class A {
    virtual void foo() = 0;
};
class B : virtual public A {  };
class C : virtual public A {  };
class D: public B, public C {
    virtual void foo() { }
};

void go()
{
    D* d = new D();
    // Upcast
    A* a = static_cast<A*>(d); // ok
    A* a2 = dynamic_cast<A*>(d); // ok
    // Downcast
    D* d2 = static_cast<D*>(a); // error: cannot convert from base ‘A’ to derived type ‘D’ via virtual base ‘A’
    D* d3 = dynamic_cast<D*>(a); // slow
}