design, I cannot derive a specialised State from either the StateMachine or a DerivedStateMachine.

class StateMachine; //fwd    class State {  public:      virtual void action(StateMachine *engine) = 0;        virtual ~State() = default;  };    class StateMachine {  public:      StateMachine() = default;        explicit StateMachine(State &state) : state_(&state) {};        virtual void setState(State &state) {          state_ = &state;      }        [[nodiscard]] virtual State *getState() const {          return state_;      }    protected:      State *state_ = nullptr;    };  

class DerivedStateMachine : public StateMachine {  public:      using StateMachine::StateMachine;        int doSpecializedThings(){          return special_thing_;      }        void setSpecialState(int specialState) {          special_thing_ = specialState;      }    private:      int special_thing_;  };  

The issue with DerivedStateDependsOnGeneralStateMachine is that a StateMachine* doesn't have access to the doSpecializedThings() method.

class DerivedStateDependsOnGeneralStateMachine : public State {        virtual void action(StateMachine *engine) {          int important_value = engine->doSpecializedThings();      };    };    

And the problem with DerivedStateDependsOnDerivedStateMachine is that you can't override a method with different arguments to the parent.

class DerivedStateDependsOnDerivedStateMachine : public State {        void action(DerivedStateMachine *engine) override {          engine->doSpecializedThings();      };    };    

Can anybody see a way out of this knot I've tied myself in? Clearly I've messed up the design somewhere. What's 'normal' (for want of a better word) in such situations?

https://stackoverflow.com/questions/65744921/designing-and-implementing-an-interface-in-c-im-damned-if-i-do-and-im-damne January 16, 2021 at 07:39AM