# What is really a class instance variable compare to a class

**URL:** <https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918>\
**Category:** Ruby\
**Created:** [February 8, 2007, 5:59pm UTC](https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918 "2007-02-08T17:59:44Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![rubarachni](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/74df32/32.png) [@rubarachni](https://www.ruby-forum.com/u/rubarachni)\
**Post date:** [February 8, 2007, 5:59pm UTC](https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918/1 "2007-02-08T17:59:44Z")

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Hi all,

Given the following definition :

class A  
@@class\_var = “a class variable”  
@class\_instance\_var = “a class instance variable”

```
def a_method
   @instance_var = "an instance variable"
end

```

end

I understand what is a class variable and what is an instance  
variable. The problem I have is what is a class instance variable like  
@class\_instance\_var ? Where does this variable live ?

A class variable is shared by all instances of the class. But what  
about the class instance variable ?

Thanks for any explanation.

CM.

---

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**Author:** ![rubarachni](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/74df32/32.png) [@rubarachni](https://www.ruby-forum.com/u/rubarachni)\
**Post date:** [February 8, 2007, 6:50pm UTC](https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918/2 "2007-02-08T18:50:48Z")

</div>

HI –

On Fri, 9 Feb 2007, Ruby A. wrote:

> end  
> end
> 
> I understand what is a class variable and what is an instance  
> variable. The problem I have is what is a class instance variable  
> like @class\_instance\_var ? Where does this variable live ?
> 
> A class variable is shared by all instances of the class. But what  
> about the class instance variable ?

Every instance variable you ever see belongs to whatever object is, at  
the moment you see the instance variable, playing the role of ‘self’.  
Inside a class definition block, ‘self’ is the class object itself:

class A  
p self # A  
end

So if you have an instance variable at that point in execution:

class A  
@x = 1  
end

it belongs to the object A, which is a class object.

Similarly, that variable @x will only be visible when self is A. When  
self is not A, @x will not be that same instance variable. You can  
always establish ownership of instance variables by calculating what  
self is at the point that the instance variable appears.

Keep in mind, too, that the phrase “class instance variable” is just a  
way of clarifying that you don’t mean “instance variable defined in  
this class’s instance methods”. There’s no separate “class instance  
variable” construct at the language level; there are just objects,  
some of which are class objects and all of which can be self and  
therefore have instance variables.

David

---

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**Author:** ![rubarachni](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/74df32/32.png) [@rubarachni](https://www.ruby-forum.com/u/rubarachni)\
**Post date:** [February 9, 2007, 1:07pm UTC](https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918/3 "2007-02-09T13:07:27Z")

</div>

On Fri, Feb 09, 2007 at 07:48:37PM +0900, Ruby A. wrote:

> =\> So, an object instance variable is shared/seen by the derived object instances.

My understanding is:

(1) An instance variable is only visible to that object. It doesn’t  
matter  
what “class” the object is. It’s not visible by any other objects,  
either of  
the same class or a different class.

(2) A class instance variable is an instance variable of an object which  
happens to be a class (its an object whose class is “Class”). This is  
also  
private to that object, i.e. to the class. A subclass is a different  
object,  
so has its own class instance variables.

(3) Class variables, i.e. @@foo, are voodoo. But yes I believe they are  
shared by groups of classes in the same ancestry somehow.

B.

---

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**Author:** ![rubarachni](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/74df32/32.png) [@rubarachni](https://www.ruby-forum.com/u/rubarachni)\
**Post date:** [February 9, 2007, 11:49am UTC](https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918/4 "2007-02-09T11:49:21Z")

</div>

David,

Thanks a lot for your reply. But this raises another understanding  
problem for me. It seems that regarding inheritance there are different  
behaviours for :

- an object instance variable
- a class variable
- a class instance variable

For the following code (I’m running Ruby 1.8.5p12) :

class TestObject  
def initialize  
@var = “an object instance variable”  
end  
def print  
puts @var  
end  
end  
class DerivedTestObject \< TestObject  
def set\_var  
@var = “an object instance variable : changed”  
end  
end

o = DerivedTestObject.new  
o.print  
o.set\_var  
o.print

The output is :  
an object instance variable  
an object instance variable : changed

=\> So, an object instance variable is shared/seen by the derived  
object instances.

For the following code :

class TestClass  
@@var = “a class variable”  
@var = “a class instance variable”  
def self.print  
puts @@var  
puts @var  
end  
end  
class DerivedTestClass \< TestClass  
def self.set\_var  
@@var = “a class variable : changed”  
@var = “a class instance variable : changed”  
end  
end  
TestClass.print  
puts “-------------”  
DerivedTestClass.print  
puts “-------------”  
DerivedTestClass.set\_var  
DerivedTestClass.print  
puts “-------------”  
TestClass.print

## The output is : a class variable a class instance variable

## a class variable nil

## a class variable : changed a class instance variable : changed

a class variable : changed  
a class instance variable

=\> So, a class variable is shared/seen by derived classes (similar  
behaviour as an instance variable). Conversely, a class instance  
variable is not shared/seen by derived classes.

