If you're a programmer using C++, Smalltalk, or Eiffel, or if you're about to migrate to object orientation for the first time, you need to know the most important principles of object-oriented design. For example: In a reliable design, why must a variable's cone of polymorphism lie within that of a method? Is connascence good or bad? How could the inherited methods of a superclass violate the invariant of a subclass—and how can you prevent this?