Implementing partial C++ Interface in parent class

Question

I have a unique case where I need two layers of Interfaces, and would like to have two layers of classes to implement them:

class IFood {
public:
    virtual ~IFood() = default;
    virtual int GetColor() const = 0;
};

class IFruit : public IFood {
public:
    virtual int GetAverageSeedCount() const = 0;
};

class GreenFood : public IFood {
    virtual int GetColor() const override {return GREEN;}
};

class GreenApple : public GreenFood, public IFruit {
    virtual int GetAverageSeedCount() const override {return 5;}
};

(I realize these interfaces don't make perfect sense. Sorry.)

In some cases, I might have a collection of IFood objects, but in others I'll have a collection of IFruit objects. In reality the interface represented by IFood has 5-8 functions.

If this code would compile and run I'd be set. Unfortunately it fails because the compiler doesn't realize GreenApple implements the IFood API (missing GetColor). This does exist in the base GreenFood that we extend, but splitting up the interface implementation doesn't seem to make the compiler happy.

For each function in IFood, I can have the IFruit implement it and just directly call the ParentClass::functionName(). But with 5-8 functions in IFood and dozens of potential fruit types, that isn't as elegant as I'd like.

I'm curious if there is any solution to get the compiler to find these missing APIs in the parent class, or any good way for me to restructure my Interfaces to keep things elegant?

I can provide more concrete examples if needed. Thank you for any tips!

Answer 1

Use Virtual keyword for base classes in IFruit and GreenFood.

class IFood {
public:
    virtual ~IFood() = default;
    virtual int GetColor() const = 0;
};

class IFruit : public virtual IFood {
public:
    virtual int GetAverageSeedCount() const = 0;
};

class GreenFood : public virtual  IFood {
    virtual int GetColor() const override {return GREEN;}
};

class GreenApple : public GreenFood, public IFruit {
    virtual int GetAverageSeedCount() const override {return 5;}
};