| [29] | 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) |
|---|
| 8 | --> |
|---|
| 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 - PIMPL</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">PIMPL</h2> |
|---|
| 24 | </td> |
|---|
| 25 | </tr> |
|---|
| 26 | </table> |
|---|
| 27 | <hr> |
|---|
| 28 | PIMPL is a C++ programming idiom described by Herb Sutter <a href="bibliography.html#10">[10]</a> |
|---|
| 29 | which stands for "Private Implementation". It is also referred to as |
|---|
| 30 | the "Handle Body Idiom". Included in this library is a program called |
|---|
| 31 | <a href="../example/demo_pimpl.cpp" target="demo_impl_cpp">demo_pimpl.cpp</a> |
|---|
| 32 | which illustrates how this is used. The file |
|---|
| 33 | <a href="../example/demo_pimpl_A.cpp" target="demo_impl__Acpp">demo_pimpl_A.hpp</a> |
|---|
| 34 | contains the declaration of the a class that hides its implementation |
|---|
| 35 | by including a pointer to struct B that is only defined as a pointer. |
|---|
| 36 | <pre><code> |
|---|
| 37 | // class whose declaration is hidden by a pointer |
|---|
| 38 | struct B; |
|---|
| 39 | |
|---|
| 40 | struct A { |
|---|
| 41 | // class a contains a pointer to a "hidden" declaration |
|---|
| 42 | B *pimpl; |
|---|
| 43 | template<class Archive> |
|---|
| 44 | void serialize(Archive & ar, const unsigned int file_version); |
|---|
| 45 | A(); |
|---|
| 46 | }; |
|---|
| 47 | </code></pre> |
|---|
| 48 | Serialization of A requires access to the definition of B in order so it |
|---|
| 49 | is defined in the separately compiled module: |
|---|
| 50 | <a href="../example/demo_pimpl_A.cpp" target="demo_impl_A_cpp">demo_pimpl_A.cpp</a> |
|---|
| 51 | by: |
|---|
| 52 | <pre><code> |
|---|
| 53 | #include "demo_pimpl_A.hpp" |
|---|
| 54 | |
|---|
| 55 | // "hidden" definition of class B |
|---|
| 56 | struct B { |
|---|
| 57 | int b; |
|---|
| 58 | template<class Archive> |
|---|
| 59 | void serialize(Archive & ar, const unsigned int file_version){ |
|---|
| 60 | ar & b; |
|---|
| 61 | } |
|---|
| 62 | }; |
|---|
| 63 | |
|---|
| 64 | A::A() : |
|---|
| 65 | pimpl(new B) |
|---|
| 66 | {} |
|---|
| 67 | |
|---|
| 68 | A::~A(){ |
|---|
| 69 | delete pimpl; |
|---|
| 70 | } |
|---|
| 71 | |
|---|
| 72 | // now we can define the serialization for class A |
|---|
| 73 | template<class Archive> |
|---|
| 74 | void A::serialize(Archive & ar, const unsigned int file_version){ |
|---|
| 75 | ar & pimpl; |
|---|
| 76 | } |
|---|
| 77 | </code></pre> |
|---|
| 78 | As described in <a href="bibliography.html#10">[10]</a> this brings the |
|---|
| 79 | following advantages: |
|---|
| 80 | <ul> |
|---|
| 81 | <li>type "B" can be used without using its header. |
|---|
| 82 | <li>implementation of B can be changed without generating a |
|---|
| 83 | cascade of recompilations. |
|---|
| 84 | </ul> |
|---|
| 85 | So, we compile the modules and everything is fine. However when we |
|---|
| 86 | link, we get an error. Two symbols are undefined: |
|---|
| 87 | <pre><code> |
|---|
| 88 | void A::serialize(boost::archive::text_oarchive & ar, const unsigned int file_version); |
|---|
| 89 | void A::serialize(boost::archive::text_iarchive & ar, const unsigned int file_version); |
|---|
| 90 | </code></pre> |
|---|
| 91 | The problem is that when compiling the above code, |
|---|
| 92 | there is no instantiation of the <code style="white-space: normal">serialize</code> template. |
|---|
| 93 | There can't be as its not "known" what types of archives |
|---|
| 94 | the serialization is going to be used with. So these functions are "missing" |
|---|
| 95 | when an attempt to link is made. The solution is to explicitly instantiate |
|---|
| 96 | serialization code for those archives which are going to be used. In this |
|---|
| 97 | example, including the the following code in any *.cpp file does just that: |
|---|
| 98 | <pre></code> |
|---|
| 99 | #include <boost/archive/text_iarchive.hpp> |
|---|
| 100 | #include <boost/archive/text_oarchive.hpp> |
|---|
| 101 | |
|---|
| 102 | template void A::serialize<boost::archive::text_iarchive>( |
|---|
| 103 | boost::archive::text_iarchive & ar, |
|---|
| 104 | const unsigned int file_version |
|---|
| 105 | ); |
|---|
| 106 | template void A::serialize<boost::archive::text_oarchive>( |
|---|
| 107 | boost::archive::text_oarchive & ar, |
|---|
| 108 | const unsigned int file_version |
|---|
| 109 | ); |
|---|
| 110 | </code></pre> |
|---|
| 111 | The program should now link as well as compile. |
|---|
| 112 | <p> |
|---|
| 113 | The downside of this is that one has to know which archives are going |
|---|
| 114 | to be used with hidden serializations. This is an effect of using template |
|---|
| 115 | driven code. One can invoke explicity instantiation for all known templates and |
|---|
| 116 | presume that the linker will exclude unreferenced code. This should |
|---|
| 117 | work but is platform dependent. |
|---|
| 118 | <hr> |
|---|
| 119 | <p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. |
|---|
| 120 | Distributed under the Boost Software License, Version 1.0. (See |
|---|
| 121 | accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
|---|
| 122 | </i></p> |
|---|
| 123 | </body> |
|---|
| 124 | </html> |
|---|
