R: Is an Object from a Class

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, fullInfo = FALSE)

    setIs(class1, class2, test=NULL, coerce=NULL, replace=NULL,
          by = character(), where = topenv(parent.frame()), classDef =,
          extensionObject = NULL, doComplete = TRUE)

### Arguments

`object` | any R object.
`class1, class2` | the names of the classes between which `is` relations are to be defined.
`maybe, fullInfo` | In a call to `extends`, `maybe` is a flag to include/exclude conditional relations, and `fullInfo` is a flag, which if `TRUE` causes object(s) of class `classExtension` to be returned, rather than just the names of the classes or a logical value. See the details below.
`extensionObject` | alternative to the `test, coerce, replace, by` arguments; an object from class `SClassExtension` describing the relation. (Used in internal calls.)
`doComplete` | when `TRUE`, the class definitions will be augmented with indirect relations as well. (Used in internal calls.)
`test, coerce, replace` | In a call to `setIs`, 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` | In a call to `setIs`, 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` | In a call to `setIs`, where to store the metadata defining the relationship. Default is the global environment.
`classDef` | Optional class definition for `class` , required internally when `setIs` is called during the initial definition of the class by a call to `setClass`. Don't use this argument, unless you really know why you're doing so.

### Details

`extends`:

  Given two class names, `extends` by default says whether the first class extends the second; that is, it does for class names what `is` does for an object and a class. Given one class name, it returns all the classes that class extends (the ‘superclasses’ of that class), including the class itself. If the flag `fullInfo` is `TRUE`, the result is a list, each element of which is an object describing the relationship; otherwise, and by default, the value returned is only the names of the classes.

`setIs`:

  This function establishes an inheritance relation between two classes, by some means other than having one class contain the other. It should not be used for ordinary relationships: either include the second class in the `contains=` argument to `setClass` if the class is contained in the usual way, or consider `setClassUnion` to define a virtual class that is extended by several ordinary classes. A call to `setIs` makes sense, for example, if one class ought to be automatically convertible into a second class, but they have different representations, so that the conversion must be done by an explicit computation, not just be inheriting slots, for example. In this case, you will typically need to provide both a `coerce=` and `replace=` argument to `setIs`.

  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.

  The `coerce` argument is a function that turns a `class1` object into a `class2` object. The `replace` argument is a function of two arguments that modifies a `class1` object (the first argument) to replace the part of it that corresponds to `class2` (supplied as `value`, the second argument). It then returns the modified object as the value of the call. In other words, it acts as a replacement method to implement the expression `as(object, class2) <- value`.

  The easiest way to think of the `coerce` and `replace` functions is by thinking of the case that `class1` contains `class2` in the usual sense, by including the slots of the second class. (To repeat, in this situation you would not call `setIs`, but the analogy shows what happens when you do.)

  The `coerce` function in this case would just make a `class2` object by extracting the corresponding slots from the `class1` object. The `replace` function would replace in the `class1` object the slots corresponding to `class2`, and return the modified object as its value.

  The relationship can also be conditional, if a function is supplied as the `test` argument. This should be a function of one argument that returns `TRUE` or `FALSE` according to whether the object supplied satisfies the relation `is(object, class2)`. If you worry about such things, conditional relations between classes are slightly deprecated because they cannot be implemented as efficiently as ordinary relations and because they sometimes can lead to confusion (in thinking about what methods are dispatched for a particular function, for example). But they can correspond to useful distinctions, such as when two classes have the same representation, but only one of them obeys certain additional constraints.

  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.

### References

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 see `?Methods` and the links from that documentation.

### Examples

    ## a class definition (see 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)))
    ## Automatically convert an object from class "trackCurve" into
    ## "trackMultiCurve", by making the y, smooth slots into 1-column matrices
    setIs("trackCurve",
          "trackMultiCurve",
          coerce = function(obj) {
            new("trackMultiCurve",
                x = obj@x,
                y = as.matrix(obj@y),
                curve = as.matrix(obj@smooth))
          },
          replace = function(obj, value) {
            obj@y <- as.matrix(value@y)
            obj@x <- value@x
            obj@smooth <- as.matrix(value@smooth)
            obj})

    ## Automatically convert the other way, but ONLY
    ## if the y data is one variable.
    setIs("trackMultiCurve",
          "trackCurve",
          test = function(obj) {ncol(obj@y) == 1},
          coerce = function(obj) {
            new("trackCurve",
                x = slot(obj, "x"),
                y = as.numeric(obj@y),
                smooth = as.numeric(obj@smooth))
          },
          replace = function(obj, value) {
            obj@y <- matrix(value@y, ncol=1)
            obj@x <- value@x
            obj@smooth <- value@smooth
            obj})

[Package methods version 2.6.2 Index]