Fully understanding Ruby class variable

Question

In this code block,

@@y = 1
class MyClass
 @@y = 2
end
p @@y # => 2

naively, it seems that @@y is in top-level scope, and it isn't the same @@y as the one in MyClass's scope. Why is @@y affected by the class MyClass definition? (why is the result 2?)

Sergio Tulentsev · Accepted Answer · 2012-09-14 12:35:36Z

7

Let's look at this example. Here @@x in Bar is indeed separate from @@x in Foo.

class Foo
  @@x = 1
end

class Bar
  @@x = 2
end

Foo.class_variable_get(:@@x) # => 1
Bar.class_variable_get(:@@x) # => 2

But what happens if Bar is a child of Foo?

class Foo
  @@x = 1
end

class Bar < Foo
  @@x = 2
end

Foo.class_variable_get(:@@x) # => 2
Bar.class_variable_get(:@@x) # => 2

In this case, @@x is the same in both cases, and it is the one declared in Foo.

Now, back to your example:

@@y = 1
class MyClass
  @@y = 2
end
p @@y

The first line declares class variable in the root scope. Root is a special object main which is of type Object. So, essentially, you're defining a class variable on Object class. Since everything is an Object, this is how definition of MyClass also inherits @@y and is able to change it.

edited Sep 14, 2012 at 12:35

answered Sep 14, 2012 at 12:26

Sergio Tulentsev

231k43 gold badges381 silver badges373 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

sploiber Over a year ago

Sergio, thanks very much! That is a fine answer. Here is a corollary question: @@y = 1 class MyClass end p MyClass.class_variable_get(:@@y) p MyClass.class_variables Why doesn't #class_variables show @@y?

Sergio Tulentsev Over a year ago

@sploiber: Because it shows only class variables that this class owns. See the documentation: Module.class_variables

Sébastien Le Callonnec Over a year ago

If Bar is able to change the value of @@y in its superclass Object, why doesn't Bar change the value of @@y in its superclass Foo?

Sergio Tulentsev Over a year ago

@SébastienLeCallonnec: it does, look closer :)

Sébastien Le Callonnec Over a year ago

@SergioTulentsev Darn, you're right, time to clean my glasses. (=

steenslag · Accepted Answer · 2012-09-14 12:31:38Z

1

When you do

@@y = 1

you are defining it on Object. Since MyClass is a subclass of Object, it has access to it's class variables.

@@y = 1
class MyClass
  @@y = 2
end
p @@y
puts MyClass.superclass #=> Object
puts Object.class_variables #=> @@y

answered Sep 14, 2012 at 12:31

steenslag

80.2k16 gold badges144 silver badges174 bronze badges

1 Comment

sploiber Over a year ago

I like both, actually; yours is more complete and adds a fuller picture. But all of this helped me. Thanks.

Sergio Tulentsev · Accepted Answer · 2012-09-14 12:27:23Z

-1

Global vars are like this:

$y = 1

and class vars are like this:

@@y = 1

So actually your using the class vars

here is a link: vars

edited Sep 14, 2012 at 12:27

Sergio Tulentsev

231k43 gold badges381 silver badges373 bronze badges

answered Sep 14, 2012 at 12:26

Extranion

6081 gold badge9 silver badges25 bronze badges

3 Comments

Sergio Tulentsev Over a year ago

There's no word "global" in the question :)

Dave Newton Over a year ago

OP asked why the class vars were behaving as observed, so must have known they were class vars. The problem was not understand the nature of Ruby class var inheritance.

sploiber Over a year ago

Dave, yes, that is right. This exchange helped me with a fuller understanding of class variables.

Collectives™ on Stack Overflow

Fully understanding Ruby class variable

3 Answers 3

5 Comments

1 Comment

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

5 Comments

1 Comment

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related