Summarizing the differences between instance variables, class variables, and class instance variables in Ruby.

Instance Variables

Variables that hold different values for each instance, using variable names that start with @.

They are mainly used to manage the state of each object declared within a class.

class Person
  def initialize(name)
    @name = name # Instance variable
  end

  def name
    @name
  end
end

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

puts p1.name # => Alice
puts p2.name # => Bob Different instances hold different values

Class Variables

Variables shared across the entire class, using variable names that start with @@.

They are used to hold values shared between different instances of the same class.

class Person
  @@count = 0 # Class variable

  def initialize(name)
    @name = name
    @@count += 1
  end

  def self.count
    @@count
  end
end

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

puts Person.count # => 2 Shared across all instances

They are also shared with subclasses.

class Parent
  @@var = "Parent"

  def self.var
    @@var
  end
end

class Child < Parent
  @@var = "Child"
end
puts Parent.var # => "Child" Overwritten by subclass

Class Instance Variables

Unlike class variables, they are not shared with subclasses.

They are used as instance variables of the class itself.

Access them within class methods using self.

class Person
  @count = 0 # Class instance variable

  def self.increment_count
    @count ||= 0
    @count += 1
  end

  def self.count
    @count
  end
end

class Student < Person
  @count = 0
end

Person.increment_count
Student.increment_count

puts Person.count # => 1 Count for Person class
puts Student.count # => 1 Count for Student class

Summary of Differences