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Class Methods and Constants

"class methods" are actually

  • instance methods
  • on the eigenclass
  • of the class object

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
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