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

Constructors

• 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!"
      end
    end
  
    thing = Thing.new  # *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
  end
  def age
    @age
  end
end

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

Setter sugar

alice.age=(17)

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"

Attributes

• 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
    end

Attribute Shortcuts (cont.)

• Can also take multiple arguments

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

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
  end
end

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
  end
end

• "!" 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

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

  def name
    "#{@first_name} #{@last_name}"
  end
end

• Why "elegant"?
  • initial state established by constructor
  • internal state used by methods, not exposed by getters