Quick question about @ and @@

ruby-talk
1

What's the difference between @ and @@? Specifically, which should I use if
I need a global variable in a non-OOP Ruby program?

Thanks!

Mike Steiner

2

Hi --

What's the difference between @ and @@? Specifically, which should I use if
I need a global variable in a non-OOP Ruby program?

@var is an instance variable (i.e., it represents the private state of
whatever object is in the role of 'self' at the point where @var
appears). @@var is a class variable, which is a kind of
de-encapsulated thing shared among a class, its descendants, and all
the instances of all those classes. There's no connection or relation
between @var and @@var.

$var is a global variable.

David

···

On Fri, 18 May 2007, Mike Steiner wrote:

--
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A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black\)
    (See what readers are saying! http://www.rubypal.com/r4rrevs.pdf\)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

3

quoth the Mike Steiner:

What's the difference between @ and @@? Specifically, which should I use if
I need a global variable in a non-OOP Ruby program?

Thanks!

Mike Steiner

As I understand it, '@' denotes an instance variable, visible only to the
instance of a class, and '@@' is a class variable, visible to all instances
of the class.

If you need a plain old global try: $foovar

-d

···

--
darren kirby :: Part of the problem since 1976 :: http://badcomputer.org
"...the number of UNIX installations has grown to 10, with more expected..."
- Dennis Ritchie and Ken Thompson, June 1972

4

Mike Steiner wrote:

What's the difference between @ and @@? Specifically, which should I use
if
I need a global variable in a non-OOP Ruby program?

Thanks!

Mike Steiner

@ = Instance variable
@@ = Class variable

If you need a global variable in a non-OOP program, I guess I'd suggest
using a constant (capitalized variable). Here's an example of @ and @@
usage:

irb(main):001:0> class Foo
irb(main):002:1> attr_accessor :a
irb(main):003:1> @@b = 0
irb(main):004:1> def initialize
irb(main):005:2> @a = 0
irb(main):006:2> end
irb(main):007:1> def add_a
irb(main):008:2> @a += 1
irb(main):009:2> end
irb(main):010:1> def add_b
irb(main):011:2> @@b += 1
irb(main):012:2> end
irb(main):013:1> def b
irb(main):014:2> @@b
irb(main):015:2> end
irb(main):016:1> end
=> nil
irb(main):017:0> one_foo = Foo.new
=> #<Foo:0x28dc078 @a=0>
irb(main):018:0> two_foo = Foo.new
=> #<Foo:0x28d9ea4 @a=0>
irb(main):019:0> one_foo.add_a
=> 1
irb(main):020:0> one_foo.a
=> 1
irb(main):021:0> two_foo.a
=> 0
irb(main):022:0> one_foo.add_b
=> 1
irb(main):023:0> one_foo.b
=> 1
irb(main):024:0> two_foo.b
=> 1

···

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Posted via http://www.ruby-forum.com/\.

5

Hi,

What's the difference between @ and @@?

  class C
    @a, @@a = 3, 33

    def set_a x, y ; @a, @@a = x, y ; end

    def dump ; puts "#@a, #@@a" ; end
    def self.dump ; puts "#@a, #@@a" ; end
  end

  C.dump
  puts

  c = C.new
  d = C.new

  c.set_a 7, 77
  d.set_a 8, 88

  c.dump
  d.dump
  C.dump

The output is:

  3, 33

  7, 88
  8, 88
  3, 88

Bertram

···

Am Freitag, 18. Mai 2007, 01:20:26 +0900 schrieb Mike Steiner:

--
Bertram Scharpf
Stuttgart, Deutschland/Germany
http://www.bertram-scharpf.de

6

That clears things up a lot. I'll use $var instead of @var for my (usually
few!) global variables.

Thanks!

Mike Steiner

···

On 5/17/07, darren kirby <bulliver@badcomputer.org> wrote:

quoth the Mike Steiner:
> What's the difference between @ and @@? Specifically, which should I use
if
> I need a global variable in a non-OOP Ruby program?
>
> Thanks!
>
> Mike Steiner

As I understand it, '@' denotes an instance variable, visible only to the
instance of a class, and '@@' is a class variable, visible to all
instances
of the class.

If you need a plain old global try: $foovar

-d
--
darren kirby :: Part of the problem since 1976 :: http://badcomputer.org
"...the number of UNIX installations has grown to 10, with more
expected..."
- Dennis Ritchie and Ken Thompson, June 1972