In the previous section, we introduced the S3 system. Unlike the object-oriented systems in most other programming languages, the S3 system is much less strict than a system in which classes are defined with a fixed structure and certain method dispatch as the program compiles. When we define an S3 class, almost nothing can be sure. We can not only add or remove methods of the class at any time but also insert or delete data elements from the object as we wish. In addition, S3 only supports single dispatch, that is, methods are chosen according to the class of only one argument, mostly the first argument.
Then, R introduces a more formal and stricter object-oriented system, S4. This system allows us to define formal classes with pre-specified definition and inheritance structure. It also supports multiple dispatch, that is, methods are chosen according to the classes of multiple arguments.
In this section, you will learn how to define S4 classes and methods.