| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Equality Predicates and Hash Functions</title> |
| <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> |
| <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> |
| <link rel="up" href="../unordered.html" title="Chapter 27. Boost.Unordered"> |
| <link rel="prev" href="buckets.html" title="The Data Structure"> |
| <link rel="next" href="comparison.html" title="Comparison with Associative Containers"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> |
| <td align="center"><a href="../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="buckets.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../unordered.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="comparison.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <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"><</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"><</span><span class="identifier">Key</span><span class="special">>,</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"><</span><span class="identifier">Key</span><span class="special">>,</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"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">Key</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Mapped</span><span class="special">></span> <span class="special">></span> <span class="special">></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"><</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">></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"><</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">></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">&</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">&</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"><</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">></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">&</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"><</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">></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">&</span> <span class="identifier">p1</span><span class="special">,</span> <span class="identifier">point</span> <span class="keyword">const</span><span class="special">&</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">&&</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"><</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">></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">&</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"><</span><span class="identifier">point</span><span class="special">,</span> <span class="identifier">point_hash</span><span class="special">></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 8. 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">&</span> <span class="identifier">p1</span><span class="special">,</span> <span class="identifier">point</span> <span class="keyword">const</span><span class="special">&</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">&&</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">&</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"><</span><span class="identifier">point</span><span class="special">></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 27.3. 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 © 2003, 2004 Jeremy B. Maitin-Shepard<br>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> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="buckets.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../unordered.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="comparison.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |