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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="unordered.buckets"></a><a class="link" href="buckets.html" title="The Data Structure">The Data Structure</a> |
| </h2></div></div></div> |
| <p> |
| The containers are made up of a number of 'buckets', each of which can contain |
| any number of elements. For example, the following diagram shows an <code class="computeroutput"><a class="link" href="../boost/unordered_set.html" title="Class template unordered_set">unordered_set</a></code> with 7 buckets containing |
| 5 elements, <code class="computeroutput"><span class="identifier">A</span></code>, <code class="computeroutput"><span class="identifier">B</span></code>, <code class="computeroutput"><span class="identifier">C</span></code>, |
| <code class="computeroutput"><span class="identifier">D</span></code> and <code class="computeroutput"><span class="identifier">E</span></code> |
| (this is just for illustration, containers will typically have more buckets). |
| </p> |
| <p> |
| <span class="inlinemediaobject"><img src="../../../libs/unordered/doc/diagrams/buckets.png" align="middle"></span> |
| </p> |
| <p> |
| In order to decide which bucket to place an element in, the container applies |
| the hash function, <code class="computeroutput"><span class="identifier">Hash</span></code>, to |
| the element's key (for <code class="computeroutput"><span class="identifier">unordered_set</span></code> |
| and <code class="computeroutput"><span class="identifier">unordered_multiset</span></code> the |
| key is the whole element, but is referred to as the key so that the same terminology |
| can be used for sets and maps). This returns a value of type <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>. |
| <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code> has a much greater range of values |
| then the number of buckets, so that container applies another transformation |
| to that value to choose a bucket to place the element in. |
| </p> |
| <p> |
| Retrieving the elements for a given key is simple. The same process is applied |
| to the key to find the correct bucket. Then the key is compared with the elements |
| in the bucket to find any elements that match (using the equality predicate |
| <code class="computeroutput"><span class="identifier">Pred</span></code>). If the hash function |
| has worked well the elements will be evenly distributed amongst the buckets |
| so only a small number of elements will need to be examined. |
| </p> |
| <p> |
| There is <a class="link" href="hash_equality.html" title="Equality Predicates and Hash Functions">more information on hash functions |
| and equality predicates in the next section</a>. |
| </p> |
| <p> |
| You can see in the diagram that <code class="computeroutput"><span class="identifier">A</span></code> |
| & <code class="computeroutput"><span class="identifier">D</span></code> have been placed in |
| the same bucket. When looking for elements in this bucket up to 2 comparisons |
| are made, making the search slower. This is known as a collision. To keep things |
| fast we try to keep collisions to a minimum. |
| </p> |
| <p> |
| </p> |
| <div class="table"> |
| <a name="id3019174"></a><p class="title"><b>Table 27.1. Methods for Accessing Buckets</b></p> |
| <div class="table-contents"><table class="table" summary="Methods for Accessing Buckets"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th><p>Method</p></th> |
| <th><p>Description</p></th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td><code class="computeroutput"><span class="identifier">size_type</span> <span class="identifier">bucket_count</span><span class="special">()</span> <span class="keyword">const</span></code></td> |
| <td>The |
| number of buckets.</td> |
| </tr> |
| <tr> |
| <td><code class="computeroutput"><span class="identifier">size_type</span> <span class="identifier">max_bucket_count</span><span class="special">()</span> |
| <span class="keyword">const</span></code></td> |
| <td>An upper bound on the number of |
| buckets.</td> |
| </tr> |
| <tr> |
| <td><code class="computeroutput"><span class="identifier">size_type</span> <span class="identifier">bucket_size</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span></code></td> |
| <td>The |
| number of elements in bucket <code class="computeroutput"><span class="identifier">n</span></code>.</td> |
| </tr> |
| <tr> |
| <td><code class="computeroutput"><span class="identifier">size_type</span> <span class="identifier">bucket</span><span class="special">(</span><span class="identifier">key_type</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">k</span><span class="special">)</span> <span class="keyword">const</span></code></td> |
| <td>Returns |
| the index of the bucket which would contain k</td> |
| </tr> |
| <tr> |
| <td><code class="computeroutput"><span class="identifier">local_iterator</span> |
| <span class="identifier">begin</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">);</span></code></td> |
| <td rowspan="6">Return begin and end iterators for bucket |
| <code class="computeroutput"><span class="identifier">n</span></code>.</td> |
| </tr> |
| <tr><td><code class="computeroutput"><span class="identifier">local_iterator</span> |
| <span class="identifier">end</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">);</span></code></td></tr> |
| <tr><td><code class="computeroutput"><span class="identifier">const_local_iterator</span> |
| <span class="identifier">begin</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></code></td></tr> |
| <tr><td><code class="computeroutput"><span class="identifier">const_local_iterator</span> <span class="identifier">end</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></code></td></tr> |
| <tr><td><code class="computeroutput"><span class="identifier">const_local_iterator</span> |
| <span class="identifier">cbegin</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></code></td></tr> |
| <tr><td><code class="computeroutput"><span class="identifier">const_local_iterator</span> <span class="identifier">cend</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></code></td></tr> |
| </tbody> |
| </table></div> |
| </div> |
| <p><br class="table-break"> |
| </p> |
| <a name="unordered.buckets.controlling_the_number_of_buckets"></a><h3> |
| <a name="id3019666"></a> |
| <a class="link" href="buckets.html#unordered.buckets.controlling_the_number_of_buckets">Controlling |
| the number of buckets</a> |
| </h3> |
| <p> |
| As more elements are added to an unordered associative container, the number |
| of elements in the buckets will increase causing performance to degrade. To |
| combat this the containers increase the bucket count as elements are inserted. |
| You can also tell the container to change the bucket count (if required) by |
| calling <code class="computeroutput"><span class="identifier">rehash</span></code>. |
| </p> |
| <p> |
| The standard leaves a lot of freedom to the implementer to decide how the number |
