1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Polymorphic casts</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../conversion.html" title="Chapter 11. The Conversion Library 1.7"> 10<link rel="prev" href="../conversion.html" title="Chapter 11. The Conversion Library 1.7"> 11<link rel="next" href="synopsis.html" title="Synopsis"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="../conversion.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../conversion.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synopsis.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="conversion.polymorphic_casts"></a><a class="link" href="polymorphic_casts.html" title="Polymorphic casts">Polymorphic casts</a> 29</h2></div></div></div> 30<div class="toc"><dl class="toc"> 31<dt><span class="section"><a href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_downcast">polymorphic_downcast</a></span></dt> 32<dt><span class="section"><a href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_cast">polymorphic_cast</a></span></dt> 33<dt><span class="section"><a href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_pointer_cast">polymorphic_pointer_cast</a></span></dt> 34</dl></div> 35<p> 36 Pointers to polymorphic objects (objects of classes which define at least one 37 virtual function) are sometimes downcast or crosscast. Downcasting means casting 38 from a base class to a derived class. Crosscasting means casting across an 39 inheritance hierarchy diagram, such as from one base to the other in a <code class="literal">Y</code> 40 diagram hierarchy. 41 </p> 42<p> 43 Such casts can be done with old-style casts, but this approach is never to 44 be recommended. Old-style casts are sorely lacking in type safety, suffer poor 45 readability, and are difficult to locate with search tools. 46 </p> 47<div class="section"> 48<div class="titlepage"><div><div><h3 class="title"> 49<a name="conversion.polymorphic_casts.polymorphic_downcast"></a><a name="polymorphic_downcast"></a><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_downcast" title="polymorphic_downcast">polymorphic_downcast</a> 50</h3></div></div></div> 51<p> 52 The C++ built-in <code class="computeroutput"><span class="keyword">static_cast</span></code> 53 can be used for efficiently downcasting pointers to polymorphic objects, 54 but provides no error detection for the case where the pointer being cast 55 actually points to the wrong derived class. The <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code> 56 template retains the efficiency of <code class="computeroutput"><span class="keyword">static_cast</span></code> 57 for non-debug compilations, but for debug compilations adds safety via an 58 <code class="computeroutput"><span class="identifier">assert</span><span class="special">()</span></code> 59 that a <code class="computeroutput"><span class="keyword">dynamic_cast</span></code> succeeds. 60 </p> 61<p> 62 A <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code> should 63 be used for downcasts that you are certain should succeed. Error checking 64 is only performed in translation units where <code class="computeroutput"><span class="identifier">NDEBUG</span></code> 65 is not defined, via 66 </p> 67<pre class="programlisting"><span class="identifier">assert</span><span class="special">(</span> <span class="keyword">dynamic_cast</span><span class="special"><</span><span class="identifier">Derived</span><span class="special">>(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">)</span> 68</pre> 69<p> 70 where <code class="computeroutput"><span class="identifier">x</span></code> is the source pointer. 71 This approach ensures that not only is a non-zero pointer returned, but also 72 that it is correct in the presence of multiple inheritance. Attempts to crosscast 73 using <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code> 74 will fail to compile. 75 </p> 76<div class="warning"><table border="0" summary="Warning"> 77<tr> 78<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td> 79<th align="left">Warning</th> 80</tr> 81<tr><td align="left" valign="top"><p> 82 Because <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code> 83 uses <code class="computeroutput"><span class="identifier">assert</span><span class="special">()</span></code>, 84 it violates the One Definition Rule (ODR) if <code class="computeroutput"><span class="identifier">NDEBUG</span></code> 85 is inconsistently defined across translation units. See ISO Std 3.2 86 </p></td></tr> 87</table></div> 88<h5> 89<a name="conversion.polymorphic_casts.polymorphic_downcast.h0"></a> 90 <span class="phrase"><a name="conversion.polymorphic_casts.polymorphic_downcast.example_"></a></span><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_downcast.example_">Example:</a> 91 </h5> 92<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">polymorphic_cast</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 93<span class="special">...</span> 94<span class="keyword">class</span> <span class="identifier">Fruit</span> <span class="special">{</span> <span class="keyword">public</span><span class="special">:</span> <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Fruit</span><span class="special">(){};</span> <span class="special">...</span> <span class="special">};</span> 95<span class="keyword">class</span> <span class="identifier">Banana</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Fruit</span> <span class="special">{</span> <span class="special">...</span> <span class="special">};</span> 96<span class="special">...</span> 97<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span> <span class="identifier">Fruit</span> <span class="special">*</span> <span class="identifier">fruit</span> <span class="special">)</span> <span class="special">{</span> 98 <span class="comment">// ... logic which leads us to believe it is a Banana</span> 99 <span class="identifier">Banana</span> <span class="special">*</span> <span class="identifier">banana</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">polymorphic_downcast</span><span class="special"><</span><span class="identifier">Banana</span><span class="special">*>(</span><span class="identifier">fruit</span><span class="special">);</span> 100 <span class="special">...