Class Methods & Variables

When calling an instance method like  withdraw_securely , the syntax generally looks something like this:

object.method_being_called(arguments)

One would therefore think it’s safe to assume that an instance method is always preceded by a  . ,
which is in turn preceded by the object that is calling the method.
Why, then, did this code work in the previous example?
 

# from inside the Customer class

def withdraw_securely(amount, password)
  if password == @password
    remove_funds(amount)
  end
end

Isn’t  remove_funds  also an instance method? Why is it suddenly exempt from following the same object. method_being_called  syntax just because it’s inside a method?
This can be compared to spoken language. If you were asking Diego to tell us his name, you might say to him “Diego, tell us your name.”
But if you were asking me to tell you my name, you’d likely say “Tell me your name”.
Yes, you could have said “You, tell me your name,” but that would have been redundant. “You” isimplicit in “Tell me your name”.
Similarly, when you call an instance method from within a class, there is an implicit object being called: itself.

# from inside the Customer class

def withdraw_securely(amount, password)
  if password == @password
    self.remove_funds(amount)
  end
end

An object can refer to itself using the  self  keyword. Think of it as an object’s way of saying “me” or “I”.
When you call  remove_funds  from within the  Customer  class, you’re saying “remove these funds from myself”.
And since Ruby is all about removing any unnecessary syntax,  self  in this context is implicit, and can be left out entirely.
 
An instance variable like  @password  is scoped to a particular instance of a class.
But what if you wanted a variable that was shared across all instances of a class? Almost like… a class variable.

@@check_out_this_cool_class_variable = "boom."

Boom is right.
A class variable’s syntax is twice as cool as an instance variable, because it has two  @ ’s instead of just one.
Let’s see how one might use a class variable.
 

class Employee
  @@bank = "Udacity International Bank"

  def bank
    @@bank
  end
end

Unfortunately, class variables can’t be accessed using  attr_accessor , so you’ll need to create your own  bank  getter method in this case.
Initialize two instances of an employee to see this working.
 

elana = Employee.new
# => #<Employee:0x007fcdb48c19d0>
corey = Employee.new
# => #<Employee:0x00nfbdm132ejd9>
elana.bank
# => "Udacity International Bank"
corey.bank
# => "Udacity International Bank"

Great, now this  @@bank  class variable is shared across all instances of an  Employee  class.

But… Why?

Class variables are used considerably less frequently than instance variables.
Unlike the  public keyword, though, there are some practical use cases for class variables. Here are two of those use cases.

class Customer
  attr_reader :id

  @@id = 1
  @@customers = []

  def initialize
    @id = @@id
    @@id += 1
    @@customers << self
  end
end

If you break this apart, you’ll see two class variables,  @@id  and  @@customers .
Every time a new customer is instantiated, an instance variable of  @id  is set to the value of theclass variable  @@id .
Immediately afterwards, the  @@id  class variable is incremented by one.
 

larry = Customer.new
# => #<Customer:0x007faaba8a6aa8 @id=1>
christine = Customer.new
# => #<Customer:0x007faaba8a6aa8 @id=2>
larry.id
# => 1
christine.id
# => 2

This way, the  Customer  class can keep track of the total number of customer objects that have been created.
By assigning the class variable to the instance variable  @id , you are capturing the current ID number from when that particular customer object was created.
Similarly,  @@customers  in this case is an Array that holds all the customer objects that have ever been created.
After a new customer is initialized,  self , the particular instance currently being used, is pushed into this  @@customers  Array.
Unfortunately, we’re left without an appropriate interface for accessing  @@customers .
It wouldn’t make sense to create a  customers  instance method, since an Array of customers isn’t really a property of a particular customer.
If only there were something else...
 
Class variables can be especially powerful when coupled with class methods.
Class methods differ from instance methods in that they are called directly on the class name, like so:

Customer.this_is_a_class_method

These are used when you need to call a method that doesn’t apply to a specific instance of a class.
Say you need a method to retrieve all the customers you’ve created so far. This method could be called  all .
Defining a class method works just like defining an instance method, except it must be preceded by  self.

class Customer
  attr_reader :id

  @@id = 1
  @@customers = []

  # ...

  def initialize
    @id = @@id
    @@id += 1
    @@customers << self
  end

  def self.all
    @@customers
  end
end

Now, if at any point you’d like to retrieve an Array of all existing customers, you can simply call the class method.

Customer.all
# => [#<Customer:0x007faaba84c148 @id=1>, #<Customer:0x007faaba82fe30 @id=2>]