Classes Defined as the Union of Other Classes

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Description

A class may be defined as the union of other classes; that is, as a virtual class defined as a superclass of several other classes. Class unions are useful in method signatures or as slots in other classes, when we want to allow one of several classes to be supplied.

Usage

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setClassUnion(name, members, where)
isClassUnion(Class)

Arguments

name

the name for the new union class.

members

the names of the classes that should be members of this union.

where

where to save the new class definition. In calls from a package's source code, should be omitted to save the definition in the package's namespace.

Class

the name or definition of a class.

Details

The classes in members must be defined before creating the union. However, members can be added later on to an existing union, as shown in the example below. Class unions can be members of other class unions.

Class unions are the only way to create a new superclass of a class whose definition is sealed. The namespace of all packages is sealed when the package is loaded, protecting the class and other definitions from being overwritten from another class or from the global environment. A call to setIs that tried to define a new superclass for class "numeric", for example, would cause an error.

Class unions are the exception; the class union "maybeNumber" in the examples defines itself as a new superclass of "numeric". Technically, it does not alter the metadata object in the other package's namespace and, of course, the effect of the class union depends on loading the package it belongs to. But, basically, class unions are sufficiently useful to justify the exemption.

The different behavior for class unions is made possible because the class definition object for class unions has itself a special class, "ClassUnionRepresentation", an extension of class classRepresentation.

References

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

Examples

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## a class for either numeric or logical data
setClassUnion("maybeNumber", c("numeric", "logical"))

## use the union as the data part of another class
setClass("withId", contains = "maybeNumber", slots = c(id = "character"))

w1 <- new("withId", 1:10, id = "test 1")
w2 <- new("withId", sqrt(w1)%%1 < .01, id = "Perfect squares")

## add class "complex" to the union "maybeNumber"
setIs("complex", "maybeNumber")

w3 <- new("withId", complex(real = 1:10, imaginary = sqrt(1:10)))

## a class union containing the existing class  union "OptionalFunction"
setClassUnion("maybeCode",
    c("expression", "language", "OptionalFunction"))

is(quote(sqrt(1:10)), "maybeCode")  ## TRUE

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