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      <h1 align="center">Serialization</h1>
      <h2 align="center">Exception Safety</h2>
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The process of loading an archive may result in the creation of new objects.  That
same process may throw an exception at some point. In order to prevent memory leaks
and invalid pointers, these situations must be considered.  Unfortunately, there is
no simple universal solution to this problem.  The manner of addressing this must
depend on the design of the data structures to be serialized.  Below, we discuss
varying scenarios in increasing order of difficulty.  This discussion presumes that
the class member functions are exception safe before considering serialization.  
That is, the destructor could be called at anytime without referencing 
an invalid pointer, or creating a memory leak.
<ol>
    <li><h4>class contains no pointers</h4>
    No problem here.
    <p>
    <li><h4>class contains only <i>owned</i> pointers</h4>
    From here on, we have to make a distinction between pointers used
    to manage heap storage (<i>owned</i> pointers) and pointers used to refer
    to related objects (<i>referenced</i> pointers).  Programs containing <i>owned</i>
    pointers must contain code for deleting these objects and returning the
    deallocated storage to the heap.  Programs containing <i>referenced</i> pointers
    must be designed to ensure that no such <i>referenced</i> pointers are de-referenced
    after the object pointed to has been destroyed and its storage returned
    to the heap. If a pointer is stored in only one place, it must be an <i>owned</i>
    pointer.
    <p>
    The load function traps any exceptions that occur between the time an object
    is created and its pointer is stored.  Should an exception occur while
    reading an archive, the created object is deleted and the de-serialized
    pointer is set to NULL.  This ensures that there are no memory leaks.
    The fact that there are no other copies of this pointer ensures that
    no pointers are left invalid. The object's destructor should
    be able to delete any other existing objects in the normal manner
    without problem.
    <a href="../test/test_delete_pointer.cpp" target="test_delete_pointer.cpp">test_delete_pointer.cpp</a>
    illustrates this case.
    <p>
    <li><h4>class contains one or more <i>referenced</i> pointers</h4>
    This situation can be further subdivided into two cases
    <p>
    <ol>
        <li><h4><i>owned</i> pointers are always serialized before <i>referenced</i> pointers</h4>
        Object tracking will ensure that no new objects will be created
        by the loading of a <i>referenced</i> pointer.
        If an exception occurs, <i>referenced</i> pointers will not need to be deleted
        so there will be no memory leaks. The destructor of this class won't attempt to
        delete these pointers so there will be no problem with dangling references.
        <i>Owned</i> pointers are handled exactly as described above.
        <p>
        <li><h4>class contains <i>referenced</i> pointers which might be created by load</h4>
        If a <i>referenced</i> pointer is loaded before its corresponding <i>owned</i>
        pointer, the object will be allocated on the heap.  In certain cases
        it cannot be known which pointers were created by their owners and which
        were created by the load function.  To address this:
        <ul>
        <li>Trap exceptions with a <code style="white-space: normal">try/catch</code> block.
        <li>Within the catch part, invoke the archive function 
        <code style="white-space: normal">delete_created_pointers()</code> to delete any pointers
        created by the class load.  Without other action, objects created in
        this way would end up as memory leaks as they are not considered <i>owned</i>
        pointers and hence aren't destroyed.
        <li>The object's destructor won't try
        to delete <i>referenced</i> pointers so any dangling references will
        cause no harm.
    </ul>
    <a href="../example/demo_exception.cpp" target="demo_exception.cpp">demo_exception.cpp</a>
        is a program that illustrates this case.
        <p>
    </ol>
    <p>
    <li><h4>Other cases</h4>
    Situations not covered above are pointers for which the classifications of 
    <i>referenced</i> and <i>owned</i> are not applicable.  This might occur where
    pointers are created by one class but consumed and deleted by another.  These
    may be addressed with an ad hoc analysis similar to the above.  As the
    situation becomes more complex this becomes more difficult and error prone.
    Eventually, it will be have to addressed by building heap management into the
    pointer itself - that is into <code style="white-space: normal">boost::shared_ptr</code>. 
    The library includes serialization of <code style="white-space: normal">boost::shared_ptr</code>.  As
    previously mentioned, this required a tiny alteration in one of the
    <code style="white-space: normal">boost::shared_ptr</code> implementation files in order to permit
    access by the serialization system.
</ol>
<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>
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Commit	Line	Data
7c673cae FG	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 .
