Class Methods¶

Class Methods in Ruby¶

A class method is a method that is associated with a class not an instance of that class. So when you call a class method is it called off of the class. For example, Dog.kennel_name.

The most common class method is the new method of class. It is invoked to create instances of a class.
SomeClass.new

When appropriate, class methods can be used to perform actions not particularly suited for instances.

More examples:

Math.sqrt(99)
Person.all

Ruby Example,

class Person
  attr_reader :age

  @@count = 0
  def initialize(name,age)
    @name,@age = name,age
    @@count += 1
  end

  def self.count
    @@count
  end

  def self.find_oldest(*people)
    people.sort_by {|person| person.age}.last
  end

  def self.find_youngest(*people)
    people.sort_by {|person| person.age}.first
  end

  def to_s
    "My name is #{@name} and my age is #{@age}" 
  end

end

person1 = Person.new("blah",33)
puts person1
puts "Number of people created so far: #{Person.count}" 

person2 = Person.new("Tim",13)
puts person2
puts "Number of people created so far: #{Person.count}" 

person3 = Person.new("Doug",83)
puts person3
puts "Number of people created so far: #{Person.count}" 

puts "Oldest person: #{Person.find_oldest(person1,person2,person3)}" 
puts "Youngest person: #{Person.find_youngest(person1,person2,person3)}" 

Additional resources¶

1. Karmen Blake: Class variables and methods screencast

Class Methods used in Rails Models¶