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();
 }