From: Phrogz <phrogz@...>
Date: 2009-03-31T05:10:02+09:00
Subject: Re: When to use instance variables @

On Mar 30, 1:52 pm, Steve Dogers <stevedog...@gmail.com> 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 can ask for any instance variable at any time - if no value
has ever been set, you will get nil back. You can set any instance
variable at any time. A common idiom is:
@foo ||= 42
which is the same as "@foo = @foo || 42" which technically means "Set
@foo to 42 if is it currently false or nil"; when you're not dealing
with an instance variable that deals in boolean values, however, it
basically means "Set @foo to 42 unless I already set it to some
value."


> 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?

It does not work without the @. You must use it everywhere, even when
using something like:
variable_name = "foo"
instance_variable_set( :"@#{variable_name}", 42 )


> 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?

Class 'declarations' aren't quite what you think they are. They're
actually code that is executed. Try this on for size:

  msg = "outside"
  puts "#{msg} class"
  class Foo
    msg = "inside"
    puts "#{msg} class"
  end
  puts "#{msg} class"

Note in particular the last message; the output "outside class" lets
you know that there are two local variables with the same name, in
their own scopes.