NAME

       classes - unique identifiers for catalog-managed objects and component views

SYNOPSIS

       #include <Unidraw/classes.h>

DESCRIPTION

       Classes   for  catalog-managed  objects,  including  components,  commands,  tools,  state
       variables,  and  transfer  functions,  must  be  associated  with  mutually-unique   class
       identifiers  that  can  be  written  onto  disk.  This allows a creator object to call the
       appropriate constructor as part of recreating an object from disk.

       Also, there must be a mechanism for creating an appropriate view given a subject; that is,
       there  must  be an association between a subject class and allowable view classes for that
       subject.  This association is established by  defining  class  identifiers  for  component
       views.  These identifiers are formed by concatenating the identifier for a given component
       subject class with an identifier that specifies a ``view  category.''   For  example,  the
       class identifier for the PostScript external view of a line component is the concatenation
       of the identifier for the line component subject  class  and  POSTSCRIPT_VIEW  identifier,
       which  specifies  the category of external views for generating PostScript.  This makes it
       possible to create an instance of the appropriate PostScript external view subclass  given
       a line component subject.

       The class identifiers for the Unidraw base and predefined derived classes are specified in
       the classes.h include file.  Applications  that  define  new  catalog-managed  objects  or
       component  views  should  define  unique  class  identifiers  in  a similar manner.  Class
       identifiers  for  catalog-managed  objects  are  simply  integers  with   mnemonic   macro
       definitions, usually the class name in all-uppercase.  For example, the identifier for the
       LineComp component subject class is

       #define LINE_COMP 9030

       View class identifiers are specified using the Combine macro, which  takes  as  its  first
       argument  the  macro  definition  of  the  corresponding component subject identifier; its
       second argument is an identifier for the view category.  The Combine macro itself is  then
       defined  with  a  mnemonic  name.  For example, the identifier for the PostScript external
       view class PSLine (which generates PostScript for the line component subject)  is  defined
       as

       #define PS_LINE Combine(LINE_COMP, POSTSCRIPT_VIEW)

       where the POSTSCRIPT_VIEW view category identifier is defined as

       #define POSTSCRIPT_VIEW 9103

       N.B.: All class identifiers in a given application must be unique.

SEE ALSO

       Catalog(3U), Creator(3U)