A software design often has aspects that apply across several classes, functions, or methods. We might have a technical aspect, examples include logging, auditing, or security, that must be implemented consistently. The general method for reuse of functionality in object-oriented programming is inheritance through a class hierarchy. However, inheritance doesn't always work out well. Some aspects of a software design are orthogonal to the class hierarchy. These are sometimes called "cross-cutting concerns". They cut across the classes, making design more complex.

A decorator provides a way to define functionality that's not bound to the inheritance hierarchy. We can use decorators to design an aspect of our application and then apply the decorators across classes, methods, or functions.

Additionally, we can use multiple inheritance in a disciplined way to create cross-cutting aspects. We'll consider a base class plus mixin class definitions...