GSwR VI: Stay Classy with Ruby

Welcome to the last post in the Getting Started with Ruby series.

In the previous post, we covered writing our own methods. In this post, we’ll learn all about how classes work: How to create them, how to create methods for them, and how to use inheritence to save recreating lots of behaviour.

Classes are quite literally the building blocks of Ruby – everything in Ruby is an object and classes are basically a list of method definitions that acts like a blueprint for objects. Classes are used to create objects, known as instances of the class. These objects have all of the methods defined in the class.

We’re going to start by looking at how to add our own methods to the built in classes such as Strings, Integers and Arrays.

Changing Built In Classes

We’ve already seen many of the built-in methods in classes like String and Integer, such as reverse and odd?. Ruby let’s us add extra methods to these classes by opening up the class definition. This is done by typing the keyword class followed by the name of the class.

Here’s an example of opening up the Integer class to add a new method called double:

class Integer
  def double
    self * 2
  end
end

Notice that the word ‘Integer’ starts with a capital letter, which is true for all class names.

This new method returns the value of the integer that calls the method multiplied by 2. The self keyword refers to the object itself, in this case the integer that calls the method. Now all integers will have this method.

We can test this code out in IRB. First of all, save the code above in a file called ‘extensions.rb’ and then load it into IRB so that the extra methods become available to us. To do this, navigate to the folder where the ‘extensions.rb’ file is saved and then launch IRB by typing irb into a terminal prompt. Once IRB starts, we just need to type the following:

load ‘./extensions.rb’
=> true

Now we can have a go at testing out our new method:

2.double
=> 4
5.double
=> 10
12345.double
=> 24690

Great, it works!

You can add new methods for all of the built in classes such as Strings and Arrays. In fact you can even redefine the methods that already exist. Here’s an example that changes the behaviour of the reverse method on the String class. Add the following code to the bottom of the ‘extensions.rb’ file:

class String
  def reverse
    "no reversing"
  end
end

As you can see, by opening up the String class we redefine the reverse method to return a string that says ‘no reversing’. Have a go at running this in IRB – you will need to save ‘extensions.rb’ and then load it again for the changes to take effect.

Changing the way built-in methods behave is very much frowned upon since people expect methods to work in a certain way. Adding new methods that add more functionality is (usually) thought of as good thing though. This is as monkey patching. Ruby on Rails has a module called Active Support which adds lots of extra methods to the String class. One of the new methods is called pluralize, which returns the plural version of a string.

Here’s an example of a similar pluralize method. Add it to the bottom of the ‘extensions.rb’ file:

class String

  def pluralize
    case self
      when "woman" then "women"
      when "person" then "people"
      when "octopus" then "octopi"
      when "sheep" then "sheep"
      else self + "s"
    end
  end

end

This uses a case statement to return the plural of some irregular words and then just puts an ‘s’ on the end for all the rest. Let’s have a look at this in action in IRB (don’t forget that you’ll need to load extensions.rb again):

octupus.pluralize
=> octupi

book.pluralize
=> books

bus.pluralize
=> buss

It’s by no means complete, as shown in the last example. It’s readily apparent, though, how this could be used to add some very useful functionality to all strings.

Creating Your Own Classes

We don’t just have to make do with playing around with the built in classes though – we can create our own!

In the last few posts, we’ve imitated a die being rolled. Let’s create a Die class that can be used to create lots of die objects. We can then “roll” these dice produce random numbers.

Create a file caled dice.rb and add the following code:

class Die
  def roll
    rand(1..6)
  end
end

Any class starts with the class keyword and is followed by the name of the class (always capitialized). It ends with the end keyword. We then add any methods inside this class definition.

To create a new Die object, we use the new method of the Die class (don’t forget to load the ‘die.rb’ file):

die = Die.new
 => #<Die:0x8e3459c>

This instantiates a new Die object represented by #<0x8e3459c>–

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