# About class and module

**URL:** <https://www.ruby-forum.com/t/about-class-and-module/180188>\
**Category:** Ruby\
**Created:** [December 14, 2009, 10:59am UTC](https://www.ruby-forum.com/t/about-class-and-module/180188 "2009-12-14T10:59:08Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\
**Post date:** [December 14, 2009, 10:59am UTC](https://www.ruby-forum.com/t/about-class-and-module/180188/1 "2009-12-14T10:59:08Z")

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Hello,

Coz Ruby’s class and module are close to Perl’s, I think it’s not hard  
to understand for them.  
I wrote a note below for learning them, if I’m not correct, please  
point it out, thanks.

1. class is defined with the keyword of “class”, module is defined  
with the keyword of “module”.

for example, define a class:

class Myclass  
def echo (a,b)  
puts a + b  
end  
end

define a module:

module Mymod  
def echo (a,b)  
puts a+b  
end  
end

1. If we want to call the methods in a class, need to instantiate the  
class with “new” (except the class method).  
If we want to call the methods in a module, need to include it  
(except the module method).

call the methods in a class:

class Myclass  
def echo (a,b)  
puts a + b  
end  
end

x = Myclass.new  
x.echo(“hello”,“world”)

call the methods in a module:

module Mymod  
def echo (a,b)  
puts a+b  
end  
end

include Mymod  
echo “hello”,“world”

1. class itself has class method, module itself has module method.  
if we call a class method or a module method, won’t use “new” or  
“include”.

the class method:

class Myclass  
def Myclass.echo(a)  
puts a  
end  
end

Myclass.echo(“hello world”)

the module method:

class Mymod  
def Mymod.echo(a)  
puts a  
end  
end

Mymod.echo “hello world”

1. the calling and called relationship between class and module:

A module can contain methods, constants, other modules, and even  
classes.  
A module can inherit from another module, but it may not inherit from a  
class.  
As a class may include a module, it may also include modules that have  
inherited other modules.

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**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\
**Post date:** [December 14, 2009, 2:37pm UTC](https://www.ruby-forum.com/t/about-class-and-module/180188/2 "2009-12-14T14:37:07Z")

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Ruby N. wrote:

> Coz Ruby’s class and module are close to Perl’s, I think it’s not hard  
> to understand for them.  
> I wrote a note below for learning them, if I’m not correct, please  
> point it out, thanks.

Worth also pointing out:

- class Class inherits from class Module

- a class _is_ a module

- the extra functionality that a class has, over and above a module, is  
that you can create an instance of a class.

> 1. If we want to call the methods in a class, need to instantiate the  
> class with “new” (except the class method).  
> If we want to call the methods in a module, need to include it  
> (except the module method).

In addition: Objects have singleton classes, which are private to that  
object. You can use ‘extend’ to add a module to an individual object’s  
singleton class.

module Foo  
def greet  
puts “Hello, #{self}!”  
end  
end  
s = “you”  
s.extend Foo  
s.greet # =\> “Hello, you!”

Note we have not touched class String at this point.

But since classes are objects too, the same applies to classes. You can  
use a module to add “class methods” to a class.

String.extend Foo  
String.greet # =\> “Hello, String!”

> 1. class itself has class method, module itself has module method.

There are also ‘module functions’, defined like this:

module Foo  
def bar  
puts “hello”  
end  
module\_function :bar  
end

Foo.bar

(There is a subtle difference between this and def Foo.bar, see if you  
can work it out…)

> 1. the calling and called relationship between class and module:
> 
> A module can contain methods, constants, other modules, and even  
> classes.

What do you mean by “contain” here?

irb(main):026:0\> module X  
irb(main):027:1\> end  
=\> nil  
irb(main):028:0\> class Y  
irb(main):029:1\> include X  
irb(main):030:1\> end  
=\> Y  
irb(main):031:0\> module Z  
irb(main):032:1\> include Y  
irb(main):033:1\> end  
TypeError: wrong argument type Class (expected Module)  
from (irb):32:in `include’  
from (irb):32  
from :0

1. modules (and hence also classes) form a namespace mechanism, which is  
entirely orthogonal to inheritance and the method call chain.

module X  
class Y  
Z = 3  
end  
end

puts X::Y::Z  
y = X::Y.new

This is maybe what you were alluding to in point 4, because you  
mentioned constants. But it must be stated this is _nothing_ to do with  
inheritance at all.

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**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\
**Post date:** [December 14, 2009, 3:10pm UTC](https://www.ruby-forum.com/t/about-class-and-module/180188/3 "2009-12-14T15:10:47Z")

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On Mon, Dec 14, 2009 at 9:37 PM, Brian C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> s = “you”

Thanks for the points. That looks very interesting.  
As you said “Objects have singleton classes”, what’s “singleton  
class”? could you show a description by examples?  
I never saw that before. Thanks again.

