Class_eval and instance_eval in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does class_eval do in Ruby?

<p><code>class_eval</code> runs code inside the context of a class. It lets you add methods or change the class from outside.</p>

intermediate

What is the difference between class_eval and instance_eval?

class_eval runs code as if inside the class (affects all instances). instance_eval runs code in the context of a single object (affects only that object).

beginner

How does instance_eval affect an object?

instance_eval changes or adds methods and variables only for that one object, not the whole class.

beginner

Give a simple example of using class_eval to add a method.

<pre>class MyClass; end

MyClass.class_eval do
  def greet
    "Hello!"
  end
end

obj = MyClass.new
puts obj.greet  # Outputs: Hello!</pre>

intermediate

Why use instance_eval instead of normal method definition?

Use instance_eval to add or change methods on a single object without affecting other objects of the same class.

What does class_eval change in Ruby?

A Only one object instance

B The superclass

C Global variables

D The class itself and all its instances

class_eval runs code inside the class context, affecting the class and all its instances.

Which method runs code in the context of a single object?

A instance_eval

B class_eval

C module_eval

D eval

instance_eval runs code on a single object, changing only that object.

If you want to add a method to all instances of a class, which should you use?

A instance_eval

B class_eval

C object_eval

D method_eval

class_eval adds methods to the class, so all instances get them.

What is a key use of instance_eval?

A Add methods to a single object

B Change global constants

C Create a new class

D Change the superclass

instance_eval lets you add or change methods on just one object.

Which of these is true about class_eval?

A It changes the value of local variables outside the class

B It only works on objects, not classes

C It can add instance methods to a class

D It runs code in the global scope

class_eval is used to add or change instance methods inside a class.

Explain in your own words how class_eval and instance_eval differ in Ruby.

Think about whether the change affects many objects or just one.

Describe a situation where you would use instance_eval instead of normal method definition.

Imagine you want one special object to behave differently.