| of buckets are chosen, but it does make some requirements based on the container's |
| 'load factor', the average number of elements per bucket. Containers also have |
| a 'maximum load factor' which they should try to keep the load factor below. |
| </p> |
| <p> |
| You can't control the bucket count directly but there are two ways to influence |
| it: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Specify the minimum number of buckets when constructing a container or |
| when calling <code class="computeroutput"><span class="identifier">rehash</span></code>. |
| </li> |
| <li class="listitem"> |
| Suggest a maximum load factor by calling <code class="computeroutput"><span class="identifier">max_load_factor</span></code>. |
| </li> |
| </ul></div> |
| <p> |
| <code class="computeroutput"><span class="identifier">max_load_factor</span></code> doesn't let |
| you set the maximum load factor yourself, it just lets you give a <span class="emphasis"><em>hint</em></span>. |
| And even then, the draft standard doesn't actually require the container to |
| pay much attention to this value. The only time the load factor is <span class="emphasis"><em>required</em></span> |
| to be less than the maximum is following a call to <code class="computeroutput"><span class="identifier">rehash</span></code>. |
| But most implementations will try to keep the number of elements below the |
| max load factor, and set the maximum load factor to be the same as or close |
| to the hint - unless your hint is unreasonably small or large. |
| </p> |
| <div class="table"> |
| <a name="id3019788"></a><p class="title"><b>Table 27.2. Methods for Controlling Bucket Size</b></p> |
| <div class="table-contents"><table class="table" summary="Methods for Controlling Bucket Size"> |
| <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">X</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Construct an empty container with at least <code class="computeroutput"><span class="identifier">n</span></code> |
| buckets (<code class="computeroutput"><span class="identifier">X</span></code> is the |
| container type). |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">X</span><span class="special">(</span><span class="identifier">InputIterator</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">InputIterator</span> |
| <span class="identifier">j</span><span class="special">,</span> |
| <span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Construct an empty container with at least <code class="computeroutput"><span class="identifier">n</span></code> |
| buckets and insert elements from the range [<code class="computeroutput"><span class="identifier">i</span></code>, |
| <code class="computeroutput"><span class="identifier">j</span></code>) (<code class="computeroutput"><span class="identifier">X</span></code> is the container type). |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">float</span> <span class="identifier">load_factor</span><span class="special">()</span> <span class="keyword">const</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The average number of elements per bucket. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">float</span> <span class="identifier">max_load_factor</span><span class="special">()</span> <span class="keyword">const</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Returns the current maximum load factor. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">float</span> <span class="identifier">max_load_factor</span><span class="special">(</span><span class="keyword">float</span> <span class="identifier">z</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Changes the container's maximum load factor, using <code class="computeroutput"><span class="identifier">z</span></code> as a hint. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">rehash</span><span class="special">(</span><span class="identifier">size_type</span> |
| <span class="identifier">n</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Changes the number of buckets so that there at least n buckets, and |
| so that the load factor is less than the maximum load factor. |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><a name="unordered.buckets.iterator_invalidation"></a><h3> |
| <a name="id3020202"></a> |
| <a class="link" href="buckets.html#unordered.buckets.iterator_invalidation">Iterator Invalidation</a> |
| </h3> |
| <p> |
| It is not specified how member functions other than <code class="computeroutput"><span class="identifier">rehash</span></code> |
| affect the bucket count, although <code class="computeroutput"><span class="identifier">insert</span></code> |
| is only allowed to invalidate iterators when the insertion causes the load |
| factor to be greater than or equal to the maximum load factor. For most implementations |
| this means that insert will only change the number of buckets when this happens. |
| While iterators can be invalidated by calls to <code class="computeroutput"><span class="identifier">insert</span></code> |
| and <code class="computeroutput"><span class="identifier">rehash</span></code>, pointers and references |
| to the container's elements are never invalidated. |
| </p> |
| <p> |
| In a similar manner to using <code class="computeroutput"><span class="identifier">reserve</span></code> |
| for <code class="computeroutput"><span class="identifier">vector</span></code>s, it can be a good |
| idea to call <code class="computeroutput"><span class="identifier">rehash</span></code> before |
| inserting a large number of elements. This will get the expensive rehashing |
| out of the way and let you store iterators, safe in the knowledge that they |
| won't be invalidated. If you are inserting <code class="computeroutput"><span class="identifier">n</span></code> |
| elements into container <code class="computeroutput"><span class="identifier">x</span></code>, |
| you could first call: |
| </p> |
| <pre class="programlisting"><span class="identifier">x</span><span class="special">.</span><span class="identifier">rehash</span><span class="special">((</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">max_load_factor</span><span class="special">()</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span> |
| </pre> |
| <div class="sidebar"> |
| <p class="title"><b></b></p> |
| <p> |
| Note: <code class="computeroutput"><span class="identifier">rehash</span></code>'s argument is |
| the minimum number of buckets, not the number of elements, which is why the |
| new size is divided by the maximum load factor. The <code class="computeroutput"><span class="special">+</span> |
| <span class="number">1</span></code> guarantees there is no invalidation; |
| without it, reallocation could occur if the number of bucket exactly divides |
| the target size, since the container is allowed to rehash when the load factor |
| is equal to the maximum load factor. |
| </p> |
| </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"><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> |
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