The R Project SVN R

\name{Classes}

\alias{Classes}

\alias{__ClassMetaData}

\title{Class Definitions}

\description{

  Class definitions are objects that contain the formal definition of a

  class of R objects.

}

\details{

  When a class is defined, an object is stored that contains the

  information about that class, including:

  \describe{

    \item{slots}{Each slot is a component object.  Like elements of a

      list these may be extracted (by name) and set.  However, they

      differ from list components in important ways.

      All the objects from a particular class have the same set of slot

      names; specifically, the slot names that are contained in the

      class definition.  Each slot in each object always has the same

      class; again, this is defined by the overall class definition.

      Classes don't need to have any slots, and many useful classes do

      not.  These objects usually extend other, simple objects, such as

      numeric or character vectors.  Finally, classes can have no data

      at all---these are known as \emph{virtual} classes and are in fact

      very important programming tools.  They are used to group together

      ordinary classes that want to share some programming behavior,

      without necessarily restricting how the behavior is implemented.

    }

    \item{extends}{The names of the classes that this class extends.  A

      class \code{Fancy}, say, extends a class \code{Simple} if an

      object from the \code{Fancy} class has all the capabilities of

      the \code{Simple} class (and probably some more as well).  In

      particular, and very usefully, any method defined to work for a

      \code{Simple} object can be applied to a \code{Fancy} object as

      well.

      In other programming languages, this relationship is sometimes

      expressed by saying that \code{Simple} is a superclass of

      \code{Fancy}, or that \code{Fancy} is a subclass of

      \code{Simple}.

      The actual class definition object contains the names of all the

      classes this class extends.  But those classes can themselves

      extend other classes also, so the complete extension can only be

      known by obtaining all those class definitions.

      Class extension is usually defined when the class itself is

      defined, by including the names of superclasses as unnamed

      elements in the representation argument to \code{\link{setClass}}.

      An object from a given class will then have all the slots

      defined for its own class \emph{and} all the slots defined for

      its superclasses as well.

      Note that \code{extends} relations can be defined in other ways

      as well, by using the \code{\link{setIs}} function.

    }

    \item{prototype}{Each class definition contains a prototype object

      from the class.  This must have all the slots, if any, defined by

      the class definition.

      The prototype most commonly just consists of the prototypes of all

      its slots.  But that need not be the case:  the definition of the

      class can specify any valid object for any of the slots.

      There are a number of \dQuote{basic} classes, corresponding to the

      ordinary kinds of data occurring in R.  For example,

      \code{"numeric"} is a class corresponding to numeric vectors.

      These classes are predefined and can then be used as slots or as

      superclasses for any other class definitions.  The prototypes for

      the vector classes are vectors of length 0 of the corresponding

      type.

      There are also a few basic virtual classes, the most important

      being \code{"vector"}, grouping together all the vector classes;

      and \code{"language"}, grouping together all the types of objects

      making up the R language.

    }

  }

}

\references{

  The web page \url{http://www.omegahat.org/RSMethods/index.html} is the

  primary documentation.

  The functions in this package emulate the facility for classes and

  methods described in \emph{Programming with Data} (John M. Chambers,

  Springer, 1998).  See this book for further details and examples.

}

\author{

  John Chambers

}

\seealso{

  \code{\link{Methods}},

  \code{\link{setClass}},

  \code{\link{is}},

  \code{\link{as}},

  \code{\link{new}},

  \code{\link{slot}}

}

\keyword{programming}

\keyword{classes}

\keyword{methods}