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<a name="intrusive.boost_intrusive_iterators"></a><a class="link" href="boost_intrusive_iterators.html" title="Boost.Intrusive Iterator features">Boost.Intrusive Iterator
features</a>
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<div class="toc"><dl class="toc">
<dt><span class="section"><a href="boost_intrusive_iterators.html#intrusive.boost_intrusive_iterators.null_forward_iterators">Null
forward iterators</a></span></dt>
<dt><span class="section"><a href="boost_intrusive_iterators.html#intrusive.boost_intrusive_iterators.scary_iterators">Scary
Iterators</a></span></dt>
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<div class="titlepage"><div><div><h3 class="title">
<a name="intrusive.boost_intrusive_iterators.null_forward_iterators"></a><a class="link" href="boost_intrusive_iterators.html#intrusive.boost_intrusive_iterators.null_forward_iterators" title="Null forward iterators">Null
forward iterators</a>
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<p>
<span class="bold"><strong>Boost.Intrusive</strong></span> implements <a href="http://www.open-std.org/JTC1/sc22/WG21/docs/papers/2013/n3644.pdf" target="_top">C++14
Null Forward Iterators</a>, a feature that was introduced with C++14.
This means that equality and inequality comparison are defined over all iterators
for the same underlying sequence and the value initialized-iterators.
</p>
<p>
Value initialized iterators behave as if they refer past the end of the same
empty sequence:
</p>
<pre class="programlisting"><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">MyType</span><span class="special">&gt;</span> <span class="identifier">l</span> <span class="special">=</span> <span class="special">{</span> <span class="special">...</span> <span class="special">};</span>
<span class="keyword">auto</span> <span class="identifier">ni</span> <span class="special">=</span> <span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">MyType</span><span class="special">&gt;::</span><span class="identifier">iterator</span><span class="special">();</span>
<span class="keyword">auto</span> <span class="identifier">nd</span> <span class="special">=</span> <span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">MyType2</span><span class="special">&gt;::</span><span class="identifier">iterator</span><span class="special">();</span>
<span class="identifier">ni</span> <span class="special">==</span> <span class="identifier">ni</span><span class="special">;</span> <span class="comment">// True.</span>
<span class="identifier">nd</span> <span class="special">!=</span> <span class="identifier">nd</span><span class="special">;</span> <span class="comment">// False.</span>
<span class="identifier">ni</span> <span class="special">==</span> <span class="identifier">nd</span><span class="special">;</span> <span class="comment">// Won't compile.</span>
</pre>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="intrusive.boost_intrusive_iterators.scary_iterators"></a><a class="link" href="boost_intrusive_iterators.html#intrusive.boost_intrusive_iterators.scary_iterators" title="Scary Iterators">Scary
Iterators</a>
</h3></div></div></div>
<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 &lt;-&gt; 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">&lt;&gt;</span></code>, <code class="computeroutput"><span class="identifier">constant_time_size</span><span class="special">&lt;&gt;</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">&lt;&gt;</span></code>,
<code class="computeroutput"><span class="identifier">linear</span><span class="special">&lt;&gt;</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">&lt;&gt;</span></code>,
<code class="computeroutput"><span class="identifier">equal</span><span class="special">&lt;&gt;</span></code>,
<code class="computeroutput"><span class="identifier">power_2_buckets</span><span class="special">&lt;&gt;</span></code>,
<code class="computeroutput"><span class="identifier">cache_begin</span><span class="special">&lt;&gt;</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">&lt;&gt;</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">&lt;&gt;</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>
</div>
</div>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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