Class instance variables as alternative in Ruby - Mini Project: Build & Apply

Class instance variables as alternative

📖 Scenario: You are creating a simple program to track the number of books created in a library system. Instead of using class variables, you will use class instance variables to keep count.

🎯 Goal: Build a Ruby class Book that uses a class instance variable to count how many book objects have been created.

📋 What You'll Learn

Create a class called Book

Use a class instance variable @count to store the number of books

Create an initialize method that increases @count by 1 each time a new book is created

Add a class method count to return the current number of books

Create 3 book instances and print the total count using Book.count

💡 Why This Matters

🌍 Real World

Class instance variables help keep data specific to a class without sharing it with subclasses, useful in real-world applications like tracking counts or settings per class.

💼 Career

Understanding class instance variables is important for Ruby developers to write clean, maintainable code and avoid bugs related to shared class variables.

1

Create the Book class with a class instance variable

Create a class called Book and inside it, create a class instance variable @count and set it to 0.

Ruby

class Book
  # Your code here
end

Need a hint?

Class instance variables start with @ but are defined directly inside the class, not inside methods.

2

Add an initialize method to increase @count

Inside the Book class, add an initialize method that increases the class instance variable @count by 1 each time a new book is created. Use self.class.instance_variable_set and self.class.instance_variable_get to update @count.

Ruby

class Book
  @count = 0

  # Your code here
end

Need a hint?

Use self.class.instance_variable_get(:@count) to read and self.class.instance_variable_set(:@count, new_value) to update the class instance variable.

3

Add a class method count to return the number of books

Add a class method called count inside the Book class that returns the value of the class instance variable @count. Use instance_variable_get to access @count.

Ruby

class Book
  @count = 0

  def initialize
    current_count = self.class.instance_variable_get(:@count)
    self.class.instance_variable_set(:@count, current_count + 1)
  end

  # Your code here
end

Need a hint?

Class methods start with self. and can access class instance variables directly.

4

Create 3 books and print the total count

Create 3 instances of the Book class named book1, book2, and book3. Then print the total number of books by calling Book.count.

Ruby

class Book
  @count = 0

  def initialize
    current_count = self.class.instance_variable_get(:@count)
    self.class.instance_variable_set(:@count, current_count + 1)
  end

  def self.count
    @count
  end
end

# Your code here

Need a hint?

Creating each new Book.new calls initialize which increases the count. Use puts Book.count to print the total.