Understanding Ruby class instance variables [duplicate]

Question

Possible Duplicate:
Why do Ruby setters need “self.” qualification within the class?

Can someone explain the difference between the following, and why it isn't as one might expect:

# version #1
class User
  def initialize(name, age)
    @name = name
    @age = age
  end
end

#version #2
class User
  attr_accessor :name, :age
  def initialize(name, age)
    @name = name
    @age = age
  end
end

#version #3
class User
  attr_accessor :name, :age
  def initialize(name, age)
    self.name = name
    self.age = age
  end
end

From what I understood, in methods, when you are assigning, you have to use the self keyword. Why can't you use this in an initialize method? Or can you? I tried using it and it didn't seem to work as expected, I'm just confused as to which technique to use and when and more importantly why.

I really hope someone can clear this up for me once and for all :)

Answer 1

Version 1: The constructor creates two instance variables, @name and @age. These two variables are private (as are all Ruby instance variables), so you can't access them outside of the class.

Version 2: Exact same thing as #1 except that you're also defining a getter and setter method for the two variables. What attr_accessor does is create two methods for each parameter that allow you to get/set the value of the instance variable with the same name.

Version 3: Exact same as #2 except that in your constructor you're not setting the instance variables directly, instead you're calling the User#name= and User#age= methods to set the value of your instance variables instead of setting them directly.

To clarify the difference between setting the instance variable directly and calling a setter method, consider this example:

user = User.new "Rob", 26
user.name = "Joe"

Here you are not actually setting the @name variable of user directly, instead you are calling a method called name= on user which sets the value of @name for you. When you did the call to attr_accessor in versions #2 and #3, it defined that method for you. However in version #1 you did not call attr_accessor, so the example above would be invalid since there is no name= method.