Overview

Master Ruby's elegant object-oriented programming features. Ruby is a pure object-oriented language where everything is an object.

Class Definition

Basic Class Structure

class Person
  # Class variable (shared across all instances)
  @@count = 0

  # Constant
  MAX_AGE = 150

  # Class method
  def self.count
    @@count
  end

  # Constructor
  def initialize(name, age)
    @name = name  # Instance variable
    @age = age
    @@count += 1
  end

  # Instance method
  def introduce
    "Hi, I'm #{@name} and I'm #{@age} years old"
  end

  # Attribute accessors (getter and setter)
  attr_accessor :name
  attr_reader :age      # Read-only
  attr_writer :email    # Write-only
end

person = Person.new("Alice", 30)
puts person.introduce
person.name = "Alicia"

Method Visibility

class BankAccount
  def initialize(balance)
    @balance = balance
  end

  # Public methods (default)
  def deposit(amount)
    @balance += amount
    log_transaction(:deposit, amount)
  end

  def balance
    format_currency(@balance)
  end

  # Protected methods - callable by instances of same class/subclass
  protected

  def log_transaction(type, amount)
    puts "[#{type}] #{amount}"
  end

  # Private methods - only callable within this instance
  private

  def format_currency(amount)
    "$#{amount}"
  end
end

Inheritance

Single Inheritance

class Animal
  def initialize(name)
    @name = name
  end

  def speak
    "Some sound"
  end
end

class Dog < Animal
  def speak
    "Woof! My name is #{@name}"
  end

  # Call parent method with super
  def introduce
    super  # Calls parent's speak method
    puts "I'm a dog"
  end
end

dog = Dog.new("Buddy")
puts dog.speak

Method Override and Super

class Vehicle
  def initialize(brand)
    @brand = brand
  end

  def start_engine
    puts "Engine starting..."
  end
end

class Car < Vehicle
  def initialize(brand, model)
    super(brand)  # Call parent constructor
    @model = model
  end

  def start_engine
    super  # Call parent method
    puts "#{@brand} #{@model} is ready to drive"
  end
end

Modules and Mixins

Module as Namespace

module MyApp
  module Utils
    def self.format_date(date)
      date.strftime("%Y-%m-%d")
    end
  end
end

MyApp::Utils.format_date(Time.now)

Module as Mixin

module Swimmable
  def swim
    "I'm swimming!"
  end
end

module Flyable
  def fly
    "I'm flying!"
  end
end

class Duck
  include Swimmable  # Instance methods
  include Flyable

  def quack
    "Quack!"
  end
end

duck = Duck.new
puts duck.swim
puts duck.fly

Extend vs Include

module Greetable
  def greet
    "Hello!"
  end
end

class Person
  include Greetable  # Adds as instance method
end

class Company
  extend Greetable   # Adds as class method
end

Person.new.greet    # Works
Company.greet       # Works

Advanced OOP Patterns

Singleton Pattern

class Database
  @instance = nil

  private_class_method :new

  def self.instance
    @instance ||= new
  end

  def connect
    puts "Connected to database"
  end
end

db1 = Database.instance
db2 = Database.instance
db1.object_id == db2.object_id  # true

Method Missing (Dynamic Methods)

class DynamicAttributes
  def method_missing(method_name, *args)
    attribute = method_name.to_s

    if attribute.end_with?("=")
      # Setter
      instance_variable_set("@#{attribute.chop}", args.first)
    else
      # Getter
      instance_variable_get("@#{attribute}")
    end
  end

  def respond_to_missing?(method_name, include_private = false)
    true
  end
end

obj = DynamicAttributes.new
obj.name = "Ruby"
puts obj.name  # "Ruby"

Class Instance Variables

class Product
  @inventory = []

  class << self
    attr_accessor :inventory

    def add(product)
      @inventory << product
    end
  end
end

Product.add("Laptop")

Struct and OpenStruct

Struct (Immutable-ish)

Person = Struct.new(:name, :age) do
  def introduce
    "I'm #{name}, #{age} years old"
  end
end

person = Person.new("Bob", 25)
puts person.name
person.age = 26

OpenStruct (Dynamic Attributes)

require 'ostruct'

person = OpenStruct.new
person.name = "Charlie"
person.age = 30
person.email = "charlie@example.com"

puts person.name

Composition Over Inheritance

class Engine
  def start
    "Engine started"
  end
end

class Wheels
  def rotate
    "Wheels rotating"
  end
end

class Car
  def initialize
    @engine = Engine.new
    @wheels = Wheels.new
  end

  def start
    @engine.start
  end

  def drive
    @wheels.rotate
  end
end

Comparable and Enumerable

Making Classes Comparable

class Person
  include Comparable

  attr_reader :age

  def initialize(name, age)
    @name = name
    @age = age
  end

  def <=>(other)
    age <=> other.age
  end
end

people = [Person.new("Alice", 30), Person.new("Bob", 25)]
puts people.sort.map(&:age)  # [25, 30]

Class Variables vs Instance Variables

class Counter
  @@count = 0      # Class variable (shared)
  @instances = []  # Class instance variable (not shared with subclasses)

  def initialize
    @@count += 1
  end

  def self.count
    @@count
  end
end

Best Practices

Prefer composition over inheritance for complex relationships
Use modules for mixins to share behavior across unrelated classes
Keep classes small and focused (Single Responsibility Principle)
Use attr_accessor/reader/writer instead of manual getters/setters
Make use of private/protected to encapsulate implementation details
Prefer instance variables over class variables to avoid unexpected sharing
Use Struct for simple data objects instead of full classes
Override to_s for debugging to provide meaningful string representations

Anti-Patterns

❌ Don't use class variables unnecessarily - they're shared across inheritance hierarchy ❌ Don't create god objects - keep classes focused and small ❌ Don't expose internal state - use methods instead of direct instance variable access ❌ Don't overuse inheritance - prefer composition or modules ❌ Don't ignore visibility modifiers - they exist for encapsulation

Related Skills

ruby-metaprogramming - For dynamic class/method generation
ruby-blocks-procs-lambdas - For functional programming patterns
ruby-modules - For advanced module usage

📖 ruby-oop