From: Artur Merke <am@...>
Date: 2006-10-21T19:55:18+09:00
Subject: instance_eval && class_eval

Hi,

I'm just studing some metaprogramming and came across some problems.
The following code:

####################
class D
  instance_eval do
    define_method("i_method") do
      print "\nin i_method"
    end
    def ii_method
      print "\nin ii_method"
    end
  end
  class_eval do
    define_method("c_method") do
      print "\nin c_method"
    end
    def cc_method
      print "\nin cc_method"
    end
  end
end

D.new.i_method
D.ii_method
D.new.c_method
D.new.cc_method

D.i_method
##################

results in

in i_method
in ii_method
in c_method
in cc_method
metaruby2.rb:163: undefined method `i_method' for D:Class (NoMethodE
rror)

so why is

D.i_method wrong and
D.new.i_method correct?

BTW, I know the right way of solving this, but I'm somehow missing
something which would explain it to me in a consitent manner

here the solution:
class D
  (class << self; self; end).instance_eval do  #class_eval also does
the job
    define_method( :iii_method ) do
      print "\nin iii_method"
    end
  end
end

D.iii_method

which works fine. 

Artur