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>Portability</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="../hash.html" title="Chapter 10. Boost.ContainerHash"> 10<link rel="prev" href="combine.html" title="Combining hash values"> 11<link rel="next" href="disable.html" title="Disabling The Extensions"> 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="combine.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../hash.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="disable.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="hash.portability"></a><a class="link" href="portability.html" title="Portability">Portability</a> 29</h2></div></div></div> 30<p> 31 <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> is written to be as portable 32 as possible, but unfortunately, several older compilers don't support argument 33 dependent lookup (ADL) - the mechanism used for customisation. On those compilers 34 custom overloads for <code class="computeroutput"><span class="identifier">hash_value</span></code> 35 needs to be declared in the boost namespace. 36 </p> 37<p> 38 On a strictly standards compliant compiler, an overload defined in the boost 39 namespace won't be found when <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> 40 is instantiated, so for these compilers the overload should only be declared 41 in the same namespace as the class. 42 </p> 43<p> 44 Let's say we have a simple custom type: 45 </p> 46<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">foo</span> 47<span class="special">{</span> 48 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 49 <span class="keyword">class</span> <span class="identifier">custom_type</span> 50 <span class="special">{</span> 51 <span class="identifier">T</span> <span class="identifier">value</span><span class="special">;</span> 52 <span class="keyword">public</span><span class="special">:</span> 53 <span class="identifier">custom_type</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">value</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span> 54 55 <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">custom_type</span> <span class="identifier">x</span><span class="special">)</span> 56 <span class="special">{</span> 57 <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">hasher</span><span class="special">;</span> 58 <span class="keyword">return</span> <span class="identifier">hasher</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">value</span><span class="special">);</span> 59 <span class="special">}</span> 60 <span class="special">};</span> 61<span class="special">}</span> 62</pre> 63<p> 64 On a compliant compiler, when <code class="computeroutput"><span class="identifier">hash_value</span></code> 65 is called for this type, it will look at the namespace inside the type and 66 find <code class="computeroutput"><span class="identifier">hash_value</span></code> but on a compiler 67 which doesn't support ADL <code class="computeroutput"><span class="identifier">hash_value</span></code> 68 won't be found. To make things worse, some compilers which do support ADL won't 69 find a friend class defined inside the class. 70 </p> 71<p> 72 So first move the member function out of the class: 73 </p> 74<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">foo</span> 75<span class="special">{</span> 76 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 77 <span class="keyword">class</span> <span class="identifier">custom_type</span> 78 <span class="special">{</span> 79 <span class="identifier">T</span> <span class="identifier">value</span><span class="special">;</span> 80 <span class="keyword">public</span><span class="special">:</span> 81 <span class="identifier">custom_type</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">value</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span> 82 83 <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash</span><span class="special">(</span><span class="identifier">custom_type</span> <span class="identifier">x</span><span class="special">)</span> 84 <span class="special">{</span> 85 <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">hasher</span><span class="special">;</span> 86 <span class="keyword">return</span> <span class="identifier">hasher</span><span class="special">(</span><span class="identifier">value</span><span class="special">);</span> 87 <span class="special">}</span> 88 <span class="special">};</span> 89 90 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 91 <span class="keyword">inline</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">custom_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">x</span><span class="special">)</span> 92 <span class="special">{</span> 93 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">hash</span><span class="special">();</span> 94 <span class="special">}</span> 95<span class="special">}</span> 96</pre> 97<p> 98 Unfortunately, I couldn't declare hash_value as a friend, as some compilers 99 don't support template friends, so instead I declared a member function to 100 calculate the hash, and called it from hash_value. 101 </p> 102<p> 103 For compilers which don't support ADL, hash_value needs to be defined in the 104 boost namespace: 105 </p> 106<pre class="programlisting"><span class="preprocessor">#ifdef</span> <span class="identifier">BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP</span> 107<span class="keyword">namespace</span> <span class="identifier">boost</span> 108<span class="preprocessor">#else</span> 109<span class="keyword">namespace</span> <span class="identifier">foo</span> 110<span class="preprocessor">#endif</span> 111<span class="special">{</span> 112 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 113 <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">foo</span><span class="special">::</span><span class="identifier">custom_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">x</span><span class="special">)</span> 114 <span class="special">{</span> 115 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">hash</span><span class="special">();</span> 116 <span class="special">}</span> 117<span class="special">}</span> 118</pre> 119<p> 120 Full code for this example is at <a href="../../../libs/container_hash/examples/portable.cpp" target="_top">/libs/container_hash/examples/portable.cpp</a>. 121 </p> 122</div> 123<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 124<td align="left"></td> 125<td align="right"><div class="copyright-footer">Copyright © 2005-2008 Daniel 126 James<p> 127 Distributed under the Boost Software License, Version 1.0. (See accompanying 128 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>) 129 </p> 130</div></td> 131</tr></table> 132<hr> 133<div class="spirit-nav"> 134<a accesskey="p" href="combine.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../hash.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="disable.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 135</div> 136</body> 137</html> 138
