The R Project SVN R

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

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

% Copyright 1995-2007 R Core Team

% Distributed under GPL 2 or later

\name{BasicClasses}

\docType{class}

\alias{character-class}

\alias{complex-class}

\alias{double-class}

\alias{expression-class}

\alias{externalptr-class}

\alias{integer-class}

\alias{list-class}

\alias{logical-class}

\alias{numeric-class}

\alias{raw-class}

\alias{single-class}

\alias{vector-class}

\alias{NULL-class}

\alias{builtin-class}

\alias{special-class}

\alias{function-class}

\alias{ANY-class}

\alias{VIRTUAL-class}

\alias{missing-class}

\alias{S4-class}

\alias{namedList-class}

\title{Classes Corresponding to Basic Data Types }

\description{

  Formal classes exist corresponding to the basic R object types, allowing

  these types to be used in method signatures, as slots in class

  definitions, and to be extended by new classes.}

\usage{

### The following are all basic vector classes.

### They can appear as class names in method signatures,

### in calls to as(), is(), and new().

"character"

"complex"

"double"

"expression"

"integer"

"list"

"logical"

"numeric"

"single"

"raw"

### the class

"vector"

### is a virtual class, extended by all the above

### the class

"S4"

### is an object type for S4 objects that do not extend

### any of the basic vector classes.  It is a virtual class.

### The following are additional basic classes

"NULL"     #  NULL objects

"function" #  function objects, including primitives

"externalptr" # raw external pointers for use in C code

"ANY"  # virtual classes used by the methods package itself

"VIRTUAL"

"missing"

"namedList" # the alternative to "list" that preserves

            # the names attribute

}

\section{Objects from the Classes}{

  If a class is not virtual (see section in \code{\link{Classes}}),

  objects can be created by calls of the form \code{new(Class, ...)},

  where \code{Class} is the quoted class name, and the remaining

  arguments if any are objects to be interpreted as vectors of this

  class.  Multiple arguments will be concatenated.

  The class \code{"expression"} is slightly odd, in that the \dots

  arguments will \emph{not} be evaluated; therefore, don't enclose them

  in a call to \code{quote()}.

  Note that class \code{"list"} is a pure vector.  Although lists with

  names go back to the earliest versions of S, they are an extension

  of the vector concept in that they have an attribute (which can now

  be a slot) and which is either \code{NULL} or a character vector of

  the same length as the vector.  If you want to guarantee that list

  names are preserved, use class \code{"namedList"}, rather than

  \code{"list"}.  Objects from this class must have a names attribute,

  corresponding to slot \code{"names"},

  of type \code{"character"}.  Internally, R treats names for

  lists  specially, which makes it impractical to have the corresponding slot in

  class \code{"namedList"} be a union of character names and \code{NULL}.

}

\section{Classes and Types}{

The basic classes include classes for the basic R types.  Note that

objects of these types will not usually be S4 objects

(\code{\link{isS4}} will return \code{FALSE}), although objects from

classes that contain the basic class will be S4 objects, still with

the same type.  The type as

returned by \code{\link{typeof}} will sometimes differ from the class,

either just from a choice of terminology (type \code{"symbol"} and

class \code{"name"}, for example) or because there is not a one-to-one

correspondence between class and type (most of the classes that

inherit from class \code{"language"} have type \code{"language"}, for example).

}

\section{Extends}{

  The vector classes extend \code{"vector"}, directly.

}

\section{Methods}{

  \describe{

    \item{coerce}{Methods are defined to coerce arbitrary objects to

      the vector classes, by calling the corresponding basic function, for

      example, \code{as(x, "numeric")} calls \code{as.numeric(x)}. }

  }

}

\keyword{classes}