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| 4 | (C) Copyright 2002-4 Robert Ramey - http://www.rrsd.com . |
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| 5 | Use, modification and distribution is subject to the Boost Software |
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| 6 | License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
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| 10 | <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> |
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| 11 | <link rel="stylesheet" type="text/css" href="../../../boost.css"> |
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| 12 | <link rel="stylesheet" type="text/css" href="style.css"> |
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| 13 | <title>Serialization - Special Considerations</title> |
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| 14 | </head> |
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| 15 | <body link="#0000ff" vlink="#800080"> |
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| 16 | <table border="0" cellpadding="7" cellspacing="0" width="100%" summary="header"> |
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| 17 | <tr> |
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| 18 | <td valign="top" width="300"> |
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| 19 | <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3> |
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| 20 | </td> |
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| 21 | <td valign="top"> |
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| 22 | <h1 align="center">Serialization</h1> |
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| 23 | <h2 align="center">Special Considerations</h2> |
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| 24 | </td> |
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| 25 | </tr> |
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| 26 | </table> |
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| 27 | <hr> |
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| 28 | <dl class="page-index"> |
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| 29 | <dt><a href="#objecttracking">Object Tracking</a> |
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| 30 | <dt><a href="#export">Exporting Class Serialization</a> |
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| 31 | <dt><a href="#classinfo">Class Information</a> |
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| 32 | <dt><a href="#portability">Archive Portability</a> |
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| 33 | <dl class="page-index"> |
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| 34 | <dt><a href="#numerics">Numerics</a> |
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| 35 | <dt><a href="#traits">Traits</a> |
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| 36 | </dl> |
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| 37 | <dt><a href="#binary_archives">Binary Archives</a> |
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| 38 | <dt><a href="#xml_archives">XML Archives</a> |
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| 39 | <dt><a href="exceptions.html">Archive Exceptions</a> |
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| 40 | <dt><a href="exception_safety.html">Exception Safety</a> |
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| 41 | </dl> |
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| 42 | |
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| 43 | <h3><a name="objecttracking">Object Tracking</a></h3> |
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| 44 | Depending on how the class is used and other factors, serialized objects |
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| 45 | may be tracked by memory address. This prevents the same object from being |
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| 46 | written to or read from an archive multiple times. These stored addresses |
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| 47 | can also be used to delete objects created during a loading process |
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| 48 | that has been interrupted by throwing of an exception. |
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| 49 | <p> |
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| 50 | This could cause problems in |
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| 51 | progams where the copies of different objects are saved from the same address. |
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| 52 | <pre><code> |
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| 53 | template<class Archive> |
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| 54 | void save(boost::basic_oarchive & ar, const unsigned int version) const |
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| 55 | { |
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| 56 | for(int i = 0; i < 10; ++i){ |
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| 57 | A x = a[i]; |
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| 58 | ar << x; |
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| 59 | } |
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| 60 | } |
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| 61 | </code></pre> |
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| 62 | In this case, the data to be saved exists on the stack. Each iteration |
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| 63 | of the loop updates the value on the stack. So although the data changes |
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| 64 | each iteration, the address of the data doesn't. If a[i] is an array of |
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| 65 | objects being tracked by memory address, the library will skip storing |
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| 66 | objects after the first as it will be assumed that objects at the same address |
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| 67 | are really the same object. |
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| 68 | <p> |
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| 69 | To help detect such cases, output archive operators expect to be passed |
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| 70 | <code style="white-space: normal">const</code> reference arguments. |
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| 71 | <p> |
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| 72 | Given this, the above code will invoke a compile time assertion. |
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| 73 | The obvious fix in this example is to use |
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| 74 | <pre><code> |
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| 75 | template<class Archive> |
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| 76 | void save(boost::basic_oarchive & ar, const unsigned int version) const |
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| 77 | { |
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| 78 | for(int i = 0; i < 10; ++i){ |
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| 79 | ar << a[i]; |
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| 80 | } |
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| 81 | } |
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| 82 | </code></pre> |
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| 83 | which will compile and run without problem. |
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| 84 | The usage of <code style="white-space: normal">const</code> by the output archive operators |
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| 85 | will ensure that the process of serialization doesn't |
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| 86 | change the state of the objects being serialized. An attempt to do this |
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| 87 | would constitute augmentation of the concept of saving of state with |
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| 88 | some sort of non-obvious side effect. This would almost surely be a mistake |
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| 89 | and a likely source of very subtle bugs. |
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| 90 | <p> |
