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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="intrusive.erasing_and_disposing"></a><a class="link" href="erasing_and_disposing.html" title="Erasing and disposing values from Boost.Intrusive containers">Erasing and disposing |
| values from Boost.Intrusive containers</a> |
| </h2></div></div></div> |
| <p> |
| One of the most tedious tasks when using intrusive containers is the management |
| of the erased elements. When using STL containers, the container itself unlinks |
| and destroys the contained elements, but with intrusive containers, the user |
| must explicitly destroy the object after erasing an element from the container. |
| This makes STL-like functions erasing multiple objects unhelpful: the user |
| can't destroy every erased element. For example, let's take the function <code class="computeroutput"><span class="identifier">remove_if</span></code> from <code class="computeroutput"><a class="link" href="../boost/intrusive/list.html" title="Class template list">list</a></code>: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Pred</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">remove_if</span><span class="special">(</span><span class="identifier">Pred</span> <span class="identifier">pred</span><span class="special">);</span> |
| </pre> |
| <p> |
| How can the user destroy the elements (say, using <code class="computeroutput"><span class="keyword">operator</span> |
| <span class="keyword">delete</span></code>) that will be erased according |
| to the predicate? <span class="bold"><strong>Boost.Intrusive</strong></span> containers |
| offer additional functions that take a function object that will be called |
| after the element has been erased from the container. For example, <code class="computeroutput"><a class="link" href="../boost/intrusive/list.html" title="Class template list">list</a></code> offers: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Pred</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Disposer</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">remove_and_dispose_if</span><span class="special">(</span><span class="identifier">Pred</span> <span class="identifier">pred</span><span class="special">,</span> <span class="identifier">Disposer</span> <span class="identifier">disposer</span><span class="special">)</span> |
| </pre> |
| <p> |
| With this function the user can efficiently remove and destroy elements if |
| the disposer function destroys an object: <code class="computeroutput"><span class="identifier">remove_and_dispose_if</span></code> |
| will call the "disposer" function object for every removed element. |
| <code class="computeroutput"><a class="link" href="../boost/intrusive/list.html" title="Class template list">list</a></code> offers more functions |
| taking a disposer function object as argument, like <code class="computeroutput"><span class="identifier">erase_and_dispose</span></code>, |
| <code class="computeroutput"><span class="identifier">clear_and_dispose</span></code>, <code class="computeroutput"><span class="identifier">remove_and_dispose</span></code>, etc. |
| </p> |
| <p> |
| Note that the disposing function does not need to just destroy the object. |
| It can implement any other operation like inserting the remove object in another |
| container. Let's see a small example: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</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">></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="comment">//A class that can be inserted in an intrusive list |
| </span><span class="keyword">class</span> <span class="identifier">my_class</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">list_base_hook</span><span class="special"><></span> |
| <span class="special">{</span> |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="identifier">my_class</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> |
| <span class="special">:</span> <span class="identifier">int_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> |
| <span class="special">{}</span> |
| |
| <span class="keyword">int</span> <span class="identifier">int_</span><span class="special">;</span> |
| <span class="comment">//... |
| </span><span class="special">};</span> |
| |
| <span class="comment">//Definition of the intrusive list |
| </span><span class="keyword">typedef</span> <span class="identifier">list</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">></span> <span class="identifier">my_class_list</span><span class="special">;</span> |
| |
| <span class="comment">//The predicate function |
| </span><span class="keyword">struct</span> <span class="identifier">is_even</span> |
| <span class="special">{</span> |
| <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">my_class</span> <span class="special">&</span><span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> <span class="keyword">return</span> <span class="number">0</span> <span class="special">==</span> <span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">int_</span> <span class="special">%</span> <span class="number">2</span><span class="special">);</span> <span class="special">}</span> |
| <span class="special">};</span> |
| |
| <span class="comment">//The disposer object function |
| </span><span class="keyword">struct</span> <span class="identifier">delete_disposer</span> |
| <span class="special">{</span> |
| <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">my_class</span> <span class="special">*</span><span class="identifier">delete_this</span><span class="special">)</span> |
| <span class="special">{</span> <span class="keyword">delete</span> <span class="identifier">delete_this</span><span class="special">;</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="keyword">const</span> <span class="keyword">int</span> <span class="identifier">MaxElem</span> <span class="special">=</span> <span class="number">100</span><span class="special">;</span> |
| |
| <span class="comment">//Fill all the nodes and insert them in the list |
| </span> <span class="identifier">my_class_list</span> <span class="identifier">list</span><span class="special">;</span> |
| |
| <span class="keyword">try</span><span class="special">{</span> |
| <span class="comment">//Insert new objects in the container |
| </span> <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">MaxElem</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="identifier">list</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(*</span><span class="keyword">new</span> <span class="identifier">my_class</span><span class="special">(</span><span class="identifier">i</span><span class="special">));</span> |
| |
| <span class="comment">//Now use remove_and_dispose_if to erase and delete the objects |
| </span> <span class="identifier">list</span><span class="special">.</span><span class="identifier">remove_and_dispose_if</span><span class="special">(</span><span class="identifier">is_even</span><span class="special">(),</span> <span class="identifier">delete_disposer</span><span class="special">());</span> |
| <span class="special">}</span> |
| <span class="keyword">catch</span><span class="special">(...){</span> |
| <span class="comment">//If something throws, make sure that all the memory is freed |
| </span> <span class="identifier">list</span><span class="special">.</span><span class="identifier">clear_and_dispose</span><span class="special">(</span><span class="identifier">delete_disposer</span><span class="special">());</span> |
| <span class="keyword">throw</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="comment">//Dispose remaining elements |
| </span> <span class="identifier">list</span><span class="special">.</span><span class="identifier">erase_and_dispose</span><span class="special">(</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">list</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">delete_disposer</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> |
| All <span class="bold"><strong>Boost.Intrusive</strong></span> containers offer these |
| "erase + dispose" additional members for all functions that erase |
| an element from the container. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2005 Olaf Krzikalla, 2006-2010 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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