Classes Are Like Modules

You can think about a module as a specialized hash that can store Ruby code so you can access it with the . operator. Ruby also has another construct that serves a similar purpose called a class. A class is a way to take a grouping of functions and data and place them inside a container so you can access them with the . (dot) operator.

If I were to create a class just like the mystuff module, I'd do something like this:

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class MyStuff

    def initialize()
        @tangerine = "And now a thousand years between"
    end

    attr_reader :tangerine

    def apple()
        puts "I AM CLASSY APPLES!"
    end

end

That looks complicated compared to modules, and there is definitely a lot going on by comparison, but you should be able to make out how this is like a "mini-module" with MyStuff having an apple() function in it. What is probably confusing is the initialize() function and use of @tangerine for setting the tangerine instance variable.

Here's why classes are used instead of modules: You can take this MyStuff class and use it to craft many of them, millions at a time if you want, and each one won't interfere with each other. When you import a module there is only one for the entire program unless you do some monster hacks.

Before you can understand this though, you need to know what an "object" is and how to work with MyStuff just like you do with the mystuff.rb module.

Objects Are Like Require

If a class is like a "mini-module," then there has to be a concept similar to require but for classes. That concept is called "instantiate", which is just a fancy, obnoxious, overly smart way to say "create." When you instantiate a class what you get is called an object.

You instantiate (create) a class by calling the class's new function, like this:

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thing = MyStuff.new()
thing.apple()
puts thing.tangerine

The first line is the "instantiate" operation, and it's a lot like calling a function. However, Ruby coordinates a sequence of events for you behind the scenes. I'll go through these steps using the preceding code for MyStuff:

1. Ruby looks for MyStuff and sees that it is a class you've defined.
2. Ruby crafts an empty object with all the functions you've specified in the class using def because you did MyStuff.new().
3. Ruby then looks to see if you made a "magic" initialize function, and if you have it calls that function to initialize your newly created empty object.
4. In the MyStuff function initialize I then use @tangerine which is telling Ruby, "I want the tangerine variable that is part of this object." Ruby uses operators like @ and $ to say where a variable is located. When you did $stdin you were saying, "the global stdin," because $ means global. When you do @tangerine you are saying, "the object's tangerine", because @ means "this object."
5. In this case, I set @tangerine to a song lyric and then I've initialized this object.
6. Now Ruby can take this newly minted object and assign it to the thing variable for me to work with.

That's the basics of how Ruby does this "mini-import" when you call a class like a function. Remember that this is not giving you the class but instead is using the class as a blueprint for building a copy of that type of thing.

Keep in mind that I'm giving you a slightly inaccurate idea of how these work so that you can start to build up an understanding of classes based on what you know about modules. The truth is, classes and objects suddenly diverge from modules at this point. If I were being totally honest, I'd say something more like this:

• Classes are like blueprints or definitions for creating new mini-modules.
• Instantiation is how you make one of these mini-modules and require it at the same time. "Instantiate" just means to create an object from the class.
• The resulting created mini-module is called an object, and you then assign it to a variable to work with it.

At this point objects behave differently from modules, and this should only serve as a way for you to bridge over to understanding classes and objects.

Getting Things from Things

I now have three ways to get things from things:

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# dict style
mystuff['apples']

# module style
MyStuff.apples()
puts MyStuff::TANGERINE

# class style
thing = MyStuff.new()
thing.apples()
puts thing.tangerine

A First Class Example

You should start seeing the similarities in these three key=value container types and probably have a bunch of questions. Hang on with the questions, as the next exercise will hammer home your "object-oriented vocabulary." In this exercise, I just want you to type in this code and get it working so that you have some experience before moving on.

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class Song

  def initialize(lyrics)
    @lyrics = lyrics
  end

  def sing_me_a_song()
    @lyrics.each {|line| puts line }
  end
end

happy_bday = Song.new(["Happy birthday to you",
           "I don't want to get sued",
           "So I'll stop right there"])

bulls_on_parade = Song.new(["They rally around tha family",
            "With pockets full of shells"])

happy_bday.sing_me_a_song()

bulls_on_parade.sing_me_a_song()