Class Level Instance Variables in Ruby

Question

Please help me to understand Class Level Instance Variables.

@@ is a class variable and is an equivalent to instance variable (@) in a class instance.

But what is an instance variable (@) when used on class level? If it puts a definition to class instance then why not to define it in an initializer?

class MyClass
  cattr_reader :class_variable

  def self.new_instance(cv, cliv, iv)
    @@class_variable = cv
    @class_level_instance_variable = cliv
    self.new(iv)
  end

  def initialize(iv)
    @instance_variable = iv
  end

  def use
    puts "class_var=#{self.class.class_variable.inspect}\ninst_var=#{@instance_variable.inspect}\ncliv=#{@class_level_instance_variable.inspect}"
  end
end

c = []
c << MyClass.new_instance(1,2,3)
c[0].use
c << MyClass.new_instance(4,5,6)
c[1].use
c << MyClass.new_instance(7,8,9)
c[2].use

c[0].use
c[1].use
c[2].use

When you set instance variable on class level, it becomes class instance variable, which is something slightly different (and not really equivalent) to class variable. More info here: railstips.org/blog/archives/2006/11/18/… — Marek Lipka
– Marek Lipka, Commented Aug 29, 2014 at 9:54
@Marek Lipka: so, if I understood right, class level instance variables are fully equivalent to pre-initialized instance level instance variables and if I make multiple class instances in a class method each of these instances becomes initialized with this variable. — Paul
– Paul, Commented Aug 29, 2014 at 10:02
No, there is no such behavior at all. I only mean class level instance variable is regular instance variable, but bound to the class object (remember in Ruby classes are also objects). — Marek Lipka
– Marek Lipka, Commented Aug 29, 2014 at 10:09
@Paul: Why don't you throw in some code to illustrate what you mean? — Sergio Tulentsev
– Sergio Tulentsev, Commented Aug 29, 2014 at 10:17

jan.zikan · Accepted Answer · 2014-08-29 10:21:28Z

3

Hopefully this example will explain the difference between @ (instance) and @@ (class) variables.

class Animal
  @@total_count = 0

  def self.total_count
    @@total_count
  end

  def initialize
    @@total_count += 1
  end
end

class Cat < Animal
end

Animal.new
Animal.new
Cat.new

Animal.total_count # => 3
Cat.total_count # => 3

As you can see the @@ variable is shared between class and its children. If I really want to count the number of instances of the classes I have to use the following code.

class Animal
  class << self
    attr_accessor :total_count
  end

  @total_count = 0

  def self.total_count
    @total_count
  end

  def initialize
    self.class.total_count += 1
  end
end

class Cat < Animal
  @total_count = 0
end

Animal.new
Animal.new
Cat.new

Animal.total_count # => 2
Cat.total_count # => 1

answered Aug 29, 2014 at 10:21

jan.zikan

1,31610 silver badges14 bronze badges

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

Comments

Sergio Tulentsev · Accepted Answer · 2014-08-29 10:34:33Z

2

In your answer you don't output the class level instance variable. Besides the usual syntax (@foo), an instance variable can be accessed via a method (instance_variable_get(:@foo)). You can use this method to read instance variables of other objects, not only self.

Here's a modified version of your code

require 'active_support/core_ext'

class MyClass
  cattr_reader :class_variable

  def self.new_instance(cv, cliv, iv)
    @@class_variable = cv
    @class_level_instance_variable = cliv
    self.new(iv)
  end

  def initialize(iv)
    @instance_variable = iv
  end

  def use
    puts "class_var=#{self.class.class_variable.inspect}"
    puts "class inst var: #{self.class.instance_variable_get(:@class_level_instance_variable)}"
    puts "inst_var=#{@instance_variable.inspect}"
  end
end

c = []
c << MyClass.new_instance(1,2,3)
c << MyClass.new_instance(4,5,6)
c << MyClass.new_instance(7,8,9)

c[0].use
c[1].use
c[2].use
# >> class_var=7
# >> class inst var: 8
# >> inst_var=3
# >> class_var=7
# >> class inst var: 8
# >> inst_var=6
# >> class_var=7
# >> class inst var: 8
# >> inst_var=9

See, class inst var is always 8 (just as class var is always 7). This is because you output values after all your modifications are made. And since class level variables are shared, the last modification wins.

c << MyClass.new_instance(7,8,9)

If you were to output from initializer (as was in your first version of code), you'd see different results.

# >> class_var=1
# >> class inst var: 2
# >> inst_var=3
# >> class_var=4
# >> class inst var: 5
# >> inst_var=6
# >> class_var=7
# >> class inst var: 8
# >> inst_var=9

edited Aug 29, 2014 at 10:34

answered Aug 29, 2014 at 10:23

Sergio Tulentsev

231k43 gold badges381 silver badges373 bronze badges

2 Comments

Paul Over a year ago

So, a class level instance variable gives a way to communicate between children because is passed by reference to each instance. Right?

Sergio Tulentsev Over a year ago

Yep, it's shared between all instances of the class.

Collectives™ on Stack Overflow

Class Level Instance Variables in Ruby

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related