is {methods} | R Documentation

## Is an Object from a Class?

### Description

Functions to test inheritance relationships between an object and a class or between two classes (`extends`).

### Usage

    is(object, class2)

    extends(class1, class2, maybe = TRUE, fullInfo = FALSE)

### Arguments

`object` | any R object.
`class1, class2` | the names of the classes between which `is` relations are to be examined defined, or (more efficiently) the class definition objects for the classes.
`fullInfo` | In a call to `extends`, with `class2` missing, `fullInfo` is a flag, which if `TRUE` causes a list of objects of class `SClassExtension` to be returned, rather than just the names of the classes. Only the distance slot is likely to be useful in practice; see the ‘Selecting Superclasses’ section;
`maybe` | What to return for conditional inheritance. But such relationships are rarely used and not recommended, so this argument should not be needed.

### Selecting Superclasses

A call to `selectSuperClasses(cl)` returns a list of superclasses, similarly to `extends(cl)`. Additional arguments restrict the class names returned to direct superclasses and/or to non-virtual classes.

Either way, programming with the result, particularly using `sapply`, can be useful.

To find superclasses with more generally defined properties, one can program with the result returned by `extends` when called with one class as argument. By default, the call returns a character vector including the name of the class itself and of all its superclasses. Alternatively, if `extends` is called with `fullInfo = TRUE`, the return value is a named list, its names being the previous character vector. The elements of the list corresponding to superclasses are objects of class `SClassExtension`. Of the information in these objects, one piece can be useful: the number of generations between the classes, given by the `"distance"` slot.

Programming with the result of the call to `extends`, particularly using `sapply`, can select superclasses. The programming technique is to define a test function that returns `TRUE` for superclasses or relationships obeying some requirement. For example, to find only next-to-direct superclasses, use this function with the list of extension objects:

`function(what) is(what, "SClassExtension") && what@distance == 2`

or, to find only superclasses from `"myPkg"`, use this function with the simple vector of names:

`function(what) getClassDef(what)@package == "myPkg"`

Giving such functions as an argument to `sapply` called on the output of `extends` allows you to find superclasses with desired properties. See the examples below.

Note that the function using extension objects must test the class of its argument since, unfortunately for this purpose, the list returned by `extends` includes `class1` itself, as the object `TRUE`.

### References

Chambers, John M. (2016) Extending R, Chapman & Hall. (Chapters 9 and 10.)

### See Also

Although `inherits` is defined for S3 classes, it has been modified so that the result returned is nearly always equivalent to `is`, both for S4 and non-S4 objects. Since it is implemented in C, it is somewhat faster. The only non-equivalences arise from use of `setIs`, which should rarely be encountered.

### Examples

    ## Not run:
    ## this example can be run if package XRPython from CRAN is installed.
    supers <- extends("PythonInterface")
    ## find all the superclasses from package XR
    fromXR <- sapply(supers,
        function(what) getClassDef(what)@package == "XR")
    ## print them
    supers[fromXR]

    ## find all the superclasses at distance 2
    superRelations <- extends("PythonInterface", fullInfo = TRUE)
    dist2 <- sapply(superRelations,
        function(what) is(what, "SClassExtension") && what@distance == 2)
    ## print them
    names(superRelations)[dist2]

    ## End(Not run)

[Package methods version 3.3.2 Index]