Creator(3U)                                                 InterViews Reference Manual                                                Creator(3U)

NAME

       Creator - recreates catalog-managed objects from disk and instantiates component views from their class identifiers

SYNOPSIS

       #include <Unidraw/creator.h>

DESCRIPTION

       A  creator object can reconstruct a catalog-managed object from disk and can instantiate a component view given a class identifier.  Gener-
       ally only the Catalog class uses the creator to reconstruct objects from disk, but it may be useful for other classes (e.g., the  Component
       base class) to use the creator to create views of component subjects.

       Applications  that derive new catalog-managed classes or component views must also derive a subclass of Creator that extends the base class
       operations to support the new classes.

PUBLIC OPERATIONS

       Creator()
              Instantiate an new creator object.  The Catalog constructor requires an instance of a creator object, either an instance of the Cre-
              ator base class or an instance of an application-specific Creator subclass.

       virtual void* Create(
           ClassId, istream& in, ObjectMap* objmap, int objid
       )
              Create  an  instance  of the class corresponding to the given class identifier.  Normally this operation will be used by the Catalog
              class exclusively.  This operation contains a switch statement that calls a CREATE macro for each possible  class  identifier.   The
              CREATE  macro  takes  the  istream, ObjectMap, and int arguments as parameters; it is not important to understand how the macro uses
              these arguments.  For example, the entry for the LineComp line component subject class is

              case LINE_COMP: CREATE(LineComp, in, objmap, objid);

              Derived Creator classes must redefine this operation to include a  switch  statement  for  application-specific  classes,  with  the
              default case calling the parent class's Create operation.

       virtual void* Create(ClassId id)
              Create  an instance of the view class corresponding to the given view class identifier.  This operation is implemented with a set of
              consecutive if statements of the form

              if (id == <identifier name>) return new <class name>;

              For example, the entry for the PostScript external view class identifier for line components is

              if (id == PS_LINE) return new PSLine;

              Derived Creator classes must redefine this operation to include if statements for application-specific view classes, with the  final
              statement being a call to the parent classes's Create operation.

SEE ALSO

       Catalog(3U), classes(3U), globals(3U)

Unidraw                                                            12 June 1990                                                        Creator(3U)