Chapter 6. The Singleton pattern
In this chapter, we'll take up the Singleton pattern. This pattern is grouped with the other Creational patterns, although it is to some extent a pattern that limits, rather than promotes, the creation of classes. Specifically, it ensures that there is one and only one instance of a class and provides a global point of access to that instance. Any number of cases in programming in which you need to ensure that there can be one and only one instance of a class are possible. For example, your system might have only one window manager or print spooler or one point of access to a database engine. Or, your computer might have several serial ports, but there can only be one instance of COM1.