From: Justin Collins <collinsj@...>
Date: 2006-06-10T14:12:09+09:00
Subject: Re: Ruby Class question

Peer Allan wrote:
> 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?
>   

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