	5	Use, modification and distribution is subject to the Boost Software
	6	License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
	7	http://www.boost.org/LICENSE_1_0.txt)
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	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>Serialization - Reference</title>
	14	</head>
	15	<body link="#0000ff" vlink="#800080">
	16	<table border="0" cellpadding="7" cellspacing="0" width="100%" summary="header">
	17	<tr>
	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>
	20	</td>
	21	<td valign="top">
	22	<h1 align="center">Serialization</h1>
	23	<h2 align="center">Exception Safety</h2>
	24	</td>
	25	</tr>
	26	</table>
	27	<hr>
	28	The process of loading an archive may result in the creation of new objects. That
	29	same process may throw an exception at some point. In order to prevent memory leaks
	30	and invalid pointers, these situations must be considered. Unfortunately, there is
	31	no simple universal solution to this problem. The manner of addressing this must
	32	depend on the design of the data structures to be serialized. Below, we discuss
	33	varying scenarios in increasing order of difficulty. This discussion presumes that
	34	the class member functions are exception safe before considering serialization.
	35	That is, the destructor could be called at anytime without referencing
	36	an invalid pointer, or creating a memory leak.
	37	<ol>
	38	<li><h4>class contains no pointers</h4>
	39	No problem here.
	40	<p>
	41	<li><h4>class contains only <i>owned</i> pointers</h4>
	42	From here on, we have to make a distinction between pointers used
	43	to manage heap storage (<i>owned</i> pointers) and pointers used to refer
	44	to related objects (<i>referenced</i> pointers). Programs containing <i>owned</i>
	45	pointers must contain code for deleting these objects and returning the
	46	deallocated storage to the heap. Programs containing <i>referenced</i> pointers
	47	must be designed to ensure that no such <i>referenced</i> pointers are de-referenced
	48	after the object pointed to has been destroyed and its storage returned
	49	to the heap. If a pointer is stored in only one place, it must be an <i>owned</i>
	50	pointer.
	51	<p>
	52	The load function traps any exceptions that occur between the time an object
	53	is created and its pointer is stored. Should an exception occur while
	54	reading an archive, the created object is deleted and the de-serialized
	55	pointer is set to NULL. This ensures that there are no memory leaks.
	56	The fact that there are no other copies of this pointer ensures that
	57	no pointers are left invalid. The object's destructor should
	58	be able to delete any other existing objects in the normal manner
	59	without problem.
	60	<a href="../test/test_delete_pointer.cpp" target="test_delete_pointer.cpp">test_delete_pointer.cpp</a>
	61	illustrates this case.
	62	<p>
	63	<li><h4>class contains one or more <i>referenced</i> pointers</h4>
	64	This situation can be further subdivided into two cases
65	<p>
66	<ol>
67	<li><h4><i>owned</i> pointers are always serialized before <i>referenced</i> pointers</h4>
68	Object tracking will ensure that no new objects will be created
69	by the loading of a <i>referenced</i> pointer.
70	If an exception occurs, <i>referenced</i> pointers will not need to be deleted
71	so there will be no memory leaks. The destructor of this class won't attempt to
72	delete these pointers so there will be no problem with dangling references.
73	<i>Owned</i> pointers are handled exactly as described above.
74	<p>
75	<li><h4>class contains <i>referenced</i> pointers which might be created by load</h4>
76	If a <i>referenced</i> pointer is loaded before its corresponding <i>owned</i>
77	pointer, the object will be allocated on the heap. In certain cases
78	it cannot be known which pointers were created by their owners and which
79	were created by the load function. To address this:
80	<ul>
81	<li>Trap exceptions with a <code style="white-space: normal">try/catch</code> block.
82	<li>Within the catch part, invoke the archive function
83	<code style="white-space: normal">delete_created_pointers()</code> to delete any pointers
84	created by the class load. Without other action, objects created in
85	this way would end up as memory leaks as they are not considered <i>owned</i>
86	pointers and hence aren't destroyed.
87	<li>The object's destructor won't try
88	to delete <i>referenced</i> pointers so any dangling references will
89	cause no harm.
90	</ul>
91	<a href="../example/demo_exception.cpp" target="demo_exception.cpp">demo_exception.cpp</a>
92	is a program that illustrates this case.
93	<p>
94	</ol>
95	<p>
96	<li><h4>Other cases</h4>
97	Situations not covered above are pointers for which the classifications of
98	<i>referenced</i> and <i>owned</i> are not applicable. This might occur where
99	pointers are created by one class but consumed and deleted by another. These
100	may be addressed with an ad hoc analysis similar to the above. As the
101	situation becomes more complex this becomes more difficult and error prone.
102	Eventually, it will be have to addressed by building heap management into the
103	pointer itself - that is into <code style="white-space: normal">boost::shared_ptr</code>.
104	The library includes serialization of <code style="white-space: normal">boost::shared_ptr</code>. As
105	previously mentioned, this required a tiny alteration in one of the
106	<code style="white-space: normal">boost::shared_ptr</code> implementation files in order to permit
107	access by the serialization system.
108	</ol>
109	<hr>
110	<p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004.
111	Distributed under the Boost Software License, Version 1.0. (See
112	accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
113	</i></p>
114	</body>
115	</html>