Why does the following class have a virtual table?

Question

Suppose I have a diamond inheritance situation as follows:

class A{
    public: virtual void foo(){};
};
class B: public virtual A{
    public: virtual void foo(){};
};
class C: public virtual A{
    public: virtual void foo(){};
};
class D: B, C{};

The last line yields a compilation error citing ambiguity. As I understand it, the problem is that the compiler doesn't know which foo to place in D's vtbl, but why would there even be a vtbl for D if it doesn't define any virtual functions of its own?

Answer 1

You're inheriting classes that contain virtual functions. Therefore, your class has virtual functions. It's as simple as that.