Instance variable in ruby acting like class variable

Question

@ - instance variable @@ - class variable.

So instance variable value shouldn't be shared if no instance is created. But:

class Add

  def self.add(what)
    if not defined? @a
      @a = 0
    end
    @a += what
    puts @a.to_s
  end
end

Add.add(4)
Add.add(4)

Results in:

$ruby main.rb

4
8

Why?

Aside: @a ||= 0 is a commonly-used replacement for if not defined? @a.... The former is shorthand for @a = @a || 0, just as x += 1 is for x = x + 1. — Cary Swoveland
– Cary Swoveland, Commented Nov 13, 2013 at 16:48

Marek Lipka · Accepted Answer · 2013-11-13 16:14:14Z

3

Every class in Ruby is also an object, instance of Class class. So, your @a is simply instance variable of Add class.

answered Nov 13, 2013 at 16:14

Marek Lipka

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6 Comments

meso_2600 Over a year ago

Ok then why Add.add(4) run for the second time results in 8. @a is not global

Marek Lipka Over a year ago

@user665967 it's because @a is instance variable of Add class. When you modify instance variable value of some object, and that object persist, its instance variable persists as well.

meso_2600 Over a year ago

Correct me if I'm wrong, so basically @variables in Class methods are instance variables but automatically become Class variables on the Object level

Marek Lipka Over a year ago

@user665967 no. The point is that in Ruby every class is also an object with all consequences. One of these consequences is the fact that it has its own instance variables. So if you define "class method" and you use instance variable inside of it, this instance variable belongs to this class instance. And, what's important, it's NOT a class variable, which is a bit different concept.

meso_2600 Over a year ago

Marek, how can I contact you? :)

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kostja · Accepted Answer · 2013-11-13 21:07:11Z

2

The @a you are referring to in the singleton method is an instance variable of Add's class.

You have effectively changed the scope of the method declaration to the eigenclass when you have defined the method as def self.add instead of def add.

edited Nov 13, 2013 at 21:07

answered Nov 13, 2013 at 16:12

kostja

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4 Comments

Marek Lipka Over a year ago

Why 'eigenclass'? It's an instance variable of object itself (which is Add class), not its eigenclass: (class << Add; self end).instance_variable_get(:@a) # => nil

kostja Over a year ago

@MarekLipka please, read up the section about class methods in the second link and you might reconsider the downvote. tl;dr: all class methods are housed in the eigenclass of the particular Class instance.

Marek Lipka Over a year ago

it's not about "class method" (which is, of course, right that they are housed in eigenclassess of the particular Class instance). It's about instance variables. Please take my example into consideration.

kostja Over a year ago

@MarekLipka - I see, you're right. Leave the downvote, it is deserved. Thank you for this small instant of enlightenment :) I will remove the eigenclass.

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