setClass {methods} | R Documentation |
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.
setClass(Class, representation, prototype, contains=character(), validity, access, where=1, version=FALSE, sealed, package) removeClass(Class, where=-1, removeSubclassLinks = TRUE) isClass(Class, formal=TRUE) isClassDef(object) getClasses(where) findClass(Class) resetClass(Class, resetSubclasses = TRUE)
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. |
sealed |
If TRUE , the class definition will be sealed,
so that another call to setClass will fail on this class name.
|
package |
An optional package name for the class. By default (and usually) the package where the class definition is assigned will be used. |
formal |
Should a formal definition be required? |
object |
any R object. |
removeSubclassLinks |
When a class is removed, any links
to that class from other classes will become invalid. If
this argument is not supplied as FALSE , then
removeClass will search for all such links and delete
them. You can omit the argument, or supply it as the
positions in the search list to look for these links (by default
all attached object tables will be searched). |
resetSubclasses |
Should resetClass also reset all known
subclasses. Usually TRUE , unless you know from the context
these will be reset elsewhere.
|
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
: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
: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
:formal
is for compatibility and is ignored.)
isClassDef
:getClass
, the complete
definition of a class with its extensions, or to
getClassDef
, the local definition of the class).
getClasses
: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 packages, but have not yet been used in the
session).
findClass
:Class
is defined. (If there is more than one definition, all
corresponding elements of the search list are returned.)
unclass
:"numeric"
) the result is an object of that class.
resetClass
: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).
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.
The R package methods
implements, with a few exceptions, the
programming interface for classes and methods in the book
Programming with Data (John M. Chambers, Springer, 1998), in
particular sections 1.6, 2.7, 2.8, and chapters 7 and 8.
While the programming interface for the methods package follows the reference, the R software is an original implementation, so details in the reference that reflect the S4 implementation may appear differently in R. Also, there are extensions to the programming interface developed more recently than the reference. For a discussion of details and ongoing development, see the web page http://developer.r-project.org/methodsPackage.html and the pointers from that page.
Methods
,
makeClassRepresentation
## 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"))) })