| Classes {methods} | R Documentation |
Class definitions are objects that contain the formal definition of a class of R objects.
When a class is defined, an object is stored that contains the information about that class, including:
All the objects from a particular class have the same set of slot names; specifically, the slot names that are contained in the class definition. Each slot in each object always has the same class; again, this is defined by the overall class definition.
Classes don't need to have any slots, and many useful classes do not. These objects usually extend other, simple objects, such as numeric or character vectors. Finally, classes can have no data at all—these are known as virtual classes and are in fact very important programming tools. They are used to group together ordinary classes that want to share some programming behavior, without necessarily restricting how the behavior is implemented.
Fancy, say, extends a class Simple if an
object from the Fancy class has all the capabilities of
the Simple class (and probably some more as well). In
particular, and very usefully, any method defined to work for a
Simple object can be applied to a Fancy object as
well.
In other programming languages, this relationship is sometimes
expressed by saying that Simple is a superclass of
Fancy, or that Fancy is a subclass of
Simple.
The actual class definition object contains the names of all the classes this class extends. But those classes can themselves extend other classes also, so the complete extension can only be known by obtaining all those class definitions.
Class extension is usually defined when the class itself is
defined, by including the names of superclasses as unnamed
elements in the representation argument to setClass.
An object from a given class will then have all the slots defined for its own class and all the slots defined for its superclasses as well.
Note that extends relations can be defined in other ways
as well, by using the setIs function.
The prototype most commonly just consists of the prototypes of all its slots. But that need not be the case: the definition of the class can specify any valid object for any of the slots.
There are a number of “basic” classes, corresponding to the
ordinary kinds of data occurring in R. For example,
"numeric" is a class corresponding to numeric vectors.
These classes are predefined and can then be used as slots or as
superclasses for any other class definitions. The prototypes for
the vector classes are vectors of length 0 of the corresponding
type.
There are also a few basic virtual classes, the most important
being "vector", grouping together all the vector classes;
and "language", grouping together all the types of objects
making up the R language.
John Chambers
The web page http://www.omegahat.org/RSMethods/index.html is the primary documentation.
The functions in this package emulate the facility for classes and methods described in Programming with Data (John M. Chambers, Springer, 1998). See this book for further details and examples.
Methods,
setClass,
is,
as,
new,
slot