What is the correct way to write a singleton pattern in Ruby?

Question

I\'m trying to write the most secure singleton in Ruby that I can. I\'m new to the language, which is so elastic that I don\'t have a strong feeling that my singleton class wil

Answer 1

If you want to create a singleton, why bother creating a class? Just create an object, and add the methods and instance variables to it you want.

>> MySingleton = Object.new
=> #
>> MySingleton.instance_eval do
?>   @count = 0
>>   def next
>>     @count += 1
>>   end
>> end
=> nil
>> MySingleton.next
=> 1
>> MySingleton.next
=> 2
>> MySingleton.next
=> 3

A more standard way that people implement this pattern is to use a Module as the singleton object (rather than the more generic Object):

>> module OtherSingleton
>>   @index = -1
>>   @colors = %w{ red green blue }
>>   def self.change
>>     @colors[(@index += 1) % @colors.size]
>>   end
>> end
=> nil
>> OtherSingleton.change
=> "red"
>> OtherSingleton.change
=> "green"
>> OtherSingleton.change
=> "blue"
>> OtherSingleton.change
=> "red"

If you wanted your singleton object to inherit from some class, just make it an instance of that class. To inherit from a mixin, just use #extend. If you want a singleton object, ruby makes it really easy, and unlike other languages, it doesn't have to be defined in a class.

Ad-hoc singletons (my first example) are all over the place, and cover the majority of cases I've encountered. The module trick normally covers the rest (when I want something a little more formal).

Ruby code should (imho) use duck typing (via #respond_to?) rather than explicitly checking an object's class, so I normally don't care about the uniqueness of my singleton objects' class, since it's not its class that makes it unique, but everything I added after.