Recognize alias, undef, BEGIN and END

Learn

alias introduces another name for an existing method at that point in the program. undef makes a method unavailable through that class. These forms affect an interface, so ordinary method definitions are often clearer when no aliasing is needed. The respond_to? method checks whether an object offers the named method.

BEGIN executes before the main body of a Ruby script. END registers work for normal Ruby process exit. The browser playground evaluates code in a Worker that the host later terminates, rather than shutting down a Ruby command-line process normally. Use a local Ruby script when investigating process-exit behavior; use ensure for cleanup during normal Ruby control flow.

Example

BEGIN { puts "start" }
class Message
  def text
    "hello"
  end
  alias original_text text
  undef text
end
puts Message.new.original_text
p Message.new.respond_to?(:text)
END { puts "finish" }

Experiment

• Try Keep two method names

  Predict which names are callable on each class. Remove undef text and compare how the subclass interface changes. Does the superclass change?

• Try Redefine after aliasing

  Predict both calls. Move the alias line below the second text definition and compare which implementation message uses.

• Try Run initialization before the body

  Predict the order of the printed lines. Move BEGIN below the second puts and run again. How does BEGIN differ from an ordinary block at that position?

Explore the official manual

• Official Ruby 4.1 development documentation: Miscellaneous
• Official Ruby 4.1 development documentation: Module#alias_method