Extend for class methods in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the extend keyword do in Ruby?

It adds the methods from a module as class methods to the class or object it is called on.

beginner

How is extend different from include in Ruby?

<code>include</code> adds module methods as instance methods, while <code>extend</code> adds them as class methods.

beginner

Given <pre>module Greetings
  def hello
    "Hello!"
  end
end

class Person
  extend Greetings
end</pre> How do you call <code>hello</code>?

You call it on the class: Person.hello returns "Hello!".

intermediate

Can extend be used on individual objects in Ruby?

Yes, it adds the module's methods as singleton methods to that object only.

intermediate

Why use <code>extend</code> instead of defining class methods directly?

It promotes code reuse by sharing methods across classes without inheritance.

What does extend add to a Ruby class?

A Nothing, it just runs the module code

B Instance methods from a module

C Constants from a module

D Class methods from a module

extend adds module methods as class methods.

Which keyword adds module methods as instance methods in Ruby?

A require

B extend

C include

D import

include adds module methods as instance methods.

If a module has method foo, how do you call it after extend ModuleName in a class?

A ClassName.foo

B ClassName.new.foo

C ModuleName.foo

D You cannot call it

Methods added by extend become class methods.

Can extend be used on a single object instance?

A Yes, but adds methods to all instances

B Yes, adds methods only to that object

C No, only on classes

D No, it raises an error

extend can add methods to a single object as singleton methods.

What is a main benefit of using extend for class methods?

A Avoids code duplication by sharing methods

B Makes code run faster

C Changes instance variables

D Automatically creates instances

extend helps reuse code by sharing methods across classes.

Explain how extend works to add class methods in Ruby.

Think about where the methods become available after using extend.

Describe a scenario where using extend is better than inheritance.

Consider when you want to share behavior but classes don’t share a parent.

Practice

1. What does the extend keyword do in Ruby when used inside a class?

easy

A. Includes the module's constants only

B. Adds the module's methods as instance methods to the class

C. Adds the module's methods as class methods to the class

D. Removes methods from the class

Solution

1. Step 1: Understand extend usage

   The extend keyword adds methods from a module as class methods to the class that calls it.

2. Step 2: Differentiate from include

   Unlike include, which adds methods as instance methods, extend adds them as class methods.

3. Final Answer:

   Adds the module's methods as class methods to the class -> Option C

4. Quick Check:

   extend = class methods [OK]

Hint: Remember: extend = class methods, include = instance methods [OK]

Common Mistakes:

• Confusing extend with include
• Thinking extend adds instance methods
• Assuming extend removes methods

2. Which of the following is the correct syntax to add module methods as class methods using extend?

easy

A. class MyClass; def self.extend MyModule; end

B. class MyClass; include MyModule; end

C. class MyClass; MyModule.extend(self); end

D. class MyClass; extend MyModule; end

Solution

1. Step 1: Identify correct extend usage

   The correct syntax is to call extend inside the class body with the module name.

2. Step 2: Check each option

   class MyClass; extend MyModule; end uses extend MyModule inside the class, which is correct syntax.

3. Final Answer:

   class MyClass; extend MyModule; end -> Option D

4. Quick Check:

   Correct extend syntax = class MyClass; extend MyModule; end [OK]

Hint: Use 'extend ModuleName' inside class body for class methods [OK]

Common Mistakes:

• Using include instead of extend
• Calling extend on module instead of class
• Incorrect method definition syntax

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

module Greetings
  def hello
    "Hello from module"
  end
end

class Person
  extend Greetings
end

puts Person.hello

medium

A. NoMethodError: undefined method 'hello' for Person

B. Hello from module

C. Hello from module called on instance

D. SyntaxError

Solution

1. Step 1: Understand extend Greetings effect

   The module's method hello becomes a class method of Person.

2. Step 2: Check method call

   Person.hello calls the class method, which exists and returns "Hello from module".

3. Final Answer:

   Hello from module -> Option B

4. Quick Check:

   Class method from extend = "Hello from module" [OK]

Hint: extend adds module methods as class methods, so Person.hello works [OK]

Common Mistakes:

• Thinking hello is an instance method
• Calling hello on an instance instead of class
• Expecting NoMethodError

4. Identify the error in this Ruby code snippet:

module Tools
  def tool_name
    "Hammer"
  end
end

class Worker
  extend Tools

  def self.tool_name
    "Wrench"
  end
end

puts Worker.tool_name

medium

A. No error; output is 'Wrench'

B. NoMethodError because tool_name is undefined

C. SyntaxError due to extend usage

D. Output is 'Hammer' ignoring class method

Solution

1. Step 1: Analyze method definitions

   The module Tools defines tool_name, which is added as a class method to Worker by extend Tools.

2. Step 2: Check method override

   Worker defines its own class method tool_name, which overrides the module method.

3. Final Answer:

   No error; output is 'Wrench' -> Option A

4. Quick Check:

   Class method override works; output = 'Wrench' [OK]

Hint: Class methods override extended module methods if defined later [OK]

Common Mistakes:

• Assuming module method overrides class method
• Expecting NoMethodError
• Confusing instance and class methods

5. You want to share a method info as a class method across multiple classes without repeating code. Which approach using extend is best?

module InfoModule
  def info
    "Shared class info"
  end
end

class Alpha
  # What to do here?
end

class Beta
  # What to do here?
end

hard

A. Add extend InfoModule inside both Alpha and Beta classes

B. Add include InfoModule inside both Alpha and Beta classes

C. Define def self.info separately in Alpha and Beta

D. Use InfoModule.extend(self) outside classes

Solution

1. Step 1: Understand sharing class methods

   Using extend InfoModule inside each class adds info as a class method.

2. Step 2: Evaluate options

   Add extend InfoModule inside both Alpha and Beta classes correctly uses extend in both classes to share the method without repetition.

3. Final Answer:

   Add extend InfoModule inside both Alpha and Beta classes -> Option A

4. Quick Check:

   Use extend in each class to share class methods [OK]

Hint: Use extend inside each class to share class methods from a module [OK]

Common Mistakes:

• Using include instead of extend for class methods
• Defining methods separately instead of sharing
• Calling extend on module instead of class