Open multi-methods for c++

Abstract

Multiple dispatch -the selection of a function to be invoked based on the dynamic type of two or more arguments -is a solution to several classical problems in object-oriented programming. Open multi-methods generalize multiple dispatch towards open-class extensions, which improve separation of concerns and provisions for retroactive design. We present the rationale, design, implementation, and performance of a language feature, called open multimethods, for C++. Our open multi-methods support both repeated and virtual inheritance. Our call resolution rules generalize both virtual function dispatch and overload resolution semantics. After using all information from argument types, these rules can resolve further ambiguities by using covariant return types. Great care was taken to integrate open multi-methods with existing C++ language features and rules. We describe a model implementation and compare its performance and space requirements to existing open multi-method extensions and workaround techniques for C++. Compared to these techniques, our approach is simpler to use, catches more user mistakes, and resolves more ambiguities through link-time analysis, runs significantly faster, and requires less memory. In particular, the runtime cost of calling an open multimethod is constant and less than the cost of a double dispatch (two virtual function calls). Finally, we provide a sketch of a design for open multi-methods in the presence of dynamic loading and linking of libraries.

Open pattern matching for C++

“…In the third study, we looked at the efficiency of hashing used in type switching on up to four scrutinees on some large real-world class hierarchies. In the fourth study, we compare the performance of matching N polymorphic arguments against double, triple and quadruple dispatch via visitor design pattern as well as open multi-methods extension to C++ [1]. In the fifth study we estimate the overhead of our and existing solutions to open patterns in comparison to a handcrafted code, while in the sixth study we compare their impact on the compilation times.…”

“…• Is non-intrusive and can be retroactively applied to existing types, both built-in and user-defined. 1 The library is available at http://parasol.tamu.edu/mach7/ Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. • Generalizes open type switch to multiple scrutinees and enables patterns in case clauses.…”

Korz

“…Pirkelbauer et al have presented Open Multi-Methods for C++ and have shown them to be efficient [Pirk07].…”

Open pattern matching for C++

“…This can be performed in a number of different ways including, for example, the techniques used to implement multiple dispatch. We compare the efficiency of type switching on multiple arguments in comparison to other relational matching alternatives based on double, triple and quadruple dispatch [16], as well as our own implementation of open multi-methods for C++ [36].…”