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<title>Serialization - Overview</title>
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<h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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<h1 align="center">Serialization</h1>
<h2 align="center">Overview</h2>
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<dl class="index">
<dt><a href="#Requirements">Requirements</a></dt>
<dt><a href="#otherimplementations">Other Implementations</a></dt>
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<p>Here, we use the term <strong>"serialization"</strong> to mean
the reversible deconstruction of an arbitrary set of C++ data structures
to a sequence of bytes. Such a system can be used to reconstitute
an equivalent structure in another program context. Depending on
the context, this might used implement object persistence, remote
parameter passing or other facility. In this system we use the term
<strong>"archive"</strong> to refer to a specific rendering of this
stream of bytes. This could be a file of binary data, text data,
XML, or some other created by the user of this library.
<h2><a name="Requirements"></a>Our goals for such a system are:</h2>
<ol>
<li>Code portability - depend only on ANSI C++ facilities.
<li>Code economy - exploit features of C++ such as RTTI,
templates, and multiple inheritance, etc. where appropriate to
make code shorter and simpler to use.
<li>Independent versioning for each class definition. That
is, when a class definition changed, older files can still be
imported to the new version of the class.
<li>Deep pointer save and restore. That is, save and restore
of pointers saves and restores the data pointed to.
<li>Proper restoration of pointers to shared data.
<li>Serialization of STL containers and other commonly used
templates.
<li>Data Portability - Streams of bytes created on one platform
should be readable on any other.
<li>Orthogonal specification of class serialization and archive format.
That is, any file format should be able to store serialization
of any arbitrary set of C++ data structures without having to
alter the serialization of any class.
<li>Non-intrusive. Permit serialization to be applied to
unaltered classes. That is, don't require that classes to be
serialized be derived from a specific base class or implement
specified member functions. This is necessary to easily
permit serialization to be applied to classes from class
libraries that we cannot or don't want to have to alter.
<li> The <strong>archive</strong> interface must be simple
enough to easily permit creation of a new type of archive.
<li> The <strong>archive</strong> interface must be rich
enough to permit the creation of an <strong>archive</strong>
that presents serialized data as XML in a useful manner.
</ol>
<h2><a name="otherimplementations"></a>Other implementations</h2>
Before getting started I searched around for current
implementations. I found several.
<ul>
<li><u>MFC</u> This is the one that I am very familiar with.
I have used it for several years and have found it very useful.
However it fails requirements 1, 2, 3, 6, 7, and 9. In spite
of all the requirements not fulfilled, this is the most
useful implementation I've found. It turns out that class
versioning - partially implemented in MFC - really is
indispensable for my applications. Inevitably, version 1.x of
a shipping program needs to store more information in files
than was originally provided for. MFC is the only one of these
implementations that supports this - though only for the most
derived class. Still it's better than nothing and does the
job. MFC doesn't implement serialization of STL collections.
Though it does so for MFC collections.
<li><u>CommonC++ libraries</u> <a href="bibliography.html#1">[1]</a>
As far as I can tell, this
closely follows the MFC implementation but does address a few
of the issues. It is portable and creates portable archives but
skips versioning. It does support proper and complete
restoration of pointers and STL collections. It does address
compression though not in the way that I would prefer. The
package would also benefit from having better documentation.
So it fails to address 2, 3, 7, 8, and 9.
<li><u>Eternity</u> <a href="bibliography.html#2">[2]</a>
This is a bare bones package. It
seems well coded but it really needs documentation and
examples. It's not obvious how to use it without time
consuming study of the source code. Recent versions do support
files in XML format. This Fails 3, 6, 7?, 8, and 9.
<li><u>Holub's implementation</u> <a href="bibliography.html#3">[3]</a> This is the article that
first got me thinking about my own requirements for
a serialization implementation. Interesting and worth
the read if you can overlook the arrogant tone of the prose.
This implementation fails 2, 3, 4, 5, and 6.
<li><u>s11n</u> <a href="bibliography.html#13">[13]</a>
This library has similar goals to this one. Some aspects of the
implemenation are also similar. As of this writing, it would seem that:
<ul>
<li>Portability(1) is guarenteed only for recent versions of GCC.
<li>Versioning(3) of class definitions is not explicitly supported by
the library.
<li>it doesn't seem to automatically account for shared pointers(5).
I concluded this from the documentation as well as the statement that
serialization of graph like structures is not supported.
</ul>
Its has lots of differences - and lots in common with this implementation.
</ul>
<hr>
<p>Revised 1 November, 2004
<p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004.
Distributed under the Boost Software License, Version 1.0. (See
accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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