From: Stefano Crocco <stefano.crocco@...>
Date: 2007-03-18T01:13:47+09:00
Subject: Re: attr_accessor question

Alle sabato 17 marzo 2007, Corey Konrad ha scritto:
> Michael Guterl wrote:
> > On 3/17/07, Corey Konrad <0011@hush.com> wrote:
> >>   end
> >> end
> >>
> >> animal = Animal.new
> >
> >  ArgumentError: wrong number of arguments (0 for 2)
> >
> > Your Animal.new is expecting you to pass in color and size.
> >
> > animal = Animal.new("brown", "big")
> >
> > animal.color = "brown"
>
> ah ok i understand that i am just learning all this OO stuff i just
> thought i needed to use the initialize method but i dont even need to
> create one for a default object state at all i can just do this as well
> and the program worked fine,
>
> class Animal
>   attr_accessor :color
>   attr_accessor :size
> end
>
>  that is pretty cool i'm having a fun time learning this language
> compared to something like vb.net.

Whenever you call the new method of a class, it calls the initialize method of 
the instance it just created. If you didn't define an initialize method for 
that class, the initialize method of the parent class will be called, or the 
parent class's parent... and so on. Going on the class hyerarchy, we reach 
Object#initialize, which (I think) does nothing.

If, as it often happens, you need instance variables, it's usually wise to 
define an initialize method in order to set them to a sensible value (or to a 
value passed by the caller using parameters). If you don't, the first time 
you reference an instance variable, it will be created and set to nil, which 
may not be what you want.

For example, take the following class

class C

 def greater? number
  @var > number
 end

end

As you can see, this class defines an instance method which tells whether its 
instance variable @var is greater than the given number or not.

Let's see what happens when we run this code:

c=C.new # Object#initialize is called.
c.instance_variables # No instance variables exist
=>[]
c.greater? 3

=>NoMethodError: undefined method `>' for nil:NilClass

We got an error, because @var is created on the spot and set to nil, and nil 
doesn't have a > method.

If we add an initialize method, instead, we have:

class C

 def initialize
  @var=0
 end

 def greater? number
  @var > number
 end

end

c=C.new #now, C#initialize is called
c.instance_variables
=> ["@var"] #now, @var exists
c.greater? 3
=> false
c.greater? -1
=> true

By the way, if you define an initialize method for a class which doesn't 
derive directly from Object, you'll usually want to call the parent class's 
initialize from your own, otherwise the parent class's instance variables 
won't be initialized:

class A
 def initialize #We don't need to call Object#initialize: it does nothing
  @var=0
 end
end

class B < A
 def initialize
  super #We call the parent class's initialize; otherwise, @var won't be set
  @var1=1
 end
end

I hope this helps

Stefano