is {methods}

R Documentation

Is an Object from a Class

Description

is: With two arguments, tests whether object can be treated as from class2.

With one argument, returns all the super-classes of this object's class.

extends: Does the first class extend the second class? Returns maybe if the extension includes a test.

setIs: Defines class1 to be an extension of class2.

Usage

is(object, class2)

extends(class1, class2, maybe=TRUE)

setIs(class1, class2, test=NULL, coerce=NULL, replace=NULL,
      by = NULL, where = 1)

Arguments

object	Any R object.
class1, class2	The names of the classes between which is relations are to be defined.
maybe	What value to return if the relationship is conditional.
test, coerce, replace	Functions optionally supplied to test whether the relation is defined, to coerce the object to class2, and to alter the object so that is(object, class2) is identical to value.
by	The name of an intermediary class. Coercion will proceed by first coercing to this class and from there to the target class. (The intermediate coercions have to be valid.)
where	Where to store the metadata defining the relationship. Default is the global environment.

Details

setIs:

The relationship can be conditional, if a function is supplied as the test argument. If a function is supplied as the coerce argument, this function will be applied to any class1 object in order to turn it into a class2 object. If the relationship is to be defined indirectly through a third class, this class can be named in the by argument.

Extension may imply that a class1 object contains a class2 object. The default sense of containing is that all the slots of the simpler class are found in the more elaborate one. If the replace argument is supplied as an S replacement function, this function will be used to implement as(obj, class2) <- value.

The coerce, replace, and by arguments behave as described for the setAs function. It's unlikely you would use the by argument directly, but it is used in defining cached information about classes. The value returned (invisibly) by setIs is the extension information, as a list.

Information about setIs relations can be stored in the metadata for either class1 (in the extends information) or in the metadata for class2 (in the subclasses information). For the information to be retained for a future session, one of these classes must be defined in the global environment, since only objects assigned there are saved by save.image. If neither class is defined in environment where, setIs generates an error.

Because only global environment information is saved, it rarely makes sense to give a value other than the default for argument where. One exception is where=0, which modifies the cached (i.e., session-scope) information about the class. Class completion computations use this version, but don't use it yourself unless you are quite sure you know what you're doing.

Author(s)

John Chambers

References

The web page 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.

Examples

## a class definition (see \link{setClass} for the example)
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 = structure(list(), x=numeric(), y=matrix(0,0,0), smooth= matrix(0,0,0)))
## Define a multi-curve to extend a single curve ONLY
## if the y data is one variable.
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")), curve = as.numeric(slot(obj, "curve")))})

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