Ruby Tutorial: Classes

Classes and Instances

Ref. WGR Chapter 3, Organizing objects with classes

The Cookie Metaphor

  • a class is a cookie cutter
  • an instance is a cookie
  • memory is cookie dough
  • state is frosting
  • users are hungry!

The Linugistic Metaphor

  • Objects are nouns
  • Methods are verbs
  • Attributes are adjectives


  • To instantiate an object, call the new method on its class
  • new does some stuff then turns around and calls initialize on the new instance

      @@@ ruby
      class Thing
        def initialize
          puts "Hi!"
      thing =  # *not* Thing.initialize!

Instance methods

  • defined inside the class
  • instance methods are shared among all instances
  • same behavior, but different data

Instance variables

  • represent object state
  • names start with an @
  • only visible inside the object
    • i.e. when self is that object

Getter and setter methods

@@@ ruby
class Person
  def age=(years_old)
    @age = years_old
  def age

alice =
alice.age = 17
alice.age #=> 17
alice.@age #=> SyntaxError

Setter sugar


is the same as

alice.age = 17

  • Technically, it's not an assignment, it's a method call
  • But it looks like an assignment!

The setter gotcha

  • Inside an object, you can't call that object's setter methods directly
  • Why not?
    • Because "age = 2" looks like a local variable assignment, which takes precedence
    • It eclipses the setter method!
    • Syntax ambiguity! Oh noes!
  • Solution: use "self.age = 2"


  • An attribute is a property with named getter and/or setter methods
  • Usually corresponds to an instance variable

Attribute Shortcuts

    @@@ ruby
    class Thing
      attr_reader :foo  #=>  def foo; @foo; end
      attr_writer :foo  #=>  def foo=(x); @foo = x; end
      attr_accessor :foo  #=> both of the above

Attribute Shortcuts (cont.)

  • Can also take multiple arguments

      @@@ ruby
      class Thing
        attr_accessor :foo, :bar

Attribute Shortcuts (cont.)

  • Wait a second!
  • Q: Where are attr_reader et al. defined?
  • A: They are class methods of Object

Attribute Shortcuts (cont.)

  • Sadly, attr_accessor is misnamed
  • "accessor" means reader, but attr_accessor makes a reader and a writer
  • Should have been called just attribute

Query methods

@@@ ruby
class Person
  def child?
    @age < 18

alice.age = 16
alice.child? #=> false

Note: query methods return a boolean by convention only

Bang methods

@@@ ruby
class Person
  def birthday!
    @age = @age + 1
  • "!" is pronounced "bang"
  • usually means "watch out" or "destructive" or "side effect"
    • could also mean "may raise an exception"
    • no real rule, so watch out
  • normally there's a non-bang equivalent
  • in ActiveRecord, "!" means: raise exception if failure

Object equality

  • Many ways to compare objects
    • == params are equal (overridable)
    • .eql? params are equal and the same type
    • .equal? params are identical (same object_id)
  • == is what you want, unless you know otherwise

An Elegant Object

class Student
  def initialize first_name, last_name
    @first_name = first_name
    @last_name = last_name

  def name
    "#{@first_name} #{@last_name}"
  • Why "elegant"?
    • initial state established by constructor
    • internal state used by methods, not exposed by getters