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comp.lang.ruby

extend keyword in class definition

ruud grosmann

9/30/2008 9:24:00 AM

Hi list,

I am reading the book 'design patterns in ruby'. On page 201 I find
this code example:

require 'forwardable'

class WriterDecorator
extend Forwardable

def_delegators :@real_writer, :write_line, :rewind, :pos, :close

def initialize (real_writer)
@real_writer = real_writer
end
end


I am a bit surprised by the 'extend Forwardable' line. Can anybody
explain me what this is? Is it the same as 'include Forwardable' or is
it the same as 'class WriterDecorator < Forwardable'?

thanks in advance, Ruud
12 Answers

James Coglan

9/30/2008 9:31:00 AM

0

[Note: parts of this message were removed to make it a legal post.]

>
> I am a bit surprised by the 'extend Forwardable' line. Can anybody
> explain me what this is? Is it the same as 'include Forwardable' or is
> it the same as 'class WriterDecorator < Forwardable'?


It's neither. extend() is method that all objects have (classes are objects
too). It takes the methods from a module (in this case Forwardable) and adds
them as singleton methods to the object. Forwardable contains the method
'def_delegators', which we see here has been added as a singleton method to
WriterDecorator.

Sebastian Hungerecker

9/30/2008 9:33:00 AM

0

ruud grosmann wrote:
> I am a bit surprised by the 'extend Forwardable' line. Can anybody
> explain me what this is? Is it the same as 'include Forwardable' or is
> it the same as 'class WriterDecorator < Forwardable'?

No, it's the same as WriterDecorator.extend Forwardable or
class <<WriterDecorator
include Forwardable
end
I.e. the module gets included in the singelton class of WriterDecorator, not
in WriterDecorator itself (or in other words: the module's methods get added
as singleton methods of the class instead of as instance methods)

--
Jabber: sepp2k@jabber.org
ICQ: 205544826

David A. Black

9/30/2008 9:33:00 AM

0

Hi --

On Tue, 30 Sep 2008, ruud grosmann wrote:

> Hi list,
>
> I am reading the book 'design patterns in ruby'. On page 201 I find
> this code example:
>
> require 'forwardable'
>
> class WriterDecorator
> extend Forwardable
>
> def_delegators :@real_writer, :write_line, :rewind, :pos, :close
>
> def initialize (real_writer)
> @real_writer = real_writer
> end
> end
>
>
> I am a bit surprised by the 'extend Forwardable' line. Can anybody
> explain me what this is? Is it the same as 'include Forwardable' or is
> it the same as 'class WriterDecorator < Forwardable'?

It's like a per-object include operation.

module M
def speak
puts "Hi!"
end
end

obj = Object.new
obj.extend(M)
obj.speak # "Hi!"

The example you've got extends the class object WriterDecorator. It's
like this:

class C
extend M
end

C.speak # "Hi!"


David

--
Rails training from David A. Black and Ruby Power and Light:
Intro to Ruby on Rails January 12-15 Fort Lauderdale, FL
Advancing with Rails January 19-22 Fort Lauderdale, FL *
* Co-taught with Patrick Ewing!
See http://www.r... for details and updates!

ruud grosmann

9/30/2008 9:57:00 AM

0

> It's neither. extend() is method that all objects have (classes are objects
> too). It takes the methods from a module (in this case Forwardable) and adds
> them as singleton methods to the object. Forwardable contains the method
> 'def_delegators', which we see here has been added as a singleton method to
> WriterDecorator.
>

thank you for your replies. But I'm afraid that I need some more
explanation. Isn't a singleton method a method that applies to the
singleton class of one object? And in this case we're talking about a
class, not an object?

As such, Forwardable is mixed in, is my first thought. What have I
understood wrongly?

regards, RUud

David A. Black

9/30/2008 10:23:00 AM

0

Hi --

On Tue, 30 Sep 2008, ruud grosmann wrote:

>> It's neither. extend() is method that all objects have (classes are objects
>> too). It takes the methods from a module (in this case Forwardable) and adds
>> them as singleton methods to the object. Forwardable contains the method
>> 'def_delegators', which we see here has been added as a singleton method to
>> WriterDecorator.
>>
>
> thank you for your replies. But I'm afraid that I need some more
> explanation. Isn't a singleton method a method that applies to the
> singleton class of one object? And in this case we're talking about a
> class, not an object?
>
> As such, Forwardable is mixed in, is my first thought. What have I
> understood wrongly?

Classes *are* objects.

c = Class.new # c is an object whose class is Class
c.extend(SomeModule)

Every class is an instance of the class Class, so you can extend
classes just like other objects.

I've always said that the answer to 75% of all questions about Ruby
is: Classes are objects :-) It's definitely a very key thing.


David

--
Rails training from David A. Black and Ruby Power and Light:
Intro to Ruby on Rails January 12-15 Fort Lauderdale, FL
Advancing with Rails January 19-22 Fort Lauderdale, FL *
* Co-taught with Patrick Ewing!
See http://www.r... for details and updates!

