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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>
<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>
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>
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<td align="right"><div class="copyright-footer">Copyright © 2005 Olaf Krzikalla<br>Copyright © 2006-2015 Ion Gaztanaga<p>
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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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