| Brian Silverman | 8867871 | 2018-08-04 23:56:48 -0700 | [diff] [blame^] | 1 | <!doctype HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
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| 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) |
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| 9 | <head> |
| 10 | <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> |
| 11 | <link rel="stylesheet" type="text/css" href="../../../boost.css"> |
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| 13 | <title>Serialization - BOOST_STATIC_WARNING</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"><code>smart_cast</code></h2> |
| 24 | </td> |
| 25 | </tr> |
| 26 | </table> |
| 27 | <hr> |
| 28 | <h3>Motivation</h3> |
| 29 | To cast from one type to another related type, C++ provides the following |
| 30 | operators: |
| 31 | |
| 32 | <dl> |
| 33 | <dt><code>static_cast<T *<>(U *)<br>static_cast<T &<>(U &)</code></dt> |
| 34 | <dd> |
| 35 | <ul> |
| 36 | <li>required if neither T nor U are polymorphic |
| 37 | <li>permitted in other cases. |
| 38 | <li>fails to detect erroneous casts of polymophic pointers/references at runtime. |
| 39 | <li>does not permit "cross casting" |
| 40 | <li>inline function calls can be optimized away during compile time. |
| 41 | </ul> |
| 42 | </dd> |
| 43 | <p> |
| 44 | <dt><code>dynamic_cast<T *<>(U *)<br>dynamic_cast<T &<>(U &)</code></dt> |
| 45 | <dd> |
| 46 | <ul> |
| 47 | <li>permitted if either T or U are polymorphic |
| 48 | <li>prohibited in other cases. |
| 49 | <li>throws exception on detecting erroneous casts of polymorphic pointers/references |
| 50 | at runtime. |
| 51 | <li>permits "cross casting" |
| 52 | <li>cannot optimise inline virtual functions at compile time. |
| 53 | </ul> |
| 54 | </dd> |
| 55 | </dl> |
| 56 | |
| 57 | These rules can make it difficult to use casting with a function template argument. |
| 58 | Consider the following example: |
| 59 | |
| 60 | <pre><code> |
| 61 | #include <boost/serialization/smart_cast.hpp> |
| 62 | |
| 63 | struct top { |
| 64 | }; |
| 65 | |
| 66 | struct base1 : public top { |
| 67 | bool is_storable() const { |
| 68 | return true; |
| 69 | } |
| 70 | virtual ~base1(); |
| 71 | }; |
| 72 | |
| 73 | struct base2 { |
| 74 | virtual ~base2(); |
| 75 | }; |
| 76 | |
| 77 | struct derived1 : |
| 78 | public base1 |
| 79 | { |
| 80 | derived1(); |
| 81 | }; |
| 82 | |
| 83 | struct derived2 : |
| 84 | public base1, |
| 85 | public base2 |
| 86 | { |
| 87 | derived2(); |
| 88 | }; |
| 89 | |
| 90 | template<class T> |
| 91 | bool is_storable(T &t){ |
| 92 | // what type of cast to use here? |
| 93 | |
| 94 | // this fails at compile time when T == base2 |
| 95 | // return static_cast<base1 &>(t).is_storable(); |
| 96 | |
| 97 | // this fails at compile time when T == top |
| 98 | // otherwise it works but cannot optimize inline function call |
| 99 | // return dynamic_cast<base1 &>(t).is_storable(); |
| 100 | |
| 101 | // this always works - and is guaranteed to generate the fastest code ! |
| 102 | return (boost::smart_cast_reference<base1 &>(t)).is_storable(); |
| 103 | } |
| 104 | |
| 105 | int main(){ |
| 106 | derived1 d1; |
| 107 | top & t1 = d1; |
| 108 | derived2 d2; |
| 109 | base2 & b2 = d2; |
| 110 | |
| 111 | bool result; |
| 112 | result = is_storable(d1); |
| 113 | result = is_storable(d2); |
| 114 | result = is_storable(b2); |
| 115 | result = is_storable(b2); |
| 116 | result = is_storable(t1); |
| 117 | return 0; |
| 118 | } |
| 119 | </code></pre> |
| 120 | The serialization library includes a mix of classes which use |
| 121 | both static polymorphism (<strong>CRTP</strong>) and dynamic |
| 122 | polymorphism via virtual functions. <code style="white-space: normal">smart_cast</code> |
| 123 | was written to address the more problematic manifestations of the |
| 124 | situation exemplified above. |
| 125 | |
| 126 | <h3>Usage</h3> |
| 127 | The following syntax is supported: |
| 128 | <pre><code> |
| 129 | smart_cast<Target *, Source *>(Source * s); |
| 130 | smart_cast<Target *>(Source * s); |
| 131 | smart_cast<Target &, Source &>(Source & s); |
| 132 | </code></pre> |
| 133 | Note that the above syntax doesn't include |
| 134 | <pre><code> |
| 135 | smart_cast<Target & >(Source & s) |
| 136 | </code></pre> |
| 137 | but the same functionality is supported with the following special syntax |
| 138 | <pre><code> |
| 139 | smart_cast_reference<Target &>(Source & s) |
| 140 | </code></pre> |
| 141 | |
| 142 | <h3>Requirements</h3> |
| 143 | <code style="white-space: normal">smart_cast</code> can be used only on compilers that support partial |
| 144 | template specialization or on types for which the |
| 145 | macro <code style="white-space: normal"> |
| 146 | BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION(<type>)</code> |
| 147 | has been applied. |
| 148 | |
| 149 | <hr> |
| 150 | <p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. |
| 151 | Distributed under the Boost Software License, Version 1.0. (See |
| 152 | accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
| 153 | </i></p> |
| 154 | </body> |
| 155 | </html> |