From: Jacob Fugal <lukfugl@...>
Date: 2004-11-12T04:39:20+09:00
Subject: Re: automatically call function on attribute set

On Fri, 12 Nov 2004 04:13:30 +0900, Joe Van Dyk <joe.vandyk@boeing.com> wrote:
>
> What is the difference between self.x and @x?

self.x, sends either the :x or :x= method to self, depending if it's
an lvalue or rvalue respectively. @x just reads/writes to the instance
variable directly. If you're just using attr_accessor :x, they're the
same thing, but if you have, for instance:

class Foo
   def bar
      raise "@bar and @baz out of sync" unless @bar == @baz
      @bar
   end
   def bar=( new_bar )
      puts "setting @bar and @baz"
      @bar = @baz = new_bar
   end
   def test1
      self.bar = "test1"
   end
   def test2
      @bar = "test2"
   end
end

foo = Foo.new
foo.bar = "test"
#  => setting @bar and @baz
 
puts foo.bar
# => test

foo.test1
# => setting @bar and @baz

puts foo.bar
# => test1

foo.test2
puts foo.bar
# RuntimeError: @bar and @baz out of sync

Using self.bar= keeps @bar and @baz in sync, while setting @bar
directly doesn't update @baz. Generally, if you have non-trivial
accessors for instance variables, you should use them even inside the
object rather than accessing the instance variables directly. I
usually use them even if the accessor is trivial, so that if the
accessor becomes non-trivial later I don't have to go find and fix all
the places where the instance variables are currently accessed
directly.

Jacob Fugal