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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 .
5Use, modification and distribution is subject to the Boost Software
6License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
7http://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 - 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>
29To cast from one type to another related type, C++ provides the following
30operators:
31
32<dl>
33  <dt><code>static_cast&lt;T *<&gt;(U *)<br>static_cast&lt;T &<&gt;(U &)</code></dt>
34  <dd>
35    <ul>
36      <li>required if neither T nor U are not polymorphic
37      <li>permitted in other cases.
38      <li>fails to detect erroneas 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&lt;T *<&gt;(U *)<br>dynamic_cast&lt;T &<&gt;(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 erroneas 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
57These rules can make it difficult to use casting with a function template argument.
58Consider the following example:
59
60<pre><code>
61#include &lt;boost/smart_cast.hpp&gt;
62
63struct top {
64};
65
66struct base1 : public top {
67    bool is_storable() const {
68        return true;
69    }
70    virtual ~base1();
71};
72
73struct base2 {
74    virtual ~base2();
75};
76
77struct derived1 :
78    public base1
79{
80    derived1();
81};
82
83struct derived2 :
84    public base1,
85    public base2
86{
87    derived2();
88};
89
90template&lt;class T&gt;
91bool is_storable(T &amp;t){
92    // what type of cast to use here?
93
94    // this fails at compiler time when T == base2
95    // return static_cast&lt;base1 &amp;&gt;(t).is_storable();
96
97    // this fails at compiler time when T == top
98    // otherwise it works but cannot optimize inline function call
99    // return dynamic_cast&lt;base1 &amp;&gt;(t).is_storable();
100
101    // this always works - and is guarenteed to generate the fastest code !
102    return (boost::smart_cast_reference&lt;base1 &amp;&gt;(t)).is_storable();
103}
104
105int main(){
106    derived1 d1;
107    top &amp; t1 = d1;
108    derived2 d2;
109    base2 &amp; 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>
120The serialization library includes a mix of classes which use
121both static polymorphism (<strong>CRTP</strong>) and dynamic
122polymorphism via virtual functions.  <code style="white-space: normal">smart_cast</code> 
123was written to address the more problematic manifestations of the
124situation exmplified above.
125
126<h3>Usage</h3>
127The following syntax is supported:
128<pre><code>
129smart_cast&lt;Target *, Source *&gt;(Source * s);
130smart_cast&lt;Target *>(Source * s);
131smart_cast&lt;Target &, Source &amp;&gt(Source &amp; s);
132</code></pre>
133Note that the above syntax doesn't include
134<pre><code>
135smart_cast&lt;Target &amp; &gt(Source &amp; s)
136</code></pre>
137but the same functionality is supported the the following special syntax
138<pre><code>
139smart_cast_reference&lt;Target &amp;&gt(Source &amp; 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
144template specialization or on types for which have be specified with the
145macro <code style="white-space: normal">
146BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION(&lt;type&gt;)</code>
147has been applied.
148
149<hr>
150<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004.
151Distributed under the Boost Software License, Version 1.0. (See
152accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
153</i></p>
154</body>
155</html>
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