From: "A.Appleyard" <A DOT APPLEYARD AT fs2 DOT mt DOT umist DOT ac DOT uk>
Organization: Materials Science Centre
To: djgpp AT delorie DOT com
Date: Thu, 10 Oct 1996 16:20:02 GMT
Subject: Re: A pointer of variable type - is it possible?
Message-ID: <982A4C7275@fs2.mt.umist.ac.uk>

 paradox AT direct DOT ca (Wesley Terpstra) wrote:-
> ... the 3d object only has ... mass,
> moment of inertia, velocity, acceleration, angular acceleration,
> points, lines, polys, textures, etc). I also need to add the ability
> for 3d objects to have a dynamic section. This will let spaceships
> have data members like weapon_power, armor_power, etc. and let stars
> have stuff like radiation_output, percentage_chance_of_flare, etc.

  Derived classes supply part of this need:-
class object {public: short sort; float mass, inertia, speed, accel, ....;};
class spaceship: public object {char*name; int weapon_power; .....;};
class star: public object {int spectrum_class; float radiation_output; ....;};

  A spaceship and a star also thereby have the fields that an object has.

  If these are declared:- object *X; star Rigel,*S; spaceship Explorer,*C;
  then X=&Rigel; X=S; X=&Explorer; X=C; are possible,
  but not S=X; C=X; Rigel=*X; Explorer=*X;

  If you define these codes:-
  enum{_unknown=0, _spaceship=1, _star=2, _missile=3; .....};
  and if you make sure that spaceship.sort==1, star.sort==2, etc, always,
  then (I think) you can convert back:-

  switch(X.sort) {
  case _spaceship: S=(spaceship*) X; .... handle a spaceship ....; break;
  case _star:      C=(star*)      X; .... handle a star       ....; break;
  etc etc;};

  ..................................................................

> I considered using a signature like:
> signature object_data_ptr {
> void *object_data;
> const char *identifier;
> void load(DB *);
> void save(DB *); };

   What's a signature? What variant of C does it occur in?