Classes.cpp

class Drawbale {
  public:
  virtual void draw() const = 0;
};
class A : Drawable {
  virtual void draw() const { /*...*/ }
};
class B : Drawable {
  virtual void draw() const { /*...*/ }
};

Error.cpp

#include "Classes.cpp"
#include <vector>

void main() {
  // Example Page 38
  A a;
  B b;
  std::vector<const Drawable&> v;
  v.push_back(a);
  v.push_back(b);
  for(auto d : v) {
    d.draw();
  }
}

Working.cpp

#include "Classes.cpp"
#include <vector>
#include <memory>


void main() {
  // smart pointers or raw pointers, idc.
  auto a = std::make_unique<A>();
  auto b = std::make_unique<B>();
  std::vector<std::unique_ptr<Drawable>> v;
  v.push_back(a);
  v.push_back(b);
  for(auto d : v) {
    d->draw();
  }
}