Using Ruby Structs To Subclass

December 10, 2012

A struct is Ruby’s convenient way of letting you bundle a bunch of attributes together (complete with accessor methods) without having to define a specific class.

For example I could have:

Employee = Struct.new(:name,:id,:company)
puts "Employee is a class?: "+Employee.is_a?(Class).to_s # =>true
josh = Employee.new("josh",1,"Google")
puts "Josh is an Employee? "+josh.is_a?(Employee).to_s # =>true

With the above, I have defined an Employee class of which josh is an instance.

Subclassing with Struct

In computer science lingo, a subclass is a class that inherits data and properties from another class. Since a Struct is a class in Ruby, we can subclass on it.

But why would we want to do this?

One interesting use case I found is to use it to shorten up class definitions. For instance, here is how you would normally define a class:

Example #1 OriginalCircle Class (normal class definition)

class OriginalCircle
attr_accessor :radius
  def initialize(r)
    @radius = r
  end

  def diameter
    2*@radius
  end

  def circle_area
    Math::PI*@radius*@radius
  end
end

And here is how you would do it using Ruby’s Struct:

Example #2 Circle Class (subclassing with struct)

class Circle < Struct.new(:radius)
  def diameter
    2*radius
  end

  def circle_area
    Math::PI*radius\*radius
  end
end

Notice that by subclassing with a struct (example 2), the “initialize” method is given to you “automagically.”

You also get the accessor method free of charge

So you can do:

c=Circle.new(1)
c.radius = 2

This changes the radius from 1 to 2. Notice I didn’t have to declare “attr_accessor” like I did in example 1.

I think this is pretty nifty. What other nifty or interesting things have you found with Struct? Feel free to share in the comments below.