Static / Instance variables in ruby

Question

I'm a Ruby newbie. Have a very basic question about static and instance variables.

class Test

  def self.init
    @@var_static = 1
    @member = 2
  end

  def self.print
    puts "@@var_static: #{@@var_static}"
    puts "@member: #{@member}"
  end

end

Test.init
Test.print

Why does the code above allow initialization of a member variable: @member, inside the static method: Test::init ? My understanding was that usage of @member will throw an error because it is not tied to any instance of class Test. But no error is thrown.

rfunduk · Accepted Answer · 2012-06-18 17:49:58Z

3

A class is an instance of an object though, and can have instance variables just like any other object:

>> Fixnum.class
=> Class

Fixnum the class is an instance of Class!

answered Jun 18, 2012 at 17:49

rfunduk

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6 Comments

Kowshik Over a year ago

Which class is the @member variable tied to, and how can I find it?

rfunduk Over a year ago

I don't understand your question. When you define a class you are creating a single instance of Class. Test the class you just defined is the instance which has @member.

rfunduk Over a year ago

Try: Test.instance_variable_get('@member')

phoet Over a year ago

it's all about the context your code is evaluated in. if you define a method with self.something then you are in the context of the class you are currently defining. so the "member" variable is a member of the class.

Chloe Over a year ago

That is very much unlike Java. Trying to access a member variable from a static method will give a compile error. You have to have an instantiated object before you can get the instance variables.

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robbrit · Accepted Answer · 2012-06-18 18:20:33Z

1

A good article explaining this is here: http://railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/

answered Jun 18, 2012 at 18:20

robbrit

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