On Thu, May 12, 2011 at 2:40 AM, Brian Xue <brxue.cn / gmail.com> wrote:
> Hello,
>
> I have some puzzles, when I start irb,
>
> puts self
> => main
> puts self.class
> => Object
>
> After that, if I write the following method,
>
> def hello; end
> =>nil
>
> then where is the method "hello" defined? within Object? but
>
> self.class.instance_methods.include? :hello
> =>false

Right, because its not defined as an instance method in the Object
class, its defined as an instance method in the singleton class of the
current object (main).

So, in IRB for Ruby 1.8.7 after the above

(class <<self; end).instance_methods.include? "hello"
=> true

For some reason, IRB for Ruby 1.9.2 is weird, and I can't find the
method anywhere, even though the method works. Even respond_to?
ignores it:

ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-linux]
christopher@ubuntu:~$ irb
ruby-1.9.2-p180 :001 > def hello
ruby-1.9.2-p180 :002?>   "hello"
ruby-1.9.2-p180 :003?>   end
 => nil
ruby-1.9.2-p180 :004 > hello
 => "hello"
ruby-1.9.2-p180 :005 > self.respond_to? :hello
 => false

Its back to working again in head, which shows the cleaner 1.9 syntax:

christopher@ubuntu:~$ ruby -v
ruby 1.9.3dev (2011-05-02 trunk 31407) [x86_64-linux]
christopher@ubuntu:~$ irb
ruby-head :001 > def hello
ruby-head :002?>   "hello"
ruby-head :003?>   end
 => nil
ruby-head :004 > hello
 => "hello"
ruby-head :005 > self.respond_to? :hello
 => true
ruby-head :006 > self.singleton_class.instance_methods.include? :hello
 => true