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.

Answer 1

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]