super: Force method dispatch to use a superclass

In S7: An Object Oriented System Meant to Become a Successor to S3 and S4

Force method dispatch to use a superclass

Description

super(from, to) causes the dispatch for the next generic to use the method for the superclass to instead of the actual class of from. It's needed when you want to implement a method in terms of the implementation of its superclass.

S3 & S4

super() performs a similar role to NextMethod() in S3 or methods::callNextMethod() in S4, but is much more explicit:

• The super class that super() will use is known when write super() (i.e. statically) as opposed to when the generic is called (i.e. dynamically).

• All arguments to the generic are explicit; they are not automatically passed along.

This makes super() more verbose, but substantially easier to understand and reason about.

super() in S3 generics

Note that you can't use super() in methods for an S3 generic. For example, imagine that you have made a subclass of "integer":

MyInt <- new_class("MyInt", parent = class_integer, package = NULL)

Now you go to write a custom print method:

method(print, MyInt) <- function(x, ...) {
   cat("<MyInt>")
   print(super(x, to = class_integer))
}

MyInt(10L)
#> <MyInt>super(<MyInt>, <integer>)

This doesn't work because print() isn't an S7 generic so doesn't understand how to interpret the special object that super() produces. While you could resolve this problem with NextMethod() (because S7 is implemented on top of S3), we instead recommend using S7_data() to extract the underlying base object:

method(print, MyInt) <- function(x, ...) {
   cat("<MyInt>")
   print(S7_data(x))
}

MyInt(10L)
#> <MyInt>[1] 10

Usage

super(from, to)

Arguments

from	An S7 object to cast.
to	An S7 class specification, passed to as_class(). Must be a superclass of object.

Value

An S7_super object which should always be passed immediately to a generic. It has no other special behavior.

Examples

Foo1 <- new_class("Foo1", properties = list(x = class_numeric, y = class_numeric))
Foo2 <- new_class("Foo2", Foo1, properties = list(z = class_numeric))

total <- new_generic("total", "x")
method(total, Foo1) <- function(x) x@x + x@y

# This won't work because it'll be stuck in an infinite loop:
method(total, Foo2) <- function(x) total(x) + x@z

# We could write
method(total, Foo2) <- function(x) x@x + x@y + x@z
# but then we'd need to remember to update it if the implementation
# for total(<Foo1>) ever changed.

# So instead we use `super()` to call the method for the parent class:
method(total, Foo2) <- function(x) total(super(x, to = Foo1)) + x@z
total(Foo2(1, 2, 3))

# To see the difference between convert() and super() we need a
# method that calls another generic

bar1 <- new_generic("bar1", "x")
method(bar1, Foo1) <- function(x) 1
method(bar1, Foo2) <- function(x) 2

bar2 <- new_generic("bar2", "x")
method(bar2, Foo1) <- function(x) c(1, bar1(x))
method(bar2, Foo2) <- function(x) c(2, bar1(x))

obj <- Foo2(1, 2, 3)
bar2(obj)
# convert() affects every generic:
bar2(convert(obj, to = Foo1))
# super() only affects the _next_ call to a generic:
bar2(super(obj, to = Foo1))

S7 documentation built on April 3, 2025, 10:50 p.m.