Challenge 3: A Class Method Counter — Solution

class Book
  attr_accessor :title, :author, :pages

  @@count = 0   # class variable, shared across every instance

  def initialize(title, author, pages)
    @title = title
    @author = author
    @pages = pages
    @@count += 1
  end

  def self.total_books
    @@count
  end
end

Book.new("The Hobbit", "J.R.R. Tolkien", 310)
Book.new("Dune", "Frank Herbert", 412)
Book.new("1984", "George Orwell", 328)

puts Book.total_books

=begin
Output:
3

Notes:
- @@count (double @) is a class variable — one single value shared by the
  class itself and every instance, unlike @count (single @) which would be
  a separate instance variable per object.
- Every call to Book.new runs initialize, which increments @@count by 1
  as a side effect of constructing a new instance.
- self.total_books is a class method (defined with self., as covered in
  the chapter) — it's called on Book directly, Book.total_books, with no
  instance required, and simply returns the current shared count.
=end