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  26 <div class="section">
  27 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  28 <a name="intrusive.boost_intrusive_iterators"></a><a class="link" href="boost_intrusive_iterators.html" title="Boost.Intrusive Iterator features">Boost.Intrusive Iterator
  29     features</a>
  30 </h2></div></div></div>
  31 <div class="toc"><dl class="toc">
  32 <dt><span class="section"><a href="boost_intrusive_iterators.html#intrusive.boost_intrusive_iterators.null_forward_iterators">Null
  33       forward iterators</a></span></dt>
  34 <dt><span class="section"><a href="boost_intrusive_iterators.html#intrusive.boost_intrusive_iterators.scary_iterators">Scary
  35       Iterators</a></span></dt>
  36 </dl></div>
  37 <div class="section">
  38 <div class="titlepage"><div><div><h3 class="title">
  39 <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
  40       forward iterators</a>
  41 </h3></div></div></div>
  42 <p>
  43         <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
  44         Null Forward Iterators</a>, a feature that was introduced with C++14.
  45         This means that equality and inequality comparison are defined over all iterators
  46         for the same underlying sequence and the value initialized-iterators.
  47       </p>
  48 <p>
  49         Value initialized iterators behave as if they refer past the end of the same
  50         empty sequence:
  51       </p>
  52 <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>
  53 <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>
  54 <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>
  55 <span class="identifier">ni</span> <span class="special">==</span> <span class="identifier">ni</span><span class="special">;</span> <span class="comment">// True.</span>
  56 <span class="identifier">nd</span> <span class="special">!=</span> <span class="identifier">nd</span><span class="special">;</span> <span class="comment">// False.</span>
  57 <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>
  58 </pre>
  59 </div>
  60 <div class="section">
  61 <div class="titlepage"><div><div><h3 class="title">
  62 <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
  63       Iterators</a>
  64 </h3></div></div></div>
  65 <p>
  66         The paper N2913, titled <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2913.pdf%2c" target="_top">SCARY
  67         Iterator Assignment and Initialization</a>, proposed a requirement that
  68         a standard container's iterator types have no dependency on any type argument
  69         apart from the container's <code class="computeroutput"><span class="identifier">value_type</span></code>,
  70         <code class="computeroutput"><span class="identifier">difference_type</span></code>, <code class="computeroutput"><span class="identifier">pointer</span> <span class="identifier">type</span></code>,
  71         and <code class="computeroutput"><span class="identifier">const_pointer</span></code> type. In
  72         particular, according to the proposal, the types of a standard container's
  73         iterators should not depend on the container's <code class="computeroutput"><span class="identifier">key_compare</span></code>,
  74         <code class="computeroutput"><span class="identifier">hasher</span></code>, <code class="computeroutput"><span class="identifier">key_equal</span></code>,
  75         or <code class="computeroutput"><span class="identifier">allocator</span></code> types.
  76       </p>
  77 <p>
  78         That paper demonstrated that SCARY operations were crucial to the performant
  79         implementation of common design patterns using STL components. It showed
  80         that implementations that support SCARY operations reduce object code bloat
  81         by eliminating redundant specializations of iterator and algorithm templates.
  82       </p>
  83 <p>
  84         <span class="bold"><strong>Boost.Intrusive</strong></span> containers are a bit different
  85         from standard containers. In particular, they have no allocator parameter
  86         and they can be configured with additional options not present in STL-like
  87         containers. Thus <span class="bold"><strong>Boost.Intrusive</strong></span> offers
  88         its own <code class="computeroutput"><span class="identifier">SCARY</span> <span class="identifier">iterator</span></code>
  89         implementation, where iterator types don't change when the container is configured
  90         with an option that does not alter the value &lt;-&gt; node transformation.
  91         More concretely, the following options and conditions guarantee that iterator
  92         types are unchanged:
  93       </p>
  94 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  95 <li class="listitem">
  96             <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>,
  97           </li>
  98 <li class="listitem">
  99             <span class="bold"><strong><code class="computeroutput"><span class="identifier">slist</span></code></strong></span>:
 100             <code class="computeroutput"><span class="identifier">cache_last</span><span class="special">&lt;&gt;</span></code>,
 101             <code class="computeroutput"><span class="identifier">linear</span><span class="special">&lt;&gt;</span></code>,
 102           </li>
 103 <li class="listitem">
 104             <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>:
 105             <code class="computeroutput"><span class="identifier">hash</span><span class="special">&lt;&gt;</span></code>,
 106             <code class="computeroutput"><span class="identifier">equal</span><span class="special">&lt;&gt;</span></code>,
 107             <code class="computeroutput"><span class="identifier">power_2_buckets</span><span class="special">&lt;&gt;</span></code>,
 108             <code class="computeroutput"><span class="identifier">cache_begin</span><span class="special">&lt;&gt;</span></code>.
 109           </li>
 110 <li class="listitem">
 111             <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>,
 112             <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>,
 113             <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>,
 114             <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>.
 115           </li>
 116 <li class="listitem">
 117             <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>:
 118             <code class="computeroutput"><span class="identifier">priority</span><span class="special">&lt;&gt;</span></code>
 119           </li>
 120 <li class="listitem">
 121             <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>,
 122             <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>,
 123             <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
 124             iterator type when configured with the same options.
 125           </li>
 126 </ul></div>
 127 </div>
 128 </div>
 129 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 130 <td align="left"></td>
 131 <td align="right"><div class="copyright-footer">Copyright &#169; 2005 Olaf Krzikalla<br>Copyright &#169; 2006-2015 Ion Gaztanaga<p>
 132         Distributed under the Boost Software License, Version 1.0. (See accompanying
 133         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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