| [29] | 1 | <!doctype HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
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| 2 | <html> |
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| 3 | <!-- |
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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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| 7 | http://www.boost.org/LICENSE_1_0.txt) |
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| 8 | --> |
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| 9 | <head> |
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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 - Derivation from an Existing Archive</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">Derivation from an Existing Archive</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 target="detail" href="#portable_archives">Portable Binary Archives</a> |
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| 30 | <dt><a target="detail" href="#fast_archives">Fast Binary Archives</a> |
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| 31 | </dl> |
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| 32 | |
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| 33 | <a name=portable_archives> |
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| 34 | <h3>Portable Binary Archives</h3> |
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| 35 | It may happen that one wants to create a new archive class by derivation from one |
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| 36 | of the included ones. Included is a sample program that shows how to derive a |
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| 37 | new archive from one of the ones included with the library. The first example is |
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| 38 | <a href="../example/demo_portable_archive.cpp" target="demo_portable_archive_cpp">demo_portable_archive.cpp</a>. |
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| 39 | This binary archive save/loads integers in a portable format. To this end |
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| 40 | it is derived from the native binary archive and the save/load functions for |
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| 41 | integers are overridden with special versions which convert to big endian |
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| 42 | format if necessary. It also implements an exception for the case where an |
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| 43 | integer saved on one platform is too large for the platform which is loading |
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| 44 | the archive. This example doesn't address floating point types. |
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| 45 | This examples illustrates several issues that have to be addressed when doing |
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| 46 | something like this. The discussion below refers to the output archive only but it |
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| 47 | applies equally to input archives as well. |
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| 48 | <ol> |
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| 49 | <li><i>It is derived from</i> <code style="white-space: normal">binary_oarchive_impl<portable_binary_oarchive></code> |
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| 50 | <b>NOT</b> <code style="white-space: normal">binary_oarchive</code> <br> |
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| 51 | As described in the comments in |
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| 52 | <a href="../../../boost/archive/binary_oarchive.hpp" target="binary_oarchive_hpp">binary_oarchive.hpp</a>. |
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| 53 | <code style="white-space: normal">binary_oarchive</code> really a shorthand name for |
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| 54 | <code style="white-space: normal">binary_oarchive_impl<binary_oarchive></code>. So we should derive |
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| 55 | from <code style="white-space: normal">binary_oarchive_impl<portable_binary_oarchive></code> rather |
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| 56 | than <code style="white-space: normal">binary_oarchive</code>. |
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| 57 | <pre><code> |
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| 58 | class portable_binary_oarchive : |
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| 59 | // don't derive from binary_oarchive !!! |
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| 60 | public binary_oarchive_impl<portable_binary_oarchive> |
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| 61 | { |
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| 62 | ... |
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| 63 | </code></pre> |
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| 64 | <li><i>Note the</i> <code style="white-space: normal">portable_binary_oarchive</code> <i>between the</i> <> |
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| 65 | This is required so that base classes can downcast their <code style="white-space: normal">this</code> pointer |
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| 66 | to the most derived class. This is referred to as <b>C</b>uriously <b>R</b>ecurring |
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| 67 | <b>T</b>emplate <b>P</b>attern (<b>CRTP</b>) <a href="bibliography.html#11">[11]</a>. |
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| 68 | It is used to implement static polymorphism. |
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| 69 | <li><i>Base classes need to be explicitly given access to the derived class.</i> |
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| 70 | This can be done by making members public or by including friend declarations for |
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| 71 | the base classes. |
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| 72 | <pre><code> |
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| 73 | typedef portable_binary_oarchive derived_t; |
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| 74 | friend class detail::common_oarchive<derived_t>; |
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| 75 | friend class basic_binary_oarchive<derived_t>; |
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| 76 | friend class basic_binary_oprimitive< |
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| 77 | derived_t, |
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| 78 | std::ostream::char_type, |
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| 79 | std::ostream::traits_type |
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| 80 | >; |
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| 81 | friend class boost::serialization::save_access; |
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| 82 | </code></pre> |
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| 83 | <li><i>Base class functions will usually need to be explicitly invoked</i> |
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| 84 | We commonly overload the function name <code style="white-space: normal">save</code> for saving primitives. |
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| 85 | This is very convenient. Usage of a function name in a derived class |
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| 86 | "hides" similarly named functions of the base class. That is, |
