Creator - recreates catalog-managed objects from disk and instantiates component
views from their class identifiers
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.
- 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
- virtual void* Create(
- ClassId, istream& in, ObjectMap* objmap, int
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
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.
Catalog(3U), classes(3U), globals(3U)