#include <iostream>
#include <string>

using namespace std;

//--------------------------------------------
// Base class is abstract
//--------------------------------------------
class Base {
 protected:
  int x;
 public:
  Base() : x(0) {}
  virtual string toString() = 0; // abstract method
};

//--------------------------------------------
// Derived class must either be abstract or
// else provide a toString() implementation
// (so D1 is an abstract class as well)
//--------------------------------------------
class D1 : public Base {
 protected:
  int y;
 public:
  D1() : y(-10) {}
};

//--------------------------------------------
// A D2 can be instantiated (i.e., D2 isn't an
// abstract class).
//--------------------------------------------
class D2 : public D1 {
 protected:
  int z;
 public:
  D2(int init) : z(init) {}
  string toString() override {
    return "x = " + to_string(x) + " y = " + to_string(y) +
           " z = " + to_string(z);
  }
};

//--------------------------------------------
// main
//--------------------------------------------
int main(int argc, char **argv) {
  Base b;      // error
  D1 d1;       // error

  D2 d2(100);  // OK
  cout << "d2 = " << d2.toString() << endl;

  return 0;
}