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<div class="section" title="Comparison with Associative Containers">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="unordered.comparison"></a><a class="link" href="comparison.html" title="Comparison with Associative Containers"> Comparison with Associative Containers</a>
</h2></div></div></div>
<div class="table">
<a name="id2014043"></a><p class="title"><b>Table 26.4. Interface differences.</b></p>
<div class="table-contents"><table class="table" summary="Interface differences.">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Associative Containers
</p>
</th>
<th>
<p>
Unordered Associative Containers
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Parameterized by an ordering relation <code class="computeroutput"><span class="identifier">Compare</span></code>
</p>
</td>
<td>
<p>
Parameterized by a function object <code class="computeroutput"><span class="identifier">Hash</span></code>
and an equivalence relation <code class="computeroutput"><span class="identifier">Pred</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
Keys can be compared using <code class="computeroutput"><span class="identifier">key_compare</span></code>
which is accessed by member function <code class="computeroutput"><span class="identifier">key_comp</span><span class="special">()</span></code>, values can be compared using <code class="computeroutput"><span class="identifier">value_compare</span></code> which is accessed by
member function <code class="computeroutput"><span class="identifier">value_comp</span><span class="special">()</span></code>.
</p>
</td>
<td>
<p>
Keys can be hashed using <code class="computeroutput"><span class="identifier">hasher</span></code>
which is accessed by member function <code class="computeroutput"><span class="identifier">hash_function</span><span class="special">()</span></code>, and checked for equality using <code class="computeroutput"><span class="identifier">key_equal</span></code> which is accessed by member
function <code class="computeroutput"><span class="identifier">key_eq</span><span class="special">()</span></code>.
There is no function object for compared or hashing values.
</p>
</td>
</tr>
<tr>
<td>
<p>
Constructors have optional extra parameters for the comparison object.
</p>
</td>
<td>
<p>
Constructors have optional extra parameters for the initial minimum number
of buckets, a hash function and an equality object.
</p>
</td>
</tr>
<tr>
<td>
<p>
Keys <code class="computeroutput"><span class="identifier">k1</span></code>, <code class="computeroutput"><span class="identifier">k2</span></code> are considered equivalent if <code class="computeroutput"><span class="special">!</span><span class="identifier">Compare</span><span class="special">(</span><span class="identifier">k1</span><span class="special">,</span> <span class="identifier">k2</span><span class="special">)</span> <span class="special">&&</span>
<span class="special">!</span><span class="identifier">Compare</span><span class="special">(</span><span class="identifier">k2</span><span class="special">,</span> <span class="identifier">k1</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Keys <code class="computeroutput"><span class="identifier">k1</span></code>, <code class="computeroutput"><span class="identifier">k2</span></code> are considered equivalent if <code class="computeroutput"><span class="identifier">Pred</span><span class="special">(</span><span class="identifier">k1</span><span class="special">,</span> <span class="identifier">k2</span><span class="special">)</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
Member function <code class="computeroutput"><span class="identifier">lower_bound</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code> and <code class="computeroutput"><span class="identifier">upper_bound</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
No equivalent. Since the elements aren't ordered <code class="computeroutput"><span class="identifier">lower_bound</span></code>
and <code class="computeroutput"><span class="identifier">upper_bound</span></code> would
be meaningless.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>
returns an empty range at the position that k would be inserted if k
isn't present in the container.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>
returns a range at the end of the container if k isn't present in the
container. It can't return a positioned range as k could be inserted
into multiple place. To find out the bucket that k would be inserted
into use <code class="computeroutput"><span class="identifier">bucket</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>.
But remember that an insert can cause the container to rehash - meaning
that the element can be inserted into a different bucket.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">iterator</span></code>, <code class="computeroutput"><span class="identifier">const_iterator</span></code> are of the bidirectional
category.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">iterator</span></code>, <code class="computeroutput"><span class="identifier">const_iterator</span></code> are of at least the
forward category.
</p>
</td>
</tr>
<tr>
<td>
<p>
Iterators, pointers and references to the container's elements are never
invalidated.
</p>
</td>
<td>
<p>
<a class="link" href="buckets.html#unordered.buckets.iterator_invalidation">Iterators can
be invalidated by calls to insert or rehash</a>. Pointers and references
to the container's elements are never invalidated.
</p>
</td>
</tr>
<tr>
<td>
<p>
Iterators iterate through the container in the order defined by the comparison
object.
</p>
</td>
<td>
<p>
Iterators iterate through the container in an arbitrary order, that can
change as elements are inserted. Although, equivalent elements are always
adjacent.
</p>
</td>
</tr>
<tr>
<td>
<p>
No equivalent
</p>
</td>
<td>
<p>
Local iterators can be used to iterate through individual buckets. (I
don't think that the order of local iterators and iterators are required
to have any correspondence.)
