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<title>Scary Iterators</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="intrusive.scary_iterators"></a><a class="link" href="scary_iterators.html" title="Scary Iterators">Scary Iterators</a>
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<p>
The paper N2913, titled <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2913.pdf%2c" target="_top">SCARY
Iterator Assignment and Initialization</a>, proposed a requirement that
a standard container's iterator types have no dependency on any type argument
apart from the container's <code class="computeroutput"><span class="identifier">value_type</span></code>,
<code class="computeroutput"><span class="identifier">difference_type</span></code>, <code class="computeroutput"><span class="identifier">pointer</span> <span class="identifier">type</span></code>,
and <code class="computeroutput"><span class="identifier">const_pointer</span></code> type. In
particular, according to the proposal, the types of a standard container's
iterators should not depend on the container's <code class="computeroutput"><span class="identifier">key_compare</span></code>,
<code class="computeroutput"><span class="identifier">hasher</span></code>, <code class="computeroutput"><span class="identifier">key_equal</span></code>,
or <code class="computeroutput"><span class="identifier">allocator</span></code> types.
</p>
<p>
That paper demonstrated that SCARY operations were crucial to the performant
implementation of common design patterns using STL components. It showed that
implementations that support SCARY operations reduce object code bloat by eliminating
redundant specializations of iterator and algorithm templates.
</p>
<p>
<span class="bold"><strong>Boost.Intrusive</strong></span> containers are a bit different
from standard containers. In particular, they have no allocator parameter and
they can be configured with additional options not present in STL-like containers.
Thus <span class="bold"><strong>Boost.Intrusive</strong></span> offers its own <code class="computeroutput"><span class="identifier">SCARY</span> <span class="identifier">iterator</span></code>
implementation, where iterator types don't change when the container is configured
with an option that does not alter the value <-> node transformation.
More concretely, the following options and conditions guarantee that iterator
types are unchanged:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<span class="bold"><strong>All containers</strong></span>: <code class="computeroutput"><span class="identifier">size_type</span><span class="special"><></span></code>, <code class="computeroutput"><span class="identifier">constant_time_size</span><span class="special"><></span></code>,
</li>
<li class="listitem">
<span class="bold"><strong><code class="computeroutput"><span class="identifier">slist</span></code></strong></span>:
<code class="computeroutput"><span class="identifier">cache_last</span><span class="special"><></span></code>,
<code class="computeroutput"><span class="identifier">linear</span><span class="special"><></span></code>,
</li>
<li class="listitem">
<span class="bold"><strong><code class="computeroutput"><span class="identifier">unordered_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code></strong></span>:
<code class="computeroutput"><span class="identifier">hash</span><span class="special"><></span></code>,
<code class="computeroutput"><span class="identifier">equal</span><span class="special"><></span></code>,
<code class="computeroutput"><span class="identifier">power_2_buckets</span><span class="special"><></span></code>,
<code class="computeroutput"><span class="identifier">cache_begin</span><span class="special"><></span></code>.
</li>
<li class="listitem">
<span class="bold"><strong>All tree-like containers</strong></span> (<code class="computeroutput"><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>,
<code class="computeroutput"><span class="identifier">avl_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>, <code class="computeroutput"><span class="identifier">sg_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>,
<code class="computeroutput"><span class="identifier">bs_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>, <code class="computeroutput"><span class="identifier">splay_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>,
<code class="computeroutput"><span class="identifier">treap_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>): <code class="computeroutput"><span class="identifier">compare</span><span class="special"><></span></code>.
</li>
<li class="listitem">
<span class="bold"><strong><code class="computeroutput"><span class="identifier">treap_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code></strong></span>:
<code class="computeroutput"><span class="identifier">priority</span><span class="special"><></span></code>
</li>
<li class="listitem">
<span class="bold"><strong><code class="computeroutput"><span class="identifier">bs_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>,
<code class="computeroutput"><span class="identifier">sg_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>, <code class="computeroutput"><span class="identifier">treap_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code>,
<code class="computeroutput"><span class="identifier">splay_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span></code></strong></span>: They share the same iterator
type when configured with the same options.
</li>
</ul></div>
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<td align="right"><div class="copyright-footer">Copyright © 2005 Olaf Krzikalla<br>Copyright © 2006-2013 Ion Gaztanaga<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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