gistfile1.rb

require 'monkey'

Monkey.see(Object) do
  def metaclass
    class << self; self; end
  end
end

o1 = Object.new
o2 = Object.new

o1.metaclass rescue 'fail' # => "fail"
o2.metaclass rescue 'fail' # => "fail"

Monkey.patch(o1, :metaclass) { o1.metaclass } # => #<Class:#<Object:0x1011baf38>>

o1.metaclass rescue 'fail' # => "fail"
o2.metaclass rescue 'fail' # => "fail"

Monkey.patch(Object, :metaclass) do
  o1.metaclass # => #<Class:#<Object:0x1011baf38>>
  o2.metaclass # => #<Class:#<Object:0x1011baec0>>
end

o1.metaclass rescue 'fail' # => "fail"
o2.metaclass rescue 'fail' # => "fail"

Monkey.patch(Object, :metaclass)

o1.metaclass rescue 'fail' # => #<Class:#<Object:0x1011baf38>>
o2.metaclass rescue 'fail' # => #<Class:#<Object:0x1011baec0>>