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13<title>Serialization - Reference</title>
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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>
28The process of loading an archive may result in the creation of new objects. That
29same process may throw an exception at some point. In order to prevent memory leaks
30and invalid pointers, these situations must be considered. Unfortunately, there is
31no simple universal solution to this problem. The manner of addressing this must
32depend on the design of the data structures to be serialized. Below, we discuss
33varying scenarios in increasing order of difficulty. This discussion presumes that
34the class member functions are exception safe before considering serialization.
35That is, the destructor could be called at anytime without referencing
36an 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>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004.
111Distributed under the Boost Software License, Version 1.0. (See
112accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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