Class Methods and Class-Level Data

Instance methods belong to objects. Class methods belong to the class itself — you call them on the class name, like Dog.new or Time.now.

Syntax

Inside a class body, self is the class, so def self.name defines a class method:

class Converter
  def self.km_to_miles(km)
    (km * 0.621371).round(2)
  end
end

p Converter.km_to_miles(10)

6.21

Named constructors

A common use is an alternative way to create objects:

class User
  attr_reader :name, :email

  def initialize(name, email)
    @name = name
    @email = email
  end

  def self.from_hash(data)
    new(data.fetch(:name), data.fetch(:email))
  end
end

user = User.from_hash({ name: "Ada", email: "ada@example.com" })
puts user.email

ada@example.com

Inside a class method, new means self.new — creating an instance of this class.

Constants in classes

Values shared by every instance usually belong in a constant:

class Order
  TAX_RATE = 0.2

  def initialize(subtotal)
    @subtotal = subtotal
  end

  def total = @subtotal * (1 + TAX_RATE)
end

p Order.new(100).total
p Order::TAX_RATE

120.0
0.2

Order::TAX_RATE reads a constant from outside the class.

Ruby also has class variables (@@count), but they're shared with every subclass in surprising ways, and most style guides advise avoiding them. Use a constant, or an instance variable on the class itself, instead.

Common mistakes

• Calling a class method on an instance (user.from_hash(...)) — NoMethodError. Class methods are called on the class.