#include <iostream>
#include <string>

using namespace std;

class Base {
 public:
  Base(int x) : x_(x) { }
  virtual string toString() {
    return string("Base: x_ = ") + std::to_string(x_);
  }
  int x_;
};

class Subclass : public Base {
 public:
  Subclass(int y) : Base(16), y_(y) { }
  string toString() override {
    return string("Subclass: y_ = ") + std::to_string(y_) + " x_ = " + std::to_string(x_);
  }
  int y_;
};

int main(int argc, char *argv[]) {
  Base b(1);
  cout << "b = " << b.toString() << endl;
  Subclass d(2);
  cout << "d = " << d.toString() << endl;
  b = d;     // what happens to y_?
  // d = b;  // would be a compiler error.  Why?
  cout << "b = " << b.toString() << endl;
}