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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="hash.custom"></a><a class="link" href="custom.html" title="Extending boost::hash for a custom data type">Extending boost::hash for a custom data type</a> |
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| <p> |
| <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> is implemented by calling |
| the function <code class="computeroutput"><a class="link" href="reference.html#boost.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"><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">&</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">book</span> <span class="keyword">const</span><span class="special">&</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> |
| </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"><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">&</span> <span class="identifier">b</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">b</span><span class="special">.</span><span class="identifier">id</span><span class="special">);</span> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| And you can now use <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> with |
| book: |
| </p> |
| <pre class="programlisting"><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 & Dandelion Greens"</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">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">></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"><</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</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">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">></span> <span class="special">></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> |
| </pre> |
| <p> |
| The full example can be found in: <a href="../../../libs/functional/hash/examples/books.hpp" 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="tip"><table border="0" summary="Tip"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td> |
| <th align="left">Tip</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| 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 they should generate different hash values. In this object equality |
| was based just on the id so the hash function only hashes the id. If it was |
| based on the object's name and author then the hash function should take |
| them into account (how to do this is discussed in the next section). |
| </p></td></tr> |
| </table></div> |
| </div> |
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| <td align="left"></td> |
| <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>) |
| </p> |
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