| methods {base} | R Documentation |
R possesses a simple generic function mechanism which can be used for an object-oriented style of programming. Method despatch takes place based on the class of the first argument to the generic function.
An R “object” is a data object which has a class attribute.
A class attribute is a vector of character strings giving the names of
the classes which the object “inherits” from. When a generic
function fun is applied to an object with class attribute
c("first", "second"), the system searches for a function called
fun.first and, if it finds it, applied it to the object. If no
such function is found a function called fun.second is tried.
If no class name produces a suitable function, the function
fun.default is used.
methods can be used to find out about the methods for a
particular generic function or class. See the examples below for
details.
Now for some obscure details that need to appear somewhere. These
comments will be slightly different than those in Appendix A of the
White S Book. UseMethod creates a “new” function call with
arguments matched as they came in to the generic. Any local variables
defined before the call to UseMethod are retained (!?). Any
statements after the call to UseMethod will not be evaluated as
UseMethod does not return.
NextMethod invokes the next method (determined by the
class). It does this by creating a special call frame for that
method. The arguments will be the same in number, order and name as
those to the current method but their values will be promises to
evaluate their name in the current method and environment. Any
arguments matched to ... are handled specially. They are
passed on as the promise that was supplied as an argument to the
current environment. (S does this differently!) If they have been
evaluated in the current (or a previous environment) they remain
evaluated.
UseMethod(name)
NextMethod(name, object, ...)
methods(generic.function, class)
The methods function was written by Martin Maechler.
class
methods(summary)
cat("methods(print):\n")
print(methods(print))
cat("methods(class = data.frame):\n")
print(methods(class = data.frame))