# Instance\_eval && class\_eval **URL:** \ **Category:** ruby-talk\ **Created:** [21 October 2006 10:55 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152 "2006-10-21T10:55:18Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Artur\_Merke](https://avatars.discourse-cdn.com/v4/letter/a/f9ae1b/32.png) [@Artur\_Merke](https://rubytalk.org/u/Artur_Merke)\ **Post date:** [21 October 2006 10:55 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152/1 "2006-10-21T10:55:18Z")
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 ---
**Author:** ![Xavier\_Noria](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/xavier_noria/32/1867_2.png) [@Xavier\_Noria](https://rubytalk.org/u/Xavier_Noria)\ **Post date:** [21 October 2006 11:11 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152/2 "2006-10-21T11:11:43Z")
define\_method defines an instance method both in the instance\_eval and class\_eval sections of your code. To be able to call    D.method method needs to be a class instance method of D (i.e. a method of the singleton class of D herself). -- fxn > **···** > > On Oct 21, 2006, at 12:55 PM, Artur Merke wrote: ---
**Author:** ![Gavin\_Kistner2](https://avatars.discourse-cdn.com/v4/letter/g/b9e5f3/32.png) [@Gavin\_Kistner2](https://rubytalk.org/u/Gavin_Kistner2)\ **Post date:** [21 October 2006 13:35 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152/3 "2006-10-21T13:35:17Z")
Artur Merke wrote: > so why is > D.i\_method wrong and > D.new.i\_method correct? I'm not surprised by that. I'm surprised that standard def...end syntax inside instance\_eval behaves differently from being in the class scope, when the self object in both cases is the same. irb(main):001:0\> class Foo irb(main):002:1\> p "Defining aa in #{self}##{self.object\_id}" irb(main):003:1\> def aa; p :aa; end irb(main):004:1\> instance\_eval{ irb(main):005:2\* p "Defining bb in #{self}##{self.object\_id}" irb(main):006:2\> def bb; p :bb; end irb(main):007:2\> } irb(main):008:1\> end "Defining aa in Foo#1699170" "Defining bb in Foo#1699170" =\> nil irb(main):009:0\> Foo.new.aa :aa =\> nil irb(main):010:0\> Foo.bb :bb =\> nil ---
**Author:** ![Artur\_Merke](https://avatars.discourse-cdn.com/v4/letter/a/f9ae1b/32.png) [@Artur\_Merke](https://rubytalk.org/u/Artur_Merke)\ **Post date:** [21 October 2006 12:45 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152/4 "2006-10-21T12:45:15Z")
Xavier Noria schrieb: > define\_method defines an instance method both in the instance\_eval > and class\_eval sections of your code. To be able to call > >    D.method > > method needs to be a class instance method of D (i.e. a method of the > singleton class of D herself). yes (see also my solution I provided at the end of my msg), but this was not the question, the question was (and still is), why(!) def a end and define\_method(:a) do end behave differently in instance\_eval (and not in class\_eval and anywhere else) ... AHA Artur > **···** > > > On Oct 21, 2006, at 12:55 PM, Artur Merke wrote: ---
**Author:** ![ts1](https://avatars.discourse-cdn.com/v4/letter/t/a8b319/32.png) [@ts1](https://rubytalk.org/u/ts1)\ **Post date:** [21 October 2006 13:40 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152/5 "2006-10-21T13:40:42Z")
> I'm not surprised by that. I'm surprised that standard def...end syntax > inside instance\_eval behaves differently from being in the class scope, > when the self object in both cases is the same. def is a keyword and it don't use self to know where the method must be defined but it use an "internal" variable the "current" class. > irb(main):001:0\> class Foo here you have                     self = current class = Foo > irb(main):002:1\> p "Defining aa in #{self}##{self.object\_id}" > irb(main):003:1\> def aa; p :aa; end > irb(main):004:1\> instance\_eval{ in instance\_eval you have                                        self = Foo                       current class = Foo singleton class > irb(main):005:2\* p "Defining bb in #{self}##{self.object\_id}" > irb(main):006:2\> def bb; p :bb; end Guy Decoux ---
**Author:** ![Xavier\_Noria](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/xavier_noria/32/1867_2.png) [@Xavier\_Noria](https://rubytalk.org/u/Xavier_Noria)\ **Post date:** [21 October 2006 13:36 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152/6 "2006-10-21T13:36:35Z")
Ah yes, I see it now. I join the question, a posteriori looks like within instance\_eval we are in the context of the singleton class of D, because D.ii\_method works. But puts self does not seem to output that one there    irb(main):010:0\> class D; instance\_eval do; puts self; end; end    D    =\> nil    irb(main):011:0\> class D; puts (class \<\< self; self; end); end    #\    =\> nil so I don't understand it either. -- fxn > **···** > > On Oct 21, 2006, at 2:45 PM, Artur Merke wrote: > > > the question was (and still is), why(!) > > > > def a > > end > > > > and > > > > define\_method(:a) do > > end > > > > behave differently in instance\_eval (and not in class\_eval and anywhere > > else) ... ---
**Author:** ![Xavier\_Noria](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/xavier_noria/32/1867_2.png) [@Xavier\_Noria](https://rubytalk.org/u/Xavier_Noria)\ **Post date:** [21 October 2006 15:20 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152/7 "2006-10-21T15:20:44Z")
Good! Is the existence of that "current" class vs self documented somewhere or do you know it from the sources? Does it have a name in jargon? What about the fact that instance\_eval switches current to the singleton class of self? I saw it works for objects which are not classes as well (which is a redundant exercise to reinforce this):    class A    end    a = A.new    b = A.new    a.instance\_eval do      def croak        puts "croak!"      end    end    a.croak    b.croak gives    croak!    tmp/foo.rb:13: undefined method `croak' for #\ (NoMethodError) so the "current" class is the singleton class of the object a as well. -- fxn > **···** > > On Oct 21, 2006, at 3:40 PM, ts wrote: > > > \> I'm not surprised by that. I'm surprised that standard def...end syntax > > \> inside instance\_eval behaves differently from being in the class scope, > > \> when the self object in both cases is the same. > > > > def is a keyword and it don't use self to know where the method must be > > defined but it use an "internal" variable the "current" class. ---
**Author:** ![ts1](https://avatars.discourse-cdn.com/v4/letter/t/a8b319/32.png) [@ts1](https://rubytalk.org/u/ts1)\ **Post date:** [21 October 2006 15:38 UTC](https://rubytalk.org/t/instance-eval-class-eval/32152/8 "2006-10-21T15:38:13Z")
> Is the existence of that "current" class vs self documented somewhere > or do you know it from the sources? Does it have a name in jargon? ruby\_class : it's in the source, this mean that it's documented 🙂 > What about the fact that instance\_eval switches current to the > singleton class of self? I saw it works for objects which are not > classes as well (which is a redundant exercise to reinforce this): Well a class can be seen as an object or as a class, i.e. with a class you can define a singleton method or a method. #instance\_eval work with any object, and in this case a class is just an   object like any other and def will define a singleton method #class\_eval work only with classes (and modules) and in this case def will   define instance methods. Guy Decoux