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13<title>Serialization - Overview</title>
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16<table border="0" cellpadding="7" cellspacing="0" width="100%" summary="header">
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18 <td valign="top" width="300">
19 <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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21 <td valign="top">
22 <h1 align="center">Serialization</h1>
23 <h2 align="center">Overview</h2>
24 </td>
25 </tr>
26</table>
27<hr>
28<dl class="index">
29 <dt><a href="#Requirements">Requirements</a></dt>
30 <dt><a href="#otherimplementations">Other Implementations</a></dt>
31</dl>
32<p>Here, we use the term <strong>"serialization"</strong> to mean
33the reversible deconstruction of an arbitrary set of C++ data structures
34to a sequence of bytes. Such a system can be used to reconstitute
35an equivalent structure in another program context. Depending on
36the context, this might used implement object persistence, remote
37parameter passing or other facility. In this system we use the term
38<strong>"archive"</strong> to refer to a specific rendering of this
39stream of bytes. This could be a file of binary data, text data,
40XML, or some other created by the user of this library.
41<h2><a name="Requirements"></a>Our goals for such a system are:</h2>
42 <ol>
43 <li>Code portability - depend only on ANSI C++ facilities.
44
45 <li>Code economy - exploit features of C++ such as RTTI,
46 templates, and multiple inheritance, etc. where appropriate to
47 make code shorter and simpler to use.
48
49 <li>Independent versioning for each class definition. That
50 is, when a class definition changed, older files can still be
51 imported to the new version of the class.
52
53 <li>Deep pointer save and restore. That is, save and restore
54 of pointers saves and restores the data pointed to.
55
56 <li>Proper restoration of pointers to shared data.
57
58 <li>Serialization of STL containers and other commonly used
59 templates.
60
61 <li>Data Portability - Streams of bytes created on one platform
62 should be readable on any other.
63
64 <li>Orthogonal specification of class serialization and archive format.
65 That is, any file format should be able to store serialization
66 of any arbitrary set of C++ data structures without having to
67 alter the serialization of any class.
68
69 <li>Non-intrusive. Permit serialization to be applied to
70 unaltered classes. That is, don't require that classes to be
71 serialized be derived from a specific base class or implement
72 specified member functions. This is necessary to easily
73 permit serialization to be applied to classes from class
74 libraries that we cannot or don't want to have to alter.
75
76 <li> The <strong>archive</strong> interface must be simple
77 enough to easily permit creation of a new type of archive.
78
79 <li> The <strong>archive</strong> interface must be rich
80 enough to permit the creation of an <strong>archive</strong>
81 that presents serialized data as XML in a useful manner.
82 </ol>
83
84<h2><a name="otherimplementations"></a>Other implementations</h2>
85 Before getting started I searched around for current
86 implementations. I found several.
87
88 <ul>
89 <li><u>MFC</u> This is the one that I am very familiar with.
90 I have used it for several years and have found it very useful.
91 However it fails requirements 1, 2, 3, 6, 7, and 9. In spite
92 of all the requirements not fulfilled, this is the most
93 useful implementation I've found. It turns out that class
94 versioning - partially implemented in MFC - really is
95 indispensable for my applications. Inevitably, version 1.x of
96 a shipping program needs to store more information in files
97 than was originally provided for. MFC is the only one of these
98 implementations that supports this - though only for the most
99 derived class. Still it's better than nothing and does the
100 job. MFC doesn't implement serialization of STL collections.
101 Though it does so for MFC collections.
102
103 <li><u>CommonC++ libraries</u> <a href="bibliography.html#1">[1]</a>
104 As far as I can tell, this
105 closely follows the MFC implementation but does address a few
106 of the issues. It is portable and creates portable archives but
107 skips versioning. It does support proper and complete
108 restoration of pointers and STL collections. It does address
109 compression though not in the way that I would prefer. The
110 package would also benefit from having better documentation.
111 So it fails to address 2, 3, 7, 8, and 9.
112
113 <li><u>Eternity</u> <a href="bibliography.html#2">[2]</a>
114 This is a bare bones package. It
115 seems well coded but it really needs documentation and
116 examples. It's not obvious how to use it without time
117 consuming study of the source code. Recent versions do support
118 files in XML format. This Fails 3, 6, 7?, 8, and 9.
119
120 <li><u>Holub's implementation</u> <a href="bibliography.html#3">[3]</a> This is the article that
121 first got me thinking about my own requirements for
122 a serialization implementation. Interesting and worth
123 the read if you can overlook the arrogant tone of the prose.
124 This implementation fails 2, 3, 4, 5, and 6.
125
126 <li><u>s11n</u> <a href="bibliography.html#13">[13]</a>
127 This library has similar goals to this one. Some aspects of the
128 implemenation are also similar. As of this writing, it would seem that:
129 <ul>
130 <li>Portability(1) is guarenteed only for recent versions of GCC.
131 <li>Versioning(3) of class definitions is not explicitly supported by
132 the library.
133 <li>it doesn't seem to automatically account for shared pointers(5).
134 I concluded this from the documentation as well as the statement that
135 serialization of graph like structures is not supported.
136 </ul>
137 Its has lots of differences - and lots in common with this implementation.
138 </ul>
139<hr>
140<p>Revised 1 November, 2004
141<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004.
142Distributed under the Boost Software License, Version 1.0. (See
143accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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