The Singleton Pattern - Anti-Pattern or Solution? - Klaus Iglberger - C++ on Sea 2022

C++ on Sea Website: cpponsea.uk/
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The Singleton Pattern - Anti-Pattern or Solution? - Klaus Iglberger - C++ on Sea 2022
Slides: github.com/phi...
Singleton is indeed a rather infamous pattern: There are many voices out there that describe Singleton as a general problem in code, as an anti-pattern, as dangerous, or even as evil. Still, despite the bad reputation and the many negative aspects, if used judiciously and if implemented properly,
Singleton has proven to be a useful solution for representing the few global aspects in your code.
This talk tries to give a balanced overview of the Singleton pattern. I will show the cons as well as the pros and will demonstrate how to properly deal with Singletons to overcome the major disadvantages. In particular, I will show how to implement Singletons such that dependencies to implementation details are avoided, code can be easily changed and full testability is given.
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Klaus Iglberger
Klaus Iglberger is a freelancing C++ trainer and consultant. He has finished his PhD in computer science in 2010 and since then is focused on large-scale C++ software design. He shares his experience in popular advanced C++ courses around the world (mainly in Germany, but also the EU and US). Additionally, he is the initiator and lead designer of the Blaze C++ math library (bitbucket.org/...) and a (co-)organizer of the Munich C++ user group (www.meetup.com....
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C++ on Sea is an annual C++ and coding conference, in Folkestone, in the UK. The 2022 conference was conducted as a two part, physical and online C++ event.
Annual C++ on Sea, C++ conference: cpponsea.uk/
2022 Program: cpponsea.uk/20...
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[@user-kf8yp9vl9p]

Definitely something to use carefully, but sometimes the best solution. The example Klaus gives is what I commonly refer to as a "Supper Simple Singleton" -- in this case it simply provides a pointer to the implementing object. Another Supper Simple Singleton that I have used many times is a registry for a service. Objects that want to use the service register and then the service calls them. Note that this is for embedded systems that never stop or change registration after initialization. Thanks!

[@Roibarkan]

Very thought provoking talk, Thanks Klaus. Another c++ use case which might relate to the topic is the “predicate” member of std::map or the “hasher” of std::unordered_map. There too, the STL uses the ‘strategy’ pattern, yet specific implementations/instantiations might require (implementation detail?) only a single object used. To some extent p1169 adds the notion of such singletons to c++23 (classes with static operator(), that might themselves have static variables)

[@Roibarkan]

12:12 (slide 22) - as an “implementation pattern” I think one of the merits of hiding globals as static variables in functions is that they handle initialization order fairly well

[@sanderjobing]

Anti-pattern: Can introduce global state and does not comply to the Liskov substitution principle. So difficult testable.