\Header{methods}{Class Methods}
\alias{NextMethod}{methods}
\alias{UseMethod}{methods}
\keyword{methods}{methods}
\begin{Description}\relax
\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 or on 
the object supplied as an argument to \code{UseMethod} or \code{NextMethod}.\end{Description}
\begin{Usage}
\begin{verbatim}
UseMethod(generic, object)
NextMethod(generic = NULL, object = NULL, ...)
methods(generic.function, class)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{generic}] a character string naming a function.
\item[\code{object}] an object whose class will determine the method to be
dispatched.  Defaults to the first argument of the enclosing function.
\item[\code{...}] further arguments to be passed to the method.
\item[\code{generic.function}] a generic function, or a character string naming a
generic function.
\item[\code{class}] a symbol or character string naming a class: only used if
\code{generic.function} is not supplied.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
An \R{} ``object'' is a data object which has a \code{class} attribute.
A class attribute is a character vector giving the names of
the classes which the object ``inherits'' from.  When a generic
function \code{fun} is applied to an object with class attribute
\code{c("first", "second")}, the system searches for a function called
\code{fun.first} and, if it finds it, applied it to the object.  If no
such function is found a function called \code{fun.second} is tried.
If no class name produces a suitable function, the function
\code{fun.default} is used.

\code{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. \code{UseMethod} creates a ``new'' function call with
arguments matched as they came in to the generic.  Any local variables
defined before the call to \code{UseMethod} are retained (!?).  Any
statements after the call to \code{UseMethod} will not be evaluated as
\code{UseMethod} does not return.

\code{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 \code{...} 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.\end{Details}
\begin{Note}\relax
The \code{methods} function was written by Martin Maechler.\end{Note}
\begin{SeeAlso}\relax
\code{\Link{class}}\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
methods(summary)

methods(print)

methods(class = data.frame)

methods("[")  ##- does not list the C-internal ones...
\end{ExampleCode}
\end{Examples}

