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| 26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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| 27 | <a name="hash.custom"></a> Extending boost::hash for a custom data type</h2></div></div></div> |
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| 28 | <p> |
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| 29 | <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> is implemented by calling |
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| 30 | the function <code class="computeroutput"><a href="../boost/hash_value.html" title="Function hash_value">hash_value</a></code>. |
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| 31 | The namespace isn't specified so that it can detect overloads via argument |
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| 32 | dependant lookup. So if there is a free function <code class="computeroutput"><span class="identifier">hash_value</span></code> |
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| 33 | in the same namespace as a custom type, it will get called. |
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| 34 | </p> |
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| 35 | <p> |
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| 36 | If you have a structure <code class="computeroutput"><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span></code>, where |
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| 37 | each <code class="computeroutput"><span class="identifier">book</span></code> is uniquely defined |
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| 38 | by it's member <code class="computeroutput"><span class="identifier">id</span></code>: |
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| 39 | </p> |
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| 40 | <pre class="programlisting"> |
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| 41 | <span class="keyword">namespace</span> <span class="identifier">library</span> |
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| 42 | <span class="special">{</span> |
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| 43 | <span class="keyword">struct</span> <span class="identifier">book</span> |
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| 44 | <span class="special">{</span> |
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| 45 | <span class="keyword">int</span> <span class="identifier">id</span><span class="special">;</span> |
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| 46 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">author</span><span class="special">;</span> |
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| 47 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">title</span><span class="special">;</span> |
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| 48 | |
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| 49 | <span class="comment">// .... |
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| 50 | </span> <span class="special">};</span> |
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| 51 | |
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| 52 | <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> |
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| 53 | <span class="special">{</span> |
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| 54 | <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> |
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| 55 | <span class="special">}</span> |
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| 56 | <span class="special">}</span> |
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| 57 | </pre> |
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| 58 | <p> |
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| 59 | 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>: |
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| 60 | </p> |
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| 61 | <pre class="programlisting"> |
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| 62 | <span class="keyword">namespace</span> <span class="identifier">library</span> |
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| 63 | <span class="special">{</span> |
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| 64 | <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> |
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| 65 | <span class="special">{</span> |
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| 66 | <code class="computeroutput"><a 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> |
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| 67 | <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> |
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| 68 | <span class="special">}</span> |
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| 69 | <span class="special">}</span> |
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| 70 | </pre> |
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| 71 | <p> |
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| 72 | And you can now use <code class="computeroutput"><a href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> with |
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| 73 | book: |
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| 74 | </p> |
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| 75 | <pre class="programlisting"> |
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| 76 | <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> |
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| 77 | <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> |
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| 78 | <span class="string">"Hash & Dandelion Greens"</span><span class="special">);</span> |
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| 79 | |
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| 80 | <code class="computeroutput"><a 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> |
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| 81 | <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> |
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| 82 | |
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| 83 | <span class="comment">// If std::unordered_set is available: |
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| 84 | </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 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> |
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| 85 | <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> |
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| 86 | <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> |
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| 87 | <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> |
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| 88 | <span class="string">"Heavenly Hash: A Tasty Mix of a Mother's Meditations"</span><span class="special">));</span> |
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| 89 | |
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| 90 | <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> |
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| 91 | <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> |
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| 92 | </pre> |
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| 93 | <p> |
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| 94 | The full example can be found in: <a href="../../../libs/functional/hash/examples/books.cpp" target="_top">/libs/functional/hash/examples/books.hpp</a> |
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| 95 | and <a href="../../../libs/functional/hash/examples/books.cpp" target="_top">/libs/functional/hash/examples/books.cpp</a>. |
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| 96 | </p> |
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| 97 | <div class="tip"><table border="0" summary="Tip"> |
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| 98 | <tr> |
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| 99 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../images/tip.png"></td> |
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| 100 | <th align="left">Tip</th> |
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| 101 | </tr> |
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| 102 | <tr><td align="left" valign="top"><p> |
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| 103 | When writing a hash function, first look at how the equality function works. |
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| 104 | Objects that are equal must generate the same hash value. When objects are |
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| 105 | not equal they should generate different hash values. In this object equality |
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| 106 | was based just on the id so the hash function only hash the id. If it was |
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| 107 | based on the objects name and author then the hash function should take them |
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| 108 | into account (how to do this is discussed in the next section). |
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| 109 | </p></td></tr> |
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| 110 | </table></div> |
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| 111 | </div> |
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| 112 | <table width="100%"><tr> |
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| 113 | <td align="left"></td> |
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| 114 | <td align="right"><small>Copyright © 2005, 2006 Daniel James</small></td> |
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