Class variables (@@) and their dangers in Ruby - Mini Project: Build & Apply

Class variables (@@) and their dangers

📖 Scenario: Imagine you are creating a simple program to track how many animals have been created in a zoo. You want to use a class variable to count all animals, but you will see why this can cause problems when you have different types of animals.

🎯 Goal: You will build a Ruby program that uses a class variable @@count to count animals created in different subclasses. You will observe how the class variable is shared and why this can be dangerous.

📋 What You'll Learn

Create a base class Animal with a class variable @@count initialized to 0

Create two subclasses Dog and Cat that inherit from Animal

Increment @@count each time a new animal is created

Print the total count of animals after creating some dogs and cats

💡 Why This Matters

🌍 Real World

Class variables are sometimes used to keep track of shared data like counts or settings across related classes in programs.

💼 Career

Understanding class variables and their pitfalls helps you write safer, clearer Ruby code, which is important for software development jobs using Ruby.

1

Create the base class with class variable

Create a class called Animal with a class variable @@count set to 0. Add an initialize method that increases @@count by 1 each time an animal is created.

Ruby

class Animal
  # Your code here
end

Need a hint?

Use @@count to store the count and increase it inside initialize.

2

Create subclasses Dog and Cat

Create two subclasses called Dog and Cat that inherit from Animal. Add empty initialize methods that call super to use the base class initialization.

Ruby

class Animal
  @@count = 0

  def initialize
    @@count += 1
  end
end

class Dog < Animal
  # Your code here
end

class Cat < Animal
  # Your code here
end

Need a hint?

Use class Dog < Animal and class Cat < Animal. Inside initialize, call super.

3

Create some animals

Create two Dog objects and three Cat objects by calling their constructors.

Ruby

class Animal
  @@count = 0

  def initialize
    @@count += 1
  end
end

class Dog < Animal
  def initialize
    super
  end
end

class Cat < Animal
  def initialize
    super
  end
end

# Your code here

Need a hint?

Create objects by calling Dog.new and Cat.new.

4

Print the total animal count

Print the total number of animals created using @@count from the Animal class.

Ruby

class Animal
  @@count = 0

  def initialize
    @@count += 1
  end
end

class Dog < Animal
  def initialize
    super
  end
end

class Cat < Animal
  def initialize
    super
  end
end

dog1 = Dog.new
 dog2 = Dog.new
cat1 = Cat.new
cat2 = Cat.new
cat3 = Cat.new

# Your code here

Need a hint?

Define a class method self.count to return @@count. Then print it.