I generally avoid class variables but at the last meeting of the New
Haven Ruby Brigade a simple question about them led to a long
discussion and irb session that only served to confuse us all more.
I’m hoping someone on the list can shed some light on a couple of
issues that were raised:
class A
@@avar = ‘hello’
end
=> “hello”
A.class_variables
=> ["@@avar"]
A.class_eval { puts @@avar }
NameError: uninitialized class variable @@avar in Object
from (irb):5
from (irb):5
class A
puts @@avar
end
hello
=> nil
class A
def get_avar
@@avar
end
end
=> nil
a = A.new
=> #<A:0x4ba360>
a.get_avar
=> “hello”
a.instance_eval { puts @@avar }
NameError: uninitialized class variable @@avar in Object
from (irb):16
from (irb):16
It seems like a block evaluated by class_eval should have access to
the class variables. Similarly, it seems like if an instance method
(get_avar in the example) has access to the class variable then the
variable should also be visible via instance_eval.
In both examples above (class_eval and instance_eval) it seems like
the class variable @@avar is being looked up relative to the
top_level object and not relative to the class and instance objects
respectively.
In both examples above (class_eval and instance_eval) it seems like the
class variable @@avar is being looked up relative to the top_level object
and not relative to the class and instance objects respectively.
What am I missing?
You had me at “I generally avoid class variables”
Here’s another one you’ll like:
@@avar = 1
class A
@@avar = “hello”
end
puts @@avar # => hello
A.class_eval { puts @@avar } # => hello
I love Ruby madly, but (or “therefore”? I would be happy never to
see another class variable again. The confusingness-to-usefulness
ratio is extremely high.
Since the block acts as a closure, the class variable references seem
to be bound to its original scope; iirc, the only variable types that
will change scope in the context of a class_eval (which changes the
value of ‘self’) are instance variables. That’s why the string will
evaluate correctly, while the block fails to find the class variable.
I’m not an expert, but this has been my experience.
@@avar = 1
class A
@@avar = “hello”
end
puts @@avar # => hello
A.class_eval { puts @@avar } # => hello
Let’s see if I understand what is going on:
When @@var is set to 1 at the top-level it is associated with the
class Object (a feature of the top-level context) and as such @@var
then becomes visible to the entire class hierarchy rooted at Object
(i.e., all objects). When @@avar is evaluated in the class A block,
Ruby finds @@var already defined for the hierarchy (because A is a
subclass of Object). So in this example all three occurrences of
@@avar are associated with the same class variable. If you reverse
the assignments:
class A
@@avar = “hello”
end
@@avar = 1
A.class_eval { puts @@avar } # => 1
class A
puts @@avar # => hello
end
There are now two distinct class variables named @@avar, one is
associated with Object and another is associated with class A and
shadows the one associated with Object.
I still don’t understand why class_eval/instance_eval don’t affect
the resolution of class variables in a manner analogous to how they
affect the resolution of instance variables. Is this by design or
accident?
I love Ruby madly, but (or “therefore”? I would be happy never to
see another class variable again. The confusingness-to-usefulness
ratio is extremely high.
So, would it be fair to say that you won’t run into difficulties with
class variables (denoted with @@) if:
a. you assign class variables in the superclass before assigning
them in the sublcass, and
Assuming you understand that when you assign them in the subclass,
you’re assigning to the exact same variable, throughout the hierarchy.
b. you don’t expect class_eval to provide access to them?
Or are there other areas of confusion?
The most important lesson to learn about class variables, I believe,
is that they have nothing to do with instance variables. They’re
not a replacement for them; they’re not the equivalent for classes
(instance variables exist for classes); and the @/@@ thing is purely
cosmetic.
The reason I say this is that most of the confusion I’ve seen over the
years has to do with class variables getting in the way of
understanding instance variables, since they lead people to assume
that the idea of a class having an instance variable is meaningless
(since there are already class variables…). That, in turn, makes it
much harder to understand instance variables.
There are now two distinct class variables named @@avar, one is associated
with Object and another is associated with class A and shadows the one
associated with Object.
Yes – basically, if you create the class variable in the subclass
first, then it’s different from the one in the superclass. Mind you,
a lot of this is scheduled to change in 2.0. I fear, though that the
confusion caused by class variables will persist. A class variable is
going to be (as I understand it) more similar to an instance variable
than it is now, but with visibility to instances of the class. A lot
of the problem with class variables is that they cloud the matter of
classes being objects and having their own instance variables. I’m
not sure whether these changes will change that.
(When I say they cloud it, I’m basing this on six years of watching
person after person after person have trouble grasping this area of
Ruby, and it’s always something like, “Wait – doesn’t a class have class variables? If they’re analogous to instance variables for
instances, then what are they?” It can be explained, of course (I’ve
done it many times but there’s definitely something in the
behavior that makes this harder to get than most other language
features.)
I still don’t understand why class_eval/instance_eval don’t affect the
resolution of class variables in a manner analogous to how they affect the
resolution of instance variables. Is this by design or accident?
See David G.'s explanation; I think it’s by design. class_eval
brings about a scope that’s like a ‘class’ block in some ways but not
in others. The surrounding class and local variables can permeate
their way in.
You had me at “I generally avoid class variables”
…snip…
I love Ruby madly, but (or “therefore”? I would be happy never to
see another class variable again. The confusingness-to-usefulness
ratio is extremely high.
David and gang, thanks for pulling me out of denial.
http://rob.muhlestein.net/2007/01/ruby-class-variables-just-dont-go-there.html]
After reading the comments in the response it is probably worth
mentioning in this thread that class variable scope inheritance is
changing between 1.8 and 1.9/2.0 (as noted in the pickax). I ran an irb
confirmation of this and posted results there to make sure.
http://rob.muhlestein.net/2007/01/ruby-class-variables-just-dont-go-there.html]
I blogged it too, though mine is mostly just a quick paste of the two
bits of code from David and Gary, and a link to your blog entry. My
NubyGems series is usually like that, a big chunk of code from an IRB
session meant to make people go ‘whoa, yeah, that does suck’
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I would be happy never to