Ruby Variables Clarification

Question

I see that Ruby has the following variables: - global variables (represented by $variable_name) - class variables (represented by @@variable_name) - instance variables (represented by @variable_name) and - local variables (represented by variable_name or _variable_name)

Occasionally I see the following in the rails source code:

class SomeClass @var end

Here what exactly @var represent and what do you call it, metaclass variable? Also whats the advantage of using this kind of variables?

Its just a general question, from time to time when I read the Ruby documentation and when I come across the statement that everything is an object in Ruby, which means Classes itself are objects and they can have their own instance variables, I think about this. Also I don't remember the actual file, but I remember seeing some usage in the rails code. — Dharam Gollapudi
– Dharam Gollapudi, Commented Mar 1, 2010 at 16:26

BaroqueBobcat · Accepted Answer · 2010-03-01 16:16:32Z

1

It is one of the classes instance variables. In Ruby, everything is an object, even classes, so it isn't surprising that classes can have instance variables.

class A
  @@class_var = 1
  @instance_var = 1
end
A.class_variables
#=> ["@@class_var"]
A.instance_variables
#=>["@instance_var"]

More Info

answered Mar 1, 2010 at 16:16

BaroqueBobcat

10.2k1 gold badge34 silver badges37 bronze badges

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

5 Comments

Dharam Gollapudi Over a year ago

Thats what I thought, but whats the purpose or use case of using classes instance variables? Also are all the instances created from this particular class share these? If yes, how can a instance of a class access these variables? We cannot just say @instance_var, that would try to query for the instances instance variable, we cannot just say @@instance_var, that would try to query for the class variable.

Beanish Over a year ago

@instance is only for the current instance of the object (all objects have this variable, but it's value is unique to the instance) @@class is shared across all instances of the objects (all objects have and share this variable, value is not unique) Take a look at attr_accessor :myvariable as a quick way to set up your gets and sets for your variables.

phtrivier Over a year ago

You cannot access it because it is a private variable. If you define an accessor like this (def A.get_instance_var @instance_var end) you can access it with (A.new).class.get_instance_var. As for the utility of this ... that's the beauty of metaprogramming, you have no clue something is either possible or usefull until you try it, it works, and it solves some problem ;)

Dharam Gollapudi Over a year ago

@phtrivier: Thanks for the clarification, that explains a whole lot. Based on your example, I am assuming it behaves more or less like class variables, I mean all the instances of A will get to the same value and if one instance modifies it, it reflects in all the other instances. If that is true, I don't quite see the difference between them and class variables, correct me if I am wrong.

Dharam Gollapudi Over a year ago

@BaroqueBobcat: Thanks for the link that clarifies a lot with the use cases. Initially I didn't see that link which is why I posted several comment responses.

Collectives™ on Stack Overflow

Ruby Variables Clarification

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related