Self keyword behavior in Ruby

Introduction

The self keyword helps you know which object is currently running the code. It points to the current object.

When you want to call a method on the current object inside a class.

When you want to refer to the current object to set or get its attributes.

When you want to define class methods instead of instance methods.

When you want to clarify that a method belongs to the current object to avoid confusion.

When you want to return the current object from a method.

Syntax

Ruby

self

self always points to the object that is running the current method or code.

Inside instance methods, self is the instance (object) itself.

Examples

This prints the current object p because self inside who_am_i is the instance.

Ruby

class Person
  def who_am_i
    puts self
  end
end

p = Person.new
p.who_am_i

Here, self is the class itself, so this defines a class method.

Ruby

class Person
  def self.describe
    puts "I am a Person class"
  end
end

Person.describe

Using self.name= calls the setter method for name on the current object.

Ruby

class Person
  attr_accessor :name

  def initialize(name)
    self.name = name  # Using self to call setter method
  end
end

p = Person.new("Alice")
puts p.name

Sample Program

This program shows how self works inside instance methods and class methods.

Ruby

class Car
  attr_accessor :color

  def initialize(color)
    self.color = color
  end

  def show_self
    puts "This car is: #{self.color}"
    puts "Self is: #{self}"
  end

  def self.info
    puts "Cars have wheels"
    puts "Self inside class method is: #{self}"
  end
end

car = Car.new("red")
car.show_self
Car.info

Important Notes

Inside instance methods, self is the object you created from the class.

Inside class methods (defined with self.method_name), self is the class itself.

Using self helps Ruby know you want to call a method or set a value on the current object, not create a local variable.

Summary

self points to the current object running the code.

Inside instance methods, self is the instance (object).

Inside class methods, self is the class itself.

Practice

1. What does the self keyword represent inside an instance method in Ruby?

easy

A. The superclass of the current class

B. The class itself

C. A global variable

D. The current instance (object) of the class

Solution

1. Step 1: Understand instance methods context

   Inside an instance method, the code runs for a specific object created from the class.

2. Step 2: Identify what self points to

   In this context, self refers to that specific object (instance) running the method.

3. Final Answer:

   The current instance (object) of the class -> Option D

4. Quick Check:

   self in instance method = instance [OK]

Hint: Inside instance methods, think "self" means the object itself [OK]

Common Mistakes:

• Confusing self with the class inside instance methods
• Thinking self is a global or class variable
• Assuming self is always the class

2. Which of the following is the correct way to define a class method using self in Ruby?

easy

A. def self.my_method; 'hello'; end

B. def my_method.self; 'hello'; end

C. def my_method(self); 'hello'; end

D. def self:my_method; 'hello'; end

Solution

1. Step 1: Recall class method syntax

   Class methods are defined by prefixing the method name with self. inside the class.

2. Step 2: Check each option's syntax

   def self.my_method; 'hello'; end uses def self.my_method which is correct syntax. Others are invalid Ruby syntax.

3. Final Answer:

   def self.my_method; 'hello'; end -> Option A

4. Quick Check:

   Class method syntax = def self.method_name [OK]

Hint: Class methods start with def self.method_name [OK]

Common Mistakes:

• Placing self after method name
• Using colon instead of dot
• Passing self as a parameter

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

class Dog
  def speak
    self.class
  end
end

fido = Dog.new
puts fido.speak

medium

A. Dog

B. #

C. fido

D. Object

Solution

1. Step 1: Understand what self is inside speak

   Inside the instance method speak, self is the instance fido.

2. Step 2: Evaluate self.class

   Calling class on the instance returns its class, which is Dog.

3. Final Answer:

   Dog -> Option A

4. Quick Check:

   self.class in instance method = class name [OK]

Hint: self.class inside instance method returns the object's class [OK]

Common Mistakes:

• Thinking self.class returns the instance
• Confusing class with object name
• Expecting output to be an object instead of class name

4. Identify the error in this Ruby code snippet:

class Cat
  def self.meow
    puts "Meow!"
  end

  def meow
    puts "Meow from instance"
  end

  def self.instance_meow
    meow
  end
end

Cat.instance_meow

medium

A. Missing self in instance method definition

B. Calling instance method meow inside a class method without an instance

C. Incorrect syntax for class method definition

D. Using puts inside methods is not allowed

Solution

1. Step 1: Analyze method calls inside self.instance_meow

   This is a class method trying to call meow without specifying an instance.

2. Step 2: Understand method types

   meow without self. is an instance method, so it needs an object to call it on.

3. Step 3: Identify the error

   Calling an instance method directly inside a class method causes a NoMethodError because no instance is given.

4. Final Answer:

   Calling instance method meow inside a class method without an instance -> Option B

5. Quick Check:

   Class methods cannot call instance methods without an object [OK]

Hint: Class methods need an instance to call instance methods [OK]

Common Mistakes:

• Trying to call instance methods directly inside class methods
• Confusing self in class vs instance methods
• Ignoring need for object to call instance methods

5. Given the following Ruby code, what will be the output?

class Calculator
  def self.description
    "I am a Calculator"
  end

  def initialize(value)
    @value = value
  end

  def show_description
    self.class.description
  end
end

calc = Calculator.new(10)
puts calc.show_description

hard

A. NoMethodError

B. "Calculator"

C. "I am a Calculator"

D. "10"

Solution

1. Step 1: Understand self.class.description inside instance method

   Inside show_description, self is the instance calc. Calling class returns the class Calculator.

2. Step 2: Call class method description

   Calling description on the class returns the string "I am a Calculator".

3. Final Answer:

   "I am a Calculator" -> Option C

4. Quick Check:

   Instance calls class method via self.class = output string [OK]

Hint: Use self.class to call class methods from instances [OK]

Common Mistakes:

• Expecting instance variable value instead of class method output
• Confusing self with class inside instance methods
• Thinking NoMethodError will occur