Ruby Classes and Objects


title: Classes and Objects

Classes and Objects

Objects in Ruby

Let us quickly go over Ruby objects. In the real world, objects may be anything including a car, computer, or even a human. Each one of these objects have different states and behaviors.

For example, a car may have its state described by its make, model, and color and some of its behaviors as turning, honking, or braking.

An object in Ruby has very similar characteristics. Ruby Objects also have states and behaviors. In Ruby Objects, the state is stored in instance variables and the behaviors are stored in methods (functions within classes).

Classes in Ruby

A class is basically an object’s template. This template defines the available properties that make up the object using instance variables. Again, there are also behaviors defined in the form of methods to help change the object’s state.

A new instance of a class is created using the initialize method of a class. An instance will have properties required to initialize the instance as dictated by the class’ initialize method.

Take for example the following sample code of a class:

class Car def initialize(make, model, color) @make = make @model = model @color = color end def turn(direction) end def honk puts "beep beep" end def brake puts "brakes squealing noise" end end

As you saw, classes are defined using the class keyword and the class code block ends with an end keywork. The initialize method is the constructor which is a special function to describe initial state. When we create this object, we define the attributes @make, @model, and @color with values we pass into the constructor.

Creating an Instance of a Class

Now, to create an instance of this class you only need to call the new function which uses the class’s initalize method.

mazda3 = Car.new('Mazda', 'Mazda3', 'White')

Yay, we just created a mazda3 which is an instance of the Car object. This is great, but sometimes you may need to change some of these attributes! Most of these attributes in this example would be static. Still, imagine that you decided to get a new paintjob. How would you go about updating the state of this mazda3?

Modifying Instance State

Thankfully, it is rather simple to update the state of an object. First, we would need a setter method! Ruby defines getter and setter settings as the attr_reader and attr_accessor respectively. For both getter and setter settings on a given attribute, you can also just use attr_accessor.

To demonstrate this, I have modified the previous Car object with these newly defined settings.

class Car attr_accessor :color attr_reader :make, :model def initialize(make, model, color) @make = make @model = model @color = color end def turn(direction) end def honk puts "beep beep" end def brake puts "brakes squealing noise" end end

So now we can change state and read the state of the object.

irb(main):023:0> c = Car.new('Mazda', 'Mazda3', 'White') => #<Car:0x00007fd3ca13fdd0 @make="Mazda", @model="Mazda3", @color="White", @speed=nil> irb(main):024:0> c.color => "White" irb(main):025:0> c.make => "Mazda" irb(main):026:0> c.model => "Mazda3" irb(main):027:0> c.color = 'Brutal Blue' => "Brutal Blue" irb(main):028:0> c.make = 'Prius' Traceback (most recent call last): 2: from /usr/local/bin/irb:11:in `<main>' 1: from (irb):28 NoMethodError (undefined method `make=' for #<Car:0x00007fd3ca13fdd0>) Did you mean? make

Viewing the previous output from irb, you can see that each one of the instance variables is readable. We can write to @color, but we end up causing a NoMethodError exception when we attempt to write to @make. This is because @make was only defined using an attr_reader, so make= is not defined. This could be fixed by adding the make to the attr_accessor like we did for color.

Treating Objects like Hashes or Arrays

Lets say that a class is created, which contains an array that we would like to access directly. For instance, say our class Car had an array of nicknames that we wanted to know.

class Car attr_accessor :color attr_reader :make, :model, :nicknames def initialize(make, model, color, nicknames) @make = make @model = model @color = color @nicknames = nicknames end def turn(direction) end def honk puts "beep beep" end def brake end end


We can assign the car an array of nicknames and access them by calling the attr_reader for nicknames

c = Car.new("Volvo","V70", "Black", ["The Wagon","Long Fella","Slick"]) => #<Car:0x00005615321badf8 @make="Volvo", @model="V70", @color="Black", @nicknames=["The Wagon", "Long Fella", "Slick"]> irb(main):023:0> c.nicknames => ["The Wagon", "Long Fella", "Slick"] irb(main):024:0> c.nicknames[1] => "Long Fella"

Sometimes, we may not want to have to reference a specific variable, or the return type is implied by the object. In this case, it would be handy to call the car object and directly reference nicknames. With ruby we can overload methods such as [] like so

class Car attr_accessor :color attr_reader :make, :model, :nicknames def initialize(make, model, color, nicknames) @make = make @model = model @color = color @nicknames = nicknames end def [](key) @nicknames[key] end def []=(key,value) @nicknames[key] = value end def turn(direction) end def honk puts "beep beep" end def brake end end

Here, we have added two methods, [] and []=. These methods are special because they allow us to index the Car object directly, in this case to resolve a nickname. Here’s an example of how we can use these methods

irb(main):030:0> c = Car.new("Volvo","V70", "Black", ["The Wagon","Long Fella","Slick"]) => #<Car:0x00005648c1b35820 @make="Volvo", @model="V70", @color="Black", @nicknames=["The Wagon", "Long Fella", "Slick"]> irb(main):031:0> c[1] => "Long Fella" irb(main):032:0> c[0] => "The Wagon" irb(main):033:0> c[3] = "Speedy guy" => "Speedy guy" irb(main):034:0> c.nicknames => ["The Wagon", "Long Fella", "Slick", "Speedy guy"]

As can be seen, the Car object c can now be treated like an array to directly retrieve and set nicknames

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