ruby 来自https://www.jimmycuadra.com/posts/metaprogramming-ruby-class-eval-and-instance-eval/

Posted 2021-05-15

class Person
end

Person.class_eval do
  def say_hello
   "Hello!"
  end
end

jimmy = Person.new
jimmy.say_hello # "Hello!"

###############################################

class Person
end

Person.instance_eval do
  def human?
    true
  end
end

Person.human? # true