class_eval method in ruby

:class_eval method is one of the way to define the class re-opening code in a more flexible manner. It works on any variable that references a class whereas re-opening a class requires defining a constant.

# Classic class re-opening style
class String
  def m
    puts 'hello!'
  end
end

# Class eval style
# The extra code is used to make the example a bit more re-usable/abstracted
def add_method_to_class(the_class)
  the_class.class_eval do
    def m
      puts 'hello!'
    end
  end
end

add_method_to_class String
'abc'.m # => hello!