Class_eval and instance_eval in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class_eval modifying a class

What is the output of this Ruby code using class_eval?

Ruby

class Dog
  def speak
    "woof"
  end
end

Dog.class_eval do
  def speak
    "bark"
  end
end

puts Dog.new.speak

A NoMethodError

B woof

C nil

D bark

💡 Hint

class_eval changes methods inside the class context.

The class_eval block redefines the speak method for the Dog class. So calling Dog.new.speak returns "bark" instead of the original "woof".

❓ Predict Output

intermediate

Output of instance_eval on an object

What does this Ruby code print when using instance_eval on an object?

Ruby

class Cat
  def speak
    "meow"
  end
end

cat = Cat.new
cat.instance_eval do
  def speak
    "purr"
  end
end

puts cat.speak

A NoMethodError

B purr

C meow

D nil

💡 Hint

instance_eval changes the singleton class of the object.

The instance_eval block defines a singleton method speak on the cat object, overriding the original speak method from the Cat class for this instance. So cat.speak calls the new method and prints "purr".

🔧 Debug

advanced

Why does instance_eval not override method for all instances?

Given this code, why does instance_eval not change the method for all instances of the class?

Ruby

class Bird
  def fly
    "I can fly"
  end
end

bird1 = Bird.new
bird2 = Bird.new

bird1.instance_eval do
  def fly
    "I cannot fly"
  end
end

puts bird1.fly
puts bird2.fly

A instance_eval defines method only on bird1's singleton class, not Bird class

B instance_eval changes the Bird class method for all instances

C instance_eval raises an error when redefining methods

D instance_eval modifies the superclass methods

💡 Hint

Think about what singleton class means for an object.

instance_eval defines methods on the singleton class of the specific object (bird1), so only bird1 has the new fly method. Other instances like bird2 use the original class method.

❓ Predict Output

advanced

Output of class_eval adding a class method

What is the output of this Ruby code that uses class_eval to add a class method?

Ruby

class Fish
end

Fish.class_eval do
  def self.swim
    "swimming"
  end
end

puts Fish.swim

A swimming

B NoMethodError

C nil

D SyntaxError

💡 Hint

class_eval can add class methods using self.method_name syntax.

The class_eval block adds a class method swim to the Fish class. Calling Fish.swim returns "swimming".

❓ Predict Output

expert

Output of instance_eval with block accessing instance variables

What is the output of this Ruby code using instance_eval to access instance variables?

Ruby

class Person
  def initialize(name)
    @name = name
  end
end

person = Person.new("Alice")

result = person.instance_eval do
  @name + " Smith"
end

puts result

A NoMethodError

B Alice Smith

C nil

D NameError

💡 Hint

instance_eval runs the block in the context of the object, so it can access instance variables.

The instance_eval block runs inside the person object context, so it can access @name directly. It returns "Alice Smith".