% File src/library/methods/man/RClassUtils.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2018 R Core Team
% Copyright 2008-2018 The R Foundation
% Distributed under GPL 2 or later

\name{RClassUtils}
%% NOTE: This is {internal} for a good reason

%% FIXME the following are undocumented utilities (Sep 23, 2002)
\alias{completeSubclasses}
\alias{newClassRepresentation}
\alias{print.classRepresentation}
\alias{setExtendsMetaData}
\alias{setSubclassMetaData}
\alias{subclassesMetaName}
\alias{extendsMetaName}
\alias{classPrototypeDef-class}
%%
\alias{.classEnv}
\alias{classLabel}
\alias{testVirtual}
\alias{makePrototypeFromClassDef}
\alias{newEmptyObject}
\alias{completeClassDefinition}
\alias{getAllSuperClasses}
\alias{superClassDepth}
\alias{isVirtualClass}
\alias{assignClassDef}
\alias{newBasic}
\alias{makeExtends}
\alias{reconcilePropertiesAndPrototype}
\alias{tryNew}
\alias{empty.dump}
\alias{showClass}
\alias{showExtends}
\alias{possibleExtends}
\alias{completeExtends}
\alias{classMetaName}
\alias{methodsPackageMetaName}
\alias{metaNameUndo}
\alias{requireMethods}
\alias{checkAtAssignment}
\alias{checkSlotAssignment}
\alias{defaultPrototype}
\alias{isClassDef}
\alias{validSlotNames}
\alias{getDataPart}
\alias{setDataPart}
%% not explicitly documented here
\alias{.BasicClasses}
\alias{.BasicVectorClasses}
\alias{.InitBasicClasses}
\alias{.InitMethodsListClass}
\alias{.setCoerceGeneric}
\alias{conditionalExtension-class}

\title{Utilities for Managing Class Definitions}
\description{
  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{\link{setClass}} for normal code development.
}

\usage{
classLabel(Class)
.classEnv(Class, default = .requirePackage("methods"), mustFind = TRUE)

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, coerce, test, replace, by, package, slots,
            classDef1, classDef2)

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

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)

checkAtAssignment(cl, name, valueClass)
checkSlotAssignment(obj, name, value)

defaultPrototype()

isClassDef(object)

validSlotNames(names)

getDataPart(object, NULL.for.none = FALSE)
setDataPart(object, value, check = TRUE)
}

\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}:}{
      Construct an SClassExtension object representing the relationship
      from \code{Class} to the class defined by \code{classDef2}. 
    }

    \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{\link{show}(\link{getClass}(.))},
      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{\link{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{\linkS4class{SClassExtension}} containing various
      functions to test and/or coerce the relationship.
    }

    \item{\code{classLabel}: }{
        Returns an informative character string identifying the class
        and, if appropriate, the package from which the class came.
    }

    \item{\code{.classEnv}: }{% is called from C level  R_check_class_etc()
        Returns the environment, typically a namespace, in which the
        \code{Class} has been defined.  \code{Class} should typically be
        the result of \code{\link{class}()} (and hence contain a
        \code{"package"} attribute) or \code{\link{getClass}} (or
        \code{\link{getClassDef}}).
    }

    \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{\linkS4class{ObjectsWithPackage}}.
    }

    \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}, \code{checkAtAssignment}:}{
      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{\link{class}} and its (\R internal) type,
      \code{\link{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{.onLoad}.
    }

    \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{ describe }
}
\examples{
typeof(defaultPrototype()) #-> "S4"

## .classEnv()
meth.ns <- asNamespace("methods")
if(get4 <- !any("package:stats4" == search()))
   require("stats4")
stopifnot(TRUE
 , identical(.classEnv("data.frame"), meth.ns)
 , identical(.classEnv(class(new("data.frame"))), meth.ns)
 , identical(.classEnv(     "mle"       ), meth.ns) # <- *not* 'stats4'
 , identical(.classEnv(class(new("mle"))), asNamespace("stats4"))
 , identical(.classEnv(getClass ("mle") ), asNamespace("stats4"))
 )
if(get4) detach("package:stats4")
}
\keyword{internal}