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| 91 | Unfortunately, implementation issues currently prevent the detection of this kind of |
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| 92 | error when the data item is wrapped as a name-value pair. |
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| 93 | <p> |
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| 94 | A similar problem can occur when different objects are loaded to and address |
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| 95 | which is different from the final location: |
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| 96 | <pre><code> |
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| 97 | template<class Archive> |
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| 98 | void load(boost::basic_oarchive & ar, const unsigned int version) const |
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| 99 | { |
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| 100 | for(int i = 0; i < 10; ++i){ |
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| 101 | A x; |
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| 102 | ar >> x; |
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| 103 | std::m_set.insert(x); |
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| 104 | } |
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| 105 | } |
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| 106 | </code></pre> |
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| 107 | In this case, the address of <code>x</code> is the one that is tracked rather than |
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| 108 | the address of the new item added to the set. Left unaddressed |
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| 109 | this will break the features that depend on tracking such as loading object through a pointer. |
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| 110 | Subtle bugs will be introduced into the program. This can be |
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| 111 | addressed by altering the above code thusly: |
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| 112 | |
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| 113 | <pre><code> |
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| 114 | template<class Archive> |
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| 115 | void load(boost::basic_iarchive & ar, const unsigned int version) const |
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| 116 | { |
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| 117 | for(int i = 0; i < 10; ++i){ |
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| 118 | A x; |
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| 119 | ar >> x; |
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| 120 | std::pair<std::set::const_iterator, bool> result; |
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| 121 | result = std::m_set.insert(x); |
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| 122 | ar.reset_object_address(& (*result.first), &x); |
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| 123 | } |
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| 124 | } |
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| 125 | </code></pre> |
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| 126 | This will adjust the tracking information to reflect the final resting place of |
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| 127 | the moved variable and thereby rectify the above problem. |
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| 128 | <p> |
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| 129 | If it is known a priori that no pointer |
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| 130 | values are duplicated, overhead associated with object tracking can |
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| 131 | be eliminated by setting the object tracking class serialization trait |
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| 132 | appropriately. |
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| 133 | <p> |
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| 134 | By default, data types designated primitive by |
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| 135 | <a target="detail" href="traits.html#level">Implementation Level</a> |
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| 136 | class serialization trait are never tracked. If it is desired to |
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| 137 | track a shared primitive object through a pointer (e.g. a |
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| 138 | <code style="white-space: normal">long</code> used as a reference count), It should be wrapped |
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| 139 | in a class/struct so that it is an identifiable type. |
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| 140 | The alternative of changing the implementation level of a <code style="white-space: normal">long</code> |
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| 141 | would affect all <code style="white-space: normal">long</code>s serialized in the whole |
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| 142 | program - probably not what one would intend. |
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| 143 | <p> |
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| 144 | It is possible that we may want to track addresses even though |
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| 145 | the object is never serialized through a pointer. For example, |
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| 146 | a virtual base class need be saved/loaded only once. By setting |
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| 147 | this serialization trait to <code style="white-space: normal">track_always</code>, we can suppress |
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| 148 | redundant save/load operations. |
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| 149 | <pre><code> |
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| 150 | BOOST_CLASS_TRACKING(my_virtual_base_class, boost::serialization::track_always) |
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| 151 | </code></pre> |
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| 152 | |
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| 153 | <h3><a name="export">Exporting Class Serialization</a></h3> |
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| 154 | <a target="detail" href="traits.html#export">Elsewhere</a> in this manual, we have described |
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| 155 | <code style="white-space: normal">BOOST_CLASS_EXPORT</code>. This is used to make the serialization library aware |
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| 156 | that code should be instantiated for serialization of a given class even though the |
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| 157 | class hasn't been otherwise referred to by the program. |
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| 158 | <p> |
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| 159 | There are several ways <code style="white-space: normal">BOOST_CLASS_EXPORT</code> could have been |
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| 160 | implemented. |
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| 161 | <p> |
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| 162 | One approach would be to instantiate serialization code for all archive classes included in the library. |
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| 163 | This would add to each executable a large amount of code that is most likely never called. |
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| 164 | Also it would needlessly slow down compilations of any program that uses the library. Finally, |
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| 165 | the list of archives would be "built-in" to the library which would compilicate the addition of |
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| 166 | new or custom archive classes. |
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| 167 | <p> |
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| 168 | Another approach would be for the library user to somehow explicitly instantiate which archive classes |
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| 169 | code should be instantiated for each class to be serialized. Users would have to include |
