Another book on design patterns in C++? Why that, and why now? Hasn't everything there is to know about design patterns been written already?

There are several reasons why yet another book on design patterns has been written, but first of all, this is very much a C++ book—this is not a book on design patterns in C++ but a book on design patterns in C++, and the emphasis sets it apart. C++ has all the capabilities of a traditional object-oriented language, so all the classic object-oriented design patterns, such as Factory and Strategy, can be implemented in C++. A few of those are covered in this book. But the full power of C++ is realized when you utilize its generic programming capabilities. Remember that design patterns are frequently occurring design challenges and the commonly accepted solution—both sides are equally important in a pattern. It stands to reason that when new tools become available, new solutions become possible. Over time, the community settles on some of these solutions as the most advantageous overall, and a new variation of an old design pattern is born—the same challenge, but a different preferred solution. But expanding capabilities also open up new frontiers—with new tools at our disposal, new design challenges arise.

In this book, we focus on design patterns where C++ has something essential to add to at least one of the two sides of the pattern. On the one hand, we have patterns such as Visitor, where the generic programming capabilities of C++ allow for a better solution. That better solution was made possible by new features added in recent versions of the language, from C++11 to C++17. On the other hand, generic programming is still programming (only the execution of the program happens at compile time); programming requires design, and design has common challenges that are not all that dissimilar to the challenges of traditional programming. Thus, many of the traditional patterns have their twins, or at least close siblings, in generic programming, and we largely focus on those patterns in this book. A prime example is the Strategy pattern, better known in the generic programming community by its alternative name, the Policy pattern. Finally, a language as complex as C++ is bound to have a few idiosyncrasies of its own that often lead to C++, specific challenges that have common, or standard, solutions. While not quite deserving of being  called patterns, these C++-specific idioms are also covered in this book.

All that said, there are three main reasons why this book has been written:

• To cover C++, specific solutions for otherwise general, classic design patterns
• To show C++, specific pattern variants that occur when old design challenges arise in the new domain of generic programming
• To keep our patterns up to date with the language's evolution