116 lines
6.2 KiB
HTML
116 lines
6.2 KiB
HTML
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<!--
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(C) Copyright 2002-4 Robert Ramey - http://www.rrsd.com .
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Use, modification and distribution is subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)
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-->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<link rel="stylesheet" type="text/css" href="../../../boost.css">
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<link rel="stylesheet" type="text/css" href="style.css">
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<title>Serialization - Reference</title>
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</head>
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<body link="#0000ff" vlink="#800080">
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary="header">
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<tr>
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<td valign="top" width="300">
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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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</td>
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<td valign="top">
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<h1 align="center">Serialization</h1>
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<h2 align="center">Exception Safety</h2>
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</td>
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</tr>
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</table>
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<hr>
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The process of loading an archive may result in the creation of new objects. That
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same process may throw an exception at some point. In order to prevent memory leaks
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and invalid pointers, these situations must be considered. Unfortunately, there is
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no simple universal solution to this problem. The manner of addressing this must
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depend on the design of the data structures to be serialized. Below, we discuss
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varying scenarios in increasing order of difficulty. This discussion presumes that
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the class member functions are exception safe before considering serialization.
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That is, the destructor could be called at anytime without referencing
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an invalid pointer, or creating a memory leak.
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<ol>
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<li><h4>class contains no pointers</h4>
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No problem here.
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<p>
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<li><h4>class contains only <i>owned</i> pointers</h4>
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From here on, we have to make a distinction between pointers used
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to manage heap storage (<i>owned</i> pointers) and pointers used to refer
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to related objects (<i>referenced</i> pointers). Programs containing <i>owned</i>
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pointers must contain code for deleting these objects and returning the
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deallocated storage to the heap. Programs containing <i>referenced</i> pointers
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must be designed to ensure that no such <i>referenced</i> pointers are de-referenced
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after the object pointed to has been destroyed and its storage returned
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to the heap. If a pointer is stored in only one place, it must be an <i>owned</i>
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pointer.
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<p>
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The load function traps any exceptions that occur between the time an object
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is created and its pointer is stored. Should an exception occur while
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reading an archive, the created object is deleted and the de-serialized
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pointer is set to NULL. This ensures that there are no memory leaks.
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The fact that there are no other copies of this pointer ensures that
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no pointers are left invalid. The object's destructor should
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be able to delete any other existing objects in the normal manner
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without problem.
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<a href="../test/test_delete_pointer.cpp" target="test_delete_pointer.cpp">test_delete_pointer.cpp</a>
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illustrates this case.
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<p>
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<li><h4>class contains one or more <i>referenced</i> pointers</h4>
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This situation can be further subdivided into two cases
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<p>
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<ol>
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<li><h4><i>owned</i> pointers are always serialized before <i>referenced</i> pointers</h4>
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Object tracking will ensure that no new objects will be created
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by the loading of a <i>referenced</i> pointer.
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If an exception occurs, <i>referenced</i> pointers will not need to be deleted
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so there will be no memory leaks. The destructor of this class won't attempt to
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delete these pointers so there will be no problem with dangling references.
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<i>Owned</i> pointers are handled exactly as described above.
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<p>
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<li><h4>class contains <i>referenced</i> pointers which might be created by load</h4>
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If a <i>referenced</i> pointer is loaded before its corresponding <i>owned</i>
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pointer, the object will be allocated on the heap. In certain cases
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it cannot be known which pointers were created by their owners and which
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were created by the load function. To address this:
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<ul>
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<li>Trap exceptions with a <code style="white-space: normal">try/catch</code> block.
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<li>Within the catch part, invoke the archive function
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<code style="white-space: normal">delete_created_pointers()</code> to delete any pointers
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created by the class load. Without other action, objects created in
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this way would end up as memory leaks as they are not considered <i>owned</i>
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pointers and hence aren't destroyed.
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<li>The object's destructor won't try
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to delete <i>referenced</i> pointers so any dangling references will
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cause no harm.
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</ul>
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<a href="../example/demo_exception.cpp" target="demo_exception.cpp">demo_exception.cpp</a>
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is a program that illustrates this case.
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<p>
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</ol>
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<p>
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<li><h4>Other cases</h4>
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Situations not covered above are pointers for which the classifications of
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<i>referenced</i> and <i>owned</i> are not applicable. This might occur where
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pointers are created by one class but consumed and deleted by another. These
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may be addressed with an ad hoc analysis similar to the above. As the
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situation becomes more complex this becomes more difficult and error prone.
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Eventually, it will be have to addressed by building heap management into the
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pointer itself - that is into <code style="white-space: normal">boost::shared_ptr</code>.
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The library includes serialization of <code style="white-space: normal">boost::shared_ptr</code>. As
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previously mentioned, this required a tiny alteration in one of the
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<code style="white-space: normal">boost::shared_ptr</code> implementation files in order to permit
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access by the serialization system.
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</ol>
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<hr>
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<p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004.
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Distributed under the Boost Software License, Version 1.0. (See
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accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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</i></p>
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</body>
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</html>
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