</p>
</td>
</tr>
<tr>
<td>
<p>
Can be compared using the <code class="computeroutput"><span class="special">==</span></code>,
<code class="computeroutput"><span class="special">!=</span></code>, <code class="computeroutput"><span class="special"><</span></code>,
<code class="computeroutput"><span class="special"><=</span></code>, <code class="computeroutput"><span class="special">></span></code>,
<code class="computeroutput"><span class="special">>=</span></code> operators.
</p>
</td>
<td>
<p>
No comparison operators are defined in the standard, although <a class="link" href="rationale.html#unordered.rationale.equality_operators">implementations
might extend the containers to support <code class="computeroutput"><span class="special">==</span></code>
and <code class="computeroutput"><span class="special">!=</span></code></a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
</p>
</td>
<td>
<p>
When inserting with a hint, implementations are permitted to ignore the
hint.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">erase</span></code> never throws an
exception
</p>
</td>
<td>
<p>
The containers' hash or predicate function can throw exceptions from
<code class="computeroutput"><span class="identifier">erase</span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><div class="table">
<a name="id2014850"></a><p class="title"><b>Table 26.5. Complexity Guarantees</b></p>
<div class="table-contents"><table class="table" summary="Complexity Guarantees">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Operation
</p>
</th>
<th>
<p>
Associative Containers
</p>
</th>
<th>
<p>
Unordered Associative Containers
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Construction of empty container
</p>
</td>
<td>
<p>
constant
</p>
</td>
<td>
<p>
O(<span class="emphasis"><em>n</em></span>) where <span class="emphasis"><em>n</em></span> is the minimum
number of buckets.
</p>
</td>
</tr>
<tr>
<td>
<p>
Construction of container from a range of <span class="emphasis"><em>N</em></span> elements
</p>
</td>
<td>
<p>
O(<span class="emphasis"><em>N</em></span> log <span class="emphasis"><em>N</em></span>), O(<span class="emphasis"><em>N</em></span>)
if the range is sorted with <code class="computeroutput"><span class="identifier">value_comp</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
Average case O(<span class="emphasis"><em>N</em></span>), worst case O(<span class="emphasis"><em>N</em></span><sup>2</sup>)
</p>
</td>
</tr>
<tr>
<td>
<p>
Insert a single element
</p>
</td>
<td>
<p>
logarithmic
</p>
</td>
<td>
<p>
Average case constant, worst case linear
</p>
</td>
</tr>
<tr>
<td>
<p>
Insert a single element with a hint
</p>
</td>
<td>
<p>
Amortized constant if t elements inserted right after hint, logarithmic
otherwise
</p>
</td>
<td>
<p>
Average case constant, worst case linear (ie. the same as a normal insert).
</p>
</td>
</tr>
<tr>
<td>
<p>
Inserting a range of <span class="emphasis"><em>N</em></span> elements
</p>
</td>
<td>
<p>
<span class="emphasis"><em>N</em></span> log(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>+<span class="emphasis"><em>N</em></span>)
</p>
</td>
<td>
<p>
Average case O(<span class="emphasis"><em>N</em></span>), worst case O(<span class="emphasis"><em>N</em></span>
* <code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
</tr>
<tr>
<td>
<p>
Erase by key, <code class="computeroutput"><span class="identifier">k</span></code>
</p>
</td>
<td>
<p>
O(log(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
+ <code class="computeroutput"><span class="identifier">count</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>)
</p>
</td>
<td>
<p>
Average case: O(<code class="computeroutput"><span class="identifier">count</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
</tr>
<tr>
<td>
<p>
Erase a single element by iterator
</p>
</td>
<td>
<p>
Amortized constant
</p>
</td>
<td>
<p>
Average case: O(1), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
</tr>
<tr>
<td>
<p>
Erase a range of <span class="emphasis"><em>N</em></span> elements
</p>
</td>
<td>
<p>
O(log(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
+ <span class="emphasis"><em>N</em></span>)
</p>
</td>
<td>
<p>
Average case: O(<span class="emphasis"><em>N</em></span>), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
</tr>
<tr>
<td>
<p>
Clearing the container
</p>
</td>
<td>
<p>
O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
<td>
<p>
O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
</tr>
<tr>
<td>
<p>
Find
</p>
</td>
<td>
<p>
logarithmic
</p>
</td>
<td>
<p>
Average case: O(1), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
</tr>
<tr>
<td>
<p>
Count
</p>
</td>
<td>
<p>
O(log(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
+ <code class="computeroutput"><span class="identifier">count</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>)
</p>
</td>
<td>
<p>
Average case: O(1), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
logarithmic
</p>
</td>
<td>
<p>
Average case: O(<code class="computeroutput"><span class="identifier">count</span><span class="special">(</span><span class="identifier">k</span><span class="special">)</span></code>), Worst case: O(<code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>)
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">lower_bound</span></code>,<code class="computeroutput"><span class="identifier">upper_bound</span></code>
</p>
</td>
<td>
<p>
logarithmic
</p>
</td>
<td>
<p>
n/a
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
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<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>
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