The R Project SVN R

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

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

% Copyright 1995-2015 R Core Team

% Distributed under GPL 2 or later

\name{getClass}

\alias{getClass}

\alias{getClassDef}

\title{Get Class Definition }

\description{

  Get the definition of a class.

}

\usage{

getClass   (Class, .Force = FALSE, where)

getClassDef(Class, where, package, inherits = TRUE)

}

\arguments{

  \item{Class}{ the character-string name of the class, often with a

      \code{"package"} attribute as noted below under \code{package}.}

  \item{.Force}{ if \code{TRUE}, return \code{NULL} if the class is

    undefined; otherwise, an undefined class results in an error.}

  \item{where}{ environment from which to begin the search for the definition; by default,

    start at the top-level (global) environment and proceed through

    the search list.}

  \item{package}{ the name or environment of the package asserted to hold the

      definition.  If it is a non-empty string it is used instead of

      \code{where}, as the first place to look for the class.

      Note that the package must be loaded but need not be attached.  By

      default, the package attribute of the \code{Class} argument is

      used, if any.  There will usually be a package attribute if

      \code{Class} comes from \code{class(x)} for some object.

    }

  \item{inherits}{logical; should the class definition be retrieved from

    any enclosing environment and also from the cache?  If \code{FALSE}

    only a definition in the environment \code{where} will be returned.}

}

\details{

  Class definitions are stored in metadata objects in a package

  namespace or other environment where they are defined.  When

  packages are loaded, the class definitions in the package are cached in an internal

  table.  Therefore, most calls to \code{getClassDef} will find the

  class in the cache or fail to find it at all, unless \code{inherits}

  is \code{FALSE}, in which case only the environment(s) defined by

  \code{package} or \code{where} are searched.

  The class cache allows for multiple definitions of the

  same class name in separate environments, with of course the

  limitation that the package attribute or package name must be

  provided in the call to

}

\value{

  The object defining the class. If the class definition is not found,

  \code{getClassDef} returns \code{NULL}, while \code{getClass}, which

  calls \code{getClassDef}, either generates an error or, if

  \code{.Force} is \code{TRUE}, returns a simple definition for the

  class.  The latter case is used internally, but is not typically

  sensible in user code.

  The non-null returned value is an object of class

  \code{\linkS4class{classRepresentation}}.

  Use functions such as \code{\link{setClass}} and

  \code{\link{setClassUnion}} to create class definitions.

}

\references{

 Chambers, John M. (2016)

 \emph{Extending R},

  Chapman & Hall.

(Chapters 9 and 10.)

}

\seealso{

  \linkS4class{classRepresentation},

  \code{\link{setClass}},

  \code{\link{isClass}}.

}

\examples{

getClass("numeric") ## a built in class

cld <- getClass("thisIsAnUndefinedClass", .Force = TRUE)

cld ## a NULL prototype

## If you are really curious:

utils::str(cld)

## Whereas these generate errors:

try(getClass("thisIsAnUndefinedClass"))

try(getClassDef("thisIsAnUndefinedClass"))

}

\keyword{programming}

\keyword{classes}