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| 87 | function name overloading doesn't automatically |
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| 88 | include base classes. To address this, we can use: |
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| 89 | <pre><code> |
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| 90 | using binary_oarchive_impl<derived_t>::save; |
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| 91 | void save(const unsigned int t); |
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| 92 | ... |
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| 93 | </code></pre> |
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| 94 | which should work on conforming compilers. However, I have found |
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| 95 | that the following equivalent works on more compilers. |
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| 96 | <pre><code> |
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| 97 | // default fall through for any types not specified here |
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| 98 | template<class T> |
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| 99 | void save(const T & t){ |
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| 100 | binary_oarchive_impl<derived_t>::save(t); |
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| 101 | } |
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| 102 | void save(const unsigned int t); |
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| 103 | ... |
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| 104 | </code></pre> |
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| 105 | so it's what I use. |
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| 106 | <li><i>Template definitions of base classes may have to be included.</i> |
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| 107 | The demo includes |
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| 108 | <pre><code> |
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| 109 | // explicitly instantiate for this type of binary stream |
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| 110 | #include <boost/archive/basic_binary_oprimitive.ipp> |
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| 111 | </code></pre> |
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| 112 | for just this purpose. Failure to include required template definitions |
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| 113 | will result in undefined symbol errors when the program is linked. |
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| 114 | <li><i>Without alteration, this class cannot be further derived from</i><br> |
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| 115 | Base classes using <b>CRTP</b> must be templates with a parameter corresponding to |
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| 116 | the most derived class. As presented here, this class doesn't qualify, so |
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| 117 | it cannot be used as a base class. In order to derive further from this class, |
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| 118 | it would have to be reorganized along the lines of the original <code style="white-space: normal">binary_oarchive</code>. |
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| 119 | Specifically, it would look something like: |
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| 120 | <pre><code> |
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| 121 | template<class Archive> |
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| 122 | class portable_binary_oarchive_impl : |
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| 123 | // don't derive from binary_oarchive !!! |
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| 124 | public binary_oarchive_impl<Archive> |
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| 125 | { |
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| 126 | ... |
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| 127 | ); |
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| 128 | |
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| 129 | // do not derived from this class !!! |
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| 130 | class portable_binary_oarchive : |
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| 131 | public portable_binary_oarchive_impl<portable_binary_oarchive> |
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| 132 | { |
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| 133 | public: |
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| 134 | portable_binary_oarchive(std::ostream & os, unsigned int flags = 0) : |
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| 135 | portable_binary_oarchive_impl<binary_oarchive>(os, flags) |
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| 136 | {} |
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| 137 | }; |
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| 138 | </code></pre> |
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| 139 | |
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| 140 | </ol> |
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| 141 | |
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| 142 | <a name=fast_archives> |
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| 143 | <h3>Fast Binary Archives</h3> |
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| 144 | The second example |
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| 145 | <a href="../example/demo_fast_archive.cpp" target="demo_fast_archive_cpp">demo_fast_archive.cpp</a>. |
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| 146 | is similar to the first one. The difference is that it intercepts the serialization before |
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| 147 | the default serialization is invoked. In this case we want to replace the default |
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| 148 | serialization of C arrays of integers with a faster one. The default version |
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| 149 | will invoke serialization of each element of the array. If its an array of |
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| 150 | integers, and we're not concerned with the archive being portable to another platform |
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| 151 | we can just save/load the whole array as a binary string of bytes. This should |
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| 152 | be faster than the default element by element method. |
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| 153 | <p> |
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| 154 | The same considerations that applied when overriding the the save/load of primitives |
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| 155 | above apply here, and the code is very similar. |
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| 156 | <hr> |
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| 157 | <p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. |
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| 158 | Distributed under the Boost Software License, Version 1.0. (See |
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| 159 | accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
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| 160 | </i></p> |
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| 161 | </body> |
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| 162 | </html> |
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