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| 170 | header files corresponding the archive classes to be instantiated. |
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| 171 | The list of instantiated archive classes would have to be manually kept in sync with the |
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| 172 | archive class headers actually included. This was considered burdensome and error prone. |
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| 173 | <p> |
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| 174 | This implementation of <code style="white-space: normal">BOOST_CLASS_EXPORT</code> works in the |
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| 175 | following way: |
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| 176 | <ul> |
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| 177 | <li>All header modules of the form <boost/archive/*archive.hpp> are required to precede |
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| 178 | the header module <a href="../../../boost/serialization/export.hpp" target="export_hpp">export.hpp</a>. |
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| 179 | <li>The header <a href="../../../boost/serialization/export.hpp" target="export_hpp">export.hpp</a> |
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| 180 | builds a list of archive classes whose header modules have been previously included. |
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| 181 | It does this by checking to see which inclusion guard constants have been defined. |
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| 182 | The header <a href="../../../boost/archive/detail/known_archive_types.hpp" target="known_archive_types_hpp">known_archive_types.hpp</a> |
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| 183 | lists the archive header files which whose include guards will be checked. If you create your own |
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| 184 | archive class, you probably want to edit this file. |
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| 185 | <li><code style="white-space: normal">BOOST_CLASS_EXPORT(my_class)</code> explicitly instantiates |
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| 186 | serialization code for <code style="white-space: normal">my_class</code> for each archive in the list. |
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| 187 | </ul> |
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| 188 | Serialization code will be instantiated for a given archive class |
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| 189 | if and only if the module that defines that archive class has been included in the program. |
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| 190 | Given this, our program will contain all necessary code instantiations and no other. |
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| 191 | <p> |
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| 192 | For many styles of code organization this header sequencing requirement presents little problem. |
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| 193 | |
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| 194 | Serialization code organized by class headers that are designed to be independent of archive |
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| 195 | implementations will look something like the following: |
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| 196 | <code><pre> |
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| 197 | // A.hpp |
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| 198 | // Note:to preserve independence from any particular archive implementation, |
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| 199 | // no headers from <boost/archive/...> are included. |
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| 200 | // Headers can be included in any order. |
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| 201 | #include <boost/serialization/...> |
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| 202 | #include <boost/serialization/export.hpp> |
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| 203 | ... // include other headers that A depends upon |
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| 204 | |
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| 205 | class A { |
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| 206 | ... |
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| 207 | }; |
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| 208 | |
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| 209 | BOOST_CLASS_EXPORT(A) // note: the export name of this class |
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| 210 | </pre></code> |
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| 211 | This style: |
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| 212 | <ul> |
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| 213 | <li>permits the header to include all aspects of the serialization implementation. |
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| 214 | <li>permits the header to be included anywhere else as part of some other class declaration. |
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| 215 | <li>reflects the concept of headers as a "library of types" which |
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| 216 | can be used independently in other programs or other parts of the same program. |
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| 217 | <li>reflects a fundamental principle of the serialization library design in that the |
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| 218 | specification of serialization of any class is independent of any archive implementation. |
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| 219 | </ul> |
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| 220 | However, it might not always be possible or convenient to conform to the above style. Something |
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| 221 | like the following might be required or preferred: |
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| 222 | <code><pre> |
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| 223 | // A.hpp |
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| 224 | // headers can be included in any order |
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| 225 | #include <boost/archive/text_oarchive.hpp> |
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| 226 | #include <boost/archive/text_iarchive.hpp> |
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| 227 | ... |
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| 228 | #include <boost/serialization/...> |
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| 229 | ... |
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| 230 | // can't do the following because then A.hpp couldn't be included somewhere else |
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| 231 | // #include <boost/serialization/export.hpp> |
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| 232 | |
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| 233 | class A { |
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| 234 | ... |
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| 235 | }; |
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| 236 | // can't do the following because export.hpp is not included !! |
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| 237 | //BOOST_CLASS_EXPORT(A) // note: the export name of this class |
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| 238 | </pre></code> |
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| 239 | As noted in the comments, this would work. But |
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| 240 | <code style="white-space: normal">#include <.../export.hpp></code> can't be used |
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| 241 | without conflicting with other modules which use |
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| 242 | <code style="white-space: normal">#include <.../*archive.hpp></code>. In this |
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| 243 | case we can move the export to an implementation file: |
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| 244 | <code><pre> |
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| 245 | // A.cpp |
