| Line 18... |
Line 18... |
| 18 |
setClassUnion(name, members, where)
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18 |
setClassUnion(name, members, where)
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isClassUnion(Class)
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19 |
isClassUnion(Class)
|
| 20 |
}
|
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}
|
| 21 |
\arguments{
|
21 |
\arguments{
|
| 22 |
\item{name}{ the name for the new union class. }
|
22 |
\item{name}{ the name for the new union class. }
|
| 23 |
\item{members}{ the classes that should be members of this union.}
|
23 |
\item{members}{ the names of the classes that should be members of this union.}
|
| 24 |
\item{where}{ where to save the new class definition; by default,
|
24 |
\item{where}{ where to save the new class definition. In calls from
|
| 25 |
the environment of the package in which the \code{setClassUnion}
|
- |
|
| 26 |
call appears, or the global environment if called outside of the
|
25 |
a package's source code, should be omitted to save the definition
|
| 27 |
source of a package.}
|
26 |
in the package's namespace.}
|
| 28 |
|
27 |
|
| 29 |
|
28 |
|
| 30 |
\item{Class}{ the name or definition of a class.
|
29 |
\item{Class}{ the name or definition of a class.
|
| 31 |
}
|
30 |
}
|
| 32 |
}
|
31 |
}
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| 34 |
The classes in \code{members} must be defined before creating the
|
33 |
The classes in \code{members} must be defined before creating the
|
| 35 |
union. However, members can be added later on to an existing
|
34 |
union. However, members can be added later on to an existing
|
| 36 |
union, as shown in the example below. Class unions can be
|
35 |
union, as shown in the example below. Class unions can be
|
| 37 |
members of other class unions.
|
36 |
members of other class unions.
|
| 38 |
|
37 |
|
| 39 |
The prototype object in the class union definition will be
|
- |
|
| 40 |
\code{NULL} if class \code{"NULL"} is a member of the union and
|
- |
|
| 41 |
the prototype object of the first member class otherwise (as of
|
- |
|
| 42 |
version 2.15.0 of R; earlier versions had a \code{NULL} prototype even if
|
- |
|
| 43 |
that was not valid).
|
- |
|
| 44 |
|
38 |
|
| 45 |
Class unions are the only way to create a class that is extended by
|
39 |
Class unions are the only way to create a new superclass of
|
| 46 |
a class whose definition is sealed (for example, the
|
40 |
a class whose definition is sealed. The namespace of all
|
| 47 |
basic datatypes or other classes defined in the base or methods
|
41 |
packages is sealed when the package is loaded, protecting the
|
| 48 |
package in R are sealed). You cannot say \code{setIs("function", "other")}
|
42 |
class and other definitions from being overwritten from another
|
| 49 |
unless \code{"other"} is a class union. In general, a
|
43 |
class or from the global environment. A call to
|
| 50 |
\code{setIs} call of this form changes the definition of the
|
44 |
\code{\link{setIs}} that tried to define a new superclass for
|
| 51 |
first class mentioned (adding \code{"other"} to the list of
|
- |
|
| 52 |
superclasses contained in the definition of \code{"function"}).
|
45 |
class \code{"numeric"}, for example, would cause an error.
|
| 53 |
|
46 |
|
| 54 |
Class unions get around this by not modifying the first class
|
47 |
Class unions are the exception; the class union
|
| 55 |
definition, relying instead on storing information in the subclasses
|
48 |
\code{"maybeNumber"} in the examples defines itself as a new
|
| 56 |
slot of the class union. In order for this technique to work, the
|
49 |
superclass of \code{"numeric"}. Technically, it does not alter the
|
| 57 |
internal computations for expressions such as
|
50 |
metadata object in the other package's namespace and, of course,
|
| 58 |
\code{\link{extends}(class1, class2)} work
|
- |
|
| 59 |
differently for class unions than for regular classes; specifically,
|
51 |
the effect of the class union depends on loading the package it
|
| 60 |
they test whether any class is in common between the superclasses of
|
52 |
belongs to. But, basically, class unions are sufficiently useful
|
| 61 |
\code{class1} and the subclasses of \code{class2}.
|
53 |
to justify the exemption.
|
| 62 |
|
54 |
|
| 63 |
The different behavior for class unions is made possible because the
|
55 |
The different behavior for class unions is made possible because the
|
| 64 |
class definition object for class unions has itself a special class,
|
56 |
class definition object for class unions has itself a special class,
|
| 65 |
\code{"ClassUnionRepresentation"}, an extension of class
|
57 |
\code{"ClassUnionRepresentation"}, an extension of class
|
| 66 |
\code{\linkS4class{classRepresentation}}.
|
58 |
\code{\linkS4class{classRepresentation}}.
|
| 67 |
}
|
59 |
}
|
| 68 |
|
60 |
|
| 69 |
\references{
|
61 |
\references{
|
| 70 |
Chambers, John M. (2008)
|
62 |
Chambers, John M. (2016)
|
| 71 |
\emph{Software for Data Analysis: Programming with R}
|
63 |
\emph{Extending R},
|
| 72 |
Springer. (For the R version.)
|
64 |
Chapman & Hall.
|
| 73 |
|
- |
|
| 74 |
Chambers, John M. (1998)
|
65 |
(Chapters 9 and 10.)
|
| 75 |
\emph{Programming with Data}
|
- |
|
| 76 |
Springer (For the original S4 version.)
|
- |
|
| 77 |
}
|
66 |
}
|
| 78 |
|
67 |
|
| - |
|
68 |
|
| 79 |
\examples{
|
69 |
\examples{
|
| 80 |
## a class for either numeric or logical data
|
70 |
## a class for either numeric or logical data
|
| 81 |
setClassUnion("maybeNumber", c("numeric", "logical"))
|
71 |
setClassUnion("maybeNumber", c("numeric", "logical"))
|
| 82 |
|
72 |
|
| 83 |
## use the union as the data part of another class
|
73 |
## use the union as the data part of another class
|
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