Re: Inheritance

Only if the pilot is rated for aerobatics...

(Couldn't resist...)

Yes, that's another case. It gets trickier, and inheritance breaks down after a while.

class WheeledVehicle {
    public:
        int wheels;
};

class Unicycle : public WheeledVehicle
    public:
        Unicycle() : wheels(1) { }
};

class WingedVehicle {
        public:
             int wings;
             int engines;
};

class biplane: public WheeledVehicle, WingedVehicle {
     public:
         biplane () : wings(4), wheels(3)
};

class monoplane : public WheeledVehicle, WingedVehicle {
    public:
        monoplane() : wings(2) {}
};

class glider : public monoplane {
    public:
       glider () : engines(0), wheels(2) {}
};

class Bomber : public monoplane {
      int BombLoad;
};

class PoweredPlane {
     public
     int engines;
     typedef { Piston, Jet } EngineType;
     EngineType type;
     };

class PropPlane {
    public:
       int bladesperprop;
};

class Jet : public PoweredPlane {
     public:
        Jet() : type(Jet);
};

class B52 : public monoplane, bomber, Jet {
    public:
       B52() : engines(8), wheels(8), BombLoad(CLASSIFIED_VALUE) {}
};

class C130 : public monoplane, Jet, PropPlane {
     public:
       C130: engines(4), wheels(8), bladesperprop(4) {}
}

Given this hierarchy, I can probably come up with actual planes that don't fit it. And on
the whole, you can't do it without multiple inheritance. What about a jet-propelled race
car, or even a jet-propelled bicycle? What about model airplanes powered by rubber bands?
We used to go through exercises like this back when we were designing inheritance
languages, and it became a weekly exercise to describe vehicles. For example, how do you
describe an airplane with skis or pontoons instead of wheels? Oh, forgot about that. So
a new hierarchy comes around. What about bulldozers or tanks with treads instead of
wheels? A halftrack? A snowmobile? This was in the days before the Internet and Google,
and we would spend hours in the library looking for photos of odd vehicles that our
current hierarchy couldn't describe, and try to work out a new hierarchy that could
describe all vehicles (hot air balloons, dirigibles, hang gliders, kites, roller skates,
skateboards,helicopters, autogyros, ...wow! It's been thirty years, and what I remember
was that we couldn't get a decent solution. The really bad thing was that we couldn't
somehow attach attributes, but our whole hierarchy would disintegrate. The folks in AI
developed entirely different mechanisms for describing objects, that were
non-hierarchical.

So hierachical systems will give superficially simple solutions to trivial problems, but
they dont' give solutions to all representations.

Note that I stopped at weeks in my description, because months get trickier, and what's a
year? 365, 366, or 365.25?

Key here is that anything in class A should only reference classes that are not related,
or its superclasses, but not its subclasses. Once you start building circular
dependencies, life gets fairly miserable. And you can't do anything that requires that
the superclass have instances of the subclass, although it could have pointers to a
subclass, but at this point you're into really weird, and probably very bad, design.
                        joe

On Wed, 28 Mar 2007 09:44:07 -0700, Al <Al@discussions.microsoft.com> wrote:


Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm