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13<title>Serialization - Private Base Classes</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">Private Base Classes</h2>
24 </td>
25 </tr>
26</table>
27<hr>
28In many cases, serialization of private or protected base classes present no special problems.
29This is true for both simple classes and types as well as pointers to those
30classes and types. That is, the following program compiles and runs exactly as one would expect.
31<pre><code>
32/////////1/////////2/////////3/////////4/////////5/////////6/////////7/////////8
33// test_private_base.cpp
34
35// (C) Copyright 2009 Eric Moyer - http://www.rrsd.com .
36// Use, modification and distribution is subject to the Boost Software
37// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
38// http://www.boost.org/LICENSE_1_0.txt)
39
40#include <fstream>
41#include <boost/config.hpp>
42#if defined(BOOST_NO_STDC_NAMESPACE)
43namespace std{
44 using ::remove;
45}
46#endif
47
48#include <boost/serialization/access.hpp>
49#include <boost/serialization/base_object.hpp>
50#include <boost/serialization/export.hpp>
51
52class Base {
53 friend class boost::serialization::access;
54 int m_i;
55 template<class Archive>
56 void serialize(Archive & ar, const unsigned int version){
57 ar & BOOST_SERIALIZATION_NVP(m_i);
58 }
59protected:
60 bool equals(const Base &rhs) const {
61 return m_i == rhs.m_i;
62 }
63 Base(int i = 0) :
64 m_i(i)
65 {}
66};
67
68class Derived : private Base {
69 friend class boost::serialization::access;
70private:
71 Base & base_cast(){
72 return static_cast<Base &>(*this);
73 }
74 template<class Archive>
75 void serialize(Archive & ar, const unsigned int version){
76 ar & BOOST_SERIALIZATION_BASE_OBJECT_NVP(Base);
77 }
78public:
79 bool operator==(const Derived &rhs) const {
80 return Base::equals(static_cast<const Base &>(rhs));
81 }
82 Derived(int i = 0) :
83 Base(i)
84 {}
85};
86
87int
88main( int /* argc */, char* /* argv */[] )
89{
90 const char * testfile = boost::archive::tmpnam(NULL);
91
92 // serialize Derived and Base
93 Derived a(1), a1(2);
94 {
95 test_ostream os(testfile);
96 test_oarchive oa(os);
97 oa << boost::serialization::make_nvp("a", a);
98 }
99 {
100 test_istream is(testfile, TEST_STREAM_FLAGS);
101 test_iarchive ia(is, TEST_ARCHIVE_FLAGS);
102 ia >> boost::serialization::make_nvp("a", a1);
103 }
104 std::remove(testfile);
105
106 if(a != a1)
107 return 1;
108
109 // serialize Derived and Base
110 Derived *ta = &a;
111 Derived *ta1 = NULL;
112 {
113 test_ostream os(testfile);
114 test_oarchive oa(os);
115 oa << boost::serialization::make_nvp("ta", ta);
116 }
117 {
118 test_istream is(testfile, TEST_STREAM_FLAGS);
119 test_iarchive ia(is, TEST_ARCHIVE_FLAGS);
120 ia >> boost::serialization::make_nvp("ta", ta1);
121 }
122 std::remove(testfile);
123 if(*ta != *ta1)
124 return 1;
125
126 return 0;
127}
128</code></pre>
129Difficulties start to occur when the base class is made polymorphic by the designation
130of one or more functions as "virtual". If a class is polymorphic, the library
131presumes that one will want the ability to serialize a derived class through
132a pointer to the base class. Included in the macro
133<code>
134BOOST_SERIALIZATION_BASE_OBJECT_NVP
135</code>
136is code which links derived and base class definitions in tables used to serialize
137derived classes through pointers to a polymorphinc base class. This code requires
138the ability to invoke
139<code>
140static_cast&lt;Base &&gt;(Derived &)
141</code>
142which C++ will only permit from within the derived class if the base class is
143private or protected. The program will fail to compile with an error message
144indicating invalid cast.
145<p>
146In order for this
147code compiler the following alteration must be made:
148<pre><code>
149 template<class Archive>
150 void serialize(Archive & ar, const unsigned int version){
151 //ar & BOOST_SERIALIZATION_BASE_OBJECT_NVP(Base);
152 ar & boost::serialization::make_nvp(
153 "Base",
154 static_cast&lt;Base &&gt;(*this)
155 );
156 }
157</code></pre>
158With this change the program will now compile.
159<p>
160If we made one of the functions of <code>Base></code> <code>virtual</code>
161in order to use the "export" functionality of the serialization library and permit serialization through
162a pointer the the base class, we'll be disappointed. Without the ability to
163cast to the base class, we can't use the functionality.
164<hr>
165<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2015.
166Distributed under the Boost Software License, Version 1.0. (See
167accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
168</i></p>
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