What does it mean to “delegate to a sister class” via virtual inheritance?

Consider the following example:

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();
}

Believe it or not, when Der1::foo() calls this->bar(), it ends up calling Der2::bar(). Yes, that’s right: a class that Der1 knows nothing about will supply the override of a virtual function invoked by Der1::foo(). This “cross delegation” can be a powerful technique for customizing the behavior of polymorphic classes.