Class variables not in the scope of class functions

Question

I have the following bit of code:

class Test {
  @s = 'Bob'

  def print_bob
    p @s
  end
end

When I try to call print_bob, it prints nil. Why is this? I understood class variables are visible from anywhere in a class?

BTW, you can assign default values for your instance variables in Test#initialize, see ruby-doc.org/core-2.1.2/doc/syntax/… — Stefan
– Stefan, Commented Jun 14, 2014 at 12:29
Eden, you need to delete {. No reply, plz--I'll delete this comment once you've seen it. — Cary Swoveland
– Cary Swoveland, Commented Jun 14, 2014 at 21:53
Eden, the answers explain that @s is a class instance variable. In fact, you could have an instance variable (defined in initialize, say) with the same name (@s) and they would co-exist no differently than would @night and @day. If you needed the value of the class instance variable in your print_bob method, replace p @s with p self.class.instance_variable_get(:@s) . Then Test.new.print_bob #=> "Bob". — Cary Swoveland
– Cary Swoveland, Commented Jun 14, 2014 at 21:57

Arup Rakshit · Accepted Answer · 2014-06-14 12:03:41Z

2

@s is an class instance variable, not the instance variable of the instances of the Test class. print_bob is an instance method. As you are not defining those instance variables, while you are creating the instances of Test, thus when you will call the method print_bob on the instances of Test, you will get nil. Remember - instance and class variables, if you attempt to use them before defining, they will return nil.

Look the below code:

class Test 
  @s = 'Bob'

  def print_bob
    p @s
  end
end

test = Test.new
# Now see, here no instance variables are listed, for instance test. Because you did
# not create any.
test.instance_variables # => []
# you can see, @s is listed in the method call, because has been defined when the
# class has been created.
Test.instance_variables # => [:@s]

edited Jun 14, 2014 at 12:03

answered Jun 14, 2014 at 11:49

Arup Rakshit

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1 Comment

Lēctia Landau Over a year ago

I see. This is exactly what I've been looking for. Thank you very much!

Marek Lipka · Accepted Answer · 2014-06-14 11:50:57Z

0

@s is instance variable definition, not a class variable. So in the class body, defining @s defines class intance variable, but if you try to refer it in method body, you refer "regular" instance variable, which is unset.

Class variables are defined with @@, so this should work as you expect:

class Test {
  @@s = 'Bob'

  def print_bob
    p @@s
  end
end

answered Jun 14, 2014 at 11:50

Marek Lipka

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