|If neither the programming language, nor its
standard library, nor the operating system supports persistence,
it is still possible to implement a library for this.
This is more easily achieved in object-oriented languages
but also traditional languages attempted it.
The first idea for this was published by Herlihy (1982),
an object-oriented implementation was provided by Borchert
for Oberon (1994).
|The principal idea is that each persistent class provides
so-called marshaling procedures that convert an object
into a sequence of bytes (on some stream) and that are able to
construct an object on base of such a sequence of bytes.
|Problem: Writing such objects is easy but how to locate
a reading method? Or better: How to construct the right
object that has to be asked to read itself?
|Solution: The design pattern of object factories helps.
Maintain a registry with class names and associated constructor
objects. Each object that is written to an output stream must
be preceded by a class information (usually just a string
that specifies the name of the class). On reading, first
the class name is read, then the factory is asked to find the
constructor object associated with that class name. Afterwards
the constructor object creates an instance of that class using
the marshaling constructor which takes an input stream as
|Drawback: All persistent objects must belong to classes
that are derived from a abstract base class for persistent
|Another drawback for C++: No free matured solution for C++ is
known to me. Only commercial or experimental libraries.
|Copyright © 2001, 2002 Andreas Borchert, converted to HTML on February 21, 2002|