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Ruby Objects

This section introduces Ruby's object-oriented programming model, including instances, methods, parameters, and memory management (instances and references). Later sections cover classes and modules and further topics.

Ref: WGR Chapter 2. Objects, methods, and local variables

Note: Following WGR's lead, in this section we define methods on instances (not on classes), to keep the lessons simple.

What is an object?

What is an object?

  • An object encapsulates state and behavior.

    • encapsulate - put like things together; keep unlike things apart
    • state - data, variables, properties, attributes, constants
    • behavior - methods

The Linguistic Metaphor for Objects

One way to think about objects:

Objects are things that can be described and can do things, or...

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

Objects vs. Classes

  • objects are more fundamental than classes
  • in Ruby, you can add behavior to an object directly
  • class inheritance is complicated
    • we'll cover it later

Creating an object

cookie =
  • now cookie refers to an object instance
    • unique storage location in memory
    • instance data stored in that location

Creating an object literally

fruit = "apple"
  • "apple" is a string literal
  • fruit now refers to a new object instance

References and Instances

  • Imagine computer memory with two compartments: references and instances
    • also known as "the stack" (or "the scope") and "the heap"
  • References include parameters and local variables
  • Instances contain the "real" data
  • Each reference points at the location of an instance
    • every reference is the same size (just 32 bits, or maybe 64 bits)
Stack Heap
fruit -> "apple"

Literals create instances

fruit = "apple"
dessert = "apple"
  • fruit refers to a new object instance
  • dessert refers to a different, new object instance
Stack Heap
fruit -> "apple"
dessert -> "apple"

References are independent of instances

fruit = "apple"
dessert = fruit
fruit = "banana"
dessert = fruit

What are the values of fruit and dessert after each line?

Object Identity

How can you tell if two references point to the same instance?

fruit = "apple"
dessert = "apple"
>> fruit.object_id
=> 2165091560
>> dessert.object_id
=> 2165084200

Ref. WGR Ch.2, Section 2.3.1

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 usually what you want

Note that .equal? is not guaranteed since bizarrely, some objects override .equal? to do something else. If you really want to know if two variables reference the same instance, compare their object_ids.

Garbage Collection

fruit = "apple"
fruit = "banana"
  • Now the instance containing "apple" is unreferenced
  • So it can (and eventually will) be garbage collected

Side effects

  • a variable is a reference to an instance (persistent location in memory)
  • if you have several references to the same instance, odd things can happen

    friend = "Alice"
    teacher = friend
    => "ALICE"


Defining methods

cookie =

def cookie.bake
  puts "the oven is nice and warm"
  • bake is a method
    • aka function, procedure, subroutine
  • def cookie. ("def cookie dot") means "define a method on cookie"

Invoking methods

Behavior comes from messages and methods.


prints I'm a cookie to the console

  • the object cookie receives the message bake and executes the method bake
  • dot (.) is the message operator

Method Definition Schematic

method definition

Ref. The Well-Grounded Rubyist PDF, Fig. 2.1

The methods method

>> cookie.methods
=> [:nil?, :===, :=~, :!~, :eql?, ...]
>> cookie.methods(false)
=> [:bake]

also useful: cookie.methods.sort, cookie.methods.grep(/age/)

>> cookie =
=> #<Object:0x007f86e485c3a8>
>> cookie.methods(false)
=> []
>> def cookie.bake; puts "hi"; end
=> nil
>> cookie.methods(false)
=> [:bake]

>> "goo".methods.grep(/sub/)
=> [:sub, :gsub, :sub!, :gsub!]

The respond_to? method

if cookie.respond_to? :bake
  puts "cookie is unbakable"

Note: Usually you don't use respond_to because of duck typing.


Instance variables are stored in the object

def cookie.add_chips(num_chips)
  @chips = num_chips

def cookie.yummy?
  @chips > 100

cookie.yummy?   #=> true


  • All OO programs suffer from multiple personality disorder
  • self is the default message receiver
  • self is set invisibly to always point to the current object

Self Example

def cookie.yummy?
  @chips > 100

def cookie.add_chips(num_chips = 10)
  @chips += num_chips

def cookie.yummify
  add_chips until yummy?
  • inside yummify the call to yummy? goes to self
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