From: Robert Klemme <bob.news@...>
Date: 2004-10-15T18:34:28+09:00
Subject: Re: Querying class attributes


"DaZoner" <nobody@nowhere.com> schrieb im Newsbeitrag
news:ckmgi5$r3k$1@news.doit.wisc.edu...
>
> I'm creating a program that allows anonymous plugins to be defined.
Since
> the user can define their own plugins I want to check that they've
defined
> them properly. What I can't figure out is how to check the plugin's
> correctness without instantiating an instance of it first thus making
> checking impossible. I'm sure there is an easy way to do this but I
don't
> know what it is.
>
> In the example below I want to check that the plugin has defined a
> constructor that takes 4 parameters. I seem to only be able to check the
> instance. How can I do it?

def test_class(cl)
  raise "Wrong arity" unless cl.instance_method(:initialize).arity == 4
end

Kind regards

    robert

>
> plugin = Class.new do
>   def initialize(a,b,c,d)
>   end
> end
> instance = plugin.new(0,0,0,0)
>
> def test(object,name,method)
>   print name," responds to ",method.to_s
>   if object.respond_to?(method,true)
>     print " true\n"
>     print "  arity ",object.method(method).arity,"\n"
>   else
>     print " false\n"
>   end
> end
>
> test(plugin,"plugin",:new)
> test(plugin,"plugin",:initialize)
> test(instance,"instance",:new)
> test(instance,"instance",:initialize)
>
>
> ## resulting output
>
> plugin responds to new true
>   arity -1
> plugin responds to initialize true
>   arity -1
> instance responds to new false
> instance responds to initialize true
>   arity 4
>
>