From: anne001 <anne@...>
Date: 2006-07-10T08:45:11+09:00
Subject: Re: why does block know about one instance variable and not the other?

Thank you for your response.

It did occured to me sometime after I wrote the post that the
difference between the two instance variables was that one was outside
the define and the other inside. A look at
http://www.ruby-doc.org/docs/UsersGuide/rg/instancevars.html
and other examples, shows that the instance variable is always defined
in a method without saying why.

So I was very glad to find your response.

I thought of an instance as a copy of the class. so implicitely I
assumed it would get a copy of an instance variable. Maybe I should
think of an object as an inheritance of methods: So it inherits
methods, not variables.

But if that was the rule, it would not work for constants either
http://www.ruby-doc.org/docs/UsersGuide/rg/constants.html
in the example there, it would get a method and have no idea what the
constants were.

your example helped clarify what is going on.

   class C
     puts self
     @var = 1
     puts self
     def x
       puts self
       @var = 2
     end
   end
  p "create new instance of C"
  y= C.new
  p y
  p "run method which creates an instance variable"
  y.x
  p y

  class D
     puts self
     @@var = 1
     puts self
     def x
       puts self
       @@var = 2
     end
  end
  p "create new instance of D"
  y= D.new
  p y
  p "run method which creates an instance variable"
  y.x
  p y

In class C, running x adds @var to the object, but in D, running x does
not add @@var.

@var is an instance variable that belongs to whatever object is
"self" at that moment in runtime.

QUESTION: If that is the case, why does puts self not append @var=1 to
object C the way it
appends @var=2 to the instance?
#<C:0x25b58 @var=2>

In any case, the code in C outside of the methods is only run once when
the new method is called, and the object is C. OK. I had not thought of
it that way.

 So is this an ok way to think about objects:
1. Objects inherit methods
2. The variable environment includes, read access to Class constants,
r/w to @@var and r/w to global variables
2. @var become local to whatever object is self at the moment they are
created.