I've got two files... sdl.h and ray.h.

Two classes, Vector3 and Point3 (+ a bunch more..) are defined in sdl.h.

One class, Ray is defined in ray.h. above the definition i put two forward declarations for the classes defined elsewhere that Ray uses.

Also, sdl.h contains other class definitions that instantiate a 'Ray', but have been commented out for now.

Code:
#include "sdl.h"

class Vector3;
class Point3;

class Ray {
 public:
  //Ray();
  Point3 start;
  Vector3 dir;
  //void setStart(Point3& p) {start.x=p.x; start.y=p.y; start.z=p.z;};
  //void setDir(Vector3& v) {dir.x=v.x ; dir.y=v.y ; dir.z=v.z;};
  //other fields and methods....
};
I see the following error:
ray.h:20: error: field `start' has incomplete type
ray.h:21: error: field `dir' has incomplete type

Can someone explain what this error is and how to solve the problem?