HP Integrity NonStop J-Series - Manual (Page 171)

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©Copyright 1996 Rogue Wave Software
Levels of Persistence
An object has one of four levels of persistence:
No persistence. There is no mechanism for storage and retrieval of the object. ●
Simple persistence. A level of persistence that provides storage and retrieval of
individual objects to and from a stream or file. Simple persistence does not preserve
pointer relationships among the persisted objects. ●
Isomorphic persistence. A level of persistence that preserves the pointer relationships
among the persisted objects. ●
Polymorphic persistence. The highest level of persistence. Polymorphic persistence
preserves pointer relationships among the persisted objects and allows the restoring
process to restore an object without prior knowledge of that object's type. ●
The Class Reference indicates the level of persistence for each class. This section provides
information about each level of persistence through descriptions, examples, and procedures for
designing your own persistent classes.
A Note About Terminology
Tools.h++ provides input and output classes that let you save and restore objects. These classes
are:
RWFile : RWFile lets you save and restore objects to a file; ●
RWvostream: Classes derived from RWvostream , such as RWpostream , RWbostream,
and RWeostream , are used to save objects;
●
RWvistream : Classes derived from RWvistream, such as RWpostream , RWbistream , and
RWeistream , are used to restore objects.
●
To keep our explanations simple, we'll refer to all of these input and output classes as streams.
For a discussion of the trade-offs in using
RWvostream and RWvistream versus RWFile , see
Chapter 6 and Chapter 7 .