gistfile1.txt

$ ruby -v kw.rb
ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-darwin13.0]
=>  let's try with m(o)
#<Object:0x007f88219791d0>
=>  and with m(**o)
--- to_hash ---
{:a=>1}
70111327930320
#<Object:0x007f88219791d0>
=>  how about mk(o)
--- to_hash ---
{:a=>1}
70111327929960
1
=>  and mk(**o)
--- to_hash ---
{:a=>1}
70111327929620
--- to_hash ---
{:a=>1}
70111327929400
1

kw.rb

def m(a)
  a
end

def mk(a:)
  a
end

o = Object.new

def o.to_hash
  puts "--- to_hash ---"
  h = { a: 1 }
  p h, h.object_id
  h
end

puts "=>  let's try with m(o)"
p m(o)

puts "=>  and with m(**o)"
p m(**o)

puts "=>  how about mk(o)"
p mk(o)

puts "=>  and mk(**o)"
p mk(**o)