# Ruby Class question

**URL:** <https://www.ruby-forum.com/t/ruby-class-question/62988>\
**Category:** Ruby\
**Created:** [June 10, 2006, 4:48am UTC](https://www.ruby-forum.com/t/ruby-class-question/62988 "2006-06-10T04:48:11Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![pallan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/71c47a/32.png) [@pallan](https://www.ruby-forum.com/u/pallan)\
**Post date:** [June 10, 2006, 4:48am UTC](https://www.ruby-forum.com/t/ruby-class-question/62988/1 "2006-06-10T04:48:11Z")

</div>

Hi all,

I am really starting to get into ruby and I am currently reading the  
PickAxe book and the Agile Rails book, but there is one part of classes  
that I don’t understand and I can’t seem to find any reference to it.

Here is a basic class as defined in the Pickaxe book:

1: class Song  
2: attr\_writer :duration  
3: end

My question is, what is the whole statement on line #2? I know what it  
does, it creates a attribute-setting method for the “duration” variable,  
but what I don’t understand is how it does it. When does that line of  
code get executed?

I am used to doing classes in C++ and PHP where the only thing outside  
methods and inside a class are class variable declarations. Is this a  
way to make variable declarations with a method?

This seems to be a very common construct in Ruby and I want to  
understand what it is and how to use it correctly. Thanks

Peer

---

<div class="post-metadata">

**Author:** ![pallan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/71c47a/32.png) [@pallan](https://www.ruby-forum.com/u/pallan)\
**Post date:** [June 10, 2006, 6:10am UTC](https://www.ruby-forum.com/t/ruby-class-question/62988/2 "2006-06-10T06:10:45Z")

</div>

Peer A. wrote:

> 3: end  
> This seems to be a very common construct in Ruby and I want to  
> understand what it is and how to use it correctly. Thanks

In Ruby, objects interact with other objects my sending messages.  
Messages usually (but not always) map to methods; methods in turn can  
manipulate instance variables.

Instance variables (the @foo things) are private, but if you want to  
create the appearance that an object has public properties, you can  
create methods that match the instance variables:

def foo  
@foo  
end

def foo=( val )  
@foo=val  
end

This is such a common convention that Ruby has a method that writes  
these methods for you:

attr\_accessor :foo

This take symbol :foo and uses it to create the pair of methods that  
reference an instance variable of the same name.

Note that you can always create such methods by hand, and the names need  
not match the internal variables; such properties are just methods  
calls:

# Create the appearance of a “public property” called ‘foo’

def foo  
@bar  
end

def foo=( val )  
@bar=val  
end

And you can add whatever code you like to these methods.

A few things to note:

Ruby methods usually return the value of the last expression evaluated,  
but methods that end with ‘=’ do not follow this; they return the value  
passed to the method.

When you use attr\_accessor, attr\_reader, or attr\_writer to create these  
methods, RDoc will list them as _attributes_, not as methods; they will  
not appear in the list of methods in the class’s RDoc.

(If you create the exact same methods by hand, though, RDoc treats them  
as methods.)

–

James B.

[http://www.ruby-doc.org](http://www.ruby-doc.org) - Ruby Help & Documentation  
[Ruby Code & Style](http://www.artima.com/rubycs/) - The Journal By & For Rubyists  
[http://www.rubystuff.com](http://www.rubystuff.com) - The Ruby Store for Ruby Stuff  
[http://www.jamesbritt.com](http://www.jamesbritt.com) - Playing with Better Toys  
[http://www.30secondrule.com](http://www.30secondrule.com) - Building Better Tools

---

<div class="post-metadata">

**Author:** ![pallan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/71c47a/32.png) [@pallan](https://www.ruby-forum.com/u/pallan)\
**Post date:** [June 10, 2006, 7:14am UTC](https://www.ruby-forum.com/t/ruby-class-question/62988/3 "2006-06-10T07:14:11Z")

</div>

Peer A. wrote:

> 3: end
> 
> My question is, what is the whole statement on line #2? I know what it  
> does, it creates a attribute-setting method for the “duration” variable,  
> but what I don’t understand is how it does it. When does that line of  
> code get executed?

James already answered the question pretty well, I just want to address  
the “when does that line get executed?” question.  
It is executed when the class is ‘parsed’. For example:

irb(main):001:0\> class A  
irb(main):002:1\> puts “Hello!”  
irb(main):003:1\> end  
Hello!  
=\> nil

Notice that “Hello!” appears immediately after I define the class and it  
gets parsed. So the attr\_writer() method (it is really just a method,  
not special syntax) is executed while your class is being defined and  
adds a method to your class for you. You can execute any ol’ code you  
wanted within the class definition, as shown above.

