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% File src/library/methods/man/setClassUnion.Rd

% Part of the R package, https://www.R-project.org

% Copyright 1995-2016 R Core Team

% Distributed under GPL 2 or later

\name{setClassUnion}

\alias{setClassUnion}

\alias{isClassUnion}

\alias{ClassUnionRepresentation-class}

\title{Classes Defined as the Union of Other Classes}

\description{

  A class may be defined as the \emph{union} of other classes; that

  is, as a virtual class defined as a superclass of several other

  classes. Class unions are useful in method signatures or as slots in

  other classes, when we want to allow one of several classes to be supplied.

}

\usage{

setClassUnion(name, members, where)

isClassUnion(Class)

}

\arguments{

  \item{name}{ the name for the new union class. }

  \item{members}{ the names of the classes that should be members of this union.}

  \item{where}{ where to save the new class definition.  In calls from

    a package's source code, should be omitted to save the definition

    in the package's namespace.}

  \item{Class}{ the name or definition of a class.

    }

}

\details{

  The classes in \code{members} must be defined before creating the

      union.  However, members can be added later on to an existing

      union, as shown in the example below. Class unions can be

      members of other class unions.

  Class unions are the only way to create a new superclass of

      a class whose definition is sealed.  The namespace of all

      packages is sealed when the package is loaded, protecting the

      class and other definitions from being overwritten from another

      class or from the global environment.  A call to

      \code{\link{setIs}} that tried to define a new superclass for

      class \code{"numeric"}, for example, would cause an error.

   Class unions are the exception; the class union

   \code{"maybeNumber"} in the examples defines itself as a new

   superclass of \code{"numeric"}.  Technically, it does not alter the

   metadata object in the other package's namespace and, of course,

   the effect of the class union depends on loading the package it

   belongs to.  But, basically, class unions are sufficiently useful

   to justify the exemption.

  The different behavior for class unions is made possible because the

  class definition object for class unions has itself a special class,

  \code{"ClassUnionRepresentation"}, an extension of class

  \code{\linkS4class{classRepresentation}}.

  }

\references{

 Chambers, John M. (2016)

 \emph{Extending R},

  Chapman & Hall.

(Chapters 9 and 10.)

}

\examples{

## a class for either numeric or logical data

setClassUnion("maybeNumber", c("numeric", "logical"))

## use the union as the data part of another class

setClass("withId", contains = "maybeNumber", slots = c(id = "character"))

w1 <- new("withId", 1:10, id = "test 1")

w2 <- new("withId", sqrt(w1)\%\%1 < .01, id = "Perfect squares")

## add class "complex" to the union "maybeNumber"

setIs("complex", "maybeNumber")

w3 <- new("withId", complex(real = 1:10, imaginary = sqrt(1:10)))

## a class union containing the existing class  union "OptionalFunction"

setClassUnion("maybeCode",

    c("expression", "language", "OptionalFunction"))

is(quote(sqrt(1:10)), "maybeCode")  ## TRUE

\dontshow{

## The following test is less trivial than it looks.

## It depends on the assignment of the data part NOT performing a

## strict coerce to "numeric" on the way to satisfying

## is(ttt, "maybeNumber").

stopifnot(identical(w1@.Data, 1:10))

removeClass("withId")

removeClass("maybeNumber")

}

}

\keyword{programming}

\keyword{classes}