Defclass vs. defstruct

Common Lisp provides two ways to create new compound data types: defstruct and defclass. Defstruct creates simple cartesian record types, while defclass is part of a full object-oriented programming system. How do you decide which one to use?

It’s easy. Unless you have a compelling reason to use defstruct, just use defclass. Even if you don’t use any other features of CLOS, defclass better supports class redefinition, and this just makes life easier.

If you modify a defstruct and recompile it, the old instances of that struct type become obsolete. They probably won’t work with the new definition. You’ll most likely have to rebuild them. If things get too screwed up, you’ll end up having to restart your Lisp image.

CLOS, on the othe hard, is designed to be dynamic. You can redefine and recompile a class on the fly. You can change the class of an instance. As you develop your code, you’ll be adding and removing slots and changing the class hierarchy. defclass usually handles these sorts of dynamic changes transparently, without having to restart your Lisp image.

CLOS achieves this by adding an extra level of indirection, and perhaps you cannot tolerate the extra overhead. Then by all means use defstruct. But if you are indifferent, defclass is a better choice.