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<title>Portability</title>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="hash.portability"></a><a class="link" href="portability.html" title="Portability">Portability</a>
</h2></div></div></div>
<p>
<code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> is written to be as portable
as possible, but unfortunately, several older compilers don't support argument
dependent lookup (ADL) - the mechanism used for customisation. On those compilers
custom overloads for <code class="computeroutput"><span class="identifier">hash_value</span></code>
needs to be declared in the boost namespace.
</p>
<p>
On a strictly standards compliant compiler, an overload defined in the boost
namespace won't be found when <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code>
is instantiated, so for these compilers the overload should only be declared
in the same namespace as the class.
</p>
<p>
Let's say we have a simple custom type:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">foo</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">custom_type</span>
<span class="special">{</span>
<span class="identifier">T</span> <span class="identifier">value</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<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>
<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>
<span class="special">{</span>
<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>
<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>
<span class="special">}</span>
<span class="special">};</span>
<span class="special">}</span>
</pre>
<p>
On a compliant compiler, when <code class="computeroutput"><span class="identifier">hash_value</span></code>
is called for this type, it will look at the namespace inside the type and
find <code class="computeroutput"><span class="identifier">hash_value</span></code> but on a compiler
which doesn't support ADL <code class="computeroutput"><span class="identifier">hash_value</span></code>
won't be found. To make things worse, some compilers which do support ADL won't
find a friend class defined inside the class.
</p>
<p>
So first move the member function out of the class:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">foo</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">custom_type</span>
<span class="special">{</span>
<span class="identifier">T</span> <span class="identifier">value</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<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>
<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>
<span class="special">{</span>
<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>
<span class="keyword">return</span> <span class="identifier">hasher</span><span class="special">(</span><span class="identifier">value</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></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="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">x</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">hash</span><span class="special">();</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
Unfortunately, I couldn't declare hash_value as a friend, as some compilers
don't support template friends, so instead I declared a member function to
calculate the hash, and called it from hash_value.
</p>
<p>
For compilers which don't support ADL, hash_value needs to be defined in the
boost namespace:
</p>
<pre class="programlisting"><span class="preprocessor">#ifdef</span> <span class="identifier">BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span>
<span class="preprocessor">#else</span>
<span class="keyword">namespace</span> <span class="identifier">foo</span>
<span class="preprocessor">#endif</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></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">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>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">hash</span><span class="special">();</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
Full code for this example is at <a href="../../../libs/functional/hash/examples/portable.cpp" target="_top">/libs/functional/hash/examples/portable.cpp</a>.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2005-2008 Daniel
James<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>)
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