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| 246 | #include "A.hpp" |
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| 247 | ... |
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| 248 | // export.hpp header should be last; |
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| 249 | #include <boost/serialization/export.hpp> |
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| 250 | ... |
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| 251 | BOOST_CLASS_EXPORT(A) |
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| 252 | ... |
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| 253 | </pre></code> |
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| 254 | |
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| 255 | <h3><a name="classinfo">Class Information</a></h3> |
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| 256 | By default, for each class serialized, class information is written to the archive. |
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| 257 | This information includes version number, implementation level and tracking |
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| 258 | behavior. This is necessary so that the archive can be correctly |
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| 259 | deserialized even if a subsequent version of the program changes |
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| 260 | some of the current trait values for a class. The space overhead for |
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| 261 | this data is minimal. There is a little bit of runtime overhead |
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| 262 | since each class has to be checked to see if it has already had its |
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| 263 | class information included in the archive. In some cases, even this |
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| 264 | might be considered too much. This extra overhead can be eliminated |
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| 265 | by setting the |
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| 266 | <a target="detail" href="traits.html#level">implementation level</a> |
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| 267 | class trait to: <code style="white-space: normal">boost::serialization::object_serializable</code>. |
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| 268 | <p> |
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| 269 | <i>Turning off tracking and class information serialization will result |
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| 270 | in pure template inline code that in principle could be optimised down |
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| 271 | to a simple stream write/read.</i> Elimination of all serialization overhead |
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| 272 | in this manner comes at a cost. Once archives are released to users, the |
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| 273 | class serialization traits cannot be changed without invalidating the old |
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| 274 | archives. Including the class information in the archive assures us |
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| 275 | that they will be readable in the future even if the class definition |
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| 276 | is revised. A light weight structure such as display pixel might be |
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| 277 | declared in a header like this: |
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| 278 | |
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| 279 | <pre><code> |
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| 280 | #include <boost/serialization/serialization.hpp> |
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| 281 | #include <boost/serialization/level.hpp> |
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| 282 | #include <boost/serialization/tracking.hpp> |
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| 283 | |
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| 284 | // a pixel is a light weight struct which is used in great numbers. |
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| 285 | struct pixel |
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| 286 | { |
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| 287 | unsigned char red, green, blue; |
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| 288 | template<class Archive> |
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| 289 | void serialize(Archive & ar, const unsigned int /* version */){ |
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| 290 | ar << red << green << blue; |
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| 291 | } |
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| 292 | }; |
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| 293 | |
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| 294 | // elminate serialization overhead at the cost of |
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| 295 | // never being able to increase the version. |
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| 296 | BOOST_CLASS_IMPLEMENTATION(pixel, boost::serialization::object_serializable); |
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| 297 | |
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| 298 | // eliminate object tracking (even if serialized through a pointer) |
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| 299 | // at the risk of a programming error creating duplicate objects. |
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| 300 | BOOST_CLASS_TRACKING(pixel, boost::serialization::track_never) |
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| 301 | </code></pre> |
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| 302 | |
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| 303 | <h3><a name="portability">Archive Portability</a></h3> |
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| 304 | Several archive classes create their data in the form of text or portable a binary format. |
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| 305 | It should be possible to save such an of such a class on one platform and load it on another. |
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| 306 | This is subject to a couple of conditions. |
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| 307 | <h4><a name="numerics">Numerics</a></h4> |
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| 308 | The architecture of the machine reading the archive must be able hold the data |
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| 309 | saved. For example, the gcc compiler reserves 4 bytes to store a variable of type |
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| 310 | <code style="white-space: normal">wchar_t</code> while other compilers reserve only 2 bytes. |
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| 311 | So its possible that a value could be written that couldn't be represented by the loading program. This is a |
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| 312 | fairly obvious situation and easily handled by using the numeric types in |
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| 313 | <a target="cstding" href="../../../boost/cstdint.hpp"><boost/cstdint.hpp></a> |
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| 314 | |
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| 315 | <h4><a name="traits">Traits</a></h4> |
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| 316 | Another potential problem is illustrated by the following example: |
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| 317 | <pre><code> |
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| 318 | template<class T> |
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| 319 | struct my_wrapper { |
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| 320 | template<class Archive> |
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| 321 | Archive & serialize ... |
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| 322 | }; |
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| 323 | |
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| 324 | ... |
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| 325 | |
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| 326 | class my_class { |
