Chapter 1. Policy-Based Class Design

This chapter describes policies and policy classes, important class design techniques that enable the creation of flexible, highly reusable libraries—as Loki aims to be. In brief, policy-based class design fosters assembling a class with complex behavior out of many little classes (called policies), each of which takes care of only one behavioral or structural aspect. As the name suggests, a policy establishes an interface pertaining to a specific issue. You can implement policies in various ways as long as you respect the policy interface.

Because you can mix and match policies, you can achieve a combinatorial set of behaviors by using a small core of elementary components.

Policies are used in many chapters of this book. The generic SingletonHolder class template (Chapter 6) uses policies for managing lifetime and thread safety. SmartPtr (Chapter 7) is built almost entirely from policies. The double-dispatch engine in Chapter 11 uses policies for selecting various trade-offs. The generic Abstract Factory (Gamma et al. 1995) implementation in Chapter 9 uses a policy for choosing a creation method.

This chapter explains the problem that policies are intended to solve, provides details of policy-based class design, and gives advice on decomposing a class into policies.