Generally, the recommend way to register an S3 method is to use the S3Method() namespace directive (often generated automatically by the @export roxygen2 tag). However, this technique requires that the generic be in an imported package, and sometimes you want to suggest a package, and only provide a method when that package is loaded. s3_register() can be called from your package's .onLoad() to dynamically register a method only if the generic's package is loaded. (To avoid taking a dependency on vctrs for this one function, please feel free to copy and paste the function source into your own package.)

s3_register(generic, class, method = NULL)



Name of the generic in the form pkg::generic.


Name of the class


Optionally, the implementation of the method. By default, this will be found by looking for a function called generic.class in the package environment.

Note that providing method can be dangerous if you use devtools. When the namespace of the method is reloaded by devtools::load_all(), the function will keep inheriting from the old namespace. This might cause crashes because of dangling .Call() pointers.


For R 3.5.0 and later, s3_register() is also useful when demonstrating class creation in a vignette, since method lookup no longer always involves the lexical scope. For R 3.6.0 and later, you can achieve a similar effect by using "delayed method registration", i.e. placing the following in your NAMESPACE file:

if (getRversion() >= "3.6.0") {
  S3method(package::generic, class)


# A typical use case is to dynamically register tibble/pillar methods # for your class. That way you avoid creating a hard dependency on packages # that are not essential, while still providing finer control over # printing when they are used. .onLoad <- function(...) { s3_register("pillar::pillar_shaft", "vctrs_vctr") s3_register("tibble::type_sum", "vctrs_vctr") }