virtual inheritance

Hello everyone,


I have learned public/private/protected inheritance, but what is virtual inheritance? Here is the sample,

class Usable : public virtual Usable_lock

http://www.research.att.com/~bs/bs_f...#no-derivation


thanks in advance,
George

http://en.wikipedia.org/wiki/Diamond_problem
http://en.wikipedia.org/wiki/Virtual_inheritance