At this point, we’ve said about as much as can be said about GOOPS without having to confront the idea of the metaobject protocol. There are a couple more topics that could be discussed in isolation first — class redefinition, and changing the class of existing instances — but in practice developers using them will be advanced enough to want to understand the metaobject protocol too, and will probably be using the protocol to customize exactly what happens during these events.
So let’s plunge in. GOOPS is based on a “metaobject protocol” (aka “MOP”) derived from the ones used in CLOS (the Common Lisp Object System), tiny-clos (a small Scheme implementation of a subset of CLOS functionality) and STKlos.
The MOP underlies many possible GOOPS customizations — such as
defining an initialize method to customize the initialization of
instances of an application-defined class — and an understanding of
the MOP makes it much easier to explain such customizations in a precise
way. And at a deeper level, understanding the MOP is a key part of
understanding GOOPS, and of taking full advantage of GOOPS’ power, by
customizing the behavior of GOOPS itself.