Programming Ruby 3.3

Reopening Classes

While we’re talking about classes in Ruby, we feel like we should at least mention one of the most unique features of Ruby’s class structure: the ability to reopen a class definition and add new methods or variables to it at any time, even classes that are part of the third-party tools or the standard library.

In other words, if you have something like this in Ruby:

class Book
  attr_accessor :title

  # and a bunch of other stuff
end

Later, you can do this:

class Book
  def uppercase_title
    title.upcase
  end
end

If you declare class Book and a Book class already exists, Ruby won’t give an error, and the new definitions in the second declaration will be added to the existing class. This is true even ...