Selecting operations based on the run-time type of an object is key to many object-oriented and functional programming techniques. We present a technique for implementing open and efficient type switching on hierarchical extensible data types. The technique is general and copes well with C++ multiple inheritance.
To simplify experimentation and gain realistic performance using production-quality compilers and tool chains, we implement a type switch construct as an ISO C++11 library, called Mach7. This library-only implementation provides concise notation and outperforms the visitor design pattern, commonly used for case analysis on types in object-oriented programming. For closed sets of types, its performance roughly equals equivalent code in functional languages, such as OCaml and Haskell. The type-switching code is easier to use and is more expressive than hand-coded visitors are. The library is non-intrusive and circumvents most of the extensibility restrictions typical of the visitor design pattern. It was motivated by applications involving large, typed, abstract syntax trees.