\HeaderA{RClassUtils}{Utilities for Managing Class Definitions}{RClassUtils}
\aliasA{.BasicClasses}{RClassUtils}{.BasicClasses}
\aliasA{.BasicVectorClasses}{RClassUtils}{.BasicVectorClasses}
\aliasA{.InitBasicClasses}{RClassUtils}{.InitBasicClasses}
\aliasA{.InitMethodsListClass}{RClassUtils}{.InitMethodsListClass}
\aliasA{.setCoerceGeneric}{RClassUtils}{.setCoerceGeneric}
\aliasA{assignClassDef}{RClassUtils}{assignClassDef}
\aliasA{checkSlotAssignment}{RClassUtils}{checkSlotAssignment}
\aliasA{classMetaName}{RClassUtils}{classMetaName}
\aliasA{classPrototypeDef-class}{RClassUtils}{classPrototypeDef.Rdash.class}
\aliasA{completeClassDefinition}{RClassUtils}{completeClassDefinition}
\aliasA{completeExtends}{RClassUtils}{completeExtends}
\aliasA{completeSubclasses}{RClassUtils}{completeSubclasses}
\aliasA{defaultPrototype}{RClassUtils}{defaultPrototype}
\aliasA{empty.dump}{RClassUtils}{empty.dump}
\aliasA{extendsMetaName}{RClassUtils}{extendsMetaName}
\aliasA{getAllSuperClasses}{RClassUtils}{getAllSuperClasses}
\aliasA{getDataPart}{RClassUtils}{getDataPart}
\aliasA{isClassDef}{RClassUtils}{isClassDef}
\aliasA{isVirtualClass}{RClassUtils}{isVirtualClass}
\aliasA{makeExtends}{RClassUtils}{makeExtends}
\aliasA{makePrototypeFromClassDef}{RClassUtils}{makePrototypeFromClassDef}
\aliasA{metaNameUndo}{RClassUtils}{metaNameUndo}
\aliasA{methodsPackageMetaName}{RClassUtils}{methodsPackageMetaName}
\aliasA{newBasic}{RClassUtils}{newBasic}
\aliasA{newClassRepresentation}{RClassUtils}{newClassRepresentation}
\aliasA{newEmptyObject}{RClassUtils}{newEmptyObject}
\aliasA{possibleExtends}{RClassUtils}{possibleExtends}
\aliasA{print.classRepresentation}{RClassUtils}{print.classRepresentation}
\aliasA{reconcilePropertiesAndPrototype}{RClassUtils}{reconcilePropertiesAndPrototype}
\aliasA{requireMethods}{RClassUtils}{requireMethods}
\aliasA{setDataPart}{RClassUtils}{setDataPart}
\aliasA{setExtendsMetaData}{RClassUtils}{setExtendsMetaData}
\aliasA{setSubclassMetaData}{RClassUtils}{setSubclassMetaData}
\aliasA{showClass}{RClassUtils}{showClass}
\aliasA{showExtends}{RClassUtils}{showExtends}
\aliasA{subclassesMetaName}{RClassUtils}{subclassesMetaName}
\aliasA{superClassDepth}{RClassUtils}{superClassDepth}
\aliasA{testVirtual}{RClassUtils}{testVirtual}
\aliasA{tryNew}{RClassUtils}{tryNew}
\aliasA{trySilent}{RClassUtils}{trySilent}
\aliasA{validSlotNames}{RClassUtils}{validSlotNames}
\keyword{internal}{RClassUtils}
\begin{Description}\relax
These are various functions to support the definition and use of
formal classes.  Most of them are rarely suitable to be called
directly.

Others are somewhat experimental and/or partially implemented only. Do
refer to \code{\LinkA{setClass}{setClass}} for normal code development.
\end{Description}
\begin{Usage}
\begin{verbatim}
testVirtual(properties, extends, prototype, where)

makePrototypeFromClassDef(slots, ClassDef, extends, where)

newEmptyObject()

completeClassDefinition(Class, ClassDef, where, doExtends)

getAllSuperClasses(ClassDef, simpleOnly = TRUE)

superClassDepth(ClassDef, soFar, simpleOnly = TRUE)

isVirtualClass(Class, where)

newBasic(Class, ...)

