Common Interview Question #2: Classes vs. Modules
Full Question
What is the difference, if any, between classes and modules in Ruby? When are they used?
Ive been asked this question 3 times now; each time I felt like I instinctively knew the answer, but my explanations were clumsy.
The Basics
Modules contain related methods that you can use with with or without classes (depending on the type of method). They cannot be instantiated. They can contain constants, but not any attributes relating to ‘state’.
Classes are about objects. They can contain methods and attributes and most importantly, they can be instantiated.
Example
The example below is a module. The functions could have been written individually, but now they are organized and namespaced. The Trig module has a constant PI, but it wouldn’t make sense for it to have any attributes that could change because trigonometry doesn’t change.
module Trig PI = 3.141592654 def self.cos(x) #... end def self.sin(x) #... end end
The example below is a class. It has an initialize method and an instance variable. Now I can create all the angles!!
class Angle attr_accessor :degrees def initialize(degrees) @degrees = degrees end end
So now I can create an angle and find it’s cosine.
angle = Angle.new(0) Trig.cos(angle.degrees) #=> 1
Below I rewrote the module Trig, so that it can be mixed in to the angle class, creating a Mixin.
module Trig PI = 3.141592654 def cos # I'm kind of cheating here by calling the Math module Math::cos(@degrees) end end class Angle include Trig def initialize(degrees) @degrees = degrees end end angle = Angle.new(0) angle.cos #=> 1 The Nitty Gritty
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