Welcome back! In this part of our Ruby Class Basics Review, we’ll dive into inheritance, a core concept in Ruby’s object-oriented programming (OOP). Inheritance enables code-sharing between classes, making our code more efficient and easier to read.

This lesson covers how inheritance works in Ruby, including attribute and method inheritance, along with the super keyword to access superclass functionality. Ready to dive in? Let’s get started!

Inheritance in Ruby allows us to create a subclass that inherits attributes and methods from a superclass. This approach is especially useful when multiple classes share common features or behaviors.

Let’s explore with a superclass named Vehicle and a subclass named Car:

Ruby
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1# Define the superclass 'Vehicle'
2class Vehicle
3  # Initialize the Vehicle with color and brand attributes
4  def initialize(color, brand)
5    @color = color
6    @brand = brand
7  end
8end
9
10# Define the subclass 'Car', inheriting from 'Vehicle'
11class Car < Vehicle
12  def initialize(color, brand, doors)
13    # Use 'super' to call the superclass's initialize method
14    super(color, brand)
15    @doors = doors
16  end
17end

In this example, Car inherits properties from Vehicle. Ruby supports several types of inheritance, but here we’re focusing on single inheritance, where a subclass has a single superclass.

With attribute inheritance, a subclass can inherit instance variables from its superclass.

Ruby
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1class Artist
2  attr_reader :name  # Getter for @name
3
4  def initialize(name)
5    @name = name  # Superclass's attribute
6  end
7end
8
9class Musician < Artist
10  attr_reader :instrument  # Getter for @instrument
11
12  def initialize(name, instrument)
13    super(name)  # Inheriting superclass's attribute
14    @instrument = instrument  # Subclass's own attribute
15  end
16end
17
18john = Musician.new('John Lennon', 'Guitar')
19puts john.name        # Output: John Lennon
20puts john.instrument  # Output: Guitar

The Musician class inherits the @name attribute from Artist, while also introducing its own attribute, @instrument. Getters can be conveniently defined using attr_reader to access instance variables of each class.

Similar to attributes, method inheritance lets a subclass use methods defined in its superclass without redefining them.

In the example below, the Car class inherits the start method from the Vehicle class:

Ruby
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1class Vehicle
2  def initialize(brand)
3    @brand = brand
4  end
5
6  def start
7    puts "The #{@brand} is starting."
8  end
9end
10
11class Car < Vehicle
12  # No new methods or attributes added here
13end
14
15my_car = Car.new('BMW')
16my_car.start # Output: The BMW is starting.

In this case, Car can use the start method inherited from Vehicle.

The super keyword is essential in Ruby inheritance, allowing a subclass to call methods from its superclass. super is especially useful in method overrides or in initialize methods to ensure that superclass functionality is included in the subclass.

Here’s how super can be used to call an overridden method in the superclass and combine it with new behavior in the subclass:

Ruby
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1class Vehicle
2  def start
3    "Vehicle is starting..."
4  end
5end
6
7class Car < Vehicle
8  def start
9    "#{super} Beep! Beep!"
10  end
11end
12
13my_car = Car.new
14puts my_car.start # Output: Vehicle is starting... Beep! Beep!

Similarly, in initialize, super allows us to ensure that both superclass and subclass attributes are set up properly:

Ruby
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1class ParentClass
2  attr_reader :value  # Getter for @value
3
4  def initialize(value)
5    @value = value
6  end
7end
8
9class ChildClass < ParentClass
10  attr_reader :additional_value  # Getter for @additional_value
11
12  def initialize(value, additional_value)
13    super(value)  # Calls superclass's initialize
14    @additional_value = additional_value
15  end
16end
17
18child = ChildClass.new("value", "additional_value")
19puts child.value            # Output: value
20puts child.additional_value # Output: additional_value

In both examples, super allows the subclass to build upon the superclass’s logic, resulting in flexible and reusable code.

In Ruby, self.class allows instance methods to dynamically call class methods or access shared data. For example, we can track the total number of books checked out in a library system:

Ruby
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1class Library
2  @books_checked_out = 0
3
4  class << self
5    # Defines class-level methods for shared behavior
6    attr_reader :books_checked_out
7
8    def checkout_book
9      @books_checked_out ||= 0  # Initialize @books_checked_out if it's nil
10      @books_checked_out += 1
11    end
12  end
13end
14
15class LibraryMember < Library
16  def checkout
17    self.class.checkout_book
18    "A book has been checked out. Total checked out: #{self.class.books_checked_out}"
19  end
20end
21
22member = LibraryMember.new
23puts member.checkout  # Output: A book has been checked out. Total checked out: 1
24
25another_member = LibraryMember.new
26puts another_member.checkout  # Output: A book has been checked out. Total checked out: 2

In this example:

• @books_checked_out is a class instance variable, storing data shared among all instances of the Library class.
• class << self opens the singleton class, allowing us to define class-level methods like books_checked_out and checkout_book in a grouped and organized manner.
• The checkout method in LibraryMember uses self.class to dynamically call class methods (checkout_book and books_checked_out) on the Library class.

This design ensures shared data is managed centrally while allowing dynamic access and modifications through instance methods.

In this lesson, we explored inheritance in Ruby, covering both attribute and method inheritance. We also discussed how the super keyword allows subclasses to access superclass methods for extending or combining behaviors. Practicing inheritance will help you write more efficient, organized, and modular Ruby code. Get ready to apply these concepts in some hands-on exercises to deepen your understanding!