oop/basic_object/answers_nothing_no_parens.rb

tests/_examples/oop/basic_object/answers_nothing_no_parens.rb in the metorex repository.

# An object rooted at BasicObject answers only what BasicObject defines.
# Everything Kernel and Object add arrives through Object, which such a class
# never inherits from.
held = BasicObject.new

begin
  held.to_s
rescue NoMethodError => refused
  puts refused.message
end

# A method taken from Kernel and bound onto one still runs.
p ::Kernel.instance_method(:frozen?).bind(held).call
p ::Kernel.instance_method(:class).bind(held).call

# A class rooted there may take Kernel on, and then it answers everything
# Kernel defines.
class WithKernel < BasicObject
  include ::Kernel
end

p WithKernel.new.frozen?

# A class may also take a single one of Kernel's methods on.
class OnlyRespondTo < BasicObject
  define_method :respond_to?, ::Kernel.instance_method(:respond_to?)

  def named
    :named
  end
end

asked = OnlyRespondTo.new
p [asked.respond_to?(:named), asked.respond_to?(:missing)]

Output:

undefined method 'to_s' for an instance of BasicObject
false
BasicObject
false
[true, false]