Metaclasses are classes that are subclasses of the class of classes. Supported by [TclOO] and [XOTcl]. Can be used to do really quite fundamental changes to the way classes work.


[LV] weird - normally, I would think of a '''meta''' class as being something ''above'' (and outside) something else, handling information about the something else. Or maybe it is the term '''subclass''' that threw me - what does ''subclass of the class of classes'' mean?

So, what kinds of ''really quite fundamental changes'' can be done with metaclasses in tcloo and xotcl? And are there similar concepts in other tcl based object oriented extensions?

[DKF]: I don't know. I know that you can't do it with [[[incr Tcl]]]; classes aren't (traditionally) objects there. To my mind, the main use for a metaclass is to change the way in which you create a class. Thus, with TclOO you can do this to create something with a much more [XOTcl]-like feel:
======
oo::class create Class {
    superclass oo::class
    method constructor {argList body} {
        ::oo::define [self] constructor $argList $body
    }
    method destructor {body} {
        ::oo::define [self] destructor $body
    }
    method filter {args} {
        ::oo::define [self] filter {*}$args
    }
    method forward {name cmdName args} {
        ::oo::define [self] forward $name $cmdName {*}$args
    }
    method method {name argList body} {
        ::oo::define [self] method $name $argList $body
    }
    method mixin {args} {
        ::oo::define [self] mixin {*}$args
    }
    method superclass {className args} {
        ::oo::define [self] superclass $className {*}$args
    }
}
======

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