Delegation through pure virtual

Hi,

Please consider this example from C++ FAQ {
http://www.parashift.com/c++-faq-lite
}

class Base {
 public:
 virtual void foo() = 0;
 virtual void bar() = 0;
 };

 class Der1 : public virtual Base {
 public:
 virtual void foo();
 };

 void Der1::foo()
 { bar(); }

 class Der2 : public virtual Base {
 public:
 virtual void bar();
 };

 class Join : public Der1, public Der2 {
 public:
 ...
 };

 int main()
 {
 Join* p1 = new Join();
 Der1* p2 = p1;
 Base* p3 = p1;

 p1->foo();
 p2->foo();
 p3->foo();
 }

Could you please explain how the compiler ( in general ) would create
virtual tables for Der1, Der2 and Join classed. I really need to
understand how this->bar() in function Der1::foo() gets translated to
a call to Der2::bar() call.
As per the explanation in 20.4, the Der1's vtable should have a
pointer &Base::bar() because bar() is not implemented in Der1; by what
mechanism this pointer points to &Der2:bar()?
I realize this would be a very naive question, but I need this to be
answered by experts like you.