Am I missing something ?

Another question :  
The inheritance of a class variable and a class method is really great  
:-).  
Is it going to be removed in Ruby 2.0 ? I heard something about that  
but I hope not.

Thanks for any explanation.

CM.

removed\_email\_address@domain.invalid wrote:  
HI –

On Fri, 9 Feb 2007, Ruby A. wrote:

> end  
> end
> 
> I understand what is a class variable and what is an instance  
> variable. The problem I have is what is a class instance variable  
> like @class\_instance\_var ? Where does this variable live ?
> 
> A class variable is shared by all instances of the class. But what  
> about the class instance variable ?

Every instance variable you ever see belongs to whatever object is, at  
the moment you see the instance variable, playing the role of ‘self’.  
Inside a class definition block, ‘self’ is the class object itself:

class A  
p self # A  
end

So if you have an instance variable at that point in execution:

class A  
@x = 1  
end

it belongs to the object A, which is a class object.

Similarly, that variable @x will only be visible when self is A. When  
self is not A, @x will not be that same instance variable. You can  
always establish ownership of instance variables by calculating what  
self is at the point that the instance variable appears.

Keep in mind, too, that the phrase “class instance variable” is just a  
way of clarifying that you don’t mean “instance variable defined in  
this class’s instance methods”. There’s no separate “class instance  
variable” construct at the language level; there are just objects,  
some of which are class objects and all of which can be self and  
therefore have instance variables.

David

–  
Q. What is THE Ruby book for Rails developers?  
A. RUBY FOR RAILS by David A. Black ([http://www.manning.com/black](http://www.manning.com/black))  
(See what readers are saying! [http://www.rubypal.com/r4rrevs.pdf](http://www.rubypal.com/r4rrevs.pdf))  
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?  
A. Ruby Power and Light, LLC ([http://www.rubypal.com](http://www.rubypal.com))

---

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**Author:** ![rubarachni](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/74df32/32.png) [@rubarachni](https://www.ruby-forum.com/u/rubarachni)\
**Post date:** [February 9, 2007, 1:09pm UTC](https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918/5 "2007-02-09T13:09:27Z")

</div>

Hi –

On Fri, 9 Feb 2007, Ruby A. wrote:

> David,
> 
> Thanks a lot for your reply. But this raises another understanding problem for me. It seems that regarding inheritance there are different behaviours for :
> 
> - an object instance variable
> - a class variable
> - a class instance variable

You’ve got two very different things there:

- instance variables
- class variables

Instance variables all behave alike. Class variables are completely  
different, and unrelated. It’s best not to go into it expecting any  
similarity.

> class DerivedTestObject \< TestObject  
> The output is :  
> an object instance variable  
> an object instance variable : changed
> 
> =\> So, an object instance variable is shared/seen by the derived  
> object instances.

Not exactly. Every object has its own instance variables, which are  
not seen by any other object. Your DerivedTestObject instance  
executes the initialize method, and inside that method, its instance  
variable @var is initialized. In set\_var, that variable is changed.

The fact that initialize is defined in a superclass doesn’t mean  
anything. When initialize is executed, ‘self’ is the new  
DerivedTestObject instance. So @var, when it appears in that method,  
belongs to that instance.

It’s better always to think of instance variables in terms of what  
object is ‘self’ at the moment the instance variable appears. An  
instance variable has no knowledge of class or inheritance; it only  
knows ‘self’.

> ## class DerivedTestClass \< TestClass DerivedTestClass.print a class variable : changed a class instance variable : changed
> 
> a class variable : changed  
> a class instance variable
> 
> =\> So, a class variable is shared/seen by derived classes (similar  
> behaviour as an instance variable). Conversely, a class instance  
> variable is not shared/seen by derived classes.

No instance variable is shared with any other object. TestClass and  
DerivedTestClass are two different objects; therefore, they do not  
share instance variables. Again, instance variables have no concept  
of “class” or “inheritance”. They simply adhere to the object in  
whose context as ‘self’ they are created.

> Am I missing something ?
> 
> Another question :  
> The inheritance of a class variable and a class method is really great :-).  
> Is it going to be removed in Ruby 2.0 ? I heard something about  
> that but I hope not.

