8.2 Using Metaclasses to Capture Class-Specific Data and Behavior

Languages use two main methods to implement the notion of class-specific versus object-specific data and functionality. The first and, in my opinion, the cleanest implementation is to keep these different types of data and behavior completely separate. In pure object-oriented languages (e.g., SmallTalk), everything is considered an object. Even a class is an object. If classes are objects, then what is the class for those objects? A class of a class is called a metaclass. The basic idea is that object-specific data and behavior should be placed in the class definition, while the class-specific data and behavior should be placed in the class's associated metaclass. This provides a clear separation of concerns. When talking about a specific object of the class, we examine the class definition. When talking about all of the objects instantiated by the class, we examine the metaclass. This separation of concerns can be best illustrated by Figure 8.1, which shows the Invoice class and its associated metaclass.