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% File src/library/methods/man/SClassExtension-class.Rd

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

% Copyright 1995-2007 R Core Team

% Distributed under GPL 2 or later

\name{SClassExtension-class}

\docType{class}

\alias{SClassExtension-class}

\title{Class to Represent Inheritance (Extension) Relations }

\description{  An object from this class represents a single \sQuote{is}

  relationship; lists of these objects are used to represent all the

  extensions (superclasses) and subclasses for a given class.  The

  object contains information about how the relation is defined and

  methods to coerce, test, and replace correspondingly. }

\section{Objects from the Class}{

  Objects from this class are generated by \code{\link{setIs}},

  from direct calls and from the \code{contains=} information in a call to \code{\link{setClass}}, and from class unions created by \code{\link{setClassUnion}}.

  In the last case, the information is stored in defining the \emph{subclasses} of the union class (allowing unions to contain sealed classes).

}

\section{Slots}{

  \describe{

    \item{\code{subClass}, \code{superClass}:}{The classes being

      extended: corresponding to the \code{from}, and \code{to}

      arguments to \code{\link{setIs}}. }

    \item{\code{package}:}{The package to which that class belongs. }

    \item{\code{coerce}:}{A function to carry out the as() computation

    implied by the relation.  Note that these functions should

    \emph{not} be used directly.  They only deal with the

    \code{strict=TRUE} calls to the \code{\link{as}} function, with

    the full method constructed from this mechanically. }

    \item{\code{test}:}{The function that would test whether the

    relation holds.  Except for explicitly specified \code{test}

    arguments to \code{\link{setIs}}, this function is trivial. }

    \item{\code{replace}:}{The method used to implement \code{as(x,

    Class) <- value}.}

    \item{\code{simple}:}{A \code{"logical"} flag, \code{TRUE} if this

    is a simple relation, either because one class is contained in the

    definition of another, or because a class has been explicitly

    stated to extend a virtual class.  For simple extensions, the

    three methods are generated automatically.}

    \item{\code{by}:}{If this relation has been constructed

    transitively, the first intermediate class from the subclass. }

    \item{\code{dataPart}:}{A \code{"logical"} flag, \code{TRUE} if

  the extended class is in fact the data part of the subclass.  In

  this case the extended class is a basic class (i.e., a type). }

    \item{\code{distance}:}{The distance between the two classes,

    1 for directly contained classes, plus the number of generations between otherwise. }  }

}

\section{Methods}{

  No methods defined with class \code{"SClassExtension"} in the

  signature.

}

\seealso{

  \code{\link{is}},

  \code{\link{as}}, and the

  \code{\linkS4class{classRepresentation}} class.

}

\keyword{classes}