Singleton class vs. class with static member

 

Despite the many threads on that topic, I am still unclear as to when to choose which approach. I am hoping that by discussing a specific example, I will finally "get it."

Note: My language here is Cocoa though the general issue is not language-specific.

I have a class TaskQueue that I want to use to:

  • access from anywhere in my code in order to add or remove scheduled tasks
  • process automatically the scheduled tasks at regular intervals

    • When TaskQueue is first used, I want TaskQueue to initiate a thread that will then wake up at regular intervals to process the tasks.

    Obviously, I will need at a minimum two variables:

  • an array to store the tasks
  • an instance of the thread processing the tasks

    • Since I only want one queue of tasks and one thread to process these tasks, I have two choices:

  • Make TaskQueue a singleton class (using for example CWL_DECLARE_SINGLETON_FOR_CLASS_WITH_ACCESSOR as described in http://www.cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html, which I believe I will have to modify the CWLSynthesizeSingleton.h file to start the thread at init time.)

  • Have the array of tasks and the thread instance both be static (following the approach suggested here: How do I declare class-level properties in Objective-C?)

    • Is there clearly one approach that's better than the other one in this specific case? If so, why?

    The main differences are simple things like:

  • with a singleton you can pass around the object for delegates and callbacks
  • with a singleton you can implement interfaces and derive it
  • with a singleton you can use a factory pattern to build up your instance

    • If you don't need any of them, as with global functionality that must be accessed all around your code then you can go with static methods.

    I personally prefer using static methods unless I have an explicit reason to use a singleton instance (such as having a common interface but different implementations).

    Mind the fact that refactoring static methods to a singleton instance is quite a straightforward process so if you ever find the need for the latter you will refactor it easily (then you have the C preprocessor, a single #define would be almost enough).

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