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+<title> Extending boost::hash for a custom data type</title>
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+<div class="titlepage"><div><div><h3 class="title">
+<a name="hash.custom"></a> Extending boost::hash for a custom data type</h3></div></div></div>
+<p><code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> is implemented by calling the function <code class="computeroutput"><a href="../id1042434.html" title="Function hash_value">hash_value</a></code>.
+The namespace isn't specified so that it can detect overloads via argument
+dependant lookup. So if there is a free function <code class="computeroutput"><span class="identifier">hash_value</span></code> in the same
+namespace as a custom type, it will get called.</p>
+<p>
+If you have a structure <code class="computeroutput"><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span></code>, where each <code class="computeroutput"><span class="identifier">book</span></code> is uniquely
+defined by it's member <code class="computeroutput"><span class="identifier">id</span></code>:</p>
+<pre class="programlisting"><code class="literal"><span class="keyword">namespace</span><span class="identifier"> library</span><span class="special">
+{</span><span class="keyword">
+    struct</span><span class="identifier"> book</span><span class="special">
+    {</span><span class="keyword">
+        int</span><span class="identifier"> id</span><span class="special">;</span><span class="identifier">
+        std</span><span class="special">::</span><span class="identifier">string</span><span class="identifier"> author</span><span class="special">;</span><span class="identifier">
+        std</span><span class="special">::</span><span class="identifier">string</span><span class="identifier"> title</span><span class="special">;</span><span class="comment">
+
+        // ....
+</span><span class="special">    };</span><span class="keyword">
+
+    bool</span><span class="keyword"> operator</span><span class="special">==(</span><span class="identifier">book</span><span class="keyword"> const</span><span class="special">&amp;</span><span class="identifier"> a</span><span class="special">,</span><span class="identifier"> book</span><span class="keyword"> const</span><span class="special">&amp;</span><span class="identifier"> b</span><span class="special">)</span><span class="special">
+    {</span><span class="keyword">
+        return</span><span class="identifier"> a</span><span class="special">.</span><span class="identifier">id</span><span class="special"> ==</span><span class="identifier"> b</span><span class="special">.</span><span class="identifier">id</span><span class="special">;</span><span class="special">
+    }</span><span class="special">
+}</span></code></pre>
+<p>
+Then all you would need to do is write the function <code class="computeroutput"><span class="identifier">library</span><span class="special">::</span><span class="identifier">hash_value</span></code>:</p>
+<pre class="programlisting"><code class="literal"><span class="keyword">namespace</span><span class="identifier"> library</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">book</span><span class="keyword"> const</span><span class="special">&amp;</span><span class="identifier"> b</span><span class="special">)</span><span class="special">
+    {</span>
+        <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">
+        return</span><span class="identifier"> hasher</span><span class="special">(</span><span class="identifier">b</span><span class="special">.</span><span class="identifier">id</span><span class="special">);</span><span class="special">
+    }</span><span class="special">
+}</span></code></pre>
+<p>
+And you can now use <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> with book:</p>
+<pre class="programlisting"><code class="literal"><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="identifier"> knife</span><span class="special">(</span><span class="number">3458</span><span class="special">,</span><span class="string"> "Zane Grey"</span><span class="special">,</span><span class="string"> "The Hash Knife Outfit"</span><span class="special">);</span><span class="identifier">
+library</span><span class="special">::</span><span class="identifier">book</span><span class="identifier"> dandelion</span><span class="special">(</span><span class="number">1354</span><span class="special">,</span><span class="string"> "Paul J. Shanley"</span><span class="special">,</span><span class="string">
+    "Hash &amp; Dandelion Greens"</span><span class="special">);</span><code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special">&lt;</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">&gt;</span><span class="identifier"> book_hasher</span><span class="special">;</span><span class="identifier">
+std</span><span class="special">::</span><span class="identifier">size_t</span><span class="identifier"> knife_hash_value</span><span class="special"> =</span><span class="identifier"> book_hasher</span><span class="special">(</span><span class="identifier">knife</span><span class="special">);</span><span class="comment">
+
+// If std::unordered_set is available:
+</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unordered_set</span><span class="special">&lt;</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">,</span> <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special">&lt;</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">&gt;</span><span class="special"> &gt;</span><span class="identifier"> books</span><span class="special">;</span><span class="identifier">
+books</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">knife</span><span class="special">);</span><span class="identifier">
+books</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">(</span><span class="number">2443</span><span class="special">,</span><span class="string"> "Lindgren, Torgny"</span><span class="special">,</span><span class="string"> "Hash"</span><span class="special">));</span><span class="identifier">
+books</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">(</span><span class="number">1953</span><span class="special">,</span><span class="string"> "Snyder, Bernadette M."</span><span class="special">,</span><span class="string">
+    "Heavenly Hash: A Tasty Mix of a Mother's Meditations"</span><span class="special">));</span><span class="identifier">
+
+assert</span><span class="special">(</span><span class="identifier">books</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">knife</span><span class="special">)</span><span class="special"> !=</span><span class="identifier"> books</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span><span class="identifier">
+assert</span><span class="special">(</span><span class="identifier">books</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">dandelion</span><span class="special">)</span><span class="special"> ==</span><span class="identifier"> books</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span></code></pre>
+<p>
+The full example can be found in:
+<a href="../../../libs/functional/hash/examples/books.cpp" target="_top">/libs/functional/hash/examples/books.hpp</a>
+and
+<a href="../../../libs/functional/hash/examples/books.cpp" target="_top">/libs/functional/hash/examples/books.cpp</a>.</p>
+<div class="informaltable"><table class="table">
+<colgroup><col></colgroup>
+<tbody><tr><td class="blurb">
+When writing a hash function, first look at how the equality function works.
+Objects that are equal must generate the same hash value.
+When objects are not equal the should generate different hash values.
+In this object equality was based just on the id, if it was based
+on the objects name and author the hash function should take them into account
+(how to do this is discussed in the next section).
+</td></tr></tbody>
+</table></div>
+</div>
+<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
+<td align="left"></td>
+<td align="right"><small>Copyright © 2005 Daniel James</small></td>
+</tr></table>
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