Undefined symbols for architecture x86_64:
  "GameObject::~GameObject()", referenced from:
      Number::Number(int) in TheMathGame.cpp.o
      MovingObject::MovingObject(char, int, int, Color, Direction::value, bool, bool) in TheMathGame.cpp.o
      Number::~Number() in Number.cpp.o
  "MovingObject::~MovingObject()", referenced from:
      Player::~Player() in Player.cpp.o
      Bullet::~Bullet() in Bullet.cpp.o
      NumEater::~NumEater() in NumEater.cpp.o
      Flyer::~Flyer() in Flyer.cpp.o
  "World::World()", referenced from:
      World::getSingleton() in World.cpp.o
  "World::~World()", referenced from:
      World::getSingleton() in World.cpp.o
  "typeinfo for GameObject", referenced from:
      Player::collision(GameObject*) in Player.cpp.o
      typeinfo for Number in Number.cpp.o
      Bullet::collision(GameObject*) in Bullet.cpp.o
      Flyer::collision(GameObject*) in Flyer.cpp.o
  "typeinfo for MovingObject", referenced from:
      typeinfo for Player in Player.cpp.o
      typeinfo for Bullet in Bullet.cpp.o
      typeinfo for NumEater in NumEater.cpp.o
      typeinfo for Flyer in Flyer.cpp.o
  "vtable for GameObject", referenced from:
      GameObject::GameObject(int, int, Color, bool) in TheMathGame.cpp.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
  "vtable for MovingObject", referenced from:
      MovingObject::MovingObject(char, int, int, Color, Direction::value, bool, bool) in TheMathGame.cpp.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)