Class methods with self prefix in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the self prefix mean in a Ruby method definition inside a class?

The <code>self</code> prefix means the method is a class method. It belongs to the class itself, not to instances of the class.

beginner

How do you call a class method defined with <code>self.method_name</code> in Ruby?

You call it on the class directly, like <code>ClassName.method_name</code>, without creating an instance.

intermediate

Why use class methods instead of instance methods?

Class methods are used for actions related to the class as a whole, like creating new instances or utility functions, while instance methods work on individual objects.

beginner

Example: What does this code do?<br><pre>class Dog
  def self.bark
    puts 'Woof!'
  end
end</pre>

Defines a class method <code>bark</code> on <code>Dog</code>. You can call <code>Dog.bark</code> to print 'Woof!'.

intermediate

Can class methods access instance variables directly? Why or why not?

No, class methods cannot access instance variables directly because instance variables belong to individual objects, not the class itself.

How do you define a class method in Ruby?

A def self.method_name

B def method_name

C def @method_name

D def #method_name

Class methods are defined with def self.method_name inside the class.

How do you call a class method named info in class Car?

A info.Car

B Car.info

C car.info

D Car#info

Class methods are called on the class itself, like Car.info.

Which of these is true about class methods?

A They are only used inside instance methods.

B They can access instance variables directly.

C They are private by default.

D They can be called without creating an object.

Class methods belong to the class and can be called without creating an instance.

What will this code print?<br>

class Cat
  def self.sound
    puts 'Meow'
  end
end
Cat.sound

A Error

B sound

C Meow

D Nothing

Calling Cat.sound runs the class method and prints 'Meow'.

Can a class method access instance variables like @name?

A No, because instance variables belong to objects.

B Yes, always.

C Only if the class method is called on an instance.

D Only if the instance variable is global.

Instance variables belong to individual objects, so class methods cannot access them directly.

Explain what a class method is in Ruby and how it differs from an instance method.

Think about who owns the method: the class or the object.

Describe a situation where you would use a class method instead of an instance method.

When the action relates to the whole class, not a single object.