Adapter versus policy

The adapter and the policy (or strategy) patterns are some of the more general patterns, and C++ adds generic programming capabilities to these patterns. This tends to extend their usability and sometimes blurs the lines between the patterns. The patterns themselves are defined very distinctly—policies provide custom implementations while adapters change the interface and add functionality to the existing interface (the latter is a decorator aspect, but, as we have seen, most decorators are implemented as adapters). We also saw in the last chapter than C++ broadens the capabilities of policy-based design; in particular, policies in C++ can add or remove parts of the interface as well as control the implementation. So, while patterns are different, there is significant overlap in the types of problem they can be used for. In the last chapter, we saw...