Question

given

module Foo
  def bar
    puts "foobar"
  end
end

I can do

String.extend(Foo)

and as a consequence do

String.bar # => "foobar"

Why doesnt this work?:

a = String.new
a.bar # => NoMethodError: undefined method `bar' for "":String

Is it because ‘a’ is now and instance and .extend only works against class methods? Why does it lose the ‘new’ functionality I have given String via .extend?

Answer 1

Ruby allows you to add methods from a Module to a Class in two ways: extend and include.

Using the module you gave:

module Foo
  def bar
    puts "foobar"
  end
end

With extend the methods are added as class methods, and can be called directly on the class object itself:

class Bar
  extend Foo
end

Bar.bar # => "foobar"

Alternatively, you can call it outside of the class scope:

class Bar
end

Bar.extend Foo

Bar.bar # => "foobar"

include is slightly different. It will add the methods as instance methods. They are only callable on an instance of the class:

class Bar
  include Foo
end

Bar.bar # NoMethodError
a = Bar.new
a.bar # => "foobar"

The key difference was that we first had to make an instance a, before we could call the instance method.

As sepp2k noted, extend is can be called on any Object, not just Class objects. For example, you can go:

class Bar
end

a = Bar.new
a.extend Foo
a.bar # => "foobar"

However, bar will only be added to the single instance a. If we create a new instance, it you will not be able to call it.

b = Bar.new
b.bar # => MethodNotFoundError