self in Ruby

Everything in Ruby is an object. Every piece of code you write belongs to some object. And every object has a class. Inside those classes there is self which refers to object itself which is an instance of that class.

♥ string = "hello"
=> "hello"
♥ string.object_id
=> 70313455665560
♥ string.class
=>String

♥ arr = Array.new([1, 2, 3])
=> [1, 2, 3]
♥ arr.object_id
=> 70313455618260
♥ arr.class
=> Array

♥ 2.object_id
=> 7
♥ 2.class
=> Fixnum

self is a special variable that points to the object that owns the currently executing code within the class definition.

Below we have an instance of Transfer class. And let’s assume we stopped our code at where binding.pry is right after it is initialized with instance variables.

class Transfer
  attr_reader :amount, :sender, :receiver
  attr_accessor :status

  def initialize(sender, receiver, amount)
    @status = "pending"
    @sender = sender
    @receiver = receiver
    @amount = amount
=>  binding.pry
  end

♥ self
=> #<Transfer:0x007ff2f4062568
 @amount=50,
 @receiver=#<BankAccount:0x007ff2f40625b8 @balance=1000, @name="Avi", @status="open">,
 @sender=#<BankAccount:0x007ff2f4062680 @balance=1000, @name="Amanda", @status="open">,
 @status="pending">

♥ self.amount
=> 50

♥ self.receiver
=> #<BankAccount:0x007ff2f40625b8 @balance=1000, @name="Avi", @status="open">

♥ self.sender
=> #<BankAccount:0x007ff2f4062680 @balance=1000, @name="Amanda", @status="open">

♥ self.status
=> #<BankAccount:0x007ff2f40625b8 @balance=1000, @name="Avi", @status="open">

Using self without understanding when to use it, might make your code harder to read.

Below we have two examples of Transfer class. Both examples work just fine. However on the one on the left use of self is redundant.

optional uses of self

• when getting instance variables
• using self to indicate which method call you are referring

non-optional uses of self

• setting instance attributes inside a class definition

  self.status = "closed"
  

• passing it as an argument to a method

  some_method(self)
  

• to indicate a method within the class definition as a class method

class BankAccount
  @@customers = []

  def self.all
    @@customers
  end
end

• use self when you need to save an instance of the class

class BankAccount
  @@customers = []
  attr_reader :name
  attr_accessor :status, :balance

  def initialize(name)
    @name = name
    @balance = 1000
    @status = "open"
    save
  end

  def save
    @@customers << self
  end
end

In the end you can avoid using self in many cases, but it might be helpful to use it in some cases to increase readability of your code. And there are non-optional cases where you have to use self.

~aehmt

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