Class Objects

Description

These are the objects that hold the definition of classes of objects. They are constructed and stored as meta-data by calls to the function setClass. Don't manipulate them directly, except perhaps to look at individual slots.

Details

Class definitions are stored as metadata in various packages. Additional metadata supplies information on inheritance (the result of calls to setIs). Inheritance information implied by the class definition itself (because the class contains one or more other classes) is also constructed automatically.

When a class is to be used in an R session, this information is assembled to complete the class definition. The completion is a second object of class "classRepresentation", cached for the session or until something happens to change the information. A call to getClass returns the completed definition of a class; a call to getClassDef returns the stored definition (uncompleted).

In particular, completion fills in the upward- and downward-pointing inheritance information for the class, in slots contains and subclasses respectively. It's in principle important to note that this information can depend on which packages are installed, since these may define additional subclasses or superclasses.

Slots

slots:

A named list of the slots in this class; the elements of the list are the classes to which the slots must belong (or extend), and the names of the list gives the corresponding slot names.

contains:

A named list of the classes this class “contains”; the elements of the list are objects of SClassExtension-class. The list may be only the direct extensions or all the currently known extensions (see the details).

virtual:

Logical flag, set to TRUE if this is a virtual class.

prototype:

The object that represents the standard prototype for this class; i.e., the data and slots returned by a call to new for this class with no special arguments. Don't mess with the prototype object directly.

validity:

Optionally, a function to be used to test the validity of objects from this class. See validObject.

access:

Access control information. Not currently used.

className:

The character string name of the class.

package:

The character string name of the package to which the class belongs. Nearly always the package on which the metadata for the class is stored, but in operations such as constructing inheritance information, the internal package name rules.

subclasses:

A named list of the classes known to extend this class'; the elements of the list are objects of SClassExtension-class. The list is currently only filled in when completing the class definition (see the details).

versionKey:

Object of class "externalptr"; eventually will perhaps hold some versioning information, but not currently used.

sealed:

Object of class "logical"; is this class sealed? If so, no modifications are allowed.

See Also

See function setClass to supply the information in the class definition. See Classes for a more basic discussion of class information.