156 lines
4.6 KiB
HTML
156 lines
4.6 KiB
HTML
<!doctype HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!--
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(C) Copyright 2002-4 Robert Ramey - http://www.rrsd.com .
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Use, modification and distribution is subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)
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-->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<link rel="stylesheet" type="text/css" href="../../../boost.css">
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<link rel="stylesheet" type="text/css" href="style.css">
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<title>Serialization - BOOST_STATIC_WARNING</title>
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</head>
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<body link="#0000ff" vlink="#800080">
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary="header">
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<tr>
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<td valign="top" width="300">
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<h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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</td>
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<td valign="top">
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<h1 align="center">Serialization</h1>
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<h2 align="center"><code>smart_cast</code></h2>
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</td>
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</tr>
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</table>
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<hr>
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<h3>Motivation</h3>
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To cast from one type to another related type, C++ provides the following
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operators:
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<dl>
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<dt><code>static_cast<T *<>(U *)<br>static_cast<T &<>(U &)</code></dt>
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<dd>
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<ul>
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<li>required if neither T nor U are polymorphic
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<li>permitted in other cases.
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<li>fails to detect erroneous casts of polymophic pointers/references at runtime.
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<li>does not permit "cross casting"
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<li>inline function calls can be optimized away during compile time.
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</ul>
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</dd>
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<p>
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<dt><code>dynamic_cast<T *<>(U *)<br>dynamic_cast<T &<>(U &)</code></dt>
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<dd>
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<ul>
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<li>permitted if either T or U are polymorphic
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<li>prohibited in other cases.
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<li>throws exception on detecting erroneous casts of polymorphic pointers/references
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at runtime.
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<li>permits "cross casting"
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<li>cannot optimise inline virtual functions at compile time.
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</ul>
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</dd>
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</dl>
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These rules can make it difficult to use casting with a function template argument.
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Consider the following example:
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<pre><code>
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#include <boost/serialization/smart_cast.hpp>
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struct top {
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};
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struct base1 : public top {
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bool is_storable() const {
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return true;
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}
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virtual ~base1();
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};
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struct base2 {
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virtual ~base2();
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};
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struct derived1 :
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public base1
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{
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derived1();
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};
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struct derived2 :
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public base1,
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public base2
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{
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derived2();
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};
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template<class T>
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bool is_storable(T &t){
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// what type of cast to use here?
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// this fails at compile time when T == base2
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// return static_cast<base1 &>(t).is_storable();
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// this fails at compile time when T == top
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// otherwise it works but cannot optimize inline function call
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// return dynamic_cast<base1 &>(t).is_storable();
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// this always works - and is guaranteed to generate the fastest code !
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return (boost::smart_cast_reference<base1 &>(t)).is_storable();
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}
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int main(){
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derived1 d1;
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top & t1 = d1;
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derived2 d2;
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base2 & b2 = d2;
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bool result;
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result = is_storable(d1);
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result = is_storable(d2);
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result = is_storable(b2);
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result = is_storable(b2);
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result = is_storable(t1);
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return 0;
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}
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</code></pre>
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The serialization library includes a mix of classes which use
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both static polymorphism (<strong>CRTP</strong>) and dynamic
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polymorphism via virtual functions. <code style="white-space: normal">smart_cast</code>
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was written to address the more problematic manifestations of the
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situation exemplified above.
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<h3>Usage</h3>
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The following syntax is supported:
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<pre><code>
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smart_cast<Target *, Source *>(Source * s);
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smart_cast<Target *>(Source * s);
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smart_cast<Target &, Source &>(Source & s);
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</code></pre>
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Note that the above syntax doesn't include
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<pre><code>
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smart_cast<Target & >(Source & s)
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</code></pre>
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but the same functionality is supported with the following special syntax
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<pre><code>
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smart_cast_reference<Target &>(Source & s)
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</code></pre>
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<h3>Requirements</h3>
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<code style="white-space: normal">smart_cast</code> can be used only on compilers that support partial
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template specialization or on types for which the
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macro <code style="white-space: normal">
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BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION(<type>)</code>
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has been applied.
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<hr>
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<p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004.
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Distributed under the Boost Software License, Version 1.0. (See
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accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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</i></p>
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</body>
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</html>
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