From: Michael Edgar <adgar@...>
Date: 2011-04-18T08:11:01+09:00
Subject: Re: Undefine Method Only Marks the Method Undefined

Hi Su,

The difference between undef_method and remove_method is, as you
point out, that undef_method marks the method as undefined.

This difference is most simply demonstrated with inheritance: if method dispatch reaches an
undefined method, it stops looking for the method and gives an error. However,
remove_method eliminates the method entirely, and the next ancestor will then
be considered.

In code:
class A
  def foo; 'foo'; end
  def bar; 'bar'; end
end
class B < A
  def foo; 'B foo'; end
  def bar; 'B bar'; end

  remove_method :foo
  undef_method :bar
end
class C < B
end

C.new.foo  #=> 'foo'
C.new.bar  #=> NoMethodError: undefined method `bar' for #<C:0x0000010161f7e0>

Regarding Marshal.dump, If you use remove_method instead of undef_method, you will
in fact be able to dump the object again:

obj = Object.new
def obj.foo
end

Marshal.dump(obj)  #=> TypeError: singleton can't be dumped

class << obj
  remove_method :foo
end

Marshal.dump(obj)  #=> "\x04\bo:\vObject\x00"

The (simplified) technical reason why undef_method doesn't work and remove_method does,
regarding Marshal.dump, is that undef_method leaves the undefined flag as additional state
in the singleton class, whereas remove_method will leave `obj` with an empty singleton class,
identical to Object.

Hope this helps,

Michael Edgar
adgar@carboni.ca
http://carboni.ca/

On Apr 17, 2011, at 6:37 PM, Su Zhang wrote:

> Hello,
> 
> According to the book Programming Ruby by Dave Thomas, "an
> undefined method still exists - it is simply marked as being undefined."
> Why is it not completely removed?
> 
> I find this somewhat related to the fact that an object still cannot be
> dumped after "undefing" its singleton methods.
> 
> obj = Object.new
> 
> def obj.foo
> end
> 
> p obj.respond_to? :foo  # => true
> 
> class << obj
>  undef foo
> end
> 
> p obj.respond_to? :foo  # => true
> 
> Marshal.dump(obj)       # => error
> 
> -- 
> Posted via http://www.ruby-forum.com/.