# Class \<\< my\_instance and class variables

**URL:** <https://www.ruby-forum.com/t/class-my-instance-and-class-variables/60145>\
**Category:** Ruby\
**Created:** [May 10, 2006, 5:55pm UTC](https://www.ruby-forum.com/t/class-my-instance-and-class-variables/60145 "2006-05-10T17:55:50Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![raphael\_b](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/7c8e57/32.png) [@raphael\_b](https://www.ruby-forum.com/u/raphael_b)\
**Post date:** [May 10, 2006, 5:55pm UTC](https://www.ruby-forum.com/t/class-my-instance-and-class-variables/60145/1 "2006-05-10T17:55:50Z")

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Hi,

When running this code,

class A  
def initialize(i)  
@value = i  
end  
end

a = A.new(3)

class \<\< a  
@@v = 5  
def test  
@value\>@@v  
end  
end

a.test

I get a warning:  
“class variable access from toplevel singleton method”

From my understanding, the test method is added to the class specific  
to object a, and other instances of A don’t have access to that  
method. The class variable @@v is also added to the same object  
specific class. But then, what exactly does this warning mean, and how  
bad is it?

Thanks

Raph

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<div class="post-metadata">

**Author:** ![raphael\_b](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/7c8e57/32.png) [@raphael\_b](https://www.ruby-forum.com/u/raphael_b)\
**Post date:** [May 10, 2006, 6:05pm UTC](https://www.ruby-forum.com/t/class-my-instance-and-class-variables/60145/2 "2006-05-10T18:05:29Z")

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> > > > > “R” == Raphael B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

R\> method. The class variable @@v is also added to the same object  
R\> specific class.

Are you sure ?

moulon% ruby -e ‘class \<\< Object.new; @@a = 12 end; p @@a’  
-e:1: warning: class variable access from toplevel singleton method  
12  
moulon%

Guy Decoux

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<div class="post-metadata">

**Author:** ![raphael\_b](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/7c8e57/32.png) [@raphael\_b](https://www.ruby-forum.com/u/raphael_b)\
**Post date:** [May 10, 2006, 6:49pm UTC](https://www.ruby-forum.com/t/class-my-instance-and-class-variables/60145/3 "2006-05-10T18:49:06Z")

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On 5/10/06, ts [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > > > > > “R” == Raphael B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:
> 
> R\> method. The class variable @@v is also added to the same object  
> R\> specific class.
> 
> Are you sure ?
> 
> moulon% ruby -e ‘class \<\< Object.new; @@a = 12 end; p @@a’  
> -e:1: warning: class variable access from toplevel singleton method  
> 12

I was not sure 😉

In the mean time I read this very interesting article:  
[http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html](http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html)

So methods are added to the object specific class (the metaclass as  
identified in why’s article), but class variables are added to the  
class of which the object is an instance. Why the difference?

Further on this, if I defined a second instance and try to call test:  
b=A.new(6)  
b.test

I get the error that test is a private method:  
NoMethodError: private method `test’ called for #\<A:0xb7cbf5ec @value=6\>

so it seems it was added to A, but marked as private.

Are there any good reading about this?  
Oh, and about my initial question: how bad is the warning I get?

Raph

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<div class="post-metadata">

**Author:** ![raphael\_b](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/7c8e57/32.png) [@raphael\_b](https://www.ruby-forum.com/u/raphael_b)\
**Post date:** [May 10, 2006, 7:26pm UTC](https://www.ruby-forum.com/t/class-my-instance-and-class-variables/60145/4 "2006-05-10T19:26:48Z")

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thanks Guy for your explanations!

> ruby is trying to say that the variable @@v will be defined in Object,  
> because there is no other class where it can define it.

I didn’t (and still can’t 😉 translate the message “class variable  
access from toplevel singleton method” to your explanation of it.

Anyway, I started to play with class instance variables, and got this  
code working, if anyone’s interested:

class A  
def initialize(i)  
@value = i  
end

def test\_in\_class  
@value\>@@v  
end  
end

a = A.new(3)

class \<\< a  
@cvalue = 5  
class \<\< self  
attr\_reader :cvalue  
end  
def test  
@value\>(class \<\< self; self; end).cvalue  
end  
end

a.test

Cheers

Raph

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<div class="post-metadata">

**Author:** ![raphael\_b](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/7c8e57/32.png) [@raphael\_b](https://www.ruby-forum.com/u/raphael_b)\
**Post date:** [May 10, 2006, 7:07pm UTC](https://www.ruby-forum.com/t/class-my-instance-and-class-variables/60145/5 "2006-05-10T19:07:45Z")

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> > > > > “R” == Raphael B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

R\> So methods are added to the object specific class (the metaclass as  
R\> identified in why’s article), but class variables are added to the  
R\> class of which the object is an instance.

Well, no. class variables (i.e @@) are added in the class which is  
cuurently in scope.

R\> Further on this, if I defined a second instance and try to call test:  
R\> b=A.new(6)  
R\> b.test

When you do this, ruby find the method Kernel#test which is a private  
method. This is why it give a warning.

R\> so it seems it was added to A, but marked as private.

No, it was added to the singleton class of `a’, you have just chosen a  
bad  
name for your method (test) 🙂

R\> Oh, and about my initial question: how bad is the warning I get?

ruby is trying to say that the variable @@v will be defined in Object,  
because there is no other class where it can define it.

Guy Decoux