Singletons: good design or a crutch?

 

Singletons are a hotly debated design pattern, so I am interested in what the Stack Overflow community thought about them.

Please provide reasons for your opinions, not just "Singletons are for lazy programmers!"

Here is a fairly good article on the issue, although it is against the use of Singletons: scientificninja.com: performant-singletons.

Does anyone have any other good articles on them? Maybe in support of Singletons?

In defense of singletons:

  • They are not as bad as globals because globals have no standard-enforced initialization order, and you could easily see nondeterministic bugs due to naive or unexpected dependency orders. Singletons (assuming they're allocated on the heap) are created after all globals, and in a very predictable place in the code.
  • They're very useful for resource-lazy / -caching systems such as an interface to a slow I/O device. If you intelligently build a singleton interface to a slow device, and no one ever calls it, you won't waste any time. If another piece of code calls it from multiple places, your singleton can optimize caching for both simultaneously, and avoid any double look-ups. You can also easily avoid any deadlock condition on the singleton-controlled resource.

    • Against singletons:

  • In C++, there's no nice way to auto-clean-up after singletons. There are work-arounds, and slightly hacky ways to do it, but there's just no simple, universal way to make sure your singleton's destructor is always called. This isn't so terrible memory-wise -- just think of it as more global variables, for this purpose. But it can be bad if your singleton allocates other resources (eg locks some files) and doesn't release them.

    • My own opinion:

    I use singletons, but avoid them if there's a reasonable alternative. This has worked well for me so far, and I have found them to be testable, although slightly more work to test.

    Google has a Singleton Detector for Java that I believe started out as a tool that must be run on all code produced at Google. The nutshell reason to remove Singletons:

    because they can make testing difficult and hide problems with your design

    For a more explicit explanation see 'Why Singletons Are Controversial' from Google.

    A singleton is just a bunch of global variables in a fancy dress.

    Global variables have their uses, as do singletons, but if you think you're doing something cool and useful with a singleton instead of using a yucky global variable (everyone knows globals are bad mmkay), you're unfortunately misled.

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