| setOldClass {methods} | R Documentation |
Register an old-style (a.k.a. ‘S3’) class as a formally defined
class. The Classes argument is the character vector used as the
class attribute; in particular, if there is more than one
string, old-style class inheritance is mimicked. Registering via
setOldClass allows S3 classes to appear in method
signatures, as a slot in an S4 class, or as a superclass of an S4 class.
setOldClass(Classes, prototype, where, test = FALSE, S4Class)
Classes |
A character vector, giving the names for old-style
classes, as they would appear on the right side of an assignment of
the |
prototype |
An optional object to use as the prototype. This should be provided
as the default S3 object for the class. If omitted, the S4 class
created to register the S3 class is |
where |
Where to store the class definitions, the global or top-level environment by default. (When either function is called in the source for a package, the class definitions will be included in the package's environment by default.) |
test |
flag, if |
S4Class |
optionally, the class definition or the class name
of an S4 class. The new class will have all the slots and other
properties of this class, plus its S3 inheritance as defined by
the |
Each of the names will be defined as an S4 class, extending the
remaining classes in Classes, and the class oldClass,
which is the ‘root’ of all old-style classes. S3 classes have
no formal definition, and therefore no formally defined slots. If a
prototype argument is supplied in the call to
setOldClass(), objects from the class can be generated, by a
call to new; however, this usually not as relevant as
generating objects from subclasses (see the section on extending S3
classes below). If a prototype is not provided,
the class will be created as a virtual S4
class.
The main disadvantage is that the prototype object in an S4 class that
uses this class as a slot will have a NULL object in that slot,
which can sometimes lead to confusion.
Beginning with version 2.8.0 of R, support is provided for using a (registered) S3 class as a super class of a new S4 class. See the section on extending S3 classes below, and the examples.
See Methods for the details of method dispatch and inheritance.
Some S3 classes cannot be represented as an ordinary combination of S4
classes and superclasses, because objects from the S3 class can have a
variable set of strings in the class. It is still possible to register
such classes as S4 classes, but now the inheritance has to be verified
for each object, and you must call setOldClass with argument
test=TRUE once for each superclass.
For example, ordered factors always have the S3
class c("ordered", "factor"). This is proper behavior, and
maps simply into two S4 classes, with "ordered" extending
"factor".
But objects whose class attribute has "POSIXt" as the first
string may have either (or neither) of "POSIXct" or
"POSIXlt" as the second string. This behavior can be mapped
into S4 classes but now to evaluate is(x, "POSIXlt"), for
example, requires checking the S3 class attribute on each object.
Supplying the test=TRUE argument to setOldClass causes
an explicit test to be included in the class definitions. It's
never wrong to have this test, but since it adds significant
overhead to methods defined for the inherited classes, you should
only supply this argument if it's known that object-specific tests
are needed.
The list .OldClassesList contains the old-style classes that
are defined by the methods package. Each element of the list is an
old-style list, with multiple character strings if inheritance is
included.
Each element of the list was passed to setOldClass when
creating the methods package; therefore, these classes can be used
in setMethod calls, with the inheritance as implied by
the list.
A call to setOldClass creates formal classes corresponding
to S3 classes, allows these to be used as slots in other classes or in
a signature in setMethod, and mimics the S3
inheritance.
In documentation for the initial implementation of S4 classes in R, users were warned against defining S4 classes that contained S3 classes, even if those had been registered. The warning was based mainly on two points. 1: The S3 behavior of the objects would fail because the S3 class would not be visible, for example, when S3 methods are dispatched. 2: Because S3 classes have no formal definition, nothing can be asserted in general about the S3 part of an object from such a class. (The warning was repeated as recently as the first reference below.)
Nevertheless, defining S4 classes to contain an S3 class and extend its behavior is attractive in many applications. The alternative is to be stuck with S3 programming, without the flexibility and security of formal class and method definitions.
Beginning with version 2.8.0, R provides support for extending
registered S3 classes; that is, for new classes defined by a call to
setClass in which the contains= argument
includes an S3 class. See the examples below. The support is aimed
primarily at providing the S3 class information for all classes that
extend class oldClass, in particular by ensuring that
all objects from such classes contain the S3 class in a special
slot.
There are three different ways to indicate an extension to an existing
S3 class: setOldClass(), setClass() and
setIs(). In most cases, calling setOldClass is the
best approach, but the alternatives may be preferred in the special
circumstances described below.
Suppose "A" is any class extending "oldClass". then
setOldClass(c("B", "A"))
creates a new class "B" whose S3 class
concatenates "B" with S3Class("A"). The new class is a
virtual class. If "A" was defined with known attribute/slots,
then "B" has these slots also; therefore, you must believe that
the corresponding S3 objects from class "B" do indeed have the
claimed attributes. Notice that you can supply an S4 definition for the
new class to specify additional attributes (as described in the next
section.)
The first alternative call produces a non-virtual class.
setClass("B", contains = "A")
This creates a non-virtual class with the same slots and superclasses
as class "A". However, class "B" is not included in
the S3 class slot of the new class, unless you provide it explicitly
in the prototype.
setClass("B"); setIs("B", "A", .....)
This creates a virtual class that extends "A", but does not
contain the slots of "A". The additional arguments to
setIs should provide a coerce and replacement method.
