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% File src/library/methods/man/className.Rd
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% Part of the R package, http://www.R-project.org
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% Copyright 2011-2 R Core Team
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% Distributed under GPL 2 or later
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\name{className}
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\alias{className}
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\alias{multipleClasses}
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\alias{className-class}
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\title{
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Class names including the corresponding package
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}
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\description{
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The function \code{className()} generates a
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valid references to a class, including the name of the package
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containing the class definition. The object returned, from class \code{"className"}, is the
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unambiguous way to refer to a class, for example when calling
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\code{\link{setMethod}}, just in case multiple definitions of the
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class exist.
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Function
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\code{"multipleClasses"} returns information about multiple
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definitions of classes with the
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same name from different packages.
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}
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\usage{
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className(class, package)
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multipleClasses(details = FALSE)
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}
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\arguments{
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\item{class, package}{
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The character string name of a class and, optionally, of the package
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to which it belongs. If argument \code{package} is missing and the
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\code{class} argument has a package slot, that is used (in
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particular, passing in an object from class \code{"className"} returns
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itself in this case, but changes the package slot if the second
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argument is supplied).
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If there is no package argument or slot, a
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definition for the class must exist and will be used to define the
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package. If there are multiple definitions, one will be chosen and a
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warning printed giving the other possibilities.
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}
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\item{details}{
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If \code{FALSE}, the default, \code{multipleClasses()} returns a
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character vector of those classes currently known with multiple
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definitions.
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If \code{TRUE}, a named list of those class definitions is returned.
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Each element of the list is itself a list of the corresponding class
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definitions, with the package names as the names of the list. Note
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that identical class definitions will not be considered
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\dQuote{multiple} definitions (see the discussion of the details below).
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}
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}
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\details{
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The table of class definitions used internally can maintain multiple
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definitions for classes with the same name but coming from different
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packages.
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If identical class definitions are encountered, only one class
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definition is kept; this occurs most often with S3 classes that have
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been specified in calls to \code{\link{setOldClass}}. For true
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classes, multiple class definitions are unavoidable in general if two
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packages happen to have used the same name, independently.
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Overriding a class definition in another package with the same name deliberately is usually a bad
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idea.
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Although \R attempts to keep and use the two definitions (as of
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version 2.14.0), ambiguities are always possible. It is more
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sensible to define a new class that extends an existing class but has
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a different name.
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}
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\value{
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A call to \code{className()} returns an object from class
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\code{"className"}.
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A call to \code{multipleClasses()} returns either a character
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vector or a named list of class definitions. In either case, testing
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the length of the returned value for being greater than \code{0} is a
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check for the existence of multiply defined classes.
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}
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\section{Objects from the Class}{
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The class \code{"className"} extends \code{"character"} and has a slot
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\code{"package"}, also of class \code{"character"}.
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}
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\examples{
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\dontrun{
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className("vector") # will be found, from package "methods"
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className("vector", "magic") # OK, even though the class doesn't exist
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className("An unknown class") # Will cause an error
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}
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}
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\keyword{ classes }
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\keyword{ programming }
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