/* BAD */

class Building {
	public:
		Building();
		...
	protected:
		ThreeDeeModel model;
};

class Barracks : public Building {
	public:
		Unit TrainFootman();
		Unit TrainRifleman();
		Unit TrainKnight();
	...
};

class TownHall : public Building {
	public:
		Unit TrainPeasant();
	...
};

...


class Unit {
	public:
		Unit();
		...
	protected:
		ThreeDeeModel model;
		float size;
		float attackDamage;
		...
};

/* GOOD */

class Building {
	public:
		Building();
		
		Unit TrainUnit(int whichUnit) {
			return unitPrototypes[whichUnit].Clone();
		}
		...
	protected:
		ThreeDeeModel model;
		
		Unit unitPrototypes[4];
		int numUnits;
};

class Unit {
	public:
		Unit();
		
		Unit Clone();
		...
	protected:
		ThreeDeeModel model;
		float size;
		float attackDamage;
		...
};