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<div class="section">
<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="idm45555040906576"></a><p class="title"><b>Table 38.1. 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>
<h3>
<a name="unordered.buckets.h0"></a>
<span class="phrase"><a name="unordered.buckets.controlling_the_number_of_buckets"></a></span><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" style="list-style-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="unordered.buckets.bucket_size"></a><p class="title"><b>Table 38.2. 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"><h3>
<a name="unordered.buckets.h1"></a>
<span class="phrase"><a name="unordered.buckets.iterator_invalidation"></a></span><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">
<div class="titlepage"></div>
<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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