Subject: Re: dynamic redefinition of classes From: Erik Naggum <firstname.lastname@example.org> Date: 1998/11/05 Newsgroups: comp.lang.lisp.franz,comp.lang.lisp,comp.lang.clos Message-ID: <email@example.com> * "Mattias Högström" <pt95...@student.hk-r.se> | When do you want to change a class' behaviour run-time? well, I have this application running at a client's that needs 100% uptime from 04:00 through 20:00 every working day. when I make changes to the system, I can load new function and class definitions into the running system, and pre-existing instances of a class will update themselves to the new class definition. we had 99.994% uptime until last week, when a firewall got Alzheimer's and the backup routed croaked, so now we're down to 99.96% uptime. without the ability to make run-time changes to class definitions, we would have had to stop and restart the system several times a day, usually in response to user requests that would probably not have been possible to satisfy at all without this feature. | Why not instanciate a subclass to change the behaviour? you're thinking static applications, here. don't. think living systems that don't believe in reincarnation (reboot). | How is it possible to avoid comparing to other languages I know? just don't do it. consciously avoid comparing when you feel it coming on. force yourself to start with a clean slate and separate old from new like you separate two natural languages you need to learn. (you never learn to speak another language if you keep translating between the two or continue to think in one and use grammar and vocabular from another -- I'm sure you already know this. :) | Learning object-orientation with C++ was huge a mistake(damn my school) | Now it's harder than ever to understand true object-oriented languages. not really. the human mind has a demonstrably great capacity to repress painful experiences. it's a basic defense mechanism in any intelligent animal with memory. you don't even need to learn to use it, just do it. (and don't think I'm being facetious.) | I will probably not need it either, I was just interested in when you | need it, since it doesn't exist in C++. How does C++ cope without it? C++ doesn't cope without it. therefore, if you think in C++ terms, you will need it as much as the average fish will invent hanggliding gear. invention is the mother of need. a typical example of when you need a meta-class is when you want some behavior to apply to all instances of all classes of the meta-class, like persistent objects that are going to be stored on disk. you would code the disk-storing properties in one place, only, and use the classes just like normal classes. everything relating to persistency would be taken care of automatically. in a language without this ability to centralize, you would need to code the persistence in a lot of places, and perhaps expose it via different calling conventions where it is being used. | Now I am really interested what meta classes really are. they define the behavior of classes as classes define the behavior of their instances. it's really no big deal. instead of being taught "this is what being a class means" and having to adapt to whatever the language offers, you can now define it yourself if you need more than the language designers had in mind at the time. this is the same argument you would use to describe classes (or even structures or simple type definitions). #:Erik -- The Microsoft Dating Program -- where do you want to crash tonight?