What does it mean when "virtual" is in "class Foo : public virtual Bar" as opposed to "virtual void frob()"?

Question

What does it mean when "virtual" is in "class Foo : public virtual Bar" as opposed to "virtual void frob()"?

For a given method there are 8 cases stemming from the presence or absence of virtual in the following three locations.

1. A superclass's functions.
2. The inheritance chain for this class.
3. This classes functions.

I think I understand how numbers 1 and 3 interact but number 2 seems redundant. Is it? What am I not understanding?

Answer 1

That's virtual inheritance, you do it when you know you'll be doing multiple inheritance. That page goes into way more detail.