The circle vs ellipse problem is rather artificial and, if anything, it shows that modeling with types and everyday intuition are two different things. If the

The circle vs ellipse problem is rather artificial and, if anything, it shows that modeling with types and everyday intuition are two different things. If the rest of your program can handle general ellipses, it should also be able to handle ellipses having the same minor and major radius (i.e., circles). The obvious solution is to not have the Circle class and _maybe_ equip Ellipse with IsCircle() method. (Though, why would you care?!) See the C++ faq lite items 21.6 -- 21.8. (A quote from 21.8: "Here's how to make good inheritance decisions in OO design/programming: recognize that the derived class objects must be substitutable for the base class objects. ")