From: Michael Malone <michael.malone@...>
Date: 2009-03-31T05:29:38+09:00
Subject: Re: When to use instance variables @

Steve Dogers wrote:
> Hi, I have a couple questions about instance variables in Ruby.
> 1) do i need to declare them at the top of my class file - I do
> understand the accessors are automatic but I'm not sure if I can just
> pull a @product in the middle of a function
>   
No, you don't need to declare them at the top of your file, just before 
you use them.  It's good practice to initialise each of your class 
variables in your constructor, but this is not always possible.
in the middle of a member function, you can just pull a '@product = 
something' even if you haven't already defined it, but bugs are harder 
to trace if variables start popping up all over the place. 
> 2) do i need to use the @ to refer to them in the class. Would it work
> without the @, and if so, how does it differentiate them from local
> vars?
>   
Yes, always with the @ that's exactly how their scope is known to the 
interpreter.
> 3) I've seen code where just below the class declaration, objects are
> instantiated like product = Product.new - I don't see a @ sign, does
> that mean it's a local var? How can a local var even exist at the class
> level, outside a function?
>   
a local var existing at the class level outside a function?  That to me 
means something like this:

class Klass
  product = Product.new

  def some_function
    nil
  end
end

That doesn't make sense to me and has no accessible means  (I could be 
wrong on this one though).  It's valid syntax, but to my understanding, 
it will be instantiated and then immediately set for garbage 
collection.  A subtle note is that if the variable name starts with a 
Capital letter, then it is a constant that belongs to that class and has 
java-like public scope, so can be referred to by Klass::Constant

Also, you might see some people write code that looks procedural, but 
uses @variables and so forth.  This is because you are by default in the 
Object class's scope.  Hmmm, this makes me think that local variables at 
the class level should be treated identically as the procedural-like 
code at the Object scope.  I think I have just confused myself, can 
anyone clarify this? (Of course, I'm not actually going to use this 
feature, even if it exists, but it would be nice to understand)
>
> Many thanks!
>   


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