Chapter 11. Multimethods

This chapter defines, discusses, and implements multimethods in the context of C++.

The C++ virtual function mechanism allows dispatching of a call depending on the dynamic type of one object. The multimethods feature allows dispatching of a function call depending on the types of multiple objects. A universally good implementation requires language support, which is the route that languages such as CLOS, ML, Haskell, and Dylan have taken. C++ lacks such support, so its emulation is left to library writers.

This chapter discusses some typical solutions and some generic implementations of each. The solutions feature various trade-offs in speed, flexibility, and dependency management. To describe the technique of dispatching a function call depending on multiple objects, this book uses the terms multimethod (borrowed from CLOS) and multiple dispatch. A particularization of multiple dispatch for two objects is known as double dispatch.

Implementing multimethods is a problem that's as fascinating as it is dreaded, one that has stolen lots of hours of good, healthy sleep from designers and programmers.^[1]

^[1] If you have trouble implementing multimethods, you can look at this chapter as a sleep aid—which I hope doesn't mean it has an actual soporific effect.

The topics of this chapter include

• Defining multimethods

• Identifying situations in which the need for multiobject polymorphism appears

• Discussing and implementing three double dispatchers that foster different trade-offs

• Enhancing double-dispatch engines

After reading this chapter, you will have a firm grasp of the typical situations for which multimethods are the way to go. In addition, you will be able to use and extend several robust generic components, provided by Loki, that implement multimethods.

This chapter limits discussion to multimethods for two objects (double dispatch). You can use the underlying techniques to extend the number of supported objects to three or more. It is likely, though, that in most situations you can get away with dispatching depending on two objects, and therefore you'll be able to use Loki directly.