method_types.md

There are many different kinds of methods in ruby, including module methods, class methods, instance methods, and singleton methods.

module Animal
  def self.browse_petco # module method
    p "browsing petco"
  end
end

class Dog
  include Animal

  def self.adopt_a_dog # class method
    p "adopting a dog"
  end

  def bark # instance method
    p "barking"
  end
end

Animal.browse_petco # => "browsing petco"
Dog.adopt_a_dog # => "adopting a dog"
spot = Dog.new
spot.bark # => "barking"

def spot.roll_over # singleton method
  p "rolling over"
end

spot.roll_over # => "rolling over"

Class Methods

Class methods are called on classes and are defined with a self. or <ClassName>. Examples include .new, to create an instance of a class, and rails model methods like .find, .find_by, .all, and .where.

Module Methods

Module methods are like class methods but for modules.

Instance Methods

Instance methods are called on instances of classes. Examples include getters and setters. When referencing instance methods within a class, it's sometimes necessary to use the self. keyword. For example, consider:

class Person
  attr_accessor :age

  def initialize(opts)
    @age = opts[:age]
  end

  def grow_older
    p age  # => 45
    age = age + 10  # => undefined method `+' for nil:NilClass (NoMethodError)
  end
end

sandra = Person.new(age: 45)
sandra.grow_older

Within the grow_older instance method, we test the age getter by printing age; it works. We then use that that same method on the next line (age + 10) and get an error. Using self., we can make the error go away, but the setter than still does not work:

class Person
  attr_accessor :age

  def initialize(opts)
    @age = opts[:age]
  end

  def grow_older
    age = self.age + 10
  end
end

sandra = Person.new(age: 45)
sandra.grow_older

p sandra  # => #<Person:0x00007fd5ea13c2d0 @age=45>

Note that when sandra is printed out, her age remains 45. This is because age = is an allowed operation (assigning a value to a variable) and does not indicate to ruby that it needs to look for an instance method. self. eliminates the ambiguity:

class Person
  attr_accessor :age

  def initialize(opts)
    @age = opts[:age]
  end

  def grow_older
    self.age = self.age + 10
  end
end

sandra = Person.new(age: 45)
sandra.grow_older

p sandra  # => #<Person:0x00007fd5ea13c2d0 @age=55>

Singleton methods

These are methods that are defined for a specific instance. They are the least common of the four.