</span> 101<span class="special">}</span> 102</pre> 103</div> 104<div class="section"> 105<div class="titlepage"><div><div><h3 class="title"> 106<a name="conversion.polymorphic_casts.polymorphic_cast"></a><a name="polymorphic_cast"></a><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_cast" title="polymorphic_cast">polymorphic_cast</a> 107</h3></div></div></div> 108<p> 109 The C++ built-in <code class="computeroutput"><span class="keyword">dynamic_cast</span></code> 110 can be used for downcasts and crosscasts of pointers to polymorphic objects, 111 but error notification in the form of a returned value of 0 is inconvenient 112 to test, or worse yet, easy to forget to test. The throwing form of <code class="computeroutput"><span class="keyword">dynamic_cast</span></code>, which works on references, can 113 be used on pointers through the ugly expression <code class="computeroutput"><span class="special">&</span><span class="keyword">dynamic_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>(*</span><span class="identifier">p</span><span class="special">)</span></code>, which 114 causes undefined behavior if <code class="computeroutput"><span class="identifier">p</span></code> 115 is <code class="computeroutput"><span class="number">0</span></code>. The <code class="computeroutput"><span class="identifier">polymorphic_cast</span></code> 116 template performs a <code class="computeroutput"><span class="keyword">dynamic_cast</span></code> 117 on a pointer, and throws an exception if the <code class="computeroutput"><span class="keyword">dynamic_cast</span></code> 118 returns 0. 119 </p> 120<p> 121 For crosscasts, or when the success of a cast can only be known at runtime, 122 or when efficiency is not important, <code class="computeroutput"><span class="identifier">polymorphic_cast</span></code> 123 is preferred. 124 </p> 125<p> 126 The C++ built-in <code class="computeroutput"><span class="keyword">dynamic_cast</span></code> 127 must be used to cast references rather than pointers. It is also the only 128 cast that can be used to check whether a given interface is supported; in 129 that case a return of 0 isn't an error condition. 130 </p> 131</div> 132<div class="section"> 133<div class="titlepage"><div><div><h3 class="title"> 134<a name="conversion.polymorphic_casts.polymorphic_pointer_cast"></a><a name="polymorphic_pointer_cast"></a><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_pointer_cast" title="polymorphic_pointer_cast">polymorphic_pointer_cast</a> 135</h3></div></div></div> 136<p> 137 While <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code> 138 and <code class="computeroutput"><span class="identifier">polymorphic_cast</span></code> work 139 with built-in pointer types only, <code class="computeroutput"><span class="identifier">polymorphic_pointer_downcast</span></code> 140 and <code class="computeroutput"><span class="identifier">polymorphic_pointer_cast</span></code> 141 are more generic versions with support for any pointer type for which the 142 following expressions would be valid: 143 </p> 144<p> 145 For <code class="computeroutput"><span class="identifier">polymorphic_pointer_downcast</span></code>: 146 </p> 147<pre class="programlisting"><span class="identifier">static_pointer_cast</span><span class="special"><</span><span class="identifier">Derived</span><span class="special">>(</span><span class="identifier">p</span><span class="special">);</span> 148<span class="identifier">dynamic_pointer_cast</span><span class="special"><</span><span class="identifier">Derived</span><span class="special">>(</span><span class="identifier">p</span><span class="special">);</span> 149</pre> 150<p> 151 For <code class="computeroutput"><span class="identifier">polymorphic_pointer_cast</span></code>: 152 </p> 153<pre class="programlisting"><span class="identifier">dynamic_pointer_cast</span><span class="special"><</span><span class="identifier">Derived</span><span class="special">>(</span><span class="identifier">p</span><span class="special">);</span> 154<span class="special">!</span><span class="identifier">p</span><span class="special">;</span> <span class="comment">// conversion to bool with negation</span> 155</pre> 156<p> 157 This includes C++ built-in pointers, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span></code>, 158 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span></code>, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive_ptr</span></code>, 159 etc. 160 </p> 161<h5> 162<a name="conversion.polymorphic_casts.polymorphic_pointer_cast.h0"></a> 163 <span class="phrase"><a name="conversion.polymorphic_casts.polymorphic_pointer_cast.example_"></a></span><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_pointer_cast.example_">Example:</a> 164 </h5> 165<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">polymorphic_pointer_cast</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 166 167<span class="keyword">class</span> <span class="identifier">Fruit</span> <span class="special">{</span> <span class="keyword">public</span><span class="special">:</span> <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Fruit</span><span class="special">(){}</span> <span class="special">};</span> 168<span class="keyword">class</span> <span class="identifier">Banana</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Fruit</span> <span class="special">{};</span> 169 170<span class="comment">// Use one of these:</span> 171<span class="keyword">typedef</span> <span class="identifier">Fruit</span><span class="special">*</span> <span class="identifier">FruitPtr</span><span class="special">;</span> 172<span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">Fruit</span><span class="special">></span> <span class="identifier">FruitPtr</span><span class="special">;</span> 173<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">Fruit</span><span class="special">></span> <span class="identifier">FruitPtr</span><span class="special">;</span> 174<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive_ptr</span><span class="special"><</span><span class="identifier">Fruit</span><span class="special">></span> <span class="identifier">FruitPtr</span><span class="special">;</span> 175 176<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">FruitPtr</span> <span class="identifier">fruit</span><span class="special">)</span> <span class="special">{</span> 177 <span class="comment">// ... logic which leads us to believe it is a banana</span> 178 <span class="keyword">auto</span> <span class="identifier">banana</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">polymorphic_pointer_downcast</span><span class="special"><</span><span class="identifier">Banana</span><span class="special">>(</span><span class="identifier">fruit</span><span class="special">);</span> 179 <span class="special">...</span> 180<span class="special">}</span> 181</pre> 182</div> 183</div> 184<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 185<td align="left"></td> 186<td align="right"><div class="copyright-footer">Copyright © 2001 Beman Dawes<br>Copyright © 2014-2020 Antony Polukhin<p> 187 Distributed under the Boost Software License, Version 1.0. (See accompanying 188 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 189 </p> 190</div></td> 191</tr></table> 192<hr> 193<div class="spirit-nav"> 194<a accesskey="p" href="../conversion.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../conversion.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synopsis.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 195</div> 196</body> 197</html> 198
