From: Stefano Crocco <stefano.crocco@...>
Date: 2007-03-04T04:47:31+09:00
Subject: Re: attr_reader explained

Alle sabato 3 marzo 2007, Raj Sahae ha scritto:
> attr_reader, in it's common usage, creates instance variables and
> defines a method by which you can read them.

Actually, attr_reader doesn't create the variable, just the accessor method. 
To see this, do the following in irb:

> class C
> attr_reader :var
> def var_defined?
> defined?(@var)
> end
> end
=> nil
> c=C.new
=> #<C:0xb7a4c650>
> c.var_defined?
=> nil
> class C
> def initialize
> @var=nil
> end
> end
=> nil
> c1=C.new
=> #<C:0xb7cf1a04 @var=nil>
> c1.var_defined?
=> "instance-variable"

As you can see, in the first case (the variable called c) doesn't have the 
@var instance variable defined. Even after you call the var method defined 
using attr_reader, the variable is still not defined. To define it, you need 
to explicitly assign it to a value (using @var= something inside an instance 
method of the class or using instance_variable_set)

Stefano