---

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**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\
**Post date:** [December 14, 2009, 4:05pm UTC](https://www.ruby-forum.com/t/about-class-and-module/180188/4 "2009-12-14T16:05:12Z")

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Ruby N. wrote:

> As you said “Objects have singleton classes”, what’s “singleton  
> class”? could you show a description by examples?

s1 = “a string”  
s2 = “another string”

def s1.wibble  
puts “bibble”  
end

s1.wibble # =\> “bibble”  
s2.wibble # =\> NoMethodError: undefined method `wibble’

Every object has a ‘singleton class’ which contains methods private to  
that object.

A class, like class Foo, is an instance of class Class. So the ‘class  
methods’ of Foo are just methods in the singleton class of Foo.

class Foo; end  
def Foo.wibble  
puts “I am a singleton method”  
end  
Foo.wibble

Another way to add methods into the singleton class is like this:

s1 = “hello”  
class \<\< s1  
def hey; puts “hey!”; end  
end  
s1.hey

So of course this works for classes (and class methods) too:

class Foo  
class \<\< self  
def bar  
puts “In bar”  
end  
end  
end  
Foo.bar

The singleton class is itself an object. It’s quite hard to grab hold  
of, but the above gives you a clue to the trick:

s = “hello”  
sc = class \<\< s; self; end

So sometimes you will see code like this:

class \<\< s; self; end.class\_eval { define\_method(:bar) { puts “bar!” }  
}

An object with a singleton class loses it when it is dup’d (otherwise it  
wouldn’t be a singleton any more 🙂

a = “hello”  
def a.world; puts “world”; end  
b = a.dup  
a.world # “world”  
b.world # NoMethodError: undefined method `world’

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**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\
**Post date:** [December 14, 2009, 4:25pm UTC](https://www.ruby-forum.com/t/about-class-and-module/180188/5 "2009-12-14T16:25:42Z")

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On Mon, Dec 14, 2009 at 3:10 PM, Ruby N. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> > end  
> > String.extend Foo  
> > String.greet # =\> “Hello, String!”
> 
> Thanks for the points. That looks very interesting.  
> As you said “Objects have singleton classes”, what’s “singleton  
> class”? could you show a description by examples?  
> I never saw that before. Thanks again.

A singleton class of an object (also referred sometimes as eigenclass  
or metaclass, although there has been much debate about which term to  
use), is a special class that can be attached to each object to  
specify per-object behaviour. This means, a place where you put  
methods that can only be called on a specific instance. Brian already  
gave you an example by means of extending an object with a module.  
Here are some other examples:

irb(main):001:0\> s = “hello”  
=\> “hello”  
irb(main):006:0\> def s.to\_upper\_case  
irb(main):007:1\> self.upcase  
irb(main):008:1\> end  
=\> nil  
irb(main):009:0\> s.to\_upper\_case  
=\> “HELLO”

As you can see we define a method for s, and only s, this is the only  
object which will have the method to\_upper\_case. No other string will  
have it. As Brian pointed out, you can extend a single object with a  
module, and what this does is add the module’s methods to the  
singleton class of the object:

irb(main):010:0\> module Testing  
irb(main):011:1\> def test  
irb(main):012:2\> “what a test !”  
irb(main):013:2\> end  
irb(main):014:1\> end  
=\> nil  
irb(main):015:0\> s.extend Testing  
=\> “hello”  
irb(main):016:0\> s.test  
=\> “what a test !”

(s is the previous string). This strategy is a really good way to add  
functionality to object, since you don’t mess with core classes (which  
can break things), but only act on the specific objects you want to  
modify.

In Ruby, classes are objects too (they are instances of the Class  
class):

irb(main):017:0\> String.class  
=\> Class

This means that they also have a singleton class where to place  
methods that can only be called on that specific object. As the object  
is a class, they are usually called class methods, but they are not  
really different to singleton methods on any object:

irb(main):018:0\> def String.test  
irb(main):019:1\> “testing again”  
irb(main):020:1\> end  
=\> nil  
irb(main):021:0\> String.test  
=\> “testing again”

Hope this helps,

Jesus.

---

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**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\
**Post date:** [December 14, 2009, 4:08pm UTC](https://www.ruby-forum.com/t/about-class-and-module/180188/6 "2009-12-14T16:08:28Z")

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Brian C. wrote:

> Every object has a ‘singleton class’ which contains methods private to  
> that object.

I shouldn’t have said “private to”, because private methods are  
something else.

“methods unique to that object” would have been better.

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**Author:** ![rubynut](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/8e8cbc/32.png) [@rubynut](https://www.ruby-forum.com/u/rubynut)\
**Post date:** [December 15, 2009, 3:17am UTC](https://www.ruby-forum.com/t/about-class-and-module/180188/7 "2009-12-15T03:17:10Z")

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Thanks a lot Brian and Jesus.  
In my last years experience of programming, I never met something like  
singleton class.  
It seems so flexible and useful. I will put it into my practice.  
So ruby has some really better features than others, and thanks all  
for the always kind helps.

// Jenni