methods (version 3.3.1)

setClassUnion: Classes Defined as the Union of Other Classes


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.


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


the name for the new union class.
the classes that should be members of this union.
where to save the new class definition; by default, the environment of the package in which the setClassUnion call appears, or the global environment if called outside of the source of a package.
the name or definition of a class.


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.

The prototype object in the class union definition will be NULL if class "NULL" is a member of the union and the prototype object of the first member class otherwise (as of version 2.15.0 of R; earlier versions had a NULL prototype even if that was not valid).

Class unions are the only way to create a class that is extended by a class whose definition is sealed (for example, the basic datatypes or other classes defined in the base or methods package in R are sealed). You cannot say setIs("function", "other") unless "other" is a class union. In general, a setIs call of this form changes the definition of the first class mentioned (adding "other" to the list of superclasses contained in the definition of "function").

Class unions get around this by not modifying the first class definition, relying instead on storing information in the subclasses slot of the class union. In order for this technique to work, the internal computations for expressions such as extends(class1, class2) work differently for class unions than for regular classes; specifically, they test whether any class is in common between the superclasses of class1 and the subclasses of class2.

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.


Chambers, John M. (2008) Software for Data Analysis: Programming with R Springer. (For the R version.)

Chambers, John M. (1998) Programming with Data Springer (For the original S4 version.)


Run this code
## 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", representation("maybeNumber", 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"
    c("expression", "language", "OptionalFunction"))

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

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