Ruby: Classes & Inheritance

What you will learn

How to define classes with attr_accessor (automatic getters/setters), the initialize constructor, and Ruby's single-inheritance model.

Defining a class

class Person
  attr_accessor :name, :age

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

  def greet
    "Hi, I'm #{@name}"
  end
end

alice = Person.new("Alice", 25)
puts alice.greet     # Hi, I'm Alice
puts alice.name      # Alice (getter from attr_accessor)
alice.age = 26       # setter

attr_accessor :name creates both name (getter) and name= (setter) methods automatically. Use attr_reader for read-only, attr_writer for write-only.

Instance variables (@name)

Variables starting with @ are instance variables — they belong to the object and persist across method calls. Unlike local variables, they default to nil if not initialized.

Inheritance

class Animal
  def speak
    "..."
  end
end

class Dog < Animal
  def speak
    "Woof!"
  end
end

rex = Dog.new
puts rex.speak  # Woof!

Ruby supports single inheritance — a class can have only one parent (use modules for multiple inheritance).

Modules (mixins)

Modules are collections of methods that can be mixed into classes:

module Swimmable
  def swim
    "#{self.class} is swimming!"
  end
end

class Fish
  include Swimmable  # adds Swimmable methods as instance methods
end

nemo = Fish.new
puts nemo.swim  # Fish is swimming!

Common mistakes

• Forgetting @ before instance variable names — name = "Alice" creates a local variable, not an instance variable
• Defining getter and setter manually — use attr_accessor instead
• Thinking initialize is optional — it's not if you need to set initial state; the default one takes no arguments
• Confusing class variables (@@) with instance variables (@) — class variables are shared across the class hierarchy and can cause surprising behavior

Quick check below!