delegate.rb

require 'delegate'

class Child
  def open
    "HELLO WORLD"
  end
  def ping
    "PING"
  end
end

class Parent < SimpleDelegator
end

parent = Parent.new(Child.new)

puts parent.ping
puts parent.send(:ping)

puts parent.open
puts parent.send(:open)

gistfile1.txt

$ ruby delegate.rb

PING
PING
HELLO WORLD
delegate.rb:21:in `initialize': wrong number of arguments (0 for 1..3) (ArgumentError)
	from delegate.rb:21:in `open'
	from delegate.rb:21:in `<main>'

It works if you use public_send instead of send. There is a private method called open somewhere in the inheritance chain that causes the error...

ping, send(:ping) and #open calls go via Delegator#method_missing but #send(:open) doesn't because Delegator includes a bunch of methods from Kernel

https://github.com/rubysl/rubysl-delegate/blob/2.0/lib/rubysl/delegate/delegate.rb#L44-L50

One of these methods is Kernel#open so send(:open) sees it in the inheritance chain for your parent and calls this directly instead of going via method_missing

You can "fix it":

require 'delegate'

class Delegator
  undef_method :open
end

and file a bug report to ruby-dev I guess.

Thanks @carvil and @hosiawak. It's an external library calling send so I can't change that without monkey patching.

@hosiawak, that's what's doing it! I worked out what was happening (i.e. calling Kernel#open) but couldn't work out why