Using strategy pattern to change method and its associate state

Question

i like to create a class "track" that contains a state member and a kalman-filter method. I like to use different types of kalman filter. Since the interface for every kalman-filter is the same, i figured, i use the strategy pattern. The problem that i have, i also like to dynamically change the state member, when changing the strategy. Because the used state has to fit to the appropriat kalman filter.

Here a simplified Code snippet:

class CVState : public StateBase { ... };
class CAState : public StateBase { ... };

Kalman base:

class KalmanStrategy {
public:
  virtual void Prediction(StateBase* state) = 0;
  virtual void Correction(StateBase* state) = 0;
  virtual ~KalmanStrategy(){}

protected:
  KalmanStrategy() {}
};

Kalman Subclasses:

class KalmanCV : public KalmanStrategy {
public:
  KalmanCV(){}
  void Prediction(StateBase* state) {...}
  void Correction(StateBase* state) {...}
};
class KalmanCA : public KalmanStrategy {...}

Here my Track Class, it contains a state member, that has to be appropriate for Kalman.

class track {
public:
  track() {
    Kalman_ = new KalmanCV;
  }
  void ChangeStateModel(KalmanStrategy* K) {
     Kalman_ = K;
     //state_ = new CVState; // the state in track has to fit to the Kalman Class
                             // in this Case KalmanCV
  }
private:
  KalmanStrategy* Kalman_;
  StateBase* state_;
}

Is there a way, when changing the stratagy, to also change state_ ?

Answer 1

You could do this:

struct KalmanModelCV {
  typedef KalmanCV filter_t;
  typedef StateCV state_t;
};

class track {
public:
  track() {
    filter_ = NULL;
    state_ = NULL;
  }
  typedef<typename Model>
  void ChangeModel() {
     delete filter_;
     delete state_;
     filter_ = new typename Model::filter_t();
     state_ = new typename Model::state_t();
  }
private:
  KalmanStrategy* filter_;
  StateBase* state_;
};

track t;
t.ChangeModel<KalmanModelCV>();

However, if every filter requires its own specific state type, it would be better to move the creation of states to the filter class. For example:

class KalmanStrategy {
public:
  virtual void Prediction(StateBase* state) = 0;
  virtual void Correction(StateBase* state) = 0;
  virtual StateBase* CreateState() = 0;
  virtual ~KalmanStrategy(){}

protected:
  KalmanStrategy() {}
};

class KalmanCV : public KalmanStrategy {
public:
  KalmanCV(){}
  void Prediction(StateBase* state) {...}
  void Correction(StateBase* state) {...}
  StateBase* CreateState() { return new StateCV(); } // KalmanCV only works with StateCV!
};

class track {
public:
  track() {
    filter_ = NULL;
    state_ = NULL;
  }
  void ChangeModel(KalmanStrategy* strat) {
     delete filter_;
     delete state_;
     filter_ = strat;
     state_ = strat->CreateState();
  }
private:
  KalmanStrategy* filter_;
  StateBase* state_;
};