makeExtends(Class, to, coerce, test, replace, by, package, slots,
                 classDef1, classDef2)

reconcilePropertiesAndPrototype(name, properties, prototype,
       superClasses, where)

tryNew(Class, where)

trySilent(expr)

empty.dump()

showClass(Class, complete=TRUE, propertiesAreCalled="Slots")

showExtends(ext, printTo = stdout())

possibleExtends(class1, class2, ClassDef1 = getClassDef(class1),
                 ClassDef2 = getClassDef(class2, where =
                 .classEnv(ClassDef1)))

completeExtends(ClassDef, class2, extensionDef, where)

classMetaName(name)

methodsPackageMetaName(prefix, name, package = "")

metaNameUndo(strings, prefix, searchForm = FALSE)

requireMethods(functions, signature, message, where)

checkSlotAssignment(obj, name, value)

defaultPrototype()

isClassDef(object)

validSlotNames(names)

getDataPart(object)
setDataPart(object, value, check = TRUE)

\end{verbatim}
\end{Usage}
\begin{Section}{Summary of Functions}
\describe{
\item[\code{testVirtual}:] Test for a Virtual Class.
Figures out, as well as possible, whether the class with these
properties, extension, and prototype is a virtual class.
Can be forced to be virtual by extending \code{"VIRTUAL"}.

Otherwise, a class is virtual only if it has no slots, extends no
non-virtual classes, and has a \code{NULL} Prototype.


\item[\code{makePrototypeFromClassDef}:] Makes the prototype implied by the class definition.

The following three rules are applied in this order.
\Enumerate{
\item If the class has slots, then the prototype for each slot
is used by default, but a corresponding element in the
explicitly supplied prototype in \code{ClassDef}, if there is
one, is used instead (but it must be coercible to the class of
the slot). This includes the data part (\code{".Data"} slot) if
there is one.

\item If there are no slots but a non-null prototype was
specified, this is returned.

\item If there is a non-virtual superclass (a class in the
extends list), then its prototype is used.  The data part is
extracted if needed (it is allowed to have two superclasses
with a data part; the first is used and a warning issued on
any others).
}
If all three of the above fail, the prototype is \code{NULL}.


\item[\code{newEmptyObject}:] Utility function to create an empty object into which slots can be
set.

Currently just creates an empty list with class \code{"NULL"}.

Later version should create a special object reference that marks
an object currently with no slots and no data.


\item[\code{completeClassDefinition}:] Completes the definition of \code{Class}, relative to the class
definitions visible from environment \code{where}.  If
\code{doExtends} is \code{TRUE}, complete the super- and
sub-class information.

This function is called when a class is defined or re-defined.


\item[\code{getFromClassDef}:] Extracts one of the intrinsically defined class definition
properties (".Properties", etc.)  Strictly a utility function.

\item[\code{getSlots}:] Returns a named character vector.  The names are the names of the
slots, the values are the classes of the corresponding slots.
The argument \code{x} can either be the name of
a class or the class definition object.


\item[\code{getAllSuperClasses}, \code{superClassDepth}:] Get the names of all the classes that this class definition
extends.

\code{getAllSuperClasses} is a utility function used to complete a
class definition.  It returns all the superclasses reachable from
this class, in breadth-first order (which is the order used for
matching methods); that is, the first direct superclass followed
by all its superclasses, then the next, etc.  (The order is
relevant only in the case that some of the superclasses have
multiple inheritance.)

\code{superClassDepth}, which is called from
\code{getAllSuperClasses}, returns the same information, but as a
list with components \code{label} and \code{depth}, the latter for
the number of generations back each class is in the inheritance
tree.  The argument \code{soFar} is used to avoid loops in the
network of class relationships.


\item[\code{isVirtualClass}:] Is the named class a virtual class?

A class is virtual if explicitly declared to be, and also if the
class is not formally defined.

\item[\code{assignClassDef}:] assign the definition of the class to the specially named object


\item[\code{newBasic}:] the implementation of the function \code{new} for basic classes
that don't have a formal definition.

Any of these could have a formal definition, except for
\code{Class="NULL"} (disallowed because \code{NULL} can't have
attributes).  For all cases except \code{"NULL"}, the class of the
result will be set to \code{Class}.

See \code{new} for the interpretation of the arguments.


\item[\code{makeExtends}:] convert the argument to a list defining the extension mechanism.


