A variable prefixed with @ is an instance variable, while one prefixed with @@ is a class variable. Check out the following example; its output is in the comments at the end of the puts lines:
class Test
@@shared = 1
def value
@@shared
end
def value=(value)
@@shared = value
end
end
class AnotherTest < Test; end
t = Test.new
puts "t.value is #{t.value}" # 1
t.value = 2
puts "t.value is #{t.value}" # 2
x = Test.new
puts "x.value is #{x.value}" # 2
a = AnotherTest.new
puts "a.value is #{a.value}" # 2
a.value = 3
puts "a.value is #{a.value}" # 3
puts "t.value is #{t.value}" # 3
puts "x.value is #{x.value}" # 3
You can see that @@shared is shared between the classes; setting the value in an instance of one changes the value for all other instances of that class and even child classes, where a variable named @shared, with one @, would not be.
[Update]
As Phrogz mentions in the comments, it’s a common idiom in Ruby to track class-level data with an instance variable on the class itself. This can be a tricky subject to wrap your mind around, and there is plenty of additional reading on the subject, but think about it as modifying the Class class, but only the instance of the Class class you’re working with. An example:
class Polygon
class << self
attr_accessor :sides
end
end
class Triangle < Polygon
@sides = 3
end
class Rectangle < Polygon
@sides = 4
end
class Square < Rectangle
end
class Hexagon < Polygon
@sides = 6
end
puts "Triangle.sides: #{Triangle.sides.inspect}" # 3
puts "Rectangle.sides: #{Rectangle.sides.inspect}" # 4
puts "Square.sides: #{Square.sides.inspect}" # nil
puts "Hexagon.sides: #{Hexagon.sides.inspect}" # 6
I included the Square example (which outputs nil) to demonstrate that this may not behave 100% as you expect; the article I linked above has plenty of additional information on the subject.
Also keep in mind that, as with most data, you should be extremely careful with class variables in a multithreaded environment, as per dmarkow’s comment.