Open Is Good Fast, Orthogonal Open Multi Methods with YOMM2 - Jean Louis Leroy

Open multi-methods make it easy to add polymorphic operations to existing classes, without modifying them. Bjarne Stroustrup pushed for their inclusion in C++ circa 2007 - without success. YOMM2 implements the feature as a library. The syntax is simple and natural, and performance is excellent - within ~35% of native virtual functions. In the last few years, the library has moved beyond emulating Stroustrup's proposal, and has evolved significantly.