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.
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. 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 class
'>SClassExtension. 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.
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.
See function setClass to supply the information in the
class definition.
See Classes_Details for a more basic discussion of class information.