In order for the new class to inherit S3 methods, the coerce method must
ensure that the class "A" object produced has a suitable S3
class. The only likely reason to prefer this third approach is that
class "B" is not consistent with known attributes in class
"A".
Beginning with version 2.9.0 of R, objects from a class extending an
S3 class will be converted to the corresponding S3 class when being
passed to an S3 method defined for that class (that is, for one of the
strings in the S3 class attribute). This is intended to ensure, as
far as possible, that such methods will work if they work for ordinary
S3 objects. See Classes for details.
A further specification of an S3 class can be made if the class is guaranteed to have some attributes of known class (where as with slots, “known” means that the attribute is an object of a specified class, or a subclass of that class).
In this case, the call to setOldClass() can supply an S4 class
definition representing the known structure. Since S4 slots are
implemented as attributes (largely for just this reason), the know
attributes can be specified in the representation of the S4 class.
The usual technique will be to create an S4 class with the desired
structure, and then supply the class name or definition as the
argument S4Class to setOldClass().
See the definition of class "ts" in the examples below. The
call to setClass to create the S4 class can use the same class name, as here,
so long as the class definition is not sealed. In the example, we
define "ts" as a vector structure with a numeric slot for
"tsp". The validity of this definition relies on an
assertion that all the S3 code for this class is consistent with
that definition; specifically, that all "ts" objects will
behave as vector structures and will have a numeric "tsp"
attribute. We believe this to be true of all the base code in R,
but as always with S3 classes, no guarantee is possible.
The S4 class definition can have virtual superclasses (as in
the "ts" case) if the S3 class is asserted to behave
consistently with these (in the example, time-series objects are
asserted to be consistent with the structure class).
For another example, look at the S4 class definition for "data.frame".
Be warned that failures of the S3 class to live up to its asserted
behavior will usually go uncorrected, since S3 classes inherently
have no definition, and the resulting invalid S4 objects can cause
all sorts of grief. Many S3 classes are not candidates for known
slots, either because the presence or class of the attributes are
not guaranteed (e.g., dimnames in arrays, although these are
not even S3 classes), or because the class uses named components of
a list rather than attributes (e.g., "lm"). An attribute
that is sometimes missing cannot be represented as a slot, not even
by pretending that it is present with class "NULL", because
attributes unlike slots can not have value NULL.
One irregularity that is usually tolerated, however, is to
optionally add other attributes to those guaranteed to exist (for
example, "terms" in "data.frame" objects returned by
model.frame). As of version 2.8.0, validity checks by
validObject ignore extra attributes; even if this
check is tightened in the future, classes extending S3 classes
would likely be exempted because extra attributes are so common.
Chambers, John M. (2008) Software for Data Analysis: Programming with R Springer. (For the R version: see section 10.6 for method selection and section 13.4 for generic functions).
Chambers, John M. (1998) Programming with Data Springer (For the original S4 version.)
setClass, setMethod
require(stats)
setOldClass(c("mlm", "lm"))
setGeneric("dfResidual", function(model)standardGeneric("dfResidual"))
setMethod("dfResidual", "lm", function(model)model$df.residual)
## dfResidual will work on mlm objects as well as lm objects
myData <- data.frame(time = 1:10, y = (1:10)^.5)
myLm <- lm(cbind(y, y^3) ~ time, myData)
## two examples extending S3 class "lm", class "xlm" directly and "ylm" indirectly
setClass("xlm", representation(eps = "numeric"), contains = "lm")
setClass("ylm", representation(header = "character"), contains = "xlm")
ym1 = new("ylm", myLm, header = "Example", eps = 0.)
## for more examples, see ?\link{S3Class}.
## Examples of S3 classes with guaranteed attributes
## an S3 class "stamped" with a vector and a "date" attribute
## Here is a generator function and an S3 print method.
## NOTE: it's essential that the generator checks the attribute classes
stamped <- function(x, date = Sys.time()) {
if(!inherits(date, "POSIXt"))
stop("bad date argument")
if(!is.vector(x))
stop("x must be a vector")
attr(x, "date") <- date
class(x) <- "stamped"
x
}
print.stamped <- function(x, ...) {
print(as.vector(x))
cat("Date: ", format(attr(x,"date")), "\n")
}
## Now, an S4 class with the same structure:
setClass("stamped4", contains = "vector", representation(date = "POSIXt"))
## We can use the S4 class to register "stamped", with its attributes:
setOldClass("stamped", S4Class = "stamped4")
selectMethod("show", "stamped")
## and then remove "stamped4" to clean up
removeClass("stamped4")
someLetters <- stamped(sample(letters, 10), ISOdatetime(2008, 10, 15, 12, 0, 0))
st <- new("stamped", someLetters)
st # show() method prints the object's class, then calls the S3 print method.
stopifnot(identical(S3Part(st, TRUE), someLetters))
# creating the S4 object directly from its data part and slots
new("stamped", 1:10, date = ISOdatetime(1976, 5, 5, 15, 10, 0))
## Not run:
## The code in R that defines "ts" as an S4 class
setClass("ts", contains = "structure",
representation(tsp = "numeric"),
prototype(NA, tsp = rep(1,3))) # prototype to be a legal S3 time-series
## and now registers it as an S3 class
setOldClass("ts", S4Class = "ts", where = envir)
## End(Not run)