13.1 Introduction

S3 is R’s first and simplest OO system. S3 is informal and ad hoc, but there is a certain elegance in its minimalism: you can’t take away any part of it and still have a useful OO system. For these reasons, you should use it, unless you have a compelling reason to do otherwise. S3 is the only OO system used in the base and stats packages, and it’s the most commonly used system in CRAN packages.

S3 is very flexible, which means it allows you to do things that are quite ill-advised. If you’re coming from a strict environment like Java this will seem pretty frightening, but it gives R programmers a tremendous amount of freedom. It may be very difficult to prevent people from doing something you don’t want them to do, but your users will never be held back because there is something you haven’t implemented yet. Since S3 has few built-in constraints, the key to its successful use is applying the constraints yourself. This chapter will therefore teach you the conventions you should (almost) always follow.

The goal of this chapter is to show you how the S3 system works, not how to use it effectively to create new classes and generics. I’d recommend coupling the theoretical knowledge from this chapter with the practical knowledge encoded in the vctrs package.


  • Section 13.2 gives a rapid overview of all the main components of S3: classes, generics, and methods. You’ll also learn about sloop::s3_dispatch(), which we’ll use throughout the chapter to explore how S3 works.

  • Section 13.3 goes into the details of creating a new S3 class, including the three functions that should accompany most classes: a constructor, a helper, and a validator.

  • Section 13.4 describes how S3 generics and methods work, including the basics of method dispatch.

  • Section 13.5 discusses the four main styles of S3 objects: vector, record, data frame, and scalar.

  • Section 13.6 demonstrates how inheritance works in S3, and shows you what you need to make a class “subclassable”.

  • Section 13.7 concludes the chapter with a discussion of the finer details of method dispatch including base types, internal generics, group generics, and double dispatch.


S3 classes are implemented using attributes, so make sure you’re familiar with the details described in Section 3.3. We’ll use existing base S3 vectors for examples and exploration, so make sure that you’re familiar with the factor, Date, difftime, POSIXct, and POSIXlt classes described in Section 3.4.

We’ll use the sloop package for its interactive helpers.