ABSTRACT
In chapter 15 we’ll learn about the S4 OO system, which provides a formal approach to functional OOP. The underlying ideas are similar to S3, but the implementation is much stricter and makes use of specialised functions for creating classes (setClass()), generics (setGeneric()), and methods (setMethod()). Additionally, S4 provides both multiple inheritance (i.e. a class can have multiple parents) and multiple dispatch (i.e. method dispatch can use the class of multiple arguments).
An important new component of S4 is the slot, a named component of the object that is accessed using the specialised subsetting operator @ (pronounced at). The set of slots, and their classes, forms an important part of the definition of an S4 class.
“Basics” gives a quick overview of the main components of S4: classes, generics, and methods. “Classes” dives into the details of S4 classes, including prototypes, constructors, helpers, and validators. “Generics and methods” shows you how to create new S4 generics, and how to supply those generics with methods. You’ll also learn about accessor functions which are designed to allow users to safely inspect and modify object slots. “Method dispatch” dives into the full details of method dispatch in S4. The basic idea is simple, then it rapidly gets more complex once multiple inheritance and multiple dispatch are combined. “S4 and S3” discusses the interaction between S4 and S3, showing you how to use them together.
