Ruby—A Diamond of a Programming Language? : Page 3

Instance Variables and Attributes
Classes can also have instance variables (also known as properties in some languages). For example, objects created from the Rectangle class should all have a height and width. In Ruby, instance variables do not explicitly have to be declared in the class, they simply have to be used and annotated with a special character in their name. Specifically, all instance variable names begin with an "@." To store the height and width in the instance of rectangle when the area method is invoked, you merely have to add instance variables to the area method:

class Rectangle
	def area (hgt, wdth)
		@height=hgt
		@width = wdth
		@height*@width
	end
end

More appropriately, when an instance of Rectangle gets created, the height and width should be specified and the instance variables set at that time. Ruby provides a special method called initialize to allow you to set up or prepare new instances of the class:

class Rectangle
	def initialize (hgt, wdth)
		@height = hgt
		@width = wdth
	end

	def area ()
		@height*@width
	end
end

To create a new object or instance of Rectangle, you call on the standard Ruby constructor method "new" against the class—in this case, Rectangle:

Rectangle.new(4,7)

Or, if you prefer the same instantiation without parentheses, although again for clarity's sake, I would recommend using parentheses:

Rectangle.new 4,7

In this example, a new Rectangle object will get created, and the initialize method is called with the parameters of 4 and Rectangle class also defined methods for granting access. Notice, in the code below, the height and width methods have been added in order to share height and width information:

class Rectangle
	def initialize (hgt, wdth)
		@height = hgt
		@width = wdth
	end

	def height
		return @height
	end

	def width
		return @width
	end

	def area ()
		@height*@width
	end
end

Likewise, in order for some other object to update or set the height and width of a Rectangle object, additional mutating methods need to be defined:

class Rectangle
	def initialize (hgt, wdth)
		@height = hgt
		@width = wdth
	end

	def height
		return @height
	end

	def height=(newHgt)
		@height=newHgt
	end

	def width
		return @width
	end

	def width=(newWdth)
		@width=newWdth
	end

	def area ()
		@height*@width
	end
end

The mutator methods above ("height=" and "width=") may look tricky, but they really are just methods. Don't let the equal sign in the name fool you. The additional character at the end of the method name means nothing to Ruby, but it makes it seem more readable to humans when used. What do you think the following code looks like it is doing?

	aRectangle.height=27

That's right, a Rectangle object's height is being assigned/changed. In fact, it is just a call to the method "height=" on the Rectangle object.

Because granting access to an object's instance variable is quite common, Ruby comes with a set of keywords that essentially define the instance variable and the accessor/mutator methods all at once; making these instance variables "public" attributes. The keywords are attr_reader, attr_accessor. They help simplify the code considerably:

class Rectangle
	attr_accessor :height, :width
	
	def initialize (hgt, wdth)
		@height = hgt
		@width = wdth
	end

	def area ()
		@height*@width
	end
end

In the example code above, attr_accessor gives the Rectangle both getters and setters for the height and width attributes.

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