From: Su Zhang <zhangsu@...>
Date: 2011-01-09T02:15:08+09:00
Subject: Re: Is singleton class of an object already created?

I just found that even though a singleton method is removed, the 
receiver is still considered a singleton and cannot be dumped.

---------------

foo = Object.new

def foo.bar
  'foo.bar'
end

p foo.singleton_methods  # => [:bar]

class << foo
  undef bar
end

p foo.singleton_methods  # => []
Marshal.dump(foo)  # error

--------------

Looking inside marhsal.c, the following check routine is found:

if (RCLASS_M_TBL(klass)->num_entries ||
  (RCLASS_IV_TBL(klass) && RCLASS_IV_TBL(klass)->num_entries > 1)) {
  rb_raise(rb_eTypeError, "singleton can't be dumped");
}

It seems that the records remains in some dispatch table even if 
Object#singleton_methods returns an empty array.

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