class A
{
protected:
  virtual void v() = 0;

public:
  void f()
  {
    std::cout << "A::f()" << std::endl;
    this->v();
  }

protected:
  A()
  {
    std::cout << "A::A()" << std::endl;
    this->f();
  }
};

class B: public A
{
private:
  virtual void v()
  {
    std::cout << "B::v()" << std::endl;
  }
};

int main()
{
    A *pa = new B;
    pa->f();
    return 0;
}