composition and inheritance

composition_and_inheritance.cpp

class SomeClass{
public:
    virtual void func() {
        std::cout << "SomeClass::func called" << std::endl;
    }
};

class SubOfSomeClass : public SomeClass {
public:
    virtual void func() override {
        std::cout << "SubOfSomeClass::func called" << std::endl;
    }
};

class BadSuper {
public:
    void init() {
        someClass.func();
    }
protected:
    SomeClass someClass; // bad idea if we expect SomeClass to be inherited
};

class BadSub : public BadSuper {
public:
    void someFunc() {
        SubOfSomeClass* sosc = static_cast<SubOfSomeClass*>(&someClass); // down casting sucks
    }
};

class GoodSuper {
public:
    void init() {
        accessObject()->func(); // inherited classes of SomeClass will be called
    }
protected:
    virtual SomeClass* accessObject() {
        return &someClass;
    }
private:
    SomeClass someClass;
};

class GoodSub : public GoodSuper {
protected:
    virtual SubOfSomeClass* accessObject() override {
        return &sosc;
    }
private:
    SubOfSomeClass sosc;
};

int main(void) {
    GoodSub gs;
    gs.init();
    return 0;
}