Class Definitions

Description

Class definitions are objects that contain the formal definition of a class of R objects.

Details

When a class is defined, an object is stored that contains the information about that class, including:

slots

Each slot is a component object. Like elements of a list these may be extracted (by name) and set. However, they differ from list components in important ways.

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.

extends

The names of the classes that this class extends. A class 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.

prototype

Each class definition contains a prototype object from the class. This must have all the slots, if any, defined by the class definition.

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.

Author(s)

John Chambers

References

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.

See Also

Methods, setClass, is, as, new, slot