Question

Unless I’ve missed the entries, I can’t find include or extend in the Ruby-Doc.org documentation or in Ruby’s keyword list. I can find obj.extend but not the extend statement used in a class definition e.g.

# add module Foo's methods as class methods of TestClass
class TestClass
  extend Foo
end

or

# add module Foo's methods as instance methods of TestClass
class TestClass
  include Foo
end

As a language construct, if that’s the proper term, what are include and extend and where are they defined?

It would be nice to understand just how these two statements are implemented in the Ruby language. Thanks.

Answer 1

extend is an Object method, a convenience method for extend_object (more or less).

include is in Module, a wrapper around append_features.