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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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| 11 | <link rel="stylesheet" type="text/css" href="../../../boost.css"> |
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| 13 | <title>Serialization - PIMPL</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">PIMPL</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 | PIMPL is a C++ programming idiom described by Herb Sutter <a href="bibliography.html#10">[10]</a> |
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| 29 | which stands for "Private Implementation". It is also referred to as |
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| 30 | the "Handle Body Idiom". Included in this library is a program called |
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| 31 | <a href="../example/demo_pimpl.cpp" target="demo_impl_cpp">demo_pimpl.cpp</a> |
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| 32 | which illustrates how this is used. The file |
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| 33 | <a href="../example/demo_pimpl_A.cpp" target="demo_impl__Acpp">demo_pimpl_A.hpp</a> |
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| 34 | contains the declaration of the a class that hides its implementation |
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| 35 | by including a pointer to struct B that is only defined as a pointer. |
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| 36 | <pre><code> |
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| 37 | // class whose declaration is hidden by a pointer |
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| 38 | struct B; |
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| 39 | |
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| 40 | struct A { |
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| 41 | // class a contains a pointer to a "hidden" declaration |
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| 42 | B *pimpl; |
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| 43 | template<class Archive> |
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| 44 | void serialize(Archive & ar, const unsigned int file_version); |
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| 45 | A(); |
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| 46 | }; |
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| 47 | </code></pre> |
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| 48 | Serialization of A requires access to the definition of B in order so it |
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| 49 | is defined in the separately compiled module: |
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| 50 | <a href="../example/demo_pimpl_A.cpp" target="demo_impl_A_cpp">demo_pimpl_A.cpp</a> |
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| 51 | by: |
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| 52 | <pre><code> |
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| 53 | #include "demo_pimpl_A.hpp" |
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| 54 | |
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| 55 | // "hidden" definition of class B |
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| 56 | struct B { |
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| 57 | int b; |
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| 58 | template<class Archive> |
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| 59 | void serialize(Archive & ar, const unsigned int file_version){ |
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| 60 | ar & b; |
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| 61 | } |
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| 62 | }; |
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| 63 | |
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| 64 | A::A() : |
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| 65 | pimpl(new B) |
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| 66 | {} |
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| 67 | |
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| 68 | A::~A(){ |
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| 69 | delete pimpl; |
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| 70 | } |
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| 71 | |
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| 72 | // now we can define the serialization for class A |
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| 73 | template<class Archive> |
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| 74 | void A::serialize(Archive & ar, const unsigned int file_version){ |
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| 75 | ar & pimpl; |
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| 76 | } |
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| 77 | </code></pre> |
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| 78 | As described in <a href="bibliography.html#10">[10]</a> this brings the |
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| 79 | following advantages: |
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| 80 | <ul> |
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| 81 | <li>type "B" can be used without using its header. |
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| 82 | <li>implementation of B can be changed without generating a |
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| 83 | cascade of recompilations. |
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| 84 | </ul> |
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| 85 | So, we compile the modules and everything is fine. However when we |
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| 86 | link, we get an error. Two symbols are undefined: |
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| 87 | <pre><code> |
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| 88 | void A::serialize(boost::archive::text_oarchive & ar, const unsigned int file_version); |
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| 89 | void A::serialize(boost::archive::text_iarchive & ar, const unsigned int file_version); |
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| 90 | </code></pre> |
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| 91 | The problem is that when compiling the above code, |
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| 92 | there is no instantiation of the <code style="white-space: normal">serialize</code> template. |
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| 93 | There can't be as its not "known" what types of archives |
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| 94 | the serialization is going to be used with. So these functions are "missing" |
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| 95 | when an attempt to link is made. The solution is to explicitly instantiate |
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| 96 | serialization code for those archives which are going to be used. In this |
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| 97 | example, including the the following code in any *.cpp file does just that: |
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| 98 | <pre></code> |
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| 99 | #include <boost/archive/text_iarchive.hpp> |
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| 100 | #include <boost/archive/text_oarchive.hpp> |
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| 101 | |
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| 102 | template void A::serialize<boost::archive::text_iarchive>( |
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| 103 | boost::archive::text_iarchive & ar, |
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| 104 | const unsigned int file_version |
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| 105 | ); |
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| 106 | template void A::serialize<boost::archive::text_oarchive>( |
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| 107 | boost::archive::text_oarchive & ar, |
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| 108 | const unsigned int file_version |
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| 109 | ); |
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| 110 | </code></pre> |
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| 111 | The program should now link as well as compile. |
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| 112 | <p> |
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| 113 | The downside of this is that one has to know which archives are going |
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| 114 | to be used with hidden serializations. This is an effect of using template |
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| 115 | driven code. One can invoke explicity instantiation for all known templates and |
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| 116 | presume that the linker will exclude unreferenced code. This should |
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| 117 | work but is platform dependent. |
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| 118 | <hr> |
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| 119 | <p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. |
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| 120 | Distributed under the Boost Software License, Version 1.0. (See |
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| 121 | accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
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| 122 | </i></p> |
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| 123 | </body> |
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| 124 | </html> |
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