Design Patterns Explained: A New Perspective on Object-Oriented Design

Field Notes: Using the Singleton and Double-Checked Locking Patterns

If you know you are going to need an object and no performance issue requires you to defer instantiation of the object until it's needed, it is usually simpler to have a static member contain a reference to the object.

In multithreaded applications, Singletons typically have to be thread safe (because the single object may be shared by multiple objects). This means having no data members but using only variables whose scope is no larger than a method.