What is the use in class/static methods in ruby?

Question

Learning ruby and oop in general, I came upon class methods which as far as I understand are like instance methods but are accessed from the class rather than from an object and can have only one running at the same time.

However, I don't understand why you would use a class method versuses a normal method (outside a class) and what is even their use?

For example:

#Why would you use:
class Foo
  def self.bar
    puts "Class method"
  end
end

#Versus simply:
def bar
  puts "Normal method"
end

Foo.bar # => Class method
bar # => Normal method

As they both produce the same result? I'm quite confused about them so please correct if I'm misunderstanding any/everything here.

Answer 1

Your example isn't a good one.

Class methods might deal with managing all instances that exist of a class, and instance methods deal with a single instance at a time.

class Book
  def self.all_by_author(author)
    # made up database call
    database.find_all(:books, where: { author: author }).map do |book_data|
      new book_data # Same as: Book.new(book_data)
    end
  end

  def title
    @title
  end
end


books = Book.all_by_author('Jules Vern')
books[0].title #=> 'Journey to the Center of the Earth'

In this example we have a class named Book. It has a class method all_by_author. It queries some pretend database and returns an array of Book instances. The instance method title fetches the title of a single Book instance.

So the class method managing a collection of instances, and the instance method manages just that instance.

---

In general, if a method would operate on a group of instances, or is code related to that class but does not directly read or update a single instance, then it probably should be a class method.