static variables in ruby

I just learned about static variables in php. Is there anything like that in ruby?

For example, if we want to create a Student class and for each student object we create, its id number should get incremented automatically.

I thought creating class variable as a static will do.

Class variables are shared between all instances (which is why they're called class variables), so they will do what you want. They're also inherited which sometimes leads to rather confusing behavior, but I don't think that will be a problem here. Here's an example of a class that uses a class variable to count how many instances of it have been created:

class Foo
  @@foos = 0

  def initialize
    @@foos += 1
  end

  def self.number_of_foos
    @@foos
  end
end

Foo.new
Foo.new
Foo.number_of_foos #=> 2

Using the accepted answer as the definition of static variable can be dangerous, and it is a common error that I have seen in a lot of Ruby code.

Something like @@foos is shared among ALL subclasses. However, most programmers expect static variables to have scope only within the class where they are defined.

If you are looking for static variables in the sense of most languages, where their scope is only their own class, look at this SO answer

Also this blog post has a nice example of the surprise most programmers will get:

http://www.railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/