# Scope of class instance variables?

**URL:** <https://www.ruby-forum.com/t/scope-of-class-instance-variables/133269>\
**Category:** Ruby\
**Created:** [March 29, 2008, 4:40pm UTC](https://www.ruby-forum.com/t/scope-of-class-instance-variables/133269 "2008-03-29T16:40:52Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 29, 2008, 4:40pm UTC](https://www.ruby-forum.com/t/scope-of-class-instance-variables/133269/1 "2008-03-29T16:40:52Z")

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I am reading Hal F.'s “The Ruby Way” On page 57 he makes the  
statement:  
“Class instance variables cannot be referenced from within instance  
methods and, in general are not very useful”

huh?..  
This , in my feeble mind, contradicts everything else I have ever  
read, except that the example he gives is similar to the @y = 7  
example below, and in that case, @y is not available to the accessor  
or any other method that I have played with.

In the following code, the original assignment of @x = 7 and @y = 5  
don’t seem to do anything, even with the attr\_accessor. I know that  
both are read by the compiler as I can assign @y = 5/0 and get a  
division by zero error.

So, could someone please explain why the accessor for @y does not work  
here. If I hand write a reader and writer for @y, it works just fine.  
There are different scopes here, but I had the impression that the  
accessor would break down that barrier and make @y available  
throughout the class just as the initialize method is able to access  
the class variable @x and assign the passed in value. That value is  
then available to the @x accessor.

I thought for a while that the @y accessor might work in the singleton  
class of Myclass, but it doesn’t.

class Myclass  
attr\_accessor :y, :x

@x = 7  
@y = 5

def initialize(new\_val= l)  
@x = new\_val ? new\_val : 0  
end  
end

mc = Myclass.new(3)

puts mc.y #=\> Nil  
puts mc.x #-\> 3

I know I can make the @x and @y class variables available to instance  
methods by referencing them inside defined methods and the scope seem  
to be class wide, but it just seems like the first assignments above  
should work as written.

Thanks in advance

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**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 29, 2008, 5:10pm UTC](https://www.ruby-forum.com/t/scope-of-class-instance-variables/133269/2 "2008-03-29T17:10:31Z")

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On Mar 29, 11:40 am, Ruby F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I am reading Hal F.'s “The Ruby Way” On page 57 he makes the  
> statement:  
> “Class instance variables cannot be referenced from within instance  
> methods and, in general are not very useful”

I think that just an out-dated way of looking at things. I expect Hal  
would agree that needs to be updated to current trends.

I remember once, Hal and I got in a big argument about floating-point  
arithmetic. A tad spiteful I pointed out a flaw in his book. Despite  
the heated argument, he took it quite gracefully, saying he was always  
happy to receive comments/corrections. That was pretty damn  
respectable, IMHO. If he doesn’t see this thread, write him directly.

T.

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**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 29, 2008, 5:37pm UTC](https://www.ruby-forum.com/t/scope-of-class-instance-variables/133269/3 "2008-03-29T17:37:41Z")

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unsuscribe

2008/3/29, Trans [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

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**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 29, 2008, 5:39pm UTC](https://www.ruby-forum.com/t/scope-of-class-instance-variables/133269/4 "2008-03-29T17:39:26Z")

</div>

unsubscribe

2008/3/29, surreal [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

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**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 29, 2008, 5:52pm UTC](https://www.ruby-forum.com/t/scope-of-class-instance-variables/133269/5 "2008-03-29T17:52:29Z")

</div>

On Mar 29, 9:39 am, surreal [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > unsuscribe
> > 
> > > would agree that needs to be updated to current trends.  
> > > Harshad Joshi
> 
> –  
> Harshad Joshi

Thanks Trans.

I guess Harshad didn’t like my post! 🙂

Anyway, I would still appreciate an explanation for the difference in  
scope, or the reason the first assignment doesn’t work.

Thanks

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<div class="post-metadata">

**Author:** ![rubyfreak](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@rubyfreak](https://www.ruby-forum.com/u/rubyfreak)\
**Post date:** [March 29, 2008, 11:08pm UTC](https://www.ruby-forum.com/t/scope-of-class-instance-variables/133269/6 "2008-03-29T23:08:56Z")

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On Sat, Mar 29, 2008 at 4:40 PM, Ruby F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> or any other method that I have played with.  
> throughout the class just as the initialize method is able to access  
> the class variable @x and assign the passed in value. That value is  
> then available to the @x accessor.
> 
> I thought for a while that the @y accessor might work in the singleton  
> class of Myclass, but it doesn’t.
> 
> class Myclass

### make this

```
     class << self

```

> ```
> attr_accessor :y, :x
> 
> ```

```
     end

```

Now this is an accessor to the singleton class of the class where the  
class instance variables are stored (that is not completely correct,  
maybe I shall say from where one has access to them).

> ```
> @x = 7
> @y = 5
> 
> def initialize(new_val= l)
> @x = new_val ? new_val : 0
> 
> ```

# and this shall read

```
                self.class.x = new_val || 0

```

> ```
> end
> 
> ```
> 
> end
> 
> puts mc.y #=\> Nil  
> Myclass.x → 7  
> mc = Myclass::new( 42 )  
> Myclass.x → 42  
> mc = Myclass::new 101010  
> Myclass.x → 101010  
> there is of course no mc.x but please see below.

I do however fear that you confuse instance variables with class  
instance variables.  
The former exist on an per object base and the later on a per class  
base ( a class being an object of course ).  
For completeness I’ll show you how to use the former

class A  
attr\_accessor :a  
def initialize; @a = 42 end  
end  
a= A.new  
a.a → 42  
b = A.new  
a.a = 101010  
a.a → 101010  
b.a → 42

## HTH Robert

[http://ruby-smalltalk.blogspot.com/](http://ruby-smalltalk.blogspot.com/)

* * *

Whereof one cannot speak, thereof one must be silent.  
Ludwig Wittgenstein