\item[\code{reconcilePropertiesAndPrototype}:] makes a list or a structure look like a prototype for the given
class.

Specifically, returns a structure with attributes corresponding to
the slot names in properties and values taken from prototype if
they exist there, from \code{new(classi)} for the class,
\code{classi} of the slot if that succeeds, and \code{NULL}
otherwise.

The prototype may imply slots not in the properties list, since
properties does not include inherited slots (these are left
unresolved until the class is used in a session).


\item[\code{tryNew}:] Tries to generate a new element from this class, but if the attempt
fails (as, e.g., when the class is undefined or virtual) just
returns \code{NULL}.

This is inefficient and also not a good idea when actually
generating objects, but is useful in the initial definition of
classes.


\item[\code{showClass}:] Print the information about a class definition.

If \code{complete} is \code{TRUE}, include the indirect
information about extensions.

It is the utility called from \code{\LinkA{show}{show}(\LinkA{show}{show}(.))},
and the user should typically use \code{getClass(.)} for looking
at class definitions.


\item[\code{showExtends}:] Print the elements of the list of extensions; for \code{printTo = FALSE},
returns a list with components \code{what} and \code{how}; this is
used e.g., by \code{\LinkA{promptClass}{promptClass}()}.


\item[\code{possibleExtends}:] Find the information that says whether class1 extends class2,
directly or indirectly.

This can be either a logical value or an object of class
\code{\LinkA{SClassExtension}{SClassExtension.Rdash.class}} containing various
functions to test and/or coerce the relationship.


\item[\code{completeExtends}:] complete the extends information in the class definition, by
following transitive chains.

If \code{class2} and \code{extensionDef} are included, this
class relation is to be added.  Otherwise just use the current
\code{ClassDef}.

Both the \code{contains} and \code{subclasses} slots are
completed with any indirect relations visible.


\item[\code{classMetaName}:] a name for the object storing this class's definition


\item[\code{methodsPackageMetaName}:] a name mangling device to hide metadata defining method and
class information.

\item[\code{metaNameUndo}] As its name implies, this function undoes the
name-mangling used to produce meta-data object names, and
returns a object of class \code{\LinkA{ObjectsWithPackage}{ObjectsWithPackage.Rdash.class}}.


\item[\code{requireMethods}:] Require a subclass to implement methods for the generic functions,
for this signature.

For each generic, \code{setMethod} will be called to define a
method that throws an error, with the supplied message.

The \code{requireMethods} function allows virtual classes to
require actual classes that extend them to implement methods for
certain functions, in effect creating an API for the virtual class.

Otherwise, default methods for the corresponding function would be
called, resulting in less helpful error messages or (worse still)
silently incorrect results.


\item[\code{checkSlotAssignment}:] Check that the value provided is allowed for this slot, by
consulting the definition of the class.  Called from the C code
that assigns slots.

For privileged slots (those that can only be set by accessor
functions defined along with the class itself), the class designer
may choose to improve efficiency by validating the value to be
assigned in the accessor function and then calling \code{slot<-}
with the argument \code{check=FALSE}, to prevent the call to
\code{checkSlotAssignment}.


\item[\code{defaultPrototype}:] The prototype for a class which will have slots, is not a virtual
class, and does not extend one of the basic classes. Both its
\code{\LinkA{class}{class}} and its (\R{} internal) type,
\code{\LinkA{typeof}{typeof}()}, are \code{"S4"}.


\item[\code{.InitBasicClasses}, \code{.InitMethodsListClass}, \code{.setCoerceGeneric}:] These functions perform part of the initialization of classes and
methods, and are called (only!) from \code{.First.lib}.


\item[\code{isClassDef}:] Is \code{object} a representation of a class?


\item[\code{validSlotNames}:] Returns \code{names} unless one of the names is reserved, in which
case there is an error.  (As of writing, \code{"class"} is the
only reserved slot name.)

\item[\code{getDataPart}, \code{setDataPart}:] Utilities called to implement
\code{object@.Data}.  Calls to \code{setDataPart} are also used
to merge the data part of a superclass prototype.


}
\end{Section}
\begin{Examples}
\begin{ExampleCode}
typeof(defaultPrototype()) #-> "S4"
\end{ExampleCode}
\end{Examples}

