C ++类层次结构设计选择

Question

In my simulation I have different objects that can be sensed in three ways: object can be seen and/or heard and/or smelled. For example, Animal can be seen, heard and smelled. And piece of Meat on the ground can be seen and smelled but not heard and Wall can only be seen. Then I have different sensors that gather this information - EyeSensor , EarSensor , NoseSensor .

Before state: brief version gist.github.com link

Before I started implementing NoseSensor I had all three functionality in one class that every object inherited - CanBeSensed because although classes were different they all needed the same getDistanceMethod() and if object implemented any CanBeSensed functionality it needed a senseMask - flags if object can be heard/seen/smelled and I didn't want to use virtual inheritance. I sacrificed having data members inside this class for smell, sounds, EyeInfo because objects that can only be seen do not need smell/sound info.

Objects then were registered in corresponding Sensor.

Now I've noticed that Smell and Sound sensors are the same and only differ in a single line inside a loop - one calls float getSound() and another float getSmell() on a CanBeSensed* object. When I create one of this two sensors I know what it needs to call, but I don't know how to choose that line without a condition and it's inside a tight loop and a virtual function.

So I've decided to make a single base class for these 3 functionality using virtual inheritance for base class with getDistanceMethod() .

But now I had to make my SensorBase class a template class because of this method

virtual void sense(std::unordered_map<IdInt, CanBeSensed*>& objectsToSense) = 0;

, and it meant that I need to make SensorySubSystem class(manages sensors and objects in range) a template as well. And it meant that all my SubSystems like VisionSubSystem, HearingSubSystem and SmellSubSystem inherit from a template class, and it broke my SensorySystem class which was managing all SensorySubSystems through a vector of pointers to SensorySubSystem class std::vector<SensorySubSystem*> subSystems;

Please, could you suggest some solution for how to restructure this or how to make compiler decide at compile time(or at least decide once per call//once per object creation) what method to call inside Hearing/Smell Sensor s.

Answer 1

Looking at your original design I have a few comments:

• The class design in hierarchy.cpp looks quite ok to me.
• Unless distance is something specific to sensory information getDistance() doesn't look like a method that belongs into this class. It could be moved either into a Vec2d-class or to a helper function (calculatePositon(vec2d, vec2d)). I do not see, why getDistance() is virtual, if it does something different than calculating the distance between the given position and the objects position, then it should be renamed.
• The class CanBeSensed sounds more like a property and should probably be renamed to eg SensableObject.

Regarding your new approach:
Inheritance should primarily be used to express concepts (is-a-relations), not to share code. If you want to reuse an algorithm, consider writing an algorithm class or function (favour composition over inheritance).

In summary I propose to keep your original class design cleaning it up a little as described above. You could add virtual functions canBeSmelled/canBeHeard/canBeSeen to CanBeSensed.
Alternatively you could create a class hierachy:

• class Object{ getPosition(); }
• class ObjectWithSmell : virtual Object
• class ObjectWithSound : virtual Object
• ...

But then you'd have to deal with virtual inheritance without any noticeable benefit.
The shared calculation code could go into an algorithmic class or function.