When using a virtual base class in a multiple inheritance scenario, is it necessary for all derived classes to reference the virtual base?

Question

The US Air Force's JSF C++ coding standard requires that the virtual base class be declared for each derived class that accesses the virtual base.

For example, in the following hierarchy:

  A
 / \
B1  B2
C1  C2
 \ /
  D

... the rule they impose in this standard (AV Rule 88.1, for reference), requires the classes to be declared like so:

class A;
class B1 : virtual A;
class B2 : virtual A;
class C1 : B1, virtual A;
class C2 : B2, virtual A;
class D  : C1, C2, virtual A;

My questions are as follows:

1. Is this semantically different from only inheriting virtually in the declaration @ B1/B2, and not specifying virtual A at each subsequent class declaration?
2. If it's semantically different, why would anyone /want/ to leave it off? It seems silly to me that you'd absolutely have to do this at each layer of inheritance since that adds a potential point of failure.

Answer 1

It semantically identical, since each derived class will have exactly one virtual base of type A. Mentio­ning the virtual base explicitly is quite nice, because the most-derived class constructs the virtual base (unlike what happens for non-virtual bases), and the construction order is important to keep in mind when writing the constructors of the derived classes.

I don't have a technical answer for (2). You don't have to do it, but it would be nice if you did. Like calling your parents, I suppose. As with many things, C++ doesn't force you to be reasonable.