You can't include a class description in a method, it causes SyntaxError. But you can require a file with a class description in a method, please check https://gist.github.com/4431893

gistfile1.rb

def f
  class C
    def g
      'hello world'
    end
  end
  puts C.new.g
end

f # => SyntaxError, class definition in method body