Class_eval and instance_eval in Ruby - Mini Project: Build & Apply

Using class_eval and instance_eval in Ruby

📖 Scenario: Imagine you are working on a Ruby program that manages a simple Car class. You want to dynamically add methods and change behavior at runtime using class_eval and instance_eval.

🎯 Goal: You will create a Car class, then use class_eval to add a method to all cars, and instance_eval to add a method to a single car instance. Finally, you will call these methods to see the results.

📋 What You'll Learn

Create a Car class with an initialize method that sets a @make instance variable

Use class_eval to add a method car_info that returns a string with the car make

Use instance_eval on a car instance to add a method special_feature that returns a custom string

Call and print the results of car_info and special_feature methods

💡 Why This Matters

🌍 Real World

Dynamically changing or adding behavior to classes and objects at runtime is useful in metaprogramming, plugins, or frameworks where flexibility is needed.

💼 Career

Understanding <code>class_eval</code> and <code>instance_eval</code> helps Ruby developers write flexible, dynamic code often required in advanced Ruby applications and libraries.

1

Create the Car class with initialize method

Create a class called Car with an initialize method that takes one parameter make and sets it to an instance variable @make.

Ruby

# Define the Car class with initialize method
# Your code here

Hint

Use class Car to start the class. Inside, define def initialize(make) and set @make = make.

2

Add car_info method using class_eval

Use Car.class_eval to add a method called car_info that returns the string "This car is a #{@make}".

Ruby

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

# Use class_eval to add car_info method
# Your code here

Hint

Use Car.class_eval do and define def car_info inside the block. Return the string with #{@make}.

3

Add special_feature method to one car instance using instance_eval

Create a new Car instance called my_car with make "Toyota". Then use my_car.instance_eval to add a method called special_feature that returns the string "This car has a sunroof".

Ruby

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

Car.class_eval do
  def car_info
    "This car is a #{@make}"
  end
end

# Create my_car instance and add special_feature method using instance_eval
# Your code here

Hint

Create my_car with Car.new("Toyota"). Then use my_car.instance_eval do to define special_feature method.

4

Call and print car_info and special_feature methods

Print the result of calling car_info on my_car. Then print the result of calling special_feature on my_car.

Ruby

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

Car.class_eval do
  def car_info
    "This car is a #{@make}"
  end
end

my_car = Car.new("Toyota")
my_car.instance_eval do
  def special_feature
    "This car has a sunroof"
  end
end

# Print car_info and special_feature results
# Your code here

Hint

Use puts my_car.car_info and puts my_car.special_feature to print the messages.

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