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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 (instantiated) by using the initialize method of a class.
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.