From: "Marcin Mielżyński" <lopx@...>
Date: 2006-07-10T09:55:12+09:00
Subject: Re: why does block know about one instance variable and not the other?

anne001 wrote:
> 
> 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.
> 

Instance of a class is not a copy of it. Classes are somewhat templates 
for instances, they define what methods and variables their instances 
will have. But since a Class in Ruby is also an object it can have 
instance variable as well!.

> 
> In class C, running x adds @var to the object,

As it should do

  but in D, running x does
> not add @@var.
@@var is a special variable that is shared between class and it's 
instances (so there is only one @@var, no matter how many instances 
there are).

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

No, self is different in class context and instance context.

> 
> 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>

Class objects have different default implementation of inspect method, 
so they are printed differently using p. There is a way to query object 
about their variables:

  p  C.instance_variable_get("@var")
-> 1
  p  y.instance_variable_get("@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

Nope, classes define methods which in turn will be available to their 
instances upon construction

> 2. The variable environment includes, read access to Class constants,
> r/w to @@var and r/w to global variables
Again, depends on the context (receiver aka self)

> 2. @var become local to whatever object is self at the moment they are
> created.

I've never considered instance variables to be local to their instances 
- but in a way, yes.

lopex