Defining class methods (from outside the class)

def Person.oldest(people)
  people.sort_by {|p| p.age}.last
end

Defining class methods (from inside the class)

class Person
  def self.oldest(people)
    people.sort_by {|p| p.age}.last
  end
end

• self points to the class inside a class definition...
• ...so def self.oldest is the same as def Person.oldest

• By the way, here's an even cooler way to implement that method:

  people.max_by(:&age)
  

Uses of class methods

• utility functions
• factories
• counters

Factories

class Person
  def self.from_string(s)
    tokens = s.split    
    new(tokens[0], tokens[1], tokens[2].to_i)
  end
  def initialize(first, last, age)
    @first, @last, @age = first, last, age
  end
end

>> alice = Person.new("Alice", "Andrews", 17)
=> #<Person:0x000001009eca90 
   @first="Alice", @last="Andrews", @age=17>

>> bob = Person.from_string("Bob Barker 67")
=> #<Person:0x000001009e3af8 
   @first="Bob", @last="Barker", @age=67>

Counters

class Person
  @count = 0 # instance variable of the class Person

  def self.one_more
    @count += 1
  end

  def self.count
    @count  
  end

  def initialize(first, last, age)
    @first, @last, @age = first, last, age
    Person.one_more
  end
end

Class Constants

By convention, constants who are values are in ALL_CAPS, and constants who are modules or classes are in CamelCase

class Person
  DEFAULT_AGE = 18

  # inside the class, can be named directly 
  def initialize(first, last, age = DEFAULT_AGE)
    @first, @last, @age = first, last, age
  end
end

outside the class, must be scoped with ::

Person::DEFAULT_AGE #=> 18 ```

Predefined Constants

Math::PI
RUBY_VERSION
RUBY_RELEASE_DATE