Sebastian Hungerecker

9/30/2008 10:28:00 AM

0

ruud grosmann wrote:
> Isn't a singleton method a method that applies to the
> singleton class of one object?

It is a method that is defined on the singleton class of an object, yes.
As a singleton class by definition only has one instance that means that a
singleton method is a method that can be called on a single object and that
object only.

> And in this case we're talking about a
> class, not an object?

A class is an object:
>> String.is_a? Class
=> true
>> String.is_a? Object
=> true
>> String.object_id
=> -605959678

> As such, Forwardable is mixed in, is my first thought.

It is mixed in. It's mixed into WriterDecorator's singleton class.

> What have I understood wrongly?

I'm not sure. Maybe you mistakenly assumed that a class is its own singleton
class?

HTH,
Sebastian
--
NP: Depeche Mode - Lilian
Jabber: sepp2k@jabber.org
ICQ: 205544826

David A. Black

9/30/2008 10:37:00 AM

0

Hi --

On Tue, 30 Sep 2008, Sebastian Hungerecker wrote:

> ruud grosmann wrote:
>> Isn't a singleton method a method that applies to the
>> singleton class of one object?
>
> It is a method that is defined on the singleton class of an object, yes.
> As a singleton class by definition only has one instance that means that a
> singleton method is a method that can be called on a single object and that
> object only.

Although:

class C
end
class D < C
end

def C.x
end

D.x # subclass can call superclass's singleton methods

Technically, it's also true that if you extend an object with a
module, you're not really writing singleton methods, since the methods
in the module are available to other objects that use the module. The
singleton_method_added callback won't get called, for example.


David

--
Rails training from David A. Black and Ruby Power and Light:
Intro to Ruby on Rails January 12-15 Fort Lauderdale, FL
Advancing with Rails January 19-22 Fort Lauderdale, FL *
* Co-taught with Patrick Ewing!
See http://www.r... for details and updates!

janus

9/30/2008 10:47:00 AM

0

David

Am a little confused by your codes, from all I understood in these
threads.
module M
def speak
puts "Hi!"
end
end

obj = Object.new
obj.extend(M)
obj.speak

obj.speak should be Object.speak. Why is yours different? And your
second code agreed with singleton idiom.

Janus

David A. Black

9/30/2008 10:53:00 AM

0

Hi --

On Tue, 30 Sep 2008, janus wrote:

> David
>
> Am a little confused by your codes, from all I understood in these
> threads.
> module M
> def speak
> puts "Hi!"
> end
> end
>
> obj = Object.new
> obj.extend(M)
> obj.speak
>
> obj.speak should be Object.speak. Why is yours different? And your
> second code agreed with singleton idiom.

I'm not sure what you mean by "should be". The way it's written,
Object does not have a "speak" method, but obj does. That's the effect
of extend: it adds module-based functionality to a single object.


David

--
Rails training from David A. Black and Ruby Power and Light:
Intro to Ruby on Rails January 12-15 Fort Lauderdale, FL
Advancing with Rails January 19-22 Fort Lauderdale, FL *
* Co-taught with Patrick Ewing!
See http://www.r... for details and updates!

ruud grosmann

9/30/2008 11:02:00 AM

0

OK. I think now I get what you mean.

To make sure I understood it right:
my mistake was that there are both singleton methods on classes and
singleton methods on instances of a class. Essentially the same,
because classes are objects too. In this case, we're talking about
singleton methods on the class WriterDecorator. This is done because
the Forwardable method 'def_delegators' is used by the class object
only, not by it's instances. Which would have been the case if
Forwardable was mixed in.
Please correct me if I'm wrong.

Thanks anyway for the time spent on helping me on this!

regards, Ruud

On 30/09/2008, David A. Black <dblack@rubypal.com> wrote:
> Hi --
>
> On Tue, 30 Sep 2008, Sebastian Hungerecker wrote:
>
>> ruud grosmann wrote:
>>> Isn't a singleton method a method that applies to the
>>> singleton class of one object?
>>
>> It is a method that is defined on the singleton class of an object, yes.
>> As a singleton class by definition only has one instance that means that a
>> singleton method is a method that can be called on a single object and
>> that
>> object only.
>
> Although:
>
> class C
> end
> class D < C
> end
>
> def C.x
> end
>
> D.x # subclass can call superclass's singleton methods
>
> Technically, it's also true that if you extend an object with a
> module, you're not really writing singleton methods, since the methods
> in the module are available to other objects that use the module. The
> singleton_method_added callback won't get called, for example.
>
>
> David
>
> --
> Rails training from David A. Black and Ruby Power and Light:
> Intro to Ruby on Rails January 12-15 Fort Lauderdale, FL
> Advancing with Rails January 19-22 Fort Lauderdale, FL *
> * Co-taught with Patrick Ewing!
> See http://www.r... for details and updates!
>
>