3rdparty/boost_1_81_0/libs/serialization/doc/rationale.html

<!doctype HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!--
(C) Copyright 2002-4 Robert Ramey - http://www.rrsd.com . 
Use, modification and distribution is subject to 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)
-->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<link rel="stylesheet" type="text/css" href="../../../boost.css">
<link rel="stylesheet" type="text/css" href="style.css">
<title>Seriealization - Rationale</title>
</head>
<body link="#0000ff" vlink="#800080">
<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
    "header">
  <tr> 
    <td valign="top" width="300"> 
      <h3><a href="http://www.boost.org"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
    </td>
    <td valign="top"> 
      <h1 align="center">Serialization</h1>
      <h2 align="center">Rationale</h2>
    </td>
  </tr>
</table>
<hr>
<dl class="index">
  <dt><a href="#serialization">The term "serialization" is preferred to "persistence"</a></dt>
  <dt><a href="#archives">Archives are not streams</a></dt>
  <dt><a href="#strings">Strings are treated specially in text archives</a></dt>
  <dt><a href="#typeid"><code style="white-space: normal">typeid</code> information is not included in archives</a></dt>
  <!--
  <dt><a href="#footnotes">Footnotes</a></dt>
  -->
</dl>
<h2><a name="serialization"></a>The term "serialization" is preferred to "persistence"</h2>
<p>
I found that persistence is often used to refer
to something quite different. Examples are storage of class
instances (objects) in database schema <a href="bibliography.html#4">[4]</a>
This library will be useful in other contexts besides implementing persistence. The
most obvious case is that of marshalling data for transmission to another system.
<h2><a name="archives"></a>Archives are not streams</h2>
<p>
Archive classes are <strong>NOT</strong> derived from
streams even though they have similar syntax rules.
<ul>
    <li>Archive classes are not kinds of streams though they
    are implemented in terms of streams. This
    distinction is addressed in <a href="bibliography.html#5">[5]</a> item number 41.
    <li>We don't want users to insert/extract&nbsp;data
    directly into/from &nbsp;the stream .&nbsp; This could
    create a corrupted archive. Were archives
    derived from streams, it would possible to
    accidentally do this. So archive classes
    only define operations which are safe and necessary.
    <li>The usage of streams to implement the archive classes that
    are included in the library is merely convenient - not necessary.
    Library users may well want to define their own archive format
    which doesn't use streams at all.
</ul>
<h2><a name="primitives"></a>Archive Members are Templates 
Rather than Virtual Functions</h2>
The previous version of this library defined virtual functions for all
primitive types.  These were overridden by each archive class.  There were
two issues related to this:
</ul>
    <li>Some disliked virtual functions because of the added execution time
    overhead.
    <li>This caused implementation difficulties since the set of primitive
    data types varies between platforms.  Attempting to define the correct
    set of virtual functions, (think <code style="white-space: normal">long long</code>, 
    <code style="white-space: normal">__int64</code>, 
    etc.) resulted in messy and fragile code.  Replacing this with templates
    and letting the compiler generate the code for the primitive types actually
    used, resolved this problem.  Of course, the ripple effects of this design
    change were significant, but in the end led to smaller, faster, more
    maintainable code.
</ul>
<h2><a name="strings"></a><code style="white-space: normal">std::strings</code> are treated specially in text files</h2>
<p>
Treating strings as STL vectors would result in minimal code size. This was
not done because:
<ul>
     <li>In text archives it is convenient to be able to view strings.  Our text
     implementation stores single characters as integers.  Storing strings
     as a vector of characters would waste space and render the archives
     inconvenient for debugging.
     <li>Stream implementations have special functions for <code style="white-space: normal">std::string</code>
     and <code style="white-space: normal">std::wstring</code>.
     Presumably they optimize appropriately.
     <li>Other specializations of <code style="white-space: normal">std::basic_string</code> are in fact handled
     as vectors of the element type.
</ul>
</p>
<h2><a name="typeid"></a><code style="white-space: normal">typeid</code> information is not included in archives</h2>
<p>
I originally thought that I had to save the name of the class specified by <code style="white-space: normal">std::type_of::name()</code>
in the archive. This created difficulties as <code style="white-space: normal">std::type_of::name()</code> is not portable and
not guaranteed to return the class name. This makes it almost useless for implementing
archive portability.  This topic is explained in much more detail in
<a href="bibliography.html#6">[7] page 206</a>. It turned out that it was not necessary.
As long as objects are loaded in the exact sequence as they were saved, the type
is available when loading.  The only exception to this is the case of polymorphic
pointers never before loaded/saved.  This is addressed with the <code style="white-space: normal">register_type()</code>
and/or <code style="white-space: normal">export</code> facilities described in the reference.  
In effect, <code style="white-space: normal">export</code> generates a portable equivalent to
<code style="white-space: normal">typeid</code> information.
<!--
<h2><a name="footnotes"></a>Footnotes</h2>
<dl>
  <dt><a name="footnote1" class="footnote">(1)</a> {{text}}</dt>
  <dt><a name="footnote2" class="footnote">(2)</a> {{text}}</dt>
</dl>
-->
<hr>
<p><i>&copy; 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)
</i></p>
</body>
</html>