Also, the attr\* methods DO NOT declare any variables. They only create  
methods to access those variables.

irb(main):001:0\> class A  
irb(main):002:1\> attr\_reader :something  
irb(main):003:1\> end  
=\> nil  
irb(main):004:0\> a = A.new  
=\> #\<A:0x40223968\>  
irb(main):005:0\> a.instance\_variables  
=\> []  
irb(main):006:0\> a.methods.sort  
=\> ["==", “===”, “=~”, “ **id** ”, “ **send** ”, “class”, “clone”,  
“display”, “dup”, “eql?”, “equal?”, “extend”, “freeze”, “frozen?”,  
“hash”, “id”, “inspect”, “instance\_eval”, “instance\_of?”,  
“instance\_variable\_get”, “instance\_variable\_set”, “instance\_variables”,  
“is\_a?”, “kind\_of?”, “method”, “methods”, “nil?”, “object\_id”,  
“private\_methods”, “protected\_methods”, “public\_methods”, “respond\_to?”,  
“send”, “singleton\_methods”, “something”, “taint”, “tainted?”, “to\_a”,  
“to\_s”, “type”, “untaint”]

Notice the method called “something” which was defined by attr\_reader  
:something.

Hope that helps.

-Justin

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**Author:** ![pallan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/71c47a/32.png) [@pallan](https://www.ruby-forum.com/u/pallan)\
**Post date:** [June 10, 2006, 3:08pm UTC](https://www.ruby-forum.com/t/ruby-class-question/62988/4 "2006-06-10T15:08:01Z")

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Peer A. wrote:

> … I did not  
> realize that the code within the class definition gets executed, rather  
> than just defined. Neat!

Yes, it’s actually very neat – you can put almost anything in there:

class Foo  
if condition? then  
def bar(arg1, arg2)  
…  
end  
end

```
 [:foo, :bar, :baz].each do |name|
   define_method(name){ puts name.inspect }
 end

```

end

Cheers,  
Daniel

---

<div class="post-metadata">

**Author:** ![pallan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/71c47a/32.png) [@pallan](https://www.ruby-forum.com/u/pallan)\
**Post date:** [June 10, 2006, 3:50pm UTC](https://www.ruby-forum.com/t/ruby-class-question/62988/5 "2006-06-10T15:50:26Z")

</div>

Peer A. wrote:

> > Hello!  
> > =\> nil
> 
> Thanks for the explanations. This is what I was looking for. I did not  
> realize that the code within the class definition gets executed, rather  
> than just defined. Neat!

It’s a subtle point, though.

For example, there have been threads here on some issues that arise when  
you do not control the order in which files are loaded.

You may see this idiom a lot, too:

if **FILE** == $0

# Some code …

end

This checks to see if the file has been loaded directly, rather than  
being included in another file. It is a way to wrap stuff (such as unit  
tests or command line interaction) that should not execute if the  
source file is being used by as part of a larger application

–  
James B.

[http://www.ruby-doc.org](http://www.ruby-doc.org) - Ruby Help & Documentation  
[Ruby Code & Style](http://www.artima.com/rubycs/) - The Journal By & For Rubyists  
[http://www.rubystuff.com](http://www.rubystuff.com) - The Ruby Store for Ruby Stuff  
[http://www.30secondrule.com](http://www.30secondrule.com) - Building Better Tools

---

<div class="post-metadata">

**Author:** ![pallan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/71c47a/32.png) [@pallan](https://www.ruby-forum.com/u/pallan)\
**Post date:** [June 10, 2006, 2:21pm UTC](https://www.ruby-forum.com/t/ruby-class-question/62988/6 "2006-06-10T14:21:54Z")

</div>

Justin C. wrote:

> James already answered the question pretty well, I just want to address  
> the “when does that line get executed?” question.  
> It is executed when the class is ‘parsed’. For example:
> 
> irb(main):001:0\> class A  
> irb(main):002:1\> puts “Hello!”  
> irb(main):003:1\> end  
> Hello!  
> =\> nil

Thanks for the explanations. This is what I was looking for. I did not  
realize that the code within the class definition gets executed, rather  
than just defined. Neat!

Thanks again,

Peer