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".

Practice

1. What is the main difference between class_eval and instance_eval in Ruby?

easy

A. class_eval and instance_eval both change the whole class.

B. instance_eval changes the class and affects all instances; class_eval changes only one object.

C. class_eval changes the class and affects all instances; instance_eval changes only one object.

D. class_eval and instance_eval both change only one object.

Solution

1. Step 1: Understand class_eval behavior

   class_eval opens the class and changes it, so all instances see the change.

2. Step 2: Understand instance_eval behavior

   instance_eval runs code in the context of a single object, changing only that object.

3. Final Answer:

   class_eval changes the class and affects all instances; instance_eval changes only one object. -> Option C

4. Quick Check:

   class_eval = class-wide, instance_eval = single object [OK]

Hint: class_eval affects all objects; instance_eval affects one [OK]

Common Mistakes:

• Confusing which method affects the class vs. single object
• Thinking both methods change the whole class
• Assuming instance_eval changes all instances

2. Which of the following is the correct syntax to add a method greet to class Person using class_eval?

easy

A. Person.class_eval { def greet; 'Hi'; end }

B. Person.instance_eval { def greet; 'Hi'; end }

C. Person.class_eval def greet; 'Hi'; end

D. Person.instance_eval def greet; 'Hi'; end

Solution

1. Step 1: Recall correct block syntax for class_eval

   class_eval takes a block with method definitions inside curly braces or do-end.

2. Step 2: Check method definition placement

   Method greet must be defined inside the block passed to class_eval.

3. Final Answer:

   Person.class_eval { def greet; 'Hi'; end } -> Option A

4. Quick Check:

   class_eval uses block with method defs [OK]

Hint: Use curly braces or do-end block with class_eval [OK]

Common Mistakes:

• Omitting braces or block for class_eval
• Using instance_eval instead of class_eval for class methods
• Writing method defs outside the block

3. What will be the output of this Ruby code?

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

fido = Dog.new('Fido')
fido.instance_eval { def speak; "Woof! I'm #{@name}"; end }
puts fido.speak

rex = Dog.new('Rex')
puts rex.respond_to?(:speak)

medium

A. Woof! I'm Fido
false

B. Woof! I'm Fido
true

C. NoMethodError for fido.speak

D. Woof! I'm Fido
NoMethodError for rex.speak

Solution

1. Step 1: Understand instance_eval adds method to one object

   The speak method is added only to fido, not to other Dog instances.

2. Step 2: Check outputs

   fido.speak prints "Woof! I'm Fido". rex.respond_to?(:speak) returns false because rex has no speak method.

3. Final Answer:

   Woof! I'm Fido false -> Option A

4. Quick Check:

   instance_eval adds method to one object only [OK]

Hint: instance_eval adds method to one object only [OK]

Common Mistakes:

• Assuming all instances get the new method
• Expecting true for rex.respond_to?(:speak)
• Confusing instance_eval with class_eval

4. Identify the error in this code snippet:

class Cat
end

kitty = Cat.new
kitty.class_eval { def meow; 'Meow!'; end }
puts kitty.meow

medium

A. No error; outputs 'Meow!'

B. NoMethodError because class_eval is called on instance

C. SyntaxError due to missing block

D. RuntimeError because method already exists

Solution

1. Step 1: Check receiver of class_eval

   class_eval is a method of Class, not of instances; calling it on an instance raises NoMethodError.

2. Step 2: Verify execution

   Since kitty is an instance, kitty.class_eval raises NoMethodError; thus, puts kitty.meow is not reached.

3. Final Answer:

   NoMethodError because class_eval is called on instance -> Option B

4. Quick Check:

   class_eval is a Class method, not instance method [OK]

Hint: class_eval is called on classes, not instances [OK]

Common Mistakes:

• Thinking class_eval can be called on instances
• Expecting NoMethodError for instance.class_eval
• Confusing class_eval usage with class-only restriction

5. Given this code, what will be the output?

class Car
  def initialize(model)
    @model = model
  end
end

car1 = Car.new('Toyota')
car2 = Car.new('Honda')

Car.class_eval do
  def info
    "Model: #{@model}"
  end
end

car1.instance_eval do
  def info
    "Special Model: #{@model.upcase}"
  end
end

puts car1.info
puts car2.info

hard

A. Special Model: TOYOTA
Special Model: HONDA

B. Model: Toyota
Model: Honda

C. Model: Toyota
Special Model: HONDA

D. Special Model: TOYOTA
Model: Honda

Solution

1. Step 1: Understand class_eval adds method to all instances

   info method added by class_eval returns "Model: #{@model}" for all cars.

2. Step 2: Understand instance_eval overrides method for one object

   car1.instance_eval redefines info only for car1, returning uppercase model with "Special Model:" prefix.

3. Step 3: Check outputs

   car1.info prints "Special Model: TOYOTA"; car2.info prints "Model: Honda".

4. Final Answer:

   Special Model: TOYOTA Model: Honda -> Option D

5. Quick Check:

   class_eval affects all; instance_eval overrides one [OK]

Hint: instance_eval overrides method only on one object [OK]

Common Mistakes:

• Assuming instance_eval changes all instances
• Expecting both outputs to be 'Special Model'
• Confusing which method is called on which object