How to represent C++ object inheritance as Java object inheritance using HybridClass?

[mrousavy]

Hey all! I've been using fbjni in a lot of libraries now, and now I stumbled upon an interesting task.

I have pure, shared C++ classes that define virtual methods. E.g. Car, and SportsCar which inherits from Car, overrides a few methods and adds some methods.

// Car.cpp
class Car {
public:
  virtual double getSpeed() = 0;
  virtual bool isFast() = 0;
};

// SportsCar.cpp
class SportsCar: public Car {
public:
  virtual double getSpeed() = 0;
  virtual bool isFast() { return true; } 
};

I want to resemble that C++ inheritance in Java, without adding something to those C++ classes (they are shared between platforms).

So my understanding is that I create the same types in Java:

// Car.java
public abstract class Car {
  HybridData mHybridData;
  Car() {
    mHybridData = initHybrid();
  }

  abstract double getSpeed();
  abstract boolean isFast();
  private static native HybridData initHybrid();
}

// SportsCar.java
public abstract class SportsCar extends Car {
  HybridData mHybridData;
  SportsCar() {
    super();
    mHybridData = initHybrid();
  }

  @Override
  boolean isFast() { return true; }

  abstract double getSpeed();
  private static native HybridData initHybrid();
}

..and then how do I properly bridge them to C++ using fbjni? Can my fbjni class inherit from both the Java base, and the SportsCar base?

// JCar.cpp
struct JCar : public jni::HybridClass<JCar>, public Car {
public:
  static auto constexpr kJavaDescriptor = "Lcom/mrousavy/Car;";
  
  double getSpeed() override {
    // TODO: get `speed` from java part, with proper virtual lookup
  }
  bool isFast() override {
    // TODO: get `isFast` from java part, with proper virtual lookup
  }

  static jni::local_ref<jhybriddata> initHybrid(jni::alias_ref<jhybridobject> jThis);
  static void registerNatives();
}

// JSportsCar.cpp
struct JSportsCar : public jni::HybridClass<JSportsCar, JCar>, public SportsCar {
public:
  static auto constexpr kJavaDescriptor = "Lcom/mrousavy/SportsCar;";
  
  double getSpeed() override {
    // TODO: get `speed` from java part, with proper virtual lookup
  }
  bool isFast() override {
    // TODO: get `isFast` from java part, with proper virtual lookup
  }

  static jni::local_ref<jhybriddata> initHybrid(jni::alias_ref<jhybridobject> jThis);
  static void registerNatives();
}

Can I call initHybrid() in both base and inherited class? Do I now have two base instances of the same type (Car) in memory?

[mrousavy]

cc @passy, maybe you know the answer to this :)

[mrousavy]

I think what I want is to use virtual inheritance to avoid having Car twice as my base class.

class JCar : public HybridClass<JCar>, virtual public Car;
class JSportsCar : public HybridClass<JSportsCar, JCar>, virtual public SportsCar;

But I can't seem to properly construct that hybrid object then:

- Constructor inherited by 'HybridClass<JSportsCar, JCar>' from base class 'JCar' is implicitly deleted

🤔