I have a module and a class.

module Dog
  def speak
    "woof"
  end
end

class Daschund; end

I create two separate instances of the class.

sausage = Daschund.new
saveloy = Daschund.new

If I want to add Dog#woof as an instance method of my two new objects, I can do it in two ways:

class << sausage
  include Dog
end

> sausage.speak
=> "woof"

saveloy.extend Dog

> saveloy.speak
=> "woof"

Are the two methods equivalent? I know the first adds the module's method to the object's meta-class. Does object#extend do the same thing? Or is it doing something slightly different? Is there any way to prove this?

CodePudding user response：

According to the docs for Object, extending an object with a module means including that module to the object's singleton class:

extend: Includes the given modules in the singleton class of self.

Inspecting the ancestors of both objects' singleton classes confirms this:

sausage.singleton_class.ancestors
#=> [#<Class:#<Daschund:0x00007fa6af92e868>>, Dog, Daschund, Object, Kernel, BasicObject]

saveloy.singleton_class.ancestors
#=> [#<Class:#<Daschund:0x00007fa6af92e778>>, Dog, Daschund, Object, Kernel, BasicObject]

The actual implementation details of course depend on the Ruby implementation and version you are using. For MRI/YARV, you have rb_extend_object defined as:

void
rb_extend_object(VALUE obj, VALUE module)
{
    rb_include_module(rb_singleton_class(obj), module);
}