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<title>Recursive Boost.Intrusive containers</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="intrusive.recursive"></a><a class="link" href="recursive.html" title="Recursive Boost.Intrusive containers">Recursive Boost.Intrusive containers</a>
</h2></div></div></div>
<p>
      <span class="bold"><strong>Boost.Intrusive</strong></span> containers can be used to
      define recursive structures very easily, allowing complex data structures with
      very low overhead. Let's see an example:
    </p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cassert</span><span class="special">&gt;</span>

<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>

<span class="keyword">typedef</span> <span class="identifier">list_base_hook</span><span class="special">&lt;&gt;</span> <span class="identifier">BaseHook</span><span class="special">;</span>

<span class="comment">//A recursive class</span>
<span class="keyword">class</span> <span class="identifier">Recursive</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">BaseHook</span>
<span class="special">{</span>
   <span class="keyword">private</span><span class="special">:</span>
   <span class="identifier">Recursive</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
   <span class="identifier">Recursive</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>

   <span class="keyword">public</span><span class="special">:</span>
   <span class="identifier">Recursive</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">BaseHook</span><span class="special">(),</span> <span class="identifier">children</span><span class="special">(){}</span>
   <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">base_hook</span><span class="special">&lt;</span><span class="identifier">BaseHook</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">children</span><span class="special">;</span>
<span class="special">};</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
   <span class="identifier">Recursive</span> <span class="identifier">f</span><span class="special">,</span> <span class="identifier">f2</span><span class="special">;</span>
   <span class="comment">//A recursive list of Recursive</span>
   <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">base_hook</span><span class="special">&lt;</span><span class="identifier">BaseHook</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">l</span><span class="special">;</span>

   <span class="comment">//Insert a node in parent list</span>
   <span class="identifier">l</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f</span><span class="special">);</span>

   <span class="comment">//Insert a node in child list</span>
   <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f2</span><span class="special">);</span>

   <span class="comment">//Objects properly inserted</span>
   <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
   <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>

   <span class="comment">//Clear both lists</span>
   <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
   <span class="identifier">l</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
   <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
    </p>
<p>
      Recursive data structures using <span class="bold"><strong>Boost.Intrusive</strong></span>
      containers must avoid using hook deduction to avoid early type instantiation:
    </p>
<pre class="programlisting"><span class="comment">//This leads to compilation error (Recursive is instantiated by</span>
<span class="comment">//'list' to deduce hook properties (pointer type, tag, safe-mode...)</span>
<span class="keyword">class</span> <span class="identifier">Recursive</span>
<span class="special">{</span>  <span class="comment">//...</span>

   <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span> <span class="special">&gt;</span> <span class="identifier">l</span><span class="special">;</span>
   <span class="comment">//...</span>
<span class="special">};</span>

<span class="comment">//Ok, programmer must specify the hook type to avoid early Recursive instantiation</span>
<span class="keyword">class</span> <span class="identifier">Recursive</span>
<span class="special">{</span>  <span class="comment">//...</span>
   <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">base_hook</span><span class="special">&lt;</span><span class="identifier">BaseHook</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">l</span><span class="special">;</span>
   <span class="comment">//...</span>
<span class="special">};</span>
</pre>
<p>
      Member hooks are not suitable for recursive structures:
    </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Recursive</span>
<span class="special">{</span>
   <span class="keyword">private</span><span class="special">:</span>
   <span class="identifier">Recursive</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
   <span class="identifier">Recursive</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>

   <span class="keyword">public</span><span class="special">:</span>
   <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;</span> <span class="identifier">memhook</span><span class="special">;</span>
   <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">member_hook</span><span class="special">&lt;</span><span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;,</span> <span class="special">&amp;</span><span class="identifier">Recursive</span><span class="special">::</span><span class="identifier">memhook</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">children</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
      Specifying <code class="computeroutput"><span class="special">&amp;</span><span class="identifier">Recursive</span><span class="special">::</span><span class="identifier">memhook</span></code>
      (that is, the offset between memhook and Recursive) provokes an early instantiation
      of <code class="computeroutput"><span class="identifier">Recursive</span></code>. To define recursive
      structures using member hooks, a programmer should use <code class="computeroutput">function_hook</code>:
    </p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">parent_from_member</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>

<span class="keyword">class</span> <span class="identifier">Recursive</span><span class="special">;</span>

<span class="comment">//Declaration of the functor that converts betwen the Recursive</span>
<span class="comment">//class and the hook</span>
<span class="keyword">struct</span> <span class="identifier">Functor</span>
<span class="special">{</span>
   <span class="comment">//Required types</span>
   <span class="keyword">typedef</span> <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;</span>    <span class="identifier">hook_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="identifier">hook_type</span><span class="special">*</span>            <span class="identifier">hook_ptr</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="keyword">const</span> <span class="identifier">hook_type</span><span class="special">*</span>      <span class="identifier">const_hook_ptr</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="identifier">Recursive</span>             <span class="identifier">value_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="identifier">value_type</span><span class="special">*</span>           <span class="identifier">pointer</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="keyword">const</span> <span class="identifier">value_type</span><span class="special">*</span>     <span class="identifier">const_pointer</span><span class="special">;</span>

