Class_eval and instance_eval in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a method to the class using class_eval.

Ruby

class MyClass
  class_eval do
    def greet
      puts [1]
    end
  end
end

obj = MyClass.new
obj.greet

Drag options to blanks, or click blank then click option'

AHello from class_eval!

B"Hello from instance_eval!"

C"Hello from class_eval!"

Dputs "Hello"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string.

Using instance_eval instead of class_eval here.

The string must be quoted to be printed correctly. Using class_eval adds the method to the class.

2fill in blank

medium

Complete the code to define an instance variable using instance_eval.

Ruby

class MyClass
  def initialize
    @name = "Ruby"
  end
end

obj = MyClass.new
obj.instance_eval do
  @name = [1]
end

puts obj.instance_eval { @name }

Drag options to blanks, or click blank then click option'

AChanged by instance_eval

Bname

C:Changed

D"Changed by instance_eval"

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning a string without quotes.

Trying to access @name without instance_eval.

The instance variable @name is set to a string, so it must be quoted.

3fill in blank

hard

Fix the error in the code by completing the blank to call instance_eval correctly.

Ruby

class MyClass
  def initialize
    @value = 10
  end
end

obj = MyClass.new
result = obj.[1] { @value * 2 }
puts result

Drag options to blanks, or click blank then click option'

Ainstance_eval

Bself_eval

Ceval

Dclass_eval

Attempts:

3 left

💡 Hint

Common Mistakes

Using class_eval instead of instance_eval.

Using eval which is a Kernel method, not related to the object.

instance_eval runs the block in the context of the object, allowing access to @value.

4fill in blank

hard

Fill both blanks to add a class method using class_eval and call it.

Ruby

class MyClass
  [1].class_eval do
    def self.say_hello
      "Hello from class method"
    end
  end
end

puts MyClass.[2]

Drag options to blanks, or click blank then click option'

Aself

Binstance

Csay_hello

Dgreet

Attempts:

3 left

💡 Hint

Common Mistakes

Calling class_eval on an instance.

Calling a method that does not exist.

class_eval is called on the class object, and the method say_hello is called on MyClass.

5fill in blank

hard

Fill all three blanks to define an instance variable with instance_eval, define a method with class_eval, and call both.

Ruby

class MyClass
  [1].class_eval do
    def greet
      "Hello, #{@name}!"
    end
  end
end

obj = MyClass.new
obj.[2] do
  @name = [3]
end

puts obj.greet

Drag options to blanks, or click blank then click option'

Aself

Binstance_eval

C"Ruby Learner"

Dclass_eval

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up class_eval and instance_eval calls.

Not quoting the string for @name.

class_eval is called on the class to define the method. instance_eval is called on the object to set @name. The name must be a string.

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