1<!doctype HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<!-- 4(C) Copyright 2002-4 Robert Ramey - http://www.rrsd.com . 5Use, modification and distribution is subject to the Boost Software 6License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at 7http://www.boost.org/LICENSE_1_0.txt) 8--> 9<head> 10<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 11<link rel="stylesheet" type="text/css" href="../../../boost.css"> 12<link rel="stylesheet" type="text/css" href="style.css"> 13<title>Seriealization - Rationale</title> 14</head> 15<body link="#0000ff" vlink="#800080"> 16<table border="0" cellpadding="7" cellspacing="0" width="100%" summary= 17 "header"> 18 <tr> 19 <td valign="top" width="300"> 20 <h3><a href="http://www.boost.org"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3> 21 </td> 22 <td valign="top"> 23 <h1 align="center">Serialization</h1> 24 <h2 align="center">Rationale</h2> 25 </td> 26 </tr> 27</table> 28<hr> 29<dl class="index"> 30 <dt><a href="#serialization">The term "serialization" is preferred to "persistence"</a></dt> 31 <dt><a href="#archives">Archives are not streams</a></dt> 32 <dt><a href="#strings">Strings are treated specially in text archives</a></dt> 33 <dt><a href="#typeid"><code style="white-space: normal">typeid</code> information is not included in archives</a></dt> 34 <!-- 35 <dt><a href="#footnotes">Footnotes</a></dt> 36 --> 37</dl> 38<h2><a name="serialization"></a>The term "serialization" is preferred to "persistence"</h2> 39<p> 40I found that persistence is often used to refer 41to something quite different. Examples are storage of class 42instances (objects) in database schema <a href="bibliography.html#4">[4]</a> 43This library will be useful in other contexts besides implementing persistence. The 44most obvious case is that of marshalling data for transmission to another system. 45<h2><a name="archives"></a>Archives are not streams</h2> 46<p> 47Archive classes are <strong>NOT</strong> derived from 48streams even though they have similar syntax rules. 49<ul> 50 <li>Archive classes are not kinds of streams though they 51 are implemented in terms of streams. This 52 distinction is addressed in <a href="bibliography.html#5">[5]</a> item number 41. 53 <li>We don't want users to insert/extract data 54 directly into/from the stream . This could 55 create a corrupted archive. Were archives 56 derived from streams, it would possible to 57 accidentally do this. So archive classes 58 only define operations which are safe and necessary. 59 <li>The usage of streams to implement the archive classes that 60 are included in the library is merely convenient - not necessary. 61 Library users may well want to define their own archive format 62 which doesn't use streams at all. 63</ul> 64<h2><a name="primitives"></a>Archive Members are Templates 65Rather than Virtual Functions</h2> 66The previous version of this library defined virtual functions for all 67primitive types. These were overridden by each archive class. There were 68two issues related to this: 69</ul> 70 <li>Some disliked virtual functions because of the added execution time 71 overhead. 72 <li>This caused implementation difficulties since the set of primitive 73 data types varies between platforms. Attempting to define the correct 74 set of virtual functions, (think <code style="white-space: normal">long long</code>, 75 <code style="white-space: normal">__int64</code>, 76 etc.) resulted in messy and fragile code. Replacing this with templates 77 and letting the compiler generate the code for the primitive types actually 78 used, resolved this problem. Of course, the ripple effects of this design 79 change were significant, but in the end led to smaller, faster, more 80 maintainable code. 81</ul> 82<h2><a name="strings"></a><code style="white-space: normal">std::strings</code> are treated specially in text files</h2> 83<p> 84Treating strings as STL vectors would result in minimal code size. This was 85not done because: 86<ul> 87 <li>In text archives it is convenient to be able to view strings. Our text 88 implementation stores single characters as integers. Storing strings 89 as a vector of characters would waste space and render the archives 90 inconvenient for debugging. 91 <li>Stream implementations have special functions for <code style="white-space: normal">std::string</code> 92 and <code style="white-space: normal">std::wstring</code>. 93 Presumably they optimize appropriately. 94 <li>Other specializations of <code style="white-space: normal">std::basic_string</code> are in fact handled 95 as vectors of the element type. 96</ul> 97</p> 98<h2><a name="typeid"></a><code style="white-space: normal">typeid</code> information is not included in archives</h2> 99<p> 100I originally thought that I had to save the name of the class specified by <code style="white-space: normal">std::type_of::name()</code> 101in the archive. This created difficulties as <code style="white-space: normal">std::type_of::name()</code> is not portable and 102not guaranteed to return the class name. This makes it almost useless for implementing 103archive portability. This topic is explained in much more detail in 104<a href="bibliography.html#6">[7] page 206</a>. It turned out that it was not necessary. 105As long as objects are loaded in the exact sequence as they were saved, the type 106is available when loading. The only exception to this is the case of polymorphic 107pointers never before loaded/saved. This is addressed with the <code style="white-space: normal">register_type()</code> 108and/or <code style="white-space: normal">export</code> facilities described in the reference. 109In effect, <code style="white-space: normal">export</code> generates a portable equivalent to 110<code style="white-space: normal">typeid</code> information. 111 112<!-- 113<h2><a name="footnotes"></a>Footnotes</h2> 114<dl> 115 <dt><a name="footnote1" class="footnote">(1)</a> {{text}}</dt> 116 <dt><a name="footnote2" class="footnote">(2)</a> {{text}}</dt> 117</dl> 118--> 119<hr> 120<p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. 121Distributed under the Boost Software License, Version 1.0. (See 122accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 123</i></p> 124</body> 125</html> 126
