The R Project SVN R

  methods can then be defined.  Typically, an existing non-generic

  methods can then be defined.  Typically, an existing non-generic

  function becomes the default method, but there is much optional

  function becomes the default method, but there is much optional

  control.  See the details section.

  control.  See the details section.

}

}

\usage{

\usage{

}

}

\arguments{

\arguments{

  \item{name}{ The character string name of the generic function.  In

  \item{name}{ The character string name of the generic function.  In

    the simplest and most common case, a function of this name is

    the simplest and most common case, a function of this name is

    already defined.  The existing function may be non-generic or

    already defined.  The existing function may be non-generic or

    no current function of this name. In that case, the formal arguments

    no current function of this name. In that case, the formal arguments

    and default values for the generic are taken from \code{def}.  You

    and default values for the generic are taken from \code{def}.  You

    can also supply this argument if you want the generic function to

    can also supply this argument if you want the generic function to

    do something other than just dispatch methods (an advanced topic

    do something other than just dispatch methods (an advanced topic

    best left alone unless you are sure you want it).

    best left alone unless you are sure you want it).

    Note that \code{def} is \emph{not} the default method; use argument

    Note that \code{def} is \emph{not} the default method; use argument

    \code{useAsDefault} if you want to specify the default separately.

    \code{useAsDefault} if you want to specify the default separately.

  }

  }

  \item{group}{ Optionally, a character string giving the group of

  \item{group}{ Optionally, a character string giving the group of

    generic functions to which this function belongs.  Methods can be

    generic functions to which this function belongs.  Methods can be

    defined for the corresponding group generic, and these will then

    defined for the corresponding group generic, and these will then

    define methods for this specific generic function, if no method has

    define methods for this specific generic function, if no method has

    been explicitly defined for the corresponding signature.  See the

    been explicitly defined for the corresponding signature.  See the

    references for more discussion.

    references for more discussion.

  }

  }

      expression.  The value returned by the generic function must

      expression.  The value returned by the generic function must

      have (or extend) this class, or one of the classes; otherwise,

      have (or extend) this class, or one of the classes; otherwise,

      an error is generated.  See the

      an error is generated.  See the

      details section for supplying an expression.

      details section for supplying an expression.

   }

   }

  (though perhaps not a good idea, in that it makes the behavior of

  (though perhaps not a good idea, in that it makes the behavior of

  your function different from the usual S model).  If your explicit

  your function different from the usual S model).  If your explicit

  definition of the generic function

  definition of the generic function

  does \emph{not} call \code{standardGeneric} you are in trouble,

  does \emph{not} call \code{standardGeneric} you are in trouble,

  because none of the methods for the function will ever be

  because none of the methods for the function will ever be

      By default, the generic function can return any object.

      By default, the generic function can return any object.

   If \code{valueClass} is supplied, it should be a vector of

   If \code{valueClass} is supplied, it should be a vector of

   class names; the value returned by a method is then required to

   class names; the value returned by a method is then required to

   satisfy \code{is(object, Class)} for one of the specified classes.

   satisfy \code{is(object, Class)} for one of the specified classes.

  "Compare", "Ops", "Math", "Math2", "Summary",} and \code{"Complex"})

  "Compare", "Ops", "Math", "Math2", "Summary",} and \code{"Complex"})

  does the same for each of the functions in that group.

  does the same for each of the functions in that group.

  You usually only need to use either function if the methods are

  You usually only need to use either function if the methods are

  being defined only for the group generic.  Defining a method for a

  being defined only for the group generic.  Defining a method for a

  turns on method dispatch for that function.  But in R defining a

  turns on method dispatch for that function.  But in R defining a

  method for the corresponding group generic, \code{"Arith"}, does not

  method for the corresponding group generic, \code{"Arith"}, does not

  currently turn on method dispatch (for efficiency reasons).  If

  currently turn on method dispatch (for efficiency reasons).  If

  there are no non-group methods for the functions, you have two

  there are no non-group methods for the functions, you have two

  choices.

  choices.

  While the programming interface for the \pkg{methods} package follows

  While the programming interface for the \pkg{methods} package follows

  the reference, the R software is an original implementation, so

  the reference, the R software is an original implementation, so

  details in the reference that reflect the S4 implementation may appear

  details in the reference that reflect the S4 implementation may appear

  differently in R.  Also, there are extensions to the programming

  differently in R.  Also, there are extensions to the programming

  interface developed more recently than the reference.  For a

  interface developed more recently than the reference.  For a

  \url{http://developer.r-project.org/methodsPackage.html} and the

  \url{http://developer.r-project.org/methodsPackage.html} and the

  pointers from that page.

  pointers from that page.

}

}

\examples{

\examples{

\dontshow{

\dontshow{

###   A non-standard generic function.  It insists that the methods

###   A non-standard generic function.  It insists that the methods

###   return a non-empty character vector (a stronger requirement than

###   return a non-empty character vector (a stronger requirement than

###    valueClass = "character" in the call to setGeneric)

###    valueClass = "character" in the call to setGeneric)

    function(text) {

    function(text) {

      value <- standardGeneric("authorNames")

      value <- standardGeneric("authorNames")

      if(!(is(value, "character") && any(nchar(value)>0)))

      if(!(is(value, "character") && any(nchar(value)>0)))

        stop("authorNames methods must return non-empty strings")

        stop("authorNames methods must return non-empty strings")

      value

      value

## An example of group generic methods, using the class

## An example of group generic methods, using the class

## "track"; see the documentation of setClass for its definition

## "track"; see the documentation of setClass for its definition

#define a method for the Arith group

#define a method for the Arith group

 function(e1, e2){

 function(e1, e2){

  e1@y <- callGeneric(e1@y , e2)

  e1@y <- callGeneric(e1@y , e2)

  e1

  e1

})

})

 function(e1, e2){

 function(e1, e2){

  e2@y <- callGeneric(e1, e2@y)

  e2@y <- callGeneric(e1, e2@y)

  e2

  e2

})

})