Instance variable initialization in Ruby

Question

I think I'm going a bit crazy when trying to understand instance variables in Ruby. My only aim here is to make sure that every object created for a given class has a variable with a predetermined value without writing an initialize method for that class. Something like:

class Test

  @my = []

  attr_accessor :my

end
t = Test.new
t.my # I want [] but this shows nil

Is it possible to achieve this without touching initialize ? Thanks.

EDIT: To clarify, I'm writing some piece of code which will be executed similar to attr_accessor in the sense that it'll add an instance variable to the class in which it is executed. If I write my own initialize, I will end up clobbering the one written by the user.

Dominik Honnef · Accepted Answer · 2012-03-03 20:25:20Z

11

What you are doing is defining an instance variable on the class level (Since classes are instances of the Class class, this works just fine).

And no, there is no way around initialize.

Edit: You have a little misconception in your edit. attr_accessor doesn't add an instance variable to the class. What it does, literally, is this (using your example of my):

def my; @my; end
def my=(value); @my = value; end

It doesn't actively create/initialize any instance variable, it just defines two methods. And you could very well write your own class method that does similar things, by using define_method.

Edit 2:

To further illustrate how one would write such a method:

class Module
  def array_attr_accessor(name)
    define_method(name) do
      if instance_variable_defined?("@#{name}")
        instance_variable_get("@#{name}")
      else
        instance_variable_set("@#{name}", [])
      end
    end

    define_method("#{name}=") do |val|
      instance_variable_set("@#{name}", val)
    end
  end
end


class Test
  array_attr_accessor :my
end

t = Test.new
t.my # => []
t.my = [1,2,3]
t.my # => [1, 2, 3]

edited Mar 3, 2012 at 20:25

answered Mar 3, 2012 at 19:24

Dominik Honnef

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5 Comments

sasuke Over a year ago

My bad, I said "similar to". Basically, this new method, when added to any class, like attr_accessor, will add a new instance variable to all objects created for that class.

sasuke Over a year ago

Ah, as I thought, I can't rely on default attr_reader for such attributes. Just one last question: there is no way of initializing instance variables like the way it is done for Java etc. in Ruby?

sasuke Over a year ago

Thanks, accepted! (though I don't understand what you mean by 9 more to go...?)

Dominik Honnef Over a year ago

A comment on StackOverflow has to be at least 15 characters long. "Nope." wasn't long enough ;)

Mon ouïe Over a year ago

I'd use instance_variable_set when the variable isn't set, instead of just returning the created array. That way, test.my << "foo" will actually modify the array test.my is referring to.

Selman Ulug · Accepted Answer · 2012-03-03 19:38:49Z

2

# as instance variable without initialize
class Test1
  def my; @my ||= [] end

  attr_writer :my
end

t = Test1.new
t.my


# as class instance variable
class Test2
  @my = []

  class << self; attr_accessor :my end
end

Test2.my

answered Mar 3, 2012 at 19:38

Selman Ulug

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1 Comment

mklement0 Over a year ago

+1, esp. for the class-level variable with accessors. The caveat re instance variables without initialize is that if you attempt to set t.my = nil, t.my will revert to [] due to ||=, which is unexpected and may not be desired; methinks it's better not to avoid initialize.

TheDelChop · Accepted Answer · 2012-03-03 19:24:41Z

0

I don't think it is, why are you so hesitant to just write a quick initialize method?

answered Mar 3, 2012 at 19:24

TheDelChop

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