Question

Possible Duplicate:
What does @@variable mean in Ruby?

What is the difference when I declare an object with double ‘@’

@@lexicon = Lexicon.new()

and declaring object with single ‘@’ in Ruby?

@lexicon = Lexicon.new()

Answer 1

The difference is that the first one is a class variable and the second one is an instance variable.

An instance variable is only available to that instance of an object. i.e.

class Yasin
  def foo=(value)
    @foo = value
  end

  def foo
    @foo
  end
end

yasin = Yasin.new
yasin.foo=1
yasin.foo #=> 1
yasin_2 = Yasin.new
yasin_2.foo #> nil

A class variable is available to all instances of the class (and subclasses, iirc) where the class variable was defined.

class Yasin
  def foo=(value)
    @@foo = value
  end

  def foo
    @@foo
  end
end

yasin = Yasin.new
yasin.foo=1
yasin.foo #=> 1
yasin_2 = Yasin.new
yasin_2.foo #=> 1