6.12 Summary

The introduction to this chapter describes the most popular C++ implementations of Singleton. It's relatively easy to protect Singleton against multiple instantiations because there is good language support in this area. The most complicated problem is managing a singleton's lifetime, especially its destruction.

Detecting postdestruction access is easy and inexpensive. This dead-reference detection ought to be part of any Singleton implementation.

We discussed four main variations on the theme: the compiler-controlled singleton, the Phoenix singleton, the singleton with longevity, and the "leaking" singleton. Each of these has different strengths and weaknesses.

There are serious threading issues surrounding the Singleton design pattern. The Double-Checked Locking pattern is of great help in implementing thread-safe singletons.

In the end, we collected and classified the variations, which helped us in defining policies and decomposing Singleton along these policies. We identified three policies with Singleton: Creation, Lifetime, and ThreadingModel. We harnessed the policies in a class template SingletonHolder with four template parameters (the client type plus one parameter for each policy) that cover all the combinations among these design choices.