| callNextMethod {methods} | R Documentation |
Call an Inherited Method
Description
A call to callNextMethod can only appear inside a method
definition. It then results in a call to the first inherited method
after the current method, with the arguments to the current method
passed down to the next method. The value of that method call is the
value of callNextMethod.
Usage
callNextMethod(...)
Arguments
... |
If included, the call to the next methods uses these as its arguments
(but note that the dispatch is as described below.) The
recommendation for most applications is to use |
Details
The definition of the first inherited method is that is the method
which would have been called if the current method did not
exist.
This is more-or-less literally what happens: The current method is
deleted from a copy of the methods for the current generic, and
selectMethod is called to find the next method (the
result is cached in a special object, so the search only typically
happens once per session per combination of argument classes).
It is also legal, and often useful, for the method called by
callNextMethod to itself have a call to callNextMethod. This
works the same way, except that now two methods are deleted before
selecting the next method, and so on for further nested calls to
callNextMethod.
The statement that the method is called with the current arguments is
more precisely as follows. Arguments that were missing in the current
call are still missing (remember that "missing" is a valid
class in a method signature). For a formal argument, say x, that
appears in the original call, there is a corresponding argument in the
next method call equivalent to “x = x”. In effect, this
means that the next method sees the same actual arguments, but
arguments are evaluated only once.
Value
The value returned by the selected method.
References
The R package methods implements, with a few exceptions, the
programming interface for classes
and methods in the book Programming with Data (John
M. Chambers, Springer, 1998), in particular sections 1.6, 2.7, 2.8,
and chapters 7 and 8.
While the programming interface for the methods package follows the reference, the R software is an original implementation, so details in the reference that reflect the S4 implementation may appear differently in R. Also, there are extensions to the programming interface developed more recently than the reference. For a discussion of details and ongoing development, see the web page http://developer.r-project.org/methodsPackage.html and the pointers from that page.
See Also
Methods for the general behavior of method dispatch
Examples
## some class definitions with simple inheritance
setClass("B0" , representation(b0 = "numeric"))
setClass("B1", "B0")
setClass("B2", representation("B1", b2 = "logical"))
## and a rather silly function to illustrate callNextMethod
f <- function(x) class(x)
setMethod("f", "B0", function(x) c(x@b0, callNextMethod()))
setMethod("f", "B2", function(x) c(x@b2, callNextMethod()))
b2 <- new("B2", b2 = FALSE, b0 = 10)
b1 <- new("B1", b0 = 2)
f(b2)
f(b1)