Declare an S3 class

To use an S3 class with S7, you must explicitly declare it using new_S3_class() because S3 lacks a formal class definition. (Unless it's an important base class already defined in base_s3_classes.)

Usage

new_S3_class(class, constructor = NULL, validator = NULL)

Arguments

class

S3 class vector (i.e. what class() returns). For method registration, you can abbreviate this to a single string, the S3 class name.

constructor

An optional constructor that can be used to create objects of the specified class. This is only needed if you wish to have an S7 class inherit from an S3 class. It must be specified in the same way as a S7 constructor: the first argument should be .data (the base type whose attributes will be modified).

All arguments to the constructor should have default values so that when the constructor is called with no arguments, it returns returns an "empty", but valid, object.

validator

An optional validator used by validate() to check that the S7 object adheres to the constraints of the S3 class.

A validator is a single argument function that takes the object to validate and returns NULL if the object is valid. If the object is invalid, it returns a character vector of problems.

Value

An S7 definition of an S3 class, i.e. a list with class S7_S3_class.

Method dispatch, properties, and unions

There are three ways of using S3 with S7 that only require the S3 class vector:

• Registering a S3 method for an S7 generic.

• Restricting an S7 property to an S3 class.

• Using an S3 class in an S7 union.

This is easy, and you can usually include the new_S3_class() call inline:

method(my_generic, new_S3_class("factor")) <- function(x) "A factor"
new_class("my_class", properties = list(types = new_S3_class("factor")))
new_union("character", new_S3_class("factor"))

Extending an S3 class

Creating an S7 class that extends an S3 class requires more work. You'll also need to provide a constructor for the S3 class that follows S7 conventions. This means the first argument to the constructor should be .data, and it should be followed by one argument for each attribute used by the class.

This can be awkward because base S3 classes are usually heavily wrapped for user convenience and no low level constructor is available. For example, the factor class is an integer vector with a character vector of levels, but there's no base R function that takes an integer vector of values and character vector of levels, verifies that they are consistent, then creates a factor object.

You may optionally want to also provide a validator function which will ensure that validate() confirms the validity of any S7 classes that build on this class. Unlike an S7 validator, you are responsible for validating the types of the attributes.

The following code shows how you might wrap the base Date class. A Date is a numeric vector with class Date that can be constructed with .Date().

S3_Date <- new_S3_class("Date",
  function(.data = integer()) {
    .Date(.data)
  },
  function(self) {
    if (!is.numeric(self)) {
      "Underlying data must be numeric"
    }
  }
)

Examples

# No checking, just used for dispatch
Date <- new_S3_class("Date")

my_generic <- new_generic("my_generic", "x")
method(my_generic, Date) <- function(x) "This is a date"

my_generic(Sys.Date())
#> [1] "This is a date"