Virtual Base Class - Not Needed for Interfaces

I was going to argue that virtual base classes are logical for interfaces, because we really only need each interface once. E.g.:

class IDeletable {
public:
virtual void delete()=0;
};

class IDrawable : public virtual IDeletable {
public:
virtual void Draw()=0;
};

class ISerializable : public virtual IDeletable {
public:
virtual void Serialize()=0;
};

void PersistentDrawing : public virtual IDrawable, public virtual ISerializable {
};

Turns out we don't need to virtualize the base classes. Virtualizing the base class is used when we have multiple copies of the base class inherited from different objects and we don't want actual copies of the base class.

But Don Box points out in "Essential COM" that this isn't necessary. In particular because the interfaces are truly empty - only a virtual function table, there is no harm in having multiple copies of them in an object. The virtual functions will be overriden correctly even if that base is represented multiple times. (The memory layout for this is moderately surprising, but I'll comment on this some other time.)