Class variables (@@) and their dangers in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding class variable sharing

What is the output of this Ruby code involving class variables?

Ruby

class Parent
  @@count = 0
  def self.increment
    @@count += 1
  end
  def self.count
    @@count
  end
end

class Child < Parent
  def self.increment
    @@count += 2
  end
end

Parent.increment
Child.increment
puts Parent.count

A 0

B 1

C 2

D 3

💡 Hint

Remember that class variables (@@) are shared across the class hierarchy.

The class variable @@count is shared between Parent and Child classes. Parent.increment adds 1, then Child.increment adds 2, so total is 3.

❓ Predict Output

intermediate

Effect of modifying class variable in subclass

What will this Ruby code print?

Ruby

class A
  @@var = 'A'
  def self.var
    @@var
  end
end

class B < A
  @@var = 'B'
end

puts A.var

A B

B A

C nil

D Error

💡 Hint

Class variables are shared, so changing in subclass affects superclass.

Assigning @@var = 'B' in subclass B changes the shared class variable for both classes. So A.var returns 'B'.

🔧 Debug

advanced

Why does this class variable cause unexpected behavior?

Consider this Ruby code. Why does the output show unexpected shared state?

Ruby

class Parent
  @@data = []
  def self.add(item)
    @@data << item
  end
  def self.data
    @@data
  end
end

class Child < Parent
end

Parent.add('parent1')
Child.add('child1')
puts Parent.data.inspect
puts Child.data.inspect

A The code raises a NameError because @@data is undefined in Child

B Both Parent and Child share the same @@data array, so both show ['parent1', 'child1']

C Parent and Child have separate @@data arrays, so Parent shows ['parent1'] and Child shows ['child1']

D The code raises a TypeError because @@data is modified in Child

💡 Hint

Class variables are shared across the inheritance chain, including mutable objects.

Class variable @@data is shared between Parent and Child. Adding items from either class modifies the same array, so both show all added items.

📝 Syntax

advanced

Identifying class variable misuse causing bugs

Which option shows a misuse of class variables that can cause bugs in inheritance?

A Using @@var to store mutable data shared by subclasses

B Using instance variables @var in class methods

C Using constants for fixed values in classes

D Using local variables inside methods

💡 Hint

Mutable shared state in class variables can cause unexpected side effects.

Using @@var to hold mutable data shared by subclasses can cause bugs because changes in one subclass affect others unexpectedly.

🧠 Conceptual

expert

Why are class variables (@@) considered dangerous in Ruby?

Select the best explanation for why class variables (@@) can be dangerous in Ruby inheritance.

A Because @@ variables are private and cannot be accessed by subclasses

B Because @@ variables are only accessible inside instance methods

C Because @@ variables are shared across the entire inheritance chain, causing unexpected side effects when subclasses modify them

D Because @@ variables are immutable and cannot be changed once set

💡 Hint

Think about how shared state affects subclasses.

Class variables (@@) are shared by a class and all its subclasses, so changes in one place affect all others, leading to hard-to-track bugs.