I'm trying to access a datamember of a parent class from a child class. I am not sure how to call it. I have found a lot of info about accessing class variables but not instance variables from a child class. Here is my code:
class Shape @var = "woohoo"
That variable is known as a class instance variable, and the reason people use a class instance variable instead of a class variable, i.e. an @@variable, is precisely so that child classes cannot access it. Because class instance variables are the way class variables work in other languages, @@variables are not used much in ruby because their behavior is surprising if you come from another language that has class variables.
Your use case apparently calls for a class variable to be accessible in the child class, so have at it with an @@variable.
EDIT: I want individual instances of the child class to replace the 'woohoo' string with their own unique strings.
You can use class instance variables for that:
class Shape @var = "shapehoo" class <<self attr_accessor :var end def display_class_instance_var puts Shape.var end end class Rectangle < Shape @var = "recthoo" def display_class_instance_var puts Rectangle.var end end class Circle < Shape @var = "circlehoo" def display_class_instance_var puts Circle.var end end Shape.new.display_class_instance_var Rectangle.new.display_class_instance_var Circle.new.display_class_instance_var Rectangle.new.display_class_instance_var Shape.new.display_class_instance_var --output:-- shapehoo recthoo circlehoo recthoo shapehoo
As with regular instance variables, class instance variables are private, so you have to provide accessor methods if you want to access them. The accessors need to be in the class's singleton class, which you can open using the syntax:
class <<self end
Added:
About this code:
class Rectangle < Shape @length @width def initialize ( l,w ) @length = l @width = w end
In your initialize() method, you are not setting the @length, @width variables you declared above the initialize method. In ruby, @variables attach themselves to whatever object is self at the time the @variables are created. Here is what your code looks like with some more detail:
class Rectangle < Shape #self=Rectangle class @length @width def initialize ( l,w ) #self=a new instance of the Rectangle class created by initialize @length = l @width = w end
As a result, the @variables created in initialize() attach themselves to the new instance, while the @variables declared above initialize() attach themselves to the Rectangle class, which means they are completely different variables.
