ceph/src/boost/libs/serialization/doc/private_base.html

   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 - Private Base Classes</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">Private Base Classes</h2>
  24     </td>
  25   </tr>
  26 </table>
  27 <hr>
  28 In many cases, serialization of private or protected base classes present no special problems.
  29 This is true for both simple classes and types as well as pointers to those
  30 classes 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)
  43 namespace 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
  52 class 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     }
  59 protected:
  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
  68 class Derived : private Base {
  69     friend class boost::serialization::access;
  70 private:
  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     }
  78 public:
  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
  87 int
  88 main( 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>
 129 Difficulties start to occur when the base class is made polymorphic by the designation
 130 of one or more functions as "virtual".  If a class is polymorphic, the library
 131 presumes that one will want the ability to serialize a derived class through
 132 a pointer to the base class.  Included in the macro
 133 <code>
 134 BOOST_SERIALIZATION_BASE_OBJECT_NVP
 135 </code>
 136 is code which links derived and base class definitions in tables used to serialize
 137 derived classes through pointers to a polymorphinc base class.  This code requires
 138 the ability to invoke
 139 <code>
 140 static_cast&lt;Base &&gt;(Derived &)
 141 </code>
 142 which C++ will only permit from within the derived class if the base class is
 143 private or protected. The program will fail to compile with an error message
 144 indicating invalid cast.
 145 <p>
 146 In order for this
 147 code 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>
 158 With this change the program will now compile.
 159 <p>
 160 If we made one of the functions of <code>Base></code> <code>virtual</code>
 161 in order to use the "export" functionality of the serialization library and permit serialization through
 162 a pointer the the base class, we'll be disappointed. Without the ability to
 163 cast 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.
 166 Distributed under the Boost Software License, Version 1.0. (See
 167 accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 168 </i></p>
 169 </body>
 170 </html>