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<title>Equality Predicates and Hash Functions</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="unordered.hash_equality"></a><a class="link" href="hash_equality.html" title="Equality Predicates and Hash Functions">Equality Predicates and Hash Functions</a>
</h2></div></div></div>
<p>
While the associative containers use an ordering relation to specify how the
elements are stored, the unordered associative containers use an equality predicate
and a hash function. For example, <code class="computeroutput"><a class="link" href="../boost/unordered_map.html" title="Class template unordered_map">boost::unordered_map</a></code>
is declared as:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span>
<span class="keyword">class</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Mapped</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">Hash</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">&lt;</span><span class="identifier">Key</span><span class="special">&gt;,</span>
<span class="keyword">class</span> <span class="identifier">Pred</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special">&lt;</span><span class="identifier">Key</span><span class="special">&gt;,</span>
<span class="keyword">class</span> <span class="identifier">Alloc</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">Key</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Mapped</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="keyword">class</span> <code class="computeroutput"><a class="link" href="../boost/unordered_map.html" title="Class template unordered_map">unordered_map</a></code><span class="special">;</span>
</pre>
<p>
The hash function comes first as you might want to change the hash function
but not the equality predicate. For example, if you wanted to use the <a href="http://www.isthe.com/chongo/tech/comp/fnv/" target="_top">FNV-1 hash</a> you could
write:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">hash</span><span class="special">::</span><span class="identifier">fnv_1</span><span class="special">&gt;</span>
<span class="identifier">dictionary</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
There is an <a href="../../../libs/unordered/examples/fnv1.hpp" target="_top">implementation
of FNV-1</a> in the examples directory.
</p>
<p>
If you wish to use a different equality function, you will also need to use
a matching hash function. For example, to implement a case insensitive dictionary
you need to define a case insensitive equality predicate and hash function:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">iequal_to</span>
<span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">binary_function</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">bool</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">)</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">algorithm</span><span class="special">::</span><span class="identifier">iequals</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span><span class="special">());</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">ihash</span>
<span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unary_function</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">seed</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">locale</span><span class="special">;</span>
<span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">const_iterator</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
<span class="identifier">it</span> <span class="special">!=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">();</span> <span class="special">++</span><span class="identifier">it</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">toupper</span><span class="special">(*</span><span class="identifier">it</span><span class="special">,</span> <span class="identifier">locale</span><span class="special">));</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Which you can then use in a case insensitive dictionary:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">ihash</span><span class="special">,</span> <span class="identifier">iequal_to</span><span class="special">&gt;</span>
<span class="identifier">idictionary</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
This is a simplified version of the example at <a href="../../../libs/unordered/examples/case_insensitive.hpp" target="_top">/libs/unordered/examples/case_insensitive.hpp</a>
which supports other locales and string types.
</p>
<div class="caution"><table border="0" summary="Caution">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
Be careful when using the equality (<code class="computeroutput"><span class="special">==</span></code>)
operator with custom equality predicates, especially if you're using a function
pointer. If you compare two containers with different equality predicates
then the result is undefined. For most stateless function objects this is
impossible - since you can only compare objects with the same equality predicate
you know the equality predicates must be equal. But if you're using function
pointers or a stateful equality predicate (e.g. boost::function) then you
can get into trouble.
</p></td></tr>
</table></div>
<a name="unordered.hash_equality.custom_types"></a><h3>
<a name="id3021632"></a>
<a class="link" href="hash_equality.html#unordered.hash_equality.custom_types">Custom Types</a>
</h3>
<p>
Similarly, a custom hash function can be used for custom types:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">point</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">};</span>
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span><span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p1</span><span class="special">,</span> <span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p2</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">p1</span><span class="special">.</span><span class="identifier">x</span> <span class="special">==</span> <span class="identifier">p2</span><span class="special">.</span><span class="identifier">x</span> <span class="special">&amp;&amp;</span> <span class="identifier">p1</span><span class="special">.</span><span class="identifier">y</span> <span class="special">==</span> <span class="identifier">p2</span><span class="special">.</span><span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">struct</span> <span class="identifier">point_hash</span>
<span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unary_function</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p</span><span class="special">)</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">seed</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">x</span><span class="special">);</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">y</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_multiset</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">,</span> <span class="identifier">point_hash</span><span class="special">&gt;</span> <span class="identifier">points</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Since the default hash function is <a class="link" href="../hash.html" title="Chapter&#160;8.&#160;Boost.Functional/Hash">Boost.Hash</a>,
we can <a class="link" href="../hash/custom.html" title="Extending boost::hash for a custom data type">extend it to support the type</a> so
that the hash function doesn't need to be explicitly given:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">point</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">};</span>
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span><span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p1</span><span class="special">,</span> <span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p2</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">p1</span><span class="special">.</span><span class="identifier">x</span> <span class="special">==</span> <span class="identifier">p2</span><span class="special">.</span><span class="identifier">x</span> <span class="special">&amp;&amp;</span> <span class="identifier">p1</span><span class="special">.</span><span class="identifier">y</span> <span class="special">==</span> <span class="identifier">p2</span><span class="special">.</span><span class="identifier">y</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_value</span><span class="special">(</span><span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p</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">seed</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">x</span><span class="special">);</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">y</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// Now the default function objects work.
</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_multiset</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">&gt;</span> <span class="identifier">points</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
See the <a class="link" href="../hash/custom.html" title="Extending boost::hash for a custom data type">Boost.Hash documentation</a> for more
detail on how to do this. Remember that it relies on extensions to the draft
standard - so it won't work on other implementations of the unordered associative
containers.
</p>
<div class="table">
<a name="id3022688"></a><p class="title"><b>Table&#160;27.3.&#160;Methods for accessing the hash and equality functions.</b></p>
<div class="table-contents"><table class="table" summary="Methods for accessing the hash and equality functions.">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Method
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">hasher</span> <span class="identifier">hash_function</span><span class="special">()</span> <span class="keyword">const</span></code>
</p>
</td>
<td>
<p>
Returns the container's hash function.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">key_equal</span> <span class="identifier">key_eq</span><span class="special">()</span> <span class="keyword">const</span></code>
</p>
</td>
<td>
<p>
Returns the container's key equality function.
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break">
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2003, 2004 Jeremy B. Maitin-Shepard<br>Copyright &#169; 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>
</div></td>
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