Open Pattern Matching for C plus

Pattern matching is an abstraction mechanism that can greatly simplify source code. We present functional-style pattern matching for C++ implemented as a library, called Mach7(1). All the patterns are user-definable, can be stored in variables, passed among functions, and allow the use of class hierarchies. As an example, we implement common patterns used in functional languages. Our approach to pattern matching is based on compile-time composition of pattern objects through concepts. This is superior (in terms of performance and expressiveness) to approaches based on run-time composition of polymorphic pattern objects. In particular, our solution allows mapping functional code based on pattern matching directly into C++ and produces code that is only a few percent slower than hand-optimized C++ code. The library uses an efficient type switch construct, further extending it to multiple scrutinees and general patterns. We compare the performance of pattern matching to that of double dispatch and open multi-methods in C++.