Instance Variables

Instance Variables

Instance variables hold state for each individual object. Prefix them with @ — that's what makes them belong to the instance rather than the class.

class Player
  def initialize(name)
    @name = name
    @health = 100
  end
end

p1 = Player.new("Alice")
p2 = Player.new("Bob")

# Each player has its own @name and @health

Every object gets its own copy. Changing one doesn't affect the other.

p1.instance_variable_get(:@name)   # => "Alice"
p2.instance_variable_get(:@name)   # => "Bob"

Instance variables are private by default. You can't reach in and grab @name from outside the object — that would break encapsulation. Instead, you use methods to access them:

class Player
  attr_reader :name
  attr_accessor :health

  def initialize(name)
    @name = name
    @health = 100
  end
end

player = Player.new("Alice")
player.name     # => "Alice"
player.health   # => 100

attr_reader creates a getter, attr_writer creates a setter, and attr_accessor creates both. These are just shortcuts for writing the methods yourself.

You can list all instance variables on any object with instance_variables:

player.instance_variables
# => [:@name, :@health]

This is Ruby's way of keeping things tidy. Your internal state stays protected, and you control how the outside world interacts with it through your methods.