setClass               package:methods               R Documentation

Create a Class Definition

Description:

     Create a formally defined class with specified slots and/or
     relationships to other classes.  Also functions to remove a class
     definition, to test whether a class has been defined, to test
     whether an object is a class definition, and to reset the internal
     definition of a class.

Usage:

     setClass(Class, representation, prototype,
              contains=character(), validity, access, where=1, version=FALSE)

     removeClass(Class, where=-1)

     isClass(Class, formal=TRUE)
     isClassDef(object)

     getClasses(where)

     resetClass(Class)

Arguments:

   Class: character string name for the class 

representation: the slots that the new class should have and/or other
          classes that this class extends.  Usually a call to the
          `representation' function. 

prototype: an object (usually a list) providing the default data for
          the slots specified in the representation. 

contains: what classes does this class extend?  (These are called
          superclasses in some languages.)  When these classes have
          slots, all their slots will be contained in the new class as
          well. 

   where: What environment to use to store or remove the definition (as
          metadata). By default, uses the global environment for
          `setClass' and searches for a definition to remove, for
          `removeClass'. 

validity, access, version: Control arguments included for compatibility
          with the S-Plus API, but not currently used.  

       x: an arbitrary object.

  formal: Should a formal definition be required? 

  object: any R object. 

Details:

     These are the functions that create and manipulate formal class
     definitions.  Brief documentation is provided below.  See the
     references for an introduction and for more details.

     `setClass': Define Class to be an S-style class.  The effect is to
          create an object, of class `"classRepEnvironment"', and store
          this (hidden) in the specified environment or database. 
          Objects can be created from the class (e.g., by calling
          `new'), manipulated (e.g., by accessing the object's slots),
          and methods may be defined including the class name in the
          signature (see `setMethod').


     `removeClass': Remove the definition of this class.  Calling this
          always resets the version of the class cached for the
          session.  If `where=0', that's all it does.  Otherwise, it
          removes the version from the specified environment or
          database (from the global environment by default).


     `isClass': Is this a the name of a formally defined class?
          (Argument `formal' is for compatibility and is ignored.)


     `isClassDef': Is this object a class definition (it will be, for
          example, if it is the value of a call to `getClass', the
          complete definition of a class with its extensions, or to
          `getClassDef', the local definition  of the class).


     `getClasses': The names of all the classes formally defined on
          `where'. If called with no argument, all the classes
          currently known in the session (which does not include
          classes that may be defined on one of the attached libraries,
          but have not yet been used in the session).


     `unclass': Returns the object containing the values of all the
          slots in this object's class definition (specifically, ithe
          returned object has attributes corresponding to each slot),
          in the case that the object's class is formally defined with
          slots.  

          For classes that extend a single other class (e.g., a basic
          class such as `"numeric"') the result is an object of that
          class.

     `resetClass': Reset the internal definition of a class.  The
          effect is that the next time the definition of this class is
          needed, it will be recomputed from the information on the
          attached databases.

          This function is called when aspects of the class definition
          are changed.  You would need to call it explicitly if you
          changed the definition of a class that this class extends
          (but doing that in the  middle of a session is living
          dangerously, since it may invalidate existing objects).

Inheritance and Prototypes:

     Defining new classes that inherit from (``extend'') other classes
     is a powerful technique, but has to be used carefully and not
     over-used. Otherwise, you will often get unintended results when
     you start to compute with objects from the new class.

     As shown in the examples below, the simplest and safest form of
     inheritance is to start with an explicit class, with some slots,
     that does not extend anything else.  It only does what we say it
     does.

     Then extensions will add some new slots and new behavior.

     Another variety of extension starts with one of the basic classes,
     perhaps with the intension of modifying R's standard behavior for
     that class.  Perfectly legal and sometimes quite helpful, but you
     may need to be more careful in this case:  your new class will
     inherit much of the behavior of the basic (informally defined)
     class, and the results can be surprising.  Just proceed with
     caution and plenty of testing.

     As an example, the class `"matrix"' is included in the pre-defined
     classes, to behave essentially as matrices do without formal class
     definitions.  Suppose we don't like all of this; in particular, we
     want the default matrix to have 0 rows and columns (not 1 by 1 as
     it is now).

     `setClass("myMatrix", "matrix", prototype = matrix(0,0,0))'

     The arguments above illustrate two short-cuts relevant to such
     examples.  We abbreviated the `representation' argument to the
     single superclass, because the new class doesn't add anything to
     the representation of class `"matrix"'.  Also, we provided an
     object from the superclass as the prototype, not a list of slots.

Author(s):

     John Chambers

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 Programming with Data, (John M. Chambers,
     Springer, 1998). See this book for further details and examples.

See Also:

     `Methods', `setSClass'

Examples:

     ## A simple class with two slots
     setClass("track",
                 representation(x="numeric", y="numeric"))
     ## A class extending the previous, adding one more slot
     setClass("trackCurve",
                 representation("track", smooth = "numeric"))
     ## A class similar to "trackCurve", but with different structure
     ## allowing matrices for the "y" and "smooth" slots
     setClass("trackMultiCurve", representation(x="numeric", y="matrix", smooth="matrix"),
               prototype = list(x=numeric(), y=matrix(0,0,0), smooth= matrix(0,0,0)))
     ##
     ## Suppose we want  trackMultiCurve to be like trackCurve when there's only
     ## one column
     ## First, the wrong way. 
     try(setIs("trackMultiCurve", "trackCurve",
       test = function(obj) {ncol(slot(obj, "y")) == 1}))

     ## why didn't that work?  You can only override the slots "x", "y", and "smooth"
     ## if you provide an explicit coerce function to correct any inconsistencies:

     setIs("trackMultiCurve", "trackCurve",
       test = function(obj) {ncol(slot(obj, "y")) == 1},
       coerce = function(obj) { new("trackCurve", x = slot(obj, "x"),
             y = as.numeric(slot(obj,"y")), smooth = as.numeric(slot(obj, "smooth")))})