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| 327 | wchar_t a; |
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| 328 | short unsigned b; |
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| 329 | template<<class Archive> |
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| 330 | Archive & serialize(Archive & ar, unsigned int version){ |
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| 331 | ar & my_wrapper(a); |
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| 332 | ar & my_wrapper(b); |
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| 333 | } |
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| 334 | }; |
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| 335 | </code></pre> |
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| 336 | If <code style="white-space: normal">my_wrapper</code> uses default serialization |
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| 337 | traits there could be a problem. With the default traits, each time a new type is |
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| 338 | added to the archive, bookkeeping information is added. So in this example, the |
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| 339 | archive would include such bookkeeping information for |
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| 340 | <code style="white-space: normal">my_wrapper<wchar_t></code> and for |
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| 341 | <code style="white-space: normal">my_wrapper<short_unsigned></code>. |
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| 342 | Or would it? What about compilers that treat |
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| 343 | <code style="white-space: normal">wchar_t</code> as a |
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| 344 | synonym for <code style="white-space: normal">unsigned short</code>? |
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| 345 | In this case there is only one distinct type - not two. If archives are passed between |
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| 346 | programs with compilers that differ in their treatment |
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| 347 | of <code style="white-space: normal">wchar_t</code> the load operation will fail |
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| 348 | in a catastrophic way. |
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| 349 | <p> |
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| 350 | One remedy for this is to assign serialization traits to the template |
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| 351 | <code style="white-space: normal">my_template</code> such that class |
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| 352 | information for instantiations of this template is never serialized. This |
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| 353 | process is described <a target="detail" href="traits.html#templates">above</a> and |
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| 354 | has been used for <a target="detail" href="wrappers.html#nvp"><strong>Name-Value Pairs</strong></a>. |
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| 355 | Wrappers would typically be assigned such traits. |
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| 356 | <p> |
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| 357 | Another way to avoid this problem is to assign serialization traits |
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| 358 | to all specializations of the template <code style="white-space: normal">my_wrapper</code> |
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| 359 | for all primitive types so that class information is never saved. This is what has |
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| 360 | been done for our implementation of serializations for STL collections. |
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| 361 | |
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| 362 | <h3><a name="binary_archives">Binary Archives</a></h3> |
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| 363 | Standard stream i/o on some systems will expand linefeed characters to carriage-return/linefeed |
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| 364 | on output. This creates a problem for binary archives. The easiest way to handle this is to |
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| 365 | open streams for binary archives in "binary mode" by using the flag |
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| 366 | <code style="white-space: normal">ios::binary</code>. If this is not done, the archive generated |
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| 367 | will be unreadable. |
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| 368 | <p> |
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| 369 | Unfortunately, no way has been found to detect this error before loading the archive. Debug builds |
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| 370 | will assert when this is detected so that may be helpful in catching this error. |
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| 371 | |
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| 372 | <h3><a name="xml_archives">XML Archives</a></h3> |
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| 373 | XML archives present a somewhat special case. |
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| 374 | XML format has a nested structure that maps well to the "recursive class member visitor" pattern |
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| 375 | used by the serialization system. However, XML differs from other formats in that it |
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| 376 | requires a name for each data member. Our goal is to add this information to the |
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| 377 | class serialization specification while still permiting the the serialization code to be |
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| 378 | used with any archive. This is achived by requiring that all data serialized to an XML archive |
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| 379 | be serialized as a <a target="detail" href="wrappers.html#nvp">name-value pair</a>. |
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| 380 | The first member is the name to be used as the XML tag for the |
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| 381 | data item while the second is a reference to the data item itself. Any attempt to serialize data |
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| 382 | not wrapped in a in a <a target="detail" href="wrappers.html#nvp">name-value pair</a> will |
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| 383 | be trapped at compile time. The system is implemented in such a way that for other archive classes, |
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| 384 | just the value portion of the data is serialized. The name portion is discarded during compilation. |
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| 385 | So by always using <a target="detail" href="wrappers.html#nvp">name-value pairs</a>, it will |
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| 386 | be guarenteed that all data can be serialized to all archive classes with maximum efficiency. |
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| 387 | |
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| 388 | <h3><a href="exceptions.html">Archive Exceptions</a></h3> |
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| 389 | <h3><a href="exception_safety.html">Exception Safety</a></h3> |
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| 390 | |
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| 391 | <hr> |
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| 392 | <p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. |
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| 393 | Distributed under the Boost Software License, Version 1.0. (See |
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| 394 | accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
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| 395 | </i></p> |
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| 396 | </body> |
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| 397 | </html> |
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