We can create an empty class in Ruby by using the following syntax:

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class Animal
end

# The above class can also be written in one line like so:
class Animal; end

Next, lets initialize our class with the initialize method like so:

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class Animal
  def initialize
  end
end

Our Animal class can now hold vairables, in the form of instance variables and class vairables. Instance variables always start with @ while class varaibles start with @@. We have to make sure every Animal we create has a name by giving it @name instance varaible. This way only the Animal class and instances of that class will have access to @name.

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

Now we can create an instance of our Animal class by calling the new method on Animal class and passing an argument.

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dog = Animal.new("bruno")

Class variables are attached to the entire class, not just an instance of the class. For instance we can set up a simple BankAccount class and use class variables to track how many instance of the class have been created like so:

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class BankAccount
  @@total_accounts = 0

  def initialize
    @@total_accounts += 1
  end

  def self.accounts_created
    @@total_accounts
  end
end

savings = BankAccount.new
current = BankAccount.new
inv = BankAccount.new

puts "Number of accounts created is #{BankAccount.accounts_created}."
# => Number of accounts created is 3.

So now everytime a new instance of BankAcoount is created @@total_accounts is increamented by one. Simple right?

The above illustration also subtly introduced the concept of Class mehtods. These usually start with self. In the context of the class self referes to the current class, hence they can be called directed without creating an instance of a class.

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class Book
  def self.title
    puts "The title of this book is 'Outliers'."
  end
end

Book.title
# => The title of this book is 'Outliers'.

Instance methods, on the other hand, can only be called by creating an instance of the class.

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

  def info
    puts "The book has X number of pages and the author is #{@name}."
  end
end

audiobook = Author.new("Malcolm Gladwell")
audiobook.info

# => The book has X number of pages and the author is Malcolm Gladwell.

The def keyword creates an instance method. Hence the only way info method can be called is by creating an instance of Author.