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4<title> Portability</title>
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26<div class="titlepage"><div><div><h3 class="title">
27<a name="hash.portability"></a> Portability</h3></div></div></div>
28<p><code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> is written to be as portable as possible, but unfortunately, several
29older compilers don't support argument dependent lookup (ADL) - the mechanism
30used for customization.  On those compilers custom overloads for hash_value
31need to be declared in the boost namespace.</p>
32<p>
33On a strictly standards compliant compiler, an overload defined in the
34boost namespace won't be found when <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> is instantiated,
35so for these compilers the overload should only be declared in the same
36namespace as the class.</p>
37<p>
38Let's say we have a simple custom type:</p>
39<pre class="programlisting"><code class="literal"><span class="keyword">namespace</span><span class="identifier"> foo</span><span class="special">
40{</span><span class="keyword">
41    struct</span><span class="identifier"> custom_type</span><span class="special">
42    {</span><span class="keyword">
43        int</span><span class="identifier"> value</span><span class="special">;</span><span class="keyword">
44
45        friend</span><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="identifier"> x</span><span class="special">)</span><span class="special">
46        {</span>
47            <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span><span class="identifier"> hasher</span><span class="special">;</span><span class="keyword">
48            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><span class="special">
49        }</span><span class="special">
50    };</span><span class="special">
51}</span></code></pre>
52<p>
53On a compliant compiler, when <code class="computeroutput"><span class="identifier">hash_value</span></code> is called for this type,
54it will look at the namespace inside the type and find <code class="computeroutput"><span class="identifier">hash_value</span></code>
55but on a compiler which doesn't support ADL <code class="computeroutput"><span class="identifier">hash_value</span></code> won't be found.</p>
56<p>
57So on these compilers define a member function:</p>
58<pre class="programlisting"><code class="literal"><span class="preprocessor">#ifndef</span><span class="identifier"> BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP</span><span class="keyword">
59        friend</span><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="identifier"> x</span><span class="special">)</span><span class="special">
60        {</span>
61            <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span><span class="identifier"> hasher</span><span class="special">;</span><span class="keyword">
62            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><span class="special">
63        }</span><span class="preprocessor">
64#else</span><span class="identifier">
65        std</span><span class="special">::</span><span class="identifier">size_t</span><span class="identifier"> hash</span><span class="special">()</span><span class="keyword"> const</span><span class="special">
66        {</span>
67            <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span><span class="identifier"> hasher</span><span class="special">;</span><span class="keyword">
68            return</span><span class="identifier"> hasher</span><span class="special">(</span><span class="identifier">value</span><span class="special">);</span><span class="special">
69        }</span><span class="preprocessor">
70#endif</span></code></pre>
71<p>
72which will be called from the <code class="computeroutput"><span class="identifier">boost</span></code> namespace:</p>
73<pre class="programlisting"><code class="literal"><span class="preprocessor">#ifdef</span><span class="identifier"> BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP</span><span class="keyword">
74namespace</span><span class="identifier"> boost</span><span class="special">
75{</span><span class="identifier">
76    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="identifier"> x</span><span class="special">)</span><span class="special">
77    {</span><span class="keyword">
78        return</span><span class="identifier"> x</span><span class="special">.</span><span class="identifier">hash</span><span class="special">();</span><span class="special">
79    }</span><span class="special">
80}</span><span class="preprocessor">
81#endif</span></code></pre>
82<p>
83Full code for this example is at
84<a href="../../../libs/functional/hash/examples/portable.cpp" target="_top">/libs/functional/hash/examples/portable.cpp</a>.</p>
85<a name="portability.other_issues"></a><h2>
86<a name="id2703933"></a>Other Issues</h2>
87<p>
88On Visual C++ versions 6.5 and 7.0, <code class="computeroutput"><span class="identifier">hash_value</span></code> isn't overloaded for built in
89arrays. <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code>, <code class="computeroutput"><a href="../hash_combine.html" title="Function template hash_combine">boost::hash_combine</a></code> and <code class="computeroutput"><a href="../hash_range.html" title="Function hash_range">boost::hash_range</a></code> all use a workaround to
90support built in arrays so this shouldn't be a problem in most cases.</p>
91<p>
92On Visual C++ versions 6.5 and 7.0, function pointers aren't currently supported.</p>
93<p><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_value</span><span class="special">(</span><span class="keyword">long</span><span class="keyword"> double</span><span class="special">)</span></code> on GCC on Solaris appears to treat
94<code class="computeroutput"><span class="keyword">long</span><span class="keyword"> double</span></code>s as doubles - so the hash function doesn't take into account the
95full range of values.</p>
96</div>
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99<td align="right"><small>Copyright © 2005 Daniel James</small></td>
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