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.
the name for the new union class.
the names of the classes that should be members of this union.
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.
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.
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
"numeric", for example, would cause an error.
Class unions are the exception; the class union
"maybeNumber" in the examples defines itself as a new
"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
Chambers, John M. (2016) Extending R, Chapman & Hall. (Chapters 9 and 10.)
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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