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.  Generally 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 Creator 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)