<!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>Serialization - Derivation from an Existing Archive</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="../../../index.htm"><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">Derivation from an Existing Archive</h2> </td> </tr> </table> <hr> <dl class="page-index"> </dl> <h3>Log Archive</h3> It may happen that one wants to create a new archive class by derivation from one of the included ones. Included is a sample program that shows how to derive a new archive from one of the ones included with the library. The first example is <a href="../example/demo_log.cpp" target="demo_log_cpp"> demo_log.cpp</a>. <p> This derivation from the xml archive writes output in xml without the extra information required to read the data back into the application. It might be used to export one's data as simple xml for other applications or for logging data while debugging. <p> To this end it is derived from the included xml archive and the save functions for some types are specialized for this application. <p> The serialization library is implemented using the <b>C</b>uriously <b>R</b>ecurring <b>T</b>emplate <b>P</b>attern (<b>CRTP</b>). Also, all common code is factored out into separate modules to minimize code repetition. This makes derivation from an existing archive less straight forward than it would otherwise be. <p> This example illustrates several issues that have to be addressed when doing something like this <ol> <li><i>It is derived from</i> <code style="white-space: normal">xml_oarchive_impl<log_archive></code> <b>NOT</b> <code style="white-space: normal">xml_oarchive</code> <br> As described in the comments in <a href="../../../boost/archive/xml_oarchive.hpp" target="xml_oarchive_hpp">xml_oarchive.hpp</a>. <code style="white-space: normal">xml_oarchive</code> really a shorthand name for <code style="white-space: normal">xml_oarchive_impl<xml_oarchive></code>. So we should derive from <code style="white-space: normal">xml_oarchive_impl<log_archive></code> rather than <code style="white-space: normal">xml_oarchive</code>. <pre><code> class log_archive : // don't derive from xml_oarchive !!! public xml_oarchive_impl<log_archive> { ... </code></pre> <li><i>Note the</i> <code style="white-space: normal">log_archive</code> <i>between the</i> <> This is required so that base classes can downcast their <code style="white-space: normal">this</code> pointer to the most derived class. This is referred to as <b>C</b>uriously <b>R</b>ecurring <b>T</b>emplate <b>P</b>attern (<b>CRTP</b>) <a href="bibliography.html#11">[11]</a>. It is used to implement static polymorphism. <li><i>Base classes need to be explicitly given access to the derived class.</i> This can be done by making members public or by including friend declarations for the base classes. <pre><code> friend class detail::common_oarchive<log_archive>; friend class basic_xml_oarchive<log_archive>; friend class boost::serialization::save_access; </code></pre> <li><i></i>Reread <a target="detail" href="headers.html#archiveinternals">Archive Internals</a>. This describes the class hierarchy so that you know what to override. <li><i>Note the usage of PFTO.</i> Some compilers fail to provide support for partial function template ordering. The serialization library works around by using <a target="detail" href="implementation.html#functiontemplateordering"> <b>P</b>artial <b>F</b>unction <b>T</b>emplate <b>O</b>rdering</a> in several places. In archive classes, this takes . This is done in several places, including the archive classes themselves. <li><i>Base class functions will usually need to be explicitly invoked</i> We commonly specialize the function name <code style="white-space: normal">save_override</code> for saving primitives. Usage of a function name in a derived class "hides" similarly named functions of the base class. That is, function name overloading doesn't automatically include base classes. To address this, we can use: <pre><code> using xml_oarchive_impl<derived_t>::save; void save(const unsigned int t); ... </code></pre> which should work on conforming compilers. However, I have found that the following equivalent works on more compilers. <pre><code> // default fall through for any types not specified here template<class T> void save(const T & t){ xml_oarchive_impl<derived_t>::save(t); } void save(const unsigned int t); ... </code></pre> so it's what I use. <li><i>Template definitions of base classes may have to be explicitly instantiated.</i> The demo includes <pre><code> // explicitly instantiate for this type of binary stream #include <boost/archive/basic_binary_oprimitive.ipp> </code></pre> for just this purpose. Failure to include required template definitions will result in undefined symbol errors when the program is linked. <li><i>Without alteration, this class cannot be further derived from</i><br> Base classes using <b>CRTP</b> must be templates with a parameter corresponding to the most derived class. As presented here, this class doesn't qualify, so it cannot be used as a base class. In order to derive further from this class, it would have to be reorganized along the lines of the original <code style="white-space: normal">xml_oarchive</code>. Specifically, it would look something like: <pre><code> template<class Archive> class log_archive_impl : // don't derive from xml_oarchive !!! public xml_oarchive_impl<Archive> { ... ); // do not derived from this class !!! class log_archive : public log_archive_impl<log_archive> { public: log_archive(std::ostream & os, unsigned int flags = 0) : log_archive_impl<xml_oarchive>(os, flags) {} }; </code></pre> </ol> <hr> <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) </i></p> </body> </html>