Is this example working with virtual inheritance in C++?

Question

Could I do this?

class A {
public:
  virtual void aFoo() = 0;
};

class B : virtual public A {
public:
  virtual void aFoo() { ... }
};

class D : public A {};

class C : public B, virtual public D {};

The issue is with the implementation of aFoo() in B and the lack of it in C. When it comes to compiling I see this error:

error: no unique final overrider for ‘virtual void A::aFoo()’ in ‘C’

Shouldn't it be possible to override a pure virtual method of a virtual base class in B?

---

Just edited the example to match the actual use case. Now looking at it in this simplified way I am not quite sure if this is even possible, let alone good design.

Answer 1

You need virtual inheritance in this way:

struct A {
    virtual int f() = 0;
};
struct B : virtual A {
    int f() { return 1; }
};
struct C : virtual A {};
struct D : B, C {};

int main() {
    D d;
    return d.f();
}

In the dupe I commented you can see this relation

  A  
 / \  
B   C  
 \ /  
  D 

for virtual inheritance and

A   A  
|   |
B   C  
 \ /  
  D 

without virtual inheritance.

In the second case D contains two function definitions. One is implemented and one isn't.