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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="intrusive.usage_when"></a><a class="link" href="usage_when.html" title="When to use?">When to use?</a> |
| </h2></div></div></div> |
| <p> |
| Intrusive containers can be used for highly optimized algorithms, where speed |
| is a crucial issue and: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| additional memory management should be avoided. |
| </li> |
| <li class="listitem"> |
| the programmer needs to efficiently track the construction and destruction |
| of objects. |
| </li> |
| <li class="listitem"> |
| exception safety, especially the no-throw guarantee, is needed. |
| </li> |
| <li class="listitem"> |
| the computation of an iterator to an element from a pointer or reference |
| to that element should be a constant time operation. |
| </li> |
| <li class="listitem"> |
| it's important to achieve a well-known worst-time system response. |
| </li> |
| <li class="listitem"> |
| localization of data (e.g. for cache hit optimization) leads to measurable |
| effects. |
| </li> |
| </ul></div> |
| <p> |
| The last point is important if you have a lot of containers over a set of elements. |
| E.g. if you have a vector of objects (say, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">Object</span><span class="special">></span></code>), and you also have a list storing a subset |
| of those objects (<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">Object</span><span class="special">*></span></code>), |
| then operating on an Object from the list iterator (<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">Object</span><span class="special">*>::</span><span class="identifier">iterator</span></code>) requires two steps: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Access from the iterator (usually on the stack) to the list node storing |
| a pointer to <code class="computeroutput"><span class="identifier">Object</span></code>. |
| </li> |
| <li class="listitem"> |
| Access from the pointer to <code class="computeroutput"><span class="identifier">Object</span></code> |
| to the Object stored in the vector. |
| </li> |
| </ul></div> |
| <p> |
| While the objects themselves are tightly packed in the memory of the vector |
| (a vector's memory is guaranteed to be contiguous), and form something like |
| a data block, list nodes may be dispersed in the heap memory. Hence depending |
| on your system you might get a lot of cache misses. The same doesn't hold for |
| an intrusive list. Indeed, dereferencing an iterator from an intrusive list |
| is performed in the same two steps as described above. But the list node is |
| already embedded in the Object, so the memory is directly tracked from the |
| iterator to the Object. |
| </p> |
| <p> |
| It's also possible to use intrusive containers when the objects to be stored |
| can have different or unknown size. This allows storing base and derived objects |
| in the same container, as shown in the following 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">//An abstract class that can be inserted in an intrusive list |
| </span><span class="keyword">class</span> <span class="identifier">Window</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="comment">//This is a container those value is an abstract class: you can't do this with std::list. |
| </span> <span class="keyword">typedef</span> <span class="identifier">list</span><span class="special"><</span><span class="identifier">Window</span><span class="special">></span> <span class="identifier">win_list</span><span class="special">;</span> |
| |
| <span class="comment">//A static intrusive list declaration |
| </span> <span class="keyword">static</span> <span class="identifier">win_list</span> <span class="identifier">all_windows</span><span class="special">;</span> |
| |
| <span class="comment">//Constructor. Includes this window in the list |
| </span> <span class="identifier">Window</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">all_windows</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(*</span><span class="keyword">this</span><span class="special">);</span> <span class="special">}</span> |
| <span class="comment">//Destructor. Removes this node from the list |
| </span> <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Window</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">all_windows</span><span class="special">.</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">win_list</span><span class="special">::</span><span class="identifier">s_iterator_to</span><span class="special">(*</span><span class="keyword">this</span><span class="special">));</span> <span class="special">}</span> |
| <span class="comment">//Pure virtual function to be implemented by derived classes |
| </span> <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="identifier">Paint</span><span class="special">()</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="comment">//The static intrusive list declaration |
| </span><span class="identifier">Window</span><span class="special">::</span><span class="identifier">win_list</span> <span class="identifier">Window</span><span class="special">::</span><span class="identifier">all_windows</span><span class="special">;</span> |
| |
| <span class="comment">//Some Window derived classes |
| </span><span class="keyword">class</span> <span class="identifier">FrameWindow</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Window</span> |
| <span class="special">{</span> <span class="keyword">void</span> <span class="identifier">Paint</span><span class="special">(){/**/}</span> <span class="special">};</span> |
| |
| <span class="keyword">class</span> <span class="identifier">EditWindow</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Window</span> |
| <span class="special">{</span> <span class="keyword">void</span> <span class="identifier">Paint</span><span class="special">(){/**/}</span> <span class="special">};</span> |
| |
| <span class="keyword">class</span> <span class="identifier">CanvasWindow</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Window</span> |
| <span class="special">{</span> <span class="keyword">void</span> <span class="identifier">Paint</span><span class="special">(){/**/}</span> <span class="special">};</span> |
| |
| <span class="comment">//A function that prints all windows stored in the intrusive list |
| </span><span class="keyword">void</span> <span class="identifier">paint_all_windows</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="keyword">for</span><span class="special">(</span><span class="identifier">Window</span><span class="special">::</span><span class="identifier">win_list</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">i</span><span class="special">(</span><span class="identifier">Window</span><span class="special">::</span><span class="identifier">all_windows</span><span class="special">.</span><span class="identifier">begin</span><span class="special">())</span> |
| <span class="special">,</span> <span class="identifier">e</span><span class="special">(</span><span class="identifier">Window</span><span class="special">::</span><span class="identifier">all_windows</span><span class="special">.</span><span class="identifier">end</span><span class="special">())</span> |
| <span class="special">;</span> <span class="identifier">i</span> <span class="special">!=</span> <span class="identifier">e</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> |
| <span class="identifier">i</span><span class="special">-></span><span class="identifier">Paint</span><span class="special">();</span> |
| <span class="special">}</span> |
| |
| <span class="comment">//... |
| </span> |
| <span class="comment">//A class derived from Window |
| </span><span class="keyword">class</span> <span class="identifier">MainWindow</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Window</span> |
| <span class="special">{</span> |
| <span class="identifier">FrameWindow</span> <span class="identifier">frame_</span><span class="special">;</span> <span class="comment">//these are derived from Window too |
| </span> <span class="identifier">EditWindow</span> <span class="identifier">edit_</span><span class="special">;</span> |
| <span class="identifier">CanvasWindow</span> <span class="identifier">canvas_</span><span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="keyword">void</span> <span class="identifier">Paint</span><span class="special">(){/**/}</span> |
| <span class="comment">//... |
| </span><span class="special">};</span> |
| |
| <span class="comment">//Main function |
| </span><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="comment">//When a Window class is created, is automatically registered in the global list |
| </span> <span class="identifier">MainWindow</span> <span class="identifier">window</span><span class="special">;</span> |
| |
| <span class="comment">//Paint all the windows, sub-windows and so on |
| </span> <span class="identifier">paint_all_windows</span><span class="special">();</span> |
| |
| <span class="comment">//All the windows are automatically unregistered in their destructors. |
| </span> <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Due to certain properties of intrusive containers they are often more difficult |
| to use than their STL-counterparts. That's why you should avoid them in public |
| interfaces of libraries. Classes to be stored in intrusive containers must |
| change their implementation to store the hook and this is not always possible |
| or desirable. |
| </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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