Creator (3U)

#include <Unidraw/creator.h>

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.

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.

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