Persistent Classes

The classes described here provide the possibility of persistent objects. As in most cases it is not necessary to have all the overhead for persistency inside of a running program, no complete new class need to be designed if persistency is needed. It is sufficient to add an extension to a CNCL-kompatible class where all necessary methods are included.

To generate new persistent classes or extensions, derived from any CNCL compatible class, the utility CNpersistent is provided. It generates a class framework with all functions required by CNCL and all necesarry methods for persistency.

Usage:

CNpersistent[-l] [-t template-dir] basename

The required parameter basename is the name of the base class. This sript will produce the persistent extension to the mentioned base class. The result are two files in the current directory: Pbasename.h (header file) and Pbasename.c (implementation file).

The optional parameters are:

• [-l] : The base class is stored in the local directory, not in the standard CNCL-tree.
• [-t template-dir] : template-dir is the directory where the templates will be found (if they are not in the standard directory).

Example:

CNpersistent MyClass

creates the class/extension files PMyClass.h and PMyClass.c. MyClass and CNPObjectID will be the base classes to the new extension. The only methods that have to be defined after this are the read_from constructor and the storer-method.

NOTE:

• At some classes it will be necessary to achieve direct access to some private members of the base class, e.g. for the read_from constructor or the storer-method. In order to achieve this it is important to declare the persistent class or extension as friend class at the base class.
• All extensions have the complete Default I/O member function for CNCL classes. In most cases this I/O is not necessary because all these functions are already defined in the base class. Thus all of them can be deleted (they are marked with the note ( to be deleted if not needed )).
• Please don't wonder that the rule of no multiple inheritance inside of CNCL seems to be broken at the persistent classes. The second base class, CNPObjectID, is not derived from any other CNCL-class. So the problems of multiple inheritance can not occur.

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