My current understanding is that class variables will be visible only  
to the class or module they’re defined in, including instance methods  
of that class or module. I have mixed emotions about this. I would  
actually rather see them disappear. My experience over the years is  
that they cause more confusion than they’re worth, and that in 2.0  
they’re going to be _almost_ like instance variables of class objects  
but not quite… and therefore people are still going to wonder what’s  
going on and why it has to be there. (At least, I will 🙂

David

---

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**Author:** ![rubarachni](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/74df32/32.png) [@rubarachni](https://www.ruby-forum.com/u/rubarachni)\
**Post date:** [February 9, 2007, 1:32pm UTC](https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918/6 "2007-02-09T13:32:29Z")

</div>

Brian, David,

Thanks a lot for your replies.  
It’s now crystal clear for me.

David,

Your book is excellent !  
May be you can add these additional explanations in a next revision  
:-)!

CM.

removed\_email\_address@domain.invalid wrote:  
Hi –

On Fri, 9 Feb 2007, Ruby A. wrote:

> David,
> 
> Thanks a lot for your reply. But this raises another understanding problem for me. It seems that regarding inheritance there are different behaviours for :
> 
> - an object instance variable
> - a class variable
> - a class instance variable

You’ve got two very different things there:

- instance variables
- class variables

Instance variables all behave alike. Class variables are completely  
different, and unrelated. It’s best not to go into it expecting any  
similarity.

> class DerivedTestObject \< TestObject  
> The output is :  
> an object instance variable  
> an object instance variable : changed
> 
> =\> So, an object instance variable is shared/seen by the derived  
> object instances.

Not exactly. Every object has its own instance variables, which are  
not seen by any other object. Your DerivedTestObject instance  
executes the initialize method, and inside that method, its instance  
variable @var is initialized. In set\_var, that variable is changed.

The fact that initialize is defined in a superclass doesn’t mean  
anything. When initialize is executed, ‘self’ is the new  
DerivedTestObject instance. So @var, when it appears in that method,  
belongs to that instance.

It’s better always to think of instance variables in terms of what  
object is ‘self’ at the moment the instance variable appears. An  
instance variable has no knowledge of class or inheritance; it only  
knows ‘self’.

> ## class DerivedTestClass \< TestClass DerivedTestClass.print a class variable : changed a class instance variable : changed
> 
> a class variable : changed  
> a class instance variable
> 
> =\> So, a class variable is shared/seen by derived classes (similar  
> behaviour as an instance variable). Conversely, a class instance  
> variable is not shared/seen by derived classes.

No instance variable is shared with any other object. TestClass and  
DerivedTestClass are two different objects; therefore, they do not  
share instance variables. Again, instance variables have no concept  
of “class” or “inheritance”. They simply adhere to the object in  
whose context as ‘self’ they are created.

> Am I missing something ?
> 
> Another question :  
> The inheritance of a class variable and a class method is really great :-).  
> Is it going to be removed in Ruby 2.0 ? I heard something about  
> that but I hope not.

My current understanding is that class variables will be visible only  
to the class or module they’re defined in, including instance methods  
of that class or module. I have mixed emotions about this. I would  
actually rather see them disappear. My experience over the years is  
that they cause more confusion than they’re worth, and that in 2.0  
they’re going to be _almost_ like instance variables of class objects  
but not quite… and therefore people are still going to wonder what’s  
going on and why it has to be there. (At least, I will 🙂

David

–  
Q. What is THE Ruby book for Rails developers?  
A. RUBY FOR RAILS by David A. Black ([http://www.manning.com/black](http://www.manning.com/black))  
(See what readers are saying! [http://www.rubypal.com/r4rrevs.pdf](http://www.rubypal.com/r4rrevs.pdf))  
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?  
A. Ruby Power and Light, LLC ([http://www.rubypal.com](http://www.rubypal.com))

---

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**Author:** ![rubarachni](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/74df32/32.png) [@rubarachni](https://www.ruby-forum.com/u/rubarachni)\
**Post date:** [February 9, 2007, 2:03pm UTC](https://www.ruby-forum.com/t/what-is-really-a-class-instance-variable-compare-to-a-class/84918/7 "2007-02-09T14:03:31Z")

</div>

hi there,

not to hijack the thread or anything, but since it´s about class  
variables.

i write a lot of rails plugins and i use class variables a lot when  
doing mixins that need to have values propagated to the class that is  
getting the module mixed in.

like this:

class Foo  
end

module Bar  
@@a = “some value”  
A = “some other value”

```
    def classvar
            return @@a
    end
    def classconst
            return A
    end

```

end

dup = Foo.new  
dup.extend Bar  
puts dup.classvar  
puts dup.classconst

i find class variables really useful for that. on the other hand like in  
my code you could always use class constants which pretty much do the  
same in my understanding (please correct me if i´m wrong here)

just wanted to show this as a valid (at least from my perspective) use  
case for class variables.

kind regards,

alexander