Register a method for a suggested dependency

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.

Arguments

generic

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

class

Name of the class

method

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.

Details

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)
}

Usage in other packages

To avoid taking a dependency on vctrs, you copy the source of s3_register() into your own package. It is licensed under the permissive unlicense to make it crystal clear that we're happy for you to do this. There's no need to include the license or even credit us when using this function.

Examples

# 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")
}