template<typename T>
class Base
{
   template<typename U>
   friend void f(void *ptr) {
     static_cast<Base<U>*>(ptr)->run();
   }
   protected:
       virtual void run() = 0; 
};

class A : public Base<A>
{
   protected:
       virtual void run() {}
};

/*
class B : public Base<B>
{
   protected:
       virtual void run() {}
};
*/

int main()
{
   A a;
   f<A>(&a);
}