Setting instance variables in ruby is all about the current value of self. Just because you use @foo = bar...doesn't mean you'll get a new copy of foo for every instance of your class.

###by Antwan Wimberly###

class HiO
  def print_i
    # note...this won't print 12...even though we set
    # this variable in the `toast` method
    puts "my var is #{@var}"

    # and neither will this...failure!!!
    #puts @@var

    # but this however...will!!!
    puts self.class.instance_variable_get "@var"

    # I wonder if we can overrite `var`?
    @@var = 19

    # do you think it will print 19...or 12?
    # nope...prints 12 as we're now
    # pointing to the original @var that `toast` set
    puts self.class.instance_variable_get "@var"

    create_new_instance_and_then_print_var
    
    # this is weird...we've got an instance of a `Class` which can have instances of 
    # itself well...turns out that every time we say `self.class`, it returns the
    # same thing! ruby will make only one instance of that guy
    
    # this will print true and they both have 12 for var as they're the same instance
    # of the same `Class`
    puts "classes are equal #{self.class.object_id === self.class.object_id}"
  end

  def create_new_instance_and_then_print_var
    # do you think it will say 19?
    #
    # yes it does...twice...all instances will from this point forward because
    # we put the value on the `Class`.
    #
    # you must be wondering though...
    #
    HiO.new.print_var
    HiO.new.print_var
    
     # if we've got two variables named var on the `Class`, then why don't
    # they collide?
    #
    # *shrugs*
    
    # ok...I lied...they're not pointing to the same thing
    puts HiO.class_variables # this would be the `@var` that `toast` set
    puts HiO.instance_variables # this....would be the `@@var` guy...make sense???
    
    # I'll tell you that the @@var guys can be very dangerous...beware..any derived
    # classes will also see the same value and can manipulate that guy thus changing
    # if for any other derived classes (in addition to the original base clase)
  end

  def print_var
    puts @@var
  end

  def self.toast
    @var = 12
    puts "Current value for @var is #{@var}"
  end
end

HiO.toast
HiO.new.print_i

armw4/ruby-instance-variables.md

armw4 commented Dec 15, 2013