A.6 Optional volatile Modifier

C++ provides the volatile type modifier, with which you should qualify each variable that you share with multiple threads. However, in a single-threaded model, it's best not to use volatile because it prevents the compiler from performing some important optimizations.

That's why Loki defines the inner class VolatileType. Inside SomeThreadingModel<Widget>, VolatileType evaluates to volatile Widget for ClassLevelLockable and ObjectLevelLockable, and to plain Widget for SingleThreaded. You can see VolatileType at work in Chapter 6.