#include <iostream>

class A {
public:
  virtual void calculate() = 0;
};

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

class C : public A {
public:
  virtual void calculate() { std::cout << "C::calculate" << std::endl; }
};

void perform(A& ra) {
  ra.calculate();
}

int main() {

  B b;
  C c;

  std::cout << "perform b, c" << std::endl;
  perform(b);
  perform(c);
  std::cout << std::endl;

  A* ra = new B();

  std::cout << "perform on ra = new B()" << std::endl;
  perform(*ra);
  std::cout << std::endl;

  delete ra;

  ra = new C();

  std::cout << "perform on ra = new C()" << std::endl;
  perform(*ra);
  std::cout << std::endl;

  delete ra;

  return 0;
}