# Include vs. extend

**URL:** <https://www.ruby-forum.com/t/include-vs-extend/97886>\
**Category:** Ruby\
**Created:** [July 11, 2006, 2:16am UTC](https://www.ruby-forum.com/t/include-vs-extend/97886 "2006-07-11T02:16:34Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Daniel\_DeLorme](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9d8465/32.png) [@Daniel\_DeLorme](https://www.ruby-forum.com/u/Daniel_DeLorme)\
**Post date:** [July 11, 2006, 2:16am UTC](https://www.ruby-forum.com/t/include-vs-extend/97886/1 "2006-07-11T02:16:34Z")

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I was under the impression that include and extend were basically  
module-level  
and object-level versions of the same thing, that including a module at  
the  
class level had the same effect as extending an object. But it seems  
there is a  
big difference:

class Foo  
include Enumerable  
end  
class Bar  
def initialize  
extend Enumerable  
end  
end  
(class \<\< Foo.new;self;end).ancestors #=\> [Foo, Enumerable, Object,  
Kernel]  
(class \<\< Bar.new;self;end).ancestors #=\> [Enumerable, Bar, Object,  
Kernel]

This makes a whole world of difference when you want to wrap certain  
methods  
with additional functionality. In such cases you _have_ to use extend  
(or ugly  
techniques like aliasing the old method). Why the difference? In what  
cases  
should I use include and what cases should I use extend?

Daniel

---

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**Author:** ![Daniel\_DeLorme](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9d8465/32.png) [@Daniel\_DeLorme](https://www.ruby-forum.com/u/Daniel_DeLorme)\
**Post date:** [July 11, 2006, 2:25am UTC](https://www.ruby-forum.com/t/include-vs-extend/97886/2 "2006-07-11T02:25:52Z")

</div>

Hi –

On Tue, 11 Jul 2006, Daniel DeLorme wrote:

> extend Enumerable  
> end  
> end  
> (class \<\< Foo.new;self;end).ancestors #=\> [Foo, Enumerable, Object, Kernel]  
> (class \<\< Bar.new;self;end).ancestors #=\> [Enumerable, Bar, Object, Kernel]
> 
> This makes a whole world of difference when you want to wrap certain methods  
> with additional functionality. In such cases you _have_ to use extend (or  
> ugly techniques like aliasing the old method). Why the difference? In what  
> cases should I use include and what cases should I use extend?

It’s understood that if you extend an object, it’s because you _want_  
to put the module ahead of the object’s class in the method look-up  
path. Otherwise, you’d put the functionality you want in the class  
🙂 By the same token, it would be very unusual to put extend in  
initialize.

Basically you use extend when you want to control behavior via modules  
on a per-object basis. A common use is for classes:

module M  
def meth  
puts “Hi”  
end  
end

class C  
extend M  
end

C.meth # Hi

obj.extend(Mod) is equivalent to:

class \<\< obj  
include Mod  
end

(or very nearly equivalent; there may be some arcane difference I’m  
not remembering). So it’s all really include operations – but, when  
done with extend, the class doing the including is a singleton class.

David

---

<div class="post-metadata">

**Author:** ![Daniel\_DeLorme](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9d8465/32.png) [@Daniel\_DeLorme](https://www.ruby-forum.com/u/Daniel_DeLorme)\
**Post date:** [July 11, 2006, 2:32am UTC](https://www.ruby-forum.com/t/include-vs-extend/97886/3 "2006-07-11T02:32:39Z")

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In the first case, the proxy that refers to the Enumerable module is  
inserted between Foo class and Object class. In the second case, an  
instance of a Bar class during initialization receives a new class  
which is the same proxy object and this proxy’s superclass is Bar  
class. BTW, check out this book at [http://rhg.rubyforge.org](http://rhg.rubyforge.org). It has a  
very nice chapter about internal structure of Ruby object model.

Kent.