| setClass {methods} | R Documentation |
Create a class definition, specifying the representation (the
slots) and/or the classes contained in this one (the superclasses),
plus other optional details. As a side effect, the class definition
is stored in the specified environment. A generator function
is returned as the value of setClass(), suitable for creating
objects from the class if the class is not virtual. Of the many
arguments to the function only Class,
representation and contains are usually needed.
setClass(Class, representation, prototype, contains=character(),
validity, access, where, version, sealed, package,
S3methods = FALSE)
Class |
character string name for the class. |
representation |
a named list of the slots that the new class should have, the names giving the names of the slots and the corresponding elements being the character string names of the corresponding classes.
Usually a call to
the Backward compatibility and compatibility with S-Plus allows unnamed
elements for superclasses, but the recommended style is to use the
|
prototype |
an object providing the default
data for the slots in this class. By default, each will be the
prototype object for the superclass. If provided, using a call to
|
contains |
the names (and optionally package slots) for the superclasses of this class. |
where |
the
environment in which to store the
definition. Should not be supplied in standard use. For calls to |
validity |
if supplied, should be a validity-checking method
for objects from this class (a function that returns |
access, version |
access and version, included for historical compatibility with S-Plus, but ignored. |
sealed |
if |
package |
an optional package name for the class. Should very rarely be used. By default the name of the package in which the class definition is assigned. |
S3methods |
if |
A generator function suitable for creating objects from the class is
returned, invisibly. A call to this function generates a call to
new for the class. The call takes any number of arguments,
which will be passed on to the initialize method. If no
initialize method is defined for the class or one of its
superclasses, the default method expects named arguments with the
name of one of the slots.
Typically the generator function is assigned the name of the class,
for programming clarity. This is not a requirement and objects
from the class can also be generated directly from
new. The advantages of the generator function are a
slightly simpler and clearer call, and that the call will contain
the package name of the class (eliminating any ambiguity if two
classes from different packages have the same name).
If the class is virtual, an attempt to generate an object from
either the generator or new()
will result in an error.
The two essential arguments other than the class name are
representation and contains, defining the explicit slots
and the inheritance (superclasses). Together, these arguments define
all the information in an object from this class; that is, the names
of all the slots and the classes required for each of them.
The name of the class determines which methods apply directly to objects from this class. The inheritance information specifies which methods apply indirectly, through inheritance. See Methods.
The slots in a class definition will be the union of all the slots
specified directly by representation and all the slots in all
the contained classes.
There can only be one slot with a given name; specifically, the
direct and inherited slot names must be unique.
That does not, however, prevent the same class from being inherited
via more than one path.
One kind of element in the contains= argument is special, specifying one of the R
object types or one of a few other special R types (matrix and
array).
See the section on inheriting from object types, below.
Slot name "class" is not allowed in the current implementation but
reserved. "Class" is valid, but undesirable, as it cannot be
used in new(<cl>, Class = <slot-value>) (because of
argument name matching).
There are other slot names with a special meaning; these names start with
the "." character. To be safe, you should define all of
your own slots with names starting with an alphabetic character.
In addition to containing other S4 classes, a class definition can
contain either an S3 class (see the next section) or a built-in R pseudo-class—one
of the R
object types or one of the special R pseudo-classes "matrix" and
"array".
A class can contain at most one of the object types, directly or indirectly.
When it does, that contained class determines the “data part”
of the class.
Objects from the new class try to inherit the built in
behavior of the contained type.
In the case of normal R data types, including vectors, functions and
expressions, the implementation is relatively straightforward.
For any object x from the class,
typeof(x) will be the contained basic type; and a special
pseudo-slot, .Data, will be shown with the corresponding class.
See the "numWithId" example below.
Classes may also inherit from "vector", "matrix" or
"array".
The data part of these objects can be any vector data type.
For an object from any class that does not contain one of these
types or classes,
typeof(x) will be "S4".
Some R data types do not behave normally, in the sense that they are
non-local references or other objects that are not duplicated.
Examples include those corresponding to classes "environment", "externalptr", and "name".
These can not be the types for objects with user-defined
classes (either S4 or S3) because setting an attribute overwrites the
object in all contexts.
It is possible to define a class that inherits from such types,
through an indirect mechanism that stores the inherited object in a
reserved slot.
See the
example for class "stampedEnv" below.
S3 method dispatch and the relevant as.type()
functions should behave correctly, but code that uses the type of the
object directly will not.
Also, keep in mind that the object passed to low-level computations will be the underlying object type, without any of the slots defined in the class. To return the full information, you will usually have to define a method that sets the data part.
Old-style S3 classes have no formal definition. Objects are “from” the class when their class attribute contains the character string considered to be the class name.
Using such classes with formal classes and methods is necessarily a
risky business, since there are no guarantees about the content of the
objects or about consistency of inherited methods.
Given that, it is still possible to define a class that inherits from
an S3 class, providing that class has been registered as an old class
(see setOldClass).
Broadly speaking, both S3 and S4 method dispatch try to behave
sensibly with respect to inheritance in either system.
Given an S4 object, S3 method dispatch and the inherits
function should use the S4 inheritance information.
Given an S3 object, an S4 generic function will dispatch S4 methods
using the S3 inheritance, provided that inheritance has been declared via
setOldClass.
Class definitions normally belong to packages (but can be defined in
the global environment as well, by evaluating the expression on the
command line or in a file sourced from the command line).
The corresponding package name is part of the class definition; that
is, part of the classRepresentation object holding that
definition. Thus, two classes with the same name can exist in
different packages, for most purposes.
When a class name is supplied for a slot or a superclass in a call to
setClass, a
corresponding class definition will be found, looking from the
namespace of the current package, assuming the call in question appears directly in the source for the
package, as it should to avoid ambiguity.
The class definition
must be found in the namespace of the current package, in the imports for that
namespace or in the basic classes defined by the methods package.
(The methods package must be included in the Depends directive
of the package's "DESCRIPTION" file in order for the
"CMD check" utility to find these classes.)
When this rule does not identify a class uniquely (because it appears
in more than one imported package) then the packageSlot
of the character string name needs to be supplied with the name.
This should be a rare occurrence.
Chambers, John M. (2008) Software for Data Analysis: Programming with R Springer. (For the R version.)
Chambers, John M. (1998) Programming with Data Springer (For the original S4 version.)
Classes for a general discussion of classes,
Methods for an analogous discussion of methods,
makeClassRepresentation
## A simple class with two slots
track <- setClass("track",
representation(x="numeric", y="numeric"))
## an object from the class
t1 <- track(x = 1:10, y = 1:10 + rnorm(10))
## A class extending the previous, adding one more slot
trackCurve <- setClass("trackCurve",
representation(smooth = "numeric"),
contains = "track")
## an object containing a superclass object
t1s <- trackCurve(t1, smooth = 1:10)
## 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)))
## See ?setIs for further examples using these classes
## A class that extends the built-in data type "numeric"
numWithId <- setClass("numWithId", representation(id = "character"),
contains = "numeric")
numWithId(1:3, id = "An Example")
## inherit from reference object of type "environment"
stampedEnv <-setClass("stampedEnv", contains = "environment",
representation(update = "POSIXct"))
setMethod("[[<-", c("stampedEnv", "character", "missing"),
function(x, i, j, ..., value) {
ev <- as(x, "environment")
ev[[i]] <- value #update the object in the environment
x@update <- Sys.time() # and the update time
x})
e1 <- stampedEnv(update = Sys.time())
e1[["noise"]] <- rnorm(10)