   <span class="comment">//Required static functions</span>
   <span class="keyword">static</span> <span class="identifier">hook_ptr</span> <span class="identifier">to_hook_ptr</span> <span class="special">(</span><span class="identifier">value_type</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">);</span>
   <span class="keyword">static</span> <span class="identifier">const_hook_ptr</span> <span class="identifier">to_hook_ptr</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">value_type</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">);</span>
   <span class="keyword">static</span> <span class="identifier">pointer</span> <span class="identifier">to_value_ptr</span><span class="special">(</span><span class="identifier">hook_ptr</span> <span class="identifier">n</span><span class="special">);</span>
   <span class="keyword">static</span> <span class="identifier">const_pointer</span> <span class="identifier">to_value_ptr</span><span class="special">(</span><span class="identifier">const_hook_ptr</span> <span class="identifier">n</span><span class="special">);</span>
<span class="special">};</span>

<span class="comment">//Define the recursive class</span>
<span class="keyword">class</span> <span class="identifier">Recursive</span>
<span class="special">{</span>
   <span class="keyword">private</span><span class="special">:</span>
   <span class="identifier">Recursive</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>
   <span class="identifier">Recursive</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">Recursive</span><span class="special">&amp;);</span>

   <span class="keyword">public</span><span class="special">:</span>
   <span class="identifier">Recursive</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">hook</span><span class="special">(),</span> <span class="identifier">children</span><span class="special">()</span> <span class="special">{}</span>
   <span class="identifier">list_member_hook</span><span class="special">&lt;&gt;</span> <span class="identifier">hook</span><span class="special">;</span>
   <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">function_hook</span><span class="special">&lt;</span> <span class="identifier">Functor</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">children</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">//Definition of Functor functions</span>
<span class="keyword">inline</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">hook_ptr</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">to_hook_ptr</span> <span class="special">(</span><span class="identifier">Functor</span><span class="special">::</span><span class="identifier">value_type</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">)</span>
   <span class="special">{</span>  <span class="keyword">return</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">.</span><span class="identifier">hook</span><span class="special">;</span> <span class="special">}</span>
<span class="keyword">inline</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">const_hook_ptr</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">to_hook_ptr</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">value_type</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">)</span>
   <span class="special">{</span>  <span class="keyword">return</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">.</span><span class="identifier">hook</span><span class="special">;</span> <span class="special">}</span>
<span class="keyword">inline</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">pointer</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">to_value_ptr</span><span class="special">(</span><span class="identifier">Functor</span><span class="special">::</span><span class="identifier">hook_ptr</span> <span class="identifier">n</span><span class="special">)</span>
   <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">get_parent_from_member</span><span class="special">&lt;</span><span class="identifier">Recursive</span><span class="special">&gt;(</span><span class="identifier">n</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Recursive</span><span class="special">::</span><span class="identifier">hook</span><span class="special">);</span>  <span class="special">}</span>
<span class="keyword">inline</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">const_pointer</span> <span class="identifier">Functor</span><span class="special">::</span><span class="identifier">to_value_ptr</span><span class="special">(</span><span class="identifier">Functor</span><span class="special">::</span><span class="identifier">const_hook_ptr</span> <span class="identifier">n</span><span class="special">)</span>
   <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">get_parent_from_member</span><span class="special">&lt;</span><span class="identifier">Recursive</span><span class="special">&gt;(</span><span class="identifier">n</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Recursive</span><span class="special">::</span><span class="identifier">hook</span><span class="special">);</span>  <span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
   <span class="identifier">Recursive</span> <span class="identifier">f</span><span class="special">,</span> <span class="identifier">f2</span><span class="special">;</span>
   <span class="comment">//A recursive list of Recursive</span>
   <span class="identifier">list</span><span class="special">&lt;</span> <span class="identifier">Recursive</span><span class="special">,</span> <span class="identifier">function_hook</span><span class="special">&lt;</span> <span class="identifier">Functor</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">l</span><span class="special">;</span>

   <span class="comment">//Insert a node in parent list</span>
   <span class="identifier">l</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f</span><span class="special">);</span>

   <span class="comment">//Insert a node in child list</span>
   <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f2</span><span class="special">);</span>

   <span class="comment">//Objects properly inserted</span>
   <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
   <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>

   <span class="comment">//Clear both lists</span>
   <span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()-&gt;</span><span class="identifier">children</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
   <span class="identifier">l</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
   <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
    </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2005 Olaf Krzikalla<br>Copyright &#169; 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>)
      </p>
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