C++ Multiple Inheritance - why you no work?

Question

I am trying to figure out an interesting multiple inheritance issue.

The grandparent is an interface class with multiple methods:

class A
{
public:
          


        

Answer 1

You don't have diamond inheritance. The B and C base classes of D each have their own A base class subobject because they do not inherit virtually from A.

So, in D, there are really four pure virtual member functions that need to be implemented: the A::foo and A::bar from B and the A::foo and A::bar from C.

You probably want to use virtual inheritance. The class declarations and base class lists would look like so:

class A
class B : public virtual A
class C : public virtual A
class D : public B, public C

If you don't want to use virtual inheritance then you need to override the other two pure virtual functions in D:

class D : public B, public C
{
public:
    using B::foo;
    using C::bar;

    int B::bar() { return 0; }
    int C::foo() { return 0; }
};

Answer 2

You need to make your base classes virtual in order for them to inherit properly. The general rule is that all non-private member functions and base classes should be virtual UNLESS you know what you're doing